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elisp None ### Profiling
If your program is working correctly, but not fast enough, and you want to make it run more quickly or efficiently, the first thing to do is *profile* your code so that you know where it spends most of the execution time. If you find that one particular function is responsible for a significant portion of the execution time, you can start looking for ways to optimize that piece.
Emacs has built-in support for this. To begin profiling, type `M-x profiler-start`. You can choose to sample CPU usage periodically (`cpu`), when memory is allocated (`memory`), or both. Then run the code you’d like to speed up. After that, type `M-x profiler-report` to display a summary buffer for CPU usage sampled by each type (cpu and memory) that you chose to profile. The names of the report buffers include the times at which the reports were generated, so you can generate another report later on without erasing previous results. When you have finished profiling, type `M-x profiler-stop` (there is a small overhead associated with profiling, so we don’t recommend leaving it active except when you are actually running the code you want to examine).
The profiler report buffer shows, on each line, a function that was called, preceded by how much CPU resources it used in absolute and percentage terms since profiling started. If a given line has a ‘`+`’ symbol to the left of the function name, you can expand that line by typing `RET`, in order to see the function(s) called by the higher-level function. Use a prefix argument (`C-u RET`) to see the whole call tree below a function. Pressing `RET` again will collapse back to the original state.
Press `j` or `mouse-2` to jump to the definition of a function at point. Press `d` to view a function’s documentation. You can save a profile to a file using `C-x C-w`. You can compare two profiles using `=`.
The `elp` library offers an alternative approach, which is useful when you know in advance which Lisp function(s) you want to profile. Using that library, you begin by setting `elp-function-list` to the list of function symbols—those are the functions you want to profile. Then type `M-x elp-instrument-list RET nil RET` to arrange for profiling those functions. After running the code you want to profile, invoke `M-x elp-results` to display the current results. See the file `elp.el` for more detailed instructions. This approach is limited to profiling functions written in Lisp, it cannot profile Emacs primitives.
You can measure the time it takes to evaluate individual Emacs Lisp forms using the `benchmark` library. See the function `benchmark-call` as well as the macros `benchmark-run`, `benchmark-run-compiled`, `benchmark-progn` and `benchmark-call` in `benchmark.el`. You can also use the `benchmark` command for timing forms interactively.
To profile Emacs at the level of its C code, you can build it using the `--enable-profiling` option of `configure`. When Emacs exits, it generates a file `gmon.out` that you can examine using the `gprof` utility. This feature is mainly useful for debugging Emacs. It actually stops the Lisp-level `M-x profiler-…` commands described above from working.
---
elisp None #### Composite Types
When none of the simple types is appropriate, you can use composite types, which build new types from other types or from specified data. The specified types or data are called the *arguments* of the composite type. The composite type normally looks like this:
```
(constructor arguments…)
```
but you can also add keyword-value pairs before the arguments, like this:
```
(constructor {keyword value}… arguments…)
```
Here is a table of constructors and how to use them to write composite types:
`(cons car-type cdr-type)`
The value must be a cons cell, its CAR must fit car-type, and its CDR must fit cdr-type. For example, `(cons string
symbol)` is a customization type which matches values such as `("foo" . foo)`.
In the customization buffer, the CAR and CDR are displayed and edited separately, each according to their specified type.
`(list element-types…)`
The value must be a list with exactly as many elements as the element-types given; and each element must fit the corresponding element-type.
For example, `(list integer string function)` describes a list of three elements; the first element must be an integer, the second a string, and the third a function.
In the customization buffer, each element is displayed and edited separately, according to the type specified for it.
`(group element-types…)`
This works like `list` except for the formatting of text in the Custom buffer. `list` labels each element value with its tag; `group` does not.
`(vector element-types…)`
Like `list` except that the value must be a vector instead of a list. The elements work the same as in `list`.
`(alist :key-type key-type :value-type value-type)`
The value must be a list of cons-cells, the CAR of each cell representing a key of customization type key-type, and the CDR of the same cell representing a value of customization type value-type. The user can add and delete key/value pairs, and edit both the key and the value of each pair.
If omitted, key-type and value-type default to `sexp`.
The user can add any key matching the specified key type, but you can give some keys a preferential treatment by specifying them with the `:options` (see [Variable Definitions](variable-definitions)). The specified keys will always be shown in the customize buffer (together with a suitable value), with a checkbox to include or exclude or disable the key/value pair from the alist. The user will not be able to edit the keys specified by the `:options` keyword argument.
The argument to the `:options` keywords should be a list of specifications for reasonable keys in the alist. Ordinarily, they are simply atoms, which stand for themselves. For example:
```
:options '("foo" "bar" "baz")
```
specifies that there are three known keys, namely `"foo"`, `"bar"` and `"baz"`, which will always be shown first.
You may want to restrict the value type for specific keys, for example, the value associated with the `"bar"` key can only be an integer. You can specify this by using a list instead of an atom in the list. The first element will specify the key, like before, while the second element will specify the value type. For example:
```
:options '("foo" ("bar" integer) "baz")
```
Finally, you may want to change how the key is presented. By default, the key is simply shown as a `const`, since the user cannot change the special keys specified with the `:options` keyword. However, you may want to use a more specialized type for presenting the key, like `function-item` if you know it is a symbol with a function binding. This is done by using a customization type specification instead of a symbol for the key.
```
:options '("foo"
((function-item some-function) integer)
"baz")
```
Many alists use lists with two elements, instead of cons cells. For example,
```
(defcustom list-alist
'(("foo" 1) ("bar" 2) ("baz" 3))
"Each element is a list of the form (KEY VALUE).")
```
instead of
```
(defcustom cons-alist
'(("foo" . 1) ("bar" . 2) ("baz" . 3))
"Each element is a cons-cell (KEY . VALUE).")
```
Because of the way lists are implemented on top of cons cells, you can treat `list-alist` in the example above as a cons cell alist, where the value type is a list with a single element containing the real value.
```
(defcustom list-alist '(("foo" 1) ("bar" 2) ("baz" 3))
"Each element is a list of the form (KEY VALUE)."
:type '(alist :value-type (group integer)))
```
The `group` widget is used here instead of `list` only because the formatting is better suited for the purpose.
Similarly, you can have alists with more values associated with each key, using variations of this trick:
```
(defcustom person-data '(("brian" 50 t)
("dorith" 55 nil)
("ken" 52 t))
"Alist of basic info about people.
Each element has the form (NAME AGE MALE-FLAG)."
:type '(alist :value-type (group integer boolean)))
```
`(plist :key-type key-type :value-type value-type)`
This customization type is similar to `alist` (see above), except that (i) the information is stored as a property list, (see [Property Lists](property-lists)), and (ii) key-type, if omitted, defaults to `symbol` rather than `sexp`.
`(choice alternative-types…)`
The value must fit one of alternative-types. For example, `(choice integer string)` allows either an integer or a string.
In the customization buffer, the user selects an alternative using a menu, and can then edit the value in the usual way for that alternative.
Normally the strings in this menu are determined automatically from the choices; however, you can specify different strings for the menu by including the `:tag` keyword in the alternatives. For example, if an integer stands for a number of spaces, while a string is text to use verbatim, you might write the customization type this way,
```
(choice (integer :tag "Number of spaces")
(string :tag "Literal text"))
```
so that the menu offers ‘`Number of spaces`’ and ‘`Literal text`’.
In any alternative for which `nil` is not a valid value, other than a `const`, you should specify a valid default for that alternative using the `:value` keyword. See [Type Keywords](type-keywords).
If some values are covered by more than one of the alternatives, customize will choose the first alternative that the value fits. This means you should always list the most specific types first, and the most general last. Here’s an example of proper usage:
```
(choice (const :tag "Off" nil)
symbol (sexp :tag "Other"))
```
This way, the special value `nil` is not treated like other symbols, and symbols are not treated like other Lisp expressions.
`(radio element-types…)`
This is similar to `choice`, except that the choices are displayed using radio buttons rather than a menu. This has the advantage of displaying documentation for the choices when applicable and so is often a good choice for a choice between constant functions (`function-item` customization types).
`(const value)`
The value must be value—nothing else is allowed.
The main use of `const` is inside of `choice`. For example, `(choice integer (const nil))` allows either an integer or `nil`.
`:tag` is often used with `const`, inside of `choice`. For example,
```
(choice (const :tag "Yes" t)
(const :tag "No" nil)
(const :tag "Ask" foo))
```
describes a variable for which `t` means yes, `nil` means no, and `foo` means “ask”.
`(other value)`
This alternative can match any Lisp value, but if the user chooses this alternative, that selects the value value.
The main use of `other` is as the last element of `choice`. For example,
```
(choice (const :tag "Yes" t)
(const :tag "No" nil)
(other :tag "Ask" foo))
```
describes a variable for which `t` means yes, `nil` means no, and anything else means “ask”. If the user chooses ‘`Ask`’ from the menu of alternatives, that specifies the value `foo`; but any other value (not `t`, `nil` or `foo`) displays as ‘`Ask`’, just like `foo`.
`(function-item function)`
Like `const`, but used for values which are functions. This displays the documentation string as well as the function name. The documentation string is either the one you specify with `:doc`, or function’s own documentation string.
`(variable-item variable)`
Like `const`, but used for values which are variable names. This displays the documentation string as well as the variable name. The documentation string is either the one you specify with `:doc`, or variable’s own documentation string.
`(set types…)`
The value must be a list, and each element of the list must match one of the types specified.
This appears in the customization buffer as a checklist, so that each of types may have either one corresponding element or none. It is not possible to specify two different elements that match the same one of types. For example, `(set integer symbol)` allows one integer and/or one symbol in the list; it does not allow multiple integers or multiple symbols. As a result, it is rare to use nonspecific types such as `integer` in a `set`.
Most often, the types in a `set` are `const` types, as shown here:
```
(set (const :bold) (const :italic))
```
Sometimes they describe possible elements in an alist:
```
(set (cons :tag "Height" (const height) integer)
(cons :tag "Width" (const width) integer))
```
That lets the user specify a height value optionally and a width value optionally.
`(repeat element-type)`
The value must be a list and each element of the list must fit the type element-type. This appears in the customization buffer as a list of elements, with ‘`[INS]`’ and ‘`[DEL]`’ buttons for adding more elements or removing elements.
`(restricted-sexp :match-alternatives criteria)`
This is the most general composite type construct. The value may be any Lisp object that satisfies one of criteria. criteria should be a list, and each element should be one of these possibilities:
* A predicate—that is, a function of one argument that returns either `nil` or non-`nil` according to the argument. Using a predicate in the list says that objects for which the predicate returns non-`nil` are acceptable.
* A quoted constant—that is, `'object`. This sort of element in the list says that object itself is an acceptable value.
For example,
```
(restricted-sexp :match-alternatives
(integerp 't 'nil))
```
allows integers, `t` and `nil` as legitimate values.
The customization buffer shows all legitimate values using their read syntax, and the user edits them textually.
Here is a table of the keywords you can use in keyword-value pairs in a composite type:
`:tag tag`
Use tag as the name of this alternative, for user communication purposes. This is useful for a type that appears inside of a `choice`.
`:match-alternatives criteria`
Use criteria to match possible values. This is used only in `restricted-sexp`.
`:args argument-list`
Use the elements of argument-list as the arguments of the type construct. For instance, `(const :args (foo))` is equivalent to `(const foo)`. You rarely need to write `:args` explicitly, because normally the arguments are recognized automatically as whatever follows the last keyword-value pair.
elisp None #### Char-Table Type
A *char-table* is a one-dimensional array of elements of any type, indexed by character codes. Char-tables have certain extra features to make them more useful for many jobs that involve assigning information to character codes—for example, a char-table can have a parent to inherit from, a default value, and a small number of extra slots to use for special purposes. A char-table can also specify a single value for a whole character set.
The printed representation of a char-table is like a vector except that there is an extra ‘`#^`’ at the beginning.[1](#FOOT1)
See [Char-Tables](char_002dtables), for special functions to operate on char-tables. Uses of char-tables include:
* Case tables (see [Case Tables](case-tables)).
* Character category tables (see [Categories](categories)).
* Display tables (see [Display Tables](display-tables)).
* Syntax tables (see [Syntax Tables](syntax-tables)).
elisp None ### Byte-Compilation Functions
You can byte-compile an individual function or macro definition with the `byte-compile` function. You can compile a whole file with `byte-compile-file`, or several files with `byte-recompile-directory` or `batch-byte-compile`.
Sometimes, the byte compiler produces warning and/or error messages (see [Compiler Errors](compiler-errors), for details). These messages are normally recorded in a buffer called `\*Compile-Log\*`, which uses Compilation mode. See [Compilation Mode](https://www.gnu.org/software/emacs/manual/html_node/emacs/Compilation-Mode.html#Compilation-Mode) in The GNU Emacs Manual. However, if the variable `byte-compile-debug` is non-`nil`, error messages will be signaled as Lisp errors instead (see [Errors](errors)).
Be careful when writing macro calls in files that you intend to byte-compile. Since macro calls are expanded when they are compiled, the macros need to be loaded into Emacs or the byte compiler will not do the right thing. The usual way to handle this is with `require` forms which specify the files containing the needed macro definitions (see [Named Features](named-features)). Normally, the byte compiler does not evaluate the code that it is compiling, but it handles `require` forms specially, by loading the specified libraries. To avoid loading the macro definition files when someone *runs* the compiled program, write `eval-when-compile` around the `require` calls (see [Eval During Compile](eval-during-compile)). For more details, See [Compiling Macros](compiling-macros).
Inline (`defsubst`) functions are less troublesome; if you compile a call to such a function before its definition is known, the call will still work right, it will just run slower.
Function: **byte-compile** *symbol*
This function byte-compiles the function definition of symbol, replacing the previous definition with the compiled one. The function definition of symbol must be the actual code for the function; `byte-compile` does not handle function indirection. The return value is the byte-code function object which is the compiled definition of symbol (see [Byte-Code Objects](byte_002dcode-objects)).
```
(defun factorial (integer)
"Compute factorial of INTEGER."
(if (= 1 integer) 1
(* integer (factorial (1- integer)))))
⇒ factorial
```
```
(byte-compile 'factorial)
⇒
#[(integer)
"^H\301U\203^H^@\301\207\302^H\303^HS!\"\207"
[integer 1 * factorial]
4 "Compute factorial of INTEGER."]
```
If symbol’s definition is a byte-code function object, `byte-compile` does nothing and returns `nil`. It does not compile the symbol’s definition again, since the original (non-compiled) code has already been replaced in the symbol’s function cell by the byte-compiled code.
The argument to `byte-compile` can also be a `lambda` expression. In that case, the function returns the corresponding compiled code but does not store it anywhere.
Command: **compile-defun** *&optional arg*
This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function.
`compile-defun` normally displays the result of evaluation in the echo area, but if arg is non-`nil`, it inserts the result in the current buffer after the form it has compiled.
Command: **byte-compile-file** *filename*
This function compiles a file of Lisp code named filename into a file of byte-code. The output file’s name is made by changing the ‘`.el`’ suffix into ‘`.elc`’; if filename does not end in ‘`.el`’, it adds ‘`.elc`’ to the end of filename.
Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read.
This command returns `t` if there were no errors and `nil` otherwise. When called interactively, it prompts for the file name.
```
$ ls -l push*
-rw-r--r-- 1 lewis lewis 791 Oct 5 20:31 push.el
```
```
(byte-compile-file "~/emacs/push.el")
⇒ t
```
```
$ ls -l push*
-rw-r--r-- 1 lewis lewis 791 Oct 5 20:31 push.el
-rw-rw-rw- 1 lewis lewis 638 Oct 8 20:25 push.elc
```
Command: **byte-recompile-directory** *directory &optional flag force follow-symlinks*
This command recompiles every ‘`.el`’ file in directory (or its subdirectories) that needs recompilation. A file needs recompilation if a ‘`.elc`’ file exists but is older than the ‘`.el`’ file.
When a ‘`.el`’ file has no corresponding ‘`.elc`’ file, flag says what to do. If it is `nil`, this command ignores these files. If flag is 0, it compiles them. If it is neither `nil` nor 0, it asks the user whether to compile each such file, and asks about each subdirectory as well.
Interactively, `byte-recompile-directory` prompts for directory and flag is the prefix argument.
If force is non-`nil`, this command recompiles every ‘`.el`’ file that has a ‘`.elc`’ file.
This command will normally not compile ‘`.el`’ files that are symlinked. If the optional follow-symlink parameter is non-`nil`, symlinked ‘`.el`’ will also be compiled.
The returned value is unpredictable.
Function: **batch-byte-compile** *&optional noforce*
This function runs `byte-compile-file` on files specified on the command line. This function must be used only in a batch execution of Emacs, as it kills Emacs on completion. An error in one file does not prevent processing of subsequent files, but no output file will be generated for it, and the Emacs process will terminate with a nonzero status code.
If noforce is non-`nil`, this function does not recompile files that have an up-to-date ‘`.elc`’ file.
```
$ emacs -batch -f batch-byte-compile *.el
```
| programming_docs |
elisp None ### Introduction to Evaluation
The Lisp interpreter, or evaluator, is the part of Emacs that computes the value of an expression that is given to it. When a function written in Lisp is called, the evaluator computes the value of the function by evaluating the expressions in the function body. Thus, running any Lisp program really means running the Lisp interpreter.
A Lisp object that is intended for evaluation is called a *form* or *expression*[7](#FOOT7). The fact that forms are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often.
In subsequent sections, we will describe the details of what evaluation means for each kind of form.
It is very common to read a Lisp form and then evaluate the form, but reading and evaluation are separate activities, and either can be performed alone. Reading per se does not evaluate anything; it converts the printed representation of a Lisp object to the object itself. It is up to the caller of `read` to specify whether this object is a form to be evaluated, or serves some entirely different purpose. See [Input Functions](input-functions).
Evaluation is a recursive process, and evaluating a form often involves evaluating parts within that form. For instance, when you evaluate a *function call* form such as `(car x)`, Emacs first evaluates the argument (the subform `x`). After evaluating the argument, Emacs *executes* the function (`car`), and if the function is written in Lisp, execution works by evaluating the *body* of the function (in this example, however, `car` is not a Lisp function; it is a primitive function implemented in C). See [Functions](functions), for more information about functions and function calls.
Evaluation takes place in a context called the *environment*, which consists of the current values and bindings of all Lisp variables (see [Variables](variables)).[8](#FOOT8) Whenever a form refers to a variable without creating a new binding for it, the variable evaluates to the value given by the current environment. Evaluating a form may also temporarily alter the environment by binding variables (see [Local Variables](local-variables)).
Evaluating a form may also make changes that persist; these changes are called *side effects*. An example of a form that produces a side effect is `(setq foo 1)`.
Do not confuse evaluation with command key interpretation. The editor command loop translates keyboard input into a command (an interactively callable function) using the active keymaps, and then uses `call-interactively` to execute that command. Executing the command usually involves evaluation, if the command is written in Lisp; however, this step is not considered a part of command key interpretation. See [Command Loop](command-loop).
elisp None #### Keyboard Events
There are two kinds of input you can get from the keyboard: ordinary keys, and function keys. Ordinary keys correspond to (possibly modified) characters; the events they generate are represented in Lisp as characters. The event type of a *character event* is the character itself (an integer), which might have some modifier bits set; see [Classifying Events](classifying-events).
An input character event consists of a *basic code* between 0 and 524287, plus any or all of these *modifier bits*:
meta
The 2\*\*27 bit in the character code indicates a character typed with the meta key held down.
control
The 2\*\*26 bit in the character code indicates a non-ASCII control character.
ASCII control characters such as `C-a` have special basic codes of their own, so Emacs needs no special bit to indicate them. Thus, the code for `C-a` is just 1.
But if you type a control combination not in ASCII, such as `%` with the control key, the numeric value you get is the code for `%` plus 2\*\*26 (assuming the terminal supports non-ASCII control characters), i.e. with the 27th bit set.
shift
The 2\*\*25 bit (the 26th bit) in the character event code indicates an ASCII control character typed with the shift key held down.
For letters, the basic code itself indicates upper versus lower case; for digits and punctuation, the shift key selects an entirely different character with a different basic code. In order to keep within the ASCII character set whenever possible, Emacs avoids using the 2\*\*25 bit for those character events.
However, ASCII provides no way to distinguish `C-A` from `C-a`, so Emacs uses the 2\*\*25 bit in `C-A` and not in `C-a`.
hyper
The 2\*\*24 bit in the character event code indicates a character typed with the hyper key held down.
super
The 2\*\*23 bit in the character event code indicates a character typed with the super key held down.
alt The 2\*\*22 bit in the character event code indicates a character typed with the alt key held down. (The key labeled Alt on most keyboards is actually treated as the meta key, not this.)
It is best to avoid mentioning specific bit numbers in your program. To test the modifier bits of a character, use the function `event-modifiers` (see [Classifying Events](classifying-events)). When making key bindings, you can use the read syntax for characters with modifier bits (‘`\C-`’, ‘`\M-`’, and so on). For making key bindings with `define-key`, you can use lists such as `(control hyper ?x)` to specify the characters (see [Changing Key Bindings](changing-key-bindings)). The function `event-convert-list` converts such a list into an event type (see [Classifying Events](classifying-events)).
elisp None ### Coding Systems
When Emacs reads or writes a file, and when Emacs sends text to a subprocess or receives text from a subprocess, it normally performs character code conversion and end-of-line conversion as specified by a particular *coding system*.
How to define a coding system is an arcane matter, and is not documented here.
| | | |
| --- | --- | --- |
| • [Coding System Basics](coding-system-basics) | | Basic concepts. |
| • [Encoding and I/O](encoding-and-i_002fo) | | How file I/O functions handle coding systems. |
| • [Lisp and Coding Systems](lisp-and-coding-systems) | | Functions to operate on coding system names. |
| • [User-Chosen Coding Systems](user_002dchosen-coding-systems) | | Asking the user to choose a coding system. |
| • [Default Coding Systems](default-coding-systems) | | Controlling the default choices. |
| • [Specifying Coding Systems](specifying-coding-systems) | | Requesting a particular coding system for a single file operation. |
| • [Explicit Encoding](explicit-encoding) | | Encoding or decoding text without doing I/O. |
| • [Terminal I/O Encoding](terminal-i_002fo-encoding) | | Use of encoding for terminal I/O. |
elisp None #### Warning Options
These variables are used by users to control what happens when a Lisp program reports a warning.
User Option: **warning-minimum-level**
This user option specifies the minimum severity level that should be shown immediately to the user. The default is `:warning`, which means to immediately display all warnings except `:debug` warnings.
User Option: **warning-minimum-log-level**
This user option specifies the minimum severity level that should be logged in the warnings buffer. The default is `:warning`, which means to log all warnings except `:debug` warnings.
User Option: **warning-suppress-types**
This list specifies which warning types should not be displayed immediately for the user. Each element of the list should be a list of symbols. If its elements match the first elements in a warning type, then that warning is not displayed immediately.
User Option: **warning-suppress-log-types**
This list specifies which warning types should not be logged in the warnings buffer. Each element of the list should be a list of symbols. If it matches the first few elements in a warning type, then that warning is not logged.
elisp None #### Completion Variables
Here are some variables that can be used to alter the default completion behavior.
User Option: **completion-styles**
The value of this variable is a list of completion style (symbols) to use for performing completion. A *completion style* is a set of rules for generating completions. Each symbol occurring this list must have a corresponding entry in `completion-styles-alist`.
Variable: **completion-styles-alist**
This variable stores a list of available completion styles. Each element in the list has the form
```
(style try-completion all-completions doc)
```
Here, style is the name of the completion style (a symbol), which may be used in the `completion-styles` variable to refer to this style; try-completion is the function that does the completion; all-completions is the function that lists the completions; and doc is a string describing the completion style.
The try-completion and all-completions functions should each accept four arguments: string, collection, predicate, and point. The string, collection, and predicate arguments have the same meanings as in `try-completion` (see [Basic Completion](basic-completion)), and the point argument is the position of point within string. Each function should return a non-`nil` value if it performed its job, and `nil` if it did not (e.g., if there is no way to complete string according to the completion style).
When the user calls a completion command like `minibuffer-complete` (see [Completion Commands](completion-commands)), Emacs looks for the first style listed in `completion-styles` and calls its try-completion function. If this function returns `nil`, Emacs moves to the next listed completion style and calls its try-completion function, and so on until one of the try-completion functions successfully performs completion and returns a non-`nil` value. A similar procedure is used for listing completions, via the all-completions functions.
See [Completion Styles](https://www.gnu.org/software/emacs/manual/html_node/emacs/Completion-Styles.html#Completion-Styles) in The GNU Emacs Manual, for a description of the available completion styles.
User Option: **completion-category-overrides**
This variable specifies special completion styles and other completion behaviors to use when completing certain types of text. Its value should be an alist with elements of the form `(category
. alist)`. category is a symbol describing what is being completed; currently, the `buffer`, `file`, and `unicode-name` categories are defined, but others can be defined via specialized completion functions (see [Programmed Completion](programmed-completion)). alist is an association list describing how completion should behave for the corresponding category. The following alist keys are supported:
`styles`
The value should be a list of completion styles (symbols).
`cycle` The value should be a value for `completion-cycle-threshold` (see [Completion Options](https://www.gnu.org/software/emacs/manual/html_node/emacs/Completion-Options.html#Completion-Options) in The GNU Emacs Manual) for this category.
Additional alist entries may be defined in the future.
Variable: **completion-extra-properties**
This variable is used to specify extra properties of the current completion command. It is intended to be let-bound by specialized completion commands. Its value should be a list of property and value pairs. The following properties are supported:
`:annotation-function`
The value should be a function to add annotations in the completions buffer. This function must accept one argument, a completion, and should either return `nil` or a string to be displayed next to the completion. Unless this function puts own face on the annotation suffix string, the `completions-annotations` face is added by default to that string.
`:affixation-function`
The value should be a function to add prefixes and suffixes to completions. This function must accept one argument, a list of completions, and should return a list of annotated completions. Each element of the returned list must be a three-element list, the completion, a prefix string, and a suffix string. This function takes priority over `:annotation-function`.
`:exit-function` The value should be a function to run after performing completion. The function should accept two arguments, string and status, where string is the text to which the field was completed, and status indicates what kind of operation happened: `finished` if text is now complete, `sole` if the text cannot be further completed but completion is not finished, or `exact` if the text is a valid completion but may be further completed.
elisp None #### Defining Menus
A keymap acts as a menu if it has an *overall prompt string*, which is a string that appears as an element of the keymap. (See [Format of Keymaps](format-of-keymaps).) The string should describe the purpose of the menu’s commands. Emacs displays the overall prompt string as the menu title in some cases, depending on the toolkit (if any) used for displaying menus.[17](#FOOT17) Keyboard menus also display the overall prompt string.
The easiest way to construct a keymap with a prompt string is to specify the string as an argument when you call `make-keymap`, `make-sparse-keymap` (see [Creating Keymaps](creating-keymaps)), or `define-prefix-command` (see [Definition of define-prefix-command](prefix-keys#Definition-of-define_002dprefix_002dcommand)). If you do not want the keymap to operate as a menu, don’t specify a prompt string for it.
Function: **keymap-prompt** *keymap*
This function returns the overall prompt string of keymap, or `nil` if it has none.
The menu’s items are the bindings in the keymap. Each binding associates an event type to a definition, but the event types have no significance for the menu appearance. (Usually we use pseudo-events, symbols that the keyboard cannot generate, as the event types for menu item bindings.) The menu is generated entirely from the bindings that correspond in the keymap to these events.
The order of items in the menu is the same as the order of bindings in the keymap. Since `define-key` puts new bindings at the front, you should define the menu items starting at the bottom of the menu and moving to the top, if you care about the order. When you add an item to an existing menu, you can specify its position in the menu using `define-key-after` (see [Modifying Menus](modifying-menus)).
| | | |
| --- | --- | --- |
| • [Simple Menu Items](simple-menu-items) | | A simple kind of menu key binding. |
| • [Extended Menu Items](extended-menu-items) | | More complex menu item definitions. |
| • [Menu Separators](menu-separators) | | Drawing a horizontal line through a menu. |
| • [Alias Menu Items](alias-menu-items) | | Using command aliases in menu items. |
elisp None #### Conversion Between Lisp and Module Values
With very few exceptions, most modules need to exchange data with Lisp programs that call them: accept arguments to module functions and return values from module functions. For this purpose, the module API provides the `emacs_value` type, which represents Emacs Lisp objects communicated via the API; it is the functional equivalent of the `Lisp_Object` type used in Emacs C primitives (see [Writing Emacs Primitives](writing-emacs-primitives)). This section describes the parts of the module API that allow to create `emacs_value` objects corresponding to basic Lisp data types, and how to access from C data in `emacs_value` objects that correspond to Lisp objects.
All of the functions described below are actually *function pointers* provided via the pointer to the environment which every module function accepts. Therefore, module code should call these functions through the environment pointer, like this:
```
emacs_env *env; /* the environment pointer */
env->some_function (arguments…);
```
The `emacs_env` pointer will usually come from the first argument to the module function, or from the call to `get_environment` if you need the environment in the module initialization function.
Most of the functions described below became available in Emacs 25, the first Emacs release that supported dynamic modules. For the few functions that became available in later Emacs releases, we mention the first Emacs version that supported them.
The following API functions extract values of various C data types from `emacs_value` objects. They all raise the `wrong-type-argument` error condition (see [Type Predicates](type-predicates)) if the argument `emacs_value` object is not of the type expected by the function. See [Module Nonlocal](module-nonlocal), for details of how signaling errors works in Emacs modules, and how to catch error conditions inside the module before they are reported to Emacs. The API function `type_of` (see [type\_of](module-misc)) can be used to obtain the type of a `emacs_value` object.
Function: *intmax\_t* **extract\_integer** *(emacs\_env \*env, emacs\_value arg)*
This function returns the value of a Lisp integer specified by arg. The C data type of the return value, `intmax_t`, is the widest integer data type supported by the C compiler, typically `long long`. If the value of arg doesn’t fit into an `intmax_t`, the function signals an error using the error symbol `overflow-error`.
Function: *bool* **extract\_big\_integer** *(emacs\_env \*env, emacs\_value arg, int \*sign, ptrdiff\_t \*count, emacs\_limb\_t \*magnitude)*
This function, which is available since Emacs 27, extracts the integer value of arg. The value of arg must be an integer (fixnum or bignum). If sign is not `NULL`, it stores the sign of arg (-1, 0, or +1) into `*sign`. The magnitude is stored into magnitude as follows. If count and magnitude are both non-`NULL`, then magnitude must point to an array of at least `*count` `unsigned long` elements. If magnitude is large enough to hold the magnitude of arg, then this function writes the magnitude into the magnitude array in little-endian form, stores the number of array elements written into `*count`, and returns `true`. If magnitude is not large enough, it stores the required array size into `*count`, signals an error, and returns `false`. If count is not `NULL` and magnitude is `NULL`, then the function stores the required array size into `*count` and returns `true`.
Emacs guarantees that the maximum required value of `*count` never exceeds `min (PTRDIFF_MAX, SIZE_MAX) / sizeof
(emacs_limb_t)`, so you can use `malloc (*count * sizeof *magnitude)` to allocate the `magnitude` array without worrying about integer overflow in the size calculation.
Type alias: **emacs\_limb\_t**
This is an unsigned integer type, used as the element type for the magnitude arrays for the big integer conversion functions. The type is guaranteed to have unique object representations, i.e., no padding bits.
Macro: **EMACS\_LIMB\_MAX**
This macro expands to a constant expression specifying the maximum possible value for an `emacs_limb_t` object. The expression is suitable for use in `#if`.
Function: *double* **extract\_float** *(emacs\_env \*env, emacs\_value arg)*
This function returns the value of a Lisp float specified by arg, as a C `double` value.
Function: *struct timespec* **extract\_time** *(emacs\_env \*env, emacs\_value arg)*
This function, which is available since Emacs 27, interprets arg as an Emacs Lisp time value and returns the corresponding `struct
timespec`. See [Time of Day](time-of-day). `struct timespec` represents a timestamp with nanosecond precision. It has the following members:
`time_t tv_sec` Whole number of seconds.
`long tv_nsec` Fractional seconds as a number of nanoseconds. For timestamps returned by `extract_time`, this is always nonnegative and less than one billion. (Although POSIX requires the type of `tv_nsec` to be `long`, the type is `long long` on some nonstandard platforms.)
See [(libc)Elapsed Time](https://www.gnu.org/software/libc/manual/html_node/Elapsed-Time.html#Elapsed-Time).
If time has higher precision than nanoseconds, then this function truncates it to nanosecond precision towards negative infinity. This function signals an error if time (truncated to nanoseconds) cannot be represented by `struct timespec`. For example, if `time_t` is a 32-bit integer type, then a time value of ten billion seconds would signal an error, but a time value of 600 picoseconds would get truncated to zero.
If you need to deal with time values that are not representable by `struct timespec`, or if you want higher precision, call the Lisp function `encode-time` and work with its return value. See [Time Conversion](time-conversion).
Function: *bool* **copy\_string\_contents** *(emacs\_env \*env, emacs\_value arg, char \*buf, ptrdiff\_t \*len)*
This function stores the UTF-8 encoded text of a Lisp string specified by arg in the array of `char` pointed by buf, which should have enough space to hold at least `*len` bytes, including the terminating null byte. The argument len must not be a `NULL` pointer, and, when the function is called, it should point to a value that specifies the size of buf in bytes.
If the buffer size specified by `*len` is large enough to hold the string’s text, the function stores in `*len` the actual number of bytes copied to buf, including the terminating null byte, and returns `true`. If the buffer is too small, the function raises the `args-out-of-range` error condition, stores the required number of bytes in `*len`, and returns `false`. See [Module Nonlocal](module-nonlocal), for how to handle pending error conditions.
The argument buf can be a `NULL` pointer, in which case the function stores in `*len` the number of bytes required for storing the contents of arg, and returns `true`. This is how you can determine the size of buf needed to store a particular string: first call `copy_string_contents` with `NULL` as buf, then allocate enough memory to hold the number of bytes stored by the function in `*len`, and call the function again with non-`NULL` buf to actually perform the text copying.
Function: *emacs\_value* **vec\_get** *(emacs\_env \*env, emacs\_value vector, ptrdiff\_t index)*
This function returns the element of vector at index. The index of the first vector element is zero. The function raises the `args-out-of-range` error condition if the value of index is invalid. To extract C data from the value the function returns, use the other extraction functions described here, as appropriate for the Lisp data type stored in that element of the vector.
Function: *ptrdiff\_t* **vec\_size** *(emacs\_env \*env, emacs\_value vector)*
This function returns the number of elements in vector.
Function: *void* **vec\_set** *(emacs\_env \*env, emacs\_value vector, ptrdiff\_t index, emacs\_value value)*
This function stores value in the element of vector whose index is index. It raises the `args-out-of-range` error condition if the value of index is invalid.
The following API functions create `emacs_value` objects from basic C data types. They all return the created `emacs_value` object.
Function: *emacs\_value* **make\_integer** *(emacs\_env \*env, intmax\_t n)*
This function takes an integer argument n and returns the corresponding `emacs_value` object. It returns either a fixnum or a bignum depending on whether the value of n is inside the limits set by `most-negative-fixnum` and `most-positive-fixnum` (see [Integer Basics](integer-basics)).
Function: *emacs\_value* **make\_big\_integer** *(emacs\_env \*env, int sign, ptrdiff\_t count, const emacs\_limb\_t \*magnitude)*
This function, which is available since Emacs 27, takes an arbitrary-sized integer argument and returns a corresponding `emacs_value` object. The sign argument gives the sign of the return value. If sign is nonzero, then magnitude must point to an array of at least count elements specifying the little-endian magnitude of the return value.
The following example uses the GNU Multiprecision Library (GMP) to calculate the next probable prime after a given integer. See [(gmp)Top](https://www.gmplib.org/manual/index.html#Top), for a general overview of GMP, and see [(gmp)Integer Import and Export](https://www.gmplib.org/manual/Integer-Import-and-Export.html#Integer-Import-and-Export) for how to convert the `magnitude` array to and from GMP `mpz_t` values.
```
#include <emacs-module.h>
int plugin_is_GPL_compatible;
#include <assert.h>
#include <limits.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <gmp.h>
static void
memory_full (emacs_env *env)
{
static const char message[] = "Memory exhausted";
emacs_value data = env->make_string (env, message,
strlen (message));
env->non_local_exit_signal
(env, env->intern (env, "error"),
env->funcall (env, env->intern (env, "list"), 1, &data));
}
enum
{
order = -1, endian = 0, nails = 0,
limb_size = sizeof (emacs_limb_t),
max_nlimbs = ((SIZE_MAX < PTRDIFF_MAX ? SIZE_MAX : PTRDIFF_MAX)
/ limb_size)
};
static bool
extract_big_integer (emacs_env *env, emacs_value arg, mpz_t result)
{
ptrdiff_t nlimbs;
bool ok = env->extract_big_integer (env, arg, NULL, &nlimbs, NULL);
if (!ok)
return false;
assert (0 < nlimbs && nlimbs <= max_nlimbs);
emacs_limb_t *magnitude = malloc (nlimbs * limb_size);
if (magnitude == NULL)
{
memory_full (env);
return false;
}
int sign;
ok = env->extract_big_integer (env, arg, &sign, &nlimbs, magnitude);
assert (ok);
mpz_import (result, nlimbs, order, limb_size, endian, nails, magnitude);
free (magnitude);
if (sign < 0)
mpz_neg (result, result);
return true;
}
static emacs_value
make_big_integer (emacs_env *env, const mpz_t value)
{
size_t nbits = mpz_sizeinbase (value, 2);
int bitsperlimb = CHAR_BIT * limb_size - nails;
size_t nlimbs = nbits / bitsperlimb + (nbits % bitsperlimb != 0);
emacs_limb_t *magnitude
= nlimbs <= max_nlimbs ? malloc (nlimbs * limb_size) : NULL;
if (magnitude == NULL)
{
memory_full (env);
return NULL;
}
size_t written;
mpz_export (magnitude, &written, order, limb_size, endian, nails, value);
assert (written == nlimbs);
assert (nlimbs <= PTRDIFF_MAX);
emacs_value result = env->make_big_integer (env, mpz_sgn (value),
nlimbs, magnitude);
free (magnitude);
return result;
}
static emacs_value
next_prime (emacs_env *env, ptrdiff_t nargs, emacs_value *args,
void *data)
{
assert (nargs == 1);
mpz_t p;
mpz_init (p);
extract_big_integer (env, args[0], p);
mpz_nextprime (p, p);
emacs_value result = make_big_integer (env, p);
mpz_clear (p);
return result;
}
int
emacs_module_init (struct emacs_runtime *runtime)
{
emacs_env *env = runtime->get_environment (runtime);
emacs_value symbol = env->intern (env, "next-prime");
emacs_value func
= env->make_function (env, 1, 1, next_prime, NULL, NULL);
emacs_value args[] = {symbol, func};
env->funcall (env, env->intern (env, "defalias"), 2, args);
return 0;
}
```
Function: *emacs\_value* **make\_float** *(emacs\_env \*env, double d)*
This function takes a `double` argument d and returns the corresponding Emacs floating-point value.
Function: *emacs\_value* **make\_time** *(emacs\_env \*env, struct timespec time)*
This function, which is available since Emacs 27, takes a `struct
timespec` argument time and returns the corresponding Emacs timestamp as a pair `(ticks . hz)`. See [Time of Day](time-of-day). The return value represents exactly the same timestamp as time: all input values are representable, and there is never a loss of precision. `time.tv_sec` and `time.tv_nsec` can be arbitrary values. In particular, there’s no requirement that time be normalized. This means that `time.tv_nsec` can be negative or larger than 999,999,999.
Function: *emacs\_value* **make\_string** *(emacs\_env \*env, const char \*str, ptrdiff\_t len)*
This function creates an Emacs string from C text string pointed by str whose length in bytes, not including the terminating null byte, is len. The original string in str can be either an ASCII string or a UTF-8 encoded non-ASCII string; it can include embedded null bytes, and doesn’t have to end in a terminating null byte at `str[len]`. The function raises the `overflow-error` error condition if len is negative or exceeds the maximum length of an Emacs string. If len is zero, then str can be `NULL`, otherwise it must point to valid memory. For nonzero len, `make_string` returns unique mutable string objects.
Function: *emacs\_value* **make\_unibyte\_string** *(emacs\_env \*env, const char \*str, ptrdiff\_t len)*
This function is like `make_string`, but has no restrictions on the values of the bytes in the C string, and can be used to pass binary data to Emacs in the form of a unibyte string.
The API does not provide functions to manipulate Lisp data structures, for example, create lists with `cons` and `list` (see [Building Lists](building-lists)), extract list members with `car` and `cdr` (see [List Elements](list-elements)), create vectors with `vector` (see [Vector Functions](vector-functions)), etc. For these, use `intern` and `funcall`, described in the next subsection, to call the corresponding Lisp functions.
Normally, `emacs_value` objects have a rather short lifetime: it ends when the `emacs_env` pointer used for their creation goes out of scope. Occasionally, you may need to create *global references*: `emacs_value` objects that live as long as you wish. Use the following two functions to manage such objects.
Function: *emacs\_value* **make\_global\_ref** *(emacs\_env \*env, emacs\_value value)*
This function returns a global reference for value.
Function: *void* **free\_global\_ref** *(emacs\_env \*env, emacs\_value global\_value)*
This function frees the global\_value previously created by `make_global_ref`. The global\_value is no longer valid after the call. Your module code should pair each call to `make_global_ref` with the corresponding `free_global_ref`.
An alternative to keeping around C data structures that need to be passed to module functions later is to create *user pointer* objects. A user pointer, or `user-ptr`, object is a Lisp object that encapsulates a C pointer and can have an associated finalizer function, which is called when the object is garbage-collected (see [Garbage Collection](garbage-collection)). The module API provides functions to create and access `user-ptr` objects. These functions raise the `wrong-type-argument` error condition if they are called on `emacs_value` that doesn’t represent a `user-ptr` object.
Function: *emacs\_value* **make\_user\_ptr** *(emacs\_env \*env, emacs\_finalizer fin, void \*ptr)*
This function creates and returns a `user-ptr` object which wraps the C pointer ptr. The finalizer function fin can be a `NULL` pointer (meaning no finalizer), or it can be a function of the following signature:
```
typedef void (*emacs_finalizer) (void *ptr);
```
If fin is not a `NULL` pointer, it will be called with the ptr as the argument when the `user-ptr` object is garbage-collected. Don’t run any expensive code in a finalizer, because GC must finish quickly to keep Emacs responsive.
Function: *void \** **get\_user\_ptr** *(emacs\_env \*env, emacs\_value arg)*
This function extracts the C pointer from the Lisp object represented by arg.
Function: *void* **set\_user\_ptr** *(emacs\_env \*env, emacs\_value arg, void \*ptr)*
This function sets the C pointer embedded in the `user-ptr` object represented by arg to ptr.
Function: *emacs\_finalizer* **get\_user\_finalizer** *(emacs\_env \*env, emacs\_value arg)*
This function returns the finalizer of the `user-ptr` object represented by arg, or `NULL` if it doesn’t have a finalizer.
Function: *void* **set\_user\_finalizer** *(emacs\_env \*env, emacs\_value arg, emacs\_finalizer fin)*
This function changes the finalizer of the `user-ptr` object represented by arg to be fin. If fin is a `NULL` pointer, the `user-ptr` object will have no finalizer.
Note that the `emacs_finalizer` type works for both user pointer an module function finalizers. See [Module Function Finalizers](module-functions#Module-Function-Finalizers).
| programming_docs |
elisp None ### Random Numbers
A deterministic computer program cannot generate true random numbers. For most purposes, *pseudo-random numbers* suffice. A series of pseudo-random numbers is generated in a deterministic fashion. The numbers are not truly random, but they have certain properties that mimic a random series. For example, all possible values occur equally often in a pseudo-random series.
Pseudo-random numbers are generated from a *seed value*. Starting from any given seed, the `random` function always generates the same sequence of numbers. By default, Emacs initializes the random seed at startup, in such a way that the sequence of values of `random` (with overwhelming likelihood) differs in each Emacs run.
Sometimes you want the random number sequence to be repeatable. For example, when debugging a program whose behavior depends on the random number sequence, it is helpful to get the same behavior in each program run. To make the sequence repeat, execute `(random "")`. This sets the seed to a constant value for your particular Emacs executable (though it may differ for other Emacs builds). You can use other strings to choose various seed values.
Function: **random** *&optional limit*
This function returns a pseudo-random integer. Repeated calls return a series of pseudo-random integers.
If limit is a positive integer, the value is chosen to be nonnegative and less than limit. Otherwise, the value might be any fixnum, i.e., any integer from `most-negative-fixnum` through `most-positive-fixnum` (see [Integer Basics](integer-basics)).
If limit is `t`, it means to choose a new seed as if Emacs were restarting, typically from the system entropy. On systems lacking entropy pools, choose the seed from less-random volatile data such as the current time.
If limit is a string, it means to choose a new seed based on the string’s contents.
elisp None ### Communicating with Serial Ports
Emacs can communicate with serial ports. For interactive use, `M-x serial-term` opens a terminal window. In a Lisp program, `make-serial-process` creates a process object.
The serial port can be configured at run-time, without having to close and re-open it. The function `serial-process-configure` lets you change the speed, bytesize, and other parameters. In a terminal window created by `serial-term`, you can click on the mode line for configuration.
A serial connection is represented by a process object, which can be used in a similar way to a subprocess or network process. You can send and receive data, and configure the serial port. A serial process object has no process ID, however, and you can’t send signals to it, and the status codes are different from other types of processes. `delete-process` on the process object or `kill-buffer` on the process buffer close the connection, but this does not affect the device connected to the serial port.
The function `process-type` returns the symbol `serial` for a process object representing a serial port connection.
Serial ports are available on GNU/Linux, Unix, and MS Windows systems.
Command: **serial-term** *port speed &optional line-mode*
Start a terminal-emulator for a serial port in a new buffer. port is the name of the serial port to connect to. For example, this could be `/dev/ttyS0` on Unix. On MS Windows, this could be `COM1`, or `\\.\COM10` (double the backslashes in Lisp strings).
speed is the speed of the serial port in bits per second. 9600 is a common value. The buffer is in Term mode; see [Term Mode](https://www.gnu.org/software/emacs/manual/html_node/emacs/Term-Mode.html#Term-Mode) in The GNU Emacs Manual, for the commands to use in that buffer. You can change the speed and the configuration in the mode line menu. If line-mode is non-`nil`, `term-line-mode` is used; otherwise `term-raw-mode` is used.
Function: **make-serial-process** *&rest args*
This function creates a process and a buffer. Arguments are specified as keyword/argument pairs. Here’s the list of the meaningful keywords, with the first two (port and speed) being mandatory:
`:port port`
This is the name of the serial port. On Unix and GNU systems, this is a file name such as `/dev/ttyS0`. On Windows, this could be `COM1`, or `\\.\COM10` for ports higher than `COM9` (double the backslashes in Lisp strings).
`:speed speed`
The speed of the serial port in bits per second. This function calls `serial-process-configure` to handle the speed; see the following documentation of that function for more details.
`:name name`
The name of the process. If name is not given, port will serve as the process name as well.
`:buffer buffer`
The buffer to associate with the process. The value can be either a buffer or a string that names a buffer. Process output goes at the end of that buffer, unless you specify an output stream or filter function to handle the output. If buffer is not given, the process buffer’s name is taken from the value of the `:name` keyword.
`:coding coding`
If coding is a symbol, it specifies the coding system used for both reading and writing for this process. If coding is a cons `(decoding . encoding)`, decoding is used for reading, and encoding is used for writing. If not specified, the default is to determine the coding systems from the data itself.
`:noquery query-flag`
Initialize the process query flag to query-flag. See [Query Before Exit](query-before-exit). The flags defaults to `nil` if unspecified.
`:stop bool`
Start process in the stopped state if bool is non-`nil`. In the stopped state, a serial process does not accept incoming data, but you can send outgoing data. The stopped state is cleared by `continue-process` and set by `stop-process`.
`:filter filter`
Install filter as the process filter.
`:sentinel sentinel`
Install sentinel as the process sentinel.
`:plist plist`
Install plist as the initial plist of the process.
`:bytesize` `:parity` `:stopbits` `:flowcontrol` These are handled by `serial-process-configure`, which is called by `make-serial-process`.
The original argument list, possibly modified by later configuration, is available via the function `process-contact`.
Here is an example:
```
(make-serial-process :port "/dev/ttyS0" :speed 9600)
```
Function: **serial-process-configure** *&rest args*
This function configures a serial port connection. Arguments are specified as keyword/argument pairs. Attributes that are not given are re-initialized from the process’s current configuration (available via the function `process-contact`), or set to reasonable default values. The following arguments are defined:
`:process process` `:name name` `:buffer buffer` `:port port`
Any of these arguments can be given to identify the process that is to be configured. If none of these arguments is given, the current buffer’s process is used.
`:speed speed`
The speed of the serial port in bits per second, a.k.a. *baud rate*. The value can be any number, but most serial ports work only at a few defined values between 1200 and 115200, with 9600 being the most common value. If speed is `nil`, the function ignores all other arguments and does not configure the port. This may be useful for special serial ports such as Bluetooth-to-serial converters, which can only be configured through ‘`AT`’ commands sent through the connection. The value of `nil` for speed is valid only for connections that were already opened by a previous call to `make-serial-process` or `serial-term`.
`:bytesize bytesize`
The number of bits per byte, which can be 7 or 8. If bytesize is not given or `nil`, it defaults to 8.
`:parity parity`
The value can be `nil` (don’t use parity), the symbol `odd` (use odd parity), or the symbol `even` (use even parity). If parity is not given, it defaults to no parity.
`:stopbits stopbits`
The number of stopbits used to terminate a transmission of each byte. stopbits can be 1 or 2. If stopbits is not given or `nil`, it defaults to 1.
`:flowcontrol flowcontrol` The type of flow control to use for this connection, which is either `nil` (don’t use flow control), the symbol `hw` (use RTS/CTS hardware flow control), or the symbol `sw` (use XON/XOFF software flow control). If flowcontrol is not given, it defaults to no flow control.
Internally, `make-serial-process` calls `serial-process-configure` for the initial configuration of the serial port.
elisp None Standard Keymaps
-----------------
In this section we list some of the more general keymaps. Many of these exist when Emacs is first started, but some are loaded only when the respective feature is accessed.
There are many other, more specialized, maps than these; in particular those associated with major and minor modes. The minibuffer uses several keymaps (see [Completion Commands](completion-commands)). For more details on keymaps, see [Keymaps](keymaps).
`2C-mode-map`
A sparse keymap for subcommands of the prefix `C-x 6`. See [Two-Column Editing](https://www.gnu.org/software/emacs/manual/html_node/emacs/Two_002dColumn.html#Two_002dColumn) in The GNU Emacs Manual.
`abbrev-map`
A sparse keymap for subcommands of the prefix `C-x a`. See [Defining Abbrevs](https://www.gnu.org/software/emacs/manual/html_node/emacs/Defining-Abbrevs.html#Defining-Abbrevs) in The GNU Emacs Manual.
`button-buffer-map`
A sparse keymap useful for buffers containing buffers. You may want to use this as a parent keymap. See [Buttons](buttons).
`button-map`
A sparse keymap used by buttons.
`ctl-x-4-map`
A sparse keymap for subcommands of the prefix `C-x 4`.
`ctl-x-5-map`
A sparse keymap for subcommands of the prefix `C-x 5`.
`ctl-x-map`
A full keymap for `C-x` commands.
`ctl-x-r-map`
A sparse keymap for subcommands of the prefix `C-x r`. See [Registers](https://www.gnu.org/software/emacs/manual/html_node/emacs/Registers.html#Registers) in The GNU Emacs Manual.
`esc-map`
A full keymap for ESC (or Meta) commands.
`function-key-map`
The parent keymap of all `local-function-key-map` (q.v.) instances.
`global-map`
The full keymap containing default global key bindings. Modes should not modify the Global map.
`goto-map`
A sparse keymap used for the `M-g` prefix key.
`help-map`
A sparse keymap for the keys following the help character `C-h`. See [Help Functions](help-functions).
`Helper-help-map`
A full keymap used by the help utility package. It has the same keymap in its value cell and in its function cell.
`input-decode-map`
The keymap for translating keypad and function keys. If there are none, then it contains an empty sparse keymap. See [Translation Keymaps](translation-keymaps).
`key-translation-map`
A keymap for translating keys. This one overrides ordinary key bindings, unlike `local-function-key-map`. See [Translation Keymaps](translation-keymaps).
`kmacro-keymap`
A sparse keymap for keys that follows the `C-x C-k` prefix search. See [Keyboard Macros](https://www.gnu.org/software/emacs/manual/html_node/emacs/Keyboard-Macros.html#Keyboard-Macros) in The GNU Emacs Manual.
`local-function-key-map`
The keymap for translating key sequences to preferred alternatives. If there are none, then it contains an empty sparse keymap. See [Translation Keymaps](translation-keymaps).
`menu-bar-file-menu` `menu-bar-edit-menu` `menu-bar-options-menu` `global-buffers-menu-map` `menu-bar-tools-menu` `menu-bar-help-menu`
These keymaps display the main, top-level menus in the menu bar. Some of them contain sub-menus. For example, the Edit menu contains `menu-bar-search-menu`, etc. See [Menu Bar](menu-bar).
`minibuffer-inactive-mode-map`
A full keymap used in the minibuffer when it is not active. See [Editing in the Minibuffer](https://www.gnu.org/software/emacs/manual/html_node/emacs/Minibuffer-Edit.html#Minibuffer-Edit) in The GNU Emacs Manual.
`mode-line-coding-system-map` `mode-line-input-method-map` `mode-line-column-line-number-mode-map`
These keymaps control various areas of the mode line. See [Mode Line Format](mode-line-format).
`mode-specific-map`
The keymap for characters following `C-c`. Note, this is in the global map. This map is not actually mode-specific: its name was chosen to be informative in `C-h b` (`display-bindings`), where it describes the main use of the `C-c` prefix key.
`mouse-appearance-menu-map`
A sparse keymap used for the `S-mouse-1` key.
`mule-keymap`
The global keymap used for the `C-x RET` prefix key.
`narrow-map`
A sparse keymap for subcommands of the prefix `C-x n`.
`prog-mode-map`
The keymap used by Prog mode. See [Basic Major Modes](basic-major-modes).
`query-replace-map` `multi-query-replace-map`
A sparse keymap used for responses in `query-replace` and related commands; also for `y-or-n-p` and `map-y-or-n-p`. The functions that use this map do not support prefix keys; they look up one event at a time. `multi-query-replace-map` extends `query-replace-map` for multi-buffer replacements. See [query-replace-map](search-and-replace).
`search-map`
A sparse keymap that provides global bindings for search-related commands.
`special-mode-map`
The keymap used by Special mode. See [Basic Major Modes](basic-major-modes).
`tab-prefix-map`
The global keymap used for the `C-x t` prefix key for tab-bar related commands. See [Tab Bars](https://www.gnu.org/software/emacs/manual/html_node/emacs/Tab-Bars.html#Tab-Bars) in The GNU Emacs Manual.
`tab-bar-map`
The keymap defining the contents of the tab bar. See [Tab Bars](https://www.gnu.org/software/emacs/manual/html_node/emacs/Tab-Bars.html#Tab-Bars) in The GNU Emacs Manual.
`tool-bar-map`
The keymap defining the contents of the tool bar. See [Tool Bar](tool-bar).
`universal-argument-map`
A sparse keymap used while processing `C-u`. See [Prefix Command Arguments](prefix-command-arguments).
`vc-prefix-map`
The global keymap used for the `C-x v` prefix key.
`x-alternatives-map`
A sparse keymap used to map certain keys under graphical frames. The function `x-setup-function-keys` uses this.
elisp None ### Char-Tables
A char-table is much like a vector, except that it is indexed by character codes. Any valid character code, without modifiers, can be used as an index in a char-table. You can access a char-table’s elements with `aref` and `aset`, as with any array. In addition, a char-table can have *extra slots* to hold additional data not associated with particular character codes. Like vectors, char-tables are constants when evaluated, and can hold elements of any type.
Each char-table has a *subtype*, a symbol, which serves two purposes:
* The subtype provides an easy way to tell what the char-table is for. For instance, display tables are char-tables with `display-table` as the subtype, and syntax tables are char-tables with `syntax-table` as the subtype. The subtype can be queried using the function `char-table-subtype`, described below.
* The subtype controls the number of *extra slots* in the char-table. This number is specified by the subtype’s `char-table-extra-slots` symbol property (see [Symbol Properties](symbol-properties)), whose value should be an integer between 0 and 10. If the subtype has no such symbol property, the char-table has no extra slots.
A char-table can have a *parent*, which is another char-table. If it does, then whenever the char-table specifies `nil` for a particular character c, it inherits the value specified in the parent. In other words, `(aref char-table c)` returns the value from the parent of char-table if char-table itself specifies `nil`.
A char-table can also have a *default value*. If so, then `(aref char-table c)` returns the default value whenever the char-table does not specify any other non-`nil` value.
Function: **make-char-table** *subtype &optional init*
Return a newly-created char-table, with subtype subtype (a symbol). Each element is initialized to init, which defaults to `nil`. You cannot alter the subtype of a char-table after the char-table is created.
There is no argument to specify the length of the char-table, because all char-tables have room for any valid character code as an index.
If subtype has the `char-table-extra-slots` symbol property, that specifies the number of extra slots in the char-table. This should be an integer between 0 and 10; otherwise, `make-char-table` raises an error. If subtype has no `char-table-extra-slots` symbol property (see [Property Lists](property-lists)), the char-table has no extra slots.
Function: **char-table-p** *object*
This function returns `t` if object is a char-table, and `nil` otherwise.
Function: **char-table-subtype** *char-table*
This function returns the subtype symbol of char-table.
There is no special function to access default values in a char-table. To do that, use `char-table-range` (see below).
Function: **char-table-parent** *char-table*
This function returns the parent of char-table. The parent is always either `nil` or another char-table.
Function: **set-char-table-parent** *char-table new-parent*
This function sets the parent of char-table to new-parent.
Function: **char-table-extra-slot** *char-table n*
This function returns the contents of extra slot n (zero based) of char-table. The number of extra slots in a char-table is determined by its subtype.
Function: **set-char-table-extra-slot** *char-table n value*
This function stores value in extra slot n (zero based) of char-table.
A char-table can specify an element value for a single character code; it can also specify a value for an entire character set.
Function: **char-table-range** *char-table range*
This returns the value specified in char-table for a range of characters range. Here are the possibilities for range:
`nil`
Refers to the default value.
char
Refers to the element for character char (supposing char is a valid character code).
`(from . to)` A cons cell refers to all the characters in the inclusive range ‘`[from..to]`’.
Function: **set-char-table-range** *char-table range value*
This function sets the value in char-table for a range of characters range. Here are the possibilities for range:
`nil`
Refers to the default value.
`t`
Refers to the whole range of character codes.
char
Refers to the element for character char (supposing char is a valid character code).
`(from . to)` A cons cell refers to all the characters in the inclusive range ‘`[from..to]`’.
Function: **map-char-table** *function char-table*
This function calls its argument function for each element of char-table that has a non-`nil` value. The call to function is with two arguments, a key and a value. The key is a possible range argument for `char-table-range`—either a valid character or a cons cell `(from . to)`, specifying a range of characters that share the same value. The value is what `(char-table-range char-table key)` returns.
Overall, the key-value pairs passed to function describe all the values stored in char-table.
The return value is always `nil`; to make calls to `map-char-table` useful, function should have side effects. For example, here is how to examine the elements of the syntax table:
```
(let (accumulator)
(map-char-table
(lambda (key value)
(setq accumulator
(cons (list
(if (consp key)
(list (car key) (cdr key))
key)
value)
accumulator)))
(syntax-table))
accumulator)
⇒
(((2597602 4194303) (2)) ((2597523 2597601) (3))
... (65379 (5 . 65378)) (65378 (4 . 65379)) (65377 (1))
... (12 (0)) (11 (3)) (10 (12)) (9 (0)) ((0 8) (3)))
```
elisp None #### Stickiness of Text Properties
Self-inserting characters, the ones that get inserted into a buffer when the user types them (see [Commands for Insertion](commands-for-insertion)), normally take on the same properties as the preceding character. This is called *inheritance* of properties.
By contrast, a Lisp program can do insertion with inheritance or without, depending on the choice of insertion primitive. The ordinary text insertion functions, such as `insert`, do not inherit any properties. They insert text with precisely the properties of the string being inserted, and no others. This is correct for programs that copy text from one context to another—for example, into or out of the kill ring. To insert with inheritance, use the special primitives described in this section. Self-inserting characters inherit properties because they work using these primitives.
When you do insertion with inheritance, *which* properties are inherited, and from where, depends on which properties are *sticky*. Insertion after a character inherits those of its properties that are *rear-sticky*. Insertion before a character inherits those of its properties that are *front-sticky*. When both sides offer different sticky values for the same property, the previous character’s value takes precedence.
By default, a text property is rear-sticky but not front-sticky; thus, the default is to inherit all the properties of the preceding character, and nothing from the following character.
You can control the stickiness of various text properties with two specific text properties, `front-sticky` and `rear-nonsticky`, and with the variable `text-property-default-nonsticky`. You can use the variable to specify a different default for a given property. You can use those two text properties to make any specific properties sticky or nonsticky in any particular part of the text.
If a character’s `front-sticky` property is `t`, then all its properties are front-sticky. If the `front-sticky` property is a list, then the sticky properties of the character are those whose names are in the list. For example, if a character has a `front-sticky` property whose value is `(face read-only)`, then insertion before the character can inherit its `face` property and its `read-only` property, but no others.
The `rear-nonsticky` property works the opposite way. Most properties are rear-sticky by default, so the `rear-nonsticky` property says which properties are *not* rear-sticky. If a character’s `rear-nonsticky` property is `t`, then none of its properties are rear-sticky. If the `rear-nonsticky` property is a list, properties are rear-sticky *unless* their names are in the list.
Variable: **text-property-default-nonsticky**
This variable holds an alist which defines the default rear-stickiness of various text properties. Each element has the form `(property . nonstickiness)`, and it defines the stickiness of a particular text property, property.
If nonstickiness is non-`nil`, this means that the property property is rear-nonsticky by default. Since all properties are front-nonsticky by default, this makes property nonsticky in both directions by default.
The text properties `front-sticky` and `rear-nonsticky`, when used, take precedence over the default nonstickiness specified in `text-property-default-nonsticky`.
Here are the functions that insert text with inheritance of properties:
Function: **insert-and-inherit** *&rest strings*
Insert the strings strings, just like the function `insert`, but inherit any sticky properties from the adjoining text.
Function: **insert-before-markers-and-inherit** *&rest strings*
Insert the strings strings, just like the function `insert-before-markers`, but inherit any sticky properties from the adjoining text.
See [Insertion](insertion), for the ordinary insertion functions which do not inherit.
| programming_docs |
elisp None ### Debugging Invalid Lisp Syntax
The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error ‘`End of file during parsing`’ in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, ‘`Invalid read syntax: ")"`’ indicates an excess close parenthesis or missing open parenthesis, but does not say where the missing parenthesis belongs. How, then, to find what to change?
If the problem is not simply an imbalance of parentheses, a useful technique is to try `C-M-e` (`end-of-defun`, see [Moving by Defuns](https://www.gnu.org/software/emacs/manual/html_node/emacs/Moving-by-Defuns.html#Moving-by-Defuns) in The GNU Emacs Manual) at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun.
However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases. (In addition, just moving point through the code with Show Paren mode enabled might find the mismatch.)
| | | |
| --- | --- | --- |
| • [Excess Open](excess-open) | | How to find a spurious open paren or missing close. |
| • [Excess Close](excess-close) | | How to find a spurious close paren or missing open. |
elisp None #### The pcase macro
For background, See [Pattern-Matching Conditional](pattern_002dmatching-conditional).
Macro: **pcase** *expression &rest clauses*
Each clause in clauses has the form: `(pattern body-forms…)`.
Evaluate expression to determine its value, expval. Find the first clause in clauses whose pattern matches expval and pass control to that clause’s body-forms.
If there is a match, the value of `pcase` is the value of the last of body-forms in the successful clause. Otherwise, `pcase` evaluates to `nil`.
Each pattern has to be a *pcase pattern*, which can use either one of the core patterns defined below, or one of the patterns defined via `pcase-defmacro` (see [Extending pcase](extending-pcase)).
The rest of this subsection describes different forms of core patterns, presents some examples, and concludes with important caveats on using the let-binding facility provided by some pattern forms. A core pattern can have the following forms:
`_ (underscore)`
Matches any expval. This is also known as *don’t care* or *wildcard*.
`'val`
Matches if expval equals val. The comparison is done as if by `equal` (see [Equality Predicates](equality-predicates)).
`keyword` `integer` `string`
Matches if expval equals the literal object. This is a special case of `'val`, above, possible because literal objects of these types are self-quoting.
`symbol`
Matches any expval, and additionally let-binds symbol to expval, such that this binding is available to body-forms (see [Dynamic Binding](dynamic-binding)).
If symbol is part of a sequencing pattern seqpat (e.g., by using `and`, below), the binding is also available to the portion of seqpat following the appearance of symbol. This usage has some caveats, see [caveats](#pcase_002dsymbol_002dcaveats).
Two symbols to avoid are `t`, which behaves like `_` (above) and is deprecated, and `nil`, which signals an error. Likewise, it makes no sense to bind keyword symbols (see [Constant Variables](constant-variables)).
`(cl-type type)`
Matches if expval is of type type, which is a type descriptor as accepted by `cl-typep` (see [Type Predicates](https://www.gnu.org/software/emacs/manual/html_node/cl/Type-Predicates.html#Type-Predicates) in Common Lisp Extensions). Examples:
```
(cl-type integer)
(cl-type (integer 0 10))
```
`(pred function)`
Matches if the predicate function returns non-`nil` when called on expval. The test can be negated with the syntax `(pred (not function))`. The predicate function can have one of the following forms:
function name (a symbol)
Call the named function with one argument, expval.
Example: `integerp`
lambda expression
Call the anonymous function with one argument, expval (see [Lambda Expressions](lambda-expressions)).
Example: `(lambda (n) (= 42 n))`
function call with n args
Call the function (the first element of the function call) with n arguments (the other elements) and an additional n+1-th argument that is expval.
Example: `(= 42)` In this example, the function is `=`, n is one, and the actual function call becomes: `(= 42 expval)`.
`(app function pattern)`
Matches if function called on expval returns a value that matches pattern. function can take one of the forms described for `pred`, above. Unlike `pred`, however, `app` tests the result against pattern, rather than against a boolean truth value.
`(guard boolean-expression)`
Matches if boolean-expression evaluates to non-`nil`.
`(let pattern expr)` Evaluates expr to get exprval and matches if exprval matches pattern. (It is called `let` because pattern can bind symbols to values using symbol.)
A *sequencing pattern* (also known as seqpat) is a pattern that processes its sub-pattern arguments in sequence. There are two for `pcase`: `and` and `or`. They behave in a similar manner to the special forms that share their name (see [Combining Conditions](combining-conditions)), but instead of processing values, they process sub-patterns.
`(and pattern1…)`
Attempts to match pattern1…, in order, until one of them fails to match. In that case, `and` likewise fails to match, and the rest of the sub-patterns are not tested. If all sub-patterns match, `and` matches.
`(or pattern1 pattern2…)`
Attempts to match pattern1, pattern2, …, in order, until one of them succeeds. In that case, `or` likewise matches, and the rest of the sub-patterns are not tested.
To present a consistent environment (see [Intro Eval](intro-eval)) to body-forms (thus avoiding an evaluation error on match), the set of variables bound by the pattern is the union of the variables bound by each sub-pattern. If a variable is not bound by the sub-pattern that matched, then it is bound to `nil`.
`(rx rx-expr…)`
Matches strings against the regexp rx-expr…, using the `rx` regexp notation (see [Rx Notation](rx-notation)), as if by `string-match`.
In addition to the usual `rx` syntax, rx-expr… can contain the following constructs:
`(let ref rx-expr…)`
Bind the symbol ref to a submatch that matches rx-expr.... ref is bound in body-forms to the string of the submatch or nil, but can also be used in `backref`.
`(backref ref)` Like the standard `backref` construct, but ref can here also be a name introduced by a previous `(let ref …)` construct.
#### Example: Advantage Over `cl-case`
Here’s an example that highlights some advantages `pcase` has over `cl-case` (see [Conditionals](https://www.gnu.org/software/emacs/manual/html_node/cl/Conditionals.html#Conditionals) in Common Lisp Extensions).
```
(pcase (get-return-code x)
;; string
((and (pred stringp) msg)
(message "%s" msg))
```
```
;; symbol
('success (message "Done!"))
('would-block (message "Sorry, can't do it now"))
('read-only (message "The shmliblick is read-only"))
('access-denied (message "You do not have the needed rights"))
```
```
;; default
(code (message "Unknown return code %S" code)))
```
With `cl-case`, you would need to explicitly declare a local variable `code` to hold the return value of `get-return-code`. Also `cl-case` is difficult to use with strings because it uses `eql` for comparison.
#### Example: Using `and`
A common idiom is to write a pattern starting with `and`, with one or more symbol sub-patterns providing bindings to the sub-patterns that follow (as well as to the body forms). For example, the following pattern matches single-digit integers.
```
(and
(pred integerp)
n ; bind `n` to expval
(guard (<= -9 n 9)))
```
First, `pred` matches if `(integerp expval)` evaluates to non-`nil`. Next, `n` is a symbol pattern that matches anything and binds `n` to expval. Lastly, `guard` matches if the boolean expression `(<= -9 n 9)` (note the reference to `n`) evaluates to non-`nil`. If all these sub-patterns match, `and` matches.
#### Example: Reformulation with `pcase`
Here is another example that shows how to reformulate a simple matching task from its traditional implementation (function `grok/traditional`) to one using `pcase` (function `grok/pcase`). The docstring for both these functions is: “If OBJ is a string of the form "key:NUMBER", return NUMBER (a string). Otherwise, return the list ("149" default).” First, the traditional implementation (see [Regular Expressions](regular-expressions)):
```
(defun grok/traditional (obj)
(if (and (stringp obj)
(string-match "^key:\\([[:digit:]]+\\)$" obj))
(match-string 1 obj)
(list "149" 'default)))
```
```
(grok/traditional "key:0") ⇒ "0"
(grok/traditional "key:149") ⇒ "149"
(grok/traditional 'monolith) ⇒ ("149" default)
```
The reformulation demonstrates symbol binding as well as `or`, `and`, `pred`, `app` and `let`.
```
(defun grok/pcase (obj)
(pcase obj
((or ; line 1
(and ; line 2
(pred stringp) ; line 3
(pred (string-match ; line 4
"^key:\\([[:digit:]]+\\)$")) ; line 5
(app (match-string 1) ; line 6
val)) ; line 7
(let val (list "149" 'default))) ; line 8
val))) ; line 9
```
```
(grok/pcase "key:0") ⇒ "0"
(grok/pcase "key:149") ⇒ "149"
(grok/pcase 'monolith) ⇒ ("149" default)
```
The bulk of `grok/pcase` is a single clause of a `pcase` form, the pattern on lines 1-8, the (single) body form on line 9. The pattern is `or`, which tries to match in turn its argument sub-patterns, first `and` (lines 2-7), then `let` (line 8), until one of them succeeds.
As in the previous example (see [Example 1](#pcase_002dexample_002d1)), `and` begins with a `pred` sub-pattern to ensure the following sub-patterns work with an object of the correct type (string, in this case). If `(stringp expval)` returns `nil`, `pred` fails, and thus `and` fails, too.
The next `pred` (lines 4-5) evaluates `(string-match RX expval)` and matches if the result is non-`nil`, which means that expval has the desired form: `key:NUMBER`. Again, failing this, `pred` fails and `and`, too.
Lastly (in this series of `and` sub-patterns), `app` evaluates `(match-string 1 expval)` (line 6) to get a temporary value tmp (i.e., the “NUMBER” substring) and tries to match tmp against pattern `val` (line 7). Since that is a symbol pattern, it matches unconditionally and additionally binds `val` to tmp.
Now that `app` has matched, all `and` sub-patterns have matched, and so `and` matches. Likewise, once `and` has matched, `or` matches and does not proceed to try sub-pattern `let` (line 8).
Let’s consider the situation where `obj` is not a string, or it is a string but has the wrong form. In this case, one of the `pred` (lines 3-5) fails to match, thus `and` (line 2) fails to match, thus `or` (line 1) proceeds to try sub-pattern `let` (line 8).
First, `let` evaluates `(list "149" 'default)` to get `("149" default)`, the exprval, and then tries to match exprval against pattern `val`. Since that is a symbol pattern, it matches unconditionally and additionally binds `val` to exprval. Now that `let` has matched, `or` matches.
Note how both `and` and `let` sub-patterns finish in the same way: by trying (always successfully) to match against the symbol pattern `val`, in the process binding `val`. Thus, `or` always matches and control always passes to the body form (line 9). Because that is the last body form in a successfully matched `pcase` clause, it is the value of `pcase` and likewise the return value of `grok/pcase` (see [What Is a Function](what-is-a-function)).
#### Caveats for symbol in Sequencing Patterns
The preceding examples all use sequencing patterns which include the symbol sub-pattern in some way. Here are some important details about that usage.
1. When symbol occurs more than once in seqpat, the second and subsequent occurrences do not expand to re-binding, but instead expand to an equality test using `eq`. The following example features a `pcase` form with two clauses and two seqpat, A and B. Both A and B first check that expval is a pair (using `pred`), and then bind symbols to the `car` and `cdr` of expval (using one `app` each).
For A, because symbol `st` is mentioned twice, the second mention becomes an equality test using `eq`. On the other hand, B uses two separate symbols, `s1` and `s2`, both of which become independent bindings.
```
(defun grok (object)
(pcase object
((and (pred consp) ; seqpat A
(app car st) ; first mention: st
(app cdr st)) ; second mention: st
(list 'eq st))
```
```
((and (pred consp) ; seqpat B
(app car s1) ; first mention: s1
(app cdr s2)) ; first mention: s2
(list 'not-eq s1 s2))))
```
```
(let ((s "yow!"))
(grok (cons s s))) ⇒ (eq "yow!")
(grok (cons "yo!" "yo!")) ⇒ (not-eq "yo!" "yo!")
(grok '(4 2)) ⇒ (not-eq 4 (2))
```
2. Side-effecting code referencing symbol is undefined. Avoid. For example, here are two similar functions. Both use `and`, symbol and `guard`:
```
(defun square-double-digit-p/CLEAN (integer)
(pcase (* integer integer)
((and n (guard (< 9 n 100))) (list 'yes n))
(sorry (list 'no sorry))))
(square-double-digit-p/CLEAN 9) ⇒ (yes 81)
(square-double-digit-p/CLEAN 3) ⇒ (no 9)
```
```
(defun square-double-digit-p/MAYBE (integer)
(pcase (* integer integer)
((and n (guard (< 9 (incf n) 100))) (list 'yes n))
(sorry (list 'no sorry))))
(square-double-digit-p/MAYBE 9) ⇒ (yes 81)
(square-double-digit-p/MAYBE 3) ⇒ (yes 9) ; WRONG!
```
The difference is in boolean-expression in `guard`: `CLEAN` references `n` simply and directly, while `MAYBE` references `n` with a side-effect, in the expression `(incf n)`. When `integer` is 3, here’s what happens:
* The first `n` binds it to expval, i.e., the result of evaluating `(* 3 3)`, or 9.
* boolean-expression is evaluated:
```
start: (< 9 (incf n) 100)
becomes: (< 9 (setq n (1+ n)) 100)
becomes: (< 9 (setq n (1+ 9)) 100)
```
```
becomes: (< 9 (setq n 10) 100)
; side-effect here!
becomes: (< 9 n 100) ; `n` now bound to 10
becomes: (< 9 10 100)
becomes: t
```
* Because the result of the evaluation is non-`nil`, `guard` matches, `and` matches, and control passes to that clause’s body forms.Aside from the mathematical incorrectness of asserting that 9 is a double-digit integer, there is another problem with `MAYBE`. The body form references `n` once more, yet we do not see the updated value—10—at all. What happened to it?
To sum up, it’s best to avoid side-effecting references to symbol patterns entirely, not only in boolean-expression (in `guard`), but also in expr (in `let`) and function (in `pred` and `app`).
3. On match, the clause’s body forms can reference the set of symbols the pattern let-binds. When seqpat is `and`, this set is the union of all the symbols each of its sub-patterns let-binds. This makes sense because, for `and` to match, all the sub-patterns must match. When seqpat is `or`, things are different: `or` matches at the first sub-pattern that matches; the rest of the sub-patterns are ignored. It makes no sense for each sub-pattern to let-bind a different set of symbols because the body forms have no way to distinguish which sub-pattern matched and choose among the different sets. For example, the following is invalid:
```
(require 'cl-lib)
(pcase (read-number "Enter an integer: ")
((or (and (pred cl-evenp)
e-num) ; bind `e-num` to expval
o-num) ; bind `o-num` to expval
(list e-num o-num)))
```
```
Enter an integer: 42
error→ Symbol’s value as variable is void: o-num
```
```
Enter an integer: 149
error→ Symbol’s value as variable is void: e-num
```
Evaluating body form `(list e-num o-num)` signals error. To distinguish between sub-patterns, you can use another symbol, identical in name in all sub-patterns but differing in value. Reworking the above example:
```
(require 'cl-lib)
(pcase (read-number "Enter an integer: ")
((and num ; line 1
(or (and (pred cl-evenp) ; line 2
(let spin 'even)) ; line 3
(let spin 'odd))) ; line 4
(list spin num))) ; line 5
```
```
Enter an integer: 42
⇒ (even 42)
```
```
Enter an integer: 149
⇒ (odd 149)
```
Line 1 “factors out” the expval binding with `and` and symbol (in this case, `num`). On line 2, `or` begins in the same way as before, but instead of binding different symbols, uses `let` twice (lines 3-4) to bind the same symbol `spin` in both sub-patterns. The value of `spin` distinguishes the sub-patterns. The body form references both symbols (line 5).
elisp None #### Low-Level Kill Ring
These functions and variables provide access to the kill ring at a lower level, but are still convenient for use in Lisp programs, because they take care of interaction with window system selections (see [Window System Selections](window-system-selections)).
Function: **current-kill** *n &optional do-not-move*
The function `current-kill` rotates the yanking pointer, which designates the front of the kill ring, by n places (from newer kills to older ones), and returns the text at that place in the ring.
If the optional second argument do-not-move is non-`nil`, then `current-kill` doesn’t alter the yanking pointer; it just returns the nth kill, counting from the current yanking pointer.
If n is zero, indicating a request for the latest kill, `current-kill` calls the value of `interprogram-paste-function` (documented below) before consulting the kill ring. If that value is a function and calling it returns a string or a list of several strings, `current-kill` pushes the strings onto the kill ring and returns the first string. It also sets the yanking pointer to point to the kill-ring entry of the first string returned by `interprogram-paste-function`, regardless of the value of do-not-move. Otherwise, `current-kill` does not treat a zero value for n specially: it returns the entry pointed at by the yanking pointer and does not move the yanking pointer.
Function: **kill-new** *string &optional replace*
This function pushes the text string onto the kill ring and makes the yanking pointer point to it. It discards the oldest entry if appropriate. It also invokes the values of `interprogram-paste-function` (subject to the user option `save-interprogram-paste-before-kill`) and `interprogram-cut-function` (see below).
If replace is non-`nil`, then `kill-new` replaces the first element of the kill ring with string, rather than pushing string onto the kill ring.
Function: **kill-append** *string before-p*
This function appends the text string to the first entry in the kill ring and makes the yanking pointer point to the combined entry. Normally string goes at the end of the entry, but if before-p is non-`nil`, it goes at the beginning. This function calls `kill-new` as a subroutine, thus causing the values of `interprogram-cut-function` and possibly `interprogram-paste-function` (see below) to be invoked by extension.
Variable: **interprogram-paste-function**
This variable provides a way of transferring killed text from other programs, when you are using a window system. Its value should be `nil` or a function of no arguments.
If the value is a function, `current-kill` calls it to get the most recent kill. If the function returns a non-`nil` value, then that value is used as the most recent kill. If it returns `nil`, then the front of the kill ring is used.
To facilitate support for window systems that support multiple selections, this function may also return a list of strings. In that case, the first string is used as the most recent kill, and all the other strings are pushed onto the kill ring, for easy access by `yank-pop`.
The normal use of this function is to get the window system’s clipboard as the most recent kill, even if the selection belongs to another application. See [Window System Selections](window-system-selections). However, if the clipboard contents come from the current Emacs session, this function should return `nil`.
Variable: **interprogram-cut-function**
This variable provides a way of communicating killed text to other programs, when you are using a window system. Its value should be `nil` or a function of one required argument.
If the value is a function, `kill-new` and `kill-append` call it with the new first element of the kill ring as the argument.
The normal use of this function is to put newly killed text in the window system’s clipboard. See [Window System Selections](window-system-selections).
| programming_docs |
elisp None #### Self-Evaluating Forms
A *self-evaluating form* is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string `"foo"` evaluates to the string `"foo"`. Likewise, evaluating a vector does not cause evaluation of the elements of the vector—it returns the same vector with its contents unchanged.
```
'123 ; A number, shown without evaluation.
⇒ 123
```
```
123 ; Evaluated as usual—result is the same.
⇒ 123
```
```
(eval '123) ; Evaluated "by hand"—result is the same.
⇒ 123
```
```
(eval (eval '123)) ; Evaluating twice changes nothing.
⇒ 123
```
A self-evaluating form yields a value that becomes part of the program, and you should not try to modify it via `setcar`, `aset` or similar operations. The Lisp interpreter might unify the constants yielded by your program’s self-evaluating forms, so that these constants might share structure. See [Mutability](mutability).
It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because there’s no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example:
```
;; Build an expression containing a buffer object.
(setq print-exp (list 'print (current-buffer)))
⇒ (print #<buffer eval.texi>)
```
```
;; Evaluate it.
(eval print-exp)
-| #<buffer eval.texi>
⇒ #<buffer eval.texi>
```
elisp None #### Access to Frame Parameters
These functions let you read and change the parameter values of a frame.
Function: **frame-parameter** *frame parameter*
This function returns the value of the parameter parameter (a symbol) of frame. If frame is `nil`, it returns the selected frame’s parameter. If frame has no setting for parameter, this function returns `nil`.
Function: **frame-parameters** *&optional frame*
The function `frame-parameters` returns an alist listing all the parameters of frame and their values. If frame is `nil` or omitted, this returns the selected frame’s parameters
Function: **modify-frame-parameters** *frame alist*
This function alters the frame frame based on the elements of alist. Each element of alist has the form `(parm . value)`, where parm is a symbol naming a parameter. If you don’t mention a parameter in alist, its value doesn’t change. If frame is `nil`, it defaults to the selected frame.
Some parameters are only meaningful for frames on certain kinds of display (see [Frames](frames)). If alist includes parameters that are not meaningful for the frame’s display, this function will change its value in the frame’s parameter list, but will otherwise ignore it.
When alist specifies more than one parameter whose value can affect the new size of frame, the final size of the frame may differ according to the toolkit used. For example, specifying that a frame should from now on have a menu and/or tool bar instead of none and simultaneously specifying the new height of the frame will inevitably lead to a recalculation of the frame’s height. Conceptually, in such case, this function will try to have the explicit height specification prevail. It cannot be excluded, however, that the addition (or removal) of the menu or tool bar, when eventually performed by the toolkit, will defeat this intention.
Sometimes, binding `frame-inhibit-implied-resize` (see [Implied Frame Resizing](implied-frame-resizing)) to a non-`nil` value around calls to this function may fix the problem sketched here. Sometimes, however, exactly such binding may be hit by the problem.
Function: **set-frame-parameter** *frame parm value*
This function sets the frame parameter parm to the specified value. If frame is `nil`, it defaults to the selected frame.
Function: **modify-all-frames-parameters** *alist*
This function alters the frame parameters of all existing frames according to alist, then modifies `default-frame-alist` (and, if necessary, `initial-frame-alist`) to apply the same parameter values to frames that will be created henceforth.
elisp None ### Command Loop Overview
The first thing the command loop must do is read a key sequence, which is a sequence of input events that translates into a command. It does this by calling the function `read-key-sequence`. Lisp programs can also call this function (see [Key Sequence Input](key-sequence-input)). They can also read input at a lower level with `read-key` or `read-event` (see [Reading One Event](reading-one-event)), or discard pending input with `discard-input` (see [Event Input Misc](event-input-misc)).
The key sequence is translated into a command through the currently active keymaps. See [Key Lookup](key-lookup), for information on how this is done. The result should be a keyboard macro or an interactively callable function. If the key is `M-x`, then it reads the name of another command, which it then calls. This is done by the command `execute-extended-command` (see [Interactive Call](interactive-call)).
Prior to executing the command, Emacs runs `undo-boundary` to create an undo boundary. See [Maintaining Undo](maintaining-undo).
To execute a command, Emacs first reads its arguments by calling `command-execute` (see [Interactive Call](interactive-call)). For commands written in Lisp, the `interactive` specification says how to read the arguments. This may use the prefix argument (see [Prefix Command Arguments](prefix-command-arguments)) or may read with prompting in the minibuffer (see [Minibuffers](minibuffers)). For example, the command `find-file` has an `interactive` specification which says to read a file name using the minibuffer. The function body of `find-file` does not use the minibuffer, so if you call `find-file` as a function from Lisp code, you must supply the file name string as an ordinary Lisp function argument.
If the command is a keyboard macro (i.e., a string or vector), Emacs executes it using `execute-kbd-macro` (see [Keyboard Macros](keyboard-macros)).
Variable: **pre-command-hook**
This normal hook is run by the editor command loop before it executes each command. At that time, `this-command` contains the command that is about to run, and `last-command` describes the previous command. See [Command Loop Info](command-loop-info).
Variable: **post-command-hook**
This normal hook is run by the editor command loop after it executes each command (including commands terminated prematurely by quitting or by errors). At that time, `this-command` refers to the command that just ran, and `last-command` refers to the command before that.
This hook is also run when Emacs first enters the command loop (at which point `this-command` and `last-command` are both `nil`).
Quitting is suppressed while running `pre-command-hook` and `post-command-hook`. If an error happens while executing one of these hooks, it does not terminate execution of the hook; instead the error is silenced and the function in which the error occurred is removed from the hook.
A request coming into the Emacs server (see [Emacs Server](https://www.gnu.org/software/emacs/manual/html_node/emacs/Emacs-Server.html#Emacs-Server) in The GNU Emacs Manual) runs these two hooks just as a keyboard command does.
elisp None #### Keymap Type
A *keymap* maps keys typed by the user to commands. This mapping controls how the user’s command input is executed. A keymap is actually a list whose CAR is the symbol `keymap`.
See [Keymaps](keymaps), for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings.
elisp None ### Transposition of Text
This function can be used to transpose stretches of text:
Function: **transpose-regions** *start1 end1 start2 end2 &optional leave-markers*
This function exchanges two nonoverlapping portions of the buffer (if they overlap, the function signals an error). Arguments start1 and end1 specify the bounds of one portion and arguments start2 and end2 specify the bounds of the other portion.
Normally, `transpose-regions` relocates markers with the transposed text; a marker previously positioned within one of the two transposed portions moves along with that portion, thus remaining between the same two characters in their new position. However, if leave-markers is non-`nil`, `transpose-regions` does not do this—it leaves all markers unrelocated.
elisp None #### Specification List
A *specification list* is required for an Edebug specification if some arguments of a macro call are evaluated while others are not. Some elements in a specification list match one or more arguments, but others modify the processing of all following elements. The latter, called *specification keywords*, are symbols beginning with ‘`&`’ (such as `&optional`).
A specification list may contain sublists, which match arguments that are themselves lists, or it may contain vectors used for grouping. Sublists and groups thus subdivide the specification list into a hierarchy of levels. Specification keywords apply only to the remainder of the sublist or group they are contained in.
When a specification list involves alternatives or repetition, matching it against an actual macro call may require backtracking. For more details, see [Backtracking](backtracking).
Edebug specifications provide the power of regular expression matching, plus some context-free grammar constructs: the matching of sublists with balanced parentheses, recursive processing of forms, and recursion via indirect specifications.
Here’s a table of the possible elements of a specification list, with their meanings (see [Specification Examples](specification-examples), for the referenced examples):
`sexp`
A single unevaluated Lisp object, which is not instrumented.
`form`
A single evaluated expression, which is instrumented. If your macro wraps the expression with `lambda` before it is evaluated, use `def-form` instead. See `def-form` below.
`place`
A generalized variable. See [Generalized Variables](generalized-variables).
`body`
Short for `&rest form`. See `&rest` below. If your macro wraps its body of code with `lambda` before it is evaluated, use `def-body` instead. See `def-body` below.
`lambda-expr`
A lambda expression with no quoting.
`&optional`
All following elements in the specification list are optional; as soon as one does not match, Edebug stops matching at this level.
To make just a few elements optional, followed by non-optional elements, use `[&optional specs…]`. To specify that several elements must all match or none, use `&optional
[specs…]`. See the `defun` example.
`&rest`
All following elements in the specification list are repeated zero or more times. In the last repetition, however, it is not a problem if the expression runs out before matching all of the elements of the specification list.
To repeat only a few elements, use `[&rest specs…]`. To specify several elements that must all match on every repetition, use `&rest [specs…]`.
`&or`
Each of the following elements in the specification list is an alternative. One of the alternatives must match, or the `&or` specification fails.
Each list element following `&or` is a single alternative. To group two or more list elements as a single alternative, enclose them in `[…]`.
`¬`
Each of the following elements is matched as alternatives as if by using `&or`, but if any of them match, the specification fails. If none of them match, nothing is matched, but the `¬` specification succeeds.
`&define`
Indicates that the specification is for a defining form. Edebug’s definition of a defining form is a form containing one or more code forms which are saved and executed later, after the execution of the defining form.
The defining form itself is not instrumented (that is, Edebug does not stop before and after the defining form), but forms inside it typically will be instrumented. The `&define` keyword should be the first element in a list specification.
`nil`
This is successful when there are no more arguments to match at the current argument list level; otherwise it fails. See sublist specifications and the backquote example.
`gate`
No argument is matched but backtracking through the gate is disabled while matching the remainder of the specifications at this level. This is primarily used to generate more specific syntax error messages. See [Backtracking](backtracking), for more details. Also see the `let` example.
`&error`
`&error` should be followed by a string, an error message, in the edebug-spec; it aborts the instrumentation, displaying the message in the minibuffer.
`&interpose`
Lets a function control the parsing of the remaining code. It takes the form `&interpose spec fun args...` and means that Edebug will first match spec against the code and then call fun with the code that matched `spec`, a parsing function pf, and finally args.... The parsing function expects a single argument indicating the specification list to use to parse the remaining code. It should be called exactly once and returns the instrumented code that fun is expected to return. For example `(&interpose symbolp pcase--match-pat-args)` matches sexps whose first element is a symbol and then lets `pcase--match-pat-args` lookup the specs associated with that head symbol according to `pcase--match-pat-args` and pass them to the pf it received as argument.
`other-symbol`
Any other symbol in a specification list may be a predicate or an indirect specification.
If the symbol has an Edebug specification, this *indirect specification* should be either a list specification that is used in place of the symbol, or a function that is called to process the arguments. The specification may be defined with `def-edebug-elem-spec`:
Function: **def-edebug-elem-spec** *element specification*
Define the specification to use in place of the symbol element. specification has to be a list.
Otherwise, the symbol should be a predicate. The predicate is called with the argument, and if the predicate returns `nil`, the specification fails and the argument is not instrumented.
Some suitable predicates include `symbolp`, `integerp`, `stringp`, `vectorp`, and `atom`.
`[elements…]`
A vector of elements groups the elements into a single *group specification*. Its meaning has nothing to do with vectors.
`"string"`
The argument should be a symbol named string. This specification is equivalent to the quoted symbol, `'symbol`, where the name of symbol is the string, but the string form is preferred.
`(vector elements…)`
The argument should be a vector whose elements must match the elements in the specification. See the backquote example.
`(elements…)`
Any other list is a *sublist specification* and the argument must be a list whose elements match the specification elements.
A sublist specification may be a dotted list and the corresponding list argument may then be a dotted list. Alternatively, the last CDR of a dotted list specification may be another sublist specification (via a grouping or an indirect specification, e.g., `(spec . [(more
specs…)])`) whose elements match the non-dotted list arguments. This is useful in recursive specifications such as in the backquote example. Also see the description of a `nil` specification above for terminating such recursion.
Note that a sublist specification written as `(specs . nil)` is equivalent to `(specs)`, and `(specs .
(sublist-elements…))` is equivalent to `(specs
sublist-elements…)`.
Here is a list of additional specifications that may appear only after `&define`. See the `defun` example.
`&name`
Extracts the name of the current defining form from the code. It takes the form `&name [prestring] spec
[poststring] fun args...` and means that Edebug will match spec against the code and then call fun with the concatenation of the current name, args..., prestring, the code that matched `spec`, and poststring. If fun is absent, it defaults to a function that concatenates the arguments (with an `@` between the previous name and the new).
`name`
The argument, a symbol, is the name of the defining form. Shorthand for `[&name symbolp]`.
A defining form is not required to have a name field; and it may have multiple name fields.
`arg`
The argument, a symbol, is the name of an argument of the defining form. However, lambda-list keywords (symbols starting with ‘`&`’) are not allowed.
`lambda-list`
This matches a lambda list—the argument list of a lambda expression.
`def-body`
The argument is the body of code in a definition. This is like `body`, described above, but a definition body must be instrumented with a different Edebug call that looks up information associated with the definition. Use `def-body` for the highest level list of forms within the definition.
`def-form` The argument is a single, highest-level form in a definition. This is like `def-body`, except it is used to match a single form rather than a list of forms. As a special case, `def-form` also means that tracing information is not output when the form is executed. See the `interactive` example.
elisp None #### Byte-Code Function Type
*Byte-code function objects* are produced by byte-compiling Lisp code (see [Byte Compilation](byte-compilation)). Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears in a function call. See [Byte-Code Objects](byte_002dcode-objects).
The printed representation and read syntax for a byte-code function object is like that for a vector, with an additional ‘`#`’ before the opening ‘`[`’.
elisp None ### Bitwise Operations on Integers
In a computer, an integer is represented as a binary number, a sequence of *bits* (digits which are either zero or one). Conceptually the bit sequence is infinite on the left, with the most-significant bits being all zeros or all ones. A bitwise operation acts on the individual bits of such a sequence. For example, *shifting* moves the whole sequence left or right one or more places, reproducing the same pattern moved over.
The bitwise operations in Emacs Lisp apply only to integers.
Function: **ash** *integer1 count*
`ash` (*arithmetic shift*) shifts the bits in integer1 to the left count places, or to the right if count is negative. Left shifts introduce zero bits on the right; right shifts discard the rightmost bits. Considered as an integer operation, `ash` multiplies integer1 by 2\*\*count, and then converts the result to an integer by rounding downward, toward minus infinity.
Here are examples of `ash`, shifting a pattern of bits one place to the left and to the right. These examples show only the low-order bits of the binary pattern; leading bits all agree with the highest-order bit shown. As you can see, shifting left by one is equivalent to multiplying by two, whereas shifting right by one is equivalent to dividing by two and then rounding toward minus infinity.
```
(ash 7 1) ⇒ 14
;; Decimal 7 becomes decimal 14.
…000111
⇒
…001110
```
```
(ash 7 -1) ⇒ 3
…000111
⇒
…000011
```
```
(ash -7 1) ⇒ -14
…111001
⇒
…110010
```
```
(ash -7 -1) ⇒ -4
…111001
⇒
…111100
```
Here are examples of shifting left or right by two bits:
```
; binary values
(ash 5 2) ; 5 = …000101
⇒ 20 ; = …010100
(ash -5 2) ; -5 = …111011
⇒ -20 ; = …101100
```
```
(ash 5 -2)
⇒ 1 ; = …000001
```
```
(ash -5 -2)
⇒ -2 ; = …111110
```
Function: **lsh** *integer1 count*
`lsh`, which is an abbreviation for *logical shift*, shifts the bits in integer1 to the left count places, or to the right if count is negative, bringing zeros into the vacated bits. If count is negative, then integer1 must be either a fixnum or a positive bignum, and `lsh` treats a negative fixnum as if it were unsigned by subtracting twice `most-negative-fixnum` before shifting, producing a nonnegative result. This quirky behavior dates back to when Emacs supported only fixnums; nowadays `ash` is a better choice.
As `lsh` behaves like `ash` except when integer1 and count1 are both negative, the following examples focus on these exceptional cases. These examples assume 30-bit fixnums.
```
; binary values
(ash -7 -1) ; -7 = …111111111111111111111111111001
⇒ -4 ; = …111111111111111111111111111100
(lsh -7 -1)
⇒ 536870908 ; = …011111111111111111111111111100
```
```
(ash -5 -2) ; -5 = …111111111111111111111111111011
⇒ -2 ; = …111111111111111111111111111110
(lsh -5 -2)
⇒ 268435454 ; = …001111111111111111111111111110
```
Function: **logand** *&rest ints-or-markers*
This function returns the bitwise AND of the arguments: the nth bit is 1 in the result if, and only if, the nth bit is 1 in all the arguments.
For example, using 4-bit binary numbers, the bitwise AND of 13 and 12 is 12: 1101 combined with 1100 produces 1100. In both the binary numbers, the leftmost two bits are both 1 so the leftmost two bits of the returned value are both 1. However, for the rightmost two bits, each is 0 in at least one of the arguments, so the rightmost two bits of the returned value are both 0.
Therefore,
```
(logand 13 12)
⇒ 12
```
If `logand` is not passed any argument, it returns a value of -1. This number is an identity element for `logand` because its binary representation consists entirely of ones. If `logand` is passed just one argument, it returns that argument.
```
; binary values
(logand 14 13) ; 14 = …001110
; 13 = …001101
⇒ 12 ; 12 = …001100
```
```
(logand 14 13 4) ; 14 = …001110
; 13 = …001101
; 4 = …000100
⇒ 4 ; 4 = …000100
```
```
(logand)
⇒ -1 ; -1 = …111111
```
Function: **logior** *&rest ints-or-markers*
This function returns the bitwise inclusive OR of its arguments: the nth bit is 1 in the result if, and only if, the nth bit is 1 in at least one of the arguments. If there are no arguments, the result is 0, which is an identity element for this operation. If `logior` is passed just one argument, it returns that argument.
```
; binary values
(logior 12 5) ; 12 = …001100
; 5 = …000101
⇒ 13 ; 13 = …001101
```
```
(logior 12 5 7) ; 12 = …001100
; 5 = …000101
; 7 = …000111
⇒ 15 ; 15 = …001111
```
Function: **logxor** *&rest ints-or-markers*
This function returns the bitwise exclusive OR of its arguments: the nth bit is 1 in the result if, and only if, the nth bit is 1 in an odd number of the arguments. If there are no arguments, the result is 0, which is an identity element for this operation. If `logxor` is passed just one argument, it returns that argument.
```
; binary values
(logxor 12 5) ; 12 = …001100
; 5 = …000101
⇒ 9 ; 9 = …001001
```
```
(logxor 12 5 7) ; 12 = …001100
; 5 = …000101
; 7 = …000111
⇒ 14 ; 14 = …001110
```
Function: **lognot** *integer*
This function returns the bitwise complement of its argument: the nth bit is one in the result if, and only if, the nth bit is zero in integer, and vice-versa. The result equals -1 - integer.
```
(lognot 5)
⇒ -6
;; 5 = …000101
;; becomes
;; -6 = …111010
```
Function: **logcount** *integer*
This function returns the *Hamming weight* of integer: the number of ones in the binary representation of integer. If integer is negative, it returns the number of zero bits in its two’s complement binary representation. The result is always nonnegative.
```
(logcount 43) ; 43 = …000101011
⇒ 4
(logcount -43) ; -43 = …111010101
⇒ 3
```
| programming_docs |
elisp None #### Quoted Character Input
You can use the function `read-quoted-char` to ask the user to specify a character, and allow the user to specify a control or meta character conveniently, either literally or as an octal character code. The command `quoted-insert` uses this function.
Function: **read-quoted-char** *&optional prompt*
This function is like `read-char`, except that if the first character read is an octal digit (0–7), it reads any number of octal digits (but stopping if a non-octal digit is found), and returns the character represented by that numeric character code. If the character that terminates the sequence of octal digits is RET, it is discarded. Any other terminating character is used as input after this function returns.
Quitting is suppressed when the first character is read, so that the user can enter a `C-g`. See [Quitting](quitting).
If prompt is supplied, it specifies a string for prompting the user. The prompt string is always displayed in the echo area, followed by a single ‘`-`’.
In the following example, the user types in the octal number 177 (which is 127 in decimal).
```
(read-quoted-char "What character")
```
```
---------- Echo Area ----------
What character 1 7 7-
---------- Echo Area ----------
⇒ 127
```
elisp None #### Vector Type
A *vector* is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.)
The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation.
```
[1 "two" (three)] ; A vector of three elements.
⇒ [1 "two" (three)]
```
See [Vectors](vectors), for functions that work with vectors.
elisp None ### Introduction to Minibuffers
In most ways, a minibuffer is a normal Emacs buffer. Most operations *within* a buffer, such as editing commands, work normally in a minibuffer. However, many operations for managing buffers do not apply to minibuffers. The name of a minibuffer always has the form ‘ `\*Minibuf-number\*`’, and it cannot be changed. Minibuffers are displayed only in special windows used only for minibuffers; these windows always appear at the bottom of a frame. (Sometimes frames have no minibuffer window, and sometimes a special kind of frame contains nothing but a minibuffer window; see [Minibuffers and Frames](minibuffers-and-frames).)
The text in the minibuffer always starts with the *prompt string*, the text that was specified by the program that is using the minibuffer to tell the user what sort of input to type. This text is marked read-only so you won’t accidentally delete or change it. It is also marked as a field (see [Fields](fields)), so that certain motion functions, including `beginning-of-line`, `forward-word`, `forward-sentence`, and `forward-paragraph`, stop at the boundary between the prompt and the actual text.
The minibuffer’s window is normally a single line; it grows automatically if the contents require more space. Whilst the minibuffer is active, you can explicitly resize its window temporarily with the window sizing commands; the window reverts to its normal size when the minibuffer is exited. When the minibuffer is not active, you can resize its window permanently by using the window sizing commands in the frame’s other window, or dragging the mode line with the mouse. (Due to details of the current implementation, for this to work `resize-mini-windows` must be `nil`.) If the frame contains just a minibuffer window, you can change its size by changing the frame’s size.
Use of the minibuffer reads input events, and that alters the values of variables such as `this-command` and `last-command` (see [Command Loop Info](command-loop-info)). Your program should bind them around the code that uses the minibuffer, if you do not want that to change them.
Under some circumstances, a command can use a minibuffer even if there is an active minibuffer; such a minibuffer is called a *recursive minibuffer*. The first minibuffer is named ‘ `\*Minibuf-1\*`’. Recursive minibuffers are named by incrementing the number at the end of the name. (The names begin with a space so that they won’t show up in normal buffer lists.) Of several recursive minibuffers, the innermost (or most recently entered) is the *active minibuffer*–it is the one you can terminate by typing RET (`exit-minibuffer`) in. We usually call this *the* minibuffer. You can permit or forbid recursive minibuffers by setting the variable `enable-recursive-minibuffers`, or by putting properties of that name on command symbols (See [Recursive Mini](recursive-mini).)
Like other buffers, a minibuffer uses a local keymap (see [Keymaps](keymaps)) to specify special key bindings. The function that invokes the minibuffer also sets up its local map according to the job to be done. See [Text from Minibuffer](text-from-minibuffer), for the non-completion minibuffer local maps. See [Completion Commands](completion-commands), for the minibuffer local maps for completion.
An active minibuffer usually has major mode `minibuffer-mode`. This is an Emacs internal mode without any special features. To customize the setup of minibuffers, we suggest you use `minibuffer-setup-hook` (see [Minibuffer Misc](minibuffer-misc)) rather than `minibuffer-mode-hook`, since the former is run later, after the minibuffer has been fully initialized.
When a minibuffer is inactive, its major mode is `minibuffer-inactive-mode`, with keymap `minibuffer-inactive-mode-map`. This is only really useful if the minibuffer is in a separate frame. See [Minibuffers and Frames](minibuffers-and-frames).
When Emacs is running in batch mode, any request to read from the minibuffer actually reads a line from the standard input descriptor that was supplied when Emacs was started. This supports only basic input: none of the special minibuffer features (history, completion, etc.) are available in batch mode.
elisp None ### Frame Configurations
A *frame configuration* records the current arrangement of frames, all their properties, and the window configuration of each one. (See [Window Configurations](window-configurations).)
Function: **current-frame-configuration**
This function returns a frame configuration list that describes the current arrangement of frames and their contents.
Function: **set-frame-configuration** *configuration &optional nodelete*
This function restores the state of frames described in configuration. However, this function does not restore deleted frames.
Ordinarily, this function deletes all existing frames not listed in configuration. But if nodelete is non-`nil`, the unwanted frames are iconified instead.
elisp None ### Deleting Text
Deletion means removing part of the text in a buffer, without saving it in the kill ring (see [The Kill Ring](the-kill-ring)). Deleted text can’t be yanked, but can be reinserted using the undo mechanism (see [Undo](undo)). Some deletion functions do save text in the kill ring in some special cases.
All of the deletion functions operate on the current buffer.
Command: **erase-buffer**
This function deletes the entire text of the current buffer (*not* just the accessible portion), leaving it empty. If the buffer is read-only, it signals a `buffer-read-only` error; if some of the text in it is read-only, it signals a `text-read-only` error. Otherwise, it deletes the text without asking for any confirmation. It returns `nil`.
Normally, deleting a large amount of text from a buffer inhibits further auto-saving of that buffer because it has shrunk. However, `erase-buffer` does not do this, the idea being that the future text is not really related to the former text, and its size should not be compared with that of the former text.
Command: **delete-region** *start end*
This command deletes the text between positions start and end in the current buffer, and returns `nil`. If point was inside the deleted region, its value afterward is start. Otherwise, point relocates with the surrounding text, as markers do.
Function: **delete-and-extract-region** *start end*
This function deletes the text between positions start and end in the current buffer, and returns a string containing the text just deleted.
If point was inside the deleted region, its value afterward is start. Otherwise, point relocates with the surrounding text, as markers do.
Command: **delete-char** *count &optional killp*
This command deletes count characters directly after point, or before point if count is negative. If killp is non-`nil`, then it saves the deleted characters in the kill ring.
In an interactive call, count is the numeric prefix argument, and killp is the unprocessed prefix argument. Therefore, if a prefix argument is supplied, the text is saved in the kill ring. If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.
The value returned is always `nil`.
Command: **delete-backward-char** *count &optional killp*
This command deletes count characters directly before point, or after point if count is negative. If killp is non-`nil`, then it saves the deleted characters in the kill ring.
In an interactive call, count is the numeric prefix argument, and killp is the unprocessed prefix argument. Therefore, if a prefix argument is supplied, the text is saved in the kill ring. If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.
The value returned is always `nil`.
Command: **backward-delete-char-untabify** *count &optional killp*
This command deletes count characters backward, changing tabs into spaces. When the next character to be deleted is a tab, it is first replaced with the proper number of spaces to preserve alignment and then one of those spaces is deleted instead of the tab. If killp is non-`nil`, then the command saves the deleted characters in the kill ring.
Conversion of tabs to spaces happens only if count is positive. If it is negative, exactly -count characters after point are deleted.
In an interactive call, count is the numeric prefix argument, and killp is the unprocessed prefix argument. Therefore, if a prefix argument is supplied, the text is saved in the kill ring. If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.
The value returned is always `nil`.
User Option: **backward-delete-char-untabify-method**
This option specifies how `backward-delete-char-untabify` should deal with whitespace. Possible values include `untabify`, the default, meaning convert a tab to many spaces and delete one; `hungry`, meaning delete all tabs and spaces before point with one command; `all` meaning delete all tabs, spaces and newlines before point, and `nil`, meaning do nothing special for whitespace characters.
elisp None #### Recording Input
Function: **recent-keys** *&optional include-cmds*
This function returns a vector containing the last 300 input events from the keyboard or mouse. All input events are included, whether or not they were used as parts of key sequences. Thus, you always get the last 300 input events, not counting events generated by keyboard macros. (These are excluded because they are less interesting for debugging; it should be enough to see the events that invoked the macros.)
If include-cmds is non-`nil`, complete key sequences in the result vector are interleaved with pseudo-events of the form `(nil . COMMAND)`, where COMMAND is the binding of the key sequence (see [Command Overview](command-overview)).
A call to `clear-this-command-keys` (see [Command Loop Info](command-loop-info)) causes this function to return an empty vector immediately afterward.
Command: **open-dribble-file** *filename*
This function opens a *dribble file* named filename. When a dribble file is open, each input event from the keyboard or mouse (but not those from keyboard macros) is written in that file. A non-character event is expressed using its printed representation surrounded by ‘`<…>`’. Be aware that sensitive information (such as passwords) may end up recorded in the dribble file.
You close the dribble file by calling this function with an argument of `nil`.
See also the `open-termscript` function (see [Terminal Output](terminal-output)).
elisp None ### Creating Keymaps
Here we describe the functions for creating keymaps.
Function: **make-sparse-keymap** *&optional prompt*
This function creates and returns a new sparse keymap with no entries. (A sparse keymap is the kind of keymap you usually want.) The new keymap does not contain a char-table, unlike `make-keymap`, and does not bind any events.
```
(make-sparse-keymap)
⇒ (keymap)
```
If you specify prompt, that becomes the overall prompt string for the keymap. You should specify this only for menu keymaps (see [Defining Menus](defining-menus)). A keymap with an overall prompt string will always present a mouse menu or a keyboard menu if it is active for looking up the next input event. Don’t specify an overall prompt string for the main map of a major or minor mode, because that would cause the command loop to present a keyboard menu every time.
Function: **make-keymap** *&optional prompt*
This function creates and returns a new full keymap. That keymap contains a char-table (see [Char-Tables](char_002dtables)) with slots for all characters without modifiers. The new keymap initially binds all these characters to `nil`, and does not bind any other kind of event. The argument prompt specifies a prompt string, as in `make-sparse-keymap`.
```
(make-keymap)
⇒ (keymap #^[nil nil keymap nil nil nil …])
```
A full keymap is more efficient than a sparse keymap when it holds lots of bindings; for just a few, the sparse keymap is better.
Function: **copy-keymap** *keymap*
This function returns a copy of keymap. This is almost never needed. If you want a keymap that’s like another yet with a few changes, you should use map inheritance rather than copying. I.e., something like:
```
(let ((map (make-sparse-keymap)))
(set-keymap-parent map <theirmap>)
(define-key map ...)
...)
```
When performing `copy-keymap`, any keymaps that appear directly as bindings in keymap are also copied recursively, and so on to any number of levels. However, recursive copying does not take place when the definition of a character is a symbol whose function definition is a keymap; the same symbol appears in the new copy.
```
(setq map (copy-keymap (current-local-map)))
⇒ (keymap
```
```
;; (This implements meta characters.)
(27 keymap
(83 . center-paragraph)
(115 . center-line))
(9 . tab-to-tab-stop))
```
```
(eq map (current-local-map))
⇒ nil
```
```
(equal map (current-local-map))
⇒ t
```
elisp None #### Terminal I/O Encoding
Emacs can use coding systems to decode keyboard input and encode terminal output. This is useful for terminals that transmit or display text using a particular encoding, such as Latin-1. Emacs does not set `last-coding-system-used` when encoding or decoding terminal I/O.
Function: **keyboard-coding-system** *&optional terminal*
This function returns the coding system used for decoding keyboard input from terminal. A value of `no-conversion` means no decoding is done. If terminal is omitted or `nil`, it means the selected frame’s terminal. See [Multiple Terminals](multiple-terminals).
Command: **set-keyboard-coding-system** *coding-system &optional terminal*
This command specifies coding-system as the coding system to use for decoding keyboard input from terminal. If coding-system is `nil`, that means not to decode keyboard input. If terminal is a frame, it means that frame’s terminal; if it is `nil`, that means the currently selected frame’s terminal. See [Multiple Terminals](multiple-terminals). Note that on modern MS-Windows systems Emacs always uses Unicode input when decoding keyboard input, so the encoding set by this command has no effect on Windows.
Function: **terminal-coding-system** *&optional terminal*
This function returns the coding system that is in use for encoding terminal output from terminal. A value of `no-conversion` means no encoding is done. If terminal is a frame, it means that frame’s terminal; if it is `nil`, that means the currently selected frame’s terminal.
Command: **set-terminal-coding-system** *coding-system &optional terminal*
This command specifies coding-system as the coding system to use for encoding terminal output from terminal. If coding-system is `nil`, that means not to encode terminal output. If terminal is a frame, it means that frame’s terminal; if it is `nil`, that means the currently selected frame’s terminal.
elisp None ### Mutability
Some Lisp objects should never change. For example, the Lisp expression `"aaa"` yields a string, but you should not change its contents. And some objects cannot be changed; for example, although you can create a new number by calculating one, Lisp provides no operation to change the value of an existing number.
Other Lisp objects are *mutable*: it is safe to change their values via destructive operations involving side effects. For example, an existing marker can be changed by moving the marker to point to somewhere else.
Although numbers never change and all markers are mutable, some types have members some of which are mutable and others not. These types include conses, vectors, and strings. For example, although `"cons"` and `(symbol-name 'cons)` both yield strings that should not be changed, `(copy-sequence "cons")` and `(make-string 3 ?a)` both yield mutable strings that can be changed via later calls to `aset`.
A mutable object stops being mutable if it is part of an expression that is evaluated. For example:
```
(let* ((x (list 0.5))
(y (eval (list 'quote x))))
(setcar x 1.5) ;; The program should not do this.
y)
```
Although the list `(0.5)` was mutable when it was created, it should not have been changed via `setcar` because it was given to `eval`. The reverse does not occur: an object that should not be changed never becomes mutable afterwards.
If a program attempts to change objects that should not be changed, the resulting behavior is undefined: the Lisp interpreter might signal an error, or it might crash or behave unpredictably in other ways.[2](#FOOT2)
When similar constants occur as parts of a program, the Lisp interpreter might save time or space by reusing existing constants or their components. For example, `(eq "abc" "abc")` returns `t` if the interpreter creates only one instance of the string literal `"abc"`, and returns `nil` if it creates two instances. Lisp programs should be written so that they work regardless of whether this optimization is in use.
elisp None ### Killing Buffers
*Killing a buffer* makes its name unknown to Emacs and makes the memory space it occupied available for other use.
The buffer object for the buffer that has been killed remains in existence as long as anything refers to it, but it is specially marked so that you cannot make it current or display it. Killed buffers retain their identity, however; if you kill two distinct buffers, they remain distinct according to `eq` although both are dead.
If you kill a buffer that is current or displayed in a window, Emacs automatically selects or displays some other buffer instead. This means that killing a buffer can change the current buffer. Therefore, when you kill a buffer, you should also take the precautions associated with changing the current buffer (unless you happen to know that the buffer being killed isn’t current). See [Current Buffer](current-buffer).
If you kill a buffer that is the base buffer of one or more indirect buffers (see [Indirect Buffers](indirect-buffers)), the indirect buffers are automatically killed as well.
The `buffer-name` of a buffer is `nil` if, and only if, the buffer is killed. A buffer that has not been killed is called a *live* buffer. To test whether a buffer is live or killed, use the function `buffer-live-p` (see below).
Command: **kill-buffer** *&optional buffer-or-name*
This function kills the buffer buffer-or-name, freeing all its memory for other uses or to be returned to the operating system. If buffer-or-name is `nil` or omitted, it kills the current buffer.
Any processes that have this buffer as the `process-buffer` are sent the `SIGHUP` (hangup) signal, which normally causes them to terminate. See [Signals to Processes](signals-to-processes).
If the buffer is visiting a file and contains unsaved changes, `kill-buffer` asks the user to confirm before the buffer is killed. It does this even if not called interactively. To prevent the request for confirmation, clear the modified flag before calling `kill-buffer`. See [Buffer Modification](buffer-modification).
This function calls `replace-buffer-in-windows` for cleaning up all windows currently displaying the buffer to be killed.
Killing a buffer that is already dead has no effect.
This function returns `t` if it actually killed the buffer. It returns `nil` if the user refuses to confirm or if buffer-or-name was already dead.
```
(kill-buffer "foo.unchanged")
⇒ t
(kill-buffer "foo.changed")
---------- Buffer: Minibuffer ----------
Buffer foo.changed modified; kill anyway? (yes or no) yes
---------- Buffer: Minibuffer ----------
⇒ t
```
Variable: **kill-buffer-query-functions**
Before confirming unsaved changes, `kill-buffer` calls the functions in the list `kill-buffer-query-functions`, in order of appearance, with no arguments. The buffer being killed is the current buffer when they are called. The idea of this feature is that these functions will ask for confirmation from the user. If any of them returns `nil`, `kill-buffer` spares the buffer’s life.
This hook is not run for internal or temporary buffers created by `get-buffer-create` or `generate-new-buffer` with a non-`nil` argument inhibit-buffer-hooks.
Variable: **kill-buffer-hook**
This is a normal hook run by `kill-buffer` after asking all the questions it is going to ask, just before actually killing the buffer. The buffer to be killed is current when the hook functions run. See [Hooks](hooks). This variable is a permanent local, so its local binding is not cleared by changing major modes.
This hook is not run for internal or temporary buffers created by `get-buffer-create` or `generate-new-buffer` with a non-`nil` argument inhibit-buffer-hooks.
User Option: **buffer-offer-save**
This variable, if non-`nil` in a particular buffer, tells `save-buffers-kill-emacs` to offer to save that buffer, just as it offers to save file-visiting buffers. If `save-some-buffers` is called with the second optional argument set to `t`, it will also offer to save the buffer. Lastly, if this variable is set to the symbol `always`, both `save-buffers-kill-emacs` and `save-some-buffers` will always offer to save. See [Definition of save-some-buffers](saving-buffers#Definition-of-save_002dsome_002dbuffers). The variable `buffer-offer-save` automatically becomes buffer-local when set for any reason. See [Buffer-Local Variables](buffer_002dlocal-variables).
Variable: **buffer-save-without-query**
This variable, if non-`nil` in a particular buffer, tells `save-buffers-kill-emacs` and `save-some-buffers` to save this buffer (if it’s modified) without asking the user. The variable automatically becomes buffer-local when set for any reason.
Function: **buffer-live-p** *object*
This function returns `t` if object is a live buffer (a buffer which has not been killed), `nil` otherwise.
| programming_docs |
elisp None #### Hash Table Type
A hash table is a very fast kind of lookup table, somewhat like an alist in that it maps keys to corresponding values, but much faster. The printed representation of a hash table specifies its properties and contents, like this:
```
(make-hash-table)
⇒ #s(hash-table size 65 test eql rehash-size 1.5
rehash-threshold 0.8125 data ())
```
See [Hash Tables](hash-tables), for more information about hash tables.
elisp None ### Embedded Native Widgets
Emacs is able to display native widgets, such as GTK+ WebKit widgets, in Emacs buffers when it was built with the necessary support libraries and is running on a graphical terminal. To test whether Emacs supports display of embedded widgets, check that the `xwidget-internal` feature is available (see [Named Features](named-features)).
To display an embedded widget in a buffer, you must first create an xwidget object, and then use that object as the display specifier in a `display` text or overlay property (see [Display Property](display-property)).
Function: **make-xwidget** *type title width height arguments &optional buffer*
This creates and returns an xwidget object. If buffer is omitted or `nil`, it defaults to the current buffer. If buffer names a buffer that doesn’t exist, it will be created. The type identifies the type of the xwidget component, it can be one of the following:
`webkit` The WebKit component.
The width and height arguments specify the widget size in pixels, and title, a string, specifies its title.
Function: **xwidgetp** *object*
This function returns `t` if object is an xwidget, `nil` otherwise.
Function: **xwidget-plist** *xwidget*
This function returns the property list of xwidget.
Function: **set-xwidget-plist** *xwidget plist*
This function replaces the property list of xwidget with a new property list given by plist.
Function: **xwidget-buffer** *xwidget*
This function returns the buffer of xwidget.
Function: **get-buffer-xwidgets** *buffer*
This function returns a list of xwidget objects associated with the buffer, which can be specified as a buffer object or a name of an existing buffer, a string. The value is `nil` if buffer contains no xwidgets.
Function: **xwidget-webkit-goto-uri** *xwidget uri*
This function browses the specified uri in the given xwidget. The uri is a string that specifies the name of a file or a URL.
Function: **xwidget-webkit-execute-script** *xwidget script*
This function causes the browser widget specified by xwidget to execute the specified JavaScript `script`.
Function: **xwidget-webkit-execute-script-rv** *xwidget script &optional default*
This function executes the specified script like `xwidget-webkit-execute-script` does, but it also returns the script’s return value as a string. If script doesn’t return a value, this function returns default, or `nil` if default was omitted.
Function: **xwidget-webkit-get-title** *xwidget*
This function returns the title of xwidget as a string.
Function: **xwidget-resize** *xwidget width height*
This function resizes the specified xwidget to the size widthxheight pixels.
Function: **xwidget-size-request** *xwidget*
This function returns the desired size of xwidget as a list of the form `(width height)`. The dimensions are in pixels.
Function: **xwidget-info** *xwidget*
This function returns the attributes of xwidget as a vector of the form `[type title width height]`. The attributes are usually determined by `make-xwidget` when the xwidget is created.
Function: **set-xwidget-query-on-exit-flag** *xwidget flag*
This function allows you to arrange that Emacs will ask the user for confirmation before exiting or before killing a buffer that has xwidget associated with it. If flag is non-`nil`, Emacs will query the user, otherwise it will not.
Function: **xwidget-query-on-exit-flag** *xwidget*
This function returns the current setting of xwidgets query-on-exit flag, either `t` or `nil`.
elisp None #### Lexical Binding
Lexical binding was introduced to Emacs, as an optional feature, in version 24.1. We expect its importance to increase with time. Lexical binding opens up many more opportunities for optimization, so programs using it are likely to run faster in future Emacs versions. Lexical binding is also more compatible with concurrency, which was added to Emacs in version 26.1.
A lexically-bound variable has *lexical scope*, meaning that any reference to the variable must be located textually within the binding construct. Here is an example (see [Using Lexical Binding](using-lexical-binding), for how to actually enable lexical binding):
```
(let ((x 1)) ; `x` is lexically bound.
(+ x 3))
⇒ 4
(defun getx ()
x) ; `x` is used free in this function.
(let ((x 1)) ; `x` is lexically bound.
(getx))
error→ Symbol's value as variable is void: x
```
Here, the variable `x` has no global value. When it is lexically bound within a `let` form, it can be used in the textual confines of that `let` form. But it can *not* be used from within a `getx` function called from the `let` form, since the function definition of `getx` occurs outside the `let` form itself.
Here is how lexical binding works. Each binding construct defines a *lexical environment*, specifying the variables that are bound within the construct and their local values. When the Lisp evaluator wants the current value of a variable, it looks first in the lexical environment; if the variable is not specified in there, it looks in the symbol’s value cell, where the dynamic value is stored.
(Internally, the lexical environment is an alist of symbol-value pairs, with the final element in the alist being the symbol `t` rather than a cons cell. Such an alist can be passed as the second argument to the `eval` function, in order to specify a lexical environment in which to evaluate a form. See [Eval](eval). Most Emacs Lisp programs, however, should not interact directly with lexical environments in this way; only specialized programs like debuggers.)
Lexical bindings have indefinite extent. Even after a binding construct has finished executing, its lexical environment can be “kept around” in Lisp objects called *closures*. A closure is created when you define a named or anonymous function with lexical binding enabled. See [Closures](closures), for details.
When a closure is called as a function, any lexical variable references within its definition use the retained lexical environment. Here is an example:
```
(defvar my-ticker nil) ; We will use this dynamically bound
; variable to store a closure.
(let ((x 0)) ; `x` is lexically bound.
(setq my-ticker (lambda ()
(setq x (1+ x)))))
⇒ (closure ((x . 0) t) ()
(setq x (1+ x)))
(funcall my-ticker)
⇒ 1
(funcall my-ticker)
⇒ 2
(funcall my-ticker)
⇒ 3
x ; Note that `x` has no global value.
error→ Symbol's value as variable is void: x
```
The `let` binding defines a lexical environment in which the variable `x` is locally bound to 0. Within this binding construct, we define a lambda expression which increments `x` by one and returns the incremented value. This lambda expression is automatically turned into a closure, in which the lexical environment lives on even after the `let` binding construct has exited. Each time we evaluate the closure, it increments `x`, using the binding of `x` in that lexical environment.
Note that unlike dynamic variables which are tied to the symbol object itself, the relationship between lexical variables and symbols is only present in the interpreter (or compiler). Therefore, functions which take a symbol argument (like `symbol-value`, `boundp`, and `set`) can only retrieve or modify a variable’s dynamic binding (i.e., the contents of its symbol’s value cell).
elisp None Variables
---------
A *variable* is a name used in a program to stand for a value. In Lisp, each variable is represented by a Lisp symbol (see [Symbols](symbols)). The variable name is simply the symbol’s name, and the variable’s value is stored in the symbol’s value cell[9](#FOOT9). See [Symbol Components](symbol-components). In Emacs Lisp, the use of a symbol as a variable is independent of its use as a function name.
As previously noted in this manual, a Lisp program is represented primarily by Lisp objects, and only secondarily as text. The textual form of a Lisp program is given by the read syntax of the Lisp objects that constitute the program. Hence, the textual form of a variable in a Lisp program is written using the read syntax for the symbol representing the variable.
| | | |
| --- | --- | --- |
| • [Global Variables](global-variables) | | Variable values that exist permanently, everywhere. |
| • [Constant Variables](constant-variables) | | Variables that never change. |
| • [Local Variables](local-variables) | | Variable values that exist only temporarily. |
| • [Void Variables](void-variables) | | Symbols that lack values. |
| • [Defining Variables](defining-variables) | | A definition says a symbol is used as a variable. |
| • [Tips for Defining](tips-for-defining) | | Things you should think about when you define a variable. |
| • [Accessing Variables](accessing-variables) | | Examining values of variables whose names are known only at run time. |
| • [Setting Variables](setting-variables) | | Storing new values in variables. |
| • [Watching Variables](watching-variables) | | Running a function when a variable is changed. |
| • [Variable Scoping](variable-scoping) | | How Lisp chooses among local and global values. |
| • [Buffer-Local Variables](buffer_002dlocal-variables) | | Variable values in effect only in one buffer. |
| • [File Local Variables](file-local-variables) | | Handling local variable lists in files. |
| • [Directory Local Variables](directory-local-variables) | | Local variables common to all files in a directory. |
| • [Connection Local Variables](connection-local-variables) | | Local variables common for remote connections. |
| • [Variable Aliases](variable-aliases) | | Variables that are aliases for other variables. |
| • [Variables with Restricted Values](variables-with-restricted-values) | | Non-constant variables whose value can *not* be an arbitrary Lisp object. |
| • [Generalized Variables](generalized-variables) | | Extending the concept of variables. |
elisp None #### A Sample Function Description
In a function description, the name of the function being described appears first. It is followed on the same line by a list of argument names. These names are also used in the body of the description, to stand for the values of the arguments.
The appearance of the keyword `&optional` in the argument list indicates that the subsequent arguments may be omitted (omitted arguments default to `nil`). Do not write `&optional` when you call the function.
The keyword `&rest` (which must be followed by a single argument name) indicates that any number of arguments can follow. The single argument name following `&rest` receives, as its value, a list of all the remaining arguments passed to the function. Do not write `&rest` when you call the function.
Here is a description of an imaginary function `foo`:
Function: **foo** *integer1 &optional integer2 &rest integers*
The function `foo` subtracts integer1 from integer2, then adds all the rest of the arguments to the result. If integer2 is not supplied, then the number 19 is used by default.
```
(foo 1 5 3 9)
⇒ 16
(foo 5)
⇒ 14
```
More generally,
```
(foo w x y…)
≡
(+ (- x w) y…)
```
By convention, any argument whose name contains the name of a type (e.g., integer, integer1 or buffer) is expected to be of that type. A plural of a type (such as buffers) often means a list of objects of that type. An argument named object may be of any type. (For a list of Emacs object types, see [Lisp Data Types](lisp-data-types).) An argument with any other sort of name (e.g., new-file) is specific to the function; if the function has a documentation string, the type of the argument should be described there (see [Documentation](documentation)).
See [Lambda Expressions](lambda-expressions), for a more complete description of arguments modified by `&optional` and `&rest`.
Command, macro, and special form descriptions have the same format, but the word ‘`Function`’ is replaced by ‘`Command`’, ‘`Macro`’, or ‘`Special Form`’, respectively. Commands are simply functions that may be called interactively; macros process their arguments differently from functions (the arguments are not evaluated), but are presented the same way.
The descriptions of macros and special forms use a more complex notation to specify optional and repeated arguments, because they can break the argument list down into separate arguments in more complicated ways. ‘`[optional-arg]`’ means that optional-arg is optional and ‘`repeated-args…`’ stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure. Here is an example:
Special Form: **count-loop** *(var [from to [inc]]) body…*
This imaginary special form implements a loop that executes the body forms and then increments the variable var on each iteration. On the first iteration, the variable has the value from; on subsequent iterations, it is incremented by one (or by inc if that is given). The loop exits before executing body if var equals to. Here is an example:
```
(count-loop (i 0 10)
(prin1 i) (princ " ")
(prin1 (aref vector i))
(terpri))
```
If from and to are omitted, var is bound to `nil` before the loop begins, and the loop exits if var is non-`nil` at the beginning of an iteration. Here is an example:
```
(count-loop (done)
(if (pending)
(fixit)
(setq done t)))
```
In this special form, the arguments from and to are optional, but must both be present or both absent. If they are present, inc may optionally be specified as well. These arguments are grouped with the argument var into a list, to distinguish them from body, which includes all remaining elements of the form.
elisp None ### Compiler Errors
Error and warning messages from byte compilation are printed in a buffer named `\*Compile-Log\*`. These messages include file names and line numbers identifying the location of the problem. The usual Emacs commands for operating on compiler output can be used on these messages.
When an error is due to invalid syntax in the program, the byte compiler might get confused about the error’s exact location. One way to investigate is to switch to the buffer `\*Compiler Input\*`. (This buffer name starts with a space, so it does not show up in the Buffer Menu.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read; the cause of the error might be nearby. See [Syntax Errors](syntax-errors), for some tips for locating syntax errors.
A common type of warning issued by the byte compiler is for functions and variables that were used but not defined. Such warnings report the line number for the end of the file, not the locations where the missing functions or variables were used; to find these, you must search the file manually.
If you are sure that a warning message about a missing function or variable is unjustified, there are several ways to suppress it:
* You can suppress the warning for a specific call to a function func by conditionalizing it on an `fboundp` test, like this:
```
(if (fboundp 'func) ...(func ...)...)
```
The call to func must be in the then-form of the `if`, and func must appear quoted in the call to `fboundp`. (This feature operates for `cond` as well.)
* Likewise, you can suppress the warning for a specific use of a variable variable by conditionalizing it on a `boundp` test:
```
(if (boundp 'variable) ...variable...)
```
The reference to variable must be in the then-form of the `if`, and variable must appear quoted in the call to `boundp`.
* You can tell the compiler that a function is defined using `declare-function`. See [Declaring Functions](declaring-functions).
* Likewise, you can tell the compiler that a variable is defined using `defvar` with no initial value. (Note that this marks the variable as special, i.e. dynamically bound, but only within the current lexical scope, or file if at top-level.) See [Defining Variables](defining-variables).
You can also suppress compiler warnings within a certain expression using the `with-suppressed-warnings` macro:
Special Form: **with-suppressed-warnings** *warnings body…*
In execution, this is equivalent to `(progn body...)`, but the compiler does not issue warnings for the specified conditions in body. warnings is an associative list of warning symbols and function/variable symbols they apply to. For instance, if you wish to call an obsolete function called `foo`, but want to suppress the compilation warning, say:
```
(with-suppressed-warnings ((obsolete foo))
(foo ...))
```
For more coarse-grained suppression of compiler warnings, you can use the `with-no-warnings` construct:
Special Form: **with-no-warnings** *body…*
In execution, this is equivalent to `(progn body...)`, but the compiler does not issue warnings for anything that occurs inside body.
We recommend that you use `with-suppressed-warnings` instead, but if you do use this construct, that you use it around the smallest possible piece of code to avoid missing possible warnings other than one you intend to suppress.
Byte compiler warnings can be controlled more precisely by setting the variable `byte-compile-warnings`. See its documentation string for details.
Sometimes you may wish the byte-compiler warnings to be reported using `error`. If so, set `byte-compile-error-on-warn` to a non-`nil` value.
elisp None ### Searching for Strings
These are the primitive functions for searching through the text in a buffer. They are meant for use in programs, but you may call them interactively. If you do so, they prompt for the search string; the arguments limit and noerror are `nil`, and repeat is 1. For more details on interactive searching, see [Searching and Replacement](https://www.gnu.org/software/emacs/manual/html_node/emacs/Search.html#Search) in The GNU Emacs Manual.
These search functions convert the search string to multibyte if the buffer is multibyte; they convert the search string to unibyte if the buffer is unibyte. See [Text Representations](text-representations).
Command: **search-forward** *string &optional limit noerror count*
This function searches forward from point for an exact match for string. If successful, it sets point to the end of the occurrence found, and returns the new value of point. If no match is found, the value and side effects depend on noerror (see below).
In the following example, point is initially at the beginning of the line. Then `(search-forward "fox")` moves point after the last letter of ‘`fox`’:
```
---------- Buffer: foo ----------
∗The quick brown fox jumped over the lazy dog.
---------- Buffer: foo ----------
```
```
(search-forward "fox")
⇒ 20
---------- Buffer: foo ----------
The quick brown fox∗ jumped over the lazy dog.
---------- Buffer: foo ----------
```
The argument limit specifies the bound to the search, and should be a position in the current buffer. No match extending after that position is accepted. If limit is omitted or `nil`, it defaults to the end of the accessible portion of the buffer.
What happens when the search fails depends on the value of noerror. If noerror is `nil`, a `search-failed` error is signaled. If noerror is `t`, `search-forward` returns `nil` and does nothing. If noerror is neither `nil` nor `t`, then `search-forward` moves point to the upper bound and returns `nil`.
The argument noerror only affects valid searches which fail to find a match. Invalid arguments cause errors regardless of noerror.
If count is a positive number n, the search is done n times; each successive search starts at the end of the previous match. If all these successive searches succeed, the function call succeeds, moving point and returning its new value. Otherwise the function call fails, with results depending on the value of noerror, as described above. If count is a negative number -n, the search is done n times in the opposite (backward) direction.
Command: **search-backward** *string &optional limit noerror count*
This function searches backward from point for string. It is like `search-forward`, except that it searches backwards rather than forwards. Backward searches leave point at the beginning of the match.
Command: **word-search-forward** *string &optional limit noerror count*
This function searches forward from point for a word match for string. If it finds a match, it sets point to the end of the match found, and returns the new value of point.
Word matching regards string as a sequence of words, disregarding punctuation that separates them. It searches the buffer for the same sequence of words. Each word must be distinct in the buffer (searching for the word ‘`ball`’ does not match the word ‘`balls`’), but the details of punctuation and spacing are ignored (searching for ‘`ball boy`’ does match ‘`ball. Boy!`’).
In this example, point is initially at the beginning of the buffer; the search leaves it between the ‘`y`’ and the ‘`!`’.
```
---------- Buffer: foo ----------
∗He said "Please! Find
the ball boy!"
---------- Buffer: foo ----------
```
```
(word-search-forward "Please find the ball, boy.")
⇒ 39
---------- Buffer: foo ----------
He said "Please! Find
the ball boy∗!"
---------- Buffer: foo ----------
```
If limit is non-`nil`, it must be a position in the current buffer; it specifies the upper bound to the search. The match found must not extend after that position.
If noerror is `nil`, then `word-search-forward` signals an error if the search fails. If noerror is `t`, then it returns `nil` instead of signaling an error. If noerror is neither `nil` nor `t`, it moves point to limit (or the end of the accessible portion of the buffer) and returns `nil`.
If count is a positive number, it specifies how many successive occurrences to search for. Point is positioned at the end of the last match. If count is a negative number, the search is backward and point is positioned at the beginning of the last match.
Internally, `word-search-forward` and related functions use the function `word-search-regexp` to convert string to a regular expression that ignores punctuation.
Command: **word-search-forward-lax** *string &optional limit noerror count*
This command is identical to `word-search-forward`, except that the beginning or the end of string need not match a word boundary, unless string begins or ends in whitespace. For instance, searching for ‘`ball boy`’ matches ‘`ball boyee`’, but does not match ‘`balls boy`’.
Command: **word-search-backward** *string &optional limit noerror count*
This function searches backward from point for a word match to string. This function is just like `word-search-forward` except that it searches backward and normally leaves point at the beginning of the match.
Command: **word-search-backward-lax** *string &optional limit noerror count*
This command is identical to `word-search-backward`, except that the beginning or the end of string need not match a word boundary, unless string begins or ends in whitespace.
| programming_docs |
elisp None #### Standard File Names
Sometimes, an Emacs Lisp program needs to specify a standard file name for a particular use—typically, to hold configuration data specified by the current user. Usually, such files should be located in the directory specified by `user-emacs-directory`, which is typically `~/.config/emacs/` or `~/.emacs.d/` by default (see [How Emacs Finds Your Init File](https://www.gnu.org/software/emacs/manual/html_node/emacs/Find-Init.html#Find-Init) in The GNU Emacs Manual). For example, abbrev definitions are stored by default in `~/.config/emacs/abbrev\_defs` or `~/.emacs.d/abbrev\_defs`. The easiest way to specify such a file name is to use the function `locate-user-emacs-file`.
Function: **locate-user-emacs-file** *base-name &optional old-name*
This function returns an absolute file name for an Emacs-specific configuration or data file. The argument `base-name` should be a relative file name. The return value is the absolute name of a file in the directory specified by `user-emacs-directory`; if that directory does not exist, this function creates it.
If the optional argument old-name is non-`nil`, it specifies a file in the user’s home directory, `~/old-name`. If such a file exists, the return value is the absolute name of that file, instead of the file specified by base-name. This argument is intended to be used by Emacs packages to provide backward compatibility. For instance, prior to the introduction of `user-emacs-directory`, the abbrev file was located in `~/.abbrev\_defs`. Here is the definition of `abbrev-file-name`:
```
(defcustom abbrev-file-name
(locate-user-emacs-file "abbrev_defs" ".abbrev_defs")
"Default name of file from which to read abbrevs."
…
:type 'file)
```
A lower-level function for standardizing file names, which `locate-user-emacs-file` uses as a subroutine, is `convert-standard-filename`.
Function: **convert-standard-filename** *filename*
This function returns a file name based on filename, which fits the conventions of the current operating system.
On GNU and other POSIX-like systems, this simply returns filename. On other operating systems, it may enforce system-specific file name conventions; for example, on MS-DOS this function performs a variety of changes to enforce MS-DOS file name limitations, including converting any leading ‘`.`’ to ‘`\_`’ and truncating to three characters after the ‘`.`’.
The recommended way to use this function is to specify a name which fits the conventions of GNU and Unix systems, and pass it to `convert-standard-filename`.
elisp None #### Documentation Strings of Functions
A lambda expression may optionally have a *documentation string* just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. See [Documentation](documentation), for how the documentation string is accessed.
It is a good idea to provide documentation strings for all the functions in your program, even those that are called only from within your program. Documentation strings are like comments, except that they are easier to access.
The first line of the documentation string should stand on its own, because `apropos` displays just this first line. It should consist of one or two complete sentences that summarize the function’s purpose.
The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. *That is a mistake.* The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands.
You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation.
The last line of the documentation string can specify calling conventions different from the actual function arguments. Write text like this:
```
\(fn arglist)
```
following a blank line, at the beginning of the line, with no newline following it inside the documentation string. (The ‘`\`’ is used to avoid confusing the Emacs motion commands.) The calling convention specified in this way appears in help messages in place of the one derived from the actual arguments of the function.
This feature is particularly useful for macro definitions, since the arguments written in a macro definition often do not correspond to the way users think of the parts of the macro call.
Do not use this feature if you want to deprecate the calling convention and favor the one you advertise by the above specification. Instead, use the `advertised-calling-convention` declaration (see [Declare Form](declare-form)) or `set-advertised-calling-convention` (see [Obsolete Functions](obsolete-functions)), because these two will cause the byte compiler emit a warning message when it compiles Lisp programs which use the deprecated calling convention.
elisp None ### Inserting Text
*Insertion* means adding new text to a buffer. The inserted text goes at point—between the character before point and the character after point. Some insertion functions leave point before the inserted text, while other functions leave it after. We call the former insertion *after point* and the latter insertion *before point*.
Insertion moves markers located at positions after the insertion point, so that they stay with the surrounding text (see [Markers](markers)). When a marker points at the place of insertion, insertion may or may not relocate the marker, depending on the marker’s insertion type (see [Marker Insertion Types](marker-insertion-types)). Certain special functions such as `insert-before-markers` relocate all such markers to point after the inserted text, regardless of the markers’ insertion type.
Insertion functions signal an error if the current buffer is read-only (see [Read Only Buffers](read-only-buffers)) or if they insert within read-only text (see [Special Properties](special-properties)).
These functions copy text characters from strings and buffers along with their properties. The inserted characters have exactly the same properties as the characters they were copied from. By contrast, characters specified as separate arguments, not part of a string or buffer, inherit their text properties from the neighboring text.
The insertion functions convert text from unibyte to multibyte in order to insert in a multibyte buffer, and vice versa—if the text comes from a string or from a buffer. However, they do not convert unibyte character codes 128 through 255 to multibyte characters, not even if the current buffer is a multibyte buffer. See [Converting Representations](converting-representations).
Function: **insert** *&rest args*
This function inserts the strings and/or characters args into the current buffer, at point, moving point forward. In other words, it inserts the text before point. An error is signaled unless all args are either strings or characters. The value is `nil`.
Function: **insert-before-markers** *&rest args*
This function inserts the strings and/or characters args into the current buffer, at point, moving point forward. An error is signaled unless all args are either strings or characters. The value is `nil`.
This function is unlike the other insertion functions in that it relocates markers initially pointing at the insertion point, to point after the inserted text. If an overlay begins at the insertion point, the inserted text falls outside the overlay; if a nonempty overlay ends at the insertion point, the inserted text falls inside that overlay.
Command: **insert-char** *character &optional count inherit*
This command inserts count instances of character into the current buffer before point. The argument count must be an integer, and character must be a character.
If called interactively, this command prompts for character using its Unicode name or its code point. See [Inserting Text](https://www.gnu.org/software/emacs/manual/html_node/emacs/Inserting-Text.html#Inserting-Text) in The GNU Emacs Manual.
This function does not convert unibyte character codes 128 through 255 to multibyte characters, not even if the current buffer is a multibyte buffer. See [Converting Representations](converting-representations).
If inherit is non-`nil`, the inserted characters inherit sticky text properties from the two characters before and after the insertion point. See [Sticky Properties](sticky-properties).
Function: **insert-buffer-substring** *from-buffer-or-name &optional start end*
This function inserts a portion of buffer from-buffer-or-name into the current buffer before point. The text inserted is the region between start (inclusive) and end (exclusive). (These arguments default to the beginning and end of the accessible portion of that buffer.) This function returns `nil`.
In this example, the form is executed with buffer ‘`bar`’ as the current buffer. We assume that buffer ‘`bar`’ is initially empty.
```
---------- Buffer: foo ----------
We hold these truths to be self-evident, that all
---------- Buffer: foo ----------
```
```
(insert-buffer-substring "foo" 1 20)
⇒ nil
---------- Buffer: bar ----------
We hold these truth∗
---------- Buffer: bar ----------
```
Function: **insert-buffer-substring-no-properties** *from-buffer-or-name &optional start end*
This is like `insert-buffer-substring` except that it does not copy any text properties.
Function: **insert-into-buffer** *to-buffer &optional start end*
This is like `insert-buffer-substring`, but works in the opposite direction: The text is copied from the current buffer into to-buffer. The block of text is copied to the current point in to-buffer, and point (in that buffer) is advanced to after the end of the copied text. Is `start`/`end` is `nil`, the entire text in the current buffer is copied over.
See [Sticky Properties](sticky-properties), for other insertion functions that inherit text properties from the nearby text in addition to inserting it. Whitespace inserted by indentation functions also inherits text properties.
elisp None #### Minibuffer Commands that Do Completion
This section describes the keymaps, commands and user options used in the minibuffer to do completion.
Variable: **minibuffer-completion-table**
The value of this variable is the completion table (see [Basic Completion](basic-completion)) used for completion in the minibuffer. This is the buffer-local variable that contains what `completing-read` passes to `try-completion`. It is used by minibuffer completion commands such as `minibuffer-complete`.
Variable: **minibuffer-completion-predicate**
This variable’s value is the predicate that `completing-read` passes to `try-completion`. The variable is also used by the other minibuffer completion functions.
Variable: **minibuffer-completion-confirm**
This variable determines whether Emacs asks for confirmation before exiting the minibuffer; `completing-read` sets this variable, and the function `minibuffer-complete-and-exit` checks the value before exiting. If the value is `nil`, confirmation is not required. If the value is `confirm`, the user may exit with an input that is not a valid completion alternative, but Emacs asks for confirmation. If the value is `confirm-after-completion`, the user may exit with an input that is not a valid completion alternative, but Emacs asks for confirmation if the user submitted the input right after any of the completion commands in `minibuffer-confirm-exit-commands`.
Variable: **minibuffer-confirm-exit-commands**
This variable holds a list of commands that cause Emacs to ask for confirmation before exiting the minibuffer, if the require-match argument to `completing-read` is `confirm-after-completion`. The confirmation is requested if the user attempts to exit the minibuffer immediately after calling any command in this list.
Command: **minibuffer-complete-word**
This function completes the minibuffer contents by at most a single word. Even if the minibuffer contents have only one completion, `minibuffer-complete-word` does not add any characters beyond the first character that is not a word constituent. See [Syntax Tables](syntax-tables).
Command: **minibuffer-complete**
This function completes the minibuffer contents as far as possible.
Command: **minibuffer-complete-and-exit**
This function completes the minibuffer contents, and exits if confirmation is not required, i.e., if `minibuffer-completion-confirm` is `nil`. If confirmation *is* required, it is given by repeating this command immediately—the command is programmed to work without confirmation when run twice in succession.
Command: **minibuffer-completion-help**
This function creates a list of the possible completions of the current minibuffer contents. It works by calling `all-completions` using the value of the variable `minibuffer-completion-table` as the collection argument, and the value of `minibuffer-completion-predicate` as the predicate argument. The list of completions is displayed as text in a buffer named `\*Completions\*`.
Function: **display-completion-list** *completions*
This function displays completions to the stream in `standard-output`, usually a buffer. (See [Read and Print](read-and-print), for more information about streams.) The argument completions is normally a list of completions just returned by `all-completions`, but it does not have to be. Each element may be a symbol or a string, either of which is simply printed. It can also be a list of two strings, which is printed as if the strings were concatenated. The first of the two strings is the actual completion, the second string serves as annotation.
This function is called by `minibuffer-completion-help`. A common way to use it is together with `with-output-to-temp-buffer`, like this:
```
(with-output-to-temp-buffer "*Completions*"
(display-completion-list
(all-completions (buffer-string) my-alist)))
```
User Option: **completion-auto-help**
If this variable is non-`nil`, the completion commands automatically display a list of possible completions whenever nothing can be completed because the next character is not uniquely determined.
Variable: **minibuffer-local-completion-map**
`completing-read` uses this value as the local keymap when an exact match of one of the completions is not required. By default, this keymap makes the following bindings:
`?`
`minibuffer-completion-help`
SPC
`minibuffer-complete-word`
TAB `minibuffer-complete`
and uses `minibuffer-local-map` as its parent keymap (see [Definition of minibuffer-local-map](text-from-minibuffer#Definition-of-minibuffer_002dlocal_002dmap)).
Variable: **minibuffer-local-must-match-map**
`completing-read` uses this value as the local keymap when an exact match of one of the completions is required. Therefore, no keys are bound to `exit-minibuffer`, the command that exits the minibuffer unconditionally. By default, this keymap makes the following bindings:
`C-j`
`minibuffer-complete-and-exit`
RET `minibuffer-complete-and-exit`
and uses `minibuffer-local-completion-map` as its parent keymap.
Variable: **minibuffer-local-filename-completion-map**
This is a sparse keymap that simply unbinds SPC; because filenames can contain spaces. The function `read-file-name` combines this keymap with either `minibuffer-local-completion-map` or `minibuffer-local-must-match-map`.
Variable: **minibuffer-beginning-of-buffer-movement**
If non-`nil`, the `M-<` command will move to the end of the prompt if point is after the end of the prompt. If point is at or before the end of the prompt, move to the start of the buffer. If this variable is `nil`, the command behaves like `beginning-of-buffer`.
elisp None ### Atomic Change Groups
In database terminology, an *atomic* change is an indivisible change—it can succeed entirely or it can fail entirely, but it cannot partly succeed. A Lisp program can make a series of changes to one or several buffers as an *atomic change group*, meaning that either the entire series of changes will be installed in their buffers or, in case of an error, none of them will be.
To do this for one buffer, the one already current, simply write a call to `atomic-change-group` around the code that makes the changes, like this:
```
(atomic-change-group
(insert foo)
(delete-region x y))
```
If an error (or other nonlocal exit) occurs inside the body of `atomic-change-group`, it unmakes all the changes in that buffer that were during the execution of the body. This kind of change group has no effect on any other buffers—any such changes remain.
If you need something more sophisticated, such as to make changes in various buffers constitute one atomic group, you must directly call lower-level functions that `atomic-change-group` uses.
Function: **prepare-change-group** *&optional buffer*
This function sets up a change group for buffer buffer, which defaults to the current buffer. It returns a handle that represents the change group. You must use this handle to activate the change group and subsequently to finish it.
To use the change group, you must *activate* it. You must do this before making any changes in the text of buffer.
Function: **activate-change-group** *handle*
This function activates the change group that handle designates.
After you activate the change group, any changes you make in that buffer become part of it. Once you have made all the desired changes in the buffer, you must *finish* the change group. There are two ways to do this: you can either accept (and finalize) all the changes, or cancel them all.
Function: **accept-change-group** *handle*
This function accepts all the changes in the change group specified by handle, making them final.
Function: **cancel-change-group** *handle*
This function cancels and undoes all the changes in the change group specified by handle.
You can cause some or all of the changes in a change group to be considered as a single unit for the purposes of the `undo` commands (see [Undo](undo)) by using `undo-amalgamate-change-group`.
Function: **undo-amalgamate-change-group**
Amalgamate all the changes made in the change-group since the state identified by handle. This function removes all undo boundaries between undo records of changes since the state described by handle. Usually, handle is the handle returned by `prepare-change-group`, in which case all the changes since the beginning of the change-group are amalgamated into a single undo unit.
Your code should use `unwind-protect` to make sure the group is always finished. The call to `activate-change-group` should be inside the `unwind-protect`, in case the user types `C-g` just after it runs. (This is one reason why `prepare-change-group` and `activate-change-group` are separate functions, because normally you would call `prepare-change-group` before the start of that `unwind-protect`.) Once you finish the group, don’t use the handle again—in particular, don’t try to finish the same group twice.
To make a multibuffer change group, call `prepare-change-group` once for each buffer you want to cover, then use `nconc` to combine the returned values, like this:
```
(nconc (prepare-change-group buffer-1)
(prepare-change-group buffer-2))
```
You can then activate the multibuffer change group with a single call to `activate-change-group`, and finish it with a single call to `accept-change-group` or `cancel-change-group`.
Nested use of several change groups for the same buffer works as you would expect. Non-nested use of change groups for the same buffer will get Emacs confused, so don’t let it happen; the first change group you start for any given buffer should be the last one finished.
| programming_docs |
elisp None ### Comparing Text
This function lets you compare portions of the text in a buffer, without copying them into strings first.
Function: **compare-buffer-substrings** *buffer1 start1 end1 buffer2 start2 end2*
This function lets you compare two substrings of the same buffer or two different buffers. The first three arguments specify one substring, giving a buffer (or a buffer name) and two positions within the buffer. The last three arguments specify the other substring in the same way. You can use `nil` for buffer1, buffer2, or both to stand for the current buffer.
The value is negative if the first substring is less, positive if the first is greater, and zero if they are equal. The absolute value of the result is one plus the index of the first differing characters within the substrings.
This function ignores case when comparing characters if `case-fold-search` is non-`nil`. It always ignores text properties.
Suppose you have the text ‘`foobarbar haha!rara!`’ in the current buffer; then in this example the two substrings are ‘`rbar` ’ and ‘`rara!`’. The value is 2 because the first substring is greater at the second character.
```
(compare-buffer-substrings nil 6 11 nil 16 21)
⇒ 2
```
elisp None ### Numeric Conversions
To convert an integer to floating point, use the function `float`.
Function: **float** *number*
This returns number converted to floating point. If number is already floating point, `float` returns it unchanged.
There are four functions to convert floating-point numbers to integers; they differ in how they round. All accept an argument number and an optional argument divisor. Both arguments may be integers or floating-point numbers. divisor may also be `nil`. If divisor is `nil` or omitted, these functions convert number to an integer, or return it unchanged if it already is an integer. If divisor is non-`nil`, they divide number by divisor and convert the result to an integer. If divisor is zero (whether integer or floating point), Emacs signals an `arith-error` error.
Function: **truncate** *number &optional divisor*
This returns number, converted to an integer by rounding towards zero.
```
(truncate 1.2)
⇒ 1
(truncate 1.7)
⇒ 1
(truncate -1.2)
⇒ -1
(truncate -1.7)
⇒ -1
```
Function: **floor** *number &optional divisor*
This returns number, converted to an integer by rounding downward (towards negative infinity).
If divisor is specified, this uses the kind of division operation that corresponds to `mod`, rounding downward.
```
(floor 1.2)
⇒ 1
(floor 1.7)
⇒ 1
(floor -1.2)
⇒ -2
(floor -1.7)
⇒ -2
(floor 5.99 3)
⇒ 1
```
Function: **ceiling** *number &optional divisor*
This returns number, converted to an integer by rounding upward (towards positive infinity).
```
(ceiling 1.2)
⇒ 2
(ceiling 1.7)
⇒ 2
(ceiling -1.2)
⇒ -1
(ceiling -1.7)
⇒ -1
```
Function: **round** *number &optional divisor*
This returns number, converted to an integer by rounding towards the nearest integer. Rounding a value equidistant between two integers returns the even integer.
```
(round 1.2)
⇒ 1
(round 1.7)
⇒ 2
(round -1.2)
⇒ -1
(round -1.7)
⇒ -2
```
elisp None ### Building Cons Cells and Lists
Many functions build lists, as lists reside at the very heart of Lisp. `cons` is the fundamental list-building function; however, it is interesting to note that `list` is used more times in the source code for Emacs than `cons`.
Function: **cons** *object1 object2*
This function is the most basic function for building new list structure. It creates a new cons cell, making object1 the CAR, and object2 the CDR. It then returns the new cons cell. The arguments object1 and object2 may be any Lisp objects, but most often object2 is a list.
```
(cons 1 '(2))
⇒ (1 2)
```
```
(cons 1 '())
⇒ (1)
```
```
(cons 1 2)
⇒ (1 . 2)
```
`cons` is often used to add a single element to the front of a list. This is called *consing the element onto the list*. [5](#FOOT5) For example:
```
(setq list (cons newelt list))
```
Note that there is no conflict between the variable named `list` used in this example and the function named `list` described below; any symbol can serve both purposes.
Function: **list** *&rest objects*
This function creates a list with objects as its elements. The resulting list is always `nil`-terminated. If no objects are given, the empty list is returned.
```
(list 1 2 3 4 5)
⇒ (1 2 3 4 5)
```
```
(list 1 2 '(3 4 5) 'foo)
⇒ (1 2 (3 4 5) foo)
```
```
(list)
⇒ nil
```
Function: **make-list** *length object*
This function creates a list of length elements, in which each element is object. Compare `make-list` with `make-string` (see [Creating Strings](creating-strings)).
```
(make-list 3 'pigs)
⇒ (pigs pigs pigs)
```
```
(make-list 0 'pigs)
⇒ nil
```
```
(setq l (make-list 3 '(a b)))
⇒ ((a b) (a b) (a b))
(eq (car l) (cadr l))
⇒ t
```
Function: **append** *&rest sequences*
This function returns a list containing all the elements of sequences. The sequences may be lists, vectors, bool-vectors, or strings, but the last one should usually be a list. All arguments except the last one are copied, so none of the arguments is altered. (See `nconc` in [Rearrangement](rearrangement), for a way to join lists with no copying.)
More generally, the final argument to `append` may be any Lisp object. The final argument is not copied or converted; it becomes the CDR of the last cons cell in the new list. If the final argument is itself a list, then its elements become in effect elements of the result list. If the final element is not a list, the result is a dotted list since its final CDR is not `nil` as required in a proper list (see [Cons Cells](cons-cells)).
Here is an example of using `append`:
```
(setq trees '(pine oak))
⇒ (pine oak)
(setq more-trees (append '(maple birch) trees))
⇒ (maple birch pine oak)
```
```
trees
⇒ (pine oak)
more-trees
⇒ (maple birch pine oak)
```
```
(eq trees (cdr (cdr more-trees)))
⇒ t
```
You can see how `append` works by looking at a box diagram. The variable `trees` is set to the list `(pine oak)` and then the variable `more-trees` is set to the list `(maple birch pine
oak)`. However, the variable `trees` continues to refer to the original list:
```
more-trees trees
| |
| --- --- --- --- -> --- --- --- ---
--> | | |--> | | |--> | | |--> | | |--> nil
--- --- --- --- --- --- --- ---
| | | |
| | | |
--> maple -->birch --> pine --> oak
```
An empty sequence contributes nothing to the value returned by `append`. As a consequence of this, a final `nil` argument forces a copy of the previous argument:
```
trees
⇒ (pine oak)
```
```
(setq wood (append trees nil))
⇒ (pine oak)
```
```
wood
⇒ (pine oak)
```
```
(eq wood trees)
⇒ nil
```
This once was the usual way to copy a list, before the function `copy-sequence` was invented. See [Sequences Arrays Vectors](sequences-arrays-vectors).
Here we show the use of vectors and strings as arguments to `append`:
```
(append [a b] "cd" nil)
⇒ (a b 99 100)
```
With the help of `apply` (see [Calling Functions](calling-functions)), we can append all the lists in a list of lists:
```
(apply 'append '((a b c) nil (x y z) nil))
⇒ (a b c x y z)
```
If no sequences are given, `nil` is returned:
```
(append)
⇒ nil
```
Here are some examples where the final argument is not a list:
```
(append '(x y) 'z)
⇒ (x y . z)
(append '(x y) [z])
⇒ (x y . [z])
```
The second example shows that when the final argument is a sequence but not a list, the sequence’s elements do not become elements of the resulting list. Instead, the sequence becomes the final CDR, like any other non-list final argument.
Function: **copy-tree** *tree &optional vecp*
This function returns a copy of the tree tree. If tree is a cons cell, this makes a new cons cell with the same CAR and CDR, then recursively copies the CAR and CDR in the same way.
Normally, when tree is anything other than a cons cell, `copy-tree` simply returns tree. However, if vecp is non-`nil`, it copies vectors too (and operates recursively on their elements).
Function: **flatten-tree** *tree*
This function returns a “flattened” copy of tree, that is, a list containing all the non-`nil` terminal nodes, or leaves, of the tree of cons cells rooted at tree. Leaves in the returned list are in the same order as in tree.
```
(flatten-tree '(1 (2 . 3) nil (4 5 (6)) 7))
⇒(1 2 3 4 5 6 7)
```
Function: **ensure-list** *object*
This function returns object as a list. If object is already a list, the function returns it; otherwise, the function returns a one-element list containing object.
This is usually useful if you have a variable that may or may not be a list, and you can then say, for instance:
```
(dolist (elem (ensure-list foo))
(princ elem))
```
Function: **number-sequence** *from &optional to separation*
This function returns a list of numbers starting with from and incrementing by separation, and ending at or just before to. separation can be positive or negative and defaults to 1. If to is `nil` or numerically equal to from, the value is the one-element list `(from)`. If to is less than from with a positive separation, or greater than from with a negative separation, the value is `nil` because those arguments specify an empty sequence.
If separation is 0 and to is neither `nil` nor numerically equal to from, `number-sequence` signals an error, since those arguments specify an infinite sequence.
All arguments are numbers. Floating-point arguments can be tricky, because floating-point arithmetic is inexact. For instance, depending on the machine, it may quite well happen that `(number-sequence 0.4 0.6 0.2)` returns the one element list `(0.4)`, whereas `(number-sequence 0.4 0.8 0.2)` returns a list with three elements. The nth element of the list is computed by the exact formula `(+ from (* n separation))`. Thus, if one wants to make sure that to is included in the list, one can pass an expression of this exact type for to. Alternatively, one can replace to with a slightly larger value (or a slightly more negative value if separation is negative).
Some examples:
```
(number-sequence 4 9)
⇒ (4 5 6 7 8 9)
(number-sequence 9 4 -1)
⇒ (9 8 7 6 5 4)
(number-sequence 9 4 -2)
⇒ (9 7 5)
(number-sequence 8)
⇒ (8)
(number-sequence 8 5)
⇒ nil
(number-sequence 5 8 -1)
⇒ nil
(number-sequence 1.5 6 2)
⇒ (1.5 3.5 5.5)
```
elisp None #### Completion in Ordinary Buffers
Although completion is usually done in the minibuffer, the completion facility can also be used on the text in ordinary Emacs buffers. In many major modes, in-buffer completion is performed by the `C-M-i` or `M-TAB` command, bound to `completion-at-point`. See [Symbol Completion](https://www.gnu.org/software/emacs/manual/html_node/emacs/Symbol-Completion.html#Symbol-Completion) in The GNU Emacs Manual. This command uses the abnormal hook variable `completion-at-point-functions`:
Variable: **completion-at-point-functions**
The value of this abnormal hook should be a list of functions, which are used to compute a completion table (see [Basic Completion](basic-completion)) for completing the text at point. It can be used by major modes to provide mode-specific completion tables (see [Major Mode Conventions](major-mode-conventions)).
When the command `completion-at-point` runs, it calls the functions in the list one by one, without any argument. Each function should return `nil` unless it can and wants to take responsibility for the completion data for the text at point. Otherwise it should return a list of the following form:
```
(start end collection . props)
```
start and end delimit the text to complete (which should enclose point). collection is a completion table for completing that text, in a form suitable for passing as the second argument to `try-completion` (see [Basic Completion](basic-completion)); completion alternatives will be generated from this completion table in the usual way, via the completion styles defined in `completion-styles` (see [Completion Variables](completion-variables)). props is a property list for additional information; any of the properties in `completion-extra-properties` are recognized (see [Completion Variables](completion-variables)), as well as the following additional ones:
`:predicate`
The value should be a predicate that completion candidates need to satisfy.
`:exclusive` If the value is `no`, then if the completion table fails to match the text at point, `completion-at-point` moves on to the next function in `completion-at-point-functions` instead of reporting a completion failure.
The functions on this hook should generally return quickly, since they may be called very often (e.g., from `post-command-hook`). Supplying a function for collection is strongly recommended if generating the list of completions is an expensive operation. Emacs may internally call functions in `completion-at-point-functions` many times, but care about the value of collection for only some of these calls. By supplying a function for collection, Emacs can defer generating completions until necessary. You can use `completion-table-dynamic` to create a wrapper function:
```
;; Avoid this pattern.
(let ((beg ...) (end ...) (my-completions (my-make-completions)))
(list beg end my-completions))
;; Use this instead.
(let ((beg ...) (end ...))
(list beg
end
(completion-table-dynamic
(lambda (_)
(my-make-completions)))))
```
Additionally, the collection should generally not be pre-filtered based on the current text between start and end, because that is the responsibility of the caller of `completion-at-point-functions` to do that according to the completion styles it decides to use.
A function in `completion-at-point-functions` may also return a function instead of a list as described above. In that case, that returned function is called, with no argument, and it is entirely responsible for performing the completion. We discourage this usage; it is only intended to help convert old code to using `completion-at-point`.
The first function in `completion-at-point-functions` to return a non-`nil` value is used by `completion-at-point`. The remaining functions are not called. The exception to this is when there is an `:exclusive` specification, as described above.
The following function provides a convenient way to perform completion on an arbitrary stretch of text in an Emacs buffer:
Function: **completion-in-region** *start end collection &optional predicate*
This function completes the text in the current buffer between the positions start and end, using collection. The argument collection has the same meaning as in `try-completion` (see [Basic Completion](basic-completion)).
This function inserts the completion text directly into the current buffer. Unlike `completing-read` (see [Minibuffer Completion](minibuffer-completion)), it does not activate the minibuffer.
For this function to work, point must be somewhere between start and end.
elisp None ### Variables Affecting Output
Variable: **standard-output**
The value of this variable is the default output stream—the stream that print functions use when the stream argument is `nil`. The default is `t`, meaning display in the echo area.
Variable: **print-quoted**
If this is non-`nil`, that means to print quoted forms using abbreviated reader syntax, e.g., `(quote foo)` prints as `'foo`, and `(function foo)` as `#'foo`. The default is `t`.
Variable: **print-escape-newlines**
If this variable is non-`nil`, then newline characters in strings are printed as ‘`\n`’ and formfeeds are printed as ‘`\f`’. Normally these characters are printed as actual newlines and formfeeds.
This variable affects the print functions `prin1` and `print` that print with quoting. It does not affect `princ`. Here is an example using `prin1`:
```
(prin1 "a\nb")
-| "a
-| b"
⇒ "a
b"
```
```
(let ((print-escape-newlines t))
(prin1 "a\nb"))
-| "a\nb"
⇒ "a
b"
```
In the second expression, the local binding of `print-escape-newlines` is in effect during the call to `prin1`, but not during the printing of the result.
Variable: **print-escape-control-characters**
If this variable is non-`nil`, control characters in strings are printed as backslash sequences by the print functions `prin1` and `print` that print with quoting. If this variable and `print-escape-newlines` are both non-`nil`, the latter takes precedences for newlines and formfeeds.
Variable: **print-escape-nonascii**
If this variable is non-`nil`, then unibyte non-ASCII characters in strings are unconditionally printed as backslash sequences by the print functions `prin1` and `print` that print with quoting.
Those functions also use backslash sequences for unibyte non-ASCII characters, regardless of the value of this variable, when the output stream is a multibyte buffer or a marker pointing into one.
Variable: **print-escape-multibyte**
If this variable is non-`nil`, then multibyte non-ASCII characters in strings are unconditionally printed as backslash sequences by the print functions `prin1` and `print` that print with quoting.
Those functions also use backslash sequences for multibyte non-ASCII characters, regardless of the value of this variable, when the output stream is a unibyte buffer or a marker pointing into one.
Variable: **print-charset-text-property**
This variable controls printing of ‘charset’ text property on printing a string. The value should be `nil`, `t`, or `default`.
If the value is `nil`, `charset` text properties are never printed. If `t`, they are always printed.
If the value is `default`, only print `charset` text properties if there is an “unexpected” `charset` property. For ascii characters, all charsets are considered “expected”. Otherwise, the expected `charset` property of a character is given by `char-charset`.
Variable: **print-length**
The value of this variable is the maximum number of elements to print in any list, vector or bool-vector. If an object being printed has more than this many elements, it is abbreviated with an ellipsis.
If the value is `nil` (the default), then there is no limit.
```
(setq print-length 2)
⇒ 2
```
```
(print '(1 2 3 4 5))
-| (1 2 ...)
⇒ (1 2 ...)
```
Variable: **print-level**
The value of this variable is the maximum depth of nesting of parentheses and brackets when printed. Any list or vector at a depth exceeding this limit is abbreviated with an ellipsis. A value of `nil` (which is the default) means no limit.
User Option: **eval-expression-print-length**
User Option: **eval-expression-print-level**
These are the values for `print-length` and `print-level` used by `eval-expression`, and thus, indirectly, by many interactive evaluation commands (see [Evaluating Emacs Lisp Expressions](https://www.gnu.org/software/emacs/manual/html_node/emacs/Lisp-Eval.html#Lisp-Eval) in The GNU Emacs Manual).
These variables are used for detecting and reporting circular and shared structure:
Variable: **print-circle**
If non-`nil`, this variable enables detection of circular and shared structure in printing. See [Circular Objects](circular-objects).
Variable: **print-gensym**
If non-`nil`, this variable enables detection of uninterned symbols (see [Creating Symbols](creating-symbols)) in printing. When this is enabled, uninterned symbols print with the prefix ‘`#:`’, which tells the Lisp reader to produce an uninterned symbol.
Variable: **print-continuous-numbering**
If non-`nil`, that means number continuously across print calls. This affects the numbers printed for ‘`#n=`’ labels and ‘`#m#`’ references. Don’t set this variable with `setq`; you should only bind it temporarily to `t` with `let`. When you do that, you should also bind `print-number-table` to `nil`.
Variable: **print-number-table**
This variable holds a vector used internally by printing to implement the `print-circle` feature. You should not use it except to bind it to `nil` when you bind `print-continuous-numbering`.
Variable: **float-output-format**
This variable specifies how to print floating-point numbers. The default is `nil`, meaning use the shortest output that represents the number without losing information.
To control output format more precisely, you can put a string in this variable. The string should hold a ‘`%`’-specification to be used in the C function `sprintf`. For further restrictions on what you can use, see the variable’s documentation string.
Variable: **print-integers-as-characters**
When this variable is non-`nil`, integers that represent graphic base characters will be printed using Lisp character syntax (see [Basic Char Syntax](basic-char-syntax)). Other numbers are printed the usual way. For example, the list `(4 65 -1 10)` would be printed as ‘`(4 ?A -1 ?\n)`’.
More precisely, values printed in character syntax are those representing characters belonging to the Unicode general categories Letter, Number, Punctuation, Symbol and Private-use (see [Character Properties](character-properties)), as well as the control characters having their own escape syntax such as newline.
| programming_docs |
elisp None ### Accessing Elements of Lists
Function: **car** *cons-cell*
This function returns the value referred to by the first slot of the cons cell cons-cell. In other words, it returns the CAR of cons-cell.
As a special case, if cons-cell is `nil`, this function returns `nil`. Therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or `nil`.
```
(car '(a b c))
⇒ a
```
```
(car '())
⇒ nil
```
Function: **cdr** *cons-cell*
This function returns the value referred to by the second slot of the cons cell cons-cell. In other words, it returns the CDR of cons-cell.
As a special case, if cons-cell is `nil`, this function returns `nil`; therefore, any list is a valid argument. An error is signaled if the argument is not a cons cell or `nil`.
```
(cdr '(a b c))
⇒ (b c)
```
```
(cdr '())
⇒ nil
```
Function: **car-safe** *object*
This function lets you take the CAR of a cons cell while avoiding errors for other data types. It returns the CAR of object if object is a cons cell, `nil` otherwise. This is in contrast to `car`, which signals an error if object is not a list.
```
(car-safe object)
≡
(let ((x object))
(if (consp x)
(car x)
nil))
```
Function: **cdr-safe** *object*
This function lets you take the CDR of a cons cell while avoiding errors for other data types. It returns the CDR of object if object is a cons cell, `nil` otherwise. This is in contrast to `cdr`, which signals an error if object is not a list.
```
(cdr-safe object)
≡
(let ((x object))
(if (consp x)
(cdr x)
nil))
```
Macro: **pop** *listname*
This macro provides a convenient way to examine the CAR of a list, and take it off the list, all at once. It operates on the list stored in listname. It removes the first element from the list, saves the CDR into listname, then returns the removed element.
In the simplest case, listname is an unquoted symbol naming a list; in that case, this macro is equivalent to `(prog1 (car listname) (setq listname (cdr listname)))`.
```
x
⇒ (a b c)
(pop x)
⇒ a
x
⇒ (b c)
```
More generally, listname can be a generalized variable. In that case, this macro saves into listname using `setf`. See [Generalized Variables](generalized-variables).
For the `push` macro, which adds an element to a list, See [List Variables](list-variables).
Function: **nth** *n list*
This function returns the nth element of list. Elements are numbered starting with zero, so the CAR of list is element number zero. If the length of list is n or less, the value is `nil`.
```
(nth 2 '(1 2 3 4))
⇒ 3
```
```
(nth 10 '(1 2 3 4))
⇒ nil
(nth n x) ≡ (car (nthcdr n x))
```
The function `elt` is similar, but applies to any kind of sequence. For historical reasons, it takes its arguments in the opposite order. See [Sequence Functions](sequence-functions).
Function: **nthcdr** *n list*
This function returns the nth CDR of list. In other words, it skips past the first n links of list and returns what follows.
If n is zero, `nthcdr` returns all of list. If the length of list is n or less, `nthcdr` returns `nil`.
```
(nthcdr 1 '(1 2 3 4))
⇒ (2 3 4)
```
```
(nthcdr 10 '(1 2 3 4))
⇒ nil
```
```
(nthcdr 0 '(1 2 3 4))
⇒ (1 2 3 4)
```
Function: **last** *list &optional n*
This function returns the last link of list. The `car` of this link is the list’s last element. If list is null, `nil` is returned. If n is non-`nil`, the nth-to-last link is returned instead, or the whole of list if n is bigger than list’s length.
Function: **safe-length** *list*
This function returns the length of list, with no risk of either an error or an infinite loop. It generally returns the number of distinct cons cells in the list. However, for circular lists, the value is just an upper bound; it is often too large.
If list is not `nil` or a cons cell, `safe-length` returns 0.
The most common way to compute the length of a list, when you are not worried that it may be circular, is with `length`. See [Sequence Functions](sequence-functions).
Function: **caar** *cons-cell*
This is the same as `(car (car cons-cell))`.
Function: **cadr** *cons-cell*
This is the same as `(car (cdr cons-cell))` or `(nth 1 cons-cell)`.
Function: **cdar** *cons-cell*
This is the same as `(cdr (car cons-cell))`.
Function: **cddr** *cons-cell*
This is the same as `(cdr (cdr cons-cell))` or `(nthcdr 2 cons-cell)`.
In addition to the above, 24 additional compositions of `car` and `cdr` are defined as `cxxxr` and `cxxxxr`, where each `x` is either `a` or `d`. `cadr`, `caddr`, and `cadddr` pick out the second, third or fourth elements of a list, respectively. `cl-lib` provides the same under the names `cl-second`, `cl-third`, and `cl-fourth`. See [List Functions](https://www.gnu.org/software/emacs/manual/html_node/cl/List-Functions.html#List-Functions) in Common Lisp Extensions.
Function: **butlast** *x &optional n*
This function returns the list x with the last element, or the last n elements, removed. If n is greater than zero it makes a copy of the list so as not to damage the original list. In general, `(append (butlast x n)
(last x n))` will return a list equal to x.
Function: **nbutlast** *x &optional n*
This is a version of `butlast` that works by destructively modifying the `cdr` of the appropriate element, rather than making a copy of the list.
elisp None Records
-------
The purpose of records is to allow programmers to create objects with new types that are not built into Emacs. They are used as the underlying representation of `cl-defstruct` and `defclass` instances.
Internally, a record object is much like a vector; its slots can be accessed using `aref` and it can be copied using `copy-sequence`. However, the first slot is used to hold its type as returned by `type-of`. Also, in the current implementation records can have at most 4096 slots, whereas vectors can be much larger. Like arrays, records use zero-origin indexing: the first slot has index 0.
The type slot should be a symbol or a type descriptor. If it’s a type descriptor, the symbol naming its type will be returned; [Type Descriptors](type-descriptors). Any other kind of object is returned as-is.
The printed representation of records is ‘`#s`’ followed by a list specifying the contents. The first list element must be the record type. The following elements are the record slots.
To avoid conflicts with other type names, Lisp programs that define new types of records should normally use the naming conventions of the package where these record types are introduced for the names of the types. Note that the names of the types which could possibly conflict might not be known at the time the package defining a record type is loaded; they could be loaded at some future point in time.
A record is considered a constant for evaluation: the result of evaluating it is the same record. This does not evaluate or even examine the slots. See [Self-Evaluating Forms](self_002devaluating-forms).
| | | |
| --- | --- | --- |
| • [Record Functions](record-functions) | | Functions for records. |
| • [Backward Compatibility](backward-compatibility) | | Compatibility for cl-defstruct. |
elisp None Byte Compilation
----------------
Emacs Lisp has a *compiler* that translates functions written in Lisp into a special representation called *byte-code* that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the *byte-code interpreter*.
Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine’s hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code.
In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true.
If you do not want a Lisp file to be compiled, ever, put a file-local variable binding for `no-byte-compile` into it, like this:
```
;; -*-no-byte-compile: t; -*-
```
| | | |
| --- | --- | --- |
| • [Speed of Byte-Code](speed-of-byte_002dcode) | | An example of speedup from byte compilation. |
| • [Compilation Functions](compilation-functions) | | Byte compilation functions. |
| • [Docs and Compilation](docs-and-compilation) | | Dynamic loading of documentation strings. |
| • [Dynamic Loading](dynamic-loading) | | Dynamic loading of individual functions. |
| • [Eval During Compile](eval-during-compile) | | Code to be evaluated when you compile. |
| • [Compiler Errors](compiler-errors) | | Handling compiler error messages. |
| • [Byte-Code Objects](byte_002dcode-objects) | | The data type used for byte-compiled functions. |
| • [Disassembly](disassembly) | | Disassembling byte-code; how to read byte-code. |
elisp None ### Anonymous Functions
Although functions are usually defined with `defun` and given names at the same time, it is sometimes convenient to use an explicit lambda expression—an *anonymous function*. Anonymous functions are valid wherever function names are. They are often assigned as variable values, or as arguments to functions; for instance, you might pass one as the function argument to `mapcar`, which applies that function to each element of a list (see [Mapping Functions](mapping-functions)). See [describe-symbols example](accessing-documentation#describe_002dsymbols-example), for a realistic example of this.
When defining a lambda expression that is to be used as an anonymous function, you can in principle use any method to construct the list. But typically you should use the `lambda` macro, or the `function` special form, or the `#'` read syntax:
Macro: **lambda** *args [doc] [interactive] body…*
This macro returns an anonymous function with argument list args, documentation string doc (if any), interactive spec interactive (if any), and body forms given by body.
Under dynamic binding, this macro effectively makes `lambda` forms self-quoting: evaluating a form whose CAR is `lambda` yields the form itself:
```
(lambda (x) (* x x))
⇒ (lambda (x) (* x x))
```
Note that when evaluating under lexical binding the result is a closure object (see [Closures](closures)).
The `lambda` form has one other effect: it tells the Emacs evaluator and byte-compiler that its argument is a function, by using `function` as a subroutine (see below).
Special Form: **function** *function-object*
This special form returns function-object without evaluating it. In this, it is similar to `quote` (see [Quoting](quoting)). But unlike `quote`, it also serves as a note to the Emacs evaluator and byte-compiler that function-object is intended to be used as a function. Assuming function-object is a valid lambda expression, this has two effects:
* When the code is byte-compiled, function-object is compiled into a byte-code function object (see [Byte Compilation](byte-compilation)).
* When lexical binding is enabled, function-object is converted into a closure. See [Closures](closures).
When function-object is a symbol and the code is byte compiled, the byte-compiler will warn if that function is not defined or might not be known at run time.
The read syntax `#'` is a short-hand for using `function`. The following forms are all equivalent:
```
(lambda (x) (* x x))
(function (lambda (x) (* x x)))
#'(lambda (x) (* x x))
```
In the following example, we define a `change-property` function that takes a function as its third argument, followed by a `double-property` function that makes use of `change-property` by passing it an anonymous function:
```
(defun change-property (symbol prop function)
(let ((value (get symbol prop)))
(put symbol prop (funcall function value))))
```
```
(defun double-property (symbol prop)
(change-property symbol prop (lambda (x) (* 2 x))))
```
Note that we do not quote the `lambda` form.
If you compile the above code, the anonymous function is also compiled. This would not happen if, say, you had constructed the anonymous function by quoting it as a list:
```
(defun double-property (symbol prop)
(change-property symbol prop '(lambda (x) (* 2 x))))
```
In that case, the anonymous function is kept as a lambda expression in the compiled code. The byte-compiler cannot assume this list is a function, even though it looks like one, since it does not know that `change-property` intends to use it as a function.
elisp None #### Indentation-Based Motion Commands
These commands, primarily for interactive use, act based on the indentation in the text.
Command: **back-to-indentation**
This command moves point to the first non-whitespace character in the current line (which is the line in which point is located). It returns `nil`.
Command: **backward-to-indentation** *&optional arg*
This command moves point backward arg lines and then to the first nonblank character on that line. It returns `nil`. If arg is omitted or `nil`, it defaults to 1.
Command: **forward-to-indentation** *&optional arg*
This command moves point forward arg lines and then to the first nonblank character on that line. It returns `nil`. If arg is omitted or `nil`, it defaults to 1.
elisp None ### Change Hooks
These hook variables let you arrange to take notice of changes in buffers (or in a particular buffer, if you make them buffer-local). See also [Special Properties](special-properties), for how to detect changes to specific parts of the text.
The functions you use in these hooks should save and restore the match data if they do anything that uses regular expressions; otherwise, they will interfere in bizarre ways with the editing operations that call them.
Variable: **before-change-functions**
This variable holds a list of functions to call when Emacs is about to modify a buffer. Each function gets two arguments, the beginning and end of the region that is about to change, represented as integers. The buffer that is about to change is always the current buffer when the function is called.
Variable: **after-change-functions**
This variable holds a list of functions to call after Emacs modifies a buffer. Each function receives three arguments: the beginning and end of the region just changed, and the length of the text that existed before the change. All three arguments are integers. The buffer that has been changed is always the current buffer when the function is called.
The length of the old text is the difference between the buffer positions before and after that text as it was before the change. As for the changed text, its length is simply the difference between the first two arguments.
Output of messages into the `\*Messages\*` buffer does not call these functions, and neither do certain internal buffer changes, such as changes in buffers created by Emacs internally for certain jobs, that should not be visible to Lisp programs.
The vast majority of buffer changing primitives will call `before-change-functions` and `after-change-functions` in balanced pairs, once for each change, where the arguments to these hooks exactly delimit the change being made. Yet, hook functions should not rely on this always being the case, because some complex primitives call `before-change-functions` once before making changes, and then call `after-change-functions` zero or more times, depending on how many individual changes the primitive is making. When that happens, the arguments to `before-change-functions` will enclose a region in which the individual changes are made, but won’t necessarily be the minimal such region, and the arguments to each successive call of `after-change-functions` will then delimit the part of text being changed exactly. In general, we advise using either the before- or the after-change hook, but not both.
Macro: **combine-after-change-calls** *body…*
The macro executes body normally, but arranges to call the after-change functions just once for a series of several changes—if that seems safe.
If a program makes several text changes in the same area of the buffer, using the macro `combine-after-change-calls` around that part of the program can make it run considerably faster when after-change hooks are in use. When the after-change hooks are ultimately called, the arguments specify a portion of the buffer including all of the changes made within the `combine-after-change-calls` body.
**Warning:** You must not alter the values of `after-change-functions` within the body of a `combine-after-change-calls` form.
**Warning:** If the changes you combine occur in widely scattered parts of the buffer, this will still work, but it is not advisable, because it may lead to inefficient behavior for some change hook functions.
Macro: **combine-change-calls** *beg end body…*
This executes body normally, except any buffer changes it makes do not trigger the calls to `before-change-functions` and `after-change-functions`. Instead there is a single call of each of these hooks for the region enclosed by beg and end, the parameters supplied to `after-change-functions` reflecting the changes made to the size of the region by body.
The result of this macro is the result returned by body.
This macro is useful when a function makes a possibly large number of repetitive changes to the buffer, and the change hooks would otherwise take a long time to run, were they to be run for each individual buffer modification. Emacs itself uses this macro, for example, in the commands `comment-region` and `uncomment-region`.
**Warning:** You must not alter the values of `before-change-functions` or `after-change-function` within body.
**Warning:** You must not make any buffer changes outside of the region specified by beg and end.
Variable: **first-change-hook**
This variable is a normal hook that is run whenever a buffer is changed that was previously in the unmodified state.
Variable: **inhibit-modification-hooks**
If this variable is non-`nil`, all of the change hooks are disabled; none of them run. This affects all the hook variables described above in this section, as well as the hooks attached to certain special text properties (see [Special Properties](special-properties)) and overlay properties (see [Overlay Properties](overlay-properties)).
Also, this variable is bound to non-`nil` while running those same hook variables, so that by default modifying the buffer from a modification hook does not cause other modification hooks to be run. If you do want modification hooks to be run in a particular piece of code that is itself run from a modification hook, then rebind locally `inhibit-modification-hooks` to `nil`. However, doing this may cause recursive calls to the modification hooks, so be sure to prepare for that (for example, by binding some variable which tells your hook to do nothing).
We recommend that you only bind this variable for modifications that do not result in lasting changes to buffer text contents (for example face changes or temporary modifications). If you need to delay change hooks during a series of changes (typically for performance reasons), use `combine-change-calls` or `combine-after-change-calls` instead.
elisp None ### Textual Scrolling
*Textual scrolling* means moving the text up or down through a window. It works by changing the window’s display-start location. It may also change the value of `window-point` to keep point on the screen (see [Window Point](window-point)).
The basic textual scrolling functions are `scroll-up` (which scrolls forward) and `scroll-down` (which scrolls backward). In these function names, “up” and “down” refer to the direction of motion of the buffer text relative to the window. Imagine that the text is written on a long roll of paper and that the scrolling commands move the paper up and down. Thus, if you are looking at the middle of a buffer and repeatedly call `scroll-down`, you will eventually see the beginning of the buffer.
Unfortunately, this sometimes causes confusion, because some people tend to think in terms of the opposite convention: they imagine the window moving over text that remains in place, so that “down” commands take you to the end of the buffer. This convention is consistent with fact that such a command is bound to a key named PageDown on modern keyboards.
Textual scrolling functions (aside from `scroll-other-window`) have unpredictable results if the current buffer is not the one displayed in the selected window. See [Current Buffer](current-buffer).
If the window contains a row taller than the height of the window (for example in the presence of a large image), the scroll functions will adjust the window’s vertical scroll position to scroll the partially visible row. Lisp callers can disable this feature by binding the variable `auto-window-vscroll` to `nil` (see [Vertical Scrolling](vertical-scrolling)).
Command: **scroll-up** *&optional count*
This function scrolls forward by count lines in the selected window.
If count is negative, it scrolls backward instead. If count is `nil` (or omitted), the distance scrolled is `next-screen-context-lines` lines less than the height of the window’s body.
If the selected window cannot be scrolled any further, this function signals an error. Otherwise, it returns `nil`.
Command: **scroll-down** *&optional count*
This function scrolls backward by count lines in the selected window.
If count is negative, it scrolls forward instead. In other respects, it behaves the same way as `scroll-up` does.
Command: **scroll-up-command** *&optional count*
This behaves like `scroll-up`, except that if the selected window cannot be scrolled any further and the value of the variable `scroll-error-top-bottom` is `t`, it tries to move to the end of the buffer instead. If point is already there, it signals an error.
Command: **scroll-down-command** *&optional count*
This behaves like `scroll-down`, except that if the selected window cannot be scrolled any further and the value of the variable `scroll-error-top-bottom` is `t`, it tries to move to the beginning of the buffer instead. If point is already there, it signals an error.
Command: **scroll-other-window** *&optional count*
This function scrolls the text in another window upward count lines. Negative values of count, or `nil`, are handled as in `scroll-up`.
You can specify which buffer to scroll by setting the variable `other-window-scroll-buffer` to a buffer. If that buffer isn’t already displayed, `scroll-other-window` displays it in some window.
When the selected window is the minibuffer, the next window is normally the leftmost one immediately above it. You can specify a different window to scroll, when the minibuffer is selected, by setting the variable `minibuffer-scroll-window`. This variable has no effect when any other window is selected. When it is non-`nil` and the minibuffer is selected, it takes precedence over `other-window-scroll-buffer`. See [Definition of minibuffer-scroll-window](minibuffer-misc#Definition-of-minibuffer_002dscroll_002dwindow).
Command: **scroll-other-window-down** *&optional count*
This function scrolls the text in another window downward count lines. Negative values of count, or `nil`, are handled as in `scroll-down`. In other respects, it behaves the same way as `scroll-other-window` does.
Variable: **other-window-scroll-buffer**
If this variable is non-`nil`, it tells `scroll-other-window` which buffer’s window to scroll.
User Option: **scroll-margin**
This option specifies the size of the scroll margin—a minimum number of lines between point and the top or bottom of a window. Whenever point gets within this many lines of the top or bottom of the window, redisplay scrolls the text automatically (if possible) to move point out of the margin, closer to the center of the window.
User Option: **maximum-scroll-margin**
This variable limits the effective value of `scroll-margin` to a fraction of the current window line height. For example, if the current window has 20 lines and `maximum-scroll-margin` is 0.1, then the scroll margins will never be larger than 2 lines, no matter how big `scroll-margin` is.
`maximum-scroll-margin` itself has a maximum value of 0.5, which allows setting margins large to keep the cursor at the middle line of the window (or two middle lines if the window has an even number of lines). If it’s set to a larger value (or any value other than a float between 0.0 and 0.5) then the default value of 0.25 will be used instead.
User Option: **scroll-conservatively**
This variable controls how scrolling is done automatically when point moves off the screen (or into the scroll margin). If the value is a positive integer n, then redisplay scrolls the text up to n lines in either direction, if that will bring point back into proper view. This behavior is called *conservative scrolling*. Otherwise, scrolling happens in the usual way, under the control of other variables such as `scroll-up-aggressively` and `scroll-down-aggressively`.
The default value is zero, which means that conservative scrolling never happens.
User Option: **scroll-down-aggressively**
The value of this variable should be either `nil` or a fraction f between 0 and 1. If it is a fraction, that specifies where on the screen to put point when scrolling down. More precisely, when a window scrolls down because point is above the window start, the new start position is chosen to put point f part of the window height from the top. The larger f, the more aggressive the scrolling.
A value of `nil` is equivalent to .5, since its effect is to center point. This variable automatically becomes buffer-local when set in any fashion.
User Option: **scroll-up-aggressively**
Likewise, for scrolling up. The value, f, specifies how far point should be placed from the bottom of the window; thus, as with `scroll-down-aggressively`, a larger value scrolls more aggressively.
User Option: **scroll-step**
This variable is an older variant of `scroll-conservatively`. The difference is that if its value is n, that permits scrolling only by precisely n lines, not a smaller number. This feature does not work with `scroll-margin`. The default value is zero.
User Option: **scroll-preserve-screen-position**
If this option is `t`, whenever a scrolling command moves point off-window, Emacs tries to adjust point to keep the cursor at its old vertical position in the window, rather than the window edge.
If the value is non-`nil` and not `t`, Emacs adjusts point to keep the cursor at the same vertical position, even if the scrolling command didn’t move point off-window.
This option affects all scroll commands that have a non-`nil` `scroll-command` symbol property.
User Option: **next-screen-context-lines**
The value of this variable is the number of lines of continuity to retain when scrolling by full screens. For example, `scroll-up` with an argument of `nil` scrolls so that this many lines at the bottom of the window appear instead at the top. The default value is `2`.
User Option: **scroll-error-top-bottom**
If this option is `nil` (the default), `scroll-up-command` and `scroll-down-command` simply signal an error when no more scrolling is possible.
If the value is `t`, these commands instead move point to the beginning or end of the buffer (depending on scrolling direction); only if point is already on that position do they signal an error.
Command: **recenter** *&optional count redisplay*
This function scrolls the text in the selected window so that point is displayed at a specified vertical position within the window. It does not move point with respect to the text.
If count is a non-negative number, that puts the line containing point count lines down from the top of the window. If count is a negative number, then it counts upward from the bottom of the window, so that -1 stands for the last usable line in the window.
If count is `nil` (or a non-`nil` list), `recenter` puts the line containing point in the middle of the window. If count is `nil` and redisplay is non-`nil`, this function may redraw the frame, according to the value of `recenter-redisplay`. Thus, omitting the second argument can be used to countermand the effect of `recenter-redisplay` being non-`nil`. Interactive calls pass non-‘nil’ for redisplay.
When `recenter` is called interactively, count is the raw prefix argument. Thus, typing `C-u` as the prefix sets the count to a non-`nil` list, while typing `C-u 4` sets count to 4, which positions the current line four lines from the top.
With an argument of zero, `recenter` positions the current line at the top of the window. The command `recenter-top-bottom` offers a more convenient way to achieve this.
Function: **recenter-window-group** *&optional count*
This function is like `recenter`, except that when the selected window is part of a group of windows (see [Window Group](selecting-windows#Window-Group)), `recenter-window-group` scrolls the entire group. This condition holds when the buffer local variable `recenter-window-group-function` is set to a function. In this case, `recenter-window-group` calls the function with the argument count, then returns its result. The argument count has the same meaning as in `recenter`, but with respect to the entire window group.
User Option: **recenter-redisplay**
If this variable is non-`nil`, calling `recenter` with a `nil` count argument and non-`nil` redisplay argument redraws the frame. The default value is `tty`, which means only redraw the frame if it is a tty frame.
Command: **recenter-top-bottom** *&optional count*
This command, which is the default binding for `C-l`, acts like `recenter`, except if called with no argument. In that case, successive calls place point according to the cycling order defined by the variable `recenter-positions`.
User Option: **recenter-positions**
This variable controls how `recenter-top-bottom` behaves when called with no argument. The default value is `(middle top
bottom)`, which means that successive calls of `recenter-top-bottom` with no argument cycle between placing point at the middle, top, and bottom of the window.
| programming_docs |
elisp None #### Naming Backup Files
The functions in this section are documented mainly because you can customize the naming conventions for backup files by redefining them. If you change one, you probably need to change the rest.
Function: **backup-file-name-p** *filename*
This function returns a non-`nil` value if filename is a possible name for a backup file. It just checks the name, not whether a file with the name filename exists.
```
(backup-file-name-p "foo")
⇒ nil
```
```
(backup-file-name-p "foo~")
⇒ 3
```
The standard definition of this function is as follows:
```
(defun backup-file-name-p (file)
"Return non-nil if FILE is a backup file \
name (numeric or not)..."
(string-match "~\\'" file))
```
Thus, the function returns a non-`nil` value if the file name ends with a ‘`~`’. (We use a backslash to split the documentation string’s first line into two lines in the text, but produce just one line in the string itself.)
This simple expression is placed in a separate function to make it easy to redefine for customization.
Function: **make-backup-file-name** *filename*
This function returns a string that is the name to use for a non-numbered backup file for file filename. On Unix, this is just filename with a tilde appended.
The standard definition of this function, on most operating systems, is as follows:
```
(defun make-backup-file-name (file)
"Create the non-numeric backup file name for FILE..."
(concat file "~"))
```
You can change the backup-file naming convention by redefining this function. The following example redefines `make-backup-file-name` to prepend a ‘`.`’ in addition to appending a tilde:
```
(defun make-backup-file-name (filename)
(expand-file-name
(concat "." (file-name-nondirectory filename) "~")
(file-name-directory filename)))
```
```
(make-backup-file-name "backups.texi")
⇒ ".backups.texi~"
```
Some parts of Emacs, including some Dired commands, assume that backup file names end with ‘`~`’. If you do not follow that convention, it will not cause serious problems, but these commands may give less-than-desirable results.
Function: **find-backup-file-name** *filename*
This function computes the file name for a new backup file for filename. It may also propose certain existing backup files for deletion. `find-backup-file-name` returns a list whose CAR is the name for the new backup file and whose CDR is a list of backup files whose deletion is proposed. The value can also be `nil`, which means not to make a backup.
Two variables, `kept-old-versions` and `kept-new-versions`, determine which backup versions should be kept. This function keeps those versions by excluding them from the CDR of the value. See [Numbered Backups](numbered-backups).
In this example, the value says that `~rms/foo.~5~` is the name to use for the new backup file, and `~rms/foo.~3~` is an excess version that the caller should consider deleting now.
```
(find-backup-file-name "~rms/foo")
⇒ ("~rms/foo.~5~" "~rms/foo.~3~")
```
Function: **file-backup-file-names** *filename*
This function returns a list of all the backup file names for filename, or `nil` if there are none. The files are sorted by modification time, descending, so that the most recent files are first.
Function: **file-newest-backup** *filename*
This function returns the first element of the list returned by `file-backup-file-names`.
Some file comparison commands use this function so that they can automatically compare a file with its most recent backup.
elisp None ### Color Names
A color name is text (usually in a string) that specifies a color. Symbolic names such as ‘`black`’, ‘`white`’, ‘`red`’, etc., are allowed; use `M-x list-colors-display` to see a list of defined names. You can also specify colors numerically in forms such as ‘`#rgb`’ and ‘`RGB:r/g/b`’, where r specifies the red level, g specifies the green level, and b specifies the blue level. You can use either one, two, three, or four hex digits for r; then you must use the same number of hex digits for all g and b as well, making either 3, 6, 9 or 12 hex digits in all. (See the documentation of the X Window System for more details about numerical RGB specification of colors.)
These functions provide a way to determine which color names are valid, and what they look like. In some cases, the value depends on the *selected frame*, as described below; see [Input Focus](input-focus), for the meaning of the term “selected frame”.
To read user input of color names with completion, use `read-color` (see [read-color](high_002dlevel-completion)).
Function: **color-defined-p** *color &optional frame*
This function reports whether a color name is meaningful. It returns `t` if so; otherwise, `nil`. The argument frame says which frame’s display to ask about; if frame is omitted or `nil`, the selected frame is used.
Note that this does not tell you whether the display you are using really supports that color. When using X, you can ask for any defined color on any kind of display, and you will get some result—typically, the closest it can do. To determine whether a frame can really display a certain color, use `color-supported-p` (see below).
This function used to be called `x-color-defined-p`, and that name is still supported as an alias.
Function: **defined-colors** *&optional frame*
This function returns a list of the color names that are defined and supported on frame frame (default, the selected frame). If frame does not support colors, the value is `nil`.
This function used to be called `x-defined-colors`, and that name is still supported as an alias.
Function: **color-supported-p** *color &optional frame background-p*
This returns `t` if frame can really display the color color (or at least something close to it). If frame is omitted or `nil`, the question applies to the selected frame.
Some terminals support a different set of colors for foreground and background. If background-p is non-`nil`, that means you are asking whether color can be used as a background; otherwise you are asking whether it can be used as a foreground.
The argument color must be a valid color name.
Function: **color-gray-p** *color &optional frame*
This returns `t` if color is a shade of gray, as defined on frame’s display. If frame is omitted or `nil`, the question applies to the selected frame. If color is not a valid color name, this function returns `nil`.
Function: **color-values** *color &optional frame*
This function returns a value that describes what color should ideally look like on frame. If color is defined, the value is a list of three integers, which give the amount of red, the amount of green, and the amount of blue. Each integer ranges in principle from 0 to 65535, but some displays may not use the full range. This three-element list is called the *rgb values* of the color.
If color is not defined, the value is `nil`.
```
(color-values "black")
⇒ (0 0 0)
(color-values "white")
⇒ (65535 65535 65535)
(color-values "red")
⇒ (65535 0 0)
(color-values "pink")
⇒ (65535 49344 52171)
(color-values "hungry")
⇒ nil
```
The color values are returned for frame’s display. If frame is omitted or `nil`, the information is returned for the selected frame’s display. If the frame cannot display colors, the value is `nil`.
This function used to be called `x-color-values`, and that name is still supported as an alias.
elisp None ### Association Lists
An *association list*, or *alist* for short, records a mapping from keys to values. It is a list of cons cells called *associations*: the CAR of each cons cell is the *key*, and the CDR is the *associated value*.[6](#FOOT6)
Here is an example of an alist. The key `pine` is associated with the value `cones`; the key `oak` is associated with `acorns`; and the key `maple` is associated with `seeds`.
```
((pine . cones)
(oak . acorns)
(maple . seeds))
```
Both the values and the keys in an alist may be any Lisp objects. For example, in the following alist, the symbol `a` is associated with the number `1`, and the string `"b"` is associated with the *list* `(2 3)`, which is the CDR of the alist element:
```
((a . 1) ("b" 2 3))
```
Sometimes it is better to design an alist to store the associated value in the CAR of the CDR of the element. Here is an example of such an alist:
```
((rose red) (lily white) (buttercup yellow))
```
Here we regard `red` as the value associated with `rose`. One advantage of this kind of alist is that you can store other related information—even a list of other items—in the CDR of the CDR. One disadvantage is that you cannot use `rassq` (see below) to find the element containing a given value. When neither of these considerations is important, the choice is a matter of taste, as long as you are consistent about it for any given alist.
The same alist shown above could be regarded as having the associated value in the CDR of the element; the value associated with `rose` would be the list `(red)`.
Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one.
In Emacs Lisp, it is *not* an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases.
Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. See [Property Lists](property-lists), for a comparison of property lists and association lists.
Function: **assoc** *key alist &optional testfn*
This function returns the first association for key in alist, comparing key against the alist elements using testfn if it is a function, and `equal` otherwise (see [Equality Predicates](equality-predicates)). If testfn is a function, it is called with two arguments: the CAR of an element from alist and key. The function returns `nil` if no association in alist has a CAR equal to key, as tested by testfn. For example:
```
(setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
⇒ ((pine . cones) (oak . acorns) (maple . seeds))
(assoc 'oak trees)
⇒ (oak . acorns)
(cdr (assoc 'oak trees))
⇒ acorns
(assoc 'birch trees)
⇒ nil
```
Here is another example, in which the keys and values are not symbols:
```
(setq needles-per-cluster
'((2 "Austrian Pine" "Red Pine")
(3 "Pitch Pine")
(5 "White Pine")))
(cdr (assoc 3 needles-per-cluster))
⇒ ("Pitch Pine")
(cdr (assoc 2 needles-per-cluster))
⇒ ("Austrian Pine" "Red Pine")
```
The function `assoc-string` is much like `assoc` except that it ignores certain differences between strings. See [Text Comparison](text-comparison).
Function: **rassoc** *value alist*
This function returns the first association with value value in alist. It returns `nil` if no association in alist has a CDR `equal` to value.
`rassoc` is like `assoc` except that it compares the CDR of each alist association instead of the CAR. You can think of this as reverse `assoc`, finding the key for a given value.
Function: **assq** *key alist*
This function is like `assoc` in that it returns the first association for key in alist, but it makes the comparison using `eq`. `assq` returns `nil` if no association in alist has a CAR `eq` to key. This function is used more often than `assoc`, since `eq` is faster than `equal` and most alists use symbols as keys. See [Equality Predicates](equality-predicates).
```
(setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
⇒ ((pine . cones) (oak . acorns) (maple . seeds))
(assq 'pine trees)
⇒ (pine . cones)
```
On the other hand, `assq` is not usually useful in alists where the keys may not be symbols:
```
(setq leaves
'(("simple leaves" . oak)
("compound leaves" . horsechestnut)))
(assq "simple leaves" leaves)
⇒ Unspecified; might be `nil` or `("simple leaves" . oak)`.
(assoc "simple leaves" leaves)
⇒ ("simple leaves" . oak)
```
Function: **alist-get** *key alist &optional default remove testfn*
This function is similar to `assq`. It finds the first association `(key . value)` by comparing key with alist elements, and, if found, returns the value of that association. If no association is found, the function returns default. Comparison of key against alist elements uses the function specified by testfn, defaulting to `eq`.
This is a generalized variable (see [Generalized Variables](generalized-variables)) that can be used to change a value with `setf`. When using it to set a value, optional argument remove non-`nil` means to remove key’s association from alist if the new value is `eql` to default.
Function: **rassq** *value alist*
This function returns the first association with value value in alist. It returns `nil` if no association in alist has a CDR `eq` to value.
`rassq` is like `assq` except that it compares the CDR of each alist association instead of the CAR. You can think of this as reverse `assq`, finding the key for a given value.
For example:
```
(setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
(rassq 'acorns trees)
⇒ (oak . acorns)
(rassq 'spores trees)
⇒ nil
```
`rassq` cannot search for a value stored in the CAR of the CDR of an element:
```
(setq colors '((rose red) (lily white) (buttercup yellow)))
(rassq 'white colors)
⇒ nil
```
In this case, the CDR of the association `(lily white)` is not the symbol `white`, but rather the list `(white)`. This becomes clearer if the association is written in dotted pair notation:
```
(lily white) ≡ (lily . (white))
```
Function: **assoc-default** *key alist &optional test default*
This function searches alist for a match for key. For each element of alist, it compares the element (if it is an atom) or the element’s CAR (if it is a cons) against key, by calling test with two arguments: the element or its CAR, and key. The arguments are passed in that order so that you can get useful results using `string-match` with an alist that contains regular expressions (see [Regexp Search](regexp-search)). If test is omitted or `nil`, `equal` is used for comparison.
If an alist element matches key by this criterion, then `assoc-default` returns a value based on this element. If the element is a cons, then the value is the element’s CDR. Otherwise, the return value is default.
If no alist element matches key, `assoc-default` returns `nil`.
Function: **copy-alist** *alist*
This function returns a two-level deep copy of alist: it creates a new copy of each association, so that you can alter the associations of the new alist without changing the old one.
```
(setq needles-per-cluster
'((2 . ("Austrian Pine" "Red Pine"))
(3 . ("Pitch Pine"))
```
```
(5 . ("White Pine"))))
⇒
((2 "Austrian Pine" "Red Pine")
(3 "Pitch Pine")
(5 "White Pine"))
(setq copy (copy-alist needles-per-cluster))
⇒
((2 "Austrian Pine" "Red Pine")
(3 "Pitch Pine")
(5 "White Pine"))
(eq needles-per-cluster copy)
⇒ nil
(equal needles-per-cluster copy)
⇒ t
(eq (car needles-per-cluster) (car copy))
⇒ nil
(cdr (car (cdr needles-per-cluster)))
⇒ ("Pitch Pine")
```
```
(eq (cdr (car (cdr needles-per-cluster)))
(cdr (car (cdr copy))))
⇒ t
```
This example shows how `copy-alist` makes it possible to change the associations of one copy without affecting the other:
```
(setcdr (assq 3 copy) '("Martian Vacuum Pine"))
(cdr (assq 3 needles-per-cluster))
⇒ ("Pitch Pine")
```
Function: **assq-delete-all** *key alist*
This function deletes from alist all the elements whose CAR is `eq` to key, much as if you used `delq` to delete each such element one by one. It returns the shortened alist, and often modifies the original list structure of alist. For correct results, use the return value of `assq-delete-all` rather than looking at the saved value of alist.
```
(setq alist (list '(foo 1) '(bar 2) '(foo 3) '(lose 4)))
⇒ ((foo 1) (bar 2) (foo 3) (lose 4))
(assq-delete-all 'foo alist)
⇒ ((bar 2) (lose 4))
alist
⇒ ((foo 1) (bar 2) (lose 4))
```
Function: **assoc-delete-all** *key alist &optional test*
This function is like `assq-delete-all` except that it accepts an optional argument test, a predicate function to compare the keys in alist. If omitted or `nil`, test defaults to `equal`. As `assq-delete-all`, this function often modifies the original list structure of alist.
Function: **rassq-delete-all** *value alist*
This function deletes from alist all the elements whose CDR is `eq` to value. It returns the shortened alist, and often modifies the original list structure of alist. `rassq-delete-all` is like `assq-delete-all` except that it compares the CDR of each alist association instead of the CAR.
Macro: **let-alist** *alist body*
Creates a binding for each symbol used as keys the association list alist, prefixed with dot. This can be useful when accessing several items in the same association list, and it’s best understood through a simple example:
```
(setq colors '((rose . red) (lily . white) (buttercup . yellow)))
(let-alist colors
(if (eq .rose 'red)
.lily))
⇒ white
```
The body is inspected at compilation time, and only the symbols that appear in body with a ‘`.`’ as the first character in the symbol name will be bound. Finding the keys is done with `assq`, and the `cdr` of the return value of this `assq` is assigned as the value for the binding.
Nested association lists is supported:
```
(setq colors '((rose . red) (lily (belladonna . yellow) (brindisi . pink))))
(let-alist colors
(if (eq .rose 'red)
.lily.belladonna))
⇒ yellow
```
Nesting `let-alist` inside each other is allowed, but the code in the inner `let-alist` can’t access the variables bound by the outer `let-alist`.
elisp None #### Running Hooks
In this section, we document the `run-hooks` function, which is used to run a normal hook. We also document the functions for running various kinds of abnormal hooks.
Function: **run-hooks** *&rest hookvars*
This function takes one or more normal hook variable names as arguments, and runs each hook in turn. Each argument should be a symbol that is a normal hook variable. These arguments are processed in the order specified.
If a hook variable has a non-`nil` value, that value should be a list of functions. `run-hooks` calls all the functions, one by one, with no arguments.
The hook variable’s value can also be a single function—either a lambda expression or a symbol with a function definition—which `run-hooks` calls. But this usage is obsolete.
If the hook variable is buffer-local, the buffer-local variable will be used instead of the global variable. However, if the buffer-local variable contains the element `t`, the global hook variable will be run as well.
Function: **run-hook-with-args** *hook &rest args*
This function runs an abnormal hook by calling all the hook functions in hook, passing each one the arguments args.
Function: **run-hook-with-args-until-failure** *hook &rest args*
This function runs an abnormal hook by calling each hook function in turn, stopping if one of them fails by returning `nil`. Each hook function is passed the arguments args. If this function stops because one of the hook functions fails, it returns `nil`; otherwise it returns a non-`nil` value.
Function: **run-hook-with-args-until-success** *hook &rest args*
This function runs an abnormal hook by calling each hook function, stopping if one of them succeeds by returning a non-`nil` value. Each hook function is passed the arguments args. If this function stops because one of the hook functions returns a non-`nil` value, it returns that value; otherwise it returns `nil`.
| programming_docs |
elisp None #### Frame Layout
A visible frame occupies a rectangular area on its terminal’s display. This area may contain a number of nested rectangles, each serving a different purpose. The drawing below sketches the layout of a frame on a graphical terminal:
```
<------------ Outer Frame Width ----------->
____________________________________________
^(0) ________ External/Outer Border _______ |
| | |_____________ Title Bar ______________| |
| | (1)_____________ Menu Bar ______________| | ^
| | (2)_____________ Tool Bar ______________| | ^
| | (3)_____________ Tab Bar _______________| | ^
| | | _________ Internal Border ________ | | ^
| | | | ^ | | | |
| | | | | | | | |
Outer | | | Inner | | | Native
Frame | | | Frame | | | Frame
Height | | | Height | | | Height
| | | | | | | | |
| | | |<--+--- Inner Frame Width ------->| | | |
| | | | | | | | |
| | | |___v______________________________| | | |
| | |___________ Internal Border __________| | v
v |___________ External/Outer Border __________|
<-------- Native Frame Width -------->
```
In practice not all of the areas shown in the drawing will or may be present. The meaning of these areas is described below.
Outer Frame
The *outer frame* is a rectangle comprising all areas shown in the drawing. The edges of that rectangle are called the *outer edges* of the frame. Together, the *outer width* and *outer height* of the frame specify the *outer size* of that rectangle.
Knowing the outer size of a frame is useful for fitting a frame into the working area of its display (see [Multiple Terminals](multiple-terminals)) or for placing two frames adjacent to each other on the screen. Usually, the outer size of a frame is available only after the frame has been mapped (made visible, see [Visibility of Frames](visibility-of-frames)) at least once. For the initial frame or a frame that has not been created yet, the outer size can be only estimated or must be calculated from the window-system’s or window manager’s defaults. One workaround is to obtain the differences of the outer and native (see below) sizes of a mapped frame and use them for calculating the outer size of the new frame.
The position of the upper left corner of the outer frame (indicated by ‘`(0)`’ in the drawing above) is the *outer position* of the frame. The outer position of a graphical frame is also referred to as “the position” of the frame because it usually remains unchanged on its display whenever the frame is resized or its layout is changed.
The outer position is specified by and can be set via the `left` and `top` frame parameters (see [Position Parameters](position-parameters)). For a normal, top-level frame these parameters usually represent its absolute position (see below) with respect to its display’s origin. For a child frame (see [Child Frames](child-frames)) these parameters represent its position relative to the native position (see below) of its parent frame. For frames on text terminals the values of these parameters are meaningless and always zero.
External Border
The *external border* is part of the decorations supplied by the window manager. It is typically used for resizing the frame with the mouse and is therefore not shown on “fullboth” and maximized frames (see [Size Parameters](size-parameters)). Its width is determined by the window manager and cannot be changed by Emacs’ functions.
External borders don’t exist on text terminal frames. For graphical frames, their display can be suppressed by setting the `override-redirect` or `undecorated` frame parameter (see [Management Parameters](management-parameters)).
Outer Border
The *outer border* is a separate border whose width can be specified with the `border-width` frame parameter (see [Layout Parameters](layout-parameters)). In practice, either the external or the outer border of a frame are displayed but never both at the same time. Usually, the outer border is shown only for special frames that are not (fully) controlled by the window manager like tooltip frames (see [Tooltips](tooltips)), child frames (see [Child Frames](child-frames)) and `undecorated` or `override-redirect` frames (see [Management Parameters](management-parameters)).
Outer borders are never shown on text terminal frames and on frames generated by GTK+ routines. On MS-Windows, the outer border is emulated with the help of a one pixel wide external border. Non-toolkit builds on X allow to change the color of the outer border by setting the `border-color` frame parameter (see [Layout Parameters](layout-parameters)).
Title Bar
The *title bar*, a.k.a. *caption bar*, is also part of the window manager’s decorations and typically displays the title of the frame (see [Frame Titles](frame-titles)) as well as buttons for minimizing, maximizing and deleting the frame. It can be also used for dragging the frame with the mouse. The title bar is usually not displayed for fullboth (see [Size Parameters](size-parameters)), tooltip (see [Tooltips](tooltips)) and child frames (see [Child Frames](child-frames)) and doesn’t exist for terminal frames. Display of the title bar can be suppressed by setting the `override-redirect` or the `undecorated` frame parameters (see [Management Parameters](management-parameters)).
Menu Bar
The menu bar (see [Menu Bar](menu-bar)) can be either internal (drawn by Emacs itself) or external (drawn by the toolkit). Most builds (GTK+, Lucid, Motif and MS-Windows) rely on an external menu bar. NS also uses an external menu bar which, however, is not part of the outer frame. Non-toolkit builds can provide an internal menu bar. On text terminal frames, the menu bar is part of the frame’s root window (see [Windows and Frames](windows-and-frames)). As a rule, menu bars are never shown on child frames (see [Child Frames](child-frames)). Display of the menu bar can be suppressed by setting the `menu-bar-lines` parameter (see [Layout Parameters](layout-parameters)) to zero.
Whether the menu bar is wrapped or truncated whenever its width becomes too large to fit on its frame depends on the toolkit . Usually, only Motif and MS-Windows builds can wrap the menu bar. When they (un-)wrap the menu bar, they try to keep the outer height of the frame unchanged, so the native height of the frame (see below) will change instead.
Tool Bar
Like the menu bar, the tool bar (see [Tool Bar](tool-bar)) can be either internal (drawn by Emacs itself) or external (drawn by a toolkit). The GTK+ and NS builds have the tool bar drawn by the toolkit. The remaining builds use internal tool bars. With GTK+ the tool bar can be located on either side of the frame, immediately outside the internal border, see below. Tool bars are usually not shown for child frames (see [Child Frames](child-frames)). Display of the tool bar can be suppressed by setting the `tool-bar-lines` parameter (see [Layout Parameters](layout-parameters)) to zero.
If the variable `auto-resize-tool-bars` is non-`nil`, Emacs wraps the internal tool bar when its width becomes too large for its frame. If and when Emacs (un-)wraps the internal tool bar, it by default keeps the outer height of the frame unchanged, so the native height of the frame (see below) will change instead. Emacs built with GTK+, on the other hand, never wraps the tool bar but may automatically increase the outer width of a frame in order to accommodate an overlong tool bar.
Tab Bar
The tab bar (see [Tab Bars](https://www.gnu.org/software/emacs/manual/html_node/emacs/Tab-Bars.html#Tab-Bars) in The GNU Emacs Manual) is always drawn by Emacs itself. The tab bar appears above the tool bar in Emacs built with an internal tool bar, and below the tool bar in builds with an external tool bar. Display of the tab bar can be suppressed by setting the `tab-bar-lines` parameter (see [Layout Parameters](layout-parameters)) to zero.
Native Frame
The *native frame* is a rectangle located entirely within the outer frame. It excludes the areas occupied by an external or outer border, the title bar and any external menu or tool bar. The edges of the native frame are called the *native edges* of the frame. Together, the *native width* and *native height* of a frame specify the *native size* of the frame.
The native size of a frame is the size Emacs passes to the window-system or window manager when creating or resizing the frame from within Emacs. It is also the size Emacs receives from the window-system or window manager whenever these resize the frame’s window-system window, for example, after maximizing the frame by clicking on the corresponding button in the title bar or when dragging its external border with the mouse.
The position of the top left corner of the native frame specifies the *native position* of the frame. (1)–(3) in the drawing above indicate that position for the various builds:
* (1) non-toolkit and terminal frames
* (2) Lucid, Motif and MS-Windows frames
* (3) GTK+ and NS frames
Accordingly, the native height of a frame may include the height of the tool bar but not that of the menu bar (Lucid, Motif, MS-Windows) or those of the menu bar and the tool bar (non-toolkit and text terminal frames).
The native position of a frame is the reference position for functions that set or return the current position of the mouse (see [Mouse Position](mouse-position)) and for functions dealing with the position of windows like `window-edges`, `window-at` or `coordinates-in-window-p` (see [Coordinates and Windows](coordinates-and-windows)). It also specifies the (0, 0) origin for locating and positioning child frames within this frame (see [Child Frames](child-frames)).
Note also that the native position of a frame usually remains unaltered on its display when removing or adding the window manager decorations by changing the frame’s `override-redirect` or `undecorated` parameter (see [Management Parameters](management-parameters)).
Internal Border
The internal border is a border drawn by Emacs around the inner frame (see below). The specification of its appearance depends on whether or not the given frame is a child frame (see [Child Frames](child-frames)).
For normal frames its width is specified by the `internal-border-width` frame parameter (see [Layout Parameters](layout-parameters)), and its color is specified by the background of the `internal-border` face.
For child frames its width is specified by the `child-frame-border-width` frame parameter (but will use the `internal-border-width` parameter as fallback), and its color is specified by the background of the `child-frame-border` face.
Inner Frame
The *inner frame* is the rectangle reserved for the frame’s windows. It’s enclosed by the internal border which, however, is not part of the inner frame. Its edges are called the *inner edges* of the frame. The *inner width* and *inner height* specify the *inner size* of the rectangle. The inner frame is sometimes also referred to as the *display area* of the frame.
As a rule, the inner frame is subdivided into the frame’s root window (see [Windows and Frames](windows-and-frames)) and the frame’s minibuffer window (see [Minibuffer Windows](minibuffer-windows)). There are two notable exceptions to this rule: A *minibuffer-less frame* contains a root window only and does not contain a minibuffer window. A *minibuffer-only frame* contains only a minibuffer window which also serves as that frame’s root window. See [Initial Parameters](initial-parameters) for how to create such frame configurations.
Text Area
The *text area* of a frame is a somewhat fictitious area that can be embedded in the native frame. Its position is unspecified. Its width can be obtained by removing from that of the native width the widths of the internal border, one vertical scroll bar, and one left and one right fringe if they are specified for this frame, see [Layout Parameters](layout-parameters). Its height can be obtained by removing from that of the native height the widths of the internal border and the heights of the frame’s internal menu and tool bars, the tab bar and one horizontal scroll bar if specified for this frame.
The *absolute position* of a frame is given as a pair (X, Y) of horizontal and vertical pixel offsets relative to an origin (0, 0) of the frame’s display. Correspondingly, the *absolute edges* of a frame are given as pixel offsets from that origin.
Note that with multiple monitors, the origin of the display does not necessarily coincide with the top-left corner of the entire usable display area of the terminal. Hence the absolute position of a frame can be negative in such an environment even when that frame is completely visible.
By convention, vertical offsets increase “downwards”. This means that the height of a frame is obtained by subtracting the offset of its top edge from that of its bottom edge. Horizontal offsets increase “rightwards”, as expected, so a frame’s width is calculated by subtracting the offset of its left edge from that of its right edge.
For a frame on a graphical terminal the following function returns the sizes of the areas described above:
Function: **frame-geometry** *&optional frame*
This function returns geometric attributes of frame. The return value is an association list of the attributes listed below. All coordinate, height and width values are integers counting pixels. Note that if frame has not been mapped yet, (see [Visibility of Frames](visibility-of-frames)) some of the return values may only represent approximations of the actual values—those that can be seen after the frame has been mapped.
`outer-position`
A cons representing the absolute position of the outer frame, relative to the origin at position (0, 0) of frame’s display.
`outer-size`
A cons of the outer width and height of frame.
`external-border-size`
A cons of the horizontal and vertical width of frame’s external borders as supplied by the window manager. If the window manager doesn’t supply these values, Emacs will try to guess them from the coordinates of the outer and inner frame.
`outer-border-width`
The width of the outer border of frame. The value is meaningful for non-GTK+ X builds only.
`title-bar-size`
A cons of the width and height of the title bar of frame as supplied by the window manager or operating system. If both of them are zero, the frame has no title bar. If only the width is zero, Emacs was not able to retrieve the width information.
`menu-bar-external`
If non-`nil`, this means the menu bar is external (not part of the native frame of frame).
`menu-bar-size`
A cons of the width and height of the menu bar of frame.
`tool-bar-external`
If non-`nil`, this means the tool bar is external (not part of the native frame of frame).
`tool-bar-position`
This tells on which side the tool bar on frame is and can be one of `left`, `top`, `right` or `bottom`. The only toolkit that currently supports a value other than `top` is GTK+.
`tool-bar-size`
A cons of the width and height of the tool bar of frame.
`internal-border-width` The width of the internal border of frame.
The following function can be used to retrieve the edges of the outer, native and inner frame.
Function: **frame-edges** *&optional frame type*
This function returns the absolute edges of the outer, native or inner frame of frame. frame must be a live frame and defaults to the selected one. The returned list has the form `(left top right bottom)` where all values are in pixels relative to the origin of frame’s display. For terminal frames the values returned for left and top are always zero.
Optional argument type specifies the type of the edges to return: `outer-edges` means to return the outer edges of frame, `native-edges` (or `nil`) means to return its native edges and `inner-edges` means to return its inner edges.
By convention, the pixels of the display at the values returned for left and top are considered to be inside (part of) frame. Hence, if left and top are both zero, the pixel at the display’s origin is part of frame. The pixels at bottom and right, on the other hand, are considered to lie immediately outside frame. This means that if you have, for example, two side-by-side frames positioned such that the right outer edge of the frame on the left equals the left outer edge of the frame on the right, the pixels at that edge show a part of the frame on the right.
elisp None ### Fringes
On graphical displays, Emacs draws *fringes* next to each window: thin vertical strips down the sides which can display bitmaps indicating truncation, continuation, horizontal scrolling, and so on.
| | | |
| --- | --- | --- |
| • [Fringe Size/Pos](fringe-size_002fpos) | | Specifying where to put the window fringes. |
| • [Fringe Indicators](fringe-indicators) | | Displaying indicator icons in the window fringes. |
| • [Fringe Cursors](fringe-cursors) | | Displaying cursors in the right fringe. |
| • [Fringe Bitmaps](fringe-bitmaps) | | Specifying bitmaps for fringe indicators. |
| • [Customizing Bitmaps](customizing-bitmaps) | | Specifying your own bitmaps to use in the fringes. |
| • [Overlay Arrow](overlay-arrow) | | Display of an arrow to indicate position. |
elisp None ### User Identification
Variable: **init-file-user**
This variable says which user’s init files should be used by Emacs—or `nil` if none. `""` stands for the user who originally logged in. The value reflects command-line options such as ‘`-q`’ or ‘`-u user`’.
Lisp packages that load files of customizations, or any other sort of user profile, should obey this variable in deciding where to find it. They should load the profile of the user name found in this variable. If `init-file-user` is `nil`, meaning that the ‘`-q`’, ‘`-Q`’, or ‘`-batch`’ option was used, then Lisp packages should not load any customization files or user profile.
User Option: **user-mail-address**
This holds the email address of the user who is using Emacs.
Function: **user-login-name** *&optional uid*
This function returns the name under which the user is logged in. It uses the environment variables `LOGNAME` or `USER` if either is set. Otherwise, the value is based on the effective UID, not the real UID.
If you specify uid (a number), the result is the user name that corresponds to uid, or `nil` if there is no such user.
Function: **user-real-login-name**
This function returns the user name corresponding to Emacs’s real UID. This ignores the effective UID, and the environment variables `LOGNAME` and `USER`.
Function: **user-full-name** *&optional uid*
This function returns the full name of the logged-in user—or the value of the environment variable `NAME`, if that is set.
If the Emacs process’s user-id does not correspond to any known user (and provided `NAME` is not set), the result is `"unknown"`.
If uid is non-`nil`, then it should be a number (a user-id) or a string (a login name). Then `user-full-name` returns the full name corresponding to that user-id or login name. If you specify a user-id or login name that isn’t defined, it returns `nil`.
The symbols `user-login-name`, `user-real-login-name` and `user-full-name` are variables as well as functions. The functions return the same values that the variables hold. These variables allow you to fake out Emacs by telling the functions what to return. The variables are also useful for constructing frame titles (see [Frame Titles](frame-titles)).
Function: **user-real-uid**
This function returns the real UID of the user.
Function: **user-uid**
This function returns the effective UID of the user.
Function: **group-gid**
This function returns the effective GID of the Emacs process.
Function: **group-real-gid**
This function returns the real GID of the Emacs process.
Function: **system-users**
This function returns a list of strings, listing the user names on the system. If Emacs cannot retrieve this information, the return value is a list containing just the value of `user-real-login-name`.
Function: **system-groups**
This function returns a list of strings, listing the names of user groups on the system. If Emacs cannot retrieve this information, the return value is `nil`.
Function: **group-name** *gid*
This function returns the group name that corresponds to the numeric group ID gid, or `nil` if there is no such group.
| programming_docs |
elisp None #### Font Lock Basics
The Font Lock functionality is based on several basic functions. Each of these calls the function specified by the corresponding variable. This indirection allows major and minor modes to modify the way fontification works in the buffers of that mode, and even use the Font Lock mechanisms for features that have nothing to do with fontification. (This is why the description below says “should” when it describes what the functions do: the mode can customize the values of the corresponding variables to do something entirely different.) The variables mentioned below are described in [Other Font Lock Variables](other-font-lock-variables).
`font-lock-fontify-buffer`
This function should fontify the current buffer’s accessible portion, by calling the function specified by `font-lock-fontify-buffer-function`.
`font-lock-unfontify-buffer`
Used when turning Font Lock off to remove the fontification. Calls the function specified by `font-lock-unfontify-buffer-function`.
`font-lock-fontify-region beg end &optional loudly`
Should fontify the region between beg and end. If loudly is non-`nil`, should display status messages while fontifying. Calls the function specified by `font-lock-fontify-region-function`.
`font-lock-unfontify-region beg end`
Should remove fontification from the region between beg and end. Calls the function specified by `font-lock-unfontify-region-function`.
`font-lock-flush &optional beg end`
This function should mark the fontification of the region between beg and end as outdated. If not specified or `nil`, beg and end default to the beginning and end of the buffer’s accessible portion. Calls the function specified by `font-lock-flush-function`.
`font-lock-ensure &optional beg end`
This function should make sure the region between beg and end has been fontified. The optional arguments beg and end default to the beginning and the end of the buffer’s accessible portion. Calls the function specified by `font-lock-ensure-function`.
`font-lock-debug-fontify` This is a convenience command meant to be used when developing font locking for a mode, and should not be called from Lisp code. It recomputes all the relevant variables and then calls `font-lock-fontify-region` on the entire buffer.
There are several variables that control how Font Lock mode highlights text. But major modes should not set any of these variables directly. Instead, they should set `font-lock-defaults` as a buffer-local variable. The value assigned to this variable is used, if and when Font Lock mode is enabled, to set all the other variables.
Variable: **font-lock-defaults**
This variable is set by modes to specify how to fontify text in that mode. It automatically becomes buffer-local when set. If its value is `nil`, Font Lock mode does no highlighting, and you can use the ‘`Faces`’ menu (under ‘`Edit`’ and then ‘`Text Properties`’ in the menu bar) to assign faces explicitly to text in the buffer.
If non-`nil`, the value should look like this:
```
(keywords [keywords-only [case-fold
[syntax-alist other-vars…]]])
```
The first element, keywords, indirectly specifies the value of `font-lock-keywords` which directs search-based fontification. It can be a symbol, a variable or a function whose value is the list to use for `font-lock-keywords`. It can also be a list of several such symbols, one for each possible level of fontification. The first symbol specifies the ‘`mode default`’ level of fontification, the next symbol level 1 fontification, the next level 2, and so on. The ‘`mode default`’ level is normally the same as level 1. It is used when `font-lock-maximum-decoration` has a `nil` value. See [Levels of Font Lock](levels-of-font-lock).
The second element, keywords-only, specifies the value of the variable `font-lock-keywords-only`. If this is omitted or `nil`, syntactic fontification (of strings and comments) is also performed. If this is non-`nil`, syntactic fontification is not performed. See [Syntactic Font Lock](syntactic-font-lock).
The third element, case-fold, specifies the value of `font-lock-keywords-case-fold-search`. If it is non-`nil`, Font Lock mode ignores case during search-based fontification.
If the fourth element, syntax-alist, is non-`nil`, it should be a list of cons cells of the form `(char-or-string
. string)`. These are used to set up a syntax table for syntactic fontification; the resulting syntax table is stored in `font-lock-syntax-table`. If syntax-alist is omitted or `nil`, syntactic fontification uses the syntax table returned by the `syntax-table` function. See [Syntax Table Functions](syntax-table-functions).
All the remaining elements (if any) are collectively called other-vars. Each of these elements should have the form `(variable . value)`—which means, make variable buffer-local and then set it to value. You can use these other-vars to set other variables that affect fontification, aside from those you can control with the first five elements. See [Other Font Lock Variables](other-font-lock-variables).
If your mode fontifies text explicitly by adding `font-lock-face` properties, it can specify `(nil t)` for `font-lock-defaults` to turn off all automatic fontification. However, this is not required; it is possible to fontify some things using `font-lock-face` properties and set up automatic fontification for other parts of the text.
elisp None ### Comparison of Numbers
To test numbers for numerical equality, you should normally use `=` instead of non-numeric comparison predicates like `eq`, `eql` and `equal`. Distinct floating-point and large integer objects can be numerically equal. If you use `eq` to compare them, you test whether they are the same *object*; if you use `eql` or `equal`, you test whether their values are *indistinguishable*. In contrast, `=` uses numeric comparison, and sometimes returns `t` when a non-numeric comparison would return `nil` and vice versa. See [Float Basics](float-basics).
In Emacs Lisp, if two fixnums are numerically equal, they are the same Lisp object. That is, `eq` is equivalent to `=` on fixnums. It is sometimes convenient to use `eq` for comparing an unknown value with a fixnum, because `eq` does not report an error if the unknown value is not a number—it accepts arguments of any type. By contrast, `=` signals an error if the arguments are not numbers or markers. However, it is better programming practice to use `=` if you can, even for comparing integers.
Sometimes it is useful to compare numbers with `eql` or `equal`, which treat two numbers as equal if they have the same data type (both integers, or both floating point) and the same value. By contrast, `=` can treat an integer and a floating-point number as equal. See [Equality Predicates](equality-predicates).
There is another wrinkle: because floating-point arithmetic is not exact, it is often a bad idea to check for equality of floating-point values. Usually it is better to test for approximate equality. Here’s a function to do this:
```
(defvar fuzz-factor 1.0e-6)
(defun approx-equal (x y)
(or (= x y)
(< (/ (abs (- x y))
(max (abs x) (abs y)))
fuzz-factor)))
```
Function: **=** *number-or-marker &rest number-or-markers*
This function tests whether all its arguments are numerically equal, and returns `t` if so, `nil` otherwise.
Function: **eql** *value1 value2*
This function acts like `eq` except when both arguments are numbers. It compares numbers by type and numeric value, so that `(eql 1.0 1)` returns `nil`, but `(eql 1.0 1.0)` and `(eql 1 1)` both return `t`. This can be used to compare large integers as well as small ones. Floating-point values with the same sign, exponent and fraction are `eql`. This differs from numeric comparison: `(eql 0.0 -0.0)` returns `nil` and `(eql 0.0e+NaN 0.0e+NaN)` returns `t`, whereas `=` does the opposite.
Function: **/=** *number-or-marker1 number-or-marker2*
This function tests whether its arguments are numerically equal, and returns `t` if they are not, and `nil` if they are.
Function: **<** *number-or-marker &rest number-or-markers*
This function tests whether each argument is strictly less than the following argument. It returns `t` if so, `nil` otherwise.
Function: **<=** *number-or-marker &rest number-or-markers*
This function tests whether each argument is less than or equal to the following argument. It returns `t` if so, `nil` otherwise.
Function: **>** *number-or-marker &rest number-or-markers*
This function tests whether each argument is strictly greater than the following argument. It returns `t` if so, `nil` otherwise.
Function: **>=** *number-or-marker &rest number-or-markers*
This function tests whether each argument is greater than or equal to the following argument. It returns `t` if so, `nil` otherwise.
Function: **max** *number-or-marker &rest numbers-or-markers*
This function returns the largest of its arguments.
```
(max 20)
⇒ 20
(max 1 2.5)
⇒ 2.5
(max 1 3 2.5)
⇒ 3
```
Function: **min** *number-or-marker &rest numbers-or-markers*
This function returns the smallest of its arguments.
```
(min -4 1)
⇒ -4
```
Function: **abs** *number*
This function returns the absolute value of number.
elisp None #### Functions for Visiting Files
This section describes the functions normally used to visit files. For historical reasons, these functions have names starting with ‘`find-`’ rather than ‘`visit-`’. See [Buffer File Name](buffer-file-name), for functions and variables that access the visited file name of a buffer or that find an existing buffer by its visited file name.
In a Lisp program, if you want to look at the contents of a file but not alter it, the fastest way is to use `insert-file-contents` in a temporary buffer. Visiting the file is not necessary and takes longer. See [Reading from Files](reading-from-files).
Command: **find-file** *filename &optional wildcards*
This command selects a buffer visiting the file filename, using an existing buffer if there is one, and otherwise creating a new buffer and reading the file into it. It also returns that buffer.
Aside from some technical details, the body of the `find-file` function is basically equivalent to:
```
(switch-to-buffer (find-file-noselect filename nil nil wildcards))
```
(See `switch-to-buffer` in [Switching Buffers](switching-buffers).)
If wildcards is non-`nil`, which is always true in an interactive call, then `find-file` expands wildcard characters in filename and visits all the matching files.
When `find-file` is called interactively, it prompts for filename in the minibuffer.
Command: **find-file-literally** *filename*
This command visits filename, like `find-file` does, but it does not perform any format conversions (see [Format Conversion](format-conversion)), character code conversions (see [Coding Systems](coding-systems)), or end-of-line conversions (see [End of line conversion](coding-system-basics)). The buffer visiting the file is made unibyte, and its major mode is Fundamental mode, regardless of the file name. File local variable specifications in the file (see [File Local Variables](file-local-variables)) are ignored, and automatic decompression and adding a newline at the end of the file due to `require-final-newline` (see [require-final-newline](saving-buffers)) are also disabled.
Note that if Emacs already has a buffer visiting the same file non-literally, it will not visit the same file literally, but instead just switch to the existing buffer. If you want to be sure of accessing a file’s contents literally, you should create a temporary buffer and then read the file contents into it using `insert-file-contents-literally` (see [Reading from Files](reading-from-files)).
Function: **find-file-noselect** *filename &optional nowarn rawfile wildcards*
This function is the guts of all the file-visiting functions. It returns a buffer visiting the file filename. You may make the buffer current or display it in a window if you wish, but this function does not do so.
The function returns an existing buffer if there is one; otherwise it creates a new buffer and reads the file into it. When `find-file-noselect` uses an existing buffer, it first verifies that the file has not changed since it was last visited or saved in that buffer. If the file has changed, this function asks the user whether to reread the changed file. If the user says ‘`yes`’, any edits previously made in the buffer are lost.
Reading the file involves decoding the file’s contents (see [Coding Systems](coding-systems)), including end-of-line conversion, and format conversion (see [Format Conversion](format-conversion)). If wildcards is non-`nil`, then `find-file-noselect` expands wildcard characters in filename and visits all the matching files.
This function displays warning or advisory messages in various peculiar cases, unless the optional argument nowarn is non-`nil`. For example, if it needs to create a buffer, and there is no file named filename, it displays the message ‘`(New file)`’ in the echo area, and leaves the buffer empty.
The `find-file-noselect` function normally calls `after-find-file` after reading the file (see [Subroutines of Visiting](subroutines-of-visiting)). That function sets the buffer major mode, parses local variables, warns the user if there exists an auto-save file more recent than the file just visited, and finishes by running the functions in `find-file-hook`.
If the optional argument rawfile is non-`nil`, then `after-find-file` is not called, and the `find-file-not-found-functions` are not run in case of failure. What’s more, a non-`nil` rawfile value suppresses coding system conversion and format conversion.
The `find-file-noselect` function usually returns the buffer that is visiting the file filename. But, if wildcards are actually used and expanded, it returns a list of buffers that are visiting the various files.
```
(find-file-noselect "/etc/fstab")
⇒ #<buffer fstab>
```
Command: **find-file-other-window** *filename &optional wildcards*
This command selects a buffer visiting the file filename, but does so in a window other than the selected window. It may use another existing window or split a window; see [Switching Buffers](switching-buffers).
When this command is called interactively, it prompts for filename.
Command: **find-file-read-only** *filename &optional wildcards*
This command selects a buffer visiting the file filename, like `find-file`, but it marks the buffer as read-only. See [Read Only Buffers](read-only-buffers), for related functions and variables.
When this command is called interactively, it prompts for filename.
User Option: **find-file-wildcards**
If this variable is non-`nil`, then the various `find-file` commands check for wildcard characters and visit all the files that match them (when invoked interactively or when their wildcards argument is non-`nil`). If this option is `nil`, then the `find-file` commands ignore their wildcards argument and never treat wildcard characters specially.
User Option: **find-file-hook**
The value of this variable is a list of functions to be called after a file is visited. The file’s local-variables specification (if any) will have been processed before the hooks are run. The buffer visiting the file is current when the hook functions are run.
This variable is a normal hook. See [Hooks](hooks).
Variable: **find-file-not-found-functions**
The value of this variable is a list of functions to be called when `find-file` or `find-file-noselect` is passed a nonexistent file name. `find-file-noselect` calls these functions as soon as it detects a nonexistent file. It calls them in the order of the list, until one of them returns non-`nil`. `buffer-file-name` is already set up.
This is not a normal hook because the values of the functions are used, and in many cases only some of the functions are called.
Variable: **find-file-literally**
This buffer-local variable, if set to a non-`nil` value, makes `save-buffer` behave as if the buffer were visiting its file literally, i.e., without conversions of any kind. The command `find-file-literally` sets this variable’s local value, but other equivalent functions and commands can do that as well, e.g., to avoid automatic addition of a newline at the end of the file. This variable is permanent local, so it is unaffected by changes of major modes.
elisp None #### SMIE Setup and Features
SMIE is meant to be a one-stop shop for structural navigation and various other features which rely on the syntactic structure of code, in particular automatic indentation. The main entry point is `smie-setup` which is a function typically called while setting up a major mode.
Function: **smie-setup** *grammar rules-function &rest keywords*
Setup SMIE navigation and indentation. grammar is a grammar table generated by `smie-prec2->grammar`. rules-function is a set of indentation rules for use on `smie-rules-function`. keywords are additional arguments, which can include the following keywords:
* `:forward-token` fun: Specify the forward lexer to use.
* `:backward-token` fun: Specify the backward lexer to use.
Calling this function is sufficient to make commands such as `forward-sexp`, `backward-sexp`, and `transpose-sexps` be able to properly handle structural elements other than just the paired parentheses already handled by syntax tables. For example, if the provided grammar is precise enough, `transpose-sexps` can correctly transpose the two arguments of a `+` operator, taking into account the precedence rules of the language.
Calling `smie-setup` is also sufficient to make TAB indentation work in the expected way, extends `blink-matching-paren` to apply to elements like `begin...end`, and provides some commands that you can bind in the major mode keymap.
Command: **smie-close-block**
This command closes the most recently opened (and not yet closed) block.
Command: **smie-down-list** *&optional arg*
This command is like `down-list` but it also pays attention to nesting of tokens other than parentheses, such as `begin...end`.
elisp None #### Primitive Function Type
A *primitive function* is a function callable from Lisp but written in the C programming language. Primitive functions are also called *subrs* or *built-in functions*. (The word “subr” is derived from “subroutine”.) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a *special form* (see [Special Forms](special-forms)).
It does not matter to the caller of a function whether the function is primitive. However, this does matter if you try to redefine a primitive with a function written in Lisp. The reason is that the primitive function may be called directly from C code. Calls to the redefined function from Lisp will use the new definition, but calls from C code may still use the built-in definition. Therefore, **we discourage redefinition of primitive functions**.
The term *function* refers to all Emacs functions, whether written in Lisp or C. See [Function Type](function-type), for information about the functions written in Lisp.
Primitive functions have no read syntax and print in hash notation with the name of the subroutine.
```
(symbol-function 'car) ; Access the function cell
; of the symbol.
⇒ #<subr car>
(subrp (symbol-function 'car)) ; Is this a primitive function?
⇒ t ; Yes.
```
elisp None ### Dealing With Compressed Data
When `auto-compression-mode` is enabled, Emacs automatically uncompresses compressed files when you visit them, and automatically recompresses them if you alter and save them. See [Compressed Files](https://www.gnu.org/software/emacs/manual/html_node/emacs/Compressed-Files.html#Compressed-Files) in The GNU Emacs Manual.
The above feature works by calling an external executable (e.g., `gzip`). Emacs can also be compiled with support for built-in decompression using the zlib library, which is faster than calling an external program.
Function: **zlib-available-p**
This function returns non-`nil` if built-in zlib decompression is available.
Function: **zlib-decompress-region** *start end &optional allow-partial*
This function decompresses the region between start and end, using built-in zlib decompression. The region should contain data that were compressed with gzip or zlib. On success, the function replaces the contents of the region with the decompressed data. If allow-partial is `nil` or omitted, then on failure, the function leaves the region unchanged and returns `nil`. Otherwise, it returns the number of bytes that were not decompressed and replaces the region text by whatever data was successfully decompressed. This function can be called only in unibyte buffers.
| programming_docs |
elisp None #### Terminal Type
A *terminal* is a device capable of displaying one or more Emacs frames (see [Frame Type](frame-type)).
Terminals have no read syntax. They print in hash notation giving the terminal’s ordinal number and its TTY device file name.
```
(get-device-terminal nil)
⇒ #<terminal 1 on /dev/tty>
```
elisp None ### Reading a Password
To read a password to pass to another program, you can use the function `read-passwd`.
Function: **read-passwd** *prompt &optional confirm default*
This function reads a password, prompting with prompt. It does not echo the password as the user types it; instead, it echoes ‘`\*`’ for each character in the password. If you want to apply another character to hide the password, let-bind the variable `read-hide-char` with that character.
The optional argument confirm, if non-`nil`, says to read the password twice and insist it must be the same both times. If it isn’t the same, the user has to type it over and over until the last two times match.
The optional argument default specifies the default password to return if the user enters empty input. If default is `nil`, then `read-passwd` returns the null string in that case.
elisp None ### Arithmetic Operations
Emacs Lisp provides the traditional four arithmetic operations (addition, subtraction, multiplication, and division), as well as remainder and modulus functions, and functions to add or subtract 1. Except for `%`, each of these functions accepts both integer and floating-point arguments, and returns a floating-point number if any argument is floating point.
Function: **1+** *number-or-marker*
This function returns number-or-marker plus 1. For example,
```
(setq foo 4)
⇒ 4
(1+ foo)
⇒ 5
```
This function is not analogous to the C operator `++`—it does not increment a variable. It just computes a sum. Thus, if we continue,
```
foo
⇒ 4
```
If you want to increment the variable, you must use `setq`, like this:
```
(setq foo (1+ foo))
⇒ 5
```
Function: **1-** *number-or-marker*
This function returns number-or-marker minus 1.
Function: **+** *&rest numbers-or-markers*
This function adds its arguments together. When given no arguments, `+` returns 0.
```
(+)
⇒ 0
(+ 1)
⇒ 1
(+ 1 2 3 4)
⇒ 10
```
Function: **-** *&optional number-or-marker &rest more-numbers-or-markers*
The `-` function serves two purposes: negation and subtraction. When `-` has a single argument, the value is the negative of the argument. When there are multiple arguments, `-` subtracts each of the more-numbers-or-markers from number-or-marker, cumulatively. If there are no arguments, the result is 0.
```
(- 10 1 2 3 4)
⇒ 0
(- 10)
⇒ -10
(-)
⇒ 0
```
Function: **\*** *&rest numbers-or-markers*
This function multiplies its arguments together, and returns the product. When given no arguments, `*` returns 1.
```
(*)
⇒ 1
(* 1)
⇒ 1
(* 1 2 3 4)
⇒ 24
```
Function: **/** *number &rest divisors*
With one or more divisors, this function divides number by each divisor in divisors in turn, and returns the quotient. With no divisors, this function returns 1/number, i.e., the multiplicative inverse of number. Each argument may be a number or a marker.
If all the arguments are integers, the result is an integer, obtained by rounding the quotient towards zero after each division.
```
(/ 6 2)
⇒ 3
```
```
(/ 5 2)
⇒ 2
```
```
(/ 5.0 2)
⇒ 2.5
```
```
(/ 5 2.0)
⇒ 2.5
```
```
(/ 5.0 2.0)
⇒ 2.5
```
```
(/ 4.0)
⇒ 0.25
```
```
(/ 4)
⇒ 0
```
```
(/ 25 3 2)
⇒ 4
```
```
(/ -17 6)
⇒ -2
```
If you divide an integer by the integer 0, Emacs signals an `arith-error` error (see [Errors](errors)). Floating-point division of a nonzero number by zero yields either positive or negative infinity (see [Float Basics](float-basics)).
Function: **%** *dividend divisor*
This function returns the integer remainder after division of dividend by divisor. The arguments must be integers or markers.
For any two integers dividend and divisor,
```
(+ (% dividend divisor)
(* (/ dividend divisor) divisor))
```
always equals dividend if divisor is nonzero.
```
(% 9 4)
⇒ 1
(% -9 4)
⇒ -1
(% 9 -4)
⇒ 1
(% -9 -4)
⇒ -1
```
Function: **mod** *dividend divisor*
This function returns the value of dividend modulo divisor; in other words, the remainder after division of dividend by divisor, but with the same sign as divisor. The arguments must be numbers or markers.
Unlike `%`, `mod` permits floating-point arguments; it rounds the quotient downward (towards minus infinity) to an integer, and uses that quotient to compute the remainder.
If divisor is zero, `mod` signals an `arith-error` error if both arguments are integers, and returns a NaN otherwise.
```
(mod 9 4)
⇒ 1
```
```
(mod -9 4)
⇒ 3
```
```
(mod 9 -4)
⇒ -3
```
```
(mod -9 -4)
⇒ -1
```
```
(mod 5.5 2.5)
⇒ .5
```
For any two numbers dividend and divisor,
```
(+ (mod dividend divisor)
(* (floor dividend divisor) divisor))
```
always equals dividend, subject to rounding error if either argument is floating point and to an `arith-error` if dividend is an integer and divisor is 0. For `floor`, see [Numeric Conversions](numeric-conversions).
elisp None #### Defining Tokens
SMIE comes with a predefined lexical analyzer which uses syntax tables in the following way: any sequence of characters that have word or symbol syntax is considered a token, and so is any sequence of characters that have punctuation syntax. This default lexer is often a good starting point but is rarely actually correct for any given language. For example, it will consider `"2,+3"` to be composed of 3 tokens: `"2"`, `",+"`, and `"3"`.
To describe the lexing rules of your language to SMIE, you need 2 functions, one to fetch the next token, and another to fetch the previous token. Those functions will usually first skip whitespace and comments and then look at the next chunk of text to see if it is a special token. If so it should skip the token and return a description of this token. Usually this is simply the string extracted from the buffer, but it can be anything you want. For example:
```
(defvar sample-keywords-regexp
(regexp-opt '("+" "*" "," ";" ">" ">=" "<" "<=" ":=" "=")))
```
```
(defun sample-smie-forward-token ()
(forward-comment (point-max))
(cond
((looking-at sample-keywords-regexp)
(goto-char (match-end 0))
(match-string-no-properties 0))
(t (buffer-substring-no-properties
(point)
(progn (skip-syntax-forward "w_")
(point))))))
```
```
(defun sample-smie-backward-token ()
(forward-comment (- (point)))
(cond
((looking-back sample-keywords-regexp (- (point) 2) t)
(goto-char (match-beginning 0))
(match-string-no-properties 0))
(t (buffer-substring-no-properties
(point)
(progn (skip-syntax-backward "w_")
(point))))))
```
Notice how those lexers return the empty string when in front of parentheses. This is because SMIE automatically takes care of the parentheses defined in the syntax table. More specifically if the lexer returns `nil` or an empty string, SMIE tries to handle the corresponding text as a sexp according to syntax tables.
elisp None #### File Name Components
The operating system groups files into directories. To specify a file, you must specify the directory and the file’s name within that directory. Therefore, Emacs considers a file name as having two main parts: the *directory name* part, and the *nondirectory* part (or *file name within the directory*). Either part may be empty. Concatenating these two parts reproduces the original file name.
On most systems, the directory part is everything up to and including the last slash (backslash is also allowed in input on MS-DOS or MS-Windows); the nondirectory part is the rest.
For some purposes, the nondirectory part is further subdivided into the name proper and the *version number*. On most systems, only backup files have version numbers in their names.
Function: **file-name-directory** *filename*
This function returns the directory part of filename, as a directory name (see [Directory Names](directory-names)), or `nil` if filename does not include a directory part.
On GNU and other POSIX-like systems, a string returned by this function always ends in a slash. On MS-DOS it can also end in a colon.
```
(file-name-directory "lewis/foo") ; GNU example
⇒ "lewis/"
```
```
(file-name-directory "foo") ; GNU example
⇒ nil
```
Function: **file-name-nondirectory** *filename*
This function returns the nondirectory part of filename.
```
(file-name-nondirectory "lewis/foo")
⇒ "foo"
```
```
(file-name-nondirectory "foo")
⇒ "foo"
```
```
(file-name-nondirectory "lewis/")
⇒ ""
```
Function: **file-name-sans-versions** *filename &optional keep-backup-version*
This function returns filename with any file version numbers, backup version numbers, or trailing tildes discarded.
If keep-backup-version is non-`nil`, then true file version numbers understood as such by the file system are discarded from the return value, but backup version numbers are kept.
```
(file-name-sans-versions "~rms/foo.~1~")
⇒ "~rms/foo"
```
```
(file-name-sans-versions "~rms/foo~")
⇒ "~rms/foo"
```
```
(file-name-sans-versions "~rms/foo")
⇒ "~rms/foo"
```
Function: **file-name-extension** *filename &optional period*
This function returns filename’s final extension, if any, after applying `file-name-sans-versions` to remove any version/backup part. The extension, in a file name, is the part that follows the last ‘`.`’ in the last name component (minus any version/backup part).
This function returns `nil` for extensionless file names such as `foo`. It returns `""` for null extensions, as in `foo.`. If the last component of a file name begins with a ‘`.`’, that ‘`.`’ doesn’t count as the beginning of an extension. Thus, `.emacs`’s extension is `nil`, not ‘`.emacs`’.
If period is non-`nil`, then the returned value includes the period that delimits the extension, and if filename has no extension, the value is `""`.
Function: **file-name-with-extension** *filename extension*
This function returns filename with its extension set to extension. A single leading dot in the extension will be stripped if there is one. For example:
```
(file-name-with-extension "file" "el")
⇒ "file.el"
(file-name-with-extension "file" ".el")
⇒ "file.el"
(file-name-with-extension "file.c" "el")
⇒ "file.el"
```
Note that this function will error if filename or extension are empty, or if the filename is shaped like a directory (i.e., if `directory-name-p` returns non-`nil`).
Function: **file-name-sans-extension** *filename*
This function returns filename minus its extension, if any. The version/backup part, if present, is only removed if the file has an extension. For example,
```
(file-name-sans-extension "foo.lose.c")
⇒ "foo.lose"
(file-name-sans-extension "big.hack/foo")
⇒ "big.hack/foo"
(file-name-sans-extension "/my/home/.emacs")
⇒ "/my/home/.emacs"
(file-name-sans-extension "/my/home/.emacs.el")
⇒ "/my/home/.emacs"
(file-name-sans-extension "~/foo.el.~3~")
⇒ "~/foo"
(file-name-sans-extension "~/foo.~3~")
⇒ "~/foo.~3~"
```
Note that the ‘`.~3~`’ in the two last examples is the backup part, not an extension.
Function: **file-name-base** *filename*
This function is the composition of `file-name-sans-extension` and `file-name-nondirectory`. For example,
```
(file-name-base "/my/home/foo.c")
⇒ "foo"
```
elisp None #### Defining Derived Modes
The recommended way to define a new major mode is to derive it from an existing one using `define-derived-mode`. If there is no closely related mode, you should inherit from either `text-mode`, `special-mode`, or `prog-mode`. See [Basic Major Modes](basic-major-modes). If none of these are suitable, you can inherit from `fundamental-mode` (see [Major Modes](major-modes)).
Macro: **define-derived-mode** *variant parent name docstring keyword-args… body…*
This macro defines variant as a major mode command, using name as the string form of the mode name. variant and parent should be unquoted symbols.
The new command variant is defined to call the function parent, then override certain aspects of that parent mode:
* The new mode has its own sparse keymap, named `variant-map`. `define-derived-mode` makes the parent mode’s keymap the parent of the new map, unless `variant-map` is already set and already has a parent.
* The new mode has its own syntax table, kept in the variable `variant-syntax-table`, unless you override this using the `:syntax-table` keyword (see below). `define-derived-mode` makes the parent mode’s syntax-table the parent of `variant-syntax-table`, unless the latter is already set and already has a parent different from the standard syntax table.
* The new mode has its own abbrev table, kept in the variable `variant-abbrev-table`, unless you override this using the `:abbrev-table` keyword (see below).
* The new mode has its own mode hook, `variant-hook`. It runs this hook, after running the hooks of its ancestor modes, with `run-mode-hooks`, as the last thing it does, apart from running any `:after-hook` form it may have. See [Mode Hooks](mode-hooks).
In addition, you can specify how to override other aspects of parent with body. The command variant evaluates the forms in body after setting up all its usual overrides, just before running the mode hooks.
If parent has a non-`nil` `mode-class` symbol property, then `define-derived-mode` sets the `mode-class` property of variant to the same value. This ensures, for example, that if parent is a special mode, then variant is also a special mode (see [Major Mode Conventions](major-mode-conventions)).
You can also specify `nil` for parent. This gives the new mode no parent. Then `define-derived-mode` behaves as described above, but, of course, omits all actions connected with parent.
The argument docstring specifies the documentation string for the new mode. `define-derived-mode` adds some general information about the mode’s hook, followed by the mode’s keymap, at the end of this documentation string. If you omit docstring, `define-derived-mode` generates a documentation string.
The keyword-args are pairs of keywords and values. The values, except for `:after-hook`’s, are evaluated. The following keywords are currently supported:
`:syntax-table`
You can use this to explicitly specify a syntax table for the new mode. If you specify a `nil` value, the new mode uses the same syntax table as parent, or the standard syntax table if parent is `nil`. (Note that this does *not* follow the convention used for non-keyword arguments that a `nil` value is equivalent with not specifying the argument.)
`:abbrev-table`
You can use this to explicitly specify an abbrev table for the new mode. If you specify a `nil` value, the new mode uses the same abbrev table as parent, or `fundamental-mode-abbrev-table` if parent is `nil`. (Again, a `nil` value is *not* equivalent to not specifying this keyword.)
`:interactive`
Modes are interactive commands by default. If you specify a `nil` value, the mode defined here won’t be interactive. This is useful for modes that are never meant to be activated by users manually, but are only supposed to be used in some specially-formatted buffer.
`:group`
If this is specified, the value should be the customization group for this mode. (Not all major modes have one.) The command `customize-mode` uses this. `define-derived-mode` does *not* automatically define the specified customization group.
`:after-hook` This optional keyword specifies a single Lisp form to evaluate as the final act of the mode function, after the mode hooks have been run. It should not be quoted. Since the form might be evaluated after the mode function has terminated, it should not access any element of the mode function’s local state. An `:after-hook` form is useful for setting up aspects of the mode which depend on the user’s settings, which in turn may have been changed in a mode hook.
Here is a hypothetical example:
```
(defvar hypertext-mode-map
(let ((map (make-sparse-keymap)))
(define-key map [down-mouse-3] 'do-hyper-link)
map))
(define-derived-mode hypertext-mode
text-mode "Hypertext"
"Major mode for hypertext."
(setq-local case-fold-search nil))
```
Do not write an `interactive` spec in the definition; `define-derived-mode` does that automatically.
Function: **derived-mode-p** *&rest modes*
This function returns non-`nil` if the current major mode is derived from any of the major modes given by the symbols modes.
elisp None #### Edebug Display Update
When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from outside Edebug (see [Window Configurations](window-configurations)). When you exit Edebug, it restores the previous window configuration.
Emacs redisplays only when it pauses. Usually, when you continue execution, the program re-enters Edebug at a breakpoint or after stepping, without pausing or reading input in between. In such cases, Emacs never gets a chance to redisplay the outside configuration. Consequently, what you see is the same window configuration as the last time Edebug was active, with no interruption.
Entry to Edebug for displaying something also saves and restores the following data (though some of them are deliberately not restored if an error or quit signal occurs).
* Which buffer is current, and the positions of point and the mark in the current buffer, are saved and restored.
* The outside window configuration is saved and restored if `edebug-save-windows` is non-`nil` (see [Edebug Options](edebug-options)). The window configuration is not restored on error or quit, but the outside selected window *is* reselected even on error or quit in case a `save-excursion` is active. If the value of `edebug-save-windows` is a list, only the listed windows are saved and restored.
The window start and horizontal scrolling of the source code buffer are not restored, however, so that the display remains coherent within Edebug.
* The value of point in each displayed buffer is saved and restored if `edebug-save-displayed-buffer-points` is non-`nil`.
* The variables `overlay-arrow-position` and `overlay-arrow-string` are saved and restored, so you can safely invoke Edebug from the recursive edit elsewhere in the same buffer.
* `cursor-in-echo-area` is locally bound to `nil` so that the cursor shows up in the window.
elisp None #### Initial Frame Parameters
You can specify the parameters for the initial startup frame by setting `initial-frame-alist` in your init file (see [Init File](init-file)).
User Option: **initial-frame-alist**
This variable’s value is an alist of parameter values used when creating the initial frame. You can set this variable to specify the appearance of the initial frame without altering subsequent frames. Each element has the form:
```
(parameter . value)
```
Emacs creates the initial frame before it reads your init file. After reading that file, Emacs checks `initial-frame-alist`, and applies the parameter settings in the altered value to the already created initial frame.
If these settings affect the frame geometry and appearance, you’ll see the frame appear with the wrong ones and then change to the specified ones. If that bothers you, you can specify the same geometry and appearance with X resources; those do take effect before the frame is created. See [X Resources](https://www.gnu.org/software/emacs/manual/html_node/emacs/X-Resources.html#X-Resources) in The GNU Emacs Manual.
X resource settings typically apply to all frames. If you want to specify some X resources solely for the sake of the initial frame, and you don’t want them to apply to subsequent frames, here’s how to achieve this. Specify parameters in `default-frame-alist` to override the X resources for subsequent frames; then, to prevent these from affecting the initial frame, specify the same parameters in `initial-frame-alist` with values that match the X resources.
If these parameters include `(minibuffer . nil)`, that indicates that the initial frame should have no minibuffer. In this case, Emacs creates a separate *minibuffer-only frame* as well.
User Option: **minibuffer-frame-alist**
This variable’s value is an alist of parameter values used when creating an initial minibuffer-only frame (i.e., the minibuffer-only frame that Emacs creates if `initial-frame-alist` specifies a frame with no minibuffer).
User Option: **default-frame-alist**
This is an alist specifying default values of frame parameters for all Emacs frames—the first frame, and subsequent frames. When using the X Window System, you can get the same results by means of X resources in many cases.
Setting this variable does not affect existing frames. Furthermore, functions that display a buffer in a separate frame may override the default parameters by supplying their own parameters.
If you invoke Emacs with command-line options that specify frame appearance, those options take effect by adding elements to either `initial-frame-alist` or `default-frame-alist`. Options which affect just the initial frame, such as ‘`--geometry`’ and ‘`--maximized`’, add to `initial-frame-alist`; the others add to `default-frame-alist`. see [Command Line Arguments for Emacs Invocation](https://www.gnu.org/software/emacs/manual/html_node/emacs/Emacs-Invocation.html#Emacs-Invocation) in The GNU Emacs Manual.
| programming_docs |
elisp None #### Face Attribute Functions
This section describes functions for directly accessing and modifying the attributes of a named face.
Function: **face-attribute** *face attribute &optional frame inherit*
This function returns the value of the attribute attribute for face on frame. See [Face Attributes](face-attributes), for the supported attributes.
If frame is omitted or `nil`, that means the selected frame (see [Input Focus](input-focus)). If frame is `t`, this function returns the value of the specified attribute for newly-created frames, i.e. the value of the attribute before applying the face spec in the face’s `defface` definition (see [Defining Faces](defining-faces)) or the spec set by `face-spec-set`. This default value of attribute is normally `unspecified`, unless you have specified some other value using `set-face-attribute`; see below.
If inherit is `nil`, only attributes directly defined by face are considered, so the return value may be `unspecified`, or a relative value. If inherit is non-`nil`, face’s definition of attribute is merged with the faces specified by its `:inherit` attribute; however the return value may still be `unspecified` or relative. If inherit is a face or a list of faces, then the result is further merged with that face (or faces), until it becomes specified and absolute.
To ensure that the return value is always specified and absolute, use a value of `default` for inherit; this will resolve any unspecified or relative values by merging with the `default` face (which is always completely specified).
For example,
```
(face-attribute 'bold :weight)
⇒ bold
```
Function: **face-attribute-relative-p** *attribute value*
This function returns non-`nil` if value, when used as the value of the face attribute attribute, is relative. This means it would modify, rather than completely override, any value that comes from a subsequent face in the face list or that is inherited from another face.
`unspecified` is a relative value for all attributes. For `:height`, floating point and function values are also relative.
For example:
```
(face-attribute-relative-p :height 2.0)
⇒ t
```
Function: **face-all-attributes** *face &optional frame*
This function returns an alist of attributes of face. The elements of the result are name-value pairs of the form `(attr-name . attr-value)`. Optional argument frame specifies the frame whose definition of face to return; if omitted or `nil`, the returned value describes the default attributes of face for newly created frames, i.e. the values these attributes have before applying the face spec in the face’s `defface` definition or the spec set by `face-spec-set`. These default values of the attributes are normally `unspecified`, unless you have specified some other value using `set-face-attribute`; see below.
Function: **merge-face-attribute** *attribute value1 value2*
If value1 is a relative value for the face attribute attribute, returns it merged with the underlying value value2; otherwise, if value1 is an absolute value for the face attribute attribute, returns value1 unchanged.
Normally, Emacs uses the face specs of each face to automatically calculate its attributes on each frame (see [Defining Faces](defining-faces)). The function `set-face-attribute` can override this calculation by directly assigning attributes to a face, either on a specific frame or for all frames. This function is mostly intended for internal usage.
Function: **set-face-attribute** *face frame &rest arguments*
This function sets one or more attributes of face for frame. The attributes specified in this way override the face spec(s) belonging to face. See [Face Attributes](face-attributes), for the supported attributes.
The extra arguments arguments specify the attributes to set, and the values for them. They should consist of alternating attribute names (such as `:family` or `:underline`) and values. Thus,
```
(set-face-attribute 'foo nil :weight 'bold :slant 'italic)
```
sets the attribute `:weight` to `bold` and the attribute `:slant` to `italic`.
If frame is `t`, this function sets the default attributes for newly created frames; they will effectively override the attribute values specified by `defface`. If frame is `nil`, this function sets the attributes for all existing frames, as well as for newly created frames. However, if you want to *reset* the value of an attribute to `unspecified` in a way that also affects newly created frames, you *must* explicitly call this function with frame set to `t` and the value of the attribute set to `unspecified` (*not* `nil`!), in addition to the call with frame set to `nil`. This is because the default attributes for newly created frames are merged with the face’s spec in `defface` when a new frame is created, and so having `unspecified` in the default attributes for new frames will be unable to override `defface`; the special call to this function as described above will arrange for `defface` to be overridden.
The following commands and functions mostly provide compatibility with old versions of Emacs. They work by calling `set-face-attribute`. Values of `t` and `nil` (or omitted) for their frame argument are handled just like `set-face-attribute` and `face-attribute`. The commands read their arguments using the minibuffer, if called interactively.
Command: **set-face-foreground** *face color &optional frame*
Command: **set-face-background** *face color &optional frame*
These set the `:foreground` attribute (or `:background` attribute, respectively) of face to color.
Command: **set-face-stipple** *face pattern &optional frame*
This sets the `:stipple` attribute of face to pattern.
Command: **set-face-font** *face font &optional frame*
Change the font-related attributes of face to those of font (a string or a font object). See [face-font-attribute](face-attributes#face_002dfont_002dattribute), for the supported formats of the font argument. This function sets the attribute `:font` of the face, and indirectly also the `:family`, `:foundry`, `:width`, `:height`, `:weight`, and `:slant` attributes, as defined by the font. If frame is non-`nil`, only change the attributes on the specified frame.
Function: **set-face-bold** *face bold-p &optional frame*
This sets the `:weight` attribute of face to normal if bold-p is `nil`, and to bold otherwise.
Function: **set-face-italic** *face italic-p &optional frame*
This sets the `:slant` attribute of face to normal if italic-p is `nil`, and to italic otherwise.
Command: **set-face-underline** *face underline &optional frame*
This sets the `:underline` attribute of face to underline.
Command: **set-face-inverse-video** *face inverse-video-p &optional frame*
This sets the `:inverse-video` attribute of face to inverse-video-p.
Command: **invert-face** *face &optional frame*
This swaps the foreground and background colors of face face.
Command: **set-face-extend** *face extend &optional frame*
This sets the `:extend` attribute of face to extend.
The following functions examine the attributes of a face. They mostly provide compatibility with old versions of Emacs. If you don’t specify frame, they refer to the selected frame; `t` refers to the default data for new frames. They return `unspecified` if the face doesn’t define any value for that attribute. If inherit is `nil`, only an attribute directly defined by the face is returned. If inherit is non-`nil`, any faces specified by its `:inherit` attribute are considered as well, and if inherit is a face or a list of faces, then they are also considered, until a specified attribute is found. To ensure that the return value is always specified, use a value of `default` for inherit.
Function: **face-font** *face &optional frame character*
This function returns the name of the font used by the specified face.
If the optional argument frame is specified, it returns the name of the font of face for that frame; frame defaults to the selected frame if it is `nil` or omitted. If frame is `t`, the function reports on the font defaults for face to be used for new frames.
By default, the returned font is for displaying ASCII characters, but if frame is anything but `t`, and the optional third argument character is supplied, the function returns the font name used by face for that character.
Function: **face-foreground** *face &optional frame inherit*
Function: **face-background** *face &optional frame inherit*
These functions return the foreground color (or background color, respectively) of face face, as a string. If the color is unspecified, they return `nil`.
Function: **face-stipple** *face &optional frame inherit*
This function returns the name of the background stipple pattern of face face, or `nil` if it doesn’t have one.
Function: **face-bold-p** *face &optional frame inherit*
This function returns a non-`nil` value if the `:weight` attribute of face is bolder than normal (i.e., one of `semi-bold`, `bold`, `extra-bold`, or `ultra-bold`). Otherwise, it returns `nil`.
Function: **face-italic-p** *face &optional frame inherit*
This function returns a non-`nil` value if the `:slant` attribute of face is `italic` or `oblique`, and `nil` otherwise.
Function: **face-underline-p** *face &optional frame inherit*
This function returns non-`nil` if face face specifies a non-`nil` `:underline` attribute.
Function: **face-inverse-video-p** *face &optional frame inherit*
This function returns non-`nil` if face face specifies a non-`nil` `:inverse-video` attribute.
Function: **face-extend-p** *face &optional frame inherit*
This function returns non-`nil` if face face specifies a non-`nil` `:extend` attribute. The inherit argument is passed to `face-attribute`.
elisp None ### Abstract Display
The Ewoc package constructs buffer text that represents a structure of Lisp objects, and updates the text to follow changes in that structure. This is like the “view” component in the “model–view–controller” design paradigm. Ewoc means “Emacs’s Widget for Object Collections”.
An *ewoc* is a structure that organizes information required to construct buffer text that represents certain Lisp data. The buffer text of the ewoc has three parts, in order: first, fixed *header* text; next, textual descriptions of a series of data elements (Lisp objects that you specify); and last, fixed *footer* text. Specifically, an ewoc contains information on:
* The buffer which its text is generated in.
* The text’s start position in the buffer.
* The header and footer strings.
* A doubly-linked chain of *nodes*, each of which contains:
+ A *data element*, a single Lisp object.
+ Links to the preceding and following nodes in the chain.
* A *pretty-printer* function which is responsible for inserting the textual representation of a data element value into the current buffer.
Typically, you define an ewoc with `ewoc-create`, and then pass the resulting ewoc structure to other functions in the Ewoc package to build nodes within it, and display it in the buffer. Once it is displayed in the buffer, other functions determine the correspondence between buffer positions and nodes, move point from one node’s textual representation to another, and so forth. See [Abstract Display Functions](abstract-display-functions).
A node *encapsulates* a data element much the way a variable holds a value. Normally, encapsulation occurs as a part of adding a node to the ewoc. You can retrieve the data element value and place a new value in its place, like so:
```
(ewoc-data node)
⇒ value
(ewoc-set-data node new-value)
⇒ new-value
```
You can also use, as the data element value, a Lisp object (list or vector) that is a container for the real value, or an index into some other structure. The example (see [Abstract Display Example](abstract-display-example)) uses the latter approach.
When the data changes, you will want to update the text in the buffer. You can update all nodes by calling `ewoc-refresh`, or just specific nodes using `ewoc-invalidate`, or all nodes satisfying a predicate using `ewoc-map`. Alternatively, you can delete invalid nodes using `ewoc-delete` or `ewoc-filter`, and add new nodes in their place. Deleting a node from an ewoc deletes its associated textual description from buffer, as well.
| | | |
| --- | --- | --- |
| • [Abstract Display Functions](abstract-display-functions) | | Functions in the Ewoc package. |
| • [Abstract Display Example](abstract-display-example) | | Example of using Ewoc. |
elisp None #### Glyphless Character Display
*Glyphless characters* are characters which are displayed in a special way, e.g., as a box containing a hexadecimal code, instead of being displayed literally. These include characters which are explicitly defined to be glyphless, as well as characters for which there is no available font (on a graphical display), and characters which cannot be encoded by the terminal’s coding system (on a text terminal).
Variable: **glyphless-char-display**
The value of this variable is a char-table which defines glyphless characters and how they are displayed. Each entry must be one of the following display methods:
`nil`
Display the character in the usual way.
`zero-width`
Don’t display the character.
`thin-space`
Display a thin space, 1-pixel wide on graphical displays, or 1-character wide on text terminals.
`empty-box`
Display an empty box.
`hex-code`
Display a box containing the Unicode codepoint of the character, in hexadecimal notation.
an ASCII string
Display a box containing that string. The string should contain at most 6 ASCII characters.
a cons cell `(graphical . text)`
Display with graphical on graphical displays, and with text on text terminals. Both graphical and text must be one of the display methods described above.
The `thin-space`, `empty-box`, `hex-code`, and ASCII string display methods are drawn with the `glyphless-char` face. On text terminals, a box is emulated by square brackets, ‘`[]`’.
The char-table has one extra slot, which determines how to display any character that cannot be displayed with any available font, or cannot be encoded by the terminal’s coding system. Its value should be one of the above display methods, except `zero-width` or a cons cell.
If a character has a non-`nil` entry in an active display table, the display table takes effect; in this case, Emacs does not consult `glyphless-char-display` at all.
User Option: **glyphless-char-display-control**
This user option provides a convenient way to set `glyphless-char-display` for groups of similar characters. Do not set its value directly from Lisp code; the value takes effect only via a custom `:set` function (see [Variable Definitions](variable-definitions)), which updates `glyphless-char-display`.
Its value should be an alist of elements `(group
. method)`, where group is a symbol specifying a group of characters, and method is a symbol specifying how to display them.
group should be one of the following:
`c0-control`
ASCII control characters `U+0000` to `U+001F`, excluding the newline and tab characters (normally displayed as escape sequences like ‘`^A`’; see [How Text Is Displayed](https://www.gnu.org/software/emacs/manual/html_node/emacs/Text-Display.html#Text-Display) in The GNU Emacs Manual).
`c1-control`
Non-ASCII, non-printing characters `U+0080` to `U+009F` (normally displayed as octal escape sequences like ‘`\230`’).
`format-control`
Characters of Unicode General Category [Cf], such as U+200E LEFT-TO-RIGHT MARK, but excluding characters that have graphic images, such as U+00AD SOFT HYPHEN.
`variation-selectors`
Unicode VS-1 through VS-16 (U+FE00 through U+FE0F), which are used to select between different glyphs for the same codepoints (typically emojis).
`no-font` Characters for which there is no suitable font, or which cannot be encoded by the terminal’s coding system, or those for which the text-mode terminal has no glyphs.
The method symbol should be one of `zero-width`, `thin-space`, `empty-box`, or `hex-code`. These have the same meanings as in `glyphless-char-display`, above.
elisp None #### Making Backup Files
Function: **backup-buffer**
This function makes a backup of the file visited by the current buffer, if appropriate. It is called by `save-buffer` before saving the buffer the first time.
If a backup was made by renaming, the return value is a cons cell of the form (modes extra-alist backupname), where modes are the mode bits of the original file, as returned by `file-modes` (see [Testing Accessibility](testing-accessibility)), extra-alist is an alist describing the original file’s extended attributes, as returned by `file-extended-attributes` (see [Extended Attributes](extended-attributes)), and backupname is the name of the backup.
In all other cases (i.e., if a backup was made by copying or if no backup was made), this function returns `nil`.
Variable: **buffer-backed-up**
This buffer-local variable says whether this buffer’s file has been backed up on account of this buffer. If it is non-`nil`, the backup file has been written. Otherwise, the file should be backed up when it is next saved (if backups are enabled). This is a permanent local; `kill-all-local-variables` does not alter it.
User Option: **make-backup-files**
This variable determines whether or not to make backup files. If it is non-`nil`, then Emacs creates a backup of each file when it is saved for the first time—provided that `backup-inhibited` is `nil` (see below).
The following example shows how to change the `make-backup-files` variable only in the Rmail buffers and not elsewhere. Setting it `nil` stops Emacs from making backups of these files, which may save disk space. (You would put this code in your init file.)
```
(add-hook 'rmail-mode-hook
(lambda () (setq-local make-backup-files nil)))
```
Variable: **backup-enable-predicate**
This variable’s value is a function to be called on certain occasions to decide whether a file should have backup files. The function receives one argument, an absolute file name to consider. If the function returns `nil`, backups are disabled for that file. Otherwise, the other variables in this section say whether and how to make backups.
The default value is `normal-backup-enable-predicate`, which checks for files in `temporary-file-directory` and `small-temporary-file-directory`.
Variable: **backup-inhibited**
If this variable is non-`nil`, backups are inhibited. It records the result of testing `backup-enable-predicate` on the visited file name. It can also coherently be used by other mechanisms that inhibit backups based on which file is visited. For example, VC sets this variable non-`nil` to prevent making backups for files managed with a version control system.
This is a permanent local, so that changing the major mode does not lose its value. Major modes should not set this variable—they should set `make-backup-files` instead.
User Option: **backup-directory-alist**
This variable’s value is an alist of filename patterns and backup directories. Each element looks like
```
(regexp . directory)
```
Backups of files with names matching regexp will be made in directory. directory may be relative or absolute. If it is absolute, so that all matching files are backed up into the same directory, the file names in this directory will be the full name of the file backed up with all directory separators changed to ‘`!`’ to prevent clashes. This will not work correctly if your filesystem truncates the resulting name.
For the common case of all backups going into one directory, the alist should contain a single element pairing ‘`"."`’ with the appropriate directory.
If this variable is `nil` (the default), or it fails to match a filename, the backup is made in the original file’s directory.
On MS-DOS filesystems without long names this variable is always ignored.
User Option: **make-backup-file-name-function**
This variable’s value is a function to use for making backup file names. The function `make-backup-file-name` calls it. See [Naming Backup Files](backup-names).
This could be buffer-local to do something special for specific files. If you change it, you may need to change `backup-file-name-p` and `file-name-sans-versions` too.
| programming_docs |
elisp None #### Examples of catch and throw
One way to use `catch` and `throw` is to exit from a doubly nested loop. (In most languages, this would be done with a `goto`.) Here we compute `(foo i j)` for i and j varying from 0 to 9:
```
(defun search-foo ()
(catch 'loop
(let ((i 0))
(while (< i 10)
(let ((j 0))
(while (< j 10)
(if (foo i j)
(throw 'loop (list i j)))
(setq j (1+ j))))
(setq i (1+ i))))))
```
If `foo` ever returns non-`nil`, we stop immediately and return a list of i and j. If `foo` always returns `nil`, the `catch` returns normally, and the value is `nil`, since that is the result of the `while`.
Here are two tricky examples, slightly different, showing two return points at once. First, two return points with the same tag, `hack`:
```
(defun catch2 (tag)
(catch tag
(throw 'hack 'yes)))
⇒ catch2
```
```
(catch 'hack
(print (catch2 'hack))
'no)
-| yes
⇒ no
```
Since both return points have tags that match the `throw`, it goes to the inner one, the one established in `catch2`. Therefore, `catch2` returns normally with value `yes`, and this value is printed. Finally the second body form in the outer `catch`, which is `'no`, is evaluated and returned from the outer `catch`.
Now let’s change the argument given to `catch2`:
```
(catch 'hack
(print (catch2 'quux))
'no)
⇒ yes
```
We still have two return points, but this time only the outer one has the tag `hack`; the inner one has the tag `quux` instead. Therefore, `throw` makes the outer `catch` return the value `yes`. The function `print` is never called, and the body-form `'no` is never evaluated.
elisp None ### A Simple Example of a Macro
Suppose we would like to define a Lisp construct to increment a variable value, much like the `++` operator in C. We would like to write `(inc x)` and have the effect of `(setq x (1+ x))`. Here’s a macro definition that does the job:
```
(defmacro inc (var)
(list 'setq var (list '1+ var)))
```
When this is called with `(inc x)`, the argument var is the symbol `x`—*not* the *value* of `x`, as it would be in a function. The body of the macro uses this to construct the expansion, which is `(setq x (1+ x))`. Once the macro definition returns this expansion, Lisp proceeds to evaluate it, thus incrementing `x`.
Function: **macrop** *object*
This predicate tests whether its argument is a macro, and returns `t` if so, `nil` otherwise.
elisp None ### Preserving Window Sizes
A window can get resized explicitly by using one of the functions from the preceding section or implicitly, for example, when resizing an adjacent window, when splitting or deleting a window (see [Splitting Windows](splitting-windows), see [Deleting Windows](deleting-windows)) or when resizing the window’s frame (see [Frame Size](frame-size)).
It is possible to avoid implicit resizing of a specific window when there are one or more other resizable windows on the same frame. For this purpose, Emacs must be advised to *preserve* the size of that window. There are two basic ways to do that.
Variable: **window-size-fixed**
If this buffer-local variable is non-`nil`, the size of any window displaying the buffer cannot normally be changed. Deleting a window or changing the frame’s size may still change the window’s size, if there is no choice.
If the value is `height`, then only the window’s height is fixed; if the value is `width`, then only the window’s width is fixed. Any other non-`nil` value fixes both the width and the height.
If this variable is `nil`, this does not necessarily mean that any window showing the buffer can be resized in the desired direction. To determine that, use the function `window-resizable`. See [Resizing Windows](resizing-windows).
Often `window-size-fixed` is overly aggressive because it inhibits any attempt to explicitly resize or split an affected window as well. This may even happen after the window has been resized implicitly, for example, when deleting an adjacent window or resizing the window’s frame. The following function tries hard to never disallow resizing such a window explicitly:
Function: **window-preserve-size** *&optional window horizontal preserve*
This function (un-)marks the height of window window as preserved for future resize operations. window must be a live window and defaults to the selected one. If the optional argument horizontal is non-`nil`, it (un-)marks the width of window as preserved.
If the optional argument preserve is `t`, this means to preserve the current height/width of window’s body. The height/width of window will change only if Emacs has no better choice. Resizing a window whose height/width is preserved by this function never throws an error.
If preserve is `nil`, this means to stop preserving the height/width of window, lifting any respective restraint induced by a previous call of this function for window. Calling `enlarge-window`, `shrink-window` or `fit-window-to-buffer` with window as argument may also remove the respective restraint.
`window-preserve-size` is currently invoked by the following functions:
`fit-window-to-buffer`
If the optional argument preserve-size of that function (see [Resizing Windows](resizing-windows)) is non-`nil`, the size established by that function is preserved.
`display-buffer` If the alist argument of that function (see [Choosing Window](choosing-window)) contains a `preserve-size` entry, the size of the window produced by that function is preserved.
`window-preserve-size` installs a window parameter (see [Window Parameters](window-parameters)) called `window-preserved-size` which is consulted by the window resizing functions. This parameter will not prevent resizing the window when the window shows another buffer than the one when `window-preserve-size` was invoked or if its size has changed since then.
The following function can be used to check whether the height of a particular window is preserved:
Function: **window-preserved-size** *&optional window horizontal*
This function returns the preserved height of window window in pixels. window must be a live window and defaults to the selected one. If the optional argument horizontal is non-`nil`, it returns the preserved width of window. It returns `nil` if the size of window is not preserved.
elisp None #### Miscellaneous Event Input Features
This section describes how to peek ahead at events without using them up, how to check for pending input, and how to discard pending input. See also the function `read-passwd` (see [Reading a Password](reading-a-password)).
Variable: **unread-command-events**
This variable holds a list of events waiting to be read as command input. The events are used in the order they appear in the list, and removed one by one as they are used.
The variable is needed because in some cases a function reads an event and then decides not to use it. Storing the event in this variable causes it to be processed normally, by the command loop or by the functions to read command input.
For example, the function that implements numeric prefix arguments reads any number of digits. When it finds a non-digit event, it must unread the event so that it can be read normally by the command loop. Likewise, incremental search uses this feature to unread events with no special meaning in a search, because these events should exit the search and then execute normally.
The reliable and easy way to extract events from a key sequence so as to put them in `unread-command-events` is to use `listify-key-sequence` (see below).
Normally you add events to the front of this list, so that the events most recently unread will be reread first.
Events read from this list are not normally added to the current command’s key sequence (as returned by, e.g., `this-command-keys`), as the events will already have been added once as they were read for the first time. An element of the form `(t . event)` forces event to be added to the current command’s key sequence.
Elements read from this list are normally recorded by the record-keeping features (see [Recording Input](recording-input)) and while defining a keyboard macro (see [Keyboard Macros](keyboard-macros)). However, an element of the form `(no-record . event)` causes event to be processed normally without recording it.
Function: **listify-key-sequence** *key*
This function converts the string or vector key to a list of individual events, which you can put in `unread-command-events`.
Function: **input-pending-p** *&optional check-timers*
This function determines whether any command input is currently available to be read. It returns immediately, with value `t` if there is available input, `nil` otherwise. On rare occasions it may return `t` when no input is available.
If the optional argument check-timers is non-`nil`, then if no input is available, Emacs runs any timers which are ready. See [Timers](timers).
Variable: **last-input-event**
This variable records the last terminal input event read, whether as part of a command or explicitly by a Lisp program.
In the example below, the Lisp program reads the character `1`, ASCII code 49. It becomes the value of `last-input-event`, while `C-e` (we assume `C-x C-e` command is used to evaluate this expression) remains the value of `last-command-event`.
```
(progn (print (read-char))
(print last-command-event)
last-input-event)
-| 49
-| 5
⇒ 49
```
Macro: **while-no-input** *body…*
This construct runs the body forms and returns the value of the last one—but only if no input arrives. If any input arrives during the execution of the body forms, it aborts them (working much like a quit). The `while-no-input` form returns `nil` if aborted by a real quit, and returns `t` if aborted by arrival of other input.
If a part of body binds `inhibit-quit` to non-`nil`, arrival of input during those parts won’t cause an abort until the end of that part.
If you want to be able to distinguish all possible values computed by body from both kinds of abort conditions, write the code like this:
```
(while-no-input
(list
(progn . body)))
```
Variable: **while-no-input-ignore-events**
This variable allow setting which special events `while-no-input` should ignore. It is a list of event symbols (see [Event Examples](event-examples)).
Function: **discard-input**
This function discards the contents of the terminal input buffer and cancels any keyboard macro that might be in the process of definition. It returns `nil`.
In the following example, the user may type a number of characters right after starting the evaluation of the form. After the `sleep-for` finishes sleeping, `discard-input` discards any characters typed during the sleep.
```
(progn (sleep-for 2)
(discard-input))
⇒ nil
```
elisp None #### Drag Events
With Emacs, you can have a drag event without even changing your clothes. A *drag event* happens every time the user presses a mouse button and then moves the mouse to a different character position before releasing the button. Like all mouse events, drag events are represented in Lisp as lists. The lists record both the starting mouse position and the final position, like this:
```
(event-type
(window1 START-POSITION)
(window2 END-POSITION))
```
For a drag event, the name of the symbol event-type contains the prefix ‘`drag-`’. For example, dragging the mouse with button 2 held down generates a `drag-mouse-2` event. The second and third elements of the event give the starting and ending position of the drag, as mouse position lists (see [Click Events](click-events)). You can access the second element of any mouse event in the same way. However, the drag event may end outside the boundaries of the frame that was initially selected. In that case, the third element’s position list contains that frame in place of a window.
The ‘`drag-`’ prefix follows the modifier key prefixes such as ‘`C-`’ and ‘`M-`’.
If `read-key-sequence` receives a drag event that has no key binding, and the corresponding click event does have a binding, it changes the drag event into a click event at the drag’s starting position. This means that you don’t have to distinguish between click and drag events unless you want to.
elisp None ### Version Information
These facilities provide information about which version of Emacs is in use.
Command: **emacs-version** *&optional here*
This function returns a string describing the version of Emacs that is running. It is useful to include this string in bug reports.
```
(emacs-version)
⇒ "GNU Emacs 26.1 (build 1, x86_64-unknown-linux-gnu,
GTK+ Version 3.16) of 2017-06-01"
```
If here is non-`nil`, it inserts the text in the buffer before point, and returns `nil`. When this function is called interactively, it prints the same information in the echo area, but giving a prefix argument makes here non-`nil`.
Variable: **emacs-build-time**
The value of this variable indicates the time at which Emacs was built. It uses the style of `current-time` (see [Time of Day](time-of-day)), or is `nil` if the information is not available.
```
emacs-build-time
⇒ (20614 63694 515336 438000)
```
Variable: **emacs-version**
The value of this variable is the version of Emacs being run. It is a string such as `"26.1"`. A value with three numeric components, such as `"26.0.91"`, indicates an unreleased test version. (Prior to Emacs 26.1, the string includes an extra final component with the integer that is now stored in `emacs-build-number`; e.g., `"25.1.1"`.)
Variable: **emacs-major-version**
The major version number of Emacs, as an integer. For Emacs version 23.1, the value is 23.
Variable: **emacs-minor-version**
The minor version number of Emacs, as an integer. For Emacs version 23.1, the value is 1.
Variable: **emacs-build-number**
An integer that increments each time Emacs is built in the same directory (without cleaning). This is only of relevance when developing Emacs.
Variable: **emacs-repository-version**
A string that gives the repository revision from which Emacs was built. If Emacs was built outside revision control, the value is `nil`.
Variable: **emacs-repository-branch**
A string that gives the repository branch from which Emacs was built. In the most cases this is `"master"`. If Emacs was built outside revision control, the value is `nil`.
elisp None ### Accessing Function Cell Contents
The *function definition* of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.
See also the function `indirect-function`. See [Definition of indirect-function](function-indirection#Definition-of-indirect_002dfunction).
Function: **symbol-function** *symbol*
This returns the object in the function cell of symbol. It does not check that the returned object is a legitimate function.
If the function cell is void, the return value is `nil`. To distinguish between a function cell that is void and one set to `nil`, use `fboundp` (see below).
```
(defun bar (n) (+ n 2))
(symbol-function 'bar)
⇒ (lambda (n) (+ n 2))
```
```
(fset 'baz 'bar)
⇒ bar
```
```
(symbol-function 'baz)
⇒ bar
```
If you have never given a symbol any function definition, we say that that symbol’s function cell is *void*. In other words, the function cell does not have any Lisp object in it. If you try to call the symbol as a function, Emacs signals a `void-function` error.
Note that void is not the same as `nil` or the symbol `void`. The symbols `nil` and `void` are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with `defun`). A void function cell contains no object whatsoever.
You can test the voidness of a symbol’s function definition with `fboundp`. After you have given a symbol a function definition, you can make it void once more using `fmakunbound`.
Function: **fboundp** *symbol*
This function returns `t` if the symbol has an object in its function cell, `nil` otherwise. It does not check that the object is a legitimate function.
Function: **fmakunbound** *symbol*
This function makes symbol’s function cell void, so that a subsequent attempt to access this cell will cause a `void-function` error. It returns symbol. (See also `makunbound`, in [Void Variables](void-variables).)
```
(defun foo (x) x)
(foo 1)
⇒1
```
```
(fmakunbound 'foo)
⇒ foo
```
```
(foo 1)
error→ Symbol's function definition is void: foo
```
Function: **fset** *symbol definition*
This function stores definition in the function cell of symbol. The result is definition. Normally definition should be a function or the name of a function, but this is not checked. The argument symbol is an ordinary evaluated argument.
The primary use of this function is as a subroutine by constructs that define or alter functions, like `defun` or `advice-add` (see [Advising Functions](advising-functions)). You can also use it to give a symbol a function definition that is not a function, e.g., a keyboard macro (see [Keyboard Macros](keyboard-macros)):
```
;; Define a named keyboard macro.
(fset 'kill-two-lines "\^u2\^k")
⇒ "\^u2\^k"
```
If you wish to use `fset` to make an alternate name for a function, consider using `defalias` instead. See [Definition of defalias](defining-functions#Definition-of-defalias).
elisp None #### Alias Menu Items
Sometimes it is useful to make menu items that use the same command but with different enable conditions. The best way to do this in Emacs now is with extended menu items; before that feature existed, it could be done by defining alias commands and using them in menu items. Here’s an example that makes two aliases for `read-only-mode` and gives them different enable conditions:
```
(defalias 'make-read-only 'read-only-mode)
(put 'make-read-only 'menu-enable '(not buffer-read-only))
(defalias 'make-writable 'read-only-mode)
(put 'make-writable 'menu-enable 'buffer-read-only)
```
When using aliases in menus, often it is useful to display the equivalent key bindings for the real command name, not the aliases (which typically don’t have any key bindings except for the menu itself). To request this, give the alias symbol a non-`nil` `menu-alias` property. Thus,
```
(put 'make-read-only 'menu-alias t)
(put 'make-writable 'menu-alias t)
```
causes menu items for `make-read-only` and `make-writable` to show the keyboard bindings for `read-only-mode`.
elisp None #### Excess Close Parentheses
To deal with an excess close parenthesis, first go to the beginning of the file, then type `C-u -1 C-M-u` (`backward-up-list` with an argument of -1) to find the end of the first unbalanced defun.
Then find the actual matching close parenthesis by typing `C-M-f` (`forward-sexp`, see [Expressions](https://www.gnu.org/software/emacs/manual/html_node/emacs/Expressions.html#Expressions) in The GNU Emacs Manual) at the beginning of that defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity.
If you don’t see a problem at that point, the next thing to do is to type `C-M-q` (`indent-pp-sexp`) at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don’t assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the `C-M-q` with `C-\_` (`undo`), since the old indentation is probably appropriate to the intended parentheses.
After you think you have fixed the problem, use `C-M-q` again. If the old indentation actually fits the intended nesting of parentheses, and you have put back those parentheses, `C-M-q` should not change anything.
| programming_docs |
elisp None ### Defining Symbols
A *definition* is a special kind of Lisp expression that announces your intention to use a symbol in a particular way. It typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable.
`defvar` and `defconst` are special forms that define a symbol as a *global variable*—a variable that can be accessed at any point in a Lisp program. See [Variables](variables), for details about variables. To define a customizable variable, use the `defcustom` macro, which also calls `defvar` as a subroutine (see [Customization](customization)).
In principle, you can assign a variable value to any symbol with `setq`, whether or not it has first been defined as a variable. However, you ought to write a variable definition for each global variable that you want to use; otherwise, your Lisp program may not act correctly if it is evaluated with lexical scoping enabled (see [Variable Scoping](variable-scoping)).
`defun` defines a symbol as a function, creating a lambda expression and storing it in the function cell of the symbol. This lambda expression thus becomes the function definition of the symbol. (The term “function definition”, meaning the contents of the function cell, is derived from the idea that `defun` gives the symbol its definition as a function.) `defsubst` and `defalias` are two other ways of defining a function. See [Functions](functions).
`defmacro` defines a symbol as a macro. It creates a macro object and stores it in the function cell of the symbol. Note that a given symbol can be a macro or a function, but not both at once, because both macro and function definitions are kept in the function cell, and that cell can hold only one Lisp object at any given time. See [Macros](macros).
As previously noted, Emacs Lisp allows the same symbol to be defined both as a variable (e.g., with `defvar`) and as a function or macro (e.g., with `defun`). Such definitions do not conflict.
These definitions also act as guides for programming tools. For example, the `C-h f` and `C-h v` commands create help buffers containing links to the relevant variable, function, or macro definitions. See [Name Help](https://www.gnu.org/software/emacs/manual/html_node/emacs/Name-Help.html#Name-Help) in The GNU Emacs Manual.
elisp None #### Adapting code using the old defadvice
A lot of code uses the old `defadvice` mechanism, which is largely made obsolete by the new `advice-add`, whose implementation and semantics is significantly simpler.
An old piece of advice such as:
```
(defadvice previous-line (before next-line-at-end
(&optional arg try-vscroll))
"Insert an empty line when moving up from the top line."
(if (and next-line-add-newlines (= arg 1)
(save-excursion (beginning-of-line) (bobp)))
(progn
(beginning-of-line)
(newline))))
```
could be translated in the new advice mechanism into a plain function:
```
(defun previous-line--next-line-at-end (&optional arg try-vscroll)
"Insert an empty line when moving up from the top line."
(if (and next-line-add-newlines (= arg 1)
(save-excursion (beginning-of-line) (bobp)))
(progn
(beginning-of-line)
(newline))))
```
Obviously, this does not actually modify `previous-line`. For that the old advice needed:
```
(ad-activate 'previous-line)
```
whereas the new advice mechanism needs:
```
(advice-add 'previous-line :before #'previous-line--next-line-at-end)
```
Note that `ad-activate` had a global effect: it activated all pieces of advice enabled for that specified function. If you wanted to only activate or deactivate a particular piece, you needed to *enable* or *disable* it with `ad-enable-advice` and `ad-disable-advice`. The new mechanism does away with this distinction.
Around advice such as:
```
(defadvice foo (around foo-around)
"Ignore case in `foo'."
(let ((case-fold-search t))
ad-do-it))
(ad-activate 'foo)
```
could translate into:
```
(defun foo--foo-around (orig-fun &rest args)
"Ignore case in `foo'."
(let ((case-fold-search t))
(apply orig-fun args)))
(advice-add 'foo :around #'foo--foo-around)
```
Regarding the advice’s *class*, note that the new `:before` is not quite equivalent to the old `before`, because in the old advice you could modify the function’s arguments (e.g., with `ad-set-arg`), and that would affect the argument values seen by the original function, whereas in the new `:before`, modifying an argument via `setq` in the advice has no effect on the arguments seen by the original function. When porting `before` advice which relied on this behavior, you’ll need to turn it into new `:around` or `:filter-args` advice instead.
Similarly old `after` advice could modify the returned value by changing `ad-return-value`, whereas new `:after` advice cannot, so when porting such old `after` advice, you’ll need to turn it into new `:around` or `:filter-return` advice instead.
elisp None ### Custom Themes
*Custom themes* are collections of settings that can be enabled or disabled as a unit. See [Custom Themes](https://www.gnu.org/software/emacs/manual/html_node/emacs/Custom-Themes.html#Custom-Themes) in The GNU Emacs Manual. Each Custom theme is defined by an Emacs Lisp source file, which should follow the conventions described in this section. (Instead of writing a Custom theme by hand, you can also create one using a Customize-like interface; see [Creating Custom Themes](https://www.gnu.org/software/emacs/manual/html_node/emacs/Creating-Custom-Themes.html#Creating-Custom-Themes) in The GNU Emacs Manual.)
A Custom theme file should be named `foo-theme.el`, where foo is the theme name. The first Lisp form in the file should be a call to `deftheme`, and the last form should be a call to `provide-theme`.
Macro: **deftheme** *theme &optional doc*
This macro declares theme (a symbol) as the name of a Custom theme. The optional argument doc should be a string describing the theme; this is the description shown when the user invokes the `describe-theme` command or types `?` in the ‘`\*Custom Themes\*`’ buffer.
Two special theme names are disallowed (using them causes an error): `user` is a dummy theme that stores the user’s direct customization settings, and `changed` is a dummy theme that stores changes made outside of the Customize system.
Macro: **provide-theme** *theme*
This macro declares that the theme named theme has been fully specified.
In between `deftheme` and `provide-theme` are Lisp forms specifying the theme settings: usually a call to `custom-theme-set-variables` and/or a call to `custom-theme-set-faces`.
Function: **custom-theme-set-variables** *theme &rest args*
This function specifies the Custom theme theme’s variable settings. theme should be a symbol. Each argument in args should be a list of the form
```
(var expression [now [request [comment]]])
```
where the list entries have the same meanings as in `custom-set-variables`. See [Applying Customizations](applying-customizations).
Function: **custom-theme-set-faces** *theme &rest args*
This function specifies the Custom theme theme’s face settings. theme should be a symbol. Each argument in args should be a list of the form
```
(face spec [now [comment]])
```
where the list entries have the same meanings as in `custom-set-faces`. See [Applying Customizations](applying-customizations).
In theory, a theme file can also contain other Lisp forms, which would be evaluated when loading the theme, but that is bad form. To protect against loading themes containing malicious code, Emacs displays the source file and asks for confirmation from the user before loading any non-built-in theme for the first time. As such, themes are not ordinarily byte-compiled, and source files usually take precedence when Emacs is looking for a theme to load.
The following functions are useful for programmatically enabling and disabling themes:
Function: **custom-theme-p** *theme*
This function return a non-`nil` value if theme (a symbol) is the name of a Custom theme (i.e., a Custom theme which has been loaded into Emacs, whether or not the theme is enabled). Otherwise, it returns `nil`.
Variable: **custom-known-themes**
The value of this variable is a list of themes loaded into Emacs. Each theme is represented by a Lisp symbol (the theme name). The default value of this variable is a list containing two dummy themes: `(user changed)`. The `changed` theme stores settings made before any Custom themes are applied (e.g., variables set outside of Customize). The `user` theme stores settings the user has customized and saved. Any additional themes declared with the `deftheme` macro are added to the front of this list.
Command: **load-theme** *theme &optional no-confirm no-enable*
This function loads the Custom theme named theme from its source file, looking for the source file in the directories specified by the variable `custom-theme-load-path`. See [Custom Themes](https://www.gnu.org/software/emacs/manual/html_node/emacs/Custom-Themes.html#Custom-Themes) in The GNU Emacs Manual. It also *enables* the theme (unless the optional argument no-enable is non-`nil`), causing its variable and face settings to take effect. It prompts the user for confirmation before loading the theme, unless the optional argument no-confirm is non-`nil`.
Function: **require-theme** *feature &optional noerror*
This function searches `custom-theme-load-path` for a file that provides feature and then loads it. This is like the function `require` (see [Named Features](named-features)), except it searches `custom-theme-load-path` instead of `load-path` (see [Library Search](library-search)). This can be useful in Custom themes that need to load supporting Lisp files when `require` is unsuitable for that.
If feature, which should be a symbol, is not already present in the current Emacs session according to `featurep`, then `require-theme` searches for a file named feature with an added ‘`.elc`’ or ‘`.el`’ suffix, in that order, in the directories specified by `custom-theme-load-path`.
If a file providing feature is successfully found and loaded, then `require-theme` returns feature. The optional argument noerror determines what happens if the search or loading fails. If it is `nil`, the function signals an error; otherwise, it returns `nil`. If the file loads successfully but does not provide feature, then `require-theme` signals an error; this cannot be suppressed.
Command: **enable-theme** *theme*
This function enables the Custom theme named theme. It signals an error if no such theme has been loaded.
Command: **disable-theme** *theme*
This function disables the Custom theme named theme. The theme remains loaded, so that a subsequent call to `enable-theme` will re-enable it.
elisp None #### The Default Value of a Buffer-Local Variable
The global value of a variable with buffer-local bindings is also called the *default* value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable.
The functions `default-value` and `setq-default` access and change a variable’s default value regardless of whether the current buffer has a buffer-local binding. For example, you could use `setq-default` to change the default setting of `paragraph-start` for most buffers; and this would work even when you are in a C or Lisp mode buffer that has a buffer-local value for this variable.
The special forms `defvar` and `defconst` also set the default value (if they set the variable at all), rather than any buffer-local value.
Function: **default-value** *symbol*
This function returns symbol’s default value. This is the value that is seen in buffers and frames that do not have their own values for this variable. If symbol is not buffer-local, this is equivalent to `symbol-value` (see [Accessing Variables](accessing-variables)).
Function: **default-boundp** *symbol*
The function `default-boundp` tells you whether symbol’s default value is nonvoid. If `(default-boundp 'foo)` returns `nil`, then `(default-value 'foo)` would get an error.
`default-boundp` is to `default-value` as `boundp` is to `symbol-value`.
Special Form: **setq-default** *[symbol form]…*
This special form gives each symbol a new default value, which is the result of evaluating the corresponding form. It does not evaluate symbol, but does evaluate form. The value of the `setq-default` form is the value of the last form.
If a symbol is not buffer-local for the current buffer, and is not marked automatically buffer-local, `setq-default` has the same effect as `setq`. If symbol is buffer-local for the current buffer, then this changes the value that other buffers will see (as long as they don’t have a buffer-local value), but not the value that the current buffer sees.
```
;; In buffer ‘`foo`’:
(make-local-variable 'buffer-local)
⇒ buffer-local
```
```
(setq buffer-local 'value-in-foo)
⇒ value-in-foo
```
```
(setq-default buffer-local 'new-default)
⇒ new-default
```
```
buffer-local
⇒ value-in-foo
```
```
(default-value 'buffer-local)
⇒ new-default
```
```
;; In (the new) buffer ‘`bar`’:
buffer-local
⇒ new-default
```
```
(default-value 'buffer-local)
⇒ new-default
```
```
(setq buffer-local 'another-default)
⇒ another-default
```
```
(default-value 'buffer-local)
⇒ another-default
```
```
;; Back in buffer ‘`foo`’:
buffer-local
⇒ value-in-foo
(default-value 'buffer-local)
⇒ another-default
```
Function: **set-default** *symbol value*
This function is like `setq-default`, except that symbol is an ordinary evaluated argument.
```
(set-default (car '(a b c)) 23)
⇒ 23
```
```
(default-value 'a)
⇒ 23
```
A variable can be let-bound (see [Local Variables](local-variables)) to a value. This makes its global value shadowed by the binding; `default-value` will then return the value from that binding, not the global value, and `set-default` will be prevented from setting the global value (it will change the let-bound value instead). The following two functions allow to reference the global value even if it’s shadowed by a let-binding.
Function: **default-toplevel-value** *symbol*
This function returns the *top-level* default value of symbol, which is its value outside of any let-binding.
```
(defvar variable 'global-value)
⇒ variable
```
```
(let ((variable 'let-binding))
(default-value 'variable))
⇒ let-binding
```
```
(let ((variable 'let-binding))
(default-toplevel-value 'variable))
⇒ global-value
```
Function: **set-default-toplevel-value** *symbol value*
This function sets the top-level default value of symbol to the specified value. This comes in handy when you want to set the global value of symbol regardless of whether your code runs in the context of symbol’s let-binding.
elisp None ### The Buffer List
The *buffer list* is a list of all live buffers. The order of the buffers in this list is based primarily on how recently each buffer has been displayed in a window. Several functions, notably `other-buffer`, use this ordering. A buffer list displayed for the user also follows this order.
Creating a buffer adds it to the end of the buffer list, and killing a buffer removes it from that list. A buffer moves to the front of this list whenever it is chosen for display in a window (see [Switching Buffers](switching-buffers)) or a window displaying it is selected (see [Selecting Windows](selecting-windows)). A buffer moves to the end of the list when it is buried (see `bury-buffer`, below). There are no functions available to the Lisp programmer which directly manipulate the buffer list.
In addition to the fundamental buffer list just described, Emacs maintains a local buffer list for each frame, in which the buffers that have been displayed (or had their windows selected) in that frame come first. (This order is recorded in the frame’s `buffer-list` frame parameter; see [Buffer Parameters](buffer-parameters).) Buffers never displayed in that frame come afterward, ordered according to the fundamental buffer list.
Function: **buffer-list** *&optional frame*
This function returns the buffer list, including all buffers, even those whose names begin with a space. The elements are actual buffers, not their names.
If frame is a frame, this returns frame’s local buffer list. If frame is `nil` or omitted, the fundamental buffer list is used: the buffers appear in order of most recent display or selection, regardless of which frames they were displayed on.
```
(buffer-list)
⇒ (#<buffer buffers.texi>
#<buffer *Minibuf-1*> #<buffer buffer.c>
#<buffer *Help*> #<buffer TAGS>)
```
```
;; Note that the name of the minibuffer
;; begins with a space!
(mapcar #'buffer-name (buffer-list))
⇒ ("buffers.texi" " *Minibuf-1*"
"buffer.c" "*Help*" "TAGS")
```
The list returned by `buffer-list` is constructed specifically; it is not an internal Emacs data structure, and modifying it has no effect on the order of buffers. If you want to change the order of buffers in the fundamental buffer list, here is an easy way:
```
(defun reorder-buffer-list (new-list)
(while new-list
(bury-buffer (car new-list))
(setq new-list (cdr new-list))))
```
With this method, you can specify any order for the list, but there is no danger of losing a buffer or adding something that is not a valid live buffer.
To change the order or value of a specific frame’s buffer list, set that frame’s `buffer-list` parameter with `modify-frame-parameters` (see [Parameter Access](parameter-access)).
Function: **other-buffer** *&optional buffer visible-ok frame*
This function returns the first buffer in the buffer list other than buffer. Usually, this is the buffer appearing in the most recently selected window (in frame frame or else the selected frame, see [Input Focus](input-focus)), aside from buffer. Buffers whose names start with a space are not considered at all.
If buffer is not supplied (or if it is not a live buffer), then `other-buffer` returns the first buffer in the selected frame’s local buffer list. (If frame is non-`nil`, it returns the first buffer in frame’s local buffer list instead.)
If frame has a non-`nil` `buffer-predicate` parameter, then `other-buffer` uses that predicate to decide which buffers to consider. It calls the predicate once for each buffer, and if the value is `nil`, that buffer is ignored. See [Buffer Parameters](buffer-parameters).
If visible-ok is `nil`, `other-buffer` avoids returning a buffer visible in any window on any visible frame, except as a last resort. If visible-ok is non-`nil`, then it does not matter whether a buffer is displayed somewhere or not.
If no suitable buffer exists, the buffer `\*scratch\*` is returned (and created, if necessary).
Function: **last-buffer** *&optional buffer visible-ok frame*
This function returns the last buffer in frame’s buffer list other than buffer. If frame is omitted or `nil`, it uses the selected frame’s buffer list.
The argument visible-ok is handled as with `other-buffer`, see above. If no suitable buffer can be found, the buffer `\*scratch\*` is returned.
Command: **bury-buffer** *&optional buffer-or-name*
This command puts buffer-or-name at the end of the buffer list, without changing the order of any of the other buffers on the list. This buffer therefore becomes the least desirable candidate for `other-buffer` to return. The argument can be either a buffer itself or the name of one.
This function operates on each frame’s `buffer-list` parameter as well as the fundamental buffer list; therefore, the buffer that you bury will come last in the value of `(buffer-list frame)` and in the value of `(buffer-list)`. In addition, it also puts the buffer at the end of the list of buffers of the selected window (see [Window History](window-history)) provided it is shown in that window.
If buffer-or-name is `nil` or omitted, this means to bury the current buffer. In addition, if the current buffer is displayed in the selected window (see [Selecting Windows](selecting-windows)), this makes sure that the window is either deleted or another buffer is shown in it. More precisely, if the selected window is dedicated (see [Dedicated Windows](dedicated-windows)) and there are other windows on its frame, the window is deleted. If it is the only window on its frame and that frame is not the only frame on its terminal, the frame is dismissed by calling the function specified by `frame-auto-hide-function` (see [Quitting Windows](quitting-windows)). Otherwise, it calls `switch-to-prev-buffer` (see [Window History](window-history)) to show another buffer in that window. If buffer-or-name is displayed in some other window, it remains displayed there.
To replace a buffer in all the windows that display it, use `replace-buffer-in-windows`, See [Buffers and Windows](buffers-and-windows).
Command: **unbury-buffer**
This command switches to the last buffer in the local buffer list of the selected frame. More precisely, it calls the function `switch-to-buffer` (see [Switching Buffers](switching-buffers)), to display the buffer returned by `last-buffer` (see above), in the selected window.
Variable: **buffer-list-update-hook**
This is a normal hook run whenever the buffer list changes. Functions (implicitly) running this hook are `get-buffer-create` (see [Creating Buffers](creating-buffers)), `rename-buffer` (see [Buffer Names](buffer-names)), `kill-buffer` (see [Killing Buffers](killing-buffers)), `bury-buffer` (see above), and `select-window` (see [Selecting Windows](selecting-windows)). This hook is not run for internal or temporary buffers created by `get-buffer-create` or `generate-new-buffer` with a non-`nil` argument inhibit-buffer-hooks.
Functions run by this hook should avoid calling `select-window` with a `nil` norecord argument since this may lead to infinite recursion.
| programming_docs |
elisp None #### Moving over Balanced Expressions
Here are several functions concerned with balanced-parenthesis expressions (also called *sexps* in connection with moving across them in Emacs). The syntax table controls how these functions interpret various characters; see [Syntax Tables](syntax-tables). See [Parsing Expressions](parsing-expressions), for lower-level primitives for scanning sexps or parts of sexps. For user-level commands, see [Commands for Editing with Parentheses](https://www.gnu.org/software/emacs/manual/html_node/emacs/Parentheses.html#Parentheses) in The GNU Emacs Manual.
Command: **forward-list** *&optional arg*
This function moves forward across arg (default 1) balanced groups of parentheses. (Other syntactic entities such as words or paired string quotes are ignored.)
Command: **backward-list** *&optional arg*
This function moves backward across arg (default 1) balanced groups of parentheses. (Other syntactic entities such as words or paired string quotes are ignored.)
Command: **up-list** *&optional arg escape-strings no-syntax-crossing*
This function moves forward out of arg (default 1) levels of parentheses. A negative argument means move backward but still to a less deep spot. If escape-strings is non-`nil` (as it is interactively), move out of enclosing strings as well. If no-syntax-crossing is non-`nil` (as it is interactively), prefer to break out of any enclosing string instead of moving to the start of a list broken across multiple strings. On error, location of point is unspecified.
Command: **backward-up-list** *&optional arg escape-strings no-syntax-crossing*
This function is just like `up-list`, but with a negated argument.
Command: **down-list** *&optional arg*
This function moves forward into arg (default 1) levels of parentheses. A negative argument means move backward but still go deeper in parentheses (-arg levels).
Command: **forward-sexp** *&optional arg*
This function moves forward across arg (default 1) balanced expressions. Balanced expressions include both those delimited by parentheses and other kinds, such as words and string constants. See [Parsing Expressions](parsing-expressions). For example,
```
---------- Buffer: foo ----------
(concat∗ "foo " (car x) y z)
---------- Buffer: foo ----------
```
```
(forward-sexp 3)
⇒ nil
---------- Buffer: foo ----------
(concat "foo " (car x) y∗ z)
---------- Buffer: foo ----------
```
Command: **backward-sexp** *&optional arg*
This function moves backward across arg (default 1) balanced expressions.
Command: **beginning-of-defun** *&optional arg*
This function moves back to the argth beginning of a defun. If arg is negative, this actually moves forward, but it still moves to the beginning of a defun, not to the end of one. arg defaults to 1.
Command: **end-of-defun** *&optional arg*
This function moves forward to the argth end of a defun. If arg is negative, this actually moves backward, but it still moves to the end of a defun, not to the beginning of one. arg defaults to 1.
User Option: **defun-prompt-regexp**
If non-`nil`, this buffer-local variable holds a regular expression that specifies what text can appear before the open-parenthesis that starts a defun. That is to say, a defun begins on a line that starts with a match for this regular expression, followed by a character with open-parenthesis syntax.
User Option: **open-paren-in-column-0-is-defun-start**
If this variable’s value is non-`nil`, an open parenthesis in column 0 is considered to be the start of a defun. If it is `nil`, an open parenthesis in column 0 has no special meaning. The default is `t`. If a string literal happens to have a parenthesis in column 0, escape it with a backslash to avoid a false positive.
Variable: **beginning-of-defun-function**
If non-`nil`, this variable holds a function for finding the beginning of a defun. The function `beginning-of-defun` calls this function instead of using its normal method, passing it its optional argument. If the argument is non-`nil`, the function should move back by that many functions, like `beginning-of-defun` does.
Variable: **end-of-defun-function**
If non-`nil`, this variable holds a function for finding the end of a defun. The function `end-of-defun` calls this function instead of using its normal method.
elisp None #### Cleaning Up from Nonlocal Exits
The `unwind-protect` construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to make the data consistent again in the event of an error or throw. (Another more specific cleanup construct that is used only for changes in buffer contents is the atomic change group; [Atomic Changes](atomic-changes).)
Special Form: **unwind-protect** *body-form cleanup-forms…*
`unwind-protect` executes body-form with a guarantee that the cleanup-forms will be evaluated if control leaves body-form, no matter how that happens. body-form may complete normally, or execute a `throw` out of the `unwind-protect`, or cause an error; in all cases, the cleanup-forms will be evaluated.
If body-form finishes normally, `unwind-protect` returns the value of body-form, after it evaluates the cleanup-forms. If body-form does not finish, `unwind-protect` does not return any value in the normal sense.
Only body-form is protected by the `unwind-protect`. If any of the cleanup-forms themselves exits nonlocally (via a `throw` or an error), `unwind-protect` is *not* guaranteed to evaluate the rest of them. If the failure of one of the cleanup-forms has the potential to cause trouble, then protect it with another `unwind-protect` around that form.
The number of currently active `unwind-protect` forms counts, together with the number of local variable bindings, against the limit `max-specpdl-size` (see [Local Variables](local-variables#Definition-of-max_002dspecpdl_002dsize)).
For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:
```
(let ((buffer (get-buffer-create " *temp*")))
(with-current-buffer buffer
(unwind-protect
body-form
(kill-buffer buffer))))
```
You might think that we could just as well write `(kill-buffer
(current-buffer))` and dispense with the variable `buffer`. However, the way shown above is safer, if body-form happens to get an error after switching to a different buffer! (Alternatively, you could write a `save-current-buffer` around body-form, to ensure that the temporary buffer becomes current again in time to kill it.)
Emacs includes a standard macro called `with-temp-buffer` which expands into more or less the code shown above (see [Current Buffer](current-buffer#Definition-of-with_002dtemp_002dbuffer)). Several of the macros defined in this manual use `unwind-protect` in this way.
Here is an actual example derived from an FTP package. It creates a process (see [Processes](processes)) to try to establish a connection to a remote machine. As the function `ftp-login` is highly susceptible to numerous problems that the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, Emacs might fill up with useless subprocesses.
```
(let ((win nil))
(unwind-protect
(progn
(setq process (ftp-setup-buffer host file))
(if (setq win (ftp-login process host user password))
(message "Logged in")
(error "Ftp login failed")))
(or win (and process (delete-process process)))))
```
This example has a small bug: if the user types `C-g` to quit, and the quit happens immediately after the function `ftp-setup-buffer` returns but before the variable `process` is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely.
---
elisp None #### Reporting Operation Progress
When an operation can take a while to finish, you should inform the user about the progress it makes. This way the user can estimate remaining time and clearly see that Emacs is busy working, not hung. A convenient way to do this is to use a *progress reporter*.
Here is a working example that does nothing useful:
```
(let ((progress-reporter
(make-progress-reporter "Collecting mana for Emacs..."
0 500)))
(dotimes (k 500)
(sit-for 0.01)
(progress-reporter-update progress-reporter k))
(progress-reporter-done progress-reporter))
```
Function: **make-progress-reporter** *message &optional min-value max-value current-value min-change min-time*
This function creates and returns a progress reporter object, which you will use as an argument for the other functions listed below. The idea is to precompute as much data as possible to make progress reporting very fast.
When this progress reporter is subsequently used, it will display message in the echo area, followed by progress percentage. message is treated as a simple string. If you need it to depend on a filename, for instance, use `format-message` before calling this function.
The arguments min-value and max-value should be numbers standing for the starting and final states of the operation. For instance, an operation that scans a buffer should set these to the results of `point-min` and `point-max` correspondingly. max-value should be greater than min-value.
Alternatively, you can set min-value and max-value to `nil`. In that case, the progress reporter does not report process percentages; it instead displays a “spinner” that rotates a notch each time you update the progress reporter.
If min-value and max-value are numbers, you can give the argument current-value a numerical value specifying the initial progress; if omitted, this defaults to min-value.
The remaining arguments control the rate of echo area updates. The progress reporter will wait for at least min-change more percents of the operation to be completed before printing next message; the default is one percent. min-time specifies the minimum time in seconds to pass between successive prints; the default is 0.2 seconds. (On some operating systems, the progress reporter may handle fractions of seconds with varying precision).
This function calls `progress-reporter-update`, so the first message is printed immediately.
Function: **progress-reporter-update** *reporter &optional value suffix*
This function does the main work of reporting progress of your operation. It displays the message of reporter, followed by progress percentage determined by value. If percentage is zero, or close enough according to the min-change and min-time arguments, then it is omitted from the output.
reporter must be the result of a call to `make-progress-reporter`. value specifies the current state of your operation and must be between min-value and max-value (inclusive) as passed to `make-progress-reporter`. For instance, if you scan a buffer, then value should be the result of a call to `point`.
Optional argument suffix is a string to be displayed after reporter’s main message and progress text. If reporter is a non-numerical reporter, then value should be `nil`, or a string to use instead of suffix.
This function respects min-change and min-time as passed to `make-progress-reporter` and so does not output new messages on every invocation. It is thus very fast and normally you should not try to reduce the number of calls to it: resulting overhead will most likely negate your effort.
Function: **progress-reporter-force-update** *reporter &optional value new-message suffix*
This function is similar to `progress-reporter-update` except that it prints a message in the echo area unconditionally.
reporter, value, and suffix have the same meaning as for `progress-reporter-update`. Optional new-message allows you to change the message of the reporter. Since this function always updates the echo area, such a change will be immediately presented to the user.
Function: **progress-reporter-done** *reporter*
This function should be called when the operation is finished. It prints the message of reporter followed by word ‘`done`’ in the echo area.
You should always call this function and not hope for `progress-reporter-update` to print ‘`100%`’. Firstly, it may never print it, there are many good reasons for this not to happen. Secondly, ‘`done`’ is more explicit.
Macro: **dotimes-with-progress-reporter** *(var count [result]) reporter-or-message body…*
This is a convenience macro that works the same way as `dotimes` does, but also reports loop progress using the functions described above. It allows you to save some typing. The argument reporter-or-message can be either a string or a progress reporter object.
You can rewrite the example in the beginning of this subsection using this macro as follows:
```
(dotimes-with-progress-reporter
(k 500)
"Collecting some mana for Emacs..."
(sit-for 0.01))
```
Using a reporter object as the reporter-or-message argument is useful if you want to specify the optional arguments in make-progress-reporter. For instance, you can write the previous example as follows:
```
(dotimes-with-progress-reporter
(k 500)
(make-progress-reporter "Collecting some mana for Emacs..." 0 500 0 1 1.5)
(sit-for 0.01))
```
Macro: **dolist-with-progress-reporter** *(var count [result]) reporter-or-message body…*
This is another convenience macro that works the same way as `dolist` does, but also reports loop progress using the functions described above. As in `dotimes-with-progress-reporter`, `reporter-or-message` can be a progress reporter or a string. You can rewrite the previous example with this macro as follows:
```
(dolist-with-progress-reporter
(k (number-sequence 0 500))
"Collecting some mana for Emacs..."
(sit-for 0.01))
```
elisp None ### The Mark
Each buffer has a special marker, which is designated *the mark*. When a buffer is newly created, this marker exists but does not point anywhere; this means that the mark doesn’t exist in that buffer yet. Subsequent commands can set the mark.
The mark specifies a position to bound a range of text for many commands, such as `kill-region` and `indent-rigidly`. These commands typically act on the text between point and the mark, which is called the *region*. If you are writing a command that operates on the region, don’t examine the mark directly; instead, use `interactive` with the ‘`r`’ specification. This provides the values of point and the mark as arguments to the command in an interactive call, but permits other Lisp programs to specify arguments explicitly. See [Interactive Codes](interactive-codes).
Some commands set the mark as a side-effect. Commands should do this only if it has a potential use to the user, and never for their own internal purposes. For example, the `replace-regexp` command sets the mark to the value of point before doing any replacements, because this enables the user to move back there conveniently after the replace is finished.
Once the mark exists in a buffer, it normally never ceases to exist. However, it may become *inactive*, if Transient Mark mode is enabled. The buffer-local variable `mark-active`, if non-`nil`, means that the mark is active. A command can call the function `deactivate-mark` to deactivate the mark directly, or it can request deactivation of the mark upon return to the editor command loop by setting the variable `deactivate-mark` to a non-`nil` value.
If Transient Mark mode is enabled, certain editing commands that normally apply to text near point, apply instead to the region when the mark is active. This is the main motivation for using Transient Mark mode. (Another is that this enables highlighting of the region when the mark is active. See [Display](display).)
In addition to the mark, each buffer has a *mark ring* which is a list of markers containing previous values of the mark. When editing commands change the mark, they should normally save the old value of the mark on the mark ring. The variable `mark-ring-max` specifies the maximum number of entries in the mark ring; once the list becomes this long, adding a new element deletes the last element.
There is also a separate global mark ring, but that is used only in a few particular user-level commands, and is not relevant to Lisp programming. So we do not describe it here.
Function: **mark** *&optional force*
This function returns the current buffer’s mark position as an integer, or `nil` if no mark has ever been set in this buffer.
If Transient Mark mode is enabled, and `mark-even-if-inactive` is `nil`, `mark` signals an error if the mark is inactive. However, if force is non-`nil`, then `mark` disregards inactivity of the mark, and returns the mark position (or `nil`) anyway.
Function: **mark-marker**
This function returns the marker that represents the current buffer’s mark. It is not a copy, it is the marker used internally. Therefore, changing this marker’s position will directly affect the buffer’s mark. Don’t do that unless that is the effect you want.
```
(setq m (mark-marker))
⇒ #<marker at 3420 in markers.texi>
```
```
(set-marker m 100)
⇒ #<marker at 100 in markers.texi>
```
```
(mark-marker)
⇒ #<marker at 100 in markers.texi>
```
Like any marker, this marker can be set to point at any buffer you like. If you make it point at any buffer other than the one of which it is the mark, it will yield perfectly consistent, but rather odd, results. We recommend that you not do it!
Function: **set-mark** *position*
This function sets the mark to position, and activates the mark. The old value of the mark is *not* pushed onto the mark ring.
**Please note:** Use this function only if you want the user to see that the mark has moved, and you want the previous mark position to be lost. Normally, when a new mark is set, the old one should go on the `mark-ring`. For this reason, most applications should use `push-mark` and `pop-mark`, not `set-mark`.
Novice Emacs Lisp programmers often try to use the mark for the wrong purposes. The mark saves a location for the user’s convenience. An editing command should not alter the mark unless altering the mark is part of the user-level functionality of the command. (And, in that case, this effect should be documented.) To remember a location for internal use in the Lisp program, store it in a Lisp variable. For example:
```
(let ((beg (point)))
(forward-line 1)
(delete-region beg (point))).
```
Function: **push-mark** *&optional position nomsg activate*
This function sets the current buffer’s mark to position, and pushes a copy of the previous mark onto `mark-ring`. If position is `nil`, then the value of point is used.
The function `push-mark` normally *does not* activate the mark. To do that, specify `t` for the argument activate.
A ‘`Mark set`’ message is displayed unless nomsg is non-`nil`.
Function: **pop-mark**
This function pops off the top element of `mark-ring` and makes that mark become the buffer’s actual mark. This does not move point in the buffer, and it does nothing if `mark-ring` is empty. It deactivates the mark.
User Option: **transient-mark-mode**
This variable, if non-`nil`, enables Transient Mark mode. In Transient Mark mode, every buffer-modifying primitive sets `deactivate-mark`. As a consequence, most commands that modify the buffer also deactivate the mark.
When Transient Mark mode is enabled and the mark is active, many commands that normally apply to the text near point instead apply to the region. Such commands should use the function `use-region-p` to test whether they should operate on the region. See [The Region](the-region).
Lisp programs can set `transient-mark-mode` to non-`nil`, non-`t` values to enable Transient Mark mode temporarily. If the value is `lambda`, Transient Mark mode is automatically turned off after any action, such as buffer modification, that would normally deactivate the mark. If the value is `(only . oldval)`, then `transient-mark-mode` is set to the value oldval after any subsequent command that moves point and is not shift-translated (see [shift-translation](key-sequence-input)), or after any other action that would normally deactivate the mark. (Marking a region with the mouse will temporarily enable `transient-mark-mode` in this way.)
User Option: **mark-even-if-inactive**
If this is non-`nil`, Lisp programs and the Emacs user can use the mark even when it is inactive. This option affects the behavior of Transient Mark mode. When the option is non-`nil`, deactivation of the mark turns off region highlighting, but commands that use the mark behave as if the mark were still active.
Variable: **deactivate-mark**
If an editor command sets this variable non-`nil`, then the editor command loop deactivates the mark after the command returns (if Transient Mark mode is enabled). All the primitives that change the buffer set `deactivate-mark`, to deactivate the mark when the command is finished. Setting this variable makes it buffer-local.
To write Lisp code that modifies the buffer without causing deactivation of the mark at the end of the command, bind `deactivate-mark` to `nil` around the code that does the modification. For example:
```
(let (deactivate-mark)
(insert " "))
```
Function: **deactivate-mark** *&optional force*
If Transient Mark mode is enabled or force is non-`nil`, this function deactivates the mark and runs the normal hook `deactivate-mark-hook`. Otherwise, it does nothing.
Variable: **mark-active**
The mark is active when this variable is non-`nil`. This variable is always buffer-local in each buffer. Do *not* use the value of this variable to decide whether a command that normally operates on text near point should operate on the region instead. Use the function `use-region-p` for that (see [The Region](the-region)).
Variable: **activate-mark-hook**
Variable: **deactivate-mark-hook**
These normal hooks are run, respectively, when the mark becomes active and when it becomes inactive. The hook `activate-mark-hook` is also run when the region is reactivated, for instance after using a command that switches back to a buffer that has an active mark.
Function: **handle-shift-selection**
This function implements the shift-selection behavior of point-motion commands. See [Shift Selection](https://www.gnu.org/software/emacs/manual/html_node/emacs/Shift-Selection.html#Shift-Selection) in The GNU Emacs Manual. It is called automatically by the Emacs command loop whenever a command with a ‘`^`’ character in its `interactive` spec is invoked, before the command itself is executed (see [^](interactive-codes)).
If `shift-select-mode` is non-`nil` and the current command was invoked via shift translation (see [shift-translation](key-sequence-input)), this function sets the mark and temporarily activates the region, unless the region was already temporarily activated in this way. Otherwise, if the region has been activated temporarily, it deactivates the mark and restores the variable `transient-mark-mode` to its earlier value.
Variable: **mark-ring**
The value of this buffer-local variable is the list of saved former marks of the current buffer, most recent first.
```
mark-ring
⇒ (#<marker at 11050 in markers.texi>
#<marker at 10832 in markers.texi>
…)
```
User Option: **mark-ring-max**
The value of this variable is the maximum size of `mark-ring`. If more marks than this are pushed onto the `mark-ring`, `push-mark` discards an old mark when it adds a new one.
When Delete Selection mode (see [Delete Selection](https://www.gnu.org/software/emacs/manual/html_node/emacs/Using-Region.html#Using-Region) in The GNU Emacs Manual) is enabled, commands that operate on the active region (a.k.a. “selection”) behave slightly differently. This works by adding the function `delete-selection-pre-hook` to the `pre-command-hook` (see [Command Overview](command-overview)). That function calls `delete-selection-helper` to delete the selection as appropriate for the command. If you want to adapt a command to Delete Selection mode, put the `delete-selection` property on the function’s symbol (see [Symbol Plists](symbol-plists)); commands that don’t have this property on their symbol won’t delete the selection. This property can have one of several values to tailor the behavior to what the command is supposed to do; see the doc strings of `delete-selection-pre-hook` and `delete-selection-helper` for the details.
| programming_docs |
elisp None ### Finding All Frames
Function: **frame-list**
This function returns a list of all the live frames, i.e., those that have not been deleted. It is analogous to `buffer-list` for buffers, and includes frames on all terminals. The list that you get is newly created, so modifying the list doesn’t have any effect on the internals of Emacs.
Function: **visible-frame-list**
This function returns a list of just the currently visible frames. See [Visibility of Frames](visibility-of-frames). Frames on text terminals always count as visible, even though only the selected one is actually displayed.
Function: **frame-list-z-order** *&optional display*
This function returns a list of Emacs’ frames, in Z (stacking) order (see [Raising and Lowering](raising-and-lowering)). The optional argument display specifies which display to poll. display should be either a frame or a display name (a string). If omitted or `nil`, that stands for the selected frame’s display. It returns `nil` if display contains no Emacs frame.
Frames are listed from topmost (first) to bottommost (last). As a special case, if display is non-`nil` and specifies a live frame, it returns the child frames of that frame in Z (stacking) order.
This function is not meaningful on text terminals.
Function: **next-frame** *&optional frame minibuf*
This function lets you cycle conveniently through all the frames on a specific terminal from an arbitrary starting point. It returns the frame following frame, in the list of all live frames, on frame’s terminal. The argument frame must specify a live frame and defaults to the selected frame. It never returns a frame whose `no-other-frame` parameter (see [Frame Interaction Parameters](frame-interaction-parameters)) is non-`nil`.
The second argument, minibuf, says which frames to consider when deciding what the next frame should be:
`nil` Consider all frames except minibuffer-only frames.
`visible` Consider only visible frames.
0 Consider only visible or iconified frames.
a window Consider only the frames using that particular window as their minibuffer window.
anything else Consider all frames.
Function: **previous-frame** *&optional frame minibuf*
Like `next-frame`, but cycles through all frames in the opposite direction.
See also `next-window` and `previous-window`, in [Cyclic Window Ordering](cyclic-window-ordering).
elisp None ### Custom Format Strings
Sometimes it is useful to allow users and Lisp programs alike to control how certain text is generated via custom format control strings. For example, a format string could control how to display someone’s forename, surname, and email address. Using the function `format` described in the previous section, the format string could be something like `"%s %s <%s>"`. This approach quickly becomes impractical, however, as it can be unclear which specification character corresponds to which piece of information.
A more convenient format string for such cases would be something like `"%f %l <%e>"`, where each specification character carries more semantic information and can easily be rearranged relative to other specification characters, making such format strings more easily customizable by the user.
The function `format-spec` described in this section performs a similar function to `format`, except it operates on format control strings that use arbitrary specification characters.
Function: **format-spec** *template spec-alist &optional ignore-missing split*
This function returns a string produced from the format string template according to conversions specified in spec-alist, which is an alist (see [Association Lists](association-lists)) of the form `(letter . replacement)`. Each specification `%letter` in template will be replaced by replacement when formatting the resulting string.
The characters in template, other than the format specifications, are copied directly into the output, including their text properties, if any. Any text properties of the format specifications are copied to their replacements.
Using an alist to specify conversions gives rise to some useful properties:
* If spec-alist contains more unique letter keys than there are unique specification characters in template, the unused keys are simply ignored.
* If spec-alist contains more than one association with the same letter, the closest one to the start of the list is used.
* If template contains the same specification character more than once, then the same replacement found in spec-alist is used as a basis for all of that character’s substitutions.
* The order of specifications in template need not correspond to the order of associations in spec-alist.
The optional argument ignore-missing indicates how to handle specification characters in template that are not found in spec-alist. If it is `nil` or omitted, the function signals an error; if it is `ignore`, those format specifications are left verbatim in the output, including their text properties, if any; if it is `delete`, those format specifications are removed from the output; any other non-`nil` value is handled like `ignore`, but any occurrences of ‘`%%`’ are also left verbatim in the output.
If the optional argument split is non-`nil`, instead of returning a single string, `format-spec` will split the result into a list of strings, based on where the substitutions were performed. For instance:
```
(format-spec "foo %b bar" '((?b . "zot")) nil t)
⇒ ("foo " "zot" " bar")
```
The syntax of format specifications accepted by `format-spec` is similar, but not identical, to that accepted by `format`. In both cases, a format specification is a sequence of characters beginning with ‘`%`’ and ending with an alphabetic letter such as ‘`s`’.
Unlike `format`, which assigns specific meanings to a fixed set of specification characters, `format-spec` accepts arbitrary specification characters and treats them all equally. For example:
```
(setq my-site-info
(list (cons ?s system-name)
(cons ?t (symbol-name system-type))
(cons ?c system-configuration)
(cons ?v emacs-version)
(cons ?e invocation-name)
(cons ?p (number-to-string (emacs-pid)))
(cons ?a user-mail-address)
(cons ?n user-full-name)))
(format-spec "%e %v (%c)" my-site-info)
⇒ "emacs 27.1 (x86_64-pc-linux-gnu)"
(format-spec "%n <%a>" my-site-info)
⇒ "Emacs Developers <[email protected]>"
```
A format specification can include any number of the following flag characters immediately after the ‘`%`’ to modify aspects of the substitution.
‘`0`’
This flag causes any padding specified by the width to consist of ‘`0`’ characters instead of spaces.
‘`-`’
This flag causes any padding specified by the width to be inserted on the right rather than the left.
‘`<`’
This flag causes the substitution to be truncated on the left to the given width and precision, if specified.
‘`>`’
This flag causes the substitution to be truncated on the right to the given width, if specified.
‘`^`’
This flag converts the substituted text to upper case (see [Case Conversion](case-conversion)).
‘`\_ (underscore)`’ This flag converts the substituted text to lower case (see [Case Conversion](case-conversion)).
The result of using contradictory flags (for instance, both upper and lower case) is undefined.
As is the case with `format`, a format specification can include a width, which is a decimal number that appears after any flags, and a precision, which is a decimal-point ‘`.`’ followed by a decimal number that appears after any flags and width.
If a substitution contains fewer characters than its specified width, it is padded on the left:
```
(format-spec "%8a is padded on the left with spaces"
'((?a . "alpha")))
⇒ " alpha is padded on the left with spaces"
```
If a substitution contains more characters than its specified precision, it is truncated on the right:
```
(format-spec "%.2a is truncated on the right"
'((?a . "alpha")))
⇒ "al is truncated on the right"
```
Here is a more complicated example that combines several aforementioned features:
```
(setq my-battery-info
(list (cons ?p "73") ; Percentage
(cons ?L "Battery") ; Status
(cons ?t "2:23") ; Remaining time
(cons ?c "24330") ; Capacity
(cons ?r "10.6"))) ; Rate of discharge
(format-spec "%>^-3L : %3p%% (%05t left)" my-battery-info)
⇒ "BAT : 73% (02:23 left)"
(format-spec "%>^-3L : %3p%% (%05t left)"
(cons (cons ?L "AC")
my-battery-info))
⇒ "AC : 73% (02:23 left)"
```
As the examples in this section illustrate, `format-spec` is often used for selectively formatting an assortment of different pieces of information. This is useful in programs that provide user-customizable format strings, as the user can choose to format with a regular syntax and in any desired order only a subset of the information that the program makes available.
elisp None #### Motion Commands Based on Parsing
This section describes simple point-motion functions that operate based on parsing expressions.
Function: **scan-lists** *from count depth*
This function scans forward count balanced parenthetical groupings from position from. It returns the position where the scan stops. If count is negative, the scan moves backwards.
If depth is nonzero, treat the starting position as being depth parentheses deep. The scanner moves forward or backward through the buffer until the depth changes to zero count times. Hence, a positive value for depth has the effect of moving out depth levels of parenthesis from the starting position, while a negative depth has the effect of moving deeper by -depth levels of parenthesis.
Scanning ignores comments if `parse-sexp-ignore-comments` is non-`nil`.
If the scan reaches the beginning or end of the accessible part of the buffer before it has scanned over count parenthetical groupings, the return value is `nil` if the depth at that point is zero; if the depth is non-zero, a `scan-error` error is signaled.
Function: **scan-sexps** *from count*
This function scans forward count sexps from position from. It returns the position where the scan stops. If count is negative, the scan moves backwards.
Scanning ignores comments if `parse-sexp-ignore-comments` is non-`nil`.
If the scan reaches the beginning or end of (the accessible part of) the buffer while in the middle of a parenthetical grouping, an error is signaled. If it reaches the beginning or end between groupings but before count is used up, `nil` is returned.
Function: **forward-comment** *count*
This function moves point forward across count complete comments (that is, including the starting delimiter and the terminating delimiter if any), plus any whitespace encountered on the way. It moves backward if count is negative. If it encounters anything other than a comment or whitespace, it stops, leaving point at the place where it stopped. This includes (for instance) finding the end of a comment when moving forward and expecting the beginning of one. The function also stops immediately after moving over the specified number of complete comments. If count comments are found as expected, with nothing except whitespace between them, it returns `t`; otherwise it returns `nil`.
This function cannot tell whether the comments it traverses are embedded within a string. If they look like comments, it treats them as comments.
To move forward over all comments and whitespace following point, use `(forward-comment (buffer-size))`. `(buffer-size)` is a good argument to use, because the number of comments in the buffer cannot exceed that many.
elisp None ### Garbage Collection
When a program creates a list or the user defines a new function (such as by loading a library), that data is placed in normal storage. If normal storage runs low, then Emacs asks the operating system to allocate more memory. Different types of Lisp objects, such as symbols, cons cells, small vectors, markers, etc., are segregated in distinct blocks in memory. (Large vectors, long strings, buffers and certain other editing types, which are fairly large, are allocated in individual blocks, one per object; small strings are packed into blocks of 8k bytes, and small vectors are packed into blocks of 4k bytes).
Beyond the basic vector, a lot of objects like markers, overlays and buffers are managed as if they were vectors. The corresponding C data structures include the `union vectorlike_header` field whose `size` member contains the subtype enumerated by `enum pvec_type` and an information about how many `Lisp_Object` fields this structure contains and what the size of the rest data is. This information is needed to calculate the memory footprint of an object, and used by the vector allocation code while iterating over the vector blocks.
It is quite common to use some storage for a while, then release it by (for example) killing a buffer or deleting the last pointer to an object. Emacs provides a *garbage collector* to reclaim this abandoned storage. The garbage collector operates, in essence, by finding and marking all Lisp objects that are still accessible to Lisp programs. To begin with, it assumes all the symbols, their values and associated function definitions, and any data presently on the stack, are accessible. Any objects that can be reached indirectly through other accessible objects are also accessible, but this calculation is done “conservatively”, so it may slightly overestimate how many objects that are accessible.
When marking is finished, all objects still unmarked are garbage. No matter what the Lisp program or the user does, it is impossible to refer to them, since there is no longer a way to reach them. Their space might as well be reused, since no one will miss them. The second (sweep) phase of the garbage collector arranges to reuse them. (But since the marking was done “conservatively”, not all unused objects are guaranteed to be garbage-collected by any one sweep.)
The sweep phase puts unused cons cells onto a *free list* for future allocation; likewise for symbols and markers. It compacts the accessible strings so they occupy fewer 8k blocks; then it frees the other 8k blocks. Unreachable vectors from vector blocks are coalesced to create largest possible free areas; if a free area spans a complete 4k block, that block is freed. Otherwise, the free area is recorded in a free list array, where each entry corresponds to a free list of areas of the same size. Large vectors, buffers, and other large objects are allocated and freed individually.
> **Common Lisp note:** Unlike other Lisps, GNU Emacs Lisp does not call the garbage collector when the free list is empty. Instead, it simply requests the operating system to allocate more storage, and processing continues until `gc-cons-threshold` bytes have been used.
>
> This means that you can make sure that the garbage collector will not run during a certain portion of a Lisp program by calling the garbage collector explicitly just before it (provided that portion of the program does not use so much space as to force a second garbage collection).
>
>
>
Command: **garbage-collect**
This command runs a garbage collection, and returns information on the amount of space in use. (Garbage collection can also occur spontaneously if you use more than `gc-cons-threshold` bytes of Lisp data since the previous garbage collection.)
`garbage-collect` returns a list with information on amount of space in use, where each entry has the form ‘`(name size used)`’ or ‘`(name size used free)`’. In the entry, name is a symbol describing the kind of objects this entry represents, size is the number of bytes used by each one, used is the number of those objects that were found live in the heap, and optional free is the number of those objects that are not live but that Emacs keeps around for future allocations. So an overall result is:
```
((conses cons-size used-conses free-conses)
(symbols symbol-size used-symbols free-symbols)
(strings string-size used-strings free-strings)
(string-bytes byte-size used-bytes)
(vectors vector-size used-vectors)
(vector-slots slot-size used-slots free-slots)
(floats float-size used-floats free-floats)
(intervals interval-size used-intervals free-intervals)
(buffers buffer-size used-buffers)
(heap unit-size total-size free-size))
```
Here is an example:
```
(garbage-collect)
⇒ ((conses 16 49126 8058) (symbols 48 14607 0)
(strings 32 2942 2607)
(string-bytes 1 78607) (vectors 16 7247)
(vector-slots 8 341609 29474) (floats 8 71 102)
(intervals 56 27 26) (buffers 944 8)
(heap 1024 11715 2678))
```
Below is a table explaining each element. Note that last `heap` entry is optional and present only if an underlying `malloc` implementation provides `mallinfo` function.
cons-size
Internal size of a cons cell, i.e., `sizeof (struct Lisp_Cons)`.
used-conses
The number of cons cells in use.
free-conses
The number of cons cells for which space has been obtained from the operating system, but that are not currently being used.
symbol-size
Internal size of a symbol, i.e., `sizeof (struct Lisp_Symbol)`.
used-symbols
The number of symbols in use.
free-symbols
The number of symbols for which space has been obtained from the operating system, but that are not currently being used.
string-size
Internal size of a string header, i.e., `sizeof (struct Lisp_String)`.
used-strings
The number of string headers in use.
free-strings
The number of string headers for which space has been obtained from the operating system, but that are not currently being used.
byte-size
This is used for convenience and equals to `sizeof (char)`.
used-bytes
The total size of all string data in bytes.
vector-size
Size in bytes of a vector of length 1, including its header.
used-vectors
The number of vector headers allocated from the vector blocks.
slot-size
Internal size of a vector slot, always equal to `sizeof (Lisp_Object)`.
used-slots
The number of slots in all used vectors. Slot counts might include some or all overhead from vector headers, depending on the platform.
free-slots
The number of free slots in all vector blocks.
float-size
Internal size of a float object, i.e., `sizeof (struct Lisp_Float)`. (Do not confuse it with the native platform `float` or `double`.)
used-floats
The number of floats in use.
free-floats
The number of floats for which space has been obtained from the operating system, but that are not currently being used.
interval-size
Internal size of an interval object, i.e., `sizeof (struct interval)`.
used-intervals
The number of intervals in use.
free-intervals
The number of intervals for which space has been obtained from the operating system, but that are not currently being used.
buffer-size
Internal size of a buffer, i.e., `sizeof (struct buffer)`. (Do not confuse with the value returned by `buffer-size` function.)
used-buffers
The number of buffer objects in use. This includes killed buffers invisible to users, i.e., all buffers in `all_buffers` list.
unit-size
The unit of heap space measurement, always equal to 1024 bytes.
total-size
Total heap size, in unit-size units.
free-size Heap space which is not currently used, in unit-size units.
If there was overflow in pure space (see [Pure Storage](pure-storage)), and Emacs was dumped using the (now obsolete) `unexec` method (see [Building Emacs](building-emacs)), then `garbage-collect` returns `nil`, because a real garbage collection cannot be done in that case.
User Option: **garbage-collection-messages**
If this variable is non-`nil`, Emacs displays a message at the beginning and end of garbage collection. The default value is `nil`.
Variable: **post-gc-hook**
This is a normal hook that is run at the end of garbage collection. Garbage collection is inhibited while the hook functions run, so be careful writing them.
User Option: **gc-cons-threshold**
The value of this variable is the number of bytes of storage that must be allocated for Lisp objects after one garbage collection in order to trigger another garbage collection. You can use the result returned by `garbage-collect` to get an information about size of the particular object type; space allocated to the contents of buffers does not count.
The initial threshold value is `GC_DEFAULT_THRESHOLD`, defined in `alloc.c`. Since it’s defined in `word_size` units, the value is 400,000 for the default 32-bit configuration and 800,000 for the 64-bit one. If you specify a larger value, garbage collection will happen less often. This reduces the amount of time spent garbage collecting, but increases total memory use. You may want to do this when running a program that creates lots of Lisp data.
You can make collections more frequent by specifying a smaller value, down to 1/10th of `GC_DEFAULT_THRESHOLD`. A value less than this minimum will remain in effect only until the subsequent garbage collection, at which time `garbage-collect` will set the threshold back to the minimum.
User Option: **gc-cons-percentage**
The value of this variable specifies the amount of consing before a garbage collection occurs, as a fraction of the current heap size. This criterion and `gc-cons-threshold` apply in parallel, and garbage collection occurs only when both criteria are satisfied.
As the heap size increases, the time to perform a garbage collection increases. Thus, it can be desirable to do them less frequently in proportion.
Control over the garbage collector via `gc-cons-threshold` and `gc-cons-percentage` is only approximate. Although Emacs checks for threshold exhaustion regularly, for efficiency reasons it does not do so immediately after every change to the heap or to `gc-cons-threshold` or `gc-cons-percentage`, so exhausting the threshold does not immediately trigger garbage collection. Also, for efficiency in threshold calculations Emacs approximates the heap size, which counts the bytes used by currently-accessible objects in the heap.
The value returned by `garbage-collect` describes the amount of memory used by Lisp data, broken down by data type. By contrast, the function `memory-limit` provides information on the total amount of memory Emacs is currently using.
Function: **memory-limit**
This function returns an estimate of the total amount of bytes of virtual memory that Emacs is currently using, divided by 1024. You can use this to get a general idea of how your actions affect the memory usage.
Variable: **memory-full**
This variable is `t` if Emacs is nearly out of memory for Lisp objects, and `nil` otherwise.
Function: **memory-use-counts**
This returns a list of numbers that count the number of objects created in this Emacs session. Each of these counters increments for a certain kind of object. See the documentation string for details.
Function: **memory-info**
This functions returns an amount of total system memory and how much of it is free. On an unsupported system, the value may be `nil`.
Variable: **gcs-done**
This variable contains the total number of garbage collections done so far in this Emacs session.
Variable: **gc-elapsed**
This variable contains the total number of seconds of elapsed time during garbage collection so far in this Emacs session, as a floating-point number.
Function: **memory-report**
It can sometimes be useful to see where Emacs is using memory (in various variables, buffers, and caches). This command will open a new buffer (called ‘`"\*Memory Report\*"`’) that will give an overview, in addition to listing the “largest” buffers and variables.
All the data here is approximate, because there’s really no consistent way to compute the size of a variable. For instance, two variables may share parts of a data structure, and this will be counted twice, but this command may still give a useful high-level overview of which parts of Emacs are using memory.
| programming_docs |
elisp None ### Base 64 Encoding
Base 64 code is used in email to encode a sequence of 8-bit bytes as a longer sequence of ASCII graphic characters. It is defined in Internet RFC[19](#FOOT19)2045 and also in RFC 4648. This section describes the functions for converting to and from this code.
Command: **base64-encode-region** *beg end &optional no-line-break*
This function converts the region from beg to end into base 64 code. It returns the length of the encoded text. An error is signaled if a character in the region is multibyte, i.e., in a multibyte buffer the region must contain only characters from the charsets `ascii`, `eight-bit-control` and `eight-bit-graphic`.
Normally, this function inserts newline characters into the encoded text, to avoid overlong lines. However, if the optional argument no-line-break is non-`nil`, these newlines are not added, so the output is just one long line.
Command: **base64url-encode-region** *beg end &optional no-pad*
This function is like `base64-encode-region`, but it implements the URL variant of base 64 encoding, per RFC 4648, and it doesn’t insert newline characters into the encoded text, so the output is just one long line.
If the optional argument no-pad is non-`nil` then this function doesn’t generate the padding (`=`).
Function: **base64-encode-string** *string &optional no-line-break*
This function converts the string string into base 64 code. It returns a string containing the encoded text. As for `base64-encode-region`, an error is signaled if a character in the string is multibyte.
Normally, this function inserts newline characters into the encoded text, to avoid overlong lines. However, if the optional argument no-line-break is non-`nil`, these newlines are not added, so the result string is just one long line.
Function: **base64url-encode-string** *string &optional no-pad*
Like `base64-encode-string`, but generates the URL variant of base 64, and doesn’t insert newline characters into the encoded text, so the result is just one long line.
If the optional argument no-pad is non-`nil` then this function doesn’t generate the padding.
Command: **base64-decode-region** *beg end &optional base64url*
This function converts the region from beg to end from base 64 code into the corresponding decoded text. It returns the length of the decoded text.
The decoding functions ignore newline characters in the encoded text.
If optional argument base64url is non-`nil`, then padding is optional, and the URL variant of base 64 encoding is used.
Function: **base64-decode-string** *string &optional base64url*
This function converts the string string from base 64 code into the corresponding decoded text. It returns a unibyte string containing the decoded text.
The decoding functions ignore newline characters in the encoded text.
If optional argument base64url is non-`nil`, then padding is optional, and the URL variant of base 64 encoding is used.
elisp None ### Visibility of Frames
A frame on a graphical display may be *visible*, *invisible*, or *iconified*. If it is visible, its contents are displayed in the usual manner. If it is iconified, its contents are not displayed, but there is a little icon somewhere to bring the frame back into view (some window managers refer to this state as *minimized* rather than *iconified*, but from Emacs’ point of view they are the same thing). If a frame is invisible, it is not displayed at all.
The concept of visibility is strongly related to that of (un-)mapped frames. A frame (or, more precisely, its window-system window) is and becomes *mapped* when it is displayed for the first time and whenever it changes its state of visibility from `iconified` or `invisible` to `visible`. Conversely, a frame is and becomes *unmapped* whenever it changes its status from `visible` to `iconified` or `invisible`.
Visibility is meaningless on text terminals, since only the selected frame is actually displayed in any case.
Function: **frame-visible-p** *frame*
This function returns the visibility status of frame frame. The value is `t` if frame is visible, `nil` if it is invisible, and `icon` if it is iconified.
On a text terminal, all frames are considered visible for the purposes of this function, even though only one frame is displayed. See [Raising and Lowering](raising-and-lowering).
Command: **iconify-frame** *&optional frame*
This function iconifies frame frame. If you omit frame, it iconifies the selected frame. This usually makes all child frames of frame (and their descendants) invisible (see [Child Frames](child-frames)).
Command: **make-frame-visible** *&optional frame*
This function makes frame frame visible. If you omit frame, it makes the selected frame visible. This does not raise the frame, but you can do that with `raise-frame` if you wish (see [Raising and Lowering](raising-and-lowering)).
Making a frame visible usually makes all its child frames (and their descendants) visible as well (see [Child Frames](child-frames)).
Command: **make-frame-invisible** *&optional frame force*
This function makes frame frame invisible. If you omit frame, it makes the selected frame invisible. Usually, this makes all child frames of frame (and their descendants) invisible too (see [Child Frames](child-frames)).
Unless force is non-`nil`, this function refuses to make frame invisible if all other frames are invisible.
The visibility status of a frame is also available as a frame parameter. You can read or change it as such. See [Management Parameters](management-parameters). The user can also iconify and deiconify frames with the window manager. This happens below the level at which Emacs can exert any control, but Emacs does provide events that you can use to keep track of such changes. See [Misc Events](misc-events).
Function: **x-double-buffered-p** *&optional frame*
This function returns non-`nil` if frame is currently being rendered with double buffering. frame defaults to the selected frame.
elisp None #### Functions for Working with Faces
Here are additional functions for creating and working with faces.
Function: **face-list**
This function returns a list of all defined face names.
Function: **face-id** *face*
This function returns the *face number* of face face. This is a number that uniquely identifies a face at low levels within Emacs. It is seldom necessary to refer to a face by its face number. However, functions that manipulate glyphs, such as `make-glyph-code` and `glyph-face` (see [Glyphs](glyphs)) access the face numbers internally. Note that the face number is stored as the value of the `face` property of the face symbol, so we recommend not to set that property of a face to any value of your own.
Function: **face-documentation** *face*
This function returns the documentation string of face face, or `nil` if none was specified for it.
Function: **face-equal** *face1 face2 &optional frame*
This returns `t` if the faces face1 and face2 have the same attributes for display.
Function: **face-differs-from-default-p** *face &optional frame*
This returns non-`nil` if the face face displays differently from the default face.
A *face alias* provides an equivalent name for a face. You can define a face alias by giving the alias symbol the `face-alias` property, with a value of the target face name. The following example makes `modeline` an alias for the `mode-line` face.
```
(put 'modeline 'face-alias 'mode-line)
```
Macro: **define-obsolete-face-alias** *obsolete-face current-face when*
This macro defines `obsolete-face` as an alias for current-face, and also marks it as obsolete, indicating that it may be removed in future. when should be a string indicating when `obsolete-face` was made obsolete (usually a version number string).
elisp None #### Specifying Indentation Rules
Based on the provided grammar, SMIE will be able to provide automatic indentation without any extra effort. But in practice, this default indentation style will probably not be good enough. You will want to tweak it in many different cases.
SMIE indentation is based on the idea that indentation rules should be as local as possible. To this end, it relies on the idea of *virtual* indentation, which is the indentation that a particular program point would have if it were at the beginning of a line. Of course, if that program point is indeed at the beginning of a line, its virtual indentation is its current indentation. But if not, then SMIE uses the indentation algorithm to compute the virtual indentation of that point. Now in practice, the virtual indentation of a program point does not have to be identical to the indentation it would have if we inserted a newline before it. To see how this works, the SMIE rule for indentation after a `{` in C does not care whether the `{` is standing on a line of its own or is at the end of the preceding line. Instead, these different cases are handled in the indentation rule that decides how to indent before a `{`.
Another important concept is the notion of *parent*: The *parent* of a token, is the head token of the nearest enclosing syntactic construct. For example, the parent of an `else` is the `if` to which it belongs, and the parent of an `if`, in turn, is the lead token of the surrounding construct. The command `backward-sexp` jumps from a token to its parent, but there are some caveats: for *openers* (tokens which start a construct, like `if`), you need to start with point before the token, while for others you need to start with point after the token. `backward-sexp` stops with point before the parent token if that is the *opener* of the token of interest, and otherwise it stops with point after the parent token.
SMIE indentation rules are specified using a function that takes two arguments method and arg where the meaning of arg and the expected return value depend on method.
method can be:
* `:after`, in which case arg is a token and the function should return the offset to use for indentation after arg.
* `:before`, in which case arg is a token and the function should return the offset to use to indent arg itself.
* `:elem`, in which case the function should return either the offset to use to indent function arguments (if arg is the symbol `arg`) or the basic indentation step (if arg is the symbol `basic`).
* `:list-intro`, in which case arg is a token and the function should return non-`nil` if the token is followed by a list of expressions (not separated by any token) rather than an expression.
When arg is a token, the function is called with point just before that token. A return value of `nil` always means to fallback on the default behavior, so the function should return `nil` for arguments it does not expect.
offset can be:
* `nil`: use the default indentation rule.
* `(column . column)`: indent to column column.
* number: offset by number, relative to a base token which is the current token for `:after` and its parent for `:before`.
elisp None #### Accessing the Entire Match Data
The functions `match-data` and `set-match-data` read or write the entire match data, all at once.
Function: **match-data** *&optional integers reuse reseat*
This function returns a list of positions (markers or integers) that record all the information on the text that the last search matched. Element zero is the position of the beginning of the match for the whole expression; element one is the position of the end of the match for the expression. The next two elements are the positions of the beginning and end of the match for the first subexpression, and so on. In general, element number 2n corresponds to `(match-beginning n)`; and element number 2n + 1 corresponds to `(match-end n)`.
Normally all the elements are markers or `nil`, but if integers is non-`nil`, that means to use integers instead of markers. (In that case, the buffer itself is appended as an additional element at the end of the list, to facilitate complete restoration of the match data.) If the last match was done on a string with `string-match`, then integers are always used, since markers can’t point into a string.
If reuse is non-`nil`, it should be a list. In that case, `match-data` stores the match data in reuse. That is, reuse is destructively modified. reuse does not need to have the right length. If it is not long enough to contain the match data, it is extended. If it is too long, the length of reuse stays the same, but the elements that were not used are set to `nil`. The purpose of this feature is to reduce the need for garbage collection.
If reseat is non-`nil`, all markers on the reuse list are reseated to point to nowhere.
As always, there must be no possibility of intervening searches between the call to a search function and the call to `match-data` that is intended to access the match data for that search.
```
(match-data)
⇒ (#<marker at 9 in foo>
#<marker at 17 in foo>
#<marker at 13 in foo>
#<marker at 17 in foo>)
```
Function: **set-match-data** *match-list &optional reseat*
This function sets the match data from the elements of match-list, which should be a list that was the value of a previous call to `match-data`. (More precisely, anything that has the same format will work.)
If match-list refers to a buffer that doesn’t exist, you don’t get an error; that sets the match data in a meaningless but harmless way.
If reseat is non-`nil`, all markers on the match-list list are reseated to point to nowhere.
`store-match-data` is a semi-obsolete alias for `set-match-data`.
elisp None ### Scanning Keymaps
This section describes functions used to scan all the current keymaps for the sake of printing help information. To display the bindings in a particular keymap, you can use the `describe-keymap` command (see [Other Help Commands](https://www.gnu.org/software/emacs/manual/html_node/emacs/Misc-Help.html#Misc-Help) in The GNU Emacs Manual)
Function: **accessible-keymaps** *keymap &optional prefix*
This function returns a list of all the keymaps that can be reached (via zero or more prefix keys) from keymap. The value is an association list with elements of the form `(key .
map)`, where key is a prefix key whose definition in keymap is map.
The elements of the alist are ordered so that the key increases in length. The first element is always `([] . keymap)`, because the specified keymap is accessible from itself with a prefix of no events.
If prefix is given, it should be a prefix key sequence; then `accessible-keymaps` includes only the submaps whose prefixes start with prefix. These elements look just as they do in the value of `(accessible-keymaps)`; the only difference is that some elements are omitted.
In the example below, the returned alist indicates that the key ESC, which is displayed as ‘`^[`’, is a prefix key whose definition is the sparse keymap `(keymap (83 . center-paragraph)
(115 . foo))`.
```
(accessible-keymaps (current-local-map))
⇒(([] keymap
(27 keymap ; Note this keymap for ESC is repeated below.
(83 . center-paragraph)
(115 . center-line))
(9 . tab-to-tab-stop))
```
```
("^[" keymap
(83 . center-paragraph)
(115 . foo)))
```
In the following example, `C-h` is a prefix key that uses a sparse keymap starting with `(keymap (118 . describe-variable)…)`. Another prefix, `C-x 4`, uses a keymap which is also the value of the variable `ctl-x-4-map`. The event `mode-line` is one of several dummy events used as prefixes for mouse actions in special parts of a window.
```
(accessible-keymaps (current-global-map))
⇒ (([] keymap [set-mark-command beginning-of-line …
delete-backward-char])
```
```
("^H" keymap (118 . describe-variable) …
(8 . help-for-help))
```
```
("^X" keymap [x-flush-mouse-queue …
backward-kill-sentence])
```
```
("^[" keymap [mark-sexp backward-sexp …
backward-kill-word])
```
```
("^X4" keymap (15 . display-buffer) …)
```
```
([mode-line] keymap
(S-mouse-2 . mouse-split-window-horizontally) …))
```
These are not all the keymaps you would see in actuality.
Function: **map-keymap** *function keymap*
The function `map-keymap` calls function once for each binding in keymap. It passes two arguments, the event type and the value of the binding. If keymap has a parent, the parent’s bindings are included as well. This works recursively: if the parent has itself a parent, then the grandparent’s bindings are also included and so on.
This function is the cleanest way to examine all the bindings in a keymap.
Function: **where-is-internal** *command &optional keymap firstonly noindirect no-remap*
This function is a subroutine used by the `where-is` command (see [Help](https://www.gnu.org/software/emacs/manual/html_node/emacs/Help.html#Help) in The GNU Emacs Manual). It returns a list of all key sequences (of any length) that are bound to command in a set of keymaps.
The argument command can be any object; it is compared with all keymap entries using `eq`.
If keymap is `nil`, then the maps used are the current active keymaps, disregarding `overriding-local-map` (that is, pretending its value is `nil`). If keymap is a keymap, then the maps searched are keymap and the global keymap. If keymap is a list of keymaps, only those keymaps are searched.
Usually it’s best to use `overriding-local-map` as the expression for keymap. Then `where-is-internal` searches precisely the keymaps that are active. To search only the global map, pass the value `(keymap)` (an empty keymap) as keymap.
If firstonly is `non-ascii`, then the value is a single vector representing the first key sequence found, rather than a list of all possible key sequences. If firstonly is `t`, then the value is the first key sequence, except that key sequences consisting entirely of ASCII characters (or meta variants of ASCII characters) are preferred to all other key sequences and that the return value can never be a menu binding.
If noindirect is non-`nil`, `where-is-internal` doesn’t look inside menu-items to find their commands. This makes it possible to search for a menu-item itself.
The fifth argument, no-remap, determines how this function treats command remappings (see [Remapping Commands](remapping-commands)). There are two cases of interest:
If a command other-command is remapped to command:
If no-remap is `nil`, find the bindings for other-command and treat them as though they are also bindings for command. If no-remap is non-`nil`, include the vector `[remap other-command]` in the list of possible key sequences, instead of finding those bindings.
If command is remapped to other-command: If no-remap is `nil`, return the bindings for other-command rather than command. If no-remap is non-`nil`, return the bindings for command, ignoring the fact that it is remapped.
Command: **describe-bindings** *&optional prefix buffer-or-name*
This function creates a listing of all current key bindings, and displays it in a buffer named `\*Help\*`. The text is grouped by modes—minor modes first, then the major mode, then global bindings.
If prefix is non-`nil`, it should be a prefix key; then the listing includes only keys that start with prefix.
When several characters with consecutive ASCII codes have the same definition, they are shown together, as ‘`firstchar..lastchar`’. In this instance, you need to know the ASCII codes to understand which characters this means. For example, in the default global map, the characters ‘`SPC .. ~`’ are described by a single line. SPC is ASCII 32, `~` is ASCII 126, and the characters between them include all the normal printing characters, (e.g., letters, digits, punctuation, etc.); all these characters are bound to `self-insert-command`.
If buffer-or-name is non-`nil`, it should be a buffer or a buffer name. Then `describe-bindings` lists that buffer’s bindings, instead of the current buffer’s.
| programming_docs |
elisp None Debugging Lisp Programs
-----------------------
There are several ways to find and investigate problems in an Emacs Lisp program.
* If a problem occurs when you run the program, you can use the built-in Emacs Lisp debugger to suspend the Lisp evaluator, and examine and/or alter its internal state.
* You can use Edebug, a source-level debugger for Emacs Lisp.
* You can trace the execution of functions involved in the problem using the tracing facilities provided by the `trace.el` package. This package provides the functions `trace-function-foreground` and `trace-function-background` for tracing function calls, and `trace-values` for adding values of select variables to the trace. For the details, see the documentation of these facilities in `trace.el`.
* If a syntactic problem is preventing Lisp from even reading the program, you can locate it using Lisp editing commands.
* You can look at the error and warning messages produced by the byte compiler when it compiles the program. See [Compiler Errors](compiler-errors).
* You can use the Testcover package to perform coverage testing on the program.
* You can use the ERT package to write regression tests for the program. See [the ERT manual](https://www.gnu.org/software/emacs/manual/html_node/ert/index.html#Top) in ERT: Emacs Lisp Regression Testing.
* You can profile the program to get hints about how to make it more efficient.
Other useful tools for debugging input and output problems are the dribble file (see [Terminal Input](terminal-input)) and the `open-termscript` function (see [Terminal Output](terminal-output)).
| | | |
| --- | --- | --- |
| • [Debugger](debugger) | | A debugger for the Emacs Lisp evaluator. |
| • [Edebug](edebug) | | A source-level Emacs Lisp debugger. |
| • [Syntax Errors](syntax-errors) | | How to find syntax errors. |
| • [Test Coverage](test-coverage) | | Ensuring you have tested all branches in your code. |
| • [Profiling](profiling) | | Measuring the resources that your code uses. |
elisp None #### Examining Text Properties
The simplest way to examine text properties is to ask for the value of a particular property of a particular character. For that, use `get-text-property`. Use `text-properties-at` to get the entire property list of a character. See [Property Search](property-search), for functions to examine the properties of a number of characters at once.
These functions handle both strings and buffers. Keep in mind that positions in a string start from 0, whereas positions in a buffer start from 1. Passing a buffer other than the current buffer may be slow.
Function: **get-text-property** *pos prop &optional object*
This function returns the value of the prop property of the character after position pos in object (a buffer or string). The argument object is optional and defaults to the current buffer.
If there is no prop property strictly speaking, but the character has a property category that is a symbol, then `get-text-property` returns the prop property of that symbol.
Function: **get-char-property** *position prop &optional object*
This function is like `get-text-property`, except that it checks overlays first and then text properties. See [Overlays](overlays).
The argument object may be a string, a buffer, or a window. If it is a window, then the buffer displayed in that window is used for text properties and overlays, but only the overlays active for that window are considered. If object is a buffer, then overlays in that buffer are considered first, in order of decreasing priority, followed by the text properties. If object is a string, only text properties are considered, since strings never have overlays.
Function: **get-pos-property** *position prop &optional object*
This function is like `get-char-property`, except that it pays attention to properties’ stickiness and overlays’ advancement settings instead of the property of the character at (i.e., right after) position.
Function: **get-char-property-and-overlay** *position prop &optional object*
This is like `get-char-property`, but gives extra information about the overlay that the property value comes from.
Its value is a cons cell whose CAR is the property value, the same value `get-char-property` would return with the same arguments. Its CDR is the overlay in which the property was found, or `nil`, if it was found as a text property or not found at all.
If position is at the end of object, both the CAR and the CDR of the value are `nil`.
Variable: **char-property-alias-alist**
This variable holds an alist which maps property names to a list of alternative property names. If a character does not specify a direct value for a property, the alternative property names are consulted in order; the first non-`nil` value is used. This variable takes precedence over `default-text-properties`, and `category` properties take precedence over this variable.
Function: **text-properties-at** *position &optional object*
This function returns the entire property list of the character at position in the string or buffer object. If object is `nil`, it defaults to the current buffer.
Variable: **default-text-properties**
This variable holds a property list giving default values for text properties. Whenever a character does not specify a value for a property, neither directly, through a category symbol, or through `char-property-alias-alist`, the value stored in this list is used instead. Here is an example:
```
(setq default-text-properties '(foo 69)
char-property-alias-alist nil)
;; Make sure character 1 has no properties of its own.
(set-text-properties 1 2 nil)
;; What we get, when we ask, is the default value.
(get-text-property 1 'foo)
⇒ 69
```
Function: **object-intervals** *OBJECT*
This function returns a copy of the intervals (i.e., text properties) in object as a list of intervals. object must be a string or a buffer. Altering the structure of this list does not change the intervals in the object.
```
(object-intervals (propertize "foo" 'face 'bold))
⇒ ((0 3 (face bold)))
```
Each element in the returned list represents one interval. Each interval has three parts: The first is the start, the second is the end, and the third part is the text property itself.
elisp None ### Adaptive Fill Mode
When *Adaptive Fill Mode* is enabled, Emacs determines the fill prefix automatically from the text in each paragraph being filled rather than using a predetermined value. During filling, this fill prefix gets inserted at the start of the second and subsequent lines of the paragraph as described in [Filling](filling), and in [Auto Filling](auto-filling).
User Option: **adaptive-fill-mode**
Adaptive Fill mode is enabled when this variable is non-`nil`. It is `t` by default.
Function: **fill-context-prefix** *from to*
This function implements the heart of Adaptive Fill mode; it chooses a fill prefix based on the text between from and to, typically the start and end of a paragraph. It does this by looking at the first two lines of the paragraph, based on the variables described below.
Usually, this function returns the fill prefix, a string. However, before doing this, the function makes a final check (not specially mentioned in the following) that a line starting with this prefix wouldn’t look like the start of a paragraph. Should this happen, the function signals the anomaly by returning `nil` instead.
In detail, `fill-context-prefix` does this:
1. It takes a candidate for the fill prefix from the first line—it tries first the function in `adaptive-fill-function` (if any), then the regular expression `adaptive-fill-regexp` (see below). The first non-`nil` result of these, or the empty string if they’re both `nil`, becomes the first line’s candidate.
2. If the paragraph has as yet only one line, the function tests the validity of the prefix candidate just found. The function then returns the candidate if it’s valid, or a string of spaces otherwise. (see the description of `adaptive-fill-first-line-regexp` below).
3. When the paragraph already has two lines, the function next looks for a prefix candidate on the second line, in just the same way it did for the first line. If it doesn’t find one, it returns `nil`.
4. The function now compares the two candidate prefixes heuristically: if the non-whitespace characters in the line 2 candidate occur in the same order in the line 1 candidate, the function returns the line 2 candidate. Otherwise, it returns the largest initial substring which is common to both candidates (which might be the empty string).
User Option: **adaptive-fill-regexp**
Adaptive Fill mode matches this regular expression against the text starting after the left margin whitespace (if any) on a line; the characters it matches are that line’s candidate for the fill prefix.
The default value matches whitespace with certain punctuation characters intermingled.
User Option: **adaptive-fill-first-line-regexp**
Used only in one-line paragraphs, this regular expression acts as an additional check of the validity of the one available candidate fill prefix: the candidate must match this regular expression, or match `comment-start-skip`. If it doesn’t, `fill-context-prefix` replaces the candidate with a string of spaces of the same width as it.
The default value of this variable is `"\\`[ \t]*\\'"`, which matches only a string of whitespace. The effect of this default is to force the fill prefixes found in one-line paragraphs always to be pure whitespace.
User Option: **adaptive-fill-function**
You can specify more complex ways of choosing a fill prefix automatically by setting this variable to a function. The function is called with point after the left margin (if any) of a line, and it must preserve point. It should return either that line’s fill prefix or `nil`, meaning it has failed to determine a prefix.
elisp None ### When a Variable is Void
We say that a variable is void if its symbol has an unassigned value cell (see [Symbol Components](symbol-components)).
Under Emacs Lisp’s default dynamic scoping rule (see [Variable Scoping](variable-scoping)), the value cell stores the variable’s current (local or global) value. Note that an unassigned value cell is *not* the same as having `nil` in the value cell. The symbol `nil` is a Lisp object and can be the value of a variable, just as any other object can be; but it is still a value. If a variable is void, trying to evaluate the variable signals a `void-variable` error, instead of returning a value.
Under the optional lexical scoping rule, the value cell only holds the variable’s global value—the value outside of any lexical binding construct. When a variable is lexically bound, the local value is determined by the lexical environment; hence, variables can have local values even if their symbols’ value cells are unassigned.
Function: **makunbound** *symbol*
This function empties out the value cell of symbol, making the variable void. It returns symbol.
If symbol has a dynamic local binding, `makunbound` voids the current binding, and this voidness lasts only as long as the local binding is in effect. Afterwards, the previously shadowed local or global binding is reexposed; then the variable will no longer be void, unless the reexposed binding is void too.
Here are some examples (assuming dynamic binding is in effect):
```
(setq x 1) ; Put a value in the global binding.
⇒ 1
(let ((x 2)) ; Locally bind it.
(makunbound 'x) ; Void the local binding.
x)
error→ Symbol's value as variable is void: x
```
```
x ; The global binding is unchanged.
⇒ 1
(let ((x 2)) ; Locally bind it.
(let ((x 3)) ; And again.
(makunbound 'x) ; Void the innermost-local binding.
x)) ; And refer: it’s void.
error→ Symbol's value as variable is void: x
```
```
(let ((x 2))
(let ((x 3))
(makunbound 'x)) ; Void inner binding, then remove it.
x) ; Now outer `let` binding is visible.
⇒ 2
```
Function: **boundp** *variable*
This function returns `t` if variable (a symbol) is not void, and `nil` if it is void.
Here are some examples (assuming dynamic binding is in effect):
```
(boundp 'abracadabra) ; Starts out void.
⇒ nil
```
```
(let ((abracadabra 5)) ; Locally bind it.
(boundp 'abracadabra))
⇒ t
```
```
(boundp 'abracadabra) ; Still globally void.
⇒ nil
```
```
(setq abracadabra 5) ; Make it globally nonvoid.
⇒ 5
```
```
(boundp 'abracadabra)
⇒ t
```
elisp None #### Other Character Modifier Bits
The case of a graphic character is indicated by its character code; for example, ASCII distinguishes between the characters ‘`a`’ and ‘`A`’. But ASCII has no way to represent whether a control character is upper case or lower case. Emacs uses the 2\*\*25 bit to indicate that the shift key was used in typing a control character. This distinction is possible only on a graphical display such as a GUI display on X; text terminals do not report the distinction. The Lisp syntax for the shift bit is ‘`\S-`’; thus, ‘`?\C-\S-o`’ or ‘`?\C-\S-O`’ represents the shifted-control-o character.
The X Window System defines three other modifier bits that can be set in a character: *hyper*, *super* and *alt*. The syntaxes for these bits are ‘`\H-`’, ‘`\s-`’ and ‘`\A-`’. (Case is significant in these prefixes.) Thus, ‘`?\H-\M-\A-x`’ represents `Alt-Hyper-Meta-x`. (Note that ‘`\s`’ with no following ‘`-`’ represents the space character.) Numerically, the bit values are 2\*\*22 for alt, 2\*\*23 for super and 2\*\*24 for hyper.
elisp None ### Contents of Directories
A directory is a kind of file that contains other files entered under various names. Directories are a feature of the file system.
Emacs can list the names of the files in a directory as a Lisp list, or display the names in a buffer using the `ls` shell command. In the latter case, it can optionally display information about each file, depending on the options passed to the `ls` command.
Function: **directory-files** *directory &optional full-name match-regexp nosort count*
This function returns a list of the names of the files in the directory directory. By default, the list is in alphabetical order.
If full-name is non-`nil`, the function returns the files’ absolute file names. Otherwise, it returns the names relative to the specified directory.
If match-regexp is non-`nil`, this function returns only those file names whose non-directory part contain a match for that regular expression—the other file names are excluded from the list. On case-insensitive filesystems, the regular expression matching is case-insensitive.
If nosort is non-`nil`, `directory-files` does not sort the list, so you get the file names in no particular order. Use this if you want the utmost possible speed and don’t care what order the files are processed in. If the order of processing is visible to the user, then the user will probably be happier if you do sort the names.
If count is non-`nil`, the function will return names of first count number of files, or names of all files, whichever occurs first. count has to be an integer greater than zero.
```
(directory-files "~lewis")
⇒ ("#foo#" "#foo.el#" "." ".."
"dired-mods.el" "files.texi"
"files.texi.~1~")
```
An error is signaled if directory is not the name of a directory that can be read.
Function: **directory-empty-p** *directory*
This utility function returns `t` if given directory is an accessible directory and it does not contain any files, i.e., is an empty directory. It will ignore ‘`.`’ and ‘`..`’ on systems that return them as files in a directory.
Symbolic links to directories count as directories. See file-symlink-p to distinguish symlinks.
Function: **directory-files-recursively** *directory regexp &optional include-directories predicate follow-symlinks*
Return all files under directory whose names match regexp. This function searches the specified directory and its sub-directories, recursively, for files whose basenames (i.e., without the leading directories) match the specified regexp, and returns a list of the absolute file names of the matching files (see [absolute file names](relative-file-names)). The file names are returned in depth-first order, meaning that files in some sub-directory are returned before the files in its parent directory. In addition, matching files found in each subdirectory are sorted alphabetically by their basenames. By default, directories whose names match regexp are omitted from the list, but if the optional argument include-directories is non-`nil`, they are included.
By default, all subdirectories are descended into. If predicate is `t`, errors when trying to descend into a subdirectory (for instance, if it’s not readable by this user) are ignored. If it’s neither `nil` nor `t`, it should be a function that takes one parameter (the subdirectory name) and should return non-`nil` if the directory is to be descended into.
Symbolic links to subdirectories are not followed by default, but if follow-symlinks is non-`nil`, they are followed.
Function: **locate-dominating-file** *file name*
Starting at file, go up the directory tree hierarchy looking for the first directory where name, a string, exists, and return that directory. If file is a file, its directory will serve as the starting point for the search; otherwise file should be a directory from which to start. The function looks in the starting directory, then in its parent, then in its parent’s parent, etc., until it either finds a directory with name or reaches the root directory of the filesystem without finding name – in the latter case the function returns `nil`.
The argument `name` can also be a predicate function. The predicate is called for every directory examined by the function, starting from file (even if file is not a directory). It is called with one argument (the file or directory) and should return non-`nil` if that directory is the one it is looking for.
Function: **directory-files-and-attributes** *directory &optional full-name match-regexp nosort id-format count*
This is similar to `directory-files` in deciding which files to report on and how to report their names. However, instead of returning a list of file names, it returns for each file a list `(filename . attributes)`, where attributes is what `file-attributes` returns for that file. The optional argument id-format has the same meaning as the corresponding argument to `file-attributes` (see [Definition of file-attributes](file-attributes#Definition-of-file_002dattributes)).
Constant: **directory-files-no-dot-files-regexp**
This regular expression matches any file name except ‘`.`’ and ‘`..`’. More precisely, it matches parts of any nonempty string except those two. It is useful as the match-regexp argument to `directory-files` and `directory-files-and-attributes`:
```
(directory-files "/foo" nil directory-files-no-dot-files-regexp)
```
returns `nil`, if directory ‘`/foo`’ is empty.
Function: **file-expand-wildcards** *pattern &optional full*
This function expands the wildcard pattern pattern, returning a list of file names that match it.
If pattern is written as an absolute file name, the values are absolute also.
If pattern is written as a relative file name, it is interpreted relative to the current default directory. The file names returned are normally also relative to the current default directory. However, if full is non-`nil`, they are absolute.
Function: **insert-directory** *file switches &optional wildcard full-directory-p*
This function inserts (in the current buffer) a directory listing for directory file, formatted with `ls` according to switches. It leaves point after the inserted text. switches may be a string of options, or a list of strings representing individual options.
The argument file may be either a directory or a file specification including wildcard characters. If wildcard is non-`nil`, that means treat file as a file specification with wildcards.
If full-directory-p is non-`nil`, that means the directory listing is expected to show the full contents of a directory. You should specify `t` when file is a directory and switches do not contain ‘`-d`’. (The ‘`-d`’ option to `ls` says to describe a directory itself as a file, rather than showing its contents.)
On most systems, this function works by running a directory listing program whose name is in the variable `insert-directory-program`. If wildcard is non-`nil`, it also runs the shell specified by `shell-file-name`, to expand the wildcards.
MS-DOS and MS-Windows systems usually lack the standard Unix program `ls`, so this function emulates the standard Unix program `ls` with Lisp code.
As a technical detail, when switches contains the long ‘`--dired`’ option, `insert-directory` treats it specially, for the sake of dired. However, the normally equivalent short ‘`-D`’ option is just passed on to `insert-directory-program`, as any other option.
Variable: **insert-directory-program**
This variable’s value is the program to run to generate a directory listing for the function `insert-directory`. It is ignored on systems which generate the listing with Lisp code.
| programming_docs |
elisp None #### Edebug Recursive Edit
When Edebug is entered and actually reads commands from the user, it saves (and later restores) these additional data:
* The current match data. See [Match Data](match-data).
* The variables `last-command`, `this-command`, `last-command-event`, `last-input-event`, `last-event-frame`, `last-nonmenu-event`, and `track-mouse`. Commands in Edebug do not affect these variables outside of Edebug. Executing commands within Edebug can change the key sequence that would be returned by `this-command-keys`, and there is no way to reset the key sequence from Lisp.
Edebug cannot save and restore the value of `unread-command-events`. Entering Edebug while this variable has a nontrivial value can interfere with execution of the program you are debugging.
* Complex commands executed while in Edebug are added to the variable `command-history`. In rare cases this can alter execution.
* Within Edebug, the recursion depth appears one deeper than the recursion depth outside Edebug. This is not true of the automatically updated evaluation list window.
* `standard-output` and `standard-input` are bound to `nil` by the `recursive-edit`, but Edebug temporarily restores them during evaluations.
* The state of keyboard macro definition is saved and restored. While Edebug is active, `defining-kbd-macro` is bound to `edebug-continue-kbd-macro`.
elisp None ### Features
`provide` and `require` are an alternative to `autoload` for loading files automatically. They work in terms of named *features*. Autoloading is triggered by calling a specific function, but a feature is loaded the first time another program asks for it by name.
A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should *provide* the feature. Another program that uses them may ensure they are defined by *requiring* the feature. This loads the file of definitions if it hasn’t been loaded already.
To require the presence of a feature, call `require` with the feature name as argument. `require` looks in the global variable `features` to see whether the desired feature has been provided already. If not, it loads the feature from the appropriate file. This file should call `provide` at the top level to add the feature to `features`; if it fails to do so, `require` signals an error.
For example, in `idlwave.el`, the definition for `idlwave-complete-filename` includes the following code:
```
(defun idlwave-complete-filename ()
"Use the comint stuff to complete a file name."
(require 'comint)
(let* ((comint-file-name-chars "~/A-Za-z0-9+@:_.$#%={}\\-")
(comint-completion-addsuffix nil)
...)
(comint-dynamic-complete-filename)))
```
The expression `(require 'comint)` loads the file `comint.el` if it has not yet been loaded, ensuring that `comint-dynamic-complete-filename` is defined. Features are normally named after the files that provide them, so that `require` need not be given the file name. (Note that it is important that the `require` statement be outside the body of the `let`. Loading a library while its variables are let-bound can have unintended consequences, namely the variables becoming unbound after the let exits.)
The `comint.el` file contains the following top-level expression:
```
(provide 'comint)
```
This adds `comint` to the global `features` list, so that `(require 'comint)` will henceforth know that nothing needs to be done.
When `require` is used at top level in a file, it takes effect when you byte-compile that file (see [Byte Compilation](byte-compilation)) as well as when you load it. This is in case the required package contains macros that the byte compiler must know about. It also avoids byte compiler warnings for functions and variables defined in the file loaded with `require`.
Although top-level calls to `require` are evaluated during byte compilation, `provide` calls are not. Therefore, you can ensure that a file of definitions is loaded before it is byte-compiled by including a `provide` followed by a `require` for the same feature, as in the following example.
```
(provide 'my-feature) ; Ignored by byte compiler,
; evaluated by `load`.
(require 'my-feature) ; Evaluated by byte compiler.
```
The compiler ignores the `provide`, then processes the `require` by loading the file in question. Loading the file does execute the `provide` call, so the subsequent `require` call does nothing when the file is loaded.
Function: **provide** *feature &optional subfeatures*
This function announces that feature is now loaded, or being loaded, into the current Emacs session. This means that the facilities associated with feature are or will be available for other Lisp programs.
The direct effect of calling `provide` is to add feature to the front of `features` if it is not already in that list and call any `eval-after-load` code waiting for it (see [Hooks for Loading](hooks-for-loading)). The argument feature must be a symbol. `provide` returns feature.
If provided, subfeatures should be a list of symbols indicating a set of specific subfeatures provided by this version of feature. You can test the presence of a subfeature using `featurep`. The idea of subfeatures is that you use them when a package (which is one feature) is complex enough to make it useful to give names to various parts or functionalities of the package, which might or might not be loaded, or might or might not be present in a given version. See [Network Feature Testing](network-feature-testing), for an example.
```
features
⇒ (bar bish)
(provide 'foo)
⇒ foo
features
⇒ (foo bar bish)
```
When a file is loaded to satisfy an autoload, and it stops due to an error in the evaluation of its contents, any function definitions or `provide` calls that occurred during the load are undone. See [Autoload](autoload).
Function: **require** *feature &optional filename noerror*
This function checks whether feature is present in the current Emacs session (using `(featurep feature)`; see below). The argument feature must be a symbol.
If the feature is not present, then `require` loads filename with `load`. If filename is not supplied, then the name of the symbol feature is used as the base file name to load. However, in this case, `require` insists on finding feature with an added ‘`.el`’ or ‘`.elc`’ suffix (possibly extended with a compression suffix); a file whose name is just feature won’t be used. (The variable `load-suffixes` specifies the exact required Lisp suffixes.)
If noerror is non-`nil`, that suppresses errors from actual loading of the file. In that case, `require` returns `nil` if loading the file fails. Normally, `require` returns feature.
If loading the file succeeds but does not provide feature, `require` signals an error about the missing feature.
Function: **featurep** *feature &optional subfeature*
This function returns `t` if feature has been provided in the current Emacs session (i.e., if feature is a member of `features`.) If subfeature is non-`nil`, then the function returns `t` only if that subfeature is provided as well (i.e., if subfeature is a member of the `subfeature` property of the feature symbol.)
Variable: **features**
The value of this variable is a list of symbols that are the features loaded in the current Emacs session. Each symbol was put in this list with a call to `provide`. The order of the elements in the `features` list is not significant.
elisp None ### Imenu
*Imenu* is a feature that lets users select a definition or section in the buffer, from a menu which lists all of them, to go directly to that location in the buffer. Imenu works by constructing a buffer index which lists the names and buffer positions of the definitions, or other named portions of the buffer; then the user can choose one of them and move point to it. Major modes can add a menu bar item to use Imenu using `imenu-add-to-menubar`.
Command: **imenu-add-to-menubar** *name*
This function defines a local menu bar item named name to run Imenu.
The user-level commands for using Imenu are described in the Emacs Manual (see [Imenu](https://www.gnu.org/software/emacs/manual/html_node/emacs/Imenu.html#Imenu) in the Emacs Manual). This section explains how to customize Imenu’s method of finding definitions or buffer portions for a particular major mode.
The usual and simplest way is to set the variable `imenu-generic-expression`:
Variable: **imenu-generic-expression**
This variable, if non-`nil`, is a list that specifies regular expressions for finding definitions for Imenu. Simple elements of `imenu-generic-expression` look like this:
```
(menu-title regexp index)
```
Here, if menu-title is non-`nil`, it says that the matches for this element should go in a submenu of the buffer index; menu-title itself specifies the name for the submenu. If menu-title is `nil`, the matches for this element go directly in the top level of the buffer index.
The second item in the list, regexp, is a regular expression (see [Regular Expressions](regular-expressions)); anything in the buffer that it matches is considered a definition, something to mention in the buffer index. The third item, index, is a non-negative integer that indicates which subexpression in regexp matches the definition’s name.
An element can also look like this:
```
(menu-title regexp index function arguments…)
```
Each match for this element creates an index item, and when the index item is selected by the user, it calls function with arguments consisting of the item name, the buffer position, and arguments.
For Emacs Lisp mode, `imenu-generic-expression` could look like this:
```
((nil "^\\s-*(def\\(un\\|subst\\|macro\\|advice\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2)
```
```
("*Vars*" "^\\s-*(def\\(var\\|const\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2)
```
```
("*Types*"
"^\\s-*\
(def\\(type\\|struct\\|class\\|ine-condition\\)\
\\s-+\\([-A-Za-z0-9+]+\\)" 2))
```
Setting this variable makes it buffer-local in the current buffer.
Variable: **imenu-case-fold-search**
This variable controls whether matching against the regular expressions in the value of `imenu-generic-expression` is case-sensitive: `t`, the default, means matching should ignore case.
Setting this variable makes it buffer-local in the current buffer.
Variable: **imenu-syntax-alist**
This variable is an alist of syntax table modifiers to use while processing `imenu-generic-expression`, to override the syntax table of the current buffer. Each element should have this form:
```
(characters . syntax-description)
```
The CAR, characters, can be either a character or a string. The element says to give that character or characters the syntax specified by syntax-description, which is passed to `modify-syntax-entry` (see [Syntax Table Functions](syntax-table-functions)).
This feature is typically used to give word syntax to characters which normally have symbol syntax, and thus to simplify `imenu-generic-expression` and speed up matching. For example, Fortran mode uses it this way:
```
(setq imenu-syntax-alist '(("_$" . "w")))
```
The `imenu-generic-expression` regular expressions can then use ‘`\\sw+`’ instead of ‘`\\(\\sw\\|\\s\_\\)+`’. Note that this technique may be inconvenient when the mode needs to limit the initial character of a name to a smaller set of characters than are allowed in the rest of a name.
Setting this variable makes it buffer-local in the current buffer.
Another way to customize Imenu for a major mode is to set the variables `imenu-prev-index-position-function` and `imenu-extract-index-name-function`:
Variable: **imenu-prev-index-position-function**
If this variable is non-`nil`, its value should be a function that finds the next definition to put in the buffer index, scanning backward in the buffer from point. It should return `nil` if it doesn’t find another definition before point. Otherwise it should leave point at the place it finds a definition and return any non-`nil` value.
Setting this variable makes it buffer-local in the current buffer.
Variable: **imenu-extract-index-name-function**
If this variable is non-`nil`, its value should be a function to return the name for a definition, assuming point is in that definition as the `imenu-prev-index-position-function` function would leave it.
Setting this variable makes it buffer-local in the current buffer.
The last way to customize Imenu for a major mode is to set the variable `imenu-create-index-function`:
Variable: **imenu-create-index-function**
This variable specifies the function to use for creating a buffer index. The function should take no arguments, and return an index alist for the current buffer. It is called within `save-excursion`, so where it leaves point makes no difference.
The index alist can have three types of elements. Simple elements look like this:
```
(index-name . index-position)
```
Selecting a simple element has the effect of moving to position index-position in the buffer. Special elements look like this:
```
(index-name index-position function arguments…)
```
Selecting a special element performs:
```
(funcall function
index-name index-position arguments…)
```
A nested sub-alist element looks like this:
```
(menu-title . sub-alist)
```
It creates the submenu menu-title specified by sub-alist.
The default value of `imenu-create-index-function` is `imenu-default-create-index-function`. This function calls the value of `imenu-prev-index-position-function` and the value of `imenu-extract-index-name-function` to produce the index alist. However, if either of these two variables is `nil`, the default function uses `imenu-generic-expression` instead.
Setting this variable makes it buffer-local in the current buffer.
elisp None ### Native-Compilation Functions
Native-Compilation is implemented as a side effect of byte-compilation (see [Byte Compilation](byte-compilation)). Thus, compiling Lisp code natively always produces its byte code as well, and therefore all the rules and caveats of preparing Lisp code for byte compilation (see [Compilation Functions](compilation-functions)) are valid for native-compilation as well.
You can natively-compile either a single function or macro definition, or a whole file of Lisp code, with the `native-compile` function. Natively-compiling a file will produce both the corresponding `.elc` file with byte code and the `.eln` file with native code.
Native compilation might produce warning or error messages; these are normally recorded in the buffer called `\*Native-compile-Log\*`. In interactive sessions, it uses the special LIMPLE mode (`native-comp-limple-mode`), which sets up `font-lock` as appropriate for this log, and is otherwise the same as Fundamental mode. Logging of messages resulting from native-compilation can be controlled by the `native-comp-verbose` variable (see [Native-Compilation Variables](native_002dcompilation-variables)).
When Emacs is run non-interactively, messages produced by native-compilation are reported by calling `message` (see [Displaying Messages](displaying-messages)), and are usually displayed on the standard error stream of the terminal from which Emacs was invoked.
Function: **native-compile** *function-or-file &optional output*
This function compiles function-or-file into native code. The argument function-or-file can be a function symbol, a Lisp form, or a name (a string) of the file which contains the Emacs Lisp source code to compile. If the optional argument output is provided, it must be a string specifying the name of the file to write the compiled code into. Otherwise, if function-or-file is a function or a Lisp form, this function returns the compiled object, and if function-or-file is a file name, the function returns the full absolute name of the file it created for the compiled code. The output file is by default given the `.eln` extension.
This function runs the final phase of the native compilation, which invokes GCC via `libgccjit`, in a separate subprocess, which invokes the same Emacs executable as the process that called this function.
Function: **batch-native-compile** *&optional for-tarball*
This function runs native-compilation on files specified on the Emacs command line in batch mode. It must be used only in a batch execution of Emacs, as it kills Emacs upon completion of the compilation. If one or more of the files fail to compile, the Emacs process will attempt to compile all the other files, and will terminate with a non-zero status code. The optional argument for-tarball, if non-`nil`, tells the function to place the resulting `.eln` files in the last directory mentioned in `native-comp-eln-load-path` (see [Library Search](library-search)); this is meant to be used as part of building an Emacs source tarball for the first time, when the natively-compiled files, which are absent from the source tarball, should be generated in the build tree instead of the user’s cache directory.
Native compilation can be run entirely asynchronously, in a subprocess of the main Emacs process. This leaves the main Emacs process free to use while the compilation runs in the background. This is the method used by Emacs to natively-compile any Lisp file or byte-compiled Lisp file that is loaded into Emacs, when no natively-compiled file for it is available. Note that because of this use of a subprocess, native compilation may produce warning and errors which byte-compilation does not, and lisp code may thus need to be modified to work correctly. See `native-comp-async-report-warnings-errors` in see [Native-Compilation Variables](native_002dcompilation-variables) for more details.
Function: **native-compile-async** *files &optional recursively load selector*
This function compiles the named files asynchronously. The argument files should be a single file name (a string) or a list of one or more file and/or directory names. If directories are present in the list, the optional argument recursively should be non-`nil` to cause the compilation to recurse into those directories. If load is non-`nil`, Emacs will load each file that it succeeded to compile. The optional argument selector allows control of which of files will be compiled; it can have one of the following values:
`nil` or omitted Select all the files and directories in files.
a regular expression string Select the files and directories whose names match the regexp.
a function A predicate function, which will be called with each file and directory in files, and should return non-`nil` if the file or the directory should be selected for compilation.
On systems with multiple CPU execution units, when files names more than one file, this function will normally start several compilation subprocesses in parallel, under the control of `native-comp-async-jobs-number` (see [Native-Compilation Variables](native_002dcompilation-variables)).
The following function allows Lisp programs to test whether native-compilation is available at runtime.
Function: **native-comp-available-p**
This function returns non-`nil` if the running Emacs process has the native-compilation support compiled into it. On systems that load `libgccjit` dynamically, it also makes sure that library is available and can be loaded. Lisp programs that need to know up front whether native-compilation is available should use this predicate.
elisp None ### Network Connections
Emacs Lisp programs can open stream (TCP) and datagram (UDP) network connections (see [Datagrams](datagrams)) to other processes on the same machine or other machines. A network connection is handled by Lisp much like a subprocess, and is represented by a process object. However, the process you are communicating with is not a child of the Emacs process, has no process ID, and you can’t kill it or send it signals. All you can do is send and receive data. `delete-process` closes the connection, but does not kill the program at the other end; that program must decide what to do about closure of the connection.
Lisp programs can listen for connections by creating network servers. A network server is also represented by a kind of process object, but unlike a network connection, the network server never transfers data itself. When it receives a connection request, it creates a new network connection to represent the connection just made. (The network connection inherits certain information, including the process plist, from the server.) The network server then goes back to listening for more connection requests.
Network connections and servers are created by calling `make-network-process` with an argument list consisting of keyword/argument pairs, for example `:server t` to create a server process, or `:type 'datagram` to create a datagram connection. See [Low-Level Network](low_002dlevel-network), for details. You can also use the `open-network-stream` function described below.
To distinguish the different types of processes, the `process-type` function returns the symbol `network` for a network connection or server, `serial` for a serial port connection, `pipe` for a pipe connection, or `real` for a real subprocess.
The `process-status` function returns `open`, `closed`, `connect`, `stop`, or `failed` for network connections. For a network server, the status is always `listen`. Except for `stop`, none of those values is possible for a real subprocess. See [Process Information](process-information).
You can stop and resume operation of a network process by calling `stop-process` and `continue-process`. For a server process, being stopped means not accepting new connections. (Up to 5 connection requests will be queued for when you resume the server; you can increase this limit, unless it is imposed by the operating system—see the `:server` keyword of `make-network-process`, [Network Processes](network-processes).) For a network stream connection, being stopped means not processing input (any arriving input waits until you resume the connection). For a datagram connection, some number of packets may be queued but input may be lost. You can use the function `process-command` to determine whether a network connection or server is stopped; a non-`nil` value means yes.
Emacs can create encrypted network connections, using the built-in support for the GnuTLS Transport Layer Security Library; see [the GnuTLS project page](https://www.gnu.org/software/gnutls/). If your Emacs was compiled with GnuTLS support, the function `gnutls-available-p` is defined and returns non-`nil`. For more details, see [Overview](https://www.gnu.org/software/emacs/manual/html_node/emacs-gnutls/index.html#Top) in The Emacs-GnuTLS manual. The `open-network-stream` function can transparently handle the details of creating encrypted connections for you, using whatever support is available.
Function: **open-network-stream** *name buffer host service &rest parameters*
This function opens a TCP connection, with optional encryption, and returns a process object that represents the connection.
The name argument specifies the name for the process object. It is modified as necessary to make it unique.
The buffer argument is the buffer to associate with the connection. Output from the connection is inserted in the buffer, unless you specify your own filter function to handle the output. If buffer is `nil`, it means that the connection is not associated with any buffer.
The arguments host and service specify where to connect to; host is the host name (a string), and service is the name of a defined network service (a string) or a port number (an integer like `80` or an integer string like `"80"`).
The remaining arguments parameters are keyword/argument pairs that are mainly relevant to encrypted connections:
`:nowait boolean`
If non-`nil`, try to make an asynchronous connection.
`:coding coding`
Use this to set the coding systems used by the network process, in preference to binding `coding-system-for-read` or `coding-system-for-write`. See [Network Processes](network-processes), for details.
`:type type`
The type of connection. Options are:
`plain` An ordinary, unencrypted connection.
`tls` `ssl` A TLS (Transport Layer Security) connection.
`nil` `network` Start with a plain connection, and if parameters ‘`:success`’ and ‘`:capability-command`’ are supplied, try to upgrade to an encrypted connection via STARTTLS. If that fails, retain the unencrypted connection.
`starttls` As for `nil`, but if STARTTLS fails drop the connection.
`shell` A shell connection.
`:always-query-capabilities boolean`
If non-`nil`, always ask for the server’s capabilities, even when doing a ‘`plain`’ connection.
`:capability-command capability-command`
Command to query the host capabilities. This can either be a string (which will then be sent verbatim to the server), or a function (called with a single parameter; the "greeting" from the server when connecting), and should return a string.
`:end-of-command regexp` `:end-of-capability regexp`
Regular expression matching the end of a command, or the end of the command capability-command. The latter defaults to the former.
`:starttls-function function`
Function of one argument (the response to capability-command), which returns either `nil`, or the command to activate STARTTLS if supported.
`:success regexp`
Regular expression matching a successful STARTTLS negotiation.
`:use-starttls-if-possible boolean`
If non-`nil`, do opportunistic STARTTLS upgrades even if Emacs doesn’t have built-in TLS support.
`:warn-unless-encrypted boolean`
If non-`nil`, and `:return-value` is also non-`nil`, Emacs will warn if the connection isn’t encrypted. This is useful for protocols like IMAP and the like, where most users would expect the network traffic to be encrypted.
`:client-certificate list-or-t`
Either a list of the form `(key-file cert-file)`, naming the certificate key file and certificate file itself, or `t`, meaning to query `auth-source` for this information (see [auth-source](https://www.gnu.org/software/emacs/manual/html_node/auth/Help-for-users.html#Help-for-users) in Emacs auth-source Library). Only used for TLS or STARTTLS. To enable automatic queries of `auth-source` when `:client-certificate` is not specified customize `network-stream-use-client-certificates` to t.
`:return-list cons-or-nil`
The return value of this function. If omitted or `nil`, return a process object. Otherwise, a cons of the form `(process-object
. plist)`, where plist has keywords:
`:greeting string-or-nil` If non-`nil`, the greeting string returned by the host.
`:capabilities string-or-nil` If non-`nil`, the host’s capability string.
`:type symbol` The connection type: ‘`plain`’ or ‘`tls`’.
`:shell-command string-or-nil`
If the connection `type` is `shell`, this parameter will be interpreted as a format-spec string that will be executed to make the connection. The specs available are ‘`%s`’ for the host name and ‘`%p`’ for the port number. For instance, if you want to first ssh to ‘`gateway`’ before making a plain connection, then this parameter could be something like ‘`ssh gateway nc %s %p`’.
| programming_docs |
elisp None #### Simple Types
This section describes all the simple customization types. For several of these customization types, the customization widget provides inline completion with `C-M-i` or `M-TAB`.
`sexp`
The value may be any Lisp object that can be printed and read back. You can use `sexp` as a fall-back for any option, if you don’t want to take the time to work out a more specific type to use.
`integer`
The value must be an integer.
`natnum`
The value must be a nonnegative integer.
`number`
The value must be a number (floating point or integer).
`float`
The value must be floating point.
`string`
The value must be a string. The customization buffer shows the string without delimiting ‘`"`’ characters or ‘`\`’ quotes.
`regexp`
Like `string` except that the string must be a valid regular expression.
`character`
The value must be a character code. A character code is actually an integer, but this type shows the value by inserting the character in the buffer, rather than by showing the number.
`file`
The value must be a file name. The widget provides completion.
`(file :must-match t)`
The value must be a file name for an existing file. The widget provides completion.
`directory`
The value must be a directory. The widget provides completion.
`hook`
The value must be a list of functions. This customization type is used for hook variables. You can use the `:options` keyword in a hook variable’s `defcustom` to specify a list of functions recommended for use in the hook; See [Variable Definitions](variable-definitions).
`symbol`
The value must be a symbol. It appears in the customization buffer as the symbol name. The widget provides completion.
`function`
The value must be either a lambda expression or a function name. The widget provides completion for function names.
`variable`
The value must be a variable name. The widget provides completion.
`face`
The value must be a symbol which is a face name. The widget provides completion.
`boolean`
The value is boolean—either `nil` or `t`. Note that by using `choice` and `const` together (see the next section), you can specify that the value must be `nil` or `t`, but also specify the text to describe each value in a way that fits the specific meaning of the alternative.
`key-sequence`
The value is a key sequence. The customization buffer shows the key sequence using the same syntax as the `kbd` function. See [Key Sequences](key-sequences).
`coding-system`
The value must be a coding-system name, and you can do completion with `M-TAB`.
`color` The value must be a valid color name. The widget provides completion for color names, as well as a sample and a button for selecting a color name from a list of color names shown in a `\*Colors\*` buffer.
elisp None ### Horizontal Scrolling
*Horizontal scrolling* means shifting the image in the window left or right by a specified multiple of the normal character width. Each window has a *horizontal scroll position*, which is a number, never less than zero. It specifies how far to shift the contents left. Shifting the window contents left generally makes all or part of some characters disappear off the left, and all or part of some other characters appear at the right. The usual value is zero.
The horizontal scroll position is measured in units of the normal character width, which is the width of space in the default font. Thus, if the value is 5, that means the window contents are scrolled left by 5 times the normal character width. How many characters actually disappear off to the left depends on their width, and could vary from line to line.
Because we read from side to side in the inner loop, and from top to bottom in the outer loop, the effect of horizontal scrolling is not like that of textual or vertical scrolling. Textual scrolling involves selection of a portion of text to display, and vertical scrolling moves the window contents contiguously; but horizontal scrolling causes part of *each line* to go off screen.
Usually, no horizontal scrolling is in effect; then the leftmost column is at the left edge of the window. In this state, scrolling to the right is meaningless, since there is no data to the left of the edge to be revealed by it; so this is not allowed. Scrolling to the left is allowed; it scrolls the first columns of text off the edge of the window and can reveal additional columns on the right that were truncated before. Once a window has a nonzero amount of leftward horizontal scrolling, you can scroll it back to the right, but only so far as to reduce the net horizontal scroll to zero. There is no limit to how far left you can scroll, but eventually all the text will disappear off the left edge.
If `auto-hscroll-mode` is set, redisplay automatically alters the horizontal scrolling of a window as necessary to ensure that point is always visible. However, you can still set the horizontal scrolling value explicitly. The value you specify serves as a lower bound for automatic scrolling, i.e., automatic scrolling will not scroll a window to a column less than the specified one.
The default value of `auto-hscroll-mode` is `t`; setting it to `current-line` activates a variant of automatic horizontal scrolling whereby only the line showing the cursor is horizontally scrolled to make point visible, the rest of the window is left either unscrolled, or at the minimum scroll amount set by `scroll-left` and `scroll-right`, see below.
Command: **scroll-left** *&optional count set-minimum*
This function scrolls the selected window count columns to the left (or to the right if count is negative). The default for count is the window width, minus 2.
The return value is the total amount of leftward horizontal scrolling in effect after the change—just like the value returned by `window-hscroll` (below).
Note that text in paragraphs whose base direction is right-to-left (see [Bidirectional Display](bidirectional-display)) moves in the opposite direction: e.g., it moves to the right when `scroll-left` is invoked with a positive value of count.
Once you scroll a window as far right as it can go, back to its normal position where the total leftward scrolling is zero, attempts to scroll any farther right have no effect.
If set-minimum is non-`nil`, the new scroll amount becomes the lower bound for automatic scrolling; that is, automatic scrolling will not scroll a window to a column less than the value returned by this function. Interactive calls pass non-`nil` for set-minimum.
Command: **scroll-right** *&optional count set-minimum*
This function scrolls the selected window count columns to the right (or to the left if count is negative). The default for count is the window width, minus 2. Aside from the direction of scrolling, this works just like `scroll-left`.
Function: **window-hscroll** *&optional window*
This function returns the total leftward horizontal scrolling of window—the number of columns by which the text in window is scrolled left past the left margin. (In right-to-left paragraphs, the value is the total amount of the rightward scrolling instead.) The default for window is the selected window.
The return value is never negative. It is zero when no horizontal scrolling has been done in window (which is usually the case).
```
(window-hscroll)
⇒ 0
```
```
(scroll-left 5)
⇒ 5
```
```
(window-hscroll)
⇒ 5
```
Function: **set-window-hscroll** *window columns*
This function sets horizontal scrolling of window. The value of columns specifies the amount of scrolling, in terms of columns from the left margin (right margin in right-to-left paragraphs). The argument columns should be zero or positive; if not, it is taken as zero. Fractional values of columns are not supported at present.
Note that `set-window-hscroll` may appear not to work if you test it by evaluating a call with `M-:` in a simple way. What happens is that the function sets the horizontal scroll value and returns, but then redisplay adjusts the horizontal scrolling to make point visible, and this overrides what the function did. You can observe the function’s effect if you call it while point is sufficiently far from the left margin that it will remain visible.
The value returned is columns.
```
(set-window-hscroll (selected-window) 10)
⇒ 10
```
Here is how you can determine whether a given position position is off the screen due to horizontal scrolling:
```
(defun hscroll-on-screen (window position)
(save-excursion
(goto-char position)
(and
(>= (- (current-column) (window-hscroll window)) 0)
(< (- (current-column) (window-hscroll window))
(window-width window)))))
```
elisp None #### Autoload by Prefix
During completion for the commands `describe-variable` and `describe-function`, Emacs will try to load files which may contain definitions matching the prefix being completed. The variable `definition-prefixes` holds a hashtable which maps a prefix to the corresponding list of files to load for it. Entries to this mapping are added by calls to `register-definition-prefixes` which are generated by `update-file-autoloads` (see [Autoload](autoload)). Files which don’t contain any definitions worth loading (test files, for example), should set `autoload-compute-prefixes` to `nil` as a file-local variable.
elisp None ### Abbrev Table Properties
Like abbrevs, abbrev tables have properties, some of which influence the way they work. You can provide them as arguments to `define-abbrev-table`, and manipulate them with the functions:
Function: **abbrev-table-put** *table prop val*
Set the property prop of abbrev table table to value val.
Function: **abbrev-table-get** *table prop*
Return the property prop of abbrev table table, or `nil` if table has no such property.
The following properties have special meaning:
`:enable-function`
This is like the `:enable-function` abbrev property except that it applies to all abbrevs in the table. It is used before even trying to find the abbrev before point, so it can dynamically modify the abbrev table.
`:case-fixed`
This is like the `:case-fixed` abbrev property except that it applies to all abbrevs in the table.
`:regexp`
If non-`nil`, this property is a regular expression that indicates how to extract the name of the abbrev before point, before looking it up in the table. When the regular expression matches before point, the abbrev name is expected to be in submatch 1. If this property is `nil`, the default is to use `backward-word` and `forward-word` to find the name. This property allows the use of abbrevs whose name contains characters of non-word syntax.
`:parents`
This property holds a list of tables from which to inherit other abbrevs.
`:abbrev-table-modiff`
This property holds a counter incremented each time a new abbrev is added to the table.
elisp None ### Point
*Point* is a special buffer position used by many editing commands, including the self-inserting typed characters and text insertion functions. Other commands move point through the text to allow editing and insertion at different places.
Like other positions, point designates a place between two characters (or before the first character, or after the last character), rather than a particular character. Usually terminals display the cursor over the character that immediately follows point; point is actually before the character on which the cursor sits.
The value of point is a number no less than 1, and no greater than the buffer size plus 1. If narrowing is in effect (see [Narrowing](narrowing)), then point is constrained to fall within the accessible portion of the buffer (possibly at one end of it).
Each buffer has its own value of point, which is independent of the value of point in other buffers. Each window also has a value of point, which is independent of the value of point in other windows on the same buffer. This is why point can have different values in various windows that display the same buffer. When a buffer appears in only one window, the buffer’s point and the window’s point normally have the same value, so the distinction is rarely important. See [Window Point](window-point), for more details.
Function: **point**
This function returns the value of point in the current buffer, as an integer.
```
(point)
⇒ 175
```
Function: **point-min**
This function returns the minimum accessible value of point in the current buffer. This is normally 1, but if narrowing is in effect, it is the position of the start of the region that you narrowed to. (See [Narrowing](narrowing).)
Function: **point-max**
This function returns the maximum accessible value of point in the current buffer. This is `(1+ (buffer-size))`, unless narrowing is in effect, in which case it is the position of the end of the region that you narrowed to. (See [Narrowing](narrowing).)
Function: **buffer-end** *flag*
This function returns `(point-max)` if flag is greater than 0, `(point-min)` otherwise. The argument flag must be a number.
Function: **buffer-size** *&optional buffer*
This function returns the total number of characters in the current buffer. In the absence of any narrowing (see [Narrowing](narrowing)), `point-max` returns a value one larger than this.
If you specify a buffer, buffer, then the value is the size of buffer.
```
(buffer-size)
⇒ 35
```
```
(point-max)
⇒ 36
```
elisp None ### Overlays
You can use *overlays* to alter the appearance of a buffer’s text on the screen, for the sake of presentation features. An overlay is an object that belongs to a particular buffer, and has a specified beginning and end. It also has properties that you can examine and set; these affect the display of the text within the overlay.
The visual effect of an overlay is the same as of the corresponding text property (see [Text Properties](text-properties)). However, due to a different implementation, overlays generally don’t scale well (many operations take a time that is proportional to the number of overlays in the buffer). If you need to affect the visual appearance of many portions in the buffer, we recommend using text properties.
An overlay uses markers to record its beginning and end; thus, editing the text of the buffer adjusts the beginning and end of each overlay so that it stays with the text. When you create the overlay, you can specify whether text inserted at the beginning should be inside the overlay or outside, and likewise for the end of the overlay.
| | | |
| --- | --- | --- |
| • [Managing Overlays](managing-overlays) | | Creating and moving overlays. |
| • [Overlay Properties](overlay-properties) | | How to read and set properties. What properties do to the screen display. |
| • [Finding Overlays](finding-overlays) | | Searching for overlays. |
elisp None ### Undo
Most buffers have an *undo list*, which records all changes made to the buffer’s text so that they can be undone. (The buffers that don’t have one are usually special-purpose buffers for which Emacs assumes that undoing is not useful. In particular, any buffer whose name begins with a space has its undo recording off by default; see [Buffer Names](buffer-names).) All the primitives that modify the text in the buffer automatically add elements to the front of the undo list, which is in the variable `buffer-undo-list`.
Variable: **buffer-undo-list**
This buffer-local variable’s value is the undo list of the current buffer. A value of `t` disables the recording of undo information.
Here are the kinds of elements an undo list can have:
`position`
This kind of element records a previous value of point; undoing this element moves point to position. Ordinary cursor motion does not make any sort of undo record, but deletion operations use these entries to record where point was before the command.
`(beg . end)`
This kind of element indicates how to delete text that was inserted. Upon insertion, the text occupied the range beg–end in the buffer.
`(text . position)`
This kind of element indicates how to reinsert text that was deleted. The deleted text itself is the string text. The place to reinsert it is `(abs position)`. If position is positive, point was at the beginning of the deleted text, otherwise it was at the end. Zero or more (marker . adjustment) elements follow immediately after this element.
`(t . time-flag)`
This kind of element indicates that an unmodified buffer became modified. A time-flag that is a non-integer Lisp timestamp represents the visited file’s modification time as of when it was previously visited or saved, using the same format as `current-time`; see [Time of Day](time-of-day). A time-flag of 0 means the buffer does not correspond to any file; -1 means the visited file previously did not exist. `primitive-undo` uses these values to determine whether to mark the buffer as unmodified once again; it does so only if the file’s status matches that of time-flag.
`(nil property value beg . end)`
This kind of element records a change in a text property. Here’s how you might undo the change:
```
(put-text-property beg end property value)
```
`(marker . adjustment)`
This kind of element records the fact that the marker marker was relocated due to deletion of surrounding text, and that it moved adjustment character positions. If the marker’s location is consistent with the (text . position) element preceding it in the undo list, then undoing this element moves marker - adjustment characters.
`(apply funname . args)`
This is an extensible undo item, which is undone by calling funname with arguments args.
`(apply delta beg end funname . args)`
This is an extensible undo item, which records a change limited to the range beg to end, which increased the size of the buffer by delta characters. It is undone by calling funname with arguments args.
This kind of element enables undo limited to a region to determine whether the element pertains to that region.
`nil` This element is a boundary. The elements between two boundaries are called a *change group*; normally, each change group corresponds to one keyboard command, and undo commands normally undo an entire group as a unit.
Function: **undo-boundary**
This function places a boundary element in the undo list. The undo command stops at such a boundary, and successive undo commands undo to earlier and earlier boundaries. This function returns `nil`.
Calling this function explicitly is useful for splitting the effects of a command into more than one unit. For example, `query-replace` calls `undo-boundary` after each replacement, so that the user can undo individual replacements one by one.
Mostly, however, this function is called automatically at an appropriate time.
Function: **undo-auto-amalgamate**
The editor command loop automatically calls `undo-boundary` just before executing each key sequence, so that each undo normally undoes the effects of one command. A few exceptional commands are *amalgamating*: these commands generally cause small changes to buffers, so with these a boundary is inserted only every 20th command, allowing the changes to be undone as a group. By default, the commands `self-insert-command`, which produces self-inserting input characters (see [Commands for Insertion](commands-for-insertion)), and `delete-char`, which deletes characters (see [Deletion](deletion)), are amalgamating. Where a command affects the contents of several buffers, as may happen, for example, when a function on the `post-command-hook` affects a buffer other than the `current-buffer`, then `undo-boundary` will be called in each of the affected buffers.
This function can be called before an amalgamating command. It removes the previous `undo-boundary` if a series of such calls have been made.
The maximum number of changes that can be amalgamated is controlled by the `amalgamating-undo-limit` variable. If this variable is 1, no changes are amalgamated.
A Lisp program can amalgamate a series of changes into a single change group by calling `undo-amalgamate-change-group` (see [Atomic Changes](atomic-changes)). Note that `amalgamating-undo-limit` has no effect on the groups produced by that function.
Variable: **undo-auto-current-boundary-timer**
Some buffers, such as process buffers, can change even when no commands are executing. In these cases, `undo-boundary` is normally called periodically by the timer in this variable. Setting this variable to non-`nil` prevents this behavior.
Variable: **undo-in-progress**
This variable is normally `nil`, but the undo commands bind it to `t`. This is so that various kinds of change hooks can tell when they’re being called for the sake of undoing.
Function: **primitive-undo** *count list*
This is the basic function for undoing elements of an undo list. It undoes the first count elements of list, returning the rest of list.
`primitive-undo` adds elements to the buffer’s undo list when it changes the buffer. Undo commands avoid confusion by saving the undo list value at the beginning of a sequence of undo operations. Then the undo operations use and update the saved value. The new elements added by undoing are not part of this saved value, so they don’t interfere with continuing to undo.
This function does not bind `undo-in-progress`.
Some commands leave the region active after execution in such a way that it interferes with selective undo of that command. To make `undo` ignore the active region when invoked immediately after such a command, set the property `undo-inhibit-region` of the command’s function symbol to a non-nil value. See [Standard Properties](standard-properties).
| programming_docs |
elisp None #### Code Characters for interactive
The code character descriptions below contain a number of key words, defined here as follows:
**Completion**
Provide completion. TAB, SPC, and RET perform name completion because the argument is read using `completing-read` (see [Completion](completion)). `?` displays a list of possible completions.
**Existing**
Require the name of an existing object. An invalid name is not accepted; the commands to exit the minibuffer do not exit if the current input is not valid.
**Default**
A default value of some sort is used if the user enters no text in the minibuffer. The default depends on the code character.
**No I/O**
This code letter computes an argument without reading any input. Therefore, it does not use a prompt string, and any prompt string you supply is ignored.
Even though the code letter doesn’t use a prompt string, you must follow it with a newline if it is not the last code character in the string.
**Prompt**
A prompt immediately follows the code character. The prompt ends either with the end of the string or with a newline.
**Special** This code character is meaningful only at the beginning of the interactive string, and it does not look for a prompt or a newline. It is a single, isolated character.
Here are the code character descriptions for use with `interactive`:
‘`\*`’
Signal an error if the current buffer is read-only. Special.
‘`@`’
Select the window mentioned in the first mouse event in the key sequence that invoked this command. Special.
‘`^`’
If the command was invoked through shift-translation, set the mark and activate the region temporarily, or extend an already active region, before the command is run. If the command was invoked without shift-translation, and the region is temporarily active, deactivate the region before the command is run. Special.
‘`a`’
A function name (i.e., a symbol satisfying `fboundp`). Existing, Completion, Prompt.
‘`b`’
The name of an existing buffer. By default, uses the name of the current buffer (see [Buffers](buffers)). Existing, Completion, Default, Prompt.
‘`B`’
A buffer name. The buffer need not exist. By default, uses the name of a recently used buffer other than the current buffer. Completion, Default, Prompt.
‘`c`’
A character. The cursor does not move into the echo area. Prompt.
‘`C`’
A command name (i.e., a symbol satisfying `commandp`). Existing, Completion, Prompt.
‘`d`’
The position of point, as an integer (see [Point](point)). No I/O.
‘`D`’
A directory. The default is the current default directory of the current buffer, `default-directory` (see [File Name Expansion](file-name-expansion)). Existing, Completion, Default, Prompt.
‘`e`’
The first or next non-keyboard event in the key sequence that invoked the command. More precisely, ‘`e`’ gets events that are lists, so you can look at the data in the lists. See [Input Events](input-events). No I/O.
You use ‘`e`’ for mouse events and for special system events (see [Misc Events](misc-events)). The event list that the command receives depends on the event. See [Input Events](input-events), which describes the forms of the list for each event in the corresponding subsections.
You can use ‘`e`’ more than once in a single command’s interactive specification. If the key sequence that invoked the command has n events that are lists, the nth ‘`e`’ provides the nth such event. Events that are not lists, such as function keys and ASCII characters, do not count where ‘`e`’ is concerned.
‘`f`’
A file name of an existing file (see [File Names](file-names)). The default directory is `default-directory`. Existing, Completion, Default, Prompt.
‘`F`’
A file name. The file need not exist. Completion, Default, Prompt.
‘`G`’
A file name. The file need not exist. If the user enters just a directory name, then the value is just that directory name, with no file name within the directory added. Completion, Default, Prompt.
‘`i`’
An irrelevant argument. This code always supplies `nil` as the argument’s value. No I/O.
‘`k`’
A key sequence (see [Key Sequences](key-sequences)). This keeps reading events until a command (or undefined command) is found in the current key maps. The key sequence argument is represented as a string or vector. The cursor does not move into the echo area. Prompt.
If ‘`k`’ reads a key sequence that ends with a down-event, it also reads and discards the following up-event. You can get access to that up-event with the ‘`U`’ code character.
This kind of input is used by commands such as `describe-key` and `global-set-key`.
‘`K`’
A key sequence on a form that can be used as input to functions like `define-key`. This works like ‘`k`’, except that it suppresses, for the last input event in the key sequence, the conversions that are normally used (when necessary) to convert an undefined key into a defined one (see [Key Sequence Input](key-sequence-input)), so this form is usually used when prompting for a new key sequence that is to be bound to a command.
‘`m`’
The position of the mark, as an integer. No I/O.
‘`M`’
Arbitrary text, read in the minibuffer using the current buffer’s input method, and returned as a string (see [Input Methods](https://www.gnu.org/software/emacs/manual/html_node/emacs/Input-Methods.html#Input-Methods) in The GNU Emacs Manual). Prompt.
‘`n`’
A number, read with the minibuffer. If the input is not a number, the user has to try again. ‘`n`’ never uses the prefix argument. Prompt.
‘`N`’
The numeric prefix argument; but if there is no prefix argument, read a number as with `n`. The value is always a number. See [Prefix Command Arguments](prefix-command-arguments). Prompt.
‘`p`’
The numeric prefix argument. (Note that this ‘`p`’ is lower case.) No I/O.
‘`P`’
The raw prefix argument. (Note that this ‘`P`’ is upper case.) No I/O.
‘`r`’
Point and the mark, as two numeric arguments, smallest first. This is the only code letter that specifies two successive arguments rather than one. This will signal an error if the mark is not set in the buffer which is current when the command is invoked. If Transient Mark mode is turned on (see [The Mark](the-mark)) — as it is by default — and user option `mark-even-if-inactive` is `nil`, Emacs will signal an error even if the mark *is* set, but is inactive. No I/O.
‘`s`’
Arbitrary text, read in the minibuffer and returned as a string (see [Text from Minibuffer](text-from-minibuffer)). Terminate the input with either `C-j` or RET. (`C-q` may be used to include either of these characters in the input.) Prompt.
‘`S`’
An interned symbol whose name is read in the minibuffer. Terminate the input with either `C-j` or RET. Other characters that normally terminate a symbol (e.g., whitespace, parentheses and brackets) do not do so here. Prompt.
‘`U`’
A key sequence or `nil`. Can be used after a ‘`k`’ or ‘`K`’ argument to get the up-event that was discarded (if any) after ‘`k`’ or ‘`K`’ read a down-event. If no up-event has been discarded, ‘`U`’ provides `nil` as the argument. No I/O.
‘`v`’
A variable declared to be a user option (i.e., satisfying the predicate `custom-variable-p`). This reads the variable using `read-variable`. See [Definition of read-variable](high_002dlevel-completion#Definition-of-read_002dvariable). Existing, Completion, Prompt.
‘`x`’
A Lisp object, specified with its read syntax, terminated with a `C-j` or RET. The object is not evaluated. See [Object from Minibuffer](object-from-minibuffer). Prompt.
‘`X`’
A Lisp form’s value. ‘`X`’ reads as ‘`x`’ does, then evaluates the form so that its value becomes the argument for the command. Prompt.
‘`z`’
A coding system name (a symbol). If the user enters null input, the argument value is `nil`. See [Coding Systems](coding-systems). Completion, Existing, Prompt.
‘`Z`’ A coding system name (a symbol)—but only if this command has a prefix argument. With no prefix argument, ‘`Z`’ provides `nil` as the argument value. Completion, Existing, Prompt.
elisp None #### Specifying a Coding System for One Operation
You can specify the coding system for a specific operation by binding the variables `coding-system-for-read` and/or `coding-system-for-write`.
Variable: **coding-system-for-read**
If this variable is non-`nil`, it specifies the coding system to use for reading a file, or for input from a synchronous subprocess.
It also applies to any asynchronous subprocess or network stream, but in a different way: the value of `coding-system-for-read` when you start the subprocess or open the network stream specifies the input decoding method for that subprocess or network stream. It remains in use for that subprocess or network stream unless and until overridden.
The right way to use this variable is to bind it with `let` for a specific I/O operation. Its global value is normally `nil`, and you should not globally set it to any other value. Here is an example of the right way to use the variable:
```
;; Read the file with no character code conversion.
(let ((coding-system-for-read 'no-conversion))
(insert-file-contents filename))
```
When its value is non-`nil`, this variable takes precedence over all other methods of specifying a coding system to use for input, including `file-coding-system-alist`, `process-coding-system-alist` and `network-coding-system-alist`.
Variable: **coding-system-for-write**
This works much like `coding-system-for-read`, except that it applies to output rather than input. It affects writing to files, as well as sending output to subprocesses and net connections. It also applies to encoding command-line arguments with which Emacs invokes subprocesses.
When a single operation does both input and output, as do `call-process-region` and `start-process`, both `coding-system-for-read` and `coding-system-for-write` affect it.
Variable: **coding-system-require-warning**
Binding `coding-system-for-write` to a non-`nil` value prevents output primitives from calling the function specified by `select-safe-coding-system-function` (see [User-Chosen Coding Systems](user_002dchosen-coding-systems)). This is because `C-x RET c` (`universal-coding-system-argument`) works by binding `coding-system-for-write`, and Emacs should obey user selection. If a Lisp program binds `coding-system-for-write` to a value that might not be safe for encoding the text to be written, it can also bind `coding-system-require-warning` to a non-`nil` value, which will force the output primitives to check the encoding by calling the value of `select-safe-coding-system-function` even though `coding-system-for-write` is non-`nil`. Alternatively, call `select-safe-coding-system` explicitly before using the specified encoding.
User Option: **inhibit-eol-conversion**
When this variable is non-`nil`, no end-of-line conversion is done, no matter which coding system is specified. This applies to all the Emacs I/O and subprocess primitives, and to the explicit encoding and decoding functions (see [Explicit Encoding](explicit-encoding)).
Sometimes, you need to prefer several coding systems for some operation, rather than fix a single one. Emacs lets you specify a priority order for using coding systems. This ordering affects the sorting of lists of coding systems returned by functions such as `find-coding-systems-region` (see [Lisp and Coding Systems](lisp-and-coding-systems)).
Function: **coding-system-priority-list** *&optional highestp*
This function returns the list of coding systems in the order of their current priorities. Optional argument highestp, if non-`nil`, means return only the highest priority coding system.
Function: **set-coding-system-priority** *&rest coding-systems*
This function puts coding-systems at the beginning of the priority list for coding systems, thus making their priority higher than all the rest.
Macro: **with-coding-priority** *coding-systems &rest body*
This macro executes body, like `progn` does (see [progn](sequencing)), with coding-systems at the front of the priority list for coding systems. coding-systems should be a list of coding systems to prefer during execution of body.
elisp None ### Buffer Names
Each buffer has a unique name, which is a string. Many of the functions that work on buffers accept either a buffer or a buffer name as an argument. Any argument called buffer-or-name is of this sort, and an error is signaled if it is neither a string nor a buffer. Any argument called buffer must be an actual buffer object, not a name.
Buffers that are ephemeral and generally uninteresting to the user have names starting with a space, so that the `list-buffers` and `buffer-menu` commands don’t mention them (but if such a buffer visits a file, it **is** mentioned). A name starting with space also initially disables recording undo information; see [Undo](undo).
Function: **buffer-name** *&optional buffer*
This function returns the name of buffer as a string. buffer defaults to the current buffer.
If `buffer-name` returns `nil`, it means that buffer has been killed. See [Killing Buffers](killing-buffers).
```
(buffer-name)
⇒ "buffers.texi"
```
```
(setq foo (get-buffer "temp"))
⇒ #<buffer temp>
```
```
(kill-buffer foo)
⇒ nil
```
```
(buffer-name foo)
⇒ nil
```
```
foo
⇒ #<killed buffer>
```
Command: **rename-buffer** *newname &optional unique*
This function renames the current buffer to newname. An error is signaled if newname is not a string.
Ordinarily, `rename-buffer` signals an error if newname is already in use. However, if unique is non-`nil`, it modifies newname to make a name that is not in use. Interactively, you can make unique non-`nil` with a numeric prefix argument. (This is how the command `rename-uniquely` is implemented.)
This function returns the name actually given to the buffer.
Function: **get-buffer** *buffer-or-name*
This function returns the buffer specified by buffer-or-name. If buffer-or-name is a string and there is no buffer with that name, the value is `nil`. If buffer-or-name is a buffer, it is returned as given; that is not very useful, so the argument is usually a name. For example:
```
(setq b (get-buffer "lewis"))
⇒ #<buffer lewis>
```
```
(get-buffer b)
⇒ #<buffer lewis>
```
```
(get-buffer "Frazzle-nots")
⇒ nil
```
See also the function `get-buffer-create` in [Creating Buffers](creating-buffers).
Function: **generate-new-buffer-name** *starting-name &optional ignore*
This function returns a name that would be unique for a new buffer—but does not create the buffer. It starts with starting-name, and produces a name not currently in use for any buffer by appending a number inside of ‘`<…>`’. It starts at 2 and keeps incrementing the number until it is not the name of an existing buffer.
If the optional second argument ignore is non-`nil`, it should be a string, a potential buffer name. It means to consider that potential buffer acceptable, if it is tried, even if it is the name of an existing buffer (which would normally be rejected). Thus, if buffers named ‘`foo`’, ‘`foo<2>`’, ‘`foo<3>`’ and ‘`foo<4>`’ exist,
```
(generate-new-buffer-name "foo")
⇒ "foo<5>"
(generate-new-buffer-name "foo" "foo<3>")
⇒ "foo<3>"
(generate-new-buffer-name "foo" "foo<6>")
⇒ "foo<5>"
```
See the related function `generate-new-buffer` in [Creating Buffers](creating-buffers).
elisp None ### Hooks
A *hook* is a variable where you can store a function or functions (see [What Is a Function](what-is-a-function)) to be called on a particular occasion by an existing program. Emacs provides hooks for the sake of customization. Most often, hooks are set up in the init file (see [Init File](init-file)), but Lisp programs can set them also. See [Standard Hooks](standard-hooks), for a list of some standard hook variables.
Most of the hooks in Emacs are *normal hooks*. These variables contain lists of functions to be called with no arguments. By convention, whenever the hook name ends in ‘`-hook`’, that tells you it is normal. We try to make all hooks normal, as much as possible, so that you can use them in a uniform way.
Every major mode command is supposed to run a normal hook called the *mode hook* as one of the last steps of initialization. This makes it easy for a user to customize the behavior of the mode, by overriding the buffer-local variable assignments already made by the mode. Most minor mode functions also run a mode hook at the end. But hooks are used in other contexts too. For example, the hook `suspend-hook` runs just before Emacs suspends itself (see [Suspending Emacs](suspending-emacs)).
If the hook variable’s name does not end with ‘`-hook`’, that indicates it is probably an *abnormal hook*. These differ from normal hooks in two ways: they can be called with one or more arguments, and their return values can be used in some way. The hook’s documentation says how the functions are called and how their return values are used. Any functions added to an abnormal hook must follow the hook’s calling convention. By convention, abnormal hook names end in ‘`-functions`’.
If the name of the variable ends in ‘`-predicate`’ or ‘`-function`’ (singular) then its value must be a function, not a list of functions. As with abnormal hooks, the expected arguments and meaning of the return value vary across such *single function hooks*. The details are explained in each variable’s docstring.
Since hooks (both multi and single function) are variables, their values can be modified with `setq` or temporarily with `let`. However, it is often useful to add or remove a particular function from a hook while preserving any other functions it might have. For multi function hooks, the recommended way of doing this is with `add-hook` and `remove-hook` (see [Setting Hooks](setting-hooks)). Most normal hook variables are initially void; `add-hook` knows how to deal with this. You can add hooks either globally or buffer-locally with `add-hook`. For hooks which hold only a single function, `add-hook` is not appropriate, but you can use `add-function` (see [Advising Functions](advising-functions)) to combine new functions with the hook. Note that some single function hooks may be `nil` which `add-function` cannot deal with, so you must check for that before calling `add-function`.
| | | |
| --- | --- | --- |
| • [Running Hooks](running-hooks) | | How to run a hook. |
| • [Setting Hooks](setting-hooks) | | How to put functions on a hook, or remove them. |
elisp None #### Indentation Controlled by Major Mode
An important function of each major mode is to customize the TAB key to indent properly for the language being edited. This section describes the mechanism of the TAB key and how to control it. The functions in this section return unpredictable values.
Command: **indent-for-tab-command** *&optional rigid*
This is the command bound to TAB in most editing modes. Its usual action is to indent the current line, but it can alternatively insert a tab character or indent a region.
Here is what it does:
* First, it checks whether Transient Mark mode is enabled and the region is active. If so, it calls `indent-region` to indent all the text in the region (see [Region Indent](region-indent)).
* Otherwise, if the indentation function in `indent-line-function` is `indent-to-left-margin` (a trivial command that inserts a tab character), or if the variable `tab-always-indent` specifies that a tab character ought to be inserted (see below), then it inserts a tab character.
* Otherwise, it indents the current line; this is done by calling the function in `indent-line-function`. If the line is already indented, and the value of `tab-always-indent` is `complete` (see below), it tries completing the text at point.
If rigid is non-`nil` (interactively, with a prefix argument), then after this command indents a line or inserts a tab, it also rigidly indents the entire balanced expression which starts at the beginning of the current line, in order to reflect the new indentation. This argument is ignored if the command indents the region.
Variable: **indent-line-function**
This variable’s value is the function to be used by `indent-for-tab-command`, and various other indentation commands, to indent the current line. It is usually assigned by the major mode; for instance, Lisp mode sets it to `lisp-indent-line`, C mode sets it to `c-indent-line`, and so on. The default value is `indent-relative`. See [Auto-Indentation](auto_002dindentation).
Command: **indent-according-to-mode**
This command calls the function in `indent-line-function` to indent the current line in a way appropriate for the current major mode.
Command: **newline-and-indent**
This function inserts a newline, then indents the new line (the one following the newline just inserted) according to the major mode. It does indentation by calling `indent-according-to-mode`.
Command: **reindent-then-newline-and-indent**
This command reindents the current line, inserts a newline at point, and then indents the new line (the one following the newline just inserted). It does indentation on both lines by calling `indent-according-to-mode`.
User Option: **tab-always-indent**
This variable can be used to customize the behavior of the TAB (`indent-for-tab-command`) command. If the value is `t` (the default), the command normally just indents the current line. If the value is `nil`, the command indents the current line only if point is at the left margin or in the line’s indentation; otherwise, it inserts a tab character. If the value is `complete`, the command first tries to indent the current line, and if the line was already indented, it calls `completion-at-point` to complete the text at point (see [Completion in Buffers](completion-in-buffers)).
User Option: **tab-first-completion**
If `tab-always-indent` is `complete`, whether to expand or indent can be further customized via the `tab-first-completion` variable. The following values can be used:
`eol`
Only complete if point is at the end of a line.
`word`
Complete unless the next character has word syntax.
`word-or-paren`
Complete unless the next character has word syntax or is a parenthesis.
`word-or-paren-or-punct` Complete unless the next character has word syntax, or is a parenthesis, or is punctuation.
In any case, typing `TAB` a second time always results in completion.
Some major modes need to support embedded regions of text whose syntax belongs to a different major mode. Examples include *literate programming* source files that combine documentation and snippets of source code, Yacc/Bison programs that include snippets of Python or JS code, etc. To correctly indent the embedded chunks, the primary mode needs to delegate the indentation to another mode’s indentation engine (e.g., call `js-indent-line` for JS code or `python-indent-line` for Python), while providing it with some context to guide the indentation. Major modes, for their part, should avoid calling `widen` in their indentation code and obey `prog-first-column`.
Variable: **prog-indentation-context**
This variable, when non-`nil`, holds the indentation context for the sub-mode’s indentation engine provided by the superior major mode. The value should be a list of the form `(first-column . rest`. The members of the list have the following meaning:
first-column The column to be used for top-level constructs. This replaces the default value of the top-level column used by the sub-mode, usually zero.
rest This value is currently unused.
The following convenience function should be used by major mode’s indentation engine in support of invocations as sub-modes of another major mode.
Function: **prog-first-column**
Call this function instead of using a literal value (usually, zero) of the column number for indenting top-level program constructs. The function’s value is the column number to use for top-level constructs. When no superior mode is in effect, this function returns zero.
| programming_docs |
elisp None ### Creating a Synchronous Process
After a *synchronous process* is created, Emacs waits for the process to terminate before continuing. Starting Dired on GNU or Unix[24](#FOOT24) is an example of this: it runs `ls` in a synchronous process, then modifies the output slightly. Because the process is synchronous, the entire directory listing arrives in the buffer before Emacs tries to do anything with it.
While Emacs waits for the synchronous subprocess to terminate, the user can quit by typing `C-g`. The first `C-g` tries to kill the subprocess with a `SIGINT` signal; but it waits until the subprocess actually terminates before quitting. If during that time the user types another `C-g`, that kills the subprocess instantly with `SIGKILL` and quits immediately (except on MS-DOS, where killing other processes doesn’t work). See [Quitting](quitting).
The synchronous subprocess functions return an indication of how the process terminated.
The output from a synchronous subprocess is generally decoded using a coding system, much like text read from a file. The input sent to a subprocess by `call-process-region` is encoded using a coding system, much like text written into a file. See [Coding Systems](coding-systems).
Function: **call-process** *program &optional infile destination display &rest args*
This function calls program and waits for it to finish.
The current working directory of the subprocess is set to the current buffer’s value of `default-directory` if that is local (as determined by `unhandled-file-name-directory`), or "~" otherwise. If you want to run a process in a remote directory use `process-file`.
The standard input for the new process comes from file infile if infile is not `nil`, and from the null device otherwise. The argument destination says where to put the process output. Here are the possibilities:
a buffer
Insert the output in that buffer, before point. This includes both the standard output stream and the standard error stream of the process.
a buffer name (a string)
Insert the output in a buffer with that name, before point.
`t`
Insert the output in the current buffer, before point.
`nil`
Discard the output.
0
Discard the output, and return `nil` immediately without waiting for the subprocess to finish.
In this case, the process is not truly synchronous, since it can run in parallel with Emacs; but you can think of it as synchronous in that Emacs is essentially finished with the subprocess as soon as this function returns.
MS-DOS doesn’t support asynchronous subprocesses, so this option doesn’t work there.
`(:file file-name)`
Send the output to the file name specified, overwriting it if it already exists.
`(real-destination error-destination)`
Keep the standard output stream separate from the standard error stream; deal with the ordinary output as specified by real-destination, and dispose of the error output according to error-destination. If error-destination is `nil`, that means to discard the error output, `t` means mix it with the ordinary output, and a string specifies a file name to redirect error output into.
You can’t directly specify a buffer to put the error output in; that is too difficult to implement. But you can achieve this result by sending the error output to a temporary file and then inserting the file into a buffer when the subprocess finishes.
If display is non-`nil`, then `call-process` redisplays the buffer as output is inserted. (However, if the coding system chosen for decoding output is `undecided`, meaning deduce the encoding from the actual data, then redisplay sometimes cannot continue once non-ASCII characters are encountered. There are fundamental reasons why it is hard to fix this; see [Output from Processes](output-from-processes).)
Otherwise the function `call-process` does no redisplay, and the results become visible on the screen only when Emacs redisplays that buffer in the normal course of events.
The remaining arguments, args, are strings that specify command line arguments for the program. Each string is passed to program as a separate argument.
The value returned by `call-process` (unless you told it not to wait) indicates the reason for process termination. A number gives the exit status of the subprocess; 0 means success, and any other value means failure. If the process terminated with a signal, `call-process` returns a string describing the signal. If you told `call-process` not to wait, it returns `nil`.
In the examples below, the buffer ‘`foo`’ is current.
```
(call-process "pwd" nil t)
⇒ 0
---------- Buffer: foo ----------
/home/lewis/manual
---------- Buffer: foo ----------
```
```
(call-process "grep" nil "bar" nil "lewis" "/etc/passwd")
⇒ 0
---------- Buffer: bar ----------
lewis:x:1001:1001:Bil Lewis,,,,:/home/lewis:/bin/bash
---------- Buffer: bar ----------
```
Here is an example of the use of `call-process`, as used to be found in the definition of the `insert-directory` function:
```
(call-process insert-directory-program nil t nil switches
(if full-directory-p
(concat (file-name-as-directory file) ".")
file))
```
Function: **process-file** *program &optional infile buffer display &rest args*
This function processes files synchronously in a separate process. It is similar to `call-process`, but may invoke a file name handler based on the value of the variable `default-directory`, which specifies the current working directory of the subprocess.
The arguments are handled in almost the same way as for `call-process`, with the following differences:
Some file name handlers may not support all combinations and forms of the arguments infile, buffer, and display. For example, some file name handlers might behave as if display were `nil`, regardless of the value actually passed. As another example, some file name handlers might not support separating standard output and error output by way of the buffer argument.
If a file name handler is invoked, it determines the program to run based on the first argument program. For instance, suppose that a handler for remote files is invoked. Then the path that is used for searching for the program might be different from `exec-path`.
The second argument infile may invoke a file name handler. The file name handler could be different from the handler chosen for the `process-file` function itself. (For example, `default-directory` could be on one remote host, and infile on a different remote host. Or `default-directory` could be non-special, whereas infile is on a remote host.)
If buffer is a list of the form `(real-destination
error-destination)`, and error-destination names a file, then the same remarks as for infile apply.
The remaining arguments (args) will be passed to the process verbatim. Emacs is not involved in processing file names that are present in args. To avoid confusion, it may be best to avoid absolute file names in args, but rather to specify all file names as relative to `default-directory`. The function `file-relative-name` is useful for constructing such relative file names. Alternatively, you can use `file-local-name` (see [Magic File Names](magic-file-names)) to obtain an absolute file name as seen from the remote host’s perspective.
Variable: **process-file-side-effects**
This variable indicates whether a call of `process-file` changes remote files.
By default, this variable is always set to `t`, meaning that a call of `process-file` could potentially change any file on a remote host. When set to `nil`, a file name handler could optimize its behavior with respect to remote file attribute caching.
You should only ever change this variable with a let-binding; never with `setq`.
User Option: **process-file-return-signal-string**
This user option indicates whether a call of `process-file` returns a string describing the signal interrupting a remote process.
When a process returns an exit code greater than 128, it is interpreted as a signal. `process-file` requires to return a string describing this signal.
Since there are processes violating this rule, returning exit codes greater than 128 which are not bound to a signal, `process-file` returns always the exit code as natural number for remote processes. Setting this user option to non-nil forces `process-file` to interpret such exit codes as signals, and to return a corresponding string.
Function: **call-process-region** *start end program &optional delete destination display &rest args*
This function sends the text from start to end as standard input to a process running program. It deletes the text sent if delete is non-`nil`; this is useful when destination is `t`, to insert the output in the current buffer in place of the input.
The arguments destination and display control what to do with the output from the subprocess, and whether to update the display as it comes in. For details, see the description of `call-process`, above. If destination is the integer 0, `call-process-region` discards the output and returns `nil` immediately, without waiting for the subprocess to finish (this only works if asynchronous subprocesses are supported; i.e., not on MS-DOS).
The remaining arguments, args, are strings that specify command line arguments for the program.
The return value of `call-process-region` is just like that of `call-process`: `nil` if you told it to return without waiting; otherwise, a number or string which indicates how the subprocess terminated.
In the following example, we use `call-process-region` to run the `cat` utility, with standard input being the first five characters in buffer ‘`foo`’ (the word ‘`input`’). `cat` copies its standard input into its standard output. Since the argument destination is `t`, this output is inserted in the current buffer.
```
---------- Buffer: foo ----------
input∗
---------- Buffer: foo ----------
```
```
(call-process-region 1 6 "cat" nil t)
⇒ 0
---------- Buffer: foo ----------
inputinput∗
---------- Buffer: foo ----------
```
For example, the `shell-command-on-region` command uses `call-shell-region` in a manner similar to this:
```
(call-shell-region
start end
command ; shell command
nil ; do not delete region
buffer) ; send output to `buffer`
```
Function: **call-process-shell-command** *command &optional infile destination display*
This function executes the shell command command synchronously. The other arguments are handled as in `call-process`. An old calling convention allowed passing any number of additional arguments after display, which were concatenated to command; this is still supported, but strongly discouraged.
Function: **process-file-shell-command** *command &optional infile destination display*
This function is like `call-process-shell-command`, but uses `process-file` internally. Depending on `default-directory`, command can be executed also on remote hosts. An old calling convention allowed passing any number of additional arguments after display, which were concatenated to command; this is still supported, but strongly discouraged.
Function: **call-shell-region** *start end command &optional delete destination*
This function sends the text from start to end as standard input to an inferior shell running command. This function is similar than `call-process-region`, with process being a shell. The arguments `delete`, `destination` and the return value are like in `call-process-region`. Note that this function doesn’t accept additional arguments.
Function: **shell-command-to-string** *command*
This function executes command (a string) as a shell command, then returns the command’s output as a string.
Function: **process-lines** *program &rest args*
This function runs program, waits for it to finish, and returns its output as a list of strings. Each string in the list holds a single line of text output by the program; the end-of-line characters are stripped from each line. The arguments beyond program, args, are strings that specify command-line arguments with which to run the program.
If program exits with a non-zero exit status, this function signals an error.
This function works by calling `call-process`, so program output is decoded in the same way as for `call-process`.
Function: **process-lines-ignore-status** *program &rest args*
This function is just like `process-lines`, but does not signal an error if program exits with a non-zero exit status.
elisp None #### Using Edebug
To debug a Lisp program with Edebug, you must first *instrument* the Lisp code that you want to debug. A simple way to do this is to first move point into the definition of a function or macro and then do `C-u C-M-x` (`eval-defun` with a prefix argument). See [Instrumenting](instrumenting), for alternative ways to instrument code.
Once a function is instrumented, any call to the function activates Edebug. Depending on which Edebug execution mode you have selected, activating Edebug may stop execution and let you step through the function, or it may update the display and continue execution while checking for debugging commands. The default execution mode is step, which stops execution. See [Edebug Execution Modes](edebug-execution-modes).
Within Edebug, you normally view an Emacs buffer showing the source of the Lisp code you are debugging. This is referred to as the *source code buffer*, and it is temporarily read-only.
An arrow in the left fringe indicates the line where the function is executing. Point initially shows where within the line the function is executing, but this ceases to be true if you move point yourself.
If you instrument the definition of `fac` (shown below) and then execute `(fac 3)`, here is what you would normally see. Point is at the open-parenthesis before `if`.
```
(defun fac (n)
=>∗(if (< 0 n)
(* n (fac (1- n)))
1))
```
The places within a function where Edebug can stop execution are called *stop points*. These occur both before and after each subexpression that is a list, and also after each variable reference. Here we use periods to show the stop points in the function `fac`:
```
(defun fac (n)
.(if .(< 0 n.).
.(* n. .(fac .(1- n.).).).
1).)
```
The special commands of Edebug are available in the source code buffer in addition to the commands of Emacs Lisp mode. For example, you can type the Edebug command SPC to execute until the next stop point. If you type SPC once after entry to `fac`, here is the display you will see:
```
(defun fac (n)
=>(if ∗(< 0 n)
(* n (fac (1- n)))
1))
```
When Edebug stops execution after an expression, it displays the expression’s value in the echo area.
Other frequently used commands are `b` to set a breakpoint at a stop point, `g` to execute until a breakpoint is reached, and `q` to exit Edebug and return to the top-level command loop. Type `?` to display a list of all Edebug commands.
elisp None #### Tabulated List mode
Tabulated List mode is a major mode for displaying tabulated data, i.e., data consisting of *entries*, each entry occupying one row of text with its contents divided into columns. Tabulated List mode provides facilities for pretty-printing rows and columns, and sorting the rows according to the values in each column. It is derived from Special mode (see [Basic Major Modes](basic-major-modes)).
Tabulated List mode is intended to be used as a parent mode by a more specialized major mode. Examples include Process Menu mode (see [Process Information](process-information)) and Package Menu mode (see [Package Menu](https://www.gnu.org/software/emacs/manual/html_node/emacs/Package-Menu.html#Package-Menu) in The GNU Emacs Manual).
Such a derived mode should use `define-derived-mode` in the usual way, specifying `tabulated-list-mode` as the second argument (see [Derived Modes](derived-modes)). The body of the `define-derived-mode` form should specify the format of the tabulated data, by assigning values to the variables documented below; optionally, it can then call the function `tabulated-list-init-header`, which will populate a header with the names of the columns.
The derived mode should also define a *listing command*. This, not the mode command, is what the user calls (e.g., `M-x list-processes`). The listing command should create or switch to a buffer, turn on the derived mode, specify the tabulated data, and finally call `tabulated-list-print` to populate the buffer.
User Option: **tabulated-list-gui-sort-indicator-asc**
This variable specifies the character to be used on GUI frames as an indication that the column is sorted in the ascending order.
Whenever you change the sort direction in Tabulated List buffers, this indicator toggles between ascending (“asc”) and descending (“desc”).
User Option: **tabulated-list-gui-sort-indicator-desc**
Like `tabulated-list-gui-sort-indicator-asc`, but used when the column is sorted in the descending order.
User Option: **tabulated-list-tty-sort-indicator-asc**
Like `tabulated-list-gui-sort-indicator-asc`, but used for text-mode frames.
User Option: **tabulated-list-tty-sort-indicator-desc**
Like `tabulated-list-tty-sort-indicator-asc`, but used when the column is sorted in the descending order.
Variable: **tabulated-list-format**
This buffer-local variable specifies the format of the Tabulated List data. Its value should be a vector. Each element of the vector represents a data column, and should be a list `(name
width sort)`, where
* name is the column’s name (a string).
* width is the width to reserve for the column (an integer). This is meaningless for the last column, which runs to the end of each line.
* sort specifies how to sort entries by the column. If `nil`, the column cannot be used for sorting. If `t`, the column is sorted by comparing string values. Otherwise, this should be a predicate function for `sort` (see [Rearrangement](rearrangement)), which accepts two arguments with the same form as the elements of `tabulated-list-entries` (see below).
Variable: **tabulated-list-entries**
This buffer-local variable specifies the entries displayed in the Tabulated List buffer. Its value should be either a list, or a function.
If the value is a list, each list element corresponds to one entry, and should have the form `(id contents)`, where
* id is either `nil`, or a Lisp object that identifies the entry. If the latter, the cursor stays on the same entry when re-sorting entries. Comparison is done with `equal`.
* contents is a vector with the same number of elements as `tabulated-list-format`. Each vector element is either a string, which is inserted into the buffer as-is, or a list `(label
. properties)`, which means to insert a text button by calling `insert-text-button` with label and properties as arguments (see [Making Buttons](making-buttons)). There should be no newlines in any of these strings.
Otherwise, the value should be a function which returns a list of the above form when called with no arguments.
Variable: **tabulated-list-revert-hook**
This normal hook is run prior to reverting a Tabulated List buffer. A derived mode can add a function to this hook to recompute `tabulated-list-entries`.
Variable: **tabulated-list-printer**
The value of this variable is the function called to insert an entry at point, including its terminating newline. The function should accept two arguments, id and contents, having the same meanings as in `tabulated-list-entries`. The default value is a function which inserts an entry in a straightforward way; a mode which uses Tabulated List mode in a more complex way can specify another function.
Variable: **tabulated-list-sort-key**
The value of this variable specifies the current sort key for the Tabulated List buffer. If it is `nil`, no sorting is done. Otherwise, it should have the form `(name . flip)`, where name is a string matching one of the column names in `tabulated-list-format`, and flip, if non-`nil`, means to invert the sort order.
Function: **tabulated-list-init-header**
This function computes and sets `header-line-format` for the Tabulated List buffer (see [Header Lines](header-lines)), and assigns a keymap to the header line to allow sorting entries by clicking on column headers.
Modes derived from Tabulated List mode should call this after setting the above variables (in particular, only after setting `tabulated-list-format`).
Function: **tabulated-list-print** *&optional remember-pos update*
This function populates the current buffer with entries. It should be called by the listing command. It erases the buffer, sorts the entries specified by `tabulated-list-entries` according to `tabulated-list-sort-key`, then calls the function specified by `tabulated-list-printer` to insert each entry.
If the optional argument remember-pos is non-`nil`, this function looks for the id element on the current line, if any, and tries to move to that entry after all the entries are (re)inserted.
If the optional argument update is non-`nil`, this function will only erase or add entries that have changed since the last print. This is several times faster if most entries haven’t changed since the last time this function was called. The only difference in outcome is that tags placed via `tabulated-list-put-tag` will not be removed from entries that haven’t changed (normally all tags are removed).
Function: **tabulated-list-delete-entry**
This function deletes the entry at point.
It returns a list `(id cols)`, where id is the ID of the deleted entry and cols is a vector of its column descriptors. It moves point to the beginning of the current line. It returns `nil` if there is no entry at point.
Note that this function only changes the buffer contents; it does not alter `tabulated-list-entries`.
Function: **tabulated-list-get-id** *&optional pos*
This `defsubst` returns the ID object from `tabulated-list-entries` (if that is a list) or from the list returned by `tabulated-list-entries` (if it is a function). If omitted or `nil`, pos defaults to point.
Function: **tabulated-list-get-entry** *&optional pos*
This `defsubst` returns the entry object from `tabulated-list-entries` (if that is a list) or from the list returned by `tabulated-list-entries` (if it is a function). This will be a vector for the ID at pos. If there is no entry at pos, then the function returns `nil`.
Function: **tabulated-list-header-overlay-p** *&optional POS*
This `defsubst` returns non-nil if there is a fake header at pos. A fake header is used if `tabulated-list-use-header-line` is `nil` to put the column names at the beginning of the buffer. If omitted or `nil`, pos defaults to `point-min`.
Function: **tabulated-list-put-tag** *tag &optional advance*
This function puts tag in the padding area of the current line. The padding area can be empty space at the beginning of the line, the width of which is governed by `tabulated-list-padding`. tag should be a string, with a length less than or equal to `tabulated-list-padding`. If advance is non-nil, this function advances point by one line.
Function: **tabulated-list-clear-all-tags**
This function clears all tags from the padding area in the current buffer.
Function: **tabulated-list-set-col** *col desc &optional change-entry-data*
This function changes the tabulated list entry at point, setting col to desc. col is the column number to change, or the name of the column to change. desc is the new column descriptor, which is inserted via `tabulated-list-print-col`.
If change-entry-data is non-nil, this function modifies the underlying data (usually the column descriptor in the list `tabulated-list-entries`) by setting the column descriptor of the vector to `desc`.
| programming_docs |
elisp None ### Output Streams
An output stream specifies what to do with the characters produced by printing. Most print functions accept an output stream as an optional argument. Here are the possible types of output stream:
buffer
The output characters are inserted into buffer at point. Point advances as characters are inserted.
marker
The output characters are inserted into the buffer that marker points into, at the marker position. The marker position advances as characters are inserted. The value of point in the buffer has no effect on printing when the stream is a marker, and this kind of printing does not move point (except that if the marker points at or before the position of point, point advances with the surrounding text, as usual).
function
The output characters are passed to function, which is responsible for storing them away. It is called with a single character as argument, as many times as there are characters to be output, and is responsible for storing the characters wherever you want to put them.
`t`
The output characters are displayed in the echo area. If Emacs is running in batch mode (see [Batch Mode](batch-mode)), the output is written to the standard output descriptor instead.
`nil`
`nil` specified as an output stream means to use the value of the `standard-output` variable instead; that value is the *default output stream*, and must not be `nil`.
symbol A symbol as output stream is equivalent to the symbol’s function definition (if any).
Many of the valid output streams are also valid as input streams. The difference between input and output streams is therefore more a matter of how you use a Lisp object, than of different types of object.
Here is an example of a buffer used as an output stream. Point is initially located as shown immediately before the ‘`h`’ in ‘`the`’. At the end, point is located directly before that same ‘`h`’.
```
---------- Buffer: foo ----------
This is t∗he contents of foo.
---------- Buffer: foo ----------
```
```
(print "This is the output" (get-buffer "foo"))
⇒ "This is the output"
```
```
---------- Buffer: foo ----------
This is t
"This is the output"
∗he contents of foo.
---------- Buffer: foo ----------
```
Now we show a use of a marker as an output stream. Initially, the marker is in buffer `foo`, between the ‘`t`’ and the ‘`h`’ in the word ‘`the`’. At the end, the marker has advanced over the inserted text so that it remains positioned before the same ‘`h`’. Note that the location of point, shown in the usual fashion, has no effect.
```
---------- Buffer: foo ----------
This is the ∗output
---------- Buffer: foo ----------
```
```
(setq m (copy-marker 10))
⇒ #<marker at 10 in foo>
```
```
(print "More output for foo." m)
⇒ "More output for foo."
```
```
---------- Buffer: foo ----------
This is t
"More output for foo."
he ∗output
---------- Buffer: foo ----------
```
```
m
⇒ #<marker at 34 in foo>
```
The following example shows output to the echo area:
```
(print "Echo Area output" t)
⇒ "Echo Area output"
---------- Echo Area ----------
"Echo Area output"
---------- Echo Area ----------
```
Finally, we show the use of a function as an output stream. The function `eat-output` takes each character that it is given and conses it onto the front of the list `last-output` (see [Building Lists](building-lists)). At the end, the list contains all the characters output, but in reverse order.
```
(setq last-output nil)
⇒ nil
```
```
(defun eat-output (c)
(setq last-output (cons c last-output)))
⇒ eat-output
```
```
(print "This is the output" #'eat-output)
⇒ "This is the output"
```
```
last-output
⇒ (10 34 116 117 112 116 117 111 32 101 104
116 32 115 105 32 115 105 104 84 34 10)
```
Now we can put the output in the proper order by reversing the list:
```
(concat (nreverse last-output))
⇒ "
\"This is the output\"
"
```
Calling `concat` converts the list to a string so you can see its contents more clearly.
Function: **external-debugging-output** *character*
This function can be useful as an output stream when debugging. It writes character to the standard error stream.
For example
```
(print "This is the output" #'external-debugging-output)
-| This is the output
⇒ "This is the output"
```
elisp None #### Association List Type
An *association list* or *alist* is a specially-constructed list whose elements are cons cells. In each element, the CAR is considered a *key*, and the CDR is considered an *associated value*. (In some cases, the associated value is stored in the CAR of the CDR.) Association lists are often used as stacks, since it is easy to add or remove associations at the front of the list.
For example,
```
(setq alist-of-colors
'((rose . red) (lily . white) (buttercup . yellow)))
```
sets the variable `alist-of-colors` to an alist of three elements. In the first element, `rose` is the key and `red` is the value.
See [Association Lists](association-lists), for a further explanation of alists and for functions that work on alists. See [Hash Tables](hash-tables), for another kind of lookup table, which is much faster for handling a large number of keys.
elisp None ### Macros and Byte Compilation
You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation.
When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself—they would be computed at compile time, which is not useful.
In order for compilation of macro calls to work, the macros must already be defined in Lisp when the calls to them are compiled. The compiler has a special feature to help you do this: if a file being compiled contains a `defmacro` form, the macro is defined temporarily for the rest of the compilation of that file.
Byte-compiling a file also executes any `require` calls at top-level in the file, so you can ensure that necessary macro definitions are available during compilation by requiring the files that define them (see [Named Features](named-features)). To avoid loading the macro definition files when someone *runs* the compiled program, write `eval-when-compile` around the `require` calls (see [Eval During Compile](eval-during-compile)).
elisp None #### Simple Menu Items
The simpler (and original) way to define a menu item is to bind some event type (it doesn’t matter what event type) to a binding like this:
```
(item-string . real-binding)
```
The CAR, item-string, is the string to be displayed in the menu. It should be short—preferably one to three words. It should describe the action of the command it corresponds to. Note that not all graphical toolkits can display non-ASCII text in menus (it will work for keyboard menus and will work to a large extent with the GTK+ toolkit).
You can also supply a second string, called the help string, as follows:
```
(item-string help . real-binding)
```
help specifies a help-echo string to display while the mouse is on that item in the same way as `help-echo` text properties (see [Help display](special-properties#Help-display)).
As far as `define-key` is concerned, item-string and help-string are part of the event’s binding. However, `lookup-key` returns just real-binding, and only real-binding is used for executing the key.
If real-binding is `nil`, then item-string appears in the menu but cannot be selected.
If real-binding is a symbol and has a non-`nil` `menu-enable` property, that property is an expression that controls whether the menu item is enabled. Every time the keymap is used to display a menu, Emacs evaluates the expression, and it enables the menu item only if the expression’s value is non-`nil`. When a menu item is disabled, it is displayed in a fuzzy fashion, and cannot be selected.
The menu bar does not recalculate which items are enabled every time you look at a menu. This is because the X toolkit requires the whole tree of menus in advance. To force recalculation of the menu bar, call `force-mode-line-update` (see [Mode Line Format](mode-line-format)).
elisp None ### Receiving Output from Processes
The output that an asynchronous subprocess writes to its standard output stream is passed to a function called the *filter function*. The default filter function simply inserts the output into a buffer, which is called the associated buffer of the process (see [Process Buffers](process-buffers)). If the process has no buffer then the default filter discards the output.
If the subprocess writes to its standard error stream, by default the error output is also passed to the process filter function. If Emacs uses a pseudo-TTY (pty) for communication with the subprocess, then it is impossible to separate the standard output and standard error streams of the subprocess, because a pseudo-TTY has only one output channel. In that case, if you want to keep the output to those streams separate, you should redirect one of them to a file—for example, by using an appropriate shell command via `start-process-shell-command` or a similar function.
Alternatively, you could use the `:stderr` parameter with a non-`nil` value in a call to `make-process` (see [make-process](asynchronous-processes)) to make the destination of the error output separate from the standard output; in that case, Emacs will use pipes for communicating with the subprocess.
When a subprocess terminates, Emacs reads any pending output, then stops reading output from that subprocess. Therefore, if the subprocess has children that are still live and still producing output, Emacs won’t receive that output.
Output from a subprocess can arrive only while Emacs is waiting: when reading terminal input (see the function `waiting-for-user-input-p`), in `sit-for` and `sleep-for` (see [Waiting](waiting)), in `accept-process-output` (see [Accepting Output](accepting-output)), and in functions which send data to processes (see [Input to Processes](input-to-processes)). This minimizes the problem of timing errors that usually plague parallel programming. For example, you can safely create a process and only then specify its buffer or filter function; no output can arrive before you finish, if the code in between does not call any primitive that waits.
Variable: **process-adaptive-read-buffering**
On some systems, when Emacs reads the output from a subprocess, the output data is read in very small blocks, potentially resulting in very poor performance. This behavior can be remedied to some extent by setting the variable `process-adaptive-read-buffering` to a non-`nil` value (the default), as it will automatically delay reading from such processes, thus allowing them to produce more output before Emacs tries to read it.
| | | |
| --- | --- | --- |
| • [Process Buffers](process-buffers) | | By default, output is put in a buffer. |
| • [Filter Functions](filter-functions) | | Filter functions accept output from the process. |
| • [Decoding Output](decoding-output) | | Filters can get unibyte or multibyte strings. |
| • [Accepting Output](accepting-output) | | How to wait until process output arrives. |
| • [Processes and Threads](processes-and-threads) | | How processes and threads interact. |
elisp None ### Terminal Parameters
Each terminal has a list of associated parameters. These *terminal parameters* are mostly a convenient way of storage for terminal-local variables, but some terminal parameters have a special meaning.
This section describes functions to read and change the parameter values of a terminal. They all accept as their argument either a terminal or a frame; the latter means use that frame’s terminal. An argument of `nil` means the selected frame’s terminal.
Function: **terminal-parameters** *&optional terminal*
This function returns an alist listing all the parameters of terminal and their values.
Function: **terminal-parameter** *terminal parameter*
This function returns the value of the parameter parameter (a symbol) of terminal. If terminal has no setting for parameter, this function returns `nil`.
Function: **set-terminal-parameter** *terminal parameter value*
This function sets the parameter parameter of terminal to the specified value, and returns the previous value of that parameter.
Here’s a list of a few terminal parameters that have a special meaning:
`background-mode` The classification of the terminal’s background color, either `light` or `dark`.
`normal-erase-is-backspace` Value is either 1 or 0, depending on whether `normal-erase-is-backspace-mode` is turned on or off on this terminal. See [DEL Does Not Delete](https://www.gnu.org/software/emacs/manual/html_node/emacs/DEL-Does-Not-Delete.html#DEL-Does-Not-Delete) in The Emacs Manual.
`terminal-initted` After the terminal is initialized, this is set to the terminal-specific initialization function.
`tty-mode-set-strings` When present, a list of strings containing escape sequences that Emacs will output while configuring a tty for rendering. Emacs emits these strings only when configuring a terminal: if you want to enable a mode on a terminal that is already active (for example, while in `tty-setup-hook`), explicitly output the necessary escape sequence using `send-string-to-terminal` in addition to adding the sequence to `tty-mode-set-strings`.
`tty-mode-reset-strings` When present, a list of strings that undo the effects of the strings in `tty-mode-set-strings`. Emacs emits these strings when exiting, deleting a terminal, or suspending itself.
elisp None ### Setting Variable Values
The usual way to change the value of a variable is with the special form `setq`. When you need to compute the choice of variable at run time, use the function `set`.
Special Form: **setq** *[symbol form]…*
This special form is the most common method of changing a variable’s value. Each symbol is given a new value, which is the result of evaluating the corresponding form. The current binding of the symbol is changed.
`setq` does not evaluate symbol; it sets the symbol that you write. We say that this argument is *automatically quoted*. The ‘`q`’ in `setq` stands for “quoted”.
The value of the `setq` form is the value of the last form.
```
(setq x (1+ 2))
⇒ 3
```
```
x ; `x` now has a global value.
⇒ 3
```
```
(let ((x 5))
(setq x 6) ; The local binding of `x` is set.
x)
⇒ 6
```
```
x ; The global value is unchanged.
⇒ 3
```
Note that the first form is evaluated, then the first symbol is set, then the second form is evaluated, then the second symbol is set, and so on:
```
(setq x 10 ; Notice that `x` is set before
y (1+ x)) ; the value of `y` is computed.
⇒ 11
```
Function: **set** *symbol value*
This function puts value in the value cell of symbol. Since it is a function rather than a special form, the expression written for symbol is evaluated to obtain the symbol to set. The return value is value.
When dynamic variable binding is in effect (the default), `set` has the same effect as `setq`, apart from the fact that `set` evaluates its symbol argument whereas `setq` does not. But when a variable is lexically bound, `set` affects its *dynamic* value, whereas `setq` affects its current (lexical) value. See [Variable Scoping](variable-scoping).
```
(set one 1)
error→ Symbol's value as variable is void: one
```
```
(set 'one 1)
⇒ 1
```
```
(set 'two 'one)
⇒ one
```
```
(set two 2) ; `two` evaluates to symbol `one`.
⇒ 2
```
```
one ; So it is `one` that was set.
⇒ 2
(let ((one 1)) ; This binding of `one` is set,
(set 'one 3) ; not the global value.
one)
⇒ 3
```
```
one
⇒ 2
```
If symbol is not actually a symbol, a `wrong-type-argument` error is signaled.
```
(set '(x y) 'z)
error→ Wrong type argument: symbolp, (x y)
```
elisp None #### Position Parameters
Parameters describing the X- and Y-offsets of a frame are always measured in pixels. For a normal, non-child frame they specify the frame’s outer position (see [Frame Geometry](frame-geometry)) relative to its display’s origin. For a child frame (see [Child Frames](child-frames)) they specify the frame’s outer position relative to the native position of the frame’s parent frame. (Note that none of these parameters is meaningful on TTY frames.)
`left`
The position, in pixels, of the left outer edge of the frame with respect to the left edge of the frame’s display or parent frame. It can be specified in one of the following ways.
an integer
A positive integer always relates the left edge of the frame to the left edge of its display or parent frame. A negative integer relates the right frame edge to the right edge of the display or parent frame.
`(+ pos)`
This specifies the position of the left frame edge relative to the left edge of its display or parent frame. The integer pos may be positive or negative; a negative value specifies a position outside the screen or parent frame or on a monitor other than the primary one (for multi-monitor displays).
`(- pos)`
This specifies the position of the right frame edge relative to the right edge of the display or parent frame. The integer pos may be positive or negative; a negative value specifies a position outside the screen or parent frame or on a monitor other than the primary one (for multi-monitor displays).
a floating-point value
A floating-point value in the range 0.0 to 1.0 specifies the left edge’s offset via the *left position ratio* of the frame—the ratio of the left edge of its outer frame to the width of the frame’s workarea (see [Multiple Terminals](multiple-terminals)) or its parent’s native frame (see [Child Frames](child-frames)) minus the width of the outer frame. Thus, a left position ratio of 0.0 flushes a frame to the left, a ratio of 0.5 centers it and a ratio of 1.0 flushes it to the right of its display or parent frame. Similarly, the *top position ratio* of a frame is the ratio of the frame’s top position to the height of its workarea or parent frame minus the height of the frame.
Emacs will try to keep the position ratios of a child frame unaltered if that frame has a non-`nil` `keep-ratio` parameter (see [Frame Interaction Parameters](frame-interaction-parameters)) and its parent frame is resized.
Since the outer size of a frame (see [Frame Geometry](frame-geometry)) is usually unavailable before a frame has been made visible, it is generally not advisable to use floating-point values when creating decorated frames. Floating-point values are more suited for ensuring that an (undecorated) child frame is positioned nicely within the area of its parent frame.
Some window managers ignore program-specified positions. If you want to be sure the position you specify is not ignored, specify a non-`nil` value for the `user-position` parameter as in the following example:
```
(modify-frame-parameters
nil '((user-position . t) (left . (+ -4))))
```
In general, it is not a good idea to position a frame relative to the right or bottom edge of its display. Positioning the initial or a new frame is either not accurate (because the size of the outer frame is not yet fully known before the frame has been made visible) or will cause additional flicker (if the frame has to be repositioned after becoming visible).
Note also, that positions specified relative to the right/bottom edge of a display, workarea or parent frame as well as floating-point offsets are stored internally as integer offsets relative to the left/top edge of the display, workarea or parent frame edge. They are also returned as such by functions like `frame-parameters` and restored as such by the desktop saving routines.
`top`
The screen position of the top (or bottom) edge, in pixels, with respect to the top (or bottom) edge of the display or parent frame. It works just like `left`, except vertically instead of horizontally.
`icon-left`
The screen position of the left edge of the frame’s icon, in pixels, counting from the left edge of the screen. This takes effect when the frame is iconified, if the window manager supports this feature. If you specify a value for this parameter, then you must also specify a value for `icon-top` and vice versa.
`icon-top`
The screen position of the top edge of the frame’s icon, in pixels, counting from the top edge of the screen. This takes effect when the frame is iconified, if the window manager supports this feature.
`user-position`
When you create a frame and specify its screen position with the `left` and `top` parameters, use this parameter to say whether the specified position was user-specified (explicitly requested in some way by a human user) or merely program-specified (chosen by a program). A non-`nil` value says the position was user-specified.
Window managers generally heed user-specified positions, and some heed program-specified positions too. But many ignore program-specified positions, placing the window in a default fashion or letting the user place it with the mouse. Some window managers, including `twm`, let the user specify whether to obey program-specified positions or ignore them.
When you call `make-frame`, you should specify a non-`nil` value for this parameter if the values of the `left` and `top` parameters represent the user’s stated preference; otherwise, use `nil`.
`z-group`
This parameter specifies a relative position of the frame’s window-system window in the stacking (Z-) order of the frame’s display.
If this is `above`, the window-system will display the window that corresponds to the frame above all other window-system windows that do not have the `above` property set. If this is `nil`, the frame’s window is displayed below all windows that have the `above` property set and above all windows that have the `below` property set. If this is `below`, the frame’s window is displayed below all windows that do not have the `below` property set.
To position the frame above or below a specific other frame use the function `frame-restack` (see [Raising and Lowering](raising-and-lowering)).
| programming_docs |
elisp None ### Network Servers
You create a server by calling `make-network-process` (see [Network Processes](network-processes)) with `:server t`. The server will listen for connection requests from clients. When it accepts a client connection request, that creates a new network connection, itself a process object, with the following parameters:
* The connection’s process name is constructed by concatenating the server process’s name with a client identification string. The client identification string for an IPv4 connection looks like ‘`<a.b.c.d:p>`’, which represents an address and port number. Otherwise, it is a unique number in brackets, as in ‘`<nnn>`’. The number is unique for each connection in the Emacs session.
* If the server has a non-default filter, the connection process does not get a separate process buffer; otherwise, Emacs creates a new buffer for the purpose. The buffer name is the server’s buffer name or process name, concatenated with the client identification string. The server’s process buffer value is never used directly, but the log function can retrieve it and use it to log connections by inserting text there.
* The communication type and the process filter and sentinel are inherited from those of the server. The server never directly uses its filter and sentinel; their sole purpose is to initialize connections made to the server.
* The connection’s process contact information is set according to the client’s addressing information (typically an IP address and a port number). This information is associated with the `process-contact` keywords `:host`, `:service`, `:remote`.
* The connection’s local address is set up according to the port number used for the connection.
* The client process’s plist is initialized from the server’s plist.
elisp None #### Floating-Point Type
Floating-point numbers are the computer equivalent of scientific notation; you can think of a floating-point number as a fraction together with a power of ten. The precise number of significant figures and the range of possible exponents is machine-specific; Emacs uses the C data type `double` to store the value, and internally this records a power of 2 rather than a power of 10.
The printed representation for floating-point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, ‘`1500.0`’, ‘`+15e2`’, ‘`15.0e+2`’, ‘`+1500000e-3`’, and ‘`.15e4`’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent.
See [Numbers](numbers), for more information.
elisp None ### Defining Abbrevs
`define-abbrev` is the low-level basic function for defining an abbrev in an abbrev table.
When a major mode defines a system abbrev, it should call `define-abbrev` and specify `t` for the `:system` property. Be aware that any saved non-system abbrevs are restored at startup, i.e., before some major modes are loaded. Therefore, major modes should not assume that their abbrev tables are empty when they are first loaded.
Function: **define-abbrev** *abbrev-table name expansion &optional hook &rest props*
This function defines an abbrev named name, in abbrev-table, to expand to expansion and call hook, with properties props (see [Abbrev Properties](abbrev-properties)). The return value is name. The `:system` property in props is treated specially here: if it has the value `force`, then it will overwrite an existing definition even for a non-system abbrev of the same name.
name should be a string. The argument expansion is normally the desired expansion (a string), or `nil` to undefine the abbrev. If it is anything but a string or `nil`, then the abbreviation expands solely by running hook.
The argument hook is a function or `nil`. If hook is non-`nil`, then it is called with no arguments after the abbrev is replaced with expansion; point is located at the end of expansion when hook is called.
If hook is a non-`nil` symbol whose `no-self-insert` property is non-`nil`, hook can explicitly control whether to insert the self-inserting input character that triggered the expansion. If hook returns non-`nil` in this case, that inhibits insertion of the character. By contrast, if hook returns `nil`, `expand-abbrev` (or `abbrev-insert`) also returns `nil`, as if expansion had not really occurred.
Normally, `define-abbrev` sets the variable `abbrevs-changed` to `t`, if it actually changes the abbrev. This is so that some commands will offer to save the abbrevs. It does not do this for a system abbrev, since those aren’t saved anyway.
User Option: **only-global-abbrevs**
If this variable is non-`nil`, it means that the user plans to use global abbrevs only. This tells the commands that define mode-specific abbrevs to define global ones instead. This variable does not alter the behavior of the functions in this section; it is examined by their callers.
elisp None ### File Format Conversion
Emacs performs several steps to convert the data in a buffer (text, text properties, and possibly other information) to and from a representation suitable for storing into a file. This section describes the fundamental functions that perform this *format conversion*, namely `insert-file-contents` for reading a file into a buffer, and `write-region` for writing a buffer into a file.
| | | |
| --- | --- | --- |
| • [Overview](format-conversion-overview) | | `insert-file-contents` and `write-region`. |
| • [Round-Trip](format-conversion-round_002dtrip) | | Using `format-alist`. |
| • [Piecemeal](format-conversion-piecemeal) | | Specifying non-paired conversion. |
elisp None ### Parsing and generating JSON values
When Emacs is compiled with JSON (*JavaScript Object Notation*) support, it provides several functions to convert between Lisp objects and JSON values. Any JSON value can be converted to a Lisp object, but not vice versa. Specifically:
* JSON uses three keywords: `true`, `null`, `false`. `true` is represented by the symbol `t`. By default, the remaining two are represented, respectively, by the symbols `:null` and `:false`.
* JSON only has floating-point numbers. They can represent both Lisp integers and Lisp floating-point numbers.
* JSON strings are always Unicode strings encoded in UTF-8. Lisp strings can contain non-Unicode characters.
* JSON has only one sequence type, the array. JSON arrays are represented using Lisp vectors.
* JSON has only one map type, the object. JSON objects are represented using Lisp hashtables, alists or plists. When an alist or plist contains several elements with the same key, Emacs uses only the first element for serialization, in accordance with the behavior of `assq`.
Note that `nil`, being both a valid alist and a valid plist, represents `{}`, the empty JSON object; not `null`, `false`, or an empty array, all of which are different JSON values.
Function: **json-available-p**
This predicate returns non-`nil` if Emacs has been built with JSON support, and the library is available on the current system.
If some Lisp object can’t be represented in JSON, the serialization functions will signal an error of type `wrong-type-argument`. The parsing functions can also signal the following errors:
`json-unavailable`
Signaled when the parsing library isn’t available.
`json-end-of-file`
Signaled when encountering a premature end of the input text.
`json-trailing-content`
Signaled when encountering unexpected input after the first JSON object parsed.
`json-parse-error` Signaled when encountering invalid JSON syntax.
Top-level values and the subobjects within these top-level values can be serialized to JSON. Likewise, the parsing functions will return any of the possible types described above.
Function: **json-serialize** *object &rest args*
This function returns a new Lisp string which contains the JSON representation of object. The argument args is a list of keyword/argument pairs. The following keywords are accepted:
`:null-object`
The value decides which Lisp object to use to represent the JSON keyword `null`. It defaults to the symbol `:null`.
`:false-object` The value decides which Lisp object to use to represent the JSON keyword `false`. It defaults to the symbol `:false`.
Function: **json-insert** *object &rest args*
This function inserts the JSON representation of object into the current buffer before point. The argument args are interpreted as in `json-parse-string`.
Function: **json-parse-string** *string &rest args*
This function parses the JSON value in string, which must be a Lisp string. If string doesn’t contain a valid JSON object, this function signals the `json-parse-error` error.
The argument args is a list of keyword/argument pairs. The following keywords are accepted:
`:object-type`
The value decides which Lisp object to use for representing the key-value mappings of a JSON object. It can be either `hash-table`, the default, to make hashtables with strings as keys; `alist` to use alists with symbols as keys; or `plist` to use plists with keyword symbols as keys.
`:array-type`
The value decides which Lisp object to use for representing a JSON array. It can be either `array`, the default, to use Lisp arrays; or `list` to use lists.
`:null-object`
The value decides which Lisp object to use to represent the JSON keyword `null`. It defaults to the symbol `:null`.
`:false-object` The value decides which Lisp object to use to represent the JSON keyword `false`. It defaults to the symbol `:false`.
Function: **json-parse-buffer** *&rest args*
This function reads the next JSON value from the current buffer, starting at point. It moves point to the position immediately after the value if contains a valid JSON object; otherwise it signals the `json-parse-error` error and doesn’t move point. The arguments args are interpreted as in `json-parse-string`.
elisp None ### Modifying Strings
You can alter the contents of a mutable string via operations described in this section. See [Mutability](mutability).
The most basic way to alter the contents of an existing string is with `aset` (see [Array Functions](array-functions)). `(aset string idx char)` stores char into string at character index idx. It will automatically convert a pure-ASCII string to a multibyte string (see [Text Representations](text-representations)) if needed, but we recommend to always make sure string is multibyte (e.g., by using `string-to-multibyte`, see [Converting Representations](converting-representations)), if char is a non-ASCII character, not a raw byte.
A more powerful function is `store-substring`:
Function: **store-substring** *string idx obj*
This function alters part of the contents of the specified string, by storing obj starting at character index idx. The argument obj may be either a character (in which case the function behaves exactly as `aset`) or a (smaller) string. If obj is a multibyte string, we recommend to make sure string is also multibyte, even if it’s pure-ASCII.
Since it is impossible to change the number of characters in an existing string, it is en error if obj consists of more characters than would fit in string starting at character index idx.
To clear out a string that contained a password, use `clear-string`:
Function: **clear-string** *string*
This makes string a unibyte string and clears its contents to zeros. It may also change string’s length.
elisp None ### Substituting for a Character Code
The following functions replace characters within a specified region based on their character codes.
Function: **subst-char-in-region** *start end old-char new-char &optional noundo*
This function replaces all occurrences of the character old-char with the character new-char in the region of the current buffer defined by start and end. Both characters must have the same length of their multibyte form.
If noundo is non-`nil`, then `subst-char-in-region` does not record the change for undo and does not mark the buffer as modified. This was useful for controlling the old selective display feature (see [Selective Display](selective-display)).
`subst-char-in-region` does not move point and returns `nil`.
```
---------- Buffer: foo ----------
This is the contents of the buffer before.
---------- Buffer: foo ----------
```
```
(subst-char-in-region 1 20 ?i ?X)
⇒ nil
---------- Buffer: foo ----------
ThXs Xs the contents of the buffer before.
---------- Buffer: foo ----------
```
Function: **subst-char-in-string** *fromchar tochar string &optional inplace*
This function replaces all occurrences of the character fromchar with tochar in string. By default, substitution occurs in a copy of string, but if the optional argument inplace is non-`nil`, the function modifies the string itself. In any case, the function returns the resulting string.
Command: **translate-region** *start end table*
This function applies a translation table to the characters in the buffer between positions start and end.
The translation table table is a string or a char-table; `(aref table ochar)` gives the translated character corresponding to ochar. If table is a string, any characters with codes larger than the length of table are not altered by the translation.
The return value of `translate-region` is the number of characters that were actually changed by the translation. This does not count characters that were mapped into themselves in the translation table.
elisp None ### Standard Abbrev Tables
Here we list the variables that hold the abbrev tables for the preloaded major modes of Emacs.
Variable: **global-abbrev-table**
This is the abbrev table for mode-independent abbrevs. The abbrevs defined in it apply to all buffers. Each buffer may also have a local abbrev table, whose abbrev definitions take precedence over those in the global table.
Variable: **local-abbrev-table**
The value of this buffer-local variable is the (mode-specific) abbreviation table of the current buffer. It can also be a list of such tables.
Variable: **abbrev-minor-mode-table-alist**
The value of this variable is a list of elements of the form `(mode . abbrev-table)` where mode is the name of a variable: if the variable is bound to a non-`nil` value, then the abbrev-table is active, otherwise it is ignored. abbrev-table can also be a list of abbrev tables.
Variable: **fundamental-mode-abbrev-table**
This is the local abbrev table used in Fundamental mode; in other words, it is the local abbrev table in all buffers in Fundamental mode.
Variable: **text-mode-abbrev-table**
This is the local abbrev table used in Text mode.
Variable: **lisp-mode-abbrev-table**
This is the local abbrev table used in Lisp mode. It is the parent of the local abbrev table used in Emacs Lisp mode. See [Abbrev Table Properties](abbrev-table-properties).
elisp None ### Font Lock Mode
*Font Lock mode* is a buffer-local minor mode that automatically attaches `face` properties to certain parts of the buffer based on their syntactic role. How it parses the buffer depends on the major mode; most major modes define syntactic criteria for which faces to use in which contexts. This section explains how to customize Font Lock for a particular major mode.
Font Lock mode finds text to highlight in two ways: through syntactic parsing based on the syntax table, and through searching (usually for regular expressions). Syntactic fontification happens first; it finds comments and string constants and highlights them. Search-based fontification happens second.
| | | |
| --- | --- | --- |
| • [Font Lock Basics](font-lock-basics) | | Overview of customizing Font Lock. |
| • [Search-based Fontification](search_002dbased-fontification) | | Fontification based on regexps. |
| • [Customizing Keywords](customizing-keywords) | | Customizing search-based fontification. |
| • [Other Font Lock Variables](other-font-lock-variables) | | Additional customization facilities. |
| • [Levels of Font Lock](levels-of-font-lock) | | Each mode can define alternative levels so that the user can select more or less. |
| • [Precalculated Fontification](precalculated-fontification) | | How Lisp programs that produce the buffer contents can also specify how to fontify it. |
| • [Faces for Font Lock](faces-for-font-lock) | | Special faces specifically for Font Lock. |
| • [Syntactic Font Lock](syntactic-font-lock) | | Fontification based on syntax tables. |
| • [Multiline Font Lock](multiline-font-lock) | | How to coerce Font Lock into properly highlighting multiline constructs. |
elisp None #### Defining the Grammar of a Language
The usual way to define the SMIE grammar of a language is by defining a new global variable that holds the precedence table by giving a set of BNF rules. For example, the grammar definition for a small Pascal-like language could look like:
```
(require 'smie)
(defvar sample-smie-grammar
(smie-prec2->grammar
(smie-bnf->prec2
```
```
'((id)
(inst ("begin" insts "end")
("if" exp "then" inst "else" inst)
(id ":=" exp)
(exp))
(insts (insts ";" insts) (inst))
(exp (exp "+" exp)
(exp "*" exp)
("(" exps ")"))
(exps (exps "," exps) (exp)))
```
```
'((assoc ";"))
'((assoc ","))
'((assoc "+") (assoc "*")))))
```
A few things to note:
* The above grammar does not explicitly mention the syntax of function calls: SMIE will automatically allow any sequence of sexps, such as identifiers, balanced parentheses, or `begin ... end` blocks to appear anywhere anyway.
* The grammar category `id` has no right hand side: this does not mean that it can match only the empty string, since as mentioned any sequence of sexps can appear anywhere anyway.
* Because non terminals cannot appear consecutively in the BNF grammar, it is difficult to correctly handle tokens that act as terminators, so the above grammar treats `";"` as a statement *separator* instead, which SMIE can handle very well.
* Separators used in sequences (such as `","` and `";"` above) are best defined with BNF rules such as `(foo (foo "separator" foo) ...)` which generate precedence conflicts which are then resolved by giving them an explicit `(assoc "separator")`.
* The `("(" exps ")")` rule was not needed to pair up parens, since SMIE will pair up any characters that are marked as having paren syntax in the syntax table. What this rule does instead (together with the definition of `exps`) is to make it clear that `","` should not appear outside of parentheses.
* Rather than have a single *precs* table to resolve conflicts, it is preferable to have several tables, so as to let the BNF part of the grammar specify relative precedences where possible.
* Unless there is a very good reason to prefer `left` or `right`, it is usually preferable to mark operators as associative, using `assoc`. For that reason `"+"` and `"*"` are defined above as `assoc`, although the language defines them formally as left associative.
elisp None #### Table of Syntax Classes
Here is a table of syntax classes, the characters that designate them, their meanings, and examples of their use.
Whitespace characters: ‘’ or ‘`-`’
Characters that separate symbols and words from each other. Typically, whitespace characters have no other syntactic significance, and multiple whitespace characters are syntactically equivalent to a single one. Space, tab, and formfeed are classified as whitespace in almost all major modes.
This syntax class can be designated by either ‘’ or ‘`-`’. Both designators are equivalent.
Word constituents: ‘`w`’
Parts of words in human languages. These are typically used in variable and command names in programs. All upper- and lower-case letters, and the digits, are typically word constituents.
Symbol constituents: ‘`\_`’
Extra characters used in variable and command names along with word constituents. Examples include the characters ‘`$&\*+-\_<>`’ in Lisp mode, which may be part of a symbol name even though they are not part of English words. In standard C, the only non-word-constituent character that is valid in symbols is underscore (‘`\_`’).
Punctuation characters: ‘`.`’
Characters used as punctuation in a human language, or used in a programming language to separate symbols from one another. Some programming language modes, such as Emacs Lisp mode, have no characters in this class since the few characters that are not symbol or word constituents all have other uses. Other programming language modes, such as C mode, use punctuation syntax for operators.
Open parenthesis characters: ‘`(`’ Close parenthesis characters: ‘`)`’
Characters used in dissimilar pairs to surround sentences or expressions. Such a grouping is begun with an open parenthesis character and terminated with a close. Each open parenthesis character matches a particular close parenthesis character, and vice versa. Normally, Emacs indicates momentarily the matching open parenthesis when you insert a close parenthesis. See [Blinking](blinking).
In human languages, and in C code, the parenthesis pairs are ‘`()`’, ‘`[]`’, and ‘`{}`’. In Emacs Lisp, the delimiters for lists and vectors (‘`()`’ and ‘`[]`’) are classified as parenthesis characters.
String quotes: ‘`"`’
Characters used to delimit string constants. The same string quote character appears at the beginning and the end of a string. Such quoted strings do not nest.
The parsing facilities of Emacs consider a string as a single token. The usual syntactic meanings of the characters in the string are suppressed.
The Lisp modes have two string quote characters: double-quote (‘`"`’) and vertical bar (‘`|`’). ‘`|`’ is not used in Emacs Lisp, but it is used in Common Lisp. C also has two string quote characters: double-quote for strings, and apostrophe (‘`'`’) for character constants.
Human text has no string quote characters. We do not want quotation marks to turn off the usual syntactic properties of other characters in the quotation.
Escape-syntax characters: ‘`\`’
Characters that start an escape sequence, such as is used in string and character constants. The character ‘`\`’ belongs to this class in both C and Lisp. (In C, it is used thus only inside strings, but it turns out to cause no trouble to treat it this way throughout C code.)
Characters in this class count as part of words if `words-include-escapes` is non-`nil`. See [Word Motion](word-motion).
Character quotes: ‘`/`’
Characters used to quote the following character so that it loses its normal syntactic meaning. This differs from an escape character in that only the character immediately following is ever affected.
Characters in this class count as part of words if `words-include-escapes` is non-`nil`. See [Word Motion](word-motion).
This class is used for backslash in TeX mode.
Paired delimiters: ‘`$`’
Similar to string quote characters, except that the syntactic properties of the characters between the delimiters are not suppressed. Only TeX mode uses a paired delimiter presently—the ‘`$`’ that both enters and leaves math mode.
Expression prefixes: ‘`'`’
Characters used for syntactic operators that are considered as part of an expression if they appear next to one. In Lisp modes, these characters include the apostrophe, ‘`'`’ (used for quoting), the comma, ‘`,`’ (used in macros), and ‘`#`’ (used in the read syntax for certain data types).
Comment starters: ‘`<`’ Comment enders: ‘`>`’
Characters used in various languages to delimit comments. Human text has no comment characters. In Lisp, the semicolon (‘`;`’) starts a comment and a newline or formfeed ends one.
Inherit standard syntax: ‘`@`’
This syntax class does not specify a particular syntax. It says to look in the parent syntax table to find the syntax of this character.
Generic comment delimiters: ‘`!`’
(This syntax class is also known as “comment-fence”.) Characters that start or end a special kind of comment. *Any* generic comment delimiter matches *any* generic comment delimiter, but they cannot match a comment starter or comment ender; generic comment delimiters can only match each other.
This syntax class is primarily meant for use with the `syntax-table` text property (see [Syntax Properties](syntax-properties)). You can mark any range of characters as forming a comment, by giving the first and last characters of the range `syntax-table` properties identifying them as generic comment delimiters.
Generic string delimiters: ‘`|`’
(This syntax class is also known as “string-fence”.) Characters that start or end a string. This class differs from the string quote class in that *any* generic string delimiter can match any other generic string delimiter; but they do not match ordinary string quote characters.
This syntax class is primarily meant for use with the `syntax-table` text property (see [Syntax Properties](syntax-properties)). You can mark any range of characters as forming a string constant, by giving the first and last characters of the range `syntax-table` properties identifying them as generic string delimiters.
| programming_docs |
elisp None Backups and Auto-Saving
-----------------------
Backup files and auto-save files are two methods by which Emacs tries to protect the user from the consequences of crashes or of the user’s own errors. Auto-saving preserves the text from earlier in the current editing session; backup files preserve file contents prior to the current session.
| | | |
| --- | --- | --- |
| • [Backup Files](backup-files) | | How backup files are made; how their names are chosen. |
| • [Auto-Saving](auto_002dsaving) | | How auto-save files are made; how their names are chosen. |
| • [Reverting](reverting) | | `revert-buffer`, and how to customize what it does. |
elisp None #### Accessing Scroll Bar Events
These functions are useful for decoding scroll bar events.
Function: **scroll-bar-event-ratio** *event*
This function returns the fractional vertical position of a scroll bar event within the scroll bar. The value is a cons cell `(portion . whole)` containing two integers whose ratio is the fractional position.
Function: **scroll-bar-scale** *ratio total*
This function multiplies (in effect) ratio by total, rounding the result to an integer. The argument ratio is not a number, but rather a pair `(num . denom)`—typically a value returned by `scroll-bar-event-ratio`.
This function is handy for scaling a position on a scroll bar into a buffer position. Here’s how to do that:
```
(+ (point-min)
(scroll-bar-scale
(posn-x-y (event-start event))
(- (point-max) (point-min))))
```
Recall that scroll bar events have two integers forming a ratio, in place of a pair of x and y coordinates.
elisp None ### Repeated Loading
You can load a given file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file it came from.
When you load or reload files, bear in mind that the `load` and `load-library` functions automatically load a byte-compiled file rather than a non-compiled file of similar name. If you rewrite a file that you intend to save and reinstall, you need to byte-compile the new version; otherwise Emacs will load the older, byte-compiled file instead of your newer, non-compiled file! If that happens, the message displayed when loading the file includes, ‘`(compiled; note, source is newer)`’, to remind you to recompile it.
When writing the forms in a Lisp library file, keep in mind that the file might be loaded more than once. For example, think about whether each variable should be reinitialized when you reload the library; `defvar` does not change the value if the variable is already initialized. (See [Defining Variables](defining-variables).)
The simplest way to add an element to an alist is like this:
```
(push '(leif-mode " Leif") minor-mode-alist)
```
But this would add multiple elements if the library is reloaded. To avoid the problem, use `add-to-list` (see [List Variables](list-variables)):
```
(add-to-list 'minor-mode-alist '(leif-mode " Leif"))
```
Occasionally you will want to test explicitly whether a library has already been loaded. If the library uses `provide` to provide a named feature, you can use `featurep` earlier in the file to test whether the `provide` call has been executed before (see [Named Features](named-features)). Alternatively, you could use something like this:
```
(defvar foo-was-loaded nil)
(unless foo-was-loaded
execute-first-time-only
(setq foo-was-loaded t))
```
elisp None #### JSONRPC JSON object format
JSONRPC JSON objects are exchanged as Lisp plists (see [Property Lists](property-lists)): JSON-compatible plists are handed to the dispatcher functions and, likewise, JSON-compatible plists should be given to `jsonrpc-notify`, `jsonrpc-request`, and `jsonrpc-async-request`.
To facilitate handling plists, this library makes liberal use of `cl-lib` library (see [cl-lib](https://www.gnu.org/software/emacs/manual/html_node/cl/index.html#Top) in Common Lisp Extensions for GNU Emacs Lisp) and suggests (but doesn’t force) its clients to do the same. A macro `jsonrpc-lambda` can be used to create a lambda for destructuring a JSON-object like in this example:
```
(jsonrpc-async-request
myproc :frobnicate `(:foo "trix")
:success-fn (jsonrpc-lambda (&key bar baz &allow-other-keys)
(message "Server replied back with %s and %s!"
bar baz))
:error-fn (jsonrpc-lambda (&key code message _data)
(message "Sadly, server reports %s: %s"
code message)))
```
elisp None #### Precedence of Action Functions
From the past subsections we already know that `display-buffer` must be supplied with a number of display actions (see [Choosing Window](choosing-window)) in order to display a buffer. In a completely uncustomized Emacs, these actions are specified by `display-buffer-fallback-action` in the following order of precedence: Reuse a window, pop up a new window on the same frame, use a window previously showing the buffer, use some window and pop up a new frame. (Note that the remaining actions named by `display-buffer-fallback-action` are void in an uncustomized Emacs).
Consider the following form:
```
(display-buffer (get-buffer-create "*foo*"))
```
Evaluating this form in the buffer `\*scratch\*` of an uncustomized Emacs session will usually fail to reuse a window that shows `\*foo\*` already, but succeed in popping up a new window. Evaluating the same form again will now not cause any visible changes—`display-buffer` reused the window already showing `\*foo\*` because that action was applicable and had the highest precedence among all applicable actions.
Popping up a new window will fail if there is not enough space on the selected frame. In an uncustomized Emacs it typically fails when there are already two windows on a frame. For example, if you now type `C-x 1` followed by `C-x 2` and evaluate the form once more, `\*foo\*` should show up in the lower window—`display-buffer` just used “some” window. If, before typing `C-x 2` you had typed `C-x o`, `\*foo\*` would have been shown in the upper window because “some” window stands for the “least recently used” window and the selected window has been least recently used if and only if it is alone on its frame.
Let’s assume you did not type `C-x o` and `\*foo\*` is shown in the lower window. Type `C-x o` to get there followed by `C-x left` and evaluate the form again. This should display `\*foo\*` in the same, lower window because that window had already shown `\*foo\*` previously and was therefore chosen instead of some other window.
So far we have only observed the default behavior in an uncustomized Emacs session. To see how this behavior can be customized, let’s consider the option `display-buffer-base-action`. It provides a very coarse customization which conceptually affects the display of *any* buffer. It can be used to supplement the actions supplied by `display-buffer-fallback-action` by reordering them or by adding actions that are not present there but fit more closely the user’s editing practice. However, it can also be used to change the default behavior in a more profound way.
Let’s consider a user who, as a rule, prefers to display buffers on another frame. Such a user might provide the following customization:
```
(customize-set-variable
'display-buffer-base-action
'((display-buffer-reuse-window display-buffer-pop-up-frame)
(reusable-frames . 0)))
```
This setting will cause `display-buffer` to first try to find a window showing the buffer on a visible or iconified frame and, if no such frame exists, pop up a new frame. You can observe this behavior on a graphical system by typing `C-x 1` in the window showing `\*scratch\*` and evaluating our canonical `display-buffer` form. This will usually create (and give focus to) a new frame whose root window shows `\*foo\*`. Iconify that frame and evaluate the canonical form again: `display-buffer` will reuse the window on the new frame (usually raising the frame and giving it focus too).
Only if creating a new frame fails, `display-buffer` will apply the actions supplied by `display-buffer-fallback-action` which means to again try reusing a window, popping up a new window and so on. A trivial way to make frame creation fail is supplied by the following form:
```
(let ((pop-up-frame-function 'ignore))
(display-buffer (get-buffer-create "*foo*")))
```
We will forget about that form immediately after observing that it fails to create a new frame and uses a fallback action instead.
Note that `display-buffer-reuse-window` appears redundant in the customization of `display-buffer-base-action` because it is already part of `display-buffer-fallback-action` and should be tried there anyway. However, that would fail because due to the precedence of `display-buffer-base-action` over `display-buffer-fallback-action`, at that time `display-buffer-pop-up-frame` would have already won the race. In fact, this:
```
(customize-set-variable
'display-buffer-base-action
'(display-buffer-pop-up-frame (reusable-frames . 0)))
```
would cause `display-buffer` to *always* pop up a new frame which is probably not what our user wants.
So far, we have only shown how *users* can customize the default behavior of `display-buffer`. Let us now see how *applications* can change the course of `display-buffer`. The canonical way to do that is to use the action argument of `display-buffer` or a function that calls it, like, for example, `pop-to-buffer` (see [Switching Buffers](switching-buffers)).
Suppose an application wants to display `\*foo\*` preferably below the selected window (to immediately attract the attention of the user to the new window) or, if that fails, in a window at the bottom of the frame. It could do that with a call like this:
```
(display-buffer
(get-buffer-create "*foo*")
'((display-buffer-below-selected display-buffer-at-bottom)))
```
In order to see how this new, modified form works, delete any frame showing `\*foo\*`, type `C-x 1` followed by `C-x 2` in the window showing `\*scratch\*`, and subsequently evaluate that form. `display-buffer` should split the upper window, and show `\*foo\*` in the new window. Alternatively, if after `C-x 2` you had typed `C-x o`, `display-buffer` would have split the window at the bottom instead.
Suppose now that, before evaluating the new form, you have made the selected window as small as possible, for example, by evaluating the form `(fit-window-to-buffer)` in that window. In that case, `display-buffer` would have failed to split the selected window and would have split the frame’s root window instead, effectively displaying `\*foo\*` at the bottom of the frame.
In either case, evaluating the new form a second time should reuse the window already showing `\*foo\*` since both functions supplied by the action argument try to reuse such a window first.
By setting the action argument, an application effectively overrules any customization of `display-buffer-base-action`. Our user can now either accept the choice of the application, or redouble by customizing the option `display-buffer-alist` as follows:
```
(customize-set-variable
'display-buffer-alist
'(("\\*foo\\*"
(display-buffer-reuse-window display-buffer-pop-up-frame))))
```
Trying this with the new, modified form above in a configuration that does not show `\*foo\*` anywhere, will display `\*foo\*` on a separate frame, completely ignoring the action argument of `display-buffer`.
Note that we didn’t care to specify a `reusable-frames` action alist entry in our specification of `display-buffer-alist`. `display-buffer` always takes the first one it finds—in our case the one specified by `display-buffer-base-action`. If we wanted to use a different specification, for example, to exclude iconified frames showing `\*foo\*` from the list of reusable ones, we would have to specify that separately, however:
```
(customize-set-variable
'display-buffer-alist
'(("\\*foo\\*"
(display-buffer-reuse-window display-buffer-pop-up-frame)
(reusable-frames . visible))))
```
If you try this, you will notice that repeated attempts to display `\*foo\*` will succeed to reuse a frame only if that frame is visible.
The above example would allow the conclusion that users customize `display-buffer-alist` for the sole purpose to overrule the action argument chosen by applications. Such a conclusion would be incorrect. `display-buffer-alist` is the standard option for users to direct the course of display of specific buffers in a preferred way regardless of whether the display is also guided by an action argument.
We can, however, reasonably conclude that customizing `display-buffer-alist` differs from customizing `display-buffer-base-action` in two major aspects: it is stronger because it overrides the action argument of `display-buffer`, and it allows to explicitly specify the affected buffers. In fact, displaying other buffers is not affected in any way by a customization for `\*foo\*`. For example,
```
(display-buffer (get-buffer-create "*bar*"))
```
continues being governed by the settings of `display-buffer-base-action` and `display-buffer-fallback-action` only.
We could stop with our examples here but Lisp programs still have an ace up their sleeves which they can use to overrule any customization of `display-buffer-alist`. It’s the variable `display-buffer-overriding-action` which they can bind around `display-buffer` calls as follows:
```
(let ((display-buffer-overriding-action
'((display-buffer-same-window))))
(display-buffer
(get-buffer-create "*foo*")
'((display-buffer-below-selected display-buffer-at-bottom))))
```
Evaluating this form will usually display `\*foo\*` in the selected window regardless of the action argument and any user customizations. (Usually, an application will not bother to also provide an action argument. Here it just serves to illustrate the fact that it gets overridden.)
It might be illustrative to look at the list of action functions `display-buffer` would have tried to display `\*foo\*` with the customizations we provided here. The list (including comments explaining who added this and the subsequent elements) is:
```
(display-buffer-same-window ;; `display-buffer-overriding-action'
display-buffer-reuse-window ;; `display-buffer-alist'
display-buffer-pop-up-frame
display-buffer-below-selected ;; ACTION argument
display-buffer-at-bottom
display-buffer-reuse-window ;; `display-buffer-base-action'
display-buffer-pop-up-frame
display-buffer--maybe-same-window ;; `display-buffer-fallback-action'
display-buffer-reuse-window
display-buffer--maybe-pop-up-frame-or-window
display-buffer-in-previous-window
display-buffer-use-some-window
display-buffer-pop-up-frame)
```
Note that among the internal functions listed here, `display-buffer--maybe-same-window` is effectively ignored while `display-buffer--maybe-pop-up-frame-or-window` actually runs `display-buffer-pop-up-window`.
The action alist passed in each function call is:
```
((reusable-frames . visible)
(reusable-frames . 0))
```
which shows that we have used the second specification of `display-buffer-alist` above, overriding the specification supplied by `display-buffer-base-action`. Suppose our user had written that as
```
(customize-set-variable
'display-buffer-alist
'(("\\*foo\\*"
(display-buffer-reuse-window display-buffer-pop-up-frame)
(inhibit-same-window . t)
(reusable-frames . visible))))
```
In this case the `inhibit-same-window` alist entry will successfully invalidate the `display-buffer-same-window` specification from `display-buffer-overriding-action` and `display-buffer` will show `\*foo\*` on another frame. To make `display-buffer-overriding-action` more robust in this regard, the application would have to specify an appropriate `inhibit-same-window` entry too, for example, as follows:
```
(let ((display-buffer-overriding-action
'(display-buffer-same-window (inhibit-same-window . nil))))
(display-buffer (get-buffer-create "*foo*")))
```
This last example shows that while the precedence order of action functions is fixed, as described in [Choosing Window](choosing-window), an action alist entry specified by a display action ranked lower in that order can affect the execution of a higher ranked display action.
elisp None #### Generic Modes
*Generic modes* are simple major modes with basic support for comment syntax and Font Lock mode. To define a generic mode, use the macro `define-generic-mode`. See the file `generic-x.el` for some examples of the use of `define-generic-mode`.
Macro: **define-generic-mode** *mode comment-list keyword-list font-lock-list auto-mode-list function-list &optional docstring*
This macro defines a generic mode command named mode (a symbol, not quoted). The optional argument docstring is the documentation for the mode command. If you do not supply it, `define-generic-mode` generates one by default.
The argument comment-list is a list in which each element is either a character, a string of one or two characters, or a cons cell. A character or a string is set up in the mode’s syntax table as a comment starter. If the entry is a cons cell, the CAR is set up as a comment starter and the CDR as a comment ender. (Use `nil` for the latter if you want comments to end at the end of the line.) Note that the syntax table mechanism has limitations about what comment starters and enders are actually possible. See [Syntax Tables](syntax-tables).
The argument keyword-list is a list of keywords to highlight with `font-lock-keyword-face`. Each keyword should be a string. Meanwhile, font-lock-list is a list of additional expressions to highlight. Each element of this list should have the same form as an element of `font-lock-keywords`. See [Search-based Fontification](search_002dbased-fontification).
The argument auto-mode-list is a list of regular expressions to add to the variable `auto-mode-alist`. They are added by the execution of the `define-generic-mode` form, not by expanding the macro call.
Finally, function-list is a list of functions for the mode command to call for additional setup. It calls these functions just before it runs the mode hook variable `mode-hook`.
elisp None #### Complex Regexp Example
Here is a complicated regexp which was formerly used by Emacs to recognize the end of a sentence together with any whitespace that follows. (Nowadays Emacs uses a similar but more complex default regexp constructed by the function `sentence-end`. See [Standard Regexps](standard-regexps).)
Below, we show first the regexp as a string in Lisp syntax (to distinguish spaces from tab characters), and then the result of evaluating it. The string constant begins and ends with a double-quote. ‘`\"`’ stands for a double-quote as part of the string, ‘`\\`’ for a backslash as part of the string, ‘`\t`’ for a tab and ‘`\n`’ for a newline.
```
"[.?!][]\"')}]*\\($\\| $\\|\t\\| \\)[ \t\n]*"
⇒ "[.?!][]\"')}]*\\($\\| $\\| \\| \\)[
]*"
```
In the output, tab and newline appear as themselves.
This regular expression contains four parts in succession and can be deciphered as follows:
`[.?!]`
The first part of the pattern is a character alternative that matches any one of three characters: period, question mark, and exclamation mark. The match must begin with one of these three characters. (This is one point where the new default regexp used by Emacs differs from the old. The new value also allows some non-ASCII characters that end a sentence without any following whitespace.)
`[]\"')}]*`
The second part of the pattern matches any closing braces and quotation marks, zero or more of them, that may follow the period, question mark or exclamation mark. The `\"` is Lisp syntax for a double-quote in a string. The ‘`\*`’ at the end indicates that the immediately preceding regular expression (a character alternative, in this case) may be repeated zero or more times.
`\\($\\| $\\|\t\\| \\)`
The third part of the pattern matches the whitespace that follows the end of a sentence: the end of a line (optionally with a space), or a tab, or two spaces. The double backslashes mark the parentheses and vertical bars as regular expression syntax; the parentheses delimit a group and the vertical bars separate alternatives. The dollar sign is used to match the end of a line.
`[ \t\n]*` Finally, the last part of the pattern matches any additional whitespace beyond the minimum needed to end a sentence.
In the `rx` notation (see [Rx Notation](rx-notation)), the regexp could be written
```
(rx (any ".?!") ; Punctuation ending sentence.
(zero-or-more (any "\"')]}")) ; Closing quotes or brackets.
(or line-end
(seq " " line-end)
"\t"
" ") ; Two spaces.
(zero-or-more (any "\t\n "))) ; Optional extra whitespace.
```
Since `rx` regexps are just S-expressions, they can be formatted and commented as such.
| programming_docs |
elisp None #### Why Text Properties are not Intervals
Some editors that support adding attributes to text in the buffer do so by letting the user specify intervals within the text, and adding the properties to the intervals. Those editors permit the user or the programmer to determine where individual intervals start and end. We deliberately provided a different sort of interface in Emacs Lisp to avoid certain paradoxical behavior associated with text modification.
If the actual subdivision into intervals is meaningful, that means you can distinguish between a buffer that is just one interval with a certain property, and a buffer containing the same text subdivided into two intervals, both of which have that property.
Suppose you take the buffer with just one interval and kill part of the text. The text remaining in the buffer is one interval, and the copy in the kill ring (and the undo list) becomes a separate interval. Then if you yank back the killed text, you get two intervals with the same properties. Thus, editing does not preserve the distinction between one interval and two.
Suppose we attempt to fix this problem by coalescing the two intervals when the text is inserted. That works fine if the buffer originally was a single interval. But suppose instead that we have two adjacent intervals with the same properties, and we kill the text of one interval and yank it back. The same interval-coalescence feature that rescues the other case causes trouble in this one: after yanking, we have just one interval. Once again, editing does not preserve the distinction between one interval and two.
Insertion of text at the border between intervals also raises questions that have no satisfactory answer.
However, it is easy to arrange for editing to behave consistently for questions of the form, “What are the properties of text at this buffer or string position?” So we have decided these are the only questions that make sense; we have not implemented asking questions about where intervals start or end.
In practice, you can usually use the text property search functions in place of explicit interval boundaries. You can think of them as finding the boundaries of intervals, assuming that intervals are always coalesced whenever possible. See [Property Search](property-search).
Emacs also provides explicit intervals as a presentation feature; see [Overlays](overlays).
elisp None #### Logging Messages in \*Messages\*
Almost all the messages displayed in the echo area are also recorded in the `\*Messages\*` buffer so that the user can refer back to them. This includes all the messages that are output with `message`. By default, this buffer is read-only and uses the major mode `messages-buffer-mode`. Nothing prevents the user from killing the `\*Messages\*` buffer, but the next display of a message recreates it. Any Lisp code that needs to access the `\*Messages\*` buffer directly and wants to ensure that it exists should use the function `messages-buffer`.
Function: **messages-buffer**
This function returns the `\*Messages\*` buffer. If it does not exist, it creates it, and switches it to `messages-buffer-mode`.
User Option: **message-log-max**
This variable specifies how many lines to keep in the `\*Messages\*` buffer. The value `t` means there is no limit on how many lines to keep. The value `nil` disables message logging entirely. Here’s how to display a message and prevent it from being logged:
```
(let (message-log-max)
(message …))
```
To make `\*Messages\*` more convenient for the user, the logging facility combines successive identical messages. It also combines successive related messages for the sake of two cases: question followed by answer, and a series of progress messages.
A question followed by an answer has two messages like the ones produced by `y-or-n-p`: the first is ‘`question`’, and the second is ‘`question...answer`’. The first message conveys no additional information beyond what’s in the second, so logging the second message discards the first from the log.
A series of progress messages has successive messages like those produced by `make-progress-reporter`. They have the form ‘`base...how-far`’, where base is the same each time, while how-far varies. Logging each message in the series discards the previous one, provided they are consecutive.
The functions `make-progress-reporter` and `y-or-n-p` don’t have to do anything special to activate the message log combination feature. It operates whenever two consecutive messages are logged that share a common prefix ending in ‘`...`’.
elisp None #### Accessing Mouse Events
This section describes convenient functions for accessing the data in a mouse button or motion event. Keyboard event data can be accessed using the same functions, but data elements that aren’t applicable to keyboard events are zero or `nil`.
The following two functions return a mouse position list (see [Click Events](click-events)), specifying the position of a mouse event.
Function: **event-start** *event*
This returns the starting position of event.
If event is a click or button-down event, this returns the location of the event. If event is a drag event, this returns the drag’s starting position.
Function: **event-end** *event*
This returns the ending position of event.
If event is a drag event, this returns the position where the user released the mouse button. If event is a click or button-down event, the value is actually the starting position, which is the only position such events have.
Function: **posnp** *object*
This function returns non-`nil` if object is a mouse position list, in the format documented in [Click Events](click-events)); and `nil` otherwise.
These functions take a mouse position list as argument, and return various parts of it:
Function: **posn-window** *position*
Return the window that position is in. If position represents a location outside the frame where the event was initiated, return that frame instead.
Function: **posn-area** *position*
Return the window area recorded in position. It returns `nil` when the event occurred in the text area of the window; otherwise, it is a symbol identifying the area in which the event occurred.
Function: **posn-point** *position*
Return the buffer position in position. When the event occurred in the text area of the window, in a marginal area, or on a fringe, this is an integer specifying a buffer position. Otherwise, the value is undefined.
Function: **posn-x-y** *position*
Return the pixel-based x and y coordinates in position, as a cons cell `(x . y)`. These coordinates are relative to the window given by `posn-window`.
This example shows how to convert the window-relative coordinates in the text area of a window into frame-relative coordinates:
```
(defun frame-relative-coordinates (position)
"Return frame-relative coordinates from POSITION.
POSITION is assumed to lie in a window text area."
(let* ((x-y (posn-x-y position))
(window (posn-window position))
(edges (window-inside-pixel-edges window)))
(cons (+ (car x-y) (car edges))
(+ (cdr x-y) (cadr edges)))))
```
Function: **posn-col-row** *position*
This function returns a cons cell `(col . row)`, containing the estimated column and row corresponding to buffer position described by position. The return value is given in units of the frame’s default character width and default line height (including spacing), as computed from the x and y values corresponding to position. (So, if the actual characters have non-default sizes, the actual row and column may differ from these computed values.)
Note that row is counted from the top of the text area. If the window given by position possesses a header line (see [Header Lines](header-lines)) or a tab line, they are *not* included in the row count.
Function: **posn-actual-col-row** *position*
Return the actual row and column in position, as a cons cell `(col . row)`. The values are the actual row and column numbers in the window given by position. See [Click Events](click-events), for details. The function returns `nil` if position does not include actual position values; in that case `posn-col-row` can be used to get approximate values.
Note that this function doesn’t account for the visual width of characters on display, like the number of visual columns taken by a tab character or an image. If you need the coordinates in canonical character units, use `posn-col-row` instead.
Function: **posn-string** *position*
Return the string object described by position, either `nil` (which means position describes buffer text), or a cons cell `(string . string-pos)`.
Function: **posn-image** *position*
Return the image object in position, either `nil` (if there’s no image at position), or an image spec `(image …)`.
Function: **posn-object** *position*
Return the image or string object described by position, either `nil` (which means position describes buffer text), an image `(image …)`, or a cons cell `(string . string-pos)`.
Function: **posn-object-x-y** *position*
Return the pixel-based x and y coordinates relative to the upper left corner of the object described by position, as a cons cell `(dx . dy)`. If the position describes buffer text, return the relative coordinates of the buffer-text character closest to that position.
Function: **posn-object-width-height** *position*
Return the pixel width and height of the object described by position, as a cons cell `(width . height)`. If the position describes a buffer position, return the size of the character at that position.
Function: **posn-timestamp** *position*
Return the timestamp in position. This is the time at which the event occurred, in milliseconds.
These functions compute a position list given particular buffer position or screen position. You can access the data in this position list with the functions described above.
Function: **posn-at-point** *&optional pos window*
This function returns a position list for position pos in window. pos defaults to point in window; window defaults to the selected window.
`posn-at-point` returns `nil` if pos is not visible in window.
Function: **posn-at-x-y** *x y &optional frame-or-window whole*
This function returns position information corresponding to pixel coordinates x and y in a specified frame or window, frame-or-window, which defaults to the selected window. The coordinates x and y are relative to the text area of the selected window. If whole is `non-nil`, the x coordinate is relative to the entire window area including scroll bars, margins and fringes.
elisp None ### Kinds of Forms
A Lisp object that is intended to be evaluated is called a *form* (or an *expression*). How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and all other types. This section describes all three kinds, one by one, starting with the other types, which are self-evaluating forms.
| | | |
| --- | --- | --- |
| • [Self-Evaluating Forms](self_002devaluating-forms) | | Forms that evaluate to themselves. |
| • [Symbol Forms](symbol-forms) | | Symbols evaluate as variables. |
| • [Classifying Lists](classifying-lists) | | How to distinguish various sorts of list forms. |
| • [Function Indirection](function-indirection) | | When a symbol appears as the car of a list, we find the real function via the symbol. |
| • [Function Forms](function-forms) | | Forms that call functions. |
| • [Macro Forms](macro-forms) | | Forms that call macros. |
| • [Special Forms](special-forms) | | Special forms are idiosyncratic primitives, most of them extremely important. |
| • [Autoloading](autoloading) | | Functions set up to load files containing their real definitions. |
elisp None #### Extending pcase
The `pcase` macro supports several kinds of patterns (see [Pattern-Matching Conditional](pattern_002dmatching-conditional)). You can add support for other kinds of patterns using the `pcase-defmacro` macro.
Macro: **pcase-defmacro** *name args [doc] &rest body*
Define a new kind of pattern for `pcase`, to be invoked as `(name actual-args)`. The `pcase` macro expands this into a function call that evaluates body, whose job it is to rewrite the invoked pattern into some other pattern, in an environment where args are bound to actual-args.
Additionally, arrange to display doc along with the docstring of `pcase`. By convention, doc should use `EXPVAL` to stand for the result of evaluating expression (first arg to `pcase`).
Typically, body rewrites the invoked pattern to use more basic patterns. Although all patterns eventually reduce to core patterns, `body` need not use core patterns straight away. The following example defines two patterns, named `less-than` and `integer-less-than`.
```
(pcase-defmacro less-than (n)
"Matches if EXPVAL is a number less than N."
`(pred (> ,n)))
```
```
(pcase-defmacro integer-less-than (n)
"Matches if EXPVAL is an integer less than N."
`(and (pred integerp)
(less-than ,n)))
```
Note that the docstrings mention args (in this case, only one: `n`) in the usual way, and also mention `EXPVAL` by convention. The first rewrite (i.e., body for `less-than`) uses one core pattern: `pred`. The second uses two core patterns: `and` and `pred`, as well as the newly-defined pattern `less-than`. Both use a single backquote construct (see [Backquote](backquote)).
elisp None ### Abbrev Properties
Abbrevs have properties, some of which influence the way they work. You can provide them as arguments to `define-abbrev`, and manipulate them with the following functions:
Function: **abbrev-put** *abbrev prop val*
Set the property prop of abbrev to value val.
Function: **abbrev-get** *abbrev prop*
Return the property prop of abbrev, or `nil` if the abbrev has no such property.
The following properties have special meanings:
`:count`
This property counts the number of times the abbrev has been expanded. If not explicitly set, it is initialized to 0 by `define-abbrev`.
`:system`
If non-`nil`, this property marks the abbrev as a system abbrev. Such abbrevs are not saved (see [Abbrev Files](abbrev-files)).
`:enable-function`
If non-`nil`, this property should be a function of no arguments which returns `nil` if the abbrev should not be used and `t` otherwise.
`:case-fixed` If non-`nil`, this property indicates that the case of the abbrev’s name is significant and should only match a text with the same pattern of capitalization. It also disables the code that modifies the capitalization of the expansion.
elisp None ### The declare Form
`declare` is a special macro which can be used to add meta properties to a function or macro: for example, marking it as obsolete, or giving its forms a special TAB indentation convention in Emacs Lisp mode.
Macro: **declare** *specs…*
This macro ignores its arguments and evaluates to `nil`; it has no run-time effect. However, when a `declare` form occurs in the declare argument of a `defun` or `defsubst` function definition (see [Defining Functions](defining-functions)) or a `defmacro` macro definition (see [Defining Macros](defining-macros)), it appends the properties specified by specs to the function or macro. This work is specially performed by `defun`, `defsubst`, and `defmacro`.
Each element in specs should have the form `(property
args…)`, which should not be quoted. These have the following effects:
`(advertised-calling-convention signature when)`
This acts like a call to `set-advertised-calling-convention` (see [Obsolete Functions](obsolete-functions)); signature specifies the correct argument list for calling the function or macro, and when should be a string indicating when the old argument list was first made obsolete.
`(debug edebug-form-spec)`
This is valid for macros only. When stepping through the macro with Edebug, use edebug-form-spec. See [Instrumenting Macro Calls](instrumenting-macro-calls).
`(doc-string n)`
This is used when defining a function or macro which itself will be used to define entities like functions, macros, or variables. It indicates that the nth argument, if any, should be considered as a documentation string.
`(indent indent-spec)`
Indent calls to this function or macro according to indent-spec. This is typically used for macros, though it works for functions too. See [Indenting Macros](indenting-macros).
`(interactive-only value)`
Set the function’s `interactive-only` property to value. See [The interactive-only property](defining-commands#The-interactive_002donly-property).
`(obsolete current-name when)`
Mark the function or macro as obsolete, similar to a call to `make-obsolete` (see [Obsolete Functions](obsolete-functions)). current-name should be a symbol (in which case the warning message says to use that instead), a string (specifying the warning message), or `nil` (in which case the warning message gives no extra details). when should be a string indicating when the function or macro was first made obsolete.
`(compiler-macro expander)`
This can only be used for functions, and tells the compiler to use expander as an optimization function. When encountering a call to the function, of the form `(function args…)`, the macro expander will call expander with that form as well as with args…, and expander can either return a new expression to use instead of the function call, or it can return just the form unchanged, to indicate that the function call should be left alone. expander can be a symbol, or it can be a form `(lambda (arg) body)` in which case arg will hold the original function call expression, and the (unevaluated) arguments to the function can be accessed using the function’s formal arguments.
`(gv-expander expander)`
Declare expander to be the function to handle calls to the macro (or function) as a generalized variable, similarly to `gv-define-expander`. expander can be a symbol or it can be of the form `(lambda
(arg) body)` in which case that function will additionally have access to the macro (or function)’s arguments.
`(gv-setter setter)`
Declare setter to be the function to handle calls to the macro (or function) as a generalized variable. setter can be a symbol in which case it will be passed to `gv-define-simple-setter`, or it can be of the form `(lambda (arg) body)` in which case that function will additionally have access to the macro (or function)’s arguments and it will be passed to `gv-define-setter`.
`(completion completion-predicate)`
Declare completion-predicate as a function to determine whether to include the symbol in the list of functions when asking for completions in `M-x`. completion-predicate is called with two parameters: The first parameter is the symbol, and the second is the current buffer.
`(modes modes)`
Specify that this command is meant to be applicable for modes only.
`(pure val)`
If val is non-`nil`, this function is *pure* (see [What Is a Function](what-is-a-function)). This is the same as the `pure` property of the function’s symbol (see [Standard Properties](standard-properties)).
`(side-effect-free val)`
If val is non-`nil`, this function is free of side effects, so the byte compiler can ignore calls whose value is ignored. This is the same as the `side-effect-free` property of the function’s symbol, see [Standard Properties](standard-properties).
`(speed n)`
Specify the value of `native-comp-speed` in effect for native compilation of this function (see [Native-Compilation Variables](native_002dcompilation-variables)). This allows function-level control of the optimization level used for native code emitted for the function. In particular, if n is -1, native compilation of the function will emit bytecode instead of native code for the function.
`no-font-lock-keyword` This is valid for macros only. Macros with this declaration are highlighted by font-lock (see [Font Lock Mode](font-lock-mode)) as normal functions, not specially as macros.
| programming_docs |
elisp None ### Regular Expressions
A *regular expression*, or *regexp* for short, is a pattern that denotes a (possibly infinite) set of strings. Searching for matches for a regexp is a very powerful operation. This section explains how to write regexps; the following section says how to search for them.
For interactive development of regular expressions, you can use the `M-x re-builder` command. It provides a convenient interface for creating regular expressions, by giving immediate visual feedback in a separate buffer. As you edit the regexp, all its matches in the target buffer are highlighted. Each parenthesized sub-expression of the regexp is shown in a distinct face, which makes it easier to verify even very complex regexps.
Note that by default Emacs search ignores case (see [Searching and Case](searching-and-case)). To enable case-sensitive regexp search and match, bind `case-fold-search` to `nil` around the code you want to be case-sensitive.
| | | |
| --- | --- | --- |
| • [Syntax of Regexps](syntax-of-regexps) | | Rules for writing regular expressions. |
| • [Regexp Example](regexp-example) | | Illustrates regular expression syntax. |
| • [Rx Notation](rx-notation) | | An alternative, structured regexp notation. |
| • [Regexp Functions](regexp-functions) | | Functions for operating on regular expressions. |
| • [Regexp Problems](regexp-problems) | | Some problems and how they may be avoided. |
elisp None ### Syntax Descriptors
The *syntax class* of a character describes its syntactic role. Each syntax table specifies the syntax class of each character. There is no necessary relationship between the class of a character in one syntax table and its class in any other table.
Each syntax class is designated by a mnemonic character, which serves as the name of the class when you need to specify a class. Usually, this designator character is one that is often assigned that class; however, its meaning as a designator is unvarying and independent of what syntax that character currently has. Thus, ‘`\`’ as a designator character always stands for escape character syntax, regardless of whether the ‘`\`’ character actually has that syntax in the current syntax table. See [Syntax Class Table](syntax-class-table), for a list of syntax classes and their designator characters.
A *syntax descriptor* is a Lisp string that describes the syntax class and other syntactic properties of a character. When you want to modify the syntax of a character, that is done by calling the function `modify-syntax-entry` and passing a syntax descriptor as one of its arguments (see [Syntax Table Functions](syntax-table-functions)).
The first character in a syntax descriptor must be a syntax class designator character. The second character, if present, specifies a matching character (e.g., in Lisp, the matching character for ‘`(`’ is ‘`)`’); a space specifies that there is no matching character. Then come characters specifying additional syntax properties (see [Syntax Flags](syntax-flags)).
If no matching character or flags are needed, only one character (specifying the syntax class) is sufficient.
For example, the syntax descriptor for the character ‘`\*`’ in C mode is `". 23"` (i.e., punctuation, matching character slot unused, second character of a comment-starter, first character of a comment-ender), and the entry for ‘`/`’ is ‘`. 14`’ (i.e., punctuation, matching character slot unused, first character of a comment-starter, second character of a comment-ender).
Emacs also defines *raw syntax descriptors*, which are used to describe syntax classes at a lower level. See [Syntax Table Internals](syntax-table-internals).
| | | |
| --- | --- | --- |
| • [Syntax Class Table](syntax-class-table) | | Table of syntax classes. |
| • [Syntax Flags](syntax-flags) | | Additional flags each character can have. |
elisp None ### Prefix Command Arguments
Most Emacs commands can use a *prefix argument*, a number specified before the command itself. (Don’t confuse prefix arguments with prefix keys.) The prefix argument is at all times represented by a value, which may be `nil`, meaning there is currently no prefix argument. Each command may use the prefix argument or ignore it.
There are two representations of the prefix argument: *raw* and *numeric*. The editor command loop uses the raw representation internally, and so do the Lisp variables that store the information, but commands can request either representation.
Here are the possible values of a raw prefix argument:
* `nil`, meaning there is no prefix argument. Its numeric value is 1, but numerous commands make a distinction between `nil` and the integer 1.
* An integer, which stands for itself.
* A list of one element, which is an integer. This form of prefix argument results from one or a succession of `C-u`s with no digits. The numeric value is the integer in the list, but some commands make a distinction between such a list and an integer alone.
* The symbol `-`. This indicates that `M--` or `C-u -` was typed, without following digits. The equivalent numeric value is -1, but some commands make a distinction between the integer -1 and the symbol `-`.
We illustrate these possibilities by calling the following function with various prefixes:
```
(defun display-prefix (arg)
"Display the value of the raw prefix arg."
(interactive "P")
(message "%s" arg))
```
Here are the results of calling `display-prefix` with various raw prefix arguments:
```
M-x display-prefix -| nil
C-u M-x display-prefix -| (4)
C-u C-u M-x display-prefix -| (16)
C-u 3 M-x display-prefix -| 3
M-3 M-x display-prefix -| 3 ; (Same as `C-u 3`.)
C-u - M-x display-prefix -| -
M-- M-x display-prefix -| - ; (Same as `C-u -`.)
C-u - 7 M-x display-prefix -| -7
M-- 7 M-x display-prefix -| -7 ; (Same as `C-u -7`.)
```
Emacs uses two variables to store the prefix argument: `prefix-arg` and `current-prefix-arg`. Commands such as `universal-argument` that set up prefix arguments for other commands store them in `prefix-arg`. In contrast, `current-prefix-arg` conveys the prefix argument to the current command, so setting it has no effect on the prefix arguments for future commands.
Normally, commands specify which representation to use for the prefix argument, either numeric or raw, in the `interactive` specification. (See [Using Interactive](using-interactive).) Alternatively, functions may look at the value of the prefix argument directly in the variable `current-prefix-arg`, but this is less clean.
Function: **prefix-numeric-value** *arg*
This function returns the numeric meaning of a valid raw prefix argument value, arg. The argument may be a symbol, a number, or a list. If it is `nil`, the value 1 is returned; if it is `-`, the value -1 is returned; if it is a number, that number is returned; if it is a list, the CAR of that list (which should be a number) is returned.
Variable: **current-prefix-arg**
This variable holds the raw prefix argument for the *current* command. Commands may examine it directly, but the usual method for accessing it is with `(interactive "P")`.
Variable: **prefix-arg**
The value of this variable is the raw prefix argument for the *next* editing command. Commands such as `universal-argument` that specify prefix arguments for the following command work by setting this variable.
Variable: **last-prefix-arg**
The raw prefix argument value used by the previous command.
The following commands exist to set up prefix arguments for the following command. Do not call them for any other reason.
Command: **universal-argument**
This command reads input and specifies a prefix argument for the following command. Don’t call this command yourself unless you know what you are doing.
Command: **digit-argument** *arg*
This command adds to the prefix argument for the following command. The argument arg is the raw prefix argument as it was before this command; it is used to compute the updated prefix argument. Don’t call this command yourself unless you know what you are doing.
Command: **negative-argument** *arg*
This command adds to the numeric argument for the next command. The argument arg is the raw prefix argument as it was before this command; its value is negated to form the new prefix argument. Don’t call this command yourself unless you know what you are doing.
elisp None ### Quitting
Typing `C-g` while a Lisp function is running causes Emacs to *quit* whatever it is doing. This means that control returns to the innermost active command loop.
Typing `C-g` while the command loop is waiting for keyboard input does not cause a quit; it acts as an ordinary input character. In the simplest case, you cannot tell the difference, because `C-g` normally runs the command `keyboard-quit`, whose effect is to quit. However, when `C-g` follows a prefix key, they combine to form an undefined key. The effect is to cancel the prefix key as well as any prefix argument.
In the minibuffer, `C-g` has a different definition: it aborts out of the minibuffer. This means, in effect, that it exits the minibuffer and then quits. (Simply quitting would return to the command loop *within* the minibuffer.) The reason why `C-g` does not quit directly when the command reader is reading input is so that its meaning can be redefined in the minibuffer in this way. `C-g` following a prefix key is not redefined in the minibuffer, and it has its normal effect of canceling the prefix key and prefix argument. This too would not be possible if `C-g` always quit directly.
When `C-g` does directly quit, it does so by setting the variable `quit-flag` to `t`. Emacs checks this variable at appropriate times and quits if it is not `nil`. Setting `quit-flag` non-`nil` in any way thus causes a quit.
At the level of C code, quitting cannot happen just anywhere; only at the special places that check `quit-flag`. The reason for this is that quitting at other places might leave an inconsistency in Emacs’s internal state. Because quitting is delayed until a safe place, quitting cannot make Emacs crash.
Certain functions such as `read-key-sequence` or `read-quoted-char` prevent quitting entirely even though they wait for input. Instead of quitting, `C-g` serves as the requested input. In the case of `read-key-sequence`, this serves to bring about the special behavior of `C-g` in the command loop. In the case of `read-quoted-char`, this is so that `C-q` can be used to quote a `C-g`.
You can prevent quitting for a portion of a Lisp function by binding the variable `inhibit-quit` to a non-`nil` value. Then, although `C-g` still sets `quit-flag` to `t` as usual, the usual result of this—a quit—is prevented. Eventually, `inhibit-quit` will become `nil` again, such as when its binding is unwound at the end of a `let` form. At that time, if `quit-flag` is still non-`nil`, the requested quit happens immediately. This behavior is ideal when you wish to make sure that quitting does not happen within a critical section of the program.
In some functions (such as `read-quoted-char`), `C-g` is handled in a special way that does not involve quitting. This is done by reading the input with `inhibit-quit` bound to `t`, and setting `quit-flag` to `nil` before `inhibit-quit` becomes `nil` again. This excerpt from the definition of `read-quoted-char` shows how this is done; it also shows that normal quitting is permitted after the first character of input.
```
(defun read-quoted-char (&optional prompt)
"…documentation…"
(let ((message-log-max nil) done (first t) (code 0) char)
(while (not done)
(let ((inhibit-quit first)
…)
(and prompt (message "%s-" prompt))
(setq char (read-event))
(if inhibit-quit (setq quit-flag nil)))
…set the variable `code`…)
code))
```
Variable: **quit-flag**
If this variable is non-`nil`, then Emacs quits immediately, unless `inhibit-quit` is non-`nil`. Typing `C-g` ordinarily sets `quit-flag` non-`nil`, regardless of `inhibit-quit`.
Variable: **inhibit-quit**
This variable determines whether Emacs should quit when `quit-flag` is set to a value other than `nil`. If `inhibit-quit` is non-`nil`, then `quit-flag` has no special effect.
Macro: **with-local-quit** *body…*
This macro executes body forms in sequence, but allows quitting, at least locally, within body even if `inhibit-quit` was non-`nil` outside this construct. It returns the value of the last form in body, unless exited by quitting, in which case it returns `nil`.
If `inhibit-quit` is `nil` on entry to `with-local-quit`, it only executes the body, and setting `quit-flag` causes a normal quit. However, if `inhibit-quit` is non-`nil` so that ordinary quitting is delayed, a non-`nil` `quit-flag` triggers a special kind of local quit. This ends the execution of body and exits the `with-local-quit` body with `quit-flag` still non-`nil`, so that another (ordinary) quit will happen as soon as that is allowed. If `quit-flag` is already non-`nil` at the beginning of body, the local quit happens immediately and the body doesn’t execute at all.
This macro is mainly useful in functions that can be called from timers, process filters, process sentinels, `pre-command-hook`, `post-command-hook`, and other places where `inhibit-quit` is normally bound to `t`.
Command: **keyboard-quit**
This function signals the `quit` condition with `(signal 'quit
nil)`. This is the same thing that quitting does. (See `signal` in [Errors](errors).)
To quit without aborting a keyboard macro definition or execution, you can signal the `minibuffer-quit` condition. This has almost the same effect as the `quit` condition except that the error handling in the command loop handles it without exiting keyboard macro definition or execution.
You can specify a character other than `C-g` to use for quitting. See the function `set-input-mode` in [Input Modes](input-modes).
elisp None #### Special Characters in Regular Expressions
Here is a list of the characters that are special in a regular expression.
‘`.`’ (Period)
is a special character that matches any single character except a newline. Using concatenation, we can make regular expressions like ‘`a.b`’, which matches any three-character string that begins with ‘`a`’ and ends with ‘`b`’.
‘`\*`’
is not a construct by itself; it is a postfix operator that means to match the preceding regular expression repetitively as many times as possible. Thus, ‘`o\*`’ matches any number of ‘`o`’s (including no ‘`o`’s).
‘`\*`’ always applies to the *smallest* possible preceding expression. Thus, ‘`fo\*`’ has a repeating ‘`o`’, not a repeating ‘`fo`’. It matches ‘`f`’, ‘`fo`’, ‘`foo`’, and so on.
The matcher processes a ‘`\*`’ construct by matching, immediately, as many repetitions as can be found. Then it continues with the rest of the pattern. If that fails, backtracking occurs, discarding some of the matches of the ‘`\*`’-modified construct in the hope that this will make it possible to match the rest of the pattern. For example, in matching ‘`ca\*ar`’ against the string ‘`caaar`’, the ‘`a\*`’ first tries to match all three ‘`a`’s; but the rest of the pattern is ‘`ar`’ and there is only ‘`r`’ left to match, so this try fails. The next alternative is for ‘`a\*`’ to match only two ‘`a`’s. With this choice, the rest of the regexp matches successfully.
‘`+`’
is a postfix operator, similar to ‘`\*`’ except that it must match the preceding expression at least once. So, for example, ‘`ca+r`’ matches the strings ‘`car`’ and ‘`caaaar`’ but not the string ‘`cr`’, whereas ‘`ca\*r`’ matches all three strings.
‘`?`’
is a postfix operator, similar to ‘`\*`’ except that it must match the preceding expression either once or not at all. For example, ‘`ca?r`’ matches ‘`car`’ or ‘`cr`’; nothing else.
‘`\*?`’, ‘`+?`’, ‘`??`’
are *non-greedy* variants of the operators ‘`\*`’, ‘`+`’ and ‘`?`’. Where those operators match the largest possible substring (consistent with matching the entire containing expression), the non-greedy variants match the smallest possible substring (consistent with matching the entire containing expression).
For example, the regular expression ‘`c[ad]\*a`’ when applied to the string ‘`cdaaada`’ matches the whole string; but the regular expression ‘`c[ad]\*?a`’, applied to that same string, matches just ‘`cda`’. (The smallest possible match here for ‘`[ad]\*?`’ that permits the whole expression to match is ‘`d`’.)
‘`[ … ]`’
is a *character alternative*, which begins with ‘`[`’ and is terminated by ‘`]`’. In the simplest case, the characters between the two brackets are what this character alternative can match.
Thus, ‘`[ad]`’ matches either one ‘`a`’ or one ‘`d`’, and ‘`[ad]\*`’ matches any string composed of just ‘`a`’s and ‘`d`’s (including the empty string). It follows that ‘`c[ad]\*r`’ matches ‘`cr`’, ‘`car`’, ‘`cdr`’, ‘`caddaar`’, etc.
You can also include character ranges in a character alternative, by writing the starting and ending characters with a ‘`-`’ between them. Thus, ‘`[a-z]`’ matches any lower-case ASCII letter. Ranges may be intermixed freely with individual characters, as in ‘`[a-z$%.]`’, which matches any lower case ASCII letter or ‘`$`’, ‘`%`’ or period. However, the ending character of one range should not be the starting point of another one; for example, ‘`[a-m-z]`’ should be avoided.
A character alternative can also specify named character classes (see [Char Classes](char-classes)). This is a POSIX feature. For example, ‘`[[:ascii:]]`’ matches any ASCII character. Using a character class is equivalent to mentioning each of the characters in that class; but the latter is not feasible in practice, since some classes include thousands of different characters. A character class should not appear as the lower or upper bound of a range.
The usual regexp special characters are not special inside a character alternative. A completely different set of characters is special: ‘`]`’, ‘`-`’ and ‘`^`’. To include ‘`]`’ in a character alternative, put it at the beginning. To include ‘`^`’, put it anywhere but at the beginning. To include ‘`-`’, put it at the end. Thus, ‘`[]^-]`’ matches all three of these special characters. You cannot use ‘`\`’ to escape these three characters, since ‘`\`’ is not special here.
The following aspects of ranges are specific to Emacs, in that POSIX allows but does not require this behavior and programs other than Emacs may behave differently:
1. If `case-fold-search` is non-`nil`, ‘`[a-z]`’ also matches upper-case letters.
2. A range is not affected by the locale’s collation sequence: it always represents the set of characters with codepoints ranging between those of its bounds, so that ‘`[a-z]`’ matches only ASCII letters, even outside the C or POSIX locale.
3. If the lower bound of a range is greater than its upper bound, the range is empty and represents no characters. Thus, ‘`[z-a]`’ always fails to match, and ‘`[^z-a]`’ matches any character, including newline. However, a reversed range should always be from the letter ‘`z`’ to the letter ‘`a`’ to make it clear that it is not a typo; for example, ‘`[+-\*/]`’ should be avoided, because it matches only ‘`/`’ rather than the likely-intended four characters.
4. If the end points of a range are raw 8-bit bytes (see [Text Representations](text-representations)), or if the range start is ASCII and the end is a raw byte (as in ‘`[a-\377]`’), the range will match only ASCII characters and raw 8-bit bytes, but not non-ASCII characters. This feature is intended for searching text in unibyte buffers and strings.
Some kinds of character alternatives are not the best style even though they have a well-defined meaning in Emacs. They include:
1. Although a range’s bound can be almost any character, it is better style to stay within natural sequences of ASCII letters and digits because most people have not memorized character code tables. For example, ‘`[.-9]`’ is less clear than ‘`[./0-9]`’, and ‘`[`-~]`’ is less clear than ‘`[`a-z{|}~]`’. Unicode character escapes can help here; for example, for most programmers ‘`[ก-ฺ฿-๛]`’ is less clear than ‘`[\u0E01-\u0E3A\u0E3F-\u0E5B]`’.
2. Although a character alternative can include duplicates, it is better style to avoid them. For example, ‘`[XYa-yYb-zX]`’ is less clear than ‘`[XYa-z]`’.
3. Although a range can denote just one, two, or three characters, it is simpler to list the characters. For example, ‘`[a-a0]`’ is less clear than ‘`[a0]`’, ‘`[i-j]`’ is less clear than ‘`[ij]`’, and ‘`[i-k]`’ is less clear than ‘`[ijk]`’.
4. Although a ‘`-`’ can appear at the beginning of a character alternative or as the upper bound of a range, it is better style to put ‘`-`’ by itself at the end of a character alternative. For example, although ‘`[-a-z]`’ is valid, ‘`[a-z-]`’ is better style; and although ‘`[\*--]`’ is valid, ‘`[\*+,-]`’ is clearer.
‘`[^ … ]`’
‘`[^`’ begins a *complemented character alternative*. This matches any character except the ones specified. Thus, ‘`[^a-z0-9A-Z]`’ matches all characters *except* ASCII letters and digits.
‘`^`’ is not special in a character alternative unless it is the first character. The character following the ‘`^`’ is treated as if it were first (in other words, ‘`-`’ and ‘`]`’ are not special there).
A complemented character alternative can match a newline, unless newline is mentioned as one of the characters not to match. This is in contrast to the handling of regexps in programs such as `grep`.
You can specify named character classes, just like in character alternatives. For instance, ‘`[^[:ascii:]]`’ matches any non-ASCII character. See [Char Classes](char-classes).
‘`^`’
When matching a buffer, ‘`^`’ matches the empty string, but only at the beginning of a line in the text being matched (or the beginning of the accessible portion of the buffer). Otherwise it fails to match anything. Thus, ‘`^foo`’ matches a ‘`foo`’ that occurs at the beginning of a line.
When matching a string instead of a buffer, ‘`^`’ matches at the beginning of the string or after a newline character.
For historical compatibility reasons, ‘`^`’ can be used only at the beginning of the regular expression, or after ‘`\(`’, ‘`\(?:`’ or ‘`\|`’.
‘`$`’
is similar to ‘`^`’ but matches only at the end of a line (or the end of the accessible portion of the buffer). Thus, ‘`x+$`’ matches a string of one ‘`x`’ or more at the end of a line.
When matching a string instead of a buffer, ‘`$`’ matches at the end of the string or before a newline character.
For historical compatibility reasons, ‘`$`’ can be used only at the end of the regular expression, or before ‘`\)`’ or ‘`\|`’.
‘`\`’
has two functions: it quotes the special characters (including ‘`\`’), and it introduces additional special constructs.
Because ‘`\`’ quotes special characters, ‘`\$`’ is a regular expression that matches only ‘`$`’, and ‘`\[`’ is a regular expression that matches only ‘`[`’, and so on.
Note that ‘`\`’ also has special meaning in the read syntax of Lisp strings (see [String Type](string-type)), and must be quoted with ‘`\`’. For example, the regular expression that matches the ‘`\`’ character is ‘`\\`’. To write a Lisp string that contains the characters ‘`\\`’, Lisp syntax requires you to quote each ‘`\`’ with another ‘`\`’. Therefore, the read syntax for a regular expression matching ‘`\`’ is `"\\\\"`.
**Please note:** For historical compatibility, special characters are treated as ordinary ones if they are in contexts where their special meanings make no sense. For example, ‘`\*foo`’ treats ‘`\*`’ as ordinary since there is no preceding expression on which the ‘`\*`’ can act. It is poor practice to depend on this behavior; quote the special character anyway, regardless of where it appears.
As a ‘`\`’ is not special inside a character alternative, it can never remove the special meaning of ‘`-`’ or ‘`]`’. So you should not quote these characters when they have no special meaning either. This would not clarify anything, since backslashes can legitimately precede these characters where they *have* special meaning, as in ‘`[^\]`’ (`"[^\\]"` for Lisp string syntax), which matches any single character except a backslash.
In practice, most ‘`]`’ that occur in regular expressions close a character alternative and hence are special. However, occasionally a regular expression may try to match a complex pattern of literal ‘`[`’ and ‘`]`’. In such situations, it sometimes may be necessary to carefully parse the regexp from the start to determine which square brackets enclose a character alternative. For example, ‘`[^][]]`’ consists of the complemented character alternative ‘`[^][]`’ (which matches any single character that is not a square bracket), followed by a literal ‘`]`’.
The exact rules are that at the beginning of a regexp, ‘`[`’ is special and ‘`]`’ not. This lasts until the first unquoted ‘`[`’, after which we are in a character alternative; ‘`[`’ is no longer special (except when it starts a character class) but ‘`]`’ is special, unless it immediately follows the special ‘`[`’ or that ‘`[`’ followed by a ‘`^`’. This lasts until the next special ‘`]`’ that does not end a character class. This ends the character alternative and restores the ordinary syntax of regular expressions; an unquoted ‘`[`’ is special again and a ‘`]`’ not.
| programming_docs |
elisp None #### Input Modes
Function: **set-input-mode** *interrupt flow meta &optional quit-char*
This function sets the mode for reading keyboard input. If interrupt is non-`nil`, then Emacs uses input interrupts. If it is `nil`, then it uses CBREAK mode. The default setting is system-dependent. Some systems always use CBREAK mode regardless of what is specified.
When Emacs communicates directly with X, it ignores this argument and uses interrupts if that is the way it knows how to communicate.
If flow is non-`nil`, then Emacs uses XON/XOFF (`C-q`, `C-s`) flow control for output to the terminal. This has no effect except in CBREAK mode.
The argument meta controls support for input character codes above 127. If meta is `t`, Emacs converts characters with the 8th bit set into Meta characters, before it decodes them as needed (see [Terminal I/O Encoding](terminal-i_002fo-encoding)). If meta is `nil`, Emacs disregards the 8th bit; this is necessary when the terminal uses it as a parity bit. If meta is the symbol `encoded`, Emacs first decodes the characters using all the 8 bits of each byte, and then converts the decoded single-byte characters into Meta characters if they have their eighth bit set. Finally, if meta is neither `t` nor `nil` nor `encoded`, Emacs uses all 8 bits of input unchanged, both before and after decoding them. This is good for terminals that use 8-bit character sets and don’t encode the Meta modifier as the eighth bit.
If quit-char is non-`nil`, it specifies the character to use for quitting. Normally this character is `C-g`. See [Quitting](quitting).
The `current-input-mode` function returns the input mode settings Emacs is currently using.
Function: **current-input-mode**
This function returns the current mode for reading keyboard input. It returns a list, corresponding to the arguments of `set-input-mode`, of the form `(interrupt flow meta quit)` in which:
interrupt is non-`nil` when Emacs is using interrupt-driven input. If `nil`, Emacs is using CBREAK mode.
flow is non-`nil` if Emacs uses XON/XOFF (`C-q`, `C-s`) flow control for output to the terminal. This value is meaningful only when interrupt is `nil`.
meta is `t` if Emacs treats the eighth bit of input characters as the Meta bit before decoding input; `encoded` if Emacs treats the eighth bit of the decoded single-byte characters as the Meta bit; `nil` if Emacs clears the eighth bit of every input character; any other value means Emacs uses all eight bits as the basic character code.
quit is the character Emacs currently uses for quitting, usually `C-g`.
elisp None ### Misc Network Facilities
These additional functions are useful for creating and operating on network connections. Note that they are supported only on some systems.
Function: **network-interface-list** *&optional full family*
This function returns a list describing the network interfaces of the machine you are using. The value is an alist whose elements have the form `(ifname . address)`. ifname is a string naming the interface, address has the same form as the local-address and remote-address arguments to `make-network-process`, i.e. a vector of integers. By default both IPv4 and IPv6 addresses are returned if possible.
Optional argument full non-`nil` means to instead return a list of one or more elements of the form `(ifname addr bcast netmask)`. ifname is a non-unique string naming the interface. addr, bcast, and netmask are vectors of integers detailing the IP address, broadcast address, and network mask.
Optional argument family specified as symbol `ipv4` or `ipv6` restricts the returned information to IPv4 and IPv6 addresses respectively, independently of the value of full. Specifying `ipv6` when IPv6 support is not available will result in an error being signaled.
Some examples:
```
(network-interface-list) ⇒
(("vmnet8" .
[172 16 76 1 0])
("vmnet1" .
[172 16 206 1 0])
("lo0" .
[65152 0 0 0 0 0 0 1 0])
("lo0" .
[0 0 0 0 0 0 0 1 0])
("lo0" .
[127 0 0 1 0]))
```
```
(network-interface-list t) ⇒
(("vmnet8"
[172 16 76 1 0]
[172 16 76 255 0]
[255 255 255 0 0])
("vmnet1"
[172 16 206 1 0]
[172 16 206 255 0]
[255 255 255 0 0])
("lo0"
[65152 0 0 0 0 0 0 1 0]
[65152 0 0 0 65535 65535 65535 65535 0]
[65535 65535 65535 65535 0 0 0 0 0])
("lo0"
[0 0 0 0 0 0 0 1 0]
[0 0 0 0 0 0 0 1 0]
[65535 65535 65535 65535 65535 65535 65535 65535 0])
("lo0"
[127 0 0 1 0]
[127 255 255 255 0]
[255 0 0 0 0]))
```
Function: **network-interface-info** *ifname*
This function returns information about the network interface named ifname. The value is a list of the form `(addr bcast netmask hwaddr flags)`.
addr The Internet protocol address.
bcast The broadcast address.
netmask The network mask.
hwaddr The layer 2 address (Ethernet MAC address, for instance).
flags The current flags of the interface.
Note that this function returns only IPv4 information.
Function: **format-network-address** *address &optional omit-port*
This function converts the Lisp representation of a network address to a string.
A five-element vector `[a b c d p]` represents an IPv4 address a.b.c.d and port number p. `format-network-address` converts that to the string `"a.b.c.d:p"`.
A nine-element vector `[a b c d e
f g h p]` represents an IPv6 address along with a port number. `format-network-address` converts that to the string `"[a:b:c:d:e:f:g:h]:p"`.
If the vector does not include the port number, p, or if omit-port is non-`nil`, the result does not include the `:p` suffix.
Function: **network-lookup-address-info** *name &optional family*
This function is used to perform hostname lookups on name, which is expected to be an ASCII-only string, otherwise an error is signaled. Call `puny-encode-domain` on name first if you wish to lookup internationalized hostnames.
If successful it returns a list of Lisp representations of network addresses, otherwise it returns `nil`. In the latter case, it also displays the error message hopefully explaining what went wrong.
By default both IPv4 and IPv6 lookups are attempted. The optional argument family controls this behavior, specifying the symbol `ipv4` or `ipv6` restricts lookups to IPv4 and IPv6 respectively.
elisp None ### Describing Characters for Help Messages
These functions convert events, key sequences, or characters to textual descriptions. These descriptions are useful for including arbitrary text characters or key sequences in messages, because they convert non-printing and whitespace characters to sequences of printing characters. The description of a non-whitespace printing character is the character itself.
Function: **key-description** *sequence &optional prefix*
This function returns a string containing the Emacs standard notation for the input events in sequence. If prefix is non-`nil`, it is a sequence of input events leading up to sequence and is included in the return value. Both arguments may be strings, vectors or lists. See [Input Events](input-events), for more information about valid events.
```
(key-description [?\M-3 delete])
⇒ "M-3 <delete>"
```
```
(key-description [delete] "\M-3")
⇒ "M-3 <delete>"
```
See also the examples for `single-key-description`, below.
Function: **single-key-description** *event &optional no-angles*
This function returns a string describing event in the standard Emacs notation for keyboard input. A normal printing character appears as itself, but a control character turns into a string starting with ‘`C-`’, a meta character turns into a string starting with ‘`M-`’, and space, tab, etc., appear as ‘`SPC`’, ‘`TAB`’, etc. A function key symbol appears inside angle brackets ‘`<…>`’. An event that is a list appears as the name of the symbol in the CAR of the list, inside angle brackets.
If the optional argument no-angles is non-`nil`, the angle brackets around function keys and event symbols are omitted; this is for compatibility with old versions of Emacs which didn’t use the brackets.
```
(single-key-description ?\C-x)
⇒ "C-x"
```
```
(key-description "\C-x \M-y \n \t \r \f123")
⇒ "C-x SPC M-y SPC C-j SPC TAB SPC RET SPC C-l 1 2 3"
```
```
(single-key-description 'delete)
⇒ "<delete>"
```
```
(single-key-description 'C-mouse-1)
⇒ "C-<mouse-1>"
```
```
(single-key-description 'C-mouse-1 t)
⇒ "C-mouse-1"
```
Function: **text-char-description** *character*
This function returns a string describing character in the standard Emacs notation for characters that can appear in text—similar to `single-key-description`, except that the argument must be a valid character code that passes a `characterp` test (see [Character Codes](character-codes)). The function produces descriptions of control characters with a leading caret (which is how Emacs usually displays control characters in buffers). Characters with modifier bits will cause this function to signal an error (ASCII characters with the Control modifier are an exception, they are represented as control characters).
```
(text-char-description ?\C-c)
⇒ "^C"
```
```
(text-char-description ?\M-m)
error→ Wrong type argument: characterp, 134217837
```
Command: **read-kbd-macro** *string &optional need-vector*
This function is used mainly for operating on keyboard macros, but it can also be used as a rough inverse for `key-description`. You call it with a string containing key descriptions, separated by spaces; it returns a string or vector containing the corresponding events. (This may or may not be a single valid key sequence, depending on what events you use; see [Key Sequences](key-sequences).) If need-vector is non-`nil`, the return value is always a vector.
elisp None #### Showing Images
You can use an image descriptor by setting up the `display` property yourself, but it is easier to use the functions in this section.
Function: **insert-image** *image &optional string area slice*
This function inserts image in the current buffer at point. The value image should be an image descriptor; it could be a value returned by `create-image`, or the value of a symbol defined with `defimage`. The argument string specifies the text to put in the buffer to hold the image. If it is omitted or `nil`, `insert-image` uses `" "` by default.
The argument area specifies whether to put the image in a margin. If it is `left-margin`, the image appears in the left margin; `right-margin` specifies the right margin. If area is `nil` or omitted, the image is displayed at point within the buffer’s text.
The argument slice specifies a slice of the image to insert. If slice is `nil` or omitted the whole image is inserted. (However, note that images are chopped on display at the window’s right edge, because wrapping images is not supported.) Otherwise, slice is a list `(x y width
height)` which specifies the x and y positions and width and height of the image area to insert. Integer values are in units of pixels. A floating-point number in the range 0.0–1.0 stands for that fraction of the width or height of the entire image.
Internally, this function inserts string in the buffer, and gives it a `display` property which specifies image. See [Display Property](display-property).
Function: **insert-sliced-image** *image &optional string area rows cols*
This function inserts image in the current buffer at point, like `insert-image`, but splits the image into rowsxcols equally sized slices.
Emacs displays each slice as a separate image, and allows more intuitive scrolling up/down, instead of jumping up/down the entire image when paging through a buffer that displays (large) images.
Function: **put-image** *image pos &optional string area*
This function puts image image in front of pos in the current buffer. The argument pos should be an integer or a marker. It specifies the buffer position where the image should appear. The argument string specifies the text that should hold the image as an alternative to the default.
The argument image must be an image descriptor, perhaps returned by `create-image` or stored by `defimage`.
The argument area specifies whether to put the image in a margin. If it is `left-margin`, the image appears in the left margin; `right-margin` specifies the right margin. If area is `nil` or omitted, the image is displayed at point within the buffer’s text.
Internally, this function creates an overlay, and gives it a `before-string` property containing text that has a `display` property whose value is the image. (Whew!)
Function: **remove-images** *start end &optional buffer*
This function removes images in buffer between positions start and end. If buffer is omitted or `nil`, images are removed from the current buffer.
This removes only images that were put into buffer the way `put-image` does it, not images that were inserted with `insert-image` or in other ways.
Function: **image-size** *spec &optional pixels frame*
This function returns the size of an image as a pair `(width . height)`. spec is an image specification. pixels non-`nil` means return sizes measured in pixels, otherwise return sizes measured in the default character size of frame (see [Frame Font](frame-font)). frame is the frame on which the image will be displayed. frame `nil` or omitted means use the selected frame (see [Input Focus](input-focus)).
Variable: **max-image-size**
This variable is used to define the maximum size of image that Emacs will load. Emacs will refuse to load (and display) any image that is larger than this limit.
If the value is an integer, it directly specifies the maximum image height and width, measured in pixels. If it is floating point, it specifies the maximum image height and width as a ratio to the frame height and width. If the value is non-numeric, there is no explicit limit on the size of images.
The purpose of this variable is to prevent unreasonably large images from accidentally being loaded into Emacs. It only takes effect the first time an image is loaded. Once an image is placed in the image cache, it can always be displayed, even if the value of `max-image-size` is subsequently changed (see [Image Cache](image-cache)).
Images inserted with the insertion functions above also get a local keymap installed in the text properties (or overlays) that span the displayed image. This keymap defines the following commands:
`+`
Increase the image size (`image-increase-size`). A prefix value of ‘`4`’ means to increase the size by 40%. The default is 20%.
`-`
Decrease the image size (`image-increase-size`). A prefix value of ‘`4`’ means to decrease the size by 40%. The default is 20%.
`r`
Rotate the image by 90 degrees clockwise (`image-rotate`). A prefix means to rotate by 90 degrees counter-clockwise instead.
`o` Save the image to a file (`image-save`).
elisp None Abbrevs and Abbrev Expansion
----------------------------
An abbreviation or *abbrev* is a string of characters that may be expanded to a longer string. The user can insert the abbrev string and find it replaced automatically with the expansion of the abbrev. This saves typing.
The set of abbrevs currently in effect is recorded in an *abbrev table*. Each buffer has a local abbrev table, but normally all buffers in the same major mode share one abbrev table. There is also a global abbrev table. Normally both are used.
An abbrev table is represented as an obarray. See [Creating Symbols](creating-symbols), for information about obarrays. Each abbreviation is represented by a symbol in the obarray. The symbol’s name is the abbreviation; its value is the expansion; its function definition is the hook function for performing the expansion (see [Defining Abbrevs](defining-abbrevs)); and its property list cell contains various additional properties, including the use count and the number of times the abbreviation has been expanded (see [Abbrev Properties](abbrev-properties)).
Certain abbrevs, called *system abbrevs*, are defined by a major mode instead of the user. A system abbrev is identified by its non-`nil` `:system` property (see [Abbrev Properties](abbrev-properties)). When abbrevs are saved to an abbrev file, system abbrevs are omitted. See [Abbrev Files](abbrev-files).
Because the symbols used for abbrevs are not interned in the usual obarray, they will never appear as the result of reading a Lisp expression; in fact, normally they are never used except by the code that handles abbrevs. Therefore, it is safe to use them in a nonstandard way.
If the minor mode Abbrev mode is enabled, the buffer-local variable `abbrev-mode` is non-`nil`, and abbrevs are automatically expanded in the buffer. For the user-level commands for abbrevs, see [Abbrev Mode](https://www.gnu.org/software/emacs/manual/html_node/emacs/Abbrevs.html#Abbrevs) in The GNU Emacs Manual.
| | | |
| --- | --- | --- |
| • [Tables](abbrev-tables) | | Creating and working with abbrev tables. |
| • [Defining Abbrevs](defining-abbrevs) | | Specifying abbreviations and their expansions. |
| • [Files](abbrev-files) | | Saving abbrevs in files. |
| • [Expansion](abbrev-expansion) | | Controlling expansion; expansion subroutines. |
| • [Standard Abbrev Tables](standard-abbrev-tables) | | Abbrev tables used by various major modes. |
| • [Abbrev Properties](abbrev-properties) | | How to read and set abbrev properties. Which properties have which effect. |
| • [Abbrev Table Properties](abbrev-table-properties) | | How to read and set abbrev table properties. Which properties have which effect. |
elisp None ### Native-Compilation Variables
This section documents the variables that control native-compilation.
User Option: **native-comp-speed**
This variable specifies the optimization level for native compilation. Its value should be a number between -1 and 3. Values between 0 and 3 specify the optimization levels equivalent to the corresponding compiler `-O0`, `-O1`, etc. command-line options of the compiler. The value -1 means disable native-compilation: functions and files will be only byte-compiled; however, the `\*.eln` files will still be produced, they will just contain the compiled code in bytecode form. (This can be achieved at function granularity by using the `(declare (speed -1))` form, see [Declare Form](declare-form).) The default value is 2.
User Option: **native-comp-debug**
This variable specifies the level of debugging information produced by native-compilation. Its value should be a number between zero and 3, with the following meaning:
0 No debugging output. This is the default.
1 Emit debugging symbols with the native code. This allows easier debugging of the native code with debuggers such as `gdb`.
2 Like 1, and in addition dump pseudo-C code.
3 Like 2, and in addition dump the GCC intermediate passes and `libgccjit` log file.
User Option: **native-comp-verbose**
This variable controls the verbosity of native-compilation by suppressing some or all of the log messages emitted by it. If its value is zero, the default, all of the log messages are suppressed. Setting it to a value between 1 and 3 will allow logging of the messages whose level is above the value. The values have the following interpretations:
0 No logging. This is the default.
1 Log the final LIMPLE representation of the code.
2 Log the LAP, the final LIMPLE, and some additional pass info.
3 Maximum verbosity: log everything.
User Option: **native-comp-async-jobs-number**
This variable determines the maximum number of native-compilation subprocesses that will be started simultaneously. It should be a non-negative number. The default value is zero, which means use half the number of the CPU execution units, or 1 if the CPU has only one execution unit.
User Option: **native-comp-async-report-warnings-errors**
If this variable’s value is non-`nil`, warnings and errors from asynchronous native-compilation subprocesses are reported in the main Emacs session in a buffer named `\*Warnings\*`. The default value `t` means display the resulting buffer. To log warnings without popping up the `\*Warnings\*` buffer, set this variable to `silent`.
A common cause for asynchronous native-compilation to produce warnings is compiling a file that is missing some `require` of a necessary feature. The feature may be loaded into the main emacs, but because native compilation always starts from a subprocess with a pristine environment, that may not be true for the subprocess.
User Option: **native-comp-async-query-on-exit**
If this variable’s value is non-nil, Emacs will query upon exiting whether to exit and kill any asynchronous native-compilation subprocesses that are still running, thus preventing the corresponding `.eln` files from being written. If the value is `nil`, the default, Emacs will kill these subprocesses without querying.
| programming_docs |
elisp None #### Directory Names
A *directory name* is a string that must name a directory if it names any file at all. A directory is actually a kind of file, and it has a file name (called the *directory file name*), which is related to the directory name but is typically not identical. (This is not quite the same as the usual POSIX terminology.) These two names for the same entity are related by a syntactic transformation. On GNU and other POSIX-like systems, this is simple: to obtain a directory name, append a ‘`/`’ to a directory file name that does not already end in ‘`/`’. On MS-DOS the relationship is more complicated.
The difference between a directory name and a directory file name is subtle but crucial. When an Emacs variable or function argument is described as being a directory name, a directory file name is not acceptable. When `file-name-directory` returns a string, that is always a directory name.
The following two functions convert between directory names and directory file names. They do nothing special with environment variable substitutions such as ‘`$HOME`’, and the constructs ‘`~`’, ‘`.`’ and ‘`..`’.
Function: **file-name-as-directory** *filename*
This function returns a string representing filename in a form that the operating system will interpret as the name of a directory (a directory name). On most systems, this means appending a slash to the string (if it does not already end in one).
```
(file-name-as-directory "~rms/lewis")
⇒ "~rms/lewis/"
```
Function: **directory-name-p** *filename*
This function returns non-`nil` if filename ends with a directory separator character. This is the forward slash ‘`/`’ on GNU and other POSIX-like systems; MS-Windows and MS-DOS recognize both the forward slash and the backslash ‘`\`’ as directory separators.
Function: **directory-file-name** *dirname*
This function returns a string representing dirname in a form that the operating system will interpret as the name of a file (a directory file name). On most systems, this means removing the final directory separators from the string, unless the string consists entirely of directory separators.
```
(directory-file-name "~lewis/")
⇒ "~lewis"
```
Function: **file-name-concat** *directory &rest components*
Concatenate components to directory, inserting a slash before the components if directory or the preceding component didn’t end with a slash.
```
(file-name-concat "/tmp" "foo")
⇒ "/tmp/foo"
```
A directory or components that are `nil` or the empty string are ignored—they are filtered out first and do not affect the results in any way.
This is almost the same as using `concat`, but dirname (and the non-final components) may or may not end with slash characters, and this function will not double those characters.
To convert a directory name to its abbreviation, use this function:
Function: **abbreviate-file-name** *filename*
This function returns an abbreviated form of filename. It applies the abbreviations specified in `directory-abbrev-alist` (see [File Aliases](https://www.gnu.org/software/emacs/manual/html_node/emacs/File-Aliases.html#File-Aliases) in The GNU Emacs Manual), then substitutes ‘`~`’ for the user’s home directory if the argument names a file in the home directory or one of its subdirectories. If the home directory is a root directory, it is not replaced with ‘`~`’, because this does not make the result shorter on many systems.
You can use this function for directory names and for file names, because it recognizes abbreviations even as part of the name.
elisp None ### Search and Replace
If you want to find all matches for a regexp in part of the buffer and replace them, the most flexible way is to write an explicit loop using `re-search-forward` and `replace-match`, like this:
```
(while (re-search-forward "foo[ \t]+bar" nil t)
(replace-match "foobar"))
```
See [Replacing the Text that Matched](replacing-match), for a description of `replace-match`.
It may be more convenient to limit the replacements to a specific region. The function `replace-regexp-in-region` does that.
Function: **replace-regexp-in-region** *regexp replacement &optional start end*
This function replaces all the occurrences of regexp with replacement in the region of buffer text between start and end; start defaults to position of point, and end defaults to the last accessible position of the buffer. The search for regexp is case-sensitive, and replacement is inserted without changing its letter-case. The replacement string can use the same special elements starting with ‘`\`’ as `replace-match` does. The function returns the number of replaced occurrences, or `nil` if regexp is not found. The function preserves the position of point.
```
(replace-regexp-in-region "foo[ \t]+bar" "foobar")
```
Function: **replace-string-in-region** *string replacement &optional start end*
This function works similarly to `replace-regexp-in-region`, but searches for, and replaces, literal strings instead of regular expressions.
Emacs also has special functions for replacing matches in a string.
Function: **replace-regexp-in-string** *regexp rep string &optional fixedcase literal subexp start*
This function copies string and searches it for matches for regexp, and replaces them with rep. It returns the modified copy. If start is non-`nil`, the search for matches starts at that index in string, and the returned value does not include the first start characters of string. To get the whole transformed string, concatenate the first start characters of string with the return value.
This function uses `replace-match` to do the replacement, and it passes the optional arguments fixedcase, literal and subexp along to `replace-match`.
Instead of a string, rep can be a function. In that case, `replace-regexp-in-string` calls rep for each match, passing the text of the match as its sole argument. It collects the value rep returns and passes that to `replace-match` as the replacement string. The match data at this point are the result of matching regexp against a substring of string.
Function: **string-replace** *from-string to-string in-string*
This function replaces all occurrences of from-string with to-string in in-string and returns the result. It may return one of its arguments unchanged, a constant string or a new string. Case is significant, and text properties are ignored.
If you want to write a command along the lines of `query-replace`, you can use `perform-replace` to do the work.
Function: **perform-replace** *from-string replacements query-flag regexp-flag delimited-flag &optional repeat-count map start end backward region-noncontiguous-p*
This function is the guts of `query-replace` and related commands. It searches for occurrences of from-string in the text between positions start and end and replaces some or all of them. If start is `nil` (or omitted), point is used instead, and the end of the buffer’s accessible portion is used for end. (If the optional argument backward is non-`nil`, the search starts at end and goes backward.)
If query-flag is `nil`, it replaces all occurrences; otherwise, it asks the user what to do about each one.
If regexp-flag is non-`nil`, then from-string is considered a regular expression; otherwise, it must match literally. If delimited-flag is non-`nil`, then only replacements surrounded by word boundaries are considered.
The argument replacements specifies what to replace occurrences with. If it is a string, that string is used. It can also be a list of strings, to be used in cyclic order.
If replacements is a cons cell, `(function . data)`, this means to call function after each match to get the replacement text. This function is called with two arguments: data, and the number of replacements already made.
If repeat-count is non-`nil`, it should be an integer. Then it specifies how many times to use each of the strings in the replacements list before advancing cyclically to the next one.
If from-string contains upper-case letters, then `perform-replace` binds `case-fold-search` to `nil`, and it uses the replacements without altering their case.
Normally, the keymap `query-replace-map` defines the possible user responses for queries. The argument map, if non-`nil`, specifies a keymap to use instead of `query-replace-map`.
Non-`nil` region-noncontiguous-p means that the region between start and end is composed of noncontiguous pieces. The most common example of this is a rectangular region, where the pieces are separated by newline characters.
This function uses one of two functions to search for the next occurrence of from-string. These functions are specified by the values of two variables: `replace-re-search-function` and `replace-search-function`. The former is called when the argument regexp-flag is non-`nil`, the latter when it is `nil`.
Variable: **query-replace-map**
This variable holds a special keymap that defines the valid user responses for `perform-replace` and the commands that use it, as well as `y-or-n-p` and `map-y-or-n-p`. This map is unusual in two ways:
* The key bindings are not commands, just symbols that are meaningful to the functions that use this map.
* Prefix keys are not supported; each key binding must be for a single-event key sequence. This is because the functions don’t use `read-key-sequence` to get the input; instead, they read a single event and look it up “by hand”.
Here are the meaningful bindings for `query-replace-map`. Several of them are meaningful only for `query-replace` and friends.
`act`
Do take the action being considered—in other words, “yes”.
`skip`
Do not take action for this question—in other words, “no”.
`exit`
Answer this question “no”, and give up on the entire series of questions, assuming that the answers will be “no”.
`exit-prefix`
Like `exit`, but add the key that was pressed to `unread-command-events` (see [Event Input Misc](event-input-misc)).
`act-and-exit`
Answer this question “yes”, and give up on the entire series of questions, assuming that subsequent answers will be “no”.
`act-and-show`
Answer this question “yes”, but show the results—don’t advance yet to the next question.
`automatic`
Answer this question and all subsequent questions in the series with “yes”, without further user interaction.
`backup`
Move back to the previous place that a question was asked about.
`undo`
Undo last replacement and move back to the place where that replacement was performed.
`undo-all`
Undo all replacements and move back to the place where the first replacement was performed.
`edit`
Enter a recursive edit to deal with this question—instead of any other action that would normally be taken.
`edit-replacement`
Edit the replacement for this question in the minibuffer.
`delete-and-edit`
Delete the text being considered, then enter a recursive edit to replace it.
`recenter` `scroll-up` `scroll-down` `scroll-other-window` `scroll-other-window-down`
Perform the specified window scroll operation, then ask the same question again. Only `y-or-n-p` and related functions use this answer.
`quit`
Perform a quit right away. Only `y-or-n-p` and related functions use this answer.
`help` Display some help, then ask again.
Variable: **multi-query-replace-map**
This variable holds a keymap that extends `query-replace-map` by providing additional keybindings that are useful in multi-buffer replacements. The additional bindings are:
`automatic-all`
Answer this question and all subsequent questions in the series with “yes”, without further user interaction, for all remaining buffers.
`exit-current` Answer this question “no”, and give up on the entire series of questions for the current buffer. Continue to the next buffer in the sequence.
Variable: **replace-search-function**
This variable specifies a function that `perform-replace` calls to search for the next string to replace. Its default value is `search-forward`. Any other value should name a function of 3 arguments: the first 3 arguments of `search-forward` (see [String Search](string-search)).
Variable: **replace-re-search-function**
This variable specifies a function that `perform-replace` calls to search for the next regexp to replace. Its default value is `re-search-forward`. Any other value should name a function of 3 arguments: the first 3 arguments of `re-search-forward` (see [Regexp Search](regexp-search)).
elisp None ### Saving Buffers
When you edit a file in Emacs, you are actually working on a buffer that is visiting that file—that is, the contents of the file are copied into the buffer and the copy is what you edit. Changes to the buffer do not change the file until you *save* the buffer, which means copying the contents of the buffer into the file. Buffers which are not visiting a file can still be “saved”, in a sense, using functions in the buffer-local `write-contents-functions` hook.
Command: **save-buffer** *&optional backup-option*
This function saves the contents of the current buffer in its visited file if the buffer has been modified since it was last visited or saved. Otherwise it does nothing.
`save-buffer` is responsible for making backup files. Normally, backup-option is `nil`, and `save-buffer` makes a backup file only if this is the first save since visiting the file. Other values for backup-option request the making of backup files in other circumstances:
* With an argument of 4 or 64, reflecting 1 or 3 `C-u`’s, the `save-buffer` function marks this version of the file to be backed up when the buffer is next saved.
* With an argument of 16 or 64, reflecting 2 or 3 `C-u`’s, the `save-buffer` function unconditionally backs up the previous version of the file before saving it.
* With an argument of 0, unconditionally do *not* make any backup file.
Command: **save-some-buffers** *&optional save-silently-p pred*
This command saves some modified file-visiting buffers. Normally it asks the user about each buffer. But if save-silently-p is non-`nil`, it saves all the file-visiting buffers without querying the user.
The optional pred argument provides a predicate that controls which buffers to ask about (or to save silently if save-silently-p is non-`nil`). If pred is `nil`, that means to use the value of `save-some-buffers-default-predicate` instead of pred. If the result is `nil`, it means ask only about file-visiting buffers. If it is `t`, that means also offer to save certain other non-file buffers—those that have a non-`nil` buffer-local value of `buffer-offer-save` (see [Killing Buffers](killing-buffers)). A user who says ‘`yes`’ to saving a non-file buffer is asked to specify the file name to use. The `save-buffers-kill-emacs` function passes the value `t` for pred.
If the predicate is neither `t` nor `nil`, then it should be a function of no arguments. It will be called in each buffer to decide whether to offer to save that buffer. If it returns a non-`nil` value in a certain buffer, that means do offer to save that buffer.
Command: **write-file** *filename &optional confirm*
This function writes the current buffer into file filename, makes the buffer visit that file, and marks it not modified. Then it renames the buffer based on filename, appending a string like ‘`<2>`’ if necessary to make a unique buffer name. It does most of this work by calling `set-visited-file-name` (see [Buffer File Name](buffer-file-name)) and `save-buffer`.
If confirm is non-`nil`, that means to ask for confirmation before overwriting an existing file. Interactively, confirmation is required, unless the user supplies a prefix argument.
If filename is a directory name (see [Directory Names](directory-names)), `write-file` uses the name of the visited file, in directory filename. If the buffer is not visiting a file, it uses the buffer name instead.
Saving a buffer runs several hooks. It also performs format conversion (see [Format Conversion](format-conversion)). Note that these hooks, described below, are only run by `save-buffer`, they are not run by other primitives and functions that write buffer text to files, and in particular auto-saving (see [Auto-Saving](auto_002dsaving)) doesn’t run these hooks.
Variable: **write-file-functions**
The value of this variable is a list of functions to be called before writing out a buffer to its visited file. If one of them returns non-`nil`, the file is considered already written and the rest of the functions are not called, nor is the usual code for writing the file executed.
If a function in `write-file-functions` returns non-`nil`, it is responsible for making a backup file (if that is appropriate). To do so, execute the following code:
```
(or buffer-backed-up (backup-buffer))
```
You might wish to save the file modes value returned by `backup-buffer` and use that (if non-`nil`) to set the mode bits of the file that you write. This is what `save-buffer` normally does. See [Making Backup Files](making-backups).
The hook functions in `write-file-functions` are also responsible for encoding the data (if desired): they must choose a suitable coding system and end-of-line conversion (see [Lisp and Coding Systems](lisp-and-coding-systems)), perform the encoding (see [Explicit Encoding](explicit-encoding)), and set `last-coding-system-used` to the coding system that was used (see [Encoding and I/O](encoding-and-i_002fo)).
If you set this hook locally in a buffer, it is assumed to be associated with the file or the way the contents of the buffer were obtained. Thus the variable is marked as a permanent local, so that changing the major mode does not alter a buffer-local value. On the other hand, calling `set-visited-file-name` will reset it. If this is not what you want, you might like to use `write-contents-functions` instead.
Even though this is not a normal hook, you can use `add-hook` and `remove-hook` to manipulate the list. See [Hooks](hooks).
Variable: **write-contents-functions**
This works just like `write-file-functions`, but it is intended for hooks that pertain to the buffer’s contents, not to the particular visited file or its location, and can be used to create arbitrary save processes for buffers that aren’t visiting files at all. Such hooks are usually set up by major modes, as buffer-local bindings for this variable. This variable automatically becomes buffer-local whenever it is set; switching to a new major mode always resets this variable, but calling `set-visited-file-name` does not.
If any of the functions in this hook returns non-`nil`, the file is considered already written and the rest are not called and neither are the functions in `write-file-functions`.
When using this hook to save buffers that are not visiting files (for instance, special-mode buffers), keep in mind that, if the function fails to save correctly and returns a `nil` value, `save-buffer` will go on to prompt the user for a file to save the buffer in. If this is undesirable, consider having the function fail by raising an error.
User Option: **before-save-hook**
This normal hook runs before a buffer is saved in its visited file, regardless of whether that is done normally or by one of the hooks described above. For instance, the `copyright.el` program uses this hook to make sure the file you are saving has the current year in its copyright notice.
User Option: **after-save-hook**
This normal hook runs after a buffer has been saved in its visited file.
User Option: **file-precious-flag**
If this variable is non-`nil`, then `save-buffer` protects against I/O errors while saving by writing the new file to a temporary name instead of the name it is supposed to have, and then renaming it to the intended name after it is clear there are no errors. This procedure prevents problems such as a lack of disk space from resulting in an invalid file.
As a side effect, backups are necessarily made by copying. See [Rename or Copy](rename-or-copy). Yet, at the same time, saving a precious file always breaks all hard links between the file you save and other file names.
Some modes give this variable a non-`nil` buffer-local value in particular buffers.
User Option: **require-final-newline**
This variable determines whether files may be written out that do *not* end with a newline. If the value of the variable is `t`, then `save-buffer` silently adds a newline at the end of the buffer whenever it does not already end in one. If the value is `visit`, Emacs adds a missing newline just after it visits the file. If the value is `visit-save`, Emacs adds a missing newline both on visiting and on saving. For any other non-`nil` value, `save-buffer` asks the user whether to add a newline each time the case arises.
If the value of the variable is `nil`, then `save-buffer` doesn’t add newlines at all. `nil` is the default value, but a few major modes set it to `t` in particular buffers.
See also the function `set-visited-file-name` (see [Buffer File Name](buffer-file-name)).
| programming_docs |
elisp None #### Image Formats
Emacs can display a number of different image formats. Some of these image formats are supported only if particular support libraries are installed. On some platforms, Emacs can load support libraries on demand; if so, the variable `dynamic-library-alist` can be used to modify the set of known names for these dynamic libraries. See [Dynamic Libraries](dynamic-libraries).
Supported image formats (and the required support libraries) include PBM and XBM (which do not depend on support libraries and are always available), XPM (`libXpm`), GIF (`libgif` or `libungif`), JPEG (`libjpeg`), TIFF (`libtiff`), PNG (`libpng`), and SVG (`librsvg`).
Each of these image formats is associated with an *image type symbol*. The symbols for the above formats are, respectively, `pbm`, `xbm`, `xpm`, `gif`, `jpeg`, `tiff`, `png`, and `svg`.
Furthermore, if you build Emacs with ImageMagick (`libMagickWand`) support, Emacs can display any image format that ImageMagick can. See [ImageMagick Images](imagemagick-images). All images displayed via ImageMagick have type symbol `imagemagick`.
Variable: **image-types**
This variable contains a list of type symbols for image formats which are potentially supported in the current configuration.
“Potentially” means that Emacs knows about the image types, not necessarily that they can be used (for example, they could depend on unavailable dynamic libraries). To know which image types are really available, use `image-type-available-p`.
Function: **image-type-available-p** *type*
This function returns non-`nil` if images of type type can be loaded and displayed. type must be an image type symbol.
For image types whose support libraries are statically linked, this function always returns `t`. For image types whose support libraries are dynamically loaded, it returns `t` if the library could be loaded and `nil` otherwise.
elisp None ### Functions for Key Lookup
Here are the functions and variables pertaining to key lookup.
Function: **lookup-key** *keymap key &optional accept-defaults*
This function returns the definition of key in keymap. All the other functions described in this chapter that look up keys use `lookup-key`. Here are examples:
```
(lookup-key (current-global-map) "\C-x\C-f")
⇒ find-file
```
```
(lookup-key (current-global-map) (kbd "C-x C-f"))
⇒ find-file
```
```
(lookup-key (current-global-map) "\C-x\C-f12345")
⇒ 2
```
If the string or vector key is not a valid key sequence according to the prefix keys specified in keymap, it must be too long and have extra events at the end that do not fit into a single key sequence. Then the value is a number, the number of events at the front of key that compose a complete key.
If accept-defaults is non-`nil`, then `lookup-key` considers default bindings as well as bindings for the specific events in key. Otherwise, `lookup-key` reports only bindings for the specific sequence key, ignoring default bindings except when you explicitly ask about them. (To do this, supply `t` as an element of key; see [Format of Keymaps](format-of-keymaps).)
If key contains a meta character (not a function key), that character is implicitly replaced by a two-character sequence: the value of `meta-prefix-char`, followed by the corresponding non-meta character. Thus, the first example below is handled by conversion into the second example.
```
(lookup-key (current-global-map) "\M-f")
⇒ forward-word
```
```
(lookup-key (current-global-map) "\ef")
⇒ forward-word
```
The keymap argument can also be a list of keymaps.
Unlike `read-key-sequence`, this function does not modify the specified events in ways that discard information (see [Key Sequence Input](key-sequence-input)). In particular, it does not convert letters to lower case and it does not change drag events to clicks.
Command: **undefined**
Used in keymaps to undefine keys. It calls `ding`, but does not cause an error.
Function: **local-key-binding** *key &optional accept-defaults*
This function returns the binding for key in the current local keymap, or `nil` if it is undefined there.
The argument accept-defaults controls checking for default bindings, as in `lookup-key` (above).
Function: **global-key-binding** *key &optional accept-defaults*
This function returns the binding for command key in the current global keymap, or `nil` if it is undefined there.
The argument accept-defaults controls checking for default bindings, as in `lookup-key` (above).
Function: **minor-mode-key-binding** *key &optional accept-defaults*
This function returns a list of all the active minor mode bindings of key. More precisely, it returns an alist of pairs `(modename . binding)`, where modename is the variable that enables the minor mode, and binding is key’s binding in that mode. If key has no minor-mode bindings, the value is `nil`.
If the first binding found is not a prefix definition (a keymap or a symbol defined as a keymap), all subsequent bindings from other minor modes are omitted, since they would be completely shadowed. Similarly, the list omits non-prefix bindings that follow prefix bindings.
The argument accept-defaults controls checking for default bindings, as in `lookup-key` (above).
User Option: **meta-prefix-char**
This variable is the meta-prefix character code. It is used for translating a meta character to a two-character sequence so it can be looked up in a keymap. For useful results, the value should be a prefix event (see [Prefix Keys](prefix-keys)). The default value is 27, which is the ASCII code for ESC.
As long as the value of `meta-prefix-char` remains 27, key lookup translates `M-b` into `ESC b`, which is normally defined as the `backward-word` command. However, if you were to set `meta-prefix-char` to 24, the code for `C-x`, then Emacs will translate `M-b` into `C-x b`, whose standard binding is the `switch-to-buffer` command. (Don’t actually do this!) Here is an illustration of what would happen:
```
meta-prefix-char ; The default value.
⇒ 27
```
```
(key-binding "\M-b")
⇒ backward-word
```
```
?\C-x ; The print representation
⇒ 24 ; of a character.
```
```
(setq meta-prefix-char 24)
⇒ 24
```
```
(key-binding "\M-b")
⇒ switch-to-buffer ; Now, typing `M-b` is
; like typing `C-x b`.
(setq meta-prefix-char 27) ; Avoid confusion!
⇒ 27 ; Restore the default value!
```
This translation of one event into two happens only for characters, not for other kinds of input events. Thus, `M-F1`, a function key, is not converted into `ESC F1`.
elisp None ### Constructs for Combining Conditions
This section describes constructs that are often used together with `if` and `cond` to express complicated conditions. The constructs `and` and `or` can also be used individually as kinds of multiple conditional constructs.
Function: **not** *condition*
This function tests for the falsehood of condition. It returns `t` if condition is `nil`, and `nil` otherwise. The function `not` is identical to `null`, and we recommend using the name `null` if you are testing for an empty list.
Special Form: **and** *conditions…*
The `and` special form tests whether all the conditions are true. It works by evaluating the conditions one by one in the order written.
If any of the conditions evaluates to `nil`, then the result of the `and` must be `nil` regardless of the remaining conditions; so `and` returns `nil` right away, ignoring the remaining conditions.
If all the conditions turn out non-`nil`, then the value of the last of them becomes the value of the `and` form. Just `(and)`, with no conditions, returns `t`, appropriate because all the conditions turned out non-`nil`. (Think about it; which one did not?)
Here is an example. The first condition returns the integer 1, which is not `nil`. Similarly, the second condition returns the integer 2, which is not `nil`. The third condition is `nil`, so the remaining condition is never evaluated.
```
(and (print 1) (print 2) nil (print 3))
-| 1
-| 2
⇒ nil
```
Here is a more realistic example of using `and`:
```
(if (and (consp foo) (eq (car foo) 'x))
(message "foo is a list starting with x"))
```
Note that `(car foo)` is not executed if `(consp foo)` returns `nil`, thus avoiding an error.
`and` expressions can also be written using either `if` or `cond`. Here’s how:
```
(and arg1 arg2 arg3)
≡
(if arg1 (if arg2 arg3))
≡
(cond (arg1 (cond (arg2 arg3))))
```
Special Form: **or** *conditions…*
The `or` special form tests whether at least one of the conditions is true. It works by evaluating all the conditions one by one in the order written.
If any of the conditions evaluates to a non-`nil` value, then the result of the `or` must be non-`nil`; so `or` returns right away, ignoring the remaining conditions. The value it returns is the non-`nil` value of the condition just evaluated.
If all the conditions turn out `nil`, then the `or` expression returns `nil`. Just `(or)`, with no conditions, returns `nil`, appropriate because all the conditions turned out `nil`. (Think about it; which one did not?)
For example, this expression tests whether `x` is either `nil` or the integer zero:
```
(or (eq x nil) (eq x 0))
```
Like the `and` construct, `or` can be written in terms of `cond`. For example:
```
(or arg1 arg2 arg3)
≡
(cond (arg1)
(arg2)
(arg3))
```
You could almost write `or` in terms of `if`, but not quite:
```
(if arg1 arg1
(if arg2 arg2
arg3))
```
This is not completely equivalent because it can evaluate arg1 or arg2 twice. By contrast, `(or arg1 arg2
arg3)` never evaluates any argument more than once.
Function: **xor** *condition1 condition2*
This function returns the boolean exclusive-or of condition1 and condition2. That is, `xor` returns `nil` if either both arguments are `nil`, or both are non-`nil`. Otherwise, it returns the value of that argument which is non-`nil`.
Note that in contrast to `or`, both arguments are always evaluated.
elisp None #### Converting to Lexical Binding
Converting an Emacs Lisp program to lexical binding is easy. First, add a file-local variable setting of `lexical-binding` to `t` in the header line of the Emacs Lisp source file (see [File Local Variables](file-local-variables)). Second, check that every variable in the program which needs to be dynamically bound has a variable definition, so that it is not inadvertently bound lexically.
A simple way to find out which variables need a variable definition is to byte-compile the source file. See [Byte Compilation](byte-compilation). If a non-special variable is used outside of a `let` form, the byte-compiler will warn about reference or assignment to a free variable. If a non-special variable is bound but not used within a `let` form, the byte-compiler will warn about an unused lexical variable. The byte-compiler will also issue a warning if you use a special variable as a function argument.
A warning about a reference or an assignment to a free variable is usually a clear sign that that variable should be marked as dynamically scoped, so you need to add an appropriate `defvar` before the first use of that variable.
A warning about an unused variable may be a good hint that the variable was intended to be dynamically scoped (because it is actually used, but in another function), but it may also be an indication that the variable is simply really not used and could simply be removed. So you need to find out which case it is, and based on that, either add a `defvar` or remove the variable altogether. If removal is not possible or not desirable (typically because it is a formal argument and that we cannot or don’t want to change all the callers), you can also add a leading underscore to the variable’s name to indicate to the compiler that this is a variable known not to be used.)
#### Cross-file variable checking
**Caution:** This is an experimental feature that may change or disappear without prior notice.
The byte-compiler can also warn about lexical variables that are special in other Emacs Lisp files, often indicating a missing `defvar` declaration. This useful but somewhat specialized check requires three steps:
1. Byte-compile all files whose special variable declarations may be of interest, with the environment variable `EMACS_GENERATE_DYNVARS` set to a nonempty string. These are typically all the files in the same package or related packages or Emacs subsystems. The process will generate a file whose name ends in `.dynvars` for each compiled Emacs Lisp file.
2. Concatenate the `.dynvars` files into a single file.
3. Byte-compile the files that need to be checked, this time with the environment variable `EMACS_DYNVARS_FILE` set to the name of the aggregated file created in step 2.
Here is an example illustrating how this could be done, assuming that a Unix shell and `make` are used for byte-compilation:
```
$ rm *.elc # force recompilation
$ EMACS_GENERATE_DYNVARS=1 make # generate .dynvars
$ cat *.dynvars > ~/my-dynvars # combine .dynvars
$ rm *.elc # force recompilation
$ EMACS_DYNVARS_FILE=~/my-dynvars make # perform checks
```
elisp None ### Condition Variables
A *condition variable* is a way for a thread to block until some event occurs. A thread can wait on a condition variable, to be woken up when some other thread notifies the condition.
A condition variable is associated with a mutex and, conceptually, with some condition. For proper operation, the mutex must be acquired, and then a waiting thread must loop, testing the condition and waiting on the condition variable. For example:
```
(with-mutex mutex
(while (not global-variable)
(condition-wait cond-var)))
```
The mutex ensures atomicity, and the loop is for robustness—there may be spurious notifications.
Similarly, the mutex must be held before notifying the condition. The typical, and best, approach is to acquire the mutex, make the changes associated with this condition, and then notify it:
```
(with-mutex mutex
(setq global-variable (some-computation))
(condition-notify cond-var))
```
Function: **make-condition-variable** *mutex &optional name*
Make a new condition variable associated with mutex. If name is specified, it is a name given to the condition variable. It must be a string. The name is for debugging purposes only; it has no meaning to Emacs.
Function: **condition-variable-p** *object*
This function returns `t` if object represents a condition variable, `nil` otherwise.
Function: **condition-wait** *cond*
Wait for another thread to notify cond, a condition variable. This function will block until the condition is notified, or until a signal is delivered to this thread using `thread-signal`.
It is an error to call `condition-wait` without holding the condition’s associated mutex.
`condition-wait` releases the associated mutex while waiting. This allows other threads to acquire the mutex in order to notify the condition.
Function: **condition-notify** *cond &optional all*
Notify cond. The mutex with cond must be held before calling this. Ordinarily a single waiting thread is woken by `condition-notify`; but if all is not `nil`, then all threads waiting on cond are notified.
`condition-notify` releases the associated mutex while waiting. This allows other threads to acquire the mutex in order to wait on the condition.
Function: **condition-name** *cond*
Return the name of cond, as passed to `make-condition-variable`.
Function: **condition-mutex** *cond*
Return the mutex associated with cond. Note that the associated mutex cannot be changed.
elisp None #### Mode Hooks
Every major mode command should finish by running the mode-independent normal hook `change-major-mode-after-body-hook`, its mode hook, and the normal hook `after-change-major-mode-hook`. It does this by calling `run-mode-hooks`. If the major mode is a derived mode, that is if it calls another major mode (the parent mode) in its body, it should do this inside `delay-mode-hooks` so that the parent won’t run these hooks itself. Instead, the derived mode’s call to `run-mode-hooks` runs the parent’s mode hook too. See [Major Mode Conventions](major-mode-conventions).
Emacs versions before Emacs 22 did not have `delay-mode-hooks`. Versions before 24 did not have `change-major-mode-after-body-hook`. When user-implemented major modes do not use `run-mode-hooks` and have not been updated to use these newer features, they won’t entirely follow these conventions: they may run the parent’s mode hook too early, or fail to run `after-change-major-mode-hook`. If you encounter such a major mode, please correct it to follow these conventions.
When you define a major mode using `define-derived-mode`, it automatically makes sure these conventions are followed. If you define a major mode “by hand”, not using `define-derived-mode`, use the following functions to handle these conventions automatically.
Function: **run-mode-hooks** *&rest hookvars*
Major modes should run their mode hook using this function. It is similar to `run-hooks` (see [Hooks](hooks)), but it also runs `change-major-mode-after-body-hook`, `hack-local-variables` (when the buffer is visiting a file) (see [File Local Variables](file-local-variables)), and `after-change-major-mode-hook`. The last thing it does is to evaluate any `:after-hook` forms declared by parent modes (see [Derived Modes](derived-modes)).
When this function is called during the execution of a `delay-mode-hooks` form, it does not run the hooks or `hack-local-variables` or evaluate the forms immediately. Instead, it arranges for the next call to `run-mode-hooks` to run them.
Macro: **delay-mode-hooks** *body…*
When one major mode command calls another, it should do so inside of `delay-mode-hooks`.
This macro executes body, but tells all `run-mode-hooks` calls during the execution of body to delay running their hooks. The hooks will actually run during the next call to `run-mode-hooks` after the end of the `delay-mode-hooks` construct.
Variable: **change-major-mode-after-body-hook**
This is a normal hook run by `run-mode-hooks`. It is run before the mode hooks.
Variable: **after-change-major-mode-hook**
This is a normal hook run by `run-mode-hooks`. It is run at the very end of every properly-written major mode command.
elisp None Customization Settings
----------------------
Users of Emacs can customize variables and faces without writing Lisp code, by using the Customize interface. See [Easy Customization](https://www.gnu.org/software/emacs/manual/html_node/emacs/Easy-Customization.html#Easy-Customization) in The GNU Emacs Manual. This chapter describes how to define *customization items* that users can interact with through the Customize interface.
Customization items include customizable variables, which are defined with the `defcustom` macro; customizable faces, which are defined with `defface` (described separately in [Defining Faces](defining-faces)); and *customization groups*, defined with `defgroup`, which act as containers for groups of related customization items.
| | | |
| --- | --- | --- |
| • [Common Keywords](common-keywords) | | Common keyword arguments for all kinds of customization declarations. |
| • [Group Definitions](group-definitions) | | Writing customization group definitions. |
| • [Variable Definitions](variable-definitions) | | Declaring user options. |
| • [Customization Types](customization-types) | | Specifying the type of a user option. |
| • [Applying Customizations](applying-customizations) | | Functions to apply customization settings. |
| • [Custom Themes](custom-themes) | | Writing Custom themes. |
| programming_docs |
elisp None ### Reading Input
The editor command loop reads key sequences using the function `read-key-sequence`, which uses `read-event`. These and other functions for event input are also available for use in Lisp programs. See also `momentary-string-display` in [Temporary Displays](temporary-displays), and `sit-for` in [Waiting](waiting). See [Terminal Input](terminal-input), for functions and variables for controlling terminal input modes and debugging terminal input.
For higher-level input facilities, see [Minibuffers](minibuffers).
| | | |
| --- | --- | --- |
| • [Key Sequence Input](key-sequence-input) | | How to read one key sequence. |
| • [Reading One Event](reading-one-event) | | How to read just one event. |
| • [Event Mod](event-mod) | | How Emacs modifies events as they are read. |
| • [Invoking the Input Method](invoking-the-input-method) | | How reading an event uses the input method. |
| • [Quoted Character Input](quoted-character-input) | | Asking the user to specify a character. |
| • [Event Input Misc](event-input-misc) | | How to reread or throw away input events. |
elisp None #### Testing Accessibility
These functions test for permission to access a file for reading, writing, or execution. Unless explicitly stated otherwise, they follow symbolic links. See [Kinds of Files](kinds-of-files).
On some operating systems, more complex sets of access permissions can be specified, via mechanisms such as Access Control Lists (ACLs). See [Extended Attributes](extended-attributes), for how to query and set those permissions.
Function: **file-exists-p** *filename*
This function returns `t` if a file named filename appears to exist. This does not mean you can necessarily read the file, only that you can probably find out its attributes. (On GNU and other POSIX-like systems, this is true if the file exists and you have execute permission on the containing directories, regardless of the permissions of the file itself.)
If the file does not exist, or if there was trouble determining whether the file exists, this function returns `nil`.
Directories are files, so `file-exists-p` can return `t` when given a directory. However, because `file-exists-p` follows symbolic links, it returns `t` for a symbolic link name only if the target file exists.
Function: **file-readable-p** *filename*
This function returns `t` if a file named filename exists and you can read it. It returns `nil` otherwise.
Function: **file-executable-p** *filename*
This function returns `t` if a file named filename exists and you can execute it. It returns `nil` otherwise. On GNU and other POSIX-like systems, if the file is a directory, execute permission means you can check the existence and attributes of files inside the directory, and open those files if their modes permit.
Function: **file-writable-p** *filename*
This function returns `t` if the file filename can be written or created by you, and `nil` otherwise. A file is writable if the file exists and you can write it. It is creatable if it does not exist, but its parent directory does exist and you can write in that directory.
In the example below, `foo` is not writable because the parent directory does not exist, even though the user could create such a directory.
```
(file-writable-p "~/no-such-dir/foo")
⇒ nil
```
Function: **file-accessible-directory-p** *dirname*
This function returns `t` if you have permission to open existing files in the directory whose name as a file is dirname; otherwise (e.g., if there is no such directory), it returns `nil`. The value of dirname may be either a directory name (such as `/foo/`) or the file name of a file which is a directory (such as `/foo`, without the final slash).
For example, from the following we deduce that any attempt to read a file in `/foo/` will give an error:
```
(file-accessible-directory-p "/foo")
⇒ nil
```
Macro: **with-existing-directory** *body…*
This macro ensures that `default-directory` is bound to an existing directory before executing body. If `default-directory` already exists, that’s preferred, and otherwise some other directory is used. This macro can be useful, for instance, when calling an external command that requires that it’s running in a directory that exists. The chosen directory is not guaranteed to be writable.
Function: **access-file** *filename string*
If you can read filename this function returns `nil`; otherwise it signals an error using string as the error message text.
Function: **file-ownership-preserved-p** *filename &optional group*
This function returns `t` if deleting the file filename and then creating it anew would keep the file’s owner unchanged. It also returns `t` for nonexistent files.
If the optional argument group is non-`nil`, this function also checks that the file’s group would be unchanged.
This function does not follow symbolic links.
Function: **file-modes** *filename &optional flag*
This function returns the *mode bits* of filename—an integer summarizing its read, write, and execution permissions. This function follows symbolic links. If the file does not exist, the return value is `nil`.
See [File permissions](https://www.gnu.org/software/coreutils/manual/html_node/File-permissions.html#File-permissions) in The GNU `Coreutils` Manual, for a description of mode bits. For example, if the low-order bit is 1, the file is executable by all users; if the second-lowest-order bit is 1, the file is writable by all users; etc. The highest possible value is 4095 (7777 octal), meaning that everyone has read, write, and execute permission, the SUID bit is set for both others and group, and the sticky bit is set.
By default this function follows symbolic links. However, if the optional argument flag is the symbol `nofollow`, this function does not follow filename if it is a symbolic link; this can help prevent inadvertently obtaining the mode bits of a file somewhere else, and is more consistent with `file-attributes` (see [File Attributes](file-attributes)).
See [Changing Files](changing-files), for the `set-file-modes` function, which can be used to set these permissions.
```
(file-modes "~/junk/diffs" 'nofollow)
⇒ 492 ; Decimal integer.
```
```
(format "%o" 492)
⇒ "754" ; Convert to octal.
```
```
(set-file-modes "~/junk/diffs" #o666 'nofollow)
⇒ nil
```
```
$ ls -l diffs
-rw-rw-rw- 1 lewis lewis 3063 Oct 30 16:00 diffs
```
**MS-DOS note:** On MS-DOS, there is no such thing as an executable file mode bit. So `file-modes` considers a file executable if its name ends in one of the standard executable extensions, such as `.com`, `.bat`, `.exe`, and some others. Files that begin with the POSIX-standard ‘`#!`’ signature, such as shell and Perl scripts, are also considered executable. Directories are also reported as executable, for compatibility with POSIX. These conventions are also followed by `file-attributes` (see [File Attributes](file-attributes)).
elisp None ### Standard Mathematical Functions
These mathematical functions allow integers as well as floating-point numbers as arguments.
Function: **sin** *arg*
Function: **cos** *arg*
Function: **tan** *arg*
These are the basic trigonometric functions, with argument arg measured in radians.
Function: **asin** *arg*
The value of `(asin arg)` is a number between -pi/2 and pi/2 (inclusive) whose sine is arg. If arg is out of range (outside [-1, 1]), `asin` returns a NaN.
Function: **acos** *arg*
The value of `(acos arg)` is a number between 0 and pi (inclusive) whose cosine is arg. If arg is out of range (outside [-1, 1]), `acos` returns a NaN.
Function: **atan** *y &optional x*
The value of `(atan y)` is a number between -pi/2 and pi/2 (exclusive) whose tangent is y. If the optional second argument x is given, the value of `(atan y x)` is the angle in radians between the vector `[x, y]` and the `X` axis.
Function: **exp** *arg*
This is the exponential function; it returns *e* to the power arg.
Function: **log** *arg &optional base*
This function returns the logarithm of arg, with base base. If you don’t specify base, the natural base *e* is used. If arg or base is negative, `log` returns a NaN.
Function: **expt** *x y*
This function returns x raised to power y. If both arguments are integers and y is nonnegative, the result is an integer; in this case, overflow signals an error, so watch out. If x is a finite negative number and y is a finite non-integer, `expt` returns a NaN.
Function: **sqrt** *arg*
This returns the square root of arg. If arg is finite and less than zero, `sqrt` returns a NaN.
In addition, Emacs defines the following common mathematical constants:
Variable: **float-e**
The mathematical constant *e* (2.71828…).
Variable: **float-pi**
The mathematical constant *pi* (3.14159…).
elisp None ### Predicates on Lists
The following predicates test whether a Lisp object is an atom, whether it is a cons cell or is a list, or whether it is the distinguished object `nil`. (Many of these predicates can be defined in terms of the others, but they are used so often that it is worth having them.)
Function: **consp** *object*
This function returns `t` if object is a cons cell, `nil` otherwise. `nil` is not a cons cell, although it *is* a list.
Function: **atom** *object*
This function returns `t` if object is an atom, `nil` otherwise. All objects except cons cells are atoms. The symbol `nil` is an atom and is also a list; it is the only Lisp object that is both.
```
(atom object) ≡ (not (consp object))
```
Function: **listp** *object*
This function returns `t` if object is a cons cell or `nil`. Otherwise, it returns `nil`.
```
(listp '(1))
⇒ t
```
```
(listp '())
⇒ t
```
Function: **nlistp** *object*
This function is the opposite of `listp`: it returns `t` if object is not a list. Otherwise, it returns `nil`.
```
(listp object) ≡ (not (nlistp object))
```
Function: **null** *object*
This function returns `t` if object is `nil`, and returns `nil` otherwise. This function is identical to `not`, but as a matter of clarity we use `null` when object is considered a list and `not` when it is considered a truth value (see `not` in [Combining Conditions](combining-conditions)).
```
(null '(1))
⇒ nil
```
```
(null '())
⇒ t
```
Function: **proper-list-p** *object*
This function returns the length of object if it is a proper list, `nil` otherwise (see [Cons Cells](cons-cells)). In addition to satisfying `listp`, a proper list is neither circular nor dotted.
```
(proper-list-p '(a b c))
⇒ 3
```
```
(proper-list-p '(a b . c))
⇒ nil
```
elisp None ### Selective Display
*Selective display* refers to a pair of related features for hiding certain lines on the screen.
The first variant, explicit selective display, was designed for use in a Lisp program: it controls which lines are hidden by altering the text. This kind of hiding is now obsolete and deprecated; instead you should use the `invisible` property (see [Invisible Text](invisible-text)) to get the same effect.
In the second variant, the choice of lines to hide is made automatically based on indentation. This variant is designed to be a user-level feature.
The way you control explicit selective display is by replacing a newline (control-j) with a carriage return (control-m). The text that was formerly a line following that newline is now hidden. Strictly speaking, it is temporarily no longer a line at all, since only newlines can separate lines; it is now part of the previous line.
Selective display does not directly affect editing commands. For example, `C-f` (`forward-char`) moves point unhesitatingly into hidden text. However, the replacement of newline characters with carriage return characters affects some editing commands. For example, `next-line` skips hidden lines, since it searches only for newlines. Modes that use selective display can also define commands that take account of the newlines, or that control which parts of the text are hidden.
When you write a selectively displayed buffer into a file, all the control-m’s are output as newlines. This means that when you next read in the file, it looks OK, with nothing hidden. The selective display effect is seen only within Emacs.
Variable: **selective-display**
This buffer-local variable enables selective display. This means that lines, or portions of lines, may be made hidden.
* If the value of `selective-display` is `t`, then the character control-m marks the start of hidden text; the control-m, and the rest of the line following it, are not displayed. This is explicit selective display.
* If the value of `selective-display` is a positive integer, then lines that start with more than that many columns of indentation are not displayed.
When some portion of a buffer is hidden, the vertical movement commands operate as if that portion did not exist, allowing a single `next-line` command to skip any number of hidden lines. However, character movement commands (such as `forward-char`) do not skip the hidden portion, and it is possible (if tricky) to insert or delete text in a hidden portion.
In the examples below, we show the *display appearance* of the buffer `foo`, which changes with the value of `selective-display`. The *contents* of the buffer do not change.
```
(setq selective-display nil)
⇒ nil
---------- Buffer: foo ----------
1 on this column
2on this column
3n this column
3n this column
2on this column
1 on this column
---------- Buffer: foo ----------
```
```
(setq selective-display 2)
⇒ 2
---------- Buffer: foo ----------
1 on this column
2on this column
2on this column
1 on this column
---------- Buffer: foo ----------
```
User Option: **selective-display-ellipses**
If this buffer-local variable is non-`nil`, then Emacs displays ‘`…`’ at the end of a line that is followed by hidden text. This example is a continuation of the previous one.
```
(setq selective-display-ellipses t)
⇒ t
---------- Buffer: foo ----------
1 on this column
2on this column ...
2on this column
1 on this column
---------- Buffer: foo ----------
```
You can use a display table to substitute other text for the ellipsis (‘`…`’). See [Display Tables](display-tables).
elisp None ### Terminal Output
The terminal output functions send output to a text terminal, or keep track of output sent to the terminal. The variable `baud-rate` tells you what Emacs thinks is the output speed of the terminal.
User Option: **baud-rate**
This variable’s value is the output speed of the terminal, as far as Emacs knows. Setting this variable does not change the speed of actual data transmission, but the value is used for calculations such as padding.
It also affects decisions about whether to scroll part of the screen or repaint on text terminals. See [Forcing Redisplay](forcing-redisplay), for the corresponding functionality on graphical terminals.
The value is measured in baud.
If you are running across a network, and different parts of the network work at different baud rates, the value returned by Emacs may be different from the value used by your local terminal. Some network protocols communicate the local terminal speed to the remote machine, so that Emacs and other programs can get the proper value, but others do not. If Emacs has the wrong value, it makes decisions that are less than optimal. To fix the problem, set `baud-rate`.
Function: **send-string-to-terminal** *string &optional terminal*
This function sends string to terminal without alteration. Control characters in string have terminal-dependent effects. (If you need to display non-ASCII text on the terminal, encode it using one of the functions described in [Explicit Encoding](explicit-encoding).) This function operates only on text terminals. terminal may be a terminal object, a frame, or `nil` for the selected frame’s terminal. In batch mode, string is sent to `stdout` when terminal is `nil`.
One use of this function is to define function keys on terminals that have downloadable function key definitions. For example, this is how (on certain terminals) to define function key 4 to move forward four characters (by transmitting the characters `C-u C-f` to the computer):
```
(send-string-to-terminal "\eF4\^U\^F")
⇒ nil
```
Command: **open-termscript** *filename*
This function is used to open a *termscript file* that will record all the characters sent by Emacs to the terminal. It returns `nil`. Termscript files are useful for investigating problems where Emacs garbles the screen, problems that are due to incorrect Termcap entries or to undesirable settings of terminal options more often than to actual Emacs bugs. Once you are certain which characters were actually output, you can determine reliably whether they correspond to the Termcap specifications in use.
```
(open-termscript "../junk/termscript")
⇒ nil
```
You close the termscript file by calling this function with an argument of `nil`.
See also `open-dribble-file` in [Recording Input](recording-input).
elisp None #### Format of Descriptions
Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category—function, variable, or whatever—appears at the beginning of the line. The description follows on succeeding lines, sometimes with examples.
| | | |
| --- | --- | --- |
| • [A Sample Function Description](a-sample-function-description) | | A description of an imaginary function, `foo`. |
| • [A Sample Variable Description](a-sample-variable-description) | | A description of an imaginary variable, `electric-future-map`. |
elisp None ### Bidirectional Display
Emacs can display text written in scripts, such as Arabic, Farsi, and Hebrew, whose natural ordering for horizontal text display runs from right to left. Furthermore, segments of Latin script and digits embedded in right-to-left text are displayed left-to-right, while segments of right-to-left script embedded in left-to-right text (e.g., Arabic or Hebrew text in comments or strings in a program source file) are appropriately displayed right-to-left. We call such mixtures of left-to-right and right-to-left text *bidirectional text*. This section describes the facilities and options for editing and displaying bidirectional text.
Text is stored in Emacs buffers and strings in *logical* (or *reading*) order, i.e., the order in which a human would read each character. In right-to-left and bidirectional text, the order in which characters are displayed on the screen (called *visual order*) is not the same as logical order; the characters’ screen positions do not increase monotonically with string or buffer position. In performing this *bidirectional reordering*, Emacs follows the Unicode Bidirectional Algorithm (a.k.a. UBA), which is described in Annex #9 of the Unicode standard (<https://www.unicode.org/reports/tr9/>). Emacs provides a “Full Bidirectionality” class implementation of the UBA, consistent with the requirements of the Unicode Standard v9.0. Note, however, that the way Emacs displays continuation lines when text direction is opposite to the base paragraph direction deviates from the UBA, which requires to perform line wrapping before reordering text for display.
Variable: **bidi-display-reordering**
If the value of this buffer-local variable is non-`nil` (the default), Emacs performs bidirectional reordering for display. The reordering affects buffer text, as well as display strings and overlay strings from text and overlay properties in the buffer (see [Overlay Properties](overlay-properties), and see [Display Property](display-property)). If the value is `nil`, Emacs does not perform bidirectional reordering in the buffer.
The default value of `bidi-display-reordering` controls the reordering of strings which are not directly supplied by a buffer, including the text displayed in mode lines (see [Mode Line Format](mode-line-format)) and header lines (see [Header Lines](header-lines)).
Emacs never reorders the text of a unibyte buffer, even if `bidi-display-reordering` is non-`nil` in the buffer. This is because unibyte buffers contain raw bytes, not characters, and thus lack the directionality properties required for reordering. Therefore, to test whether text in a buffer will be reordered for display, it is not enough to test the value of `bidi-display-reordering` alone. The correct test is this:
```
(if (and enable-multibyte-characters
bidi-display-reordering)
;; Buffer is being reordered for display
)
```
However, unibyte display and overlay strings *are* reordered if their parent buffer is reordered. This is because plain-ASCII strings are stored by Emacs as unibyte strings. If a unibyte display or overlay string includes non-ASCII characters, these characters are assumed to have left-to-right direction.
Text covered by `display` text properties, by overlays with `display` properties whose value is a string, and by any other properties that replace buffer text, is treated as a single unit when it is reordered for display. That is, the entire chunk of text covered by these properties is reordered together. Moreover, the bidirectional properties of the characters in such a chunk of text are ignored, and Emacs reorders them as if they were replaced with a single character `U+FFFC`, known as the *Object Replacement Character*. This means that placing a display property over a portion of text may change the way that the surrounding text is reordered for display. To prevent this unexpected effect, always place such properties on text whose directionality is identical with text that surrounds it.
Each paragraph of bidirectional text has a *base direction*, either right-to-left or left-to-right. Left-to-right paragraphs are displayed beginning at the left margin of the window, and are truncated or continued when the text reaches the right margin. Right-to-left paragraphs are displayed beginning at the right margin, and are continued or truncated at the left margin.
Where exactly paragraphs start and end, for the purpose of the Emacs UBA implementation, is determined by the following two buffer-local variables (note that `paragraph-start` and `paragraph-separate` have no influence on this). By default both of these variables are `nil`, and paragraphs are bounded by empty lines, i.e., lines that consist entirely of zero or more whitespace characters followed by a newline.
Variable: **bidi-paragraph-start-re**
If non-`nil`, this variable’s value should be a regular expression matching a line that starts or separates two paragraphs. The regular expression is always matched after a newline, so it is best to anchor it, i.e., begin it with a `"^"`.
Variable: **bidi-paragraph-separate-re**
If non-`nil`, this variable’s value should be a regular expression matching a line separates two paragraphs. The regular expression is always matched after a newline, so it is best to anchor it, i.e., begin it with a `"^"`.
If you modify any of these two variables, you should normally modify both, to make sure they describe paragraphs consistently. For example, to have each new line start a new paragraph for bidi-reordering purposes, set both variables to `"^"`.
By default, Emacs determines the base direction of each paragraph by looking at the text at its beginning. The precise method of determining the base direction is specified by the UBA; in a nutshell, the first character in a paragraph that has an explicit directionality determines the base direction of the paragraph. However, sometimes a buffer may need to force a certain base direction for its paragraphs. For example, buffers containing program source code should force all paragraphs to be displayed left-to-right. You can use following variable to do this:
User Option: **bidi-paragraph-direction**
If the value of this buffer-local variable is the symbol `right-to-left` or `left-to-right`, all paragraphs in the buffer are assumed to have that specified direction. Any other value is equivalent to `nil` (the default), which means to determine the base direction of each paragraph from its contents.
Modes for program source code should set this to `left-to-right`. Prog mode does this by default, so modes derived from Prog mode do not need to set this explicitly (see [Basic Major Modes](basic-major-modes)).
Function: **current-bidi-paragraph-direction** *&optional buffer*
This function returns the paragraph direction at point in the named buffer. The returned value is a symbol, either `left-to-right` or `right-to-left`. If buffer is omitted or `nil`, it defaults to the current buffer. If the buffer-local value of the variable `bidi-paragraph-direction` is non-`nil`, the returned value will be identical to that value; otherwise, the returned value reflects the paragraph direction determined dynamically by Emacs. For buffers whose value of `bidi-display-reordering` is `nil` as well as unibyte buffers, this function always returns `left-to-right`.
Sometimes there’s a need to move point in strict visual order, either to the left or to the right of its current screen position. Emacs provides a primitive to do that.
Function: **move-point-visually** *direction*
This function moves point of the currently selected window to the buffer position that appears immediately to the right or to the left of point on the screen. If direction is positive, point will move one screen position to the right, otherwise it will move one screen position to the left. Note that, depending on the surrounding bidirectional context, this could potentially move point many buffer positions away. If invoked at the end of a screen line, the function moves point to the rightmost or leftmost screen position of the next or previous screen line, as appropriate for the value of direction.
The function returns the new buffer position as its value.
Bidirectional reordering can have surprising and unpleasant effects when two strings with bidirectional content are juxtaposed in a buffer, or otherwise programmatically concatenated into a string of text. A typical problematic case is when a buffer consists of sequences of text fields separated by whitespace or punctuation characters, like Buffer Menu mode or Rmail Summary Mode. Because the punctuation characters used as separators have *weak directionality*, they take on the directionality of surrounding text. As result, a numeric field that follows a field with bidirectional content can be displayed *to the left* of the preceding field, messing up the expected layout. There are several ways to avoid this problem:
* - Append the special character U+200E LEFT-TO-RIGHT MARK, or LRM, to the end of each field that may have bidirectional content, or prepend it to the beginning of the following field. The function `bidi-string-mark-left-to-right`, described below, comes in handy for this purpose. (In a right-to-left paragraph, use U+200F RIGHT-TO-LEFT MARK, or RLM, instead.) This is one of the solutions recommended by the UBA.
* - Include the tab character in the field separator. The tab character plays the role of *segment separator* in bidirectional reordering, causing the text on either side to be reordered separately.
* - Separate fields with a `display` property or overlay with a property value of the form `(space . PROPS)` (see [Specified Space](specified-space)). Emacs treats this display specification as a *paragraph separator*, and reorders the text on either side separately.
Function: **bidi-string-mark-left-to-right** *string*
This function returns its argument string, possibly modified, such that the result can be safely concatenated with another string, or juxtaposed with another string in a buffer, without disrupting the relative layout of this string and the next one on display. If the string returned by this function is displayed as part of a left-to-right paragraph, it will always appear on display to the left of the text that follows it. The function works by examining the characters of its argument, and if any of those characters could cause reordering on display, the function appends the LRM character to the string. The appended LRM character is made invisible by giving it an `invisible` text property of `t` (see [Invisible Text](invisible-text)).
The reordering algorithm uses the bidirectional properties of the characters stored as their `bidi-class` property (see [Character Properties](character-properties)). Lisp programs can change these properties by calling the `put-char-code-property` function. However, doing this requires a thorough understanding of the UBA, and is therefore not recommended. Any changes to the bidirectional properties of a character have global effect: they affect all Emacs frames and windows.
Similarly, the `mirroring` property is used to display the appropriate mirrored character in the reordered text. Lisp programs can affect the mirrored display by changing this property. Again, any such changes affect all of Emacs display.
The bidirectional properties of characters can be overridden by inserting into the text special directional control characters, LEFT-TO-RIGHT OVERRIDE (LRO) and RIGHT-TO-LEFT OVERRIDE (RLO). Any characters between a RLO and the following newline or POP DIRECTIONAL FORMATTING (PDF) control character, whichever comes first, will be displayed as if they were strong right-to-left characters, i.e. they will be reversed on display. Similarly, any characters between LRO and PDF or newline will display as if they were strong left-to-right, and will *not* be reversed even if they are strong right-to-left characters.
These overrides are useful when you want to make some text unaffected by the reordering algorithm, and instead directly control the display order. But they can also be used for malicious purposes, known as *phishing*. Specifically, a URL on a Web page or a link in an email message can be manipulated to make its visual appearance unrecognizable, or similar to some popular benign location, while the real location, interpreted by a browser in the logical order, is very different.
Emacs provides a primitive that applications can use to detect instances of text whose bidirectional properties were overridden so as to make a left-to-right character display as if it were a right-to-left character, or vice versa.
Function: **bidi-find-overridden-directionality** *from to &optional object*
This function looks at the text of the specified object between positions from (inclusive) and to (exclusive), and returns the first position where it finds a strong left-to-right character whose directional properties were forced to display the character as right-to-left, or for a strong right-to-left character that was forced to display as left-to-right. If it finds no such characters in the specified region of text, it returns `nil`.
The optional argument object specifies which text to search, and defaults to the current buffer. If object is non-`nil`, it can be some other buffer, or it can be a string or a window. If it is a string, the function searches that string. If it is a window, the function searches the buffer displayed in that window. If a buffer whose text you want to examine is displayed in some window, we recommend to specify it by that window, rather than pass the buffer to the function. This is because telling the function about the window allows it to correctly account for window-specific overlays, which might change the result of the function if some text in the buffer is covered by overlays.
When text that includes mixed right-to-left and left-to-right characters and bidirectional controls is copied into a different location, it can change its visual appearance, and also can affect the visual appearance of the surrounding text at destination. This is because reordering of bidirectional text specified by the UBA has non-trivial context-dependent effects both on the copied text and on the text at copy destination that will surround it.
Sometimes, a Lisp program may need to preserve the exact visual appearance of the copied text at destination, and of the text that surrounds the copy. Lisp programs can use the following function to achieve that effect.
Function: **buffer-substring-with-bidi-context** *start end &optional no-properties*
This function works similar to `buffer-substring` (see [Buffer Contents](buffer-contents)), but it prepends and appends to the copied text bidi directional control characters necessary to preserve the visual appearance of the text when it is inserted at another place. Optional argument no-properties, if non-`nil`, means remove the text properties from the copy of the text.
| programming_docs |
elisp None #### Explicit Entry to the Debugger
You can cause the debugger to be called at a certain point in your program by writing the expression `(debug)` at that point. To do this, visit the source file, insert the text ‘`(debug)`’ at the proper place, and type `C-M-x` (`eval-defun`, a Lisp mode key binding). **Warning:** if you do this for temporary debugging purposes, be sure to undo this insertion before you save the file!
The place where you insert ‘`(debug)`’ must be a place where an additional form can be evaluated and its value ignored. (If the value of `(debug)` isn’t ignored, it will alter the execution of the program!) The most common suitable places are inside a `progn` or an implicit `progn` (see [Sequencing](sequencing)).
If you don’t know exactly where in the source code you want to put the debug statement, but you want to display a backtrace when a certain message is displayed, you can set `debug-on-message` to a regular expression matching the desired message.
elisp None ### User-Level Deletion Commands
This section describes higher-level commands for deleting text, commands intended primarily for the user but useful also in Lisp programs.
Command: **delete-horizontal-space** *&optional backward-only*
This function deletes all spaces and tabs around point. It returns `nil`.
If backward-only is non-`nil`, the function deletes spaces and tabs before point, but not after point.
In the following examples, we call `delete-horizontal-space` four times, once on each line, with point between the second and third characters on the line each time.
```
---------- Buffer: foo ----------
I ∗thought
I ∗ thought
We∗ thought
Yo∗u thought
---------- Buffer: foo ----------
```
```
(delete-horizontal-space) ; Four times.
⇒ nil
---------- Buffer: foo ----------
Ithought
Ithought
Wethought
You thought
---------- Buffer: foo ----------
```
Command: **delete-indentation** *&optional join-following-p beg end*
This function joins the line point is on to the previous line, deleting any whitespace at the join and in some cases replacing it with one space. If join-following-p is non-`nil`, `delete-indentation` joins this line to the following line instead. Otherwise, if beg and end are non-`nil`, this function joins all lines in the region they define.
In an interactive call, join-following-p is the prefix argument, and beg and end are, respectively, the start and end of the region if it is active, else `nil`. The function returns `nil`.
If there is a fill prefix, and the second of the lines being joined starts with the prefix, then `delete-indentation` deletes the fill prefix before joining the lines. See [Margins](margins).
In the example below, point is located on the line starting ‘`events`’, and it makes no difference if there are trailing spaces in the preceding line.
```
---------- Buffer: foo ----------
When in the course of human
∗ events, it becomes necessary
---------- Buffer: foo ----------
```
```
(delete-indentation)
⇒ nil
```
```
---------- Buffer: foo ----------
When in the course of human∗ events, it becomes necessary
---------- Buffer: foo ----------
```
After the lines are joined, the function `fixup-whitespace` is responsible for deciding whether to leave a space at the junction.
Command: **fixup-whitespace**
This function replaces all the horizontal whitespace surrounding point with either one space or no space, according to the context. It returns `nil`.
At the beginning or end of a line, the appropriate amount of space is none. Before a character with close parenthesis syntax, or after a character with open parenthesis or expression-prefix syntax, no space is also appropriate. Otherwise, one space is appropriate. See [Syntax Class Table](syntax-class-table).
In the example below, `fixup-whitespace` is called the first time with point before the word ‘`spaces`’ in the first line. For the second invocation, point is directly after the ‘`(`’.
```
---------- Buffer: foo ----------
This has too many ∗spaces
This has too many spaces at the start of (∗ this list)
---------- Buffer: foo ----------
```
```
(fixup-whitespace)
⇒ nil
(fixup-whitespace)
⇒ nil
```
```
---------- Buffer: foo ----------
This has too many spaces
This has too many spaces at the start of (this list)
---------- Buffer: foo ----------
```
Command: **just-one-space** *&optional n*
This command replaces any spaces and tabs around point with a single space, or n spaces if n is specified. It returns `nil`.
Command: **delete-blank-lines**
This function deletes blank lines surrounding point. If point is on a blank line with one or more blank lines before or after it, then all but one of them are deleted. If point is on an isolated blank line, then it is deleted. If point is on a nonblank line, the command deletes all blank lines immediately following it.
A blank line is defined as a line containing only tabs and spaces.
`delete-blank-lines` returns `nil`.
Command: **delete-trailing-whitespace** *&optional start end*
Delete trailing whitespace in the region defined by start and end.
This command deletes whitespace characters after the last non-whitespace character in each line in the region.
If this command acts on the entire buffer (i.e., if called interactively with the mark inactive, or called from Lisp with end `nil`), it also deletes all trailing lines at the end of the buffer if the variable `delete-trailing-lines` is non-`nil`.
elisp None Windows
-------
This chapter describes the functions and variables related to Emacs windows. See [Frames](frames), for how windows are assigned an area of screen available for Emacs to use. See [Display](display), for information on how text is displayed in windows.
| | | |
| --- | --- | --- |
| • [Basic Windows](basic-windows) | | Basic information on using windows. |
| • [Windows and Frames](windows-and-frames) | | Relating windows to the frame they appear on. |
| • [Selecting Windows](selecting-windows) | | The selected window is the one that you edit in. |
| • [Window Sizes](window-sizes) | | Accessing a window’s size. |
| • [Resizing Windows](resizing-windows) | | Changing the sizes of windows. |
| • [Preserving Window Sizes](preserving-window-sizes) | | Preserving the size of windows. |
| • [Splitting Windows](splitting-windows) | | Creating a new window. |
| • [Deleting Windows](deleting-windows) | | Removing a window from its frame. |
| • [Recombining Windows](recombining-windows) | | Preserving the frame layout when splitting and deleting windows. |
| • [Cyclic Window Ordering](cyclic-window-ordering) | | Moving around the existing windows. |
| • [Buffers and Windows](buffers-and-windows) | | Each window displays the contents of a buffer. |
| • [Switching Buffers](switching-buffers) | | Higher-level functions for switching to a buffer. |
| • [Displaying Buffers](displaying-buffers) | | Displaying a buffer in a suitable window. |
| • [Window History](window-history) | | Each window remembers the buffers displayed in it. |
| • [Dedicated Windows](dedicated-windows) | | How to avoid displaying another buffer in a specific window. |
| • [Quitting Windows](quitting-windows) | | How to restore the state prior to displaying a buffer. |
| • [Side Windows](side-windows) | | Special windows on a frame’s sides. |
| • [Atomic Windows](atomic-windows) | | Preserving parts of the window layout. |
| • [Window Point](window-point) | | Each window has its own location of point. |
| • [Window Start and End](window-start-and-end) | | Buffer positions indicating which text is on-screen in a window. |
| • [Textual Scrolling](textual-scrolling) | | Moving text up and down through the window. |
| • [Vertical Scrolling](vertical-scrolling) | | Moving the contents up and down on the window. |
| • [Horizontal Scrolling](horizontal-scrolling) | | Moving the contents sideways on the window. |
| • [Coordinates and Windows](coordinates-and-windows) | | Converting coordinates to windows. |
| • [Mouse Window Auto-selection](mouse-window-auto_002dselection) | | Automatically selecting windows with the mouse. |
| • [Window Configurations](window-configurations) | | Saving and restoring the state of the screen. |
| • [Window Parameters](window-parameters) | | Associating additional information with windows. |
| • [Window Hooks](window-hooks) | | Hooks for scrolling, window size changes, redisplay going past a certain point, or window configuration changes. |
elisp None ### Information from Markers
This section describes the functions for accessing the components of a marker object.
Function: **marker-position** *marker*
This function returns the position that marker points to, or `nil` if it points nowhere.
Function: **marker-buffer** *marker*
This function returns the buffer that marker points into, or `nil` if it points nowhere.
```
(setq m (make-marker))
⇒ #<marker in no buffer>
```
```
(marker-position m)
⇒ nil
```
```
(marker-buffer m)
⇒ nil
```
```
(set-marker m 3770 (current-buffer))
⇒ #<marker at 3770 in markers.texi>
```
```
(marker-buffer m)
⇒ #<buffer markers.texi>
```
```
(marker-position m)
⇒ 3770
```
elisp None ### Unloading
You can discard the functions and variables loaded by a library to reclaim memory for other Lisp objects. To do this, use the function `unload-feature`:
Command: **unload-feature** *feature &optional force*
This command unloads the library that provided feature feature. It undefines all functions, macros, and variables defined in that library with `defun`, `defalias`, `defsubst`, `defmacro`, `defconst`, `defvar`, and `defcustom`. It then restores any autoloads formerly associated with those symbols. (Loading saves these in the `autoload` property of the symbol.)
Before restoring the previous definitions, `unload-feature` runs `remove-hook` to remove functions defined by the library from certain hooks. These hooks include variables whose names end in ‘`-hook`’ (or the deprecated suffix ‘`-hooks`’), plus those listed in `unload-feature-special-hooks`, as well as `auto-mode-alist`. This is to prevent Emacs from ceasing to function because important hooks refer to functions that are no longer defined.
Standard unloading activities also undo ELP profiling of functions in that library, unprovides any features provided by the library, and cancels timers held in variables defined by the library.
If these measures are not sufficient to prevent malfunction, a library can define an explicit unloader named `feature-unload-function`. If that symbol is defined as a function, `unload-feature` calls it with no arguments before doing anything else. It can do whatever is appropriate to unload the library. If it returns `nil`, `unload-feature` proceeds to take the normal unload actions. Otherwise it considers the job to be done.
Ordinarily, `unload-feature` refuses to unload a library on which other loaded libraries depend. (A library a depends on library b if a contains a `require` for b.) If the optional argument force is non-`nil`, dependencies are ignored and you can unload any library.
The `unload-feature` function is written in Lisp; its actions are based on the variable `load-history`.
Variable: **unload-feature-special-hooks**
This variable holds a list of hooks to be scanned before unloading a library, to remove functions defined in the library.
elisp None ### Defining Global Variables
A *variable definition* is a construct that announces your intention to use a symbol as a global variable. It uses the special forms `defvar` or `defconst`, which are documented below.
A variable definition serves three purposes. First, it informs people who read the code that the symbol is *intended* to be used a certain way (as a variable). Second, it informs the Lisp system of this, optionally supplying an initial value and a documentation string. Third, it provides information to programming tools such as `etags`, allowing them to find where the variable was defined.
The difference between `defconst` and `defvar` is mainly a matter of intent, serving to inform human readers of whether the value should ever change. Emacs Lisp does not actually prevent you from changing the value of a variable defined with `defconst`. One notable difference between the two forms is that `defconst` unconditionally initializes the variable, whereas `defvar` initializes it only if it is originally void.
To define a customizable variable, you should use `defcustom` (which calls `defvar` as a subroutine). See [Variable Definitions](variable-definitions).
Special Form: **defvar** *symbol [value [doc-string]]*
This special form defines symbol as a variable. Note that symbol is not evaluated; the symbol to be defined should appear explicitly in the `defvar` form. The variable is marked as *special*, meaning that it should always be dynamically bound (see [Variable Scoping](variable-scoping)).
If value is specified, and symbol is void (i.e., it has no dynamically bound value; see [Void Variables](void-variables)), then value is evaluated and symbol is set to the result. But if symbol is not void, value is not evaluated, and symbol’s value is left unchanged. If value is omitted, the value of symbol is not changed in any case.
Note that specifying a value, even `nil`, marks the variable as special permanently. Whereas if value is omitted then the variable is only marked special locally (i.e. within the current lexical scope, or file if at the top-level). This can be useful for suppressing byte compilation warnings, see [Compiler Errors](compiler-errors).
If symbol has a buffer-local binding in the current buffer, `defvar` acts on the default value, which is buffer-independent, rather than the buffer-local binding. It sets the default value if the default value is void. See [Buffer-Local Variables](buffer_002dlocal-variables).
If symbol is already lexically bound (e.g., if the `defvar` form occurs in a `let` form with lexical binding enabled), then `defvar` sets the dynamic value. The lexical binding remains in effect until its binding construct exits. See [Variable Scoping](variable-scoping).
When you evaluate a top-level `defvar` form with `C-M-x` (`eval-defun`) or with `C-x C-e` (`eval-last-sexp`) in Emacs Lisp mode, a special feature of these two commands arranges to set the variable unconditionally, without testing whether its value is void.
If the doc-string argument is supplied, it specifies the documentation string for the variable (stored in the symbol’s `variable-documentation` property). See [Documentation](documentation).
Here are some examples. This form defines `foo` but does not initialize it:
```
(defvar foo)
⇒ foo
```
This example initializes the value of `bar` to `23`, and gives it a documentation string:
```
(defvar bar 23
"The normal weight of a bar.")
⇒ bar
```
The `defvar` form returns symbol, but it is normally used at top level in a file where its value does not matter.
For a more elaborate example of using `defvar` without a value, see [Local defvar example](using-lexical-binding#Local-defvar-example).
Special Form: **defconst** *symbol value [doc-string]*
This special form defines symbol as a value and initializes it. It informs a person reading your code that symbol has a standard global value, established here, that should not be changed by the user or by other programs. Note that symbol is not evaluated; the symbol to be defined must appear explicitly in the `defconst`.
The `defconst` form, like `defvar`, marks the variable as *special*, meaning that it should always be dynamically bound (see [Variable Scoping](variable-scoping)). In addition, it marks the variable as risky (see [File Local Variables](file-local-variables)).
`defconst` always evaluates value, and sets the value of symbol to the result. If symbol does have a buffer-local binding in the current buffer, `defconst` sets the default value, not the buffer-local value. (But you should not be making buffer-local bindings for a symbol that is defined with `defconst`.)
An example of the use of `defconst` is Emacs’s definition of `float-pi`—the mathematical constant *pi*, which ought not to be changed by anyone (attempts by the Indiana State Legislature notwithstanding). As the second form illustrates, however, `defconst` is only advisory.
```
(defconst float-pi 3.141592653589793 "The value of Pi.")
⇒ float-pi
```
```
(setq float-pi 3)
⇒ float-pi
```
```
float-pi
⇒ 3
```
**Warning:** If you use a `defconst` or `defvar` special form while the variable has a local binding (made with `let`, or a function argument), it sets the local binding rather than the global binding. This is not what you usually want. To prevent this, use these special forms at top level in a file, where normally no local binding is in effect, and make sure to load the file before making a local binding for the variable.
elisp None #### Precalculated Fontification
Some major modes such as `list-buffers` and `occur` construct the buffer text programmatically. The easiest way for them to support Font Lock mode is to specify the faces of text when they insert the text in the buffer.
The way to do this is to specify the faces in the text with the special text property `font-lock-face` (see [Special Properties](special-properties)). When Font Lock mode is enabled, this property controls the display, just like the `face` property. When Font Lock mode is disabled, `font-lock-face` has no effect on the display.
It is ok for a mode to use `font-lock-face` for some text and also use the normal Font Lock machinery. But if the mode does not use the normal Font Lock machinery, it should not set the variable `font-lock-defaults`. In this case the `face` property will not be overridden, so using the `face` property could work too. However, using `font-lock-face` is generally preferable as it allows the user to control the fontification by toggling `font-lock-mode`, and lets the code work regardless of whether the mode uses Font Lock machinery or not.
elisp None #### Excess Open Parentheses
The first step is to find the defun that is unbalanced. If there is an excess open parenthesis, the way to do this is to go to the end of the file and type `C-u C-M-u` (`backward-up-list`, see [Moving by Parens](https://www.gnu.org/software/emacs/manual/html_node/emacs/Moving-by-Parens.html#Moving-by-Parens) in The GNU Emacs Manual). This will move you to the beginning of the first defun that is unbalanced.
The next step is to determine precisely what is wrong. There is no way to be sure of this except by studying the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with `C-M-q` (`indent-pp-sexp`, see [Multi-line Indent](https://www.gnu.org/software/emacs/manual/html_node/emacs/Multi_002dline-Indent.html#Multi_002dline-Indent) in The GNU Emacs Manual) and see what moves. **But don’t do this yet!** Keep reading, first.
Before you do this, make sure the defun has enough close parentheses. Otherwise, `C-M-q` will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don’t use `C-M-e` (`end-of-defun`) to move there, since that too will fail to work until the defun is balanced.
Now you can go to the beginning of the defun and type `C-M-q`. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don’t assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the `C-M-q` with `C-\_` (`undo`), since the old indentation is probably appropriate to the intended parentheses.
After you think you have fixed the problem, use `C-M-q` again. If the old indentation actually fit the intended nesting of parentheses, and you have put back those parentheses, `C-M-q` should not change anything.
| programming_docs |
elisp None #### Implied Frame Resizing
By default, Emacs tries to keep the number of lines and columns of a frame’s text area unaltered when, for example, toggling its menu or tool bar, changing its default font or setting the width of any of its scroll bars. This means that in such case Emacs must ask the window manager to resize the frame’s window in order to accommodate the size change.
Occasionally, such *implied frame resizing* may be unwanted, for example, when a frame has been maximized or made full-screen (where it’s turned off by default). In general, users can disable implied resizing with the following option:
User Option: **frame-inhibit-implied-resize**
If this option is `nil`, changing a frame’s font, menu bar, tool bar, internal borders, fringes or scroll bars may resize its outer frame in order to keep the number of columns or lines of its text area unaltered. If this option is `t`, no such resizing is done.
The value of this option can be also a list of frame parameters. In that case, implied resizing is inhibited for the change of a parameter that appears in this list. Parameters currently handled by this option are `font`, `font-backend`, `internal-border-width`, `menu-bar-lines` and `tool-bar-lines`.
Changing any of the `scroll-bar-width`, `scroll-bar-height`, `vertical-scroll-bars`, `horizontal-scroll-bars`, `left-fringe` and `right-fringe` frame parameters is handled as if the frame contained just one live window. This means, for example, that removing vertical scroll bars on a frame containing several side by side windows will shrink the outer frame width by the width of one scroll bar provided this option is `nil` and keep it unchanged if this option is `t` or a list containing `vertical-scroll-bars`.
The default value is `(tab-bar-lines tool-bar-lines)` for Lucid, Motif and MS-Windows (which means that adding/removing a tool or tab bar there does not change the outer frame height), `(tab-bar-lines)` on all other window systems including GTK+ (which means that changing any of the parameters listed above with the exception of `tab-bar-lines` may change the size of the outer frame), and `t` otherwise (which means the outer frame size never changes implicitly when there’s no window system support).
Note that when a frame is not large enough to accommodate a change of any of the parameters listed above, Emacs may try to enlarge the frame even if this option is non-`nil`.
Note also that window managers usually do not ask for resizing a frame when they change the number of lines occupied by an external menu or tool bar. Typically, such “wrappings” occur when a user shrinks a frame horizontally, making it impossible to display all elements of its menu or tool bar. They may also result from a change of the major mode altering the number of items of a menu or tool bar. Any such wrappings may implicitly alter the number of lines of a frame’s text area and are unaffected by the setting of this option.
elisp None ### Dynamically Loaded Libraries
A *dynamically loaded library* is a library that is loaded on demand, when its facilities are first needed. Emacs supports such on-demand loading of support libraries for some of its features.
Variable: **dynamic-library-alist**
This is an alist of dynamic libraries and external library files implementing them.
Each element is a list of the form `(library files…)`, where the `car` is a symbol representing a supported external library, and the rest are strings giving alternate filenames for that library.
Emacs tries to load the library from the files in the order they appear in the list; if none is found, the Emacs session won’t have access to that library, and the features it provides will be unavailable.
Image support on some platforms uses this facility. Here’s an example of setting this variable for supporting images on MS-Windows:
```
(setq dynamic-library-alist
'((xpm "libxpm.dll" "xpm4.dll" "libXpm-nox4.dll")
(png "libpng12d.dll" "libpng12.dll" "libpng.dll"
"libpng13d.dll" "libpng13.dll")
(jpeg "jpeg62.dll" "libjpeg.dll" "jpeg-62.dll"
"jpeg.dll")
(tiff "libtiff3.dll" "libtiff.dll")
(gif "giflib4.dll" "libungif4.dll" "libungif.dll")
(svg "librsvg-2-2.dll")
(gdk-pixbuf "libgdk_pixbuf-2.0-0.dll")
(glib "libglib-2.0-0.dll")
(gobject "libgobject-2.0-0.dll")))
```
Note that image types `pbm` and `xbm` do not need entries in this variable because they do not depend on external libraries and are always available in Emacs.
Also note that this variable is not meant to be a generic facility for accessing external libraries; only those already known by Emacs can be loaded through it.
This variable is ignored if the given library is statically linked into Emacs.
elisp None ### Marker Insertion Types
When you insert text directly at the place where a marker points, there are two possible ways to relocate that marker: it can point before the inserted text, or point after it. You can specify which one a given marker should do by setting its *insertion type*. Note that use of `insert-before-markers` ignores markers’ insertion types, always relocating a marker to point after the inserted text.
Function: **set-marker-insertion-type** *marker type*
This function sets the insertion type of marker marker to type. If type is `t`, marker will advance when text is inserted at its position. If type is `nil`, marker does not advance when text is inserted there.
Function: **marker-insertion-type** *marker*
This function reports the current insertion type of marker.
All functions that create markers without accepting an argument that specifies the insertion type, create them with insertion type `nil` (see [Creating Markers](creating-markers)). Also, the mark has, by default, insertion type `nil`.
elisp None #### Ways to compose advice
Here are the different possible values for the where argument of `add-function` and `advice-add`, specifying how the advice function and the original function should be composed.
`:before`
Call function before the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (apply function r) (apply oldfun r))
```
`(add-function :before funvar function)` is comparable for single-function hooks to `(add-hook 'hookvar function)` for normal hooks.
`:after`
Call function after the old function. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (prog1 (apply oldfun r) (apply function r)))
```
`(add-function :after funvar function)` is comparable for single-function hooks to `(add-hook 'hookvar function
'append)` for normal hooks.
`:override`
This completely replaces the old function with the new one. The old function can of course be recovered if you later call `remove-function`.
`:around`
Call function instead of the old function, but provide the old function as an extra argument to function. This is the most flexible composition. For example, it lets you call the old function with different arguments, or many times, or within a let-binding, or you can sometimes delegate the work to the old function and sometimes override it completely. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (apply function oldfun r))
```
`:before-while`
Call function before the old function and don’t call the old function if function returns `nil`. Both functions receive the same arguments, and the return value of the composition is the return value of the old function. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (and (apply function r) (apply oldfun r)))
```
`(add-function :before-while funvar function)` is comparable for single-function hooks to `(add-hook 'hookvar function)` when hookvar is run via `run-hook-with-args-until-failure`.
`:before-until`
Call function before the old function and only call the old function if function returns `nil`. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (or (apply function r) (apply oldfun r)))
```
`(add-function :before-until funvar function)` is comparable for single-function hooks to `(add-hook 'hookvar function)` when hookvar is run via `run-hook-with-args-until-success`.
`:after-while`
Call function after the old function and only if the old function returned non-`nil`. Both functions receive the same arguments, and the return value of the composition is the return value of function. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (and (apply oldfun r) (apply function r)))
```
`(add-function :after-while funvar function)` is comparable for single-function hooks to `(add-hook 'hookvar function
'append)` when hookvar is run via `run-hook-with-args-until-failure`.
`:after-until`
Call function after the old function and only if the old function returned `nil`. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (or (apply oldfun r) (apply function r)))
```
`(add-function :after-until funvar function)` is comparable for single-function hooks to `(add-hook 'hookvar function
'append)` when hookvar is run via `run-hook-with-args-until-success`.
`:filter-args`
Call function first and use the result (which should be a list) as the new arguments to pass to the old function. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (apply oldfun (funcall function r)))
```
`:filter-return`
Call the old function first and pass the result to function. More specifically, the composition of the two functions behaves like:
```
(lambda (&rest r) (funcall function (apply oldfun r)))
```
elisp None #### Display Tables
A display table is a special-purpose char-table (see [Char-Tables](char_002dtables)), with `display-table` as its subtype, which is used to override the usual character display conventions. This section describes how to make, inspect, and assign elements to a display table object. The next section (see [Active Display Table](active-display-table)) describes the various standard display tables and their precedence.
Function: **make-display-table**
This creates and returns a display table. The table initially has `nil` in all elements.
The ordinary elements of the display table are indexed by character codes; the element at index c says how to display the character code c. The value should be `nil` (which means to display the character c according to the usual display conventions; see [Usual Display](usual-display)), or a vector of glyph codes (which means to display the character c as those glyphs; see [Glyphs](glyphs)).
**Warning:** if you use the display table to change the display of newline characters, the whole buffer will be displayed as one long line.
The display table also has six *extra slots* which serve special purposes. Here is a table of their meanings; `nil` in any slot means to use the default for that slot, as stated below.
0
The glyph for the end of a truncated screen line (the default for this is ‘`$`’). See [Glyphs](glyphs). On graphical terminals, Emacs by default uses arrows in the fringes to indicate truncation, so the display table has no effect, unless you disable the fringes (see [Window Fringes](https://www.gnu.org/software/emacs/manual/html_node/emacs/Fringes.html#Fringes) in the GNU Emacs Manual).
1
The glyph for the end of a continued line (the default is ‘`\`’). On graphical terminals, Emacs by default uses curved arrows in the fringes to indicate continuation, so the display table has no effect, unless you disable the fringes.
2
The glyph for indicating a character displayed as an octal character code (the default is ‘`\`’).
3
The glyph for indicating a control character (the default is ‘`^`’).
4
A vector of glyphs for indicating the presence of invisible lines (the default is ‘`...`’). See [Selective Display](selective-display).
5 The glyph used to draw the border between side-by-side windows (the default is ‘`|`’). See [Splitting Windows](splitting-windows). This currently has effect only on text terminals; on graphical terminals, if vertical scroll bars are supported and in use, a scroll bar separates the two windows, and if there are no vertical scroll bars and no dividers (see [Window Dividers](window-dividers)), Emacs uses a thin line to indicate the border.
For example, here is how to construct a display table that mimics the effect of setting `ctl-arrow` to a non-`nil` value (see [Glyphs](glyphs), for the function `make-glyph-code`):
```
(setq disptab (make-display-table))
(dotimes (i 32)
(or (= i ?\t)
(= i ?\n)
(aset disptab i
(vector (make-glyph-code ?^ 'escape-glyph)
(make-glyph-code (+ i 64) 'escape-glyph)))))
(aset disptab 127
(vector (make-glyph-code ?^ 'escape-glyph)
(make-glyph-code ?? 'escape-glyph)))))
```
Function: **display-table-slot** *display-table slot*
This function returns the value of the extra slot slot of display-table. The argument slot may be a number from 0 to 5 inclusive, or a slot name (symbol). Valid symbols are `truncation`, `wrap`, `escape`, `control`, `selective-display`, and `vertical-border`.
Function: **set-display-table-slot** *display-table slot value*
This function stores value in the extra slot slot of display-table. The argument slot may be a number from 0 to 5 inclusive, or a slot name (symbol). Valid symbols are `truncation`, `wrap`, `escape`, `control`, `selective-display`, and `vertical-border`.
Function: **describe-display-table** *display-table*
This function displays a description of the display table display-table in a help buffer.
Command: **describe-current-display-table**
This command displays a description of the current display table in a help buffer.
elisp None #### Character Classes
Below is a table of the classes you can use in a character alternative, and what they mean. Note that the ‘`[`’ and ‘`]`’ characters that enclose the class name are part of the name, so a regular expression using these classes needs one more pair of brackets. For example, a regular expression matching a sequence of one or more letters and digits would be ‘`[[:alnum:]]+`’, not ‘`[:alnum:]+`’.
‘`[:ascii:]`’ This matches any ASCII character (codes 0–127).
‘`[:alnum:]`’ This matches any letter or digit. For multibyte characters, it matches characters whose Unicode ‘`general-category`’ property (see [Character Properties](character-properties)) indicates they are alphabetic or decimal number characters.
‘`[:alpha:]`’ This matches any letter. For multibyte characters, it matches characters whose Unicode ‘`general-category`’ property (see [Character Properties](character-properties)) indicates they are alphabetic characters.
‘`[:blank:]`’ This matches horizontal whitespace, as defined by Annex C of the Unicode Technical Standard #18. In particular, it matches spaces, tabs, and other characters whose Unicode ‘`general-category`’ property (see [Character Properties](character-properties)) indicates they are spacing separators.
‘`[:cntrl:]`’ This matches any character whose code is in the range 0–31.
‘`[:digit:]`’ This matches ‘`0`’ through ‘`9`’. Thus, ‘`[-+[:digit:]]`’ matches any digit, as well as ‘`+`’ and ‘`-`’.
‘`[:graph:]`’ This matches graphic characters—everything except whitespace, ASCII and non-ASCII control characters, surrogates, and codepoints unassigned by Unicode, as indicated by the Unicode ‘`general-category`’ property (see [Character Properties](character-properties)).
‘`[:lower:]`’ This matches any lower-case letter, as determined by the current case table (see [Case Tables](case-tables)). If `case-fold-search` is non-`nil`, this also matches any upper-case letter.
‘`[:multibyte:]`’ This matches any multibyte character (see [Text Representations](text-representations)).
‘`[:nonascii:]`’ This matches any non-ASCII character.
‘`[:print:]`’ This matches any printing character—either whitespace, or a graphic character matched by ‘`[:graph:]`’.
‘`[:punct:]`’ This matches any punctuation character. (At present, for multibyte characters, it matches anything that has non-word syntax.)
‘`[:space:]`’ This matches any character that has whitespace syntax (see [Syntax Class Table](syntax-class-table)).
‘`[:unibyte:]`’ This matches any unibyte character (see [Text Representations](text-representations)).
‘`[:upper:]`’ This matches any upper-case letter, as determined by the current case table (see [Case Tables](case-tables)). If `case-fold-search` is non-`nil`, this also matches any lower-case letter.
‘`[:word:]`’ This matches any character that has word syntax (see [Syntax Class Table](syntax-class-table)).
‘`[:xdigit:]`’ This matches the hexadecimal digits: ‘`0`’ through ‘`9`’, ‘`a`’ through ‘`f`’ and ‘`A`’ through ‘`F`’.
elisp None #### Process Type
The word *process* usually means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by the Emacs process. Programs such as shells, GDB, ftp, and compilers, running in subprocesses of Emacs, extend the capabilities of Emacs. An Emacs subprocess takes textual input from Emacs and returns textual output to Emacs for further manipulation. Emacs can also send signals to the subprocess.
Process objects have no read syntax. They print in hash notation, giving the name of the process:
```
(process-list)
⇒ (#<process shell>)
```
See [Processes](processes), for information about functions that create, delete, return information about, send input or signals to, and receive output from processes.
elisp None ### Atomic Windows
Atomic windows are rectangular compositions of at least two live windows. They have the following distinctive characteristics:
* The function `split-window` (see [Splitting Windows](splitting-windows)), when applied to a constituent of an atomic window, will try to create the new window outside of the atomic window.
* The function `delete-window` (see [Deleting Windows](deleting-windows)), when applied to a constituent of an atomic window, will try to delete the entire atomic window instead.
* The function `delete-other-windows` (see [Deleting Windows](deleting-windows)), when applied to a constituent of an atomic window, will try to make the atomic window fill its frame or main window (see [Side Windows](side-windows)).
This means that the basic groups of functions that alter the window structure treat an atomic window like a live one, thus preserving the internal structure of the atomic window.
Atomic windows are useful to construct and preserve window layouts that are meaningful only when all involved buffers are shown simultaneously in a specific manner, such as when showing differences between file revisions, or the same text in different languages or markups. They can also be used to permanently display information pertinent to a specific window in bars on that window’s sides.
Atomic windows are implemented with the help of the reserved `window-atom` window parameter (see [Window Parameters](window-parameters)) and an internal window (see [Basic Windows](basic-windows)) called the root window of the atomic window. All windows that are part of the same atomic window have this root window as their common ancestor and are assigned a non-`nil` `window-atom` parameter.
The following function returns the root of the atomic window a specified window is part of:
Function: **window-atom-root** *&optional window*
This functions returns the root of the atomic window window is a part of. The specified window must be a valid window and defaults to the selected one. It returns `nil` if window is not part of an atomic window.
The most simple approach to make a new atomic window is to take an existing internal window and apply the following function:
Function: **window-make-atom** *window*
This function converts window into an atomic window. The specified window must be an internal window. All this function does is to set the `window-atom` parameter of each descendant of window to `t`.
To create a new atomic window from an existing live window or to add a new window to an existing atomic window, the following buffer display action function (see [Buffer Display Action Functions](buffer-display-action-functions)) can be used:
Function: **display-buffer-in-atom-window** *buffer alist*
This function tries to display buffer in a new window that will be combined with an existing window to form an atomic window. If the existing window is already part of an atomic window, it adds the new window to that atomic window.
The specified alist is an association list of symbols and values. The following symbols have a special meaning:
`window`
The value of such an element specifies an existing window the new window shall be combined with. If it specifies an internal window, all children of that window become part of the atomic window too. If no window is specified, the new window becomes a sibling of the selected window. The `window-atom` parameter of the existing window is set to `main` provided that window is live and its `window-atom` parameter was not already set.
`side` The value of such an element denotes the side of the existing window where the new window shall be located. Valid values are `below`, `right`, `above` and `left`. The default is `below`. The `window-atom` parameter of the new window is set to this value.
The return value is the new window, `nil` when creating that window failed.
Note that the value of the `window-atom` parameter does not really matter as long as it is non-`nil`. The values assigned by `display-buffer-in-atom-window` just allow for easy retrieval of the original and the new window after that function has been applied. Note also that the `window-atom` parameter is the only window parameter assigned by `display-buffer-in-atom-window`. Further parameters have to be set by the application explicitly via a `window-parameters` entry in alist.
Atomic windows automatically cease to exist when one of their constituents gets deleted. To dissolve an atomic window manually, reset the `window-atom` parameter of its constituents—the root of the atomic window and all its descendants.
The following code snippet, when applied to a single-window frame, first splits the selected window and makes the selected and the new window constituents of an atomic window with their parent as root. It then displays the buffer `\*Messages\*` in a new window at the frame’s bottom and makes that new window part of the atomic window just created.
```
(let ((window (split-window-right)))
(window-make-atom (window-parent window))
(display-buffer-in-atom-window
(get-buffer-create "*Messages*")
`((window . ,(window-parent window)) (window-height . 5))))
```
At this moment typing `C-x 2` in any window of that frame produces a new window at the bottom of the frame. Typing `C-x 3` instead will put the new window at the frame’s right. In either case, typing now `C-x 1` in any window of the atomic window will remove the new window only. Typing `C-x 0` in any window of the atomic window will make that new window fill the frame.
| programming_docs |
elisp None #### Decoding Process Output
When Emacs writes process output directly into a multibyte buffer, it decodes the output according to the process output coding system. If the coding system is `raw-text` or `no-conversion`, Emacs converts the unibyte output to multibyte using `string-to-multibyte`, and inserts the resulting multibyte text.
You can use `set-process-coding-system` to specify which coding system to use (see [Process Information](process-information)). Otherwise, the coding system comes from `coding-system-for-read`, if that is non-`nil`; or else from the defaulting mechanism (see [Default Coding Systems](default-coding-systems)). If the text output by a process contains null bytes, Emacs by default uses `no-conversion` for it; see [inhibit-null-byte-detection](lisp-and-coding-systems), for how to control this behavior.
**Warning:** Coding systems such as `undecided`, which determine the coding system from the data, do not work entirely reliably with asynchronous subprocess output. This is because Emacs has to process asynchronous subprocess output in batches, as it arrives. Emacs must try to detect the proper coding system from one batch at a time, and this does not always work. Therefore, if at all possible, specify a coding system that determines both the character code conversion and the end of line conversion—that is, one like `latin-1-unix`, rather than `undecided` or `latin-1`.
When Emacs calls a process filter function, it provides the process output as a multibyte string or as a unibyte string according to the process’s filter coding system. Emacs decodes the output according to the process output coding system, which usually produces a multibyte string, except for coding systems such as `binary` and `raw-text`.
elisp None ### Applying Customizations
The following functions are responsible for installing the user’s customization settings for variables and faces, respectively. When the user invokes ‘`Save for future sessions`’ in the Customize interface, that takes effect by writing a `custom-set-variables` and/or a `custom-set-faces` form into the custom file, to be evaluated the next time Emacs starts.
Function: **custom-set-variables** *&rest args*
This function installs the variable customizations specified by args. Each argument in args should have the form
```
(var expression [now [request [comment]]])
```
var is a variable name (a symbol), and expression is an expression which evaluates to the desired customized value.
If the `defcustom` form for var has been evaluated prior to this `custom-set-variables` call, expression is immediately evaluated, and the variable’s value is set to the result. Otherwise, expression is stored into the variable’s `saved-value` property, to be evaluated when the relevant `defcustom` is called (usually when the library defining that variable is loaded into Emacs).
The now, request, and comment entries are for internal use only, and may be omitted. now, if non-`nil`, means to set the variable’s value now, even if the variable’s `defcustom` form has not been evaluated. request is a list of features to be loaded immediately (see [Named Features](named-features)). comment is a string describing the customization.
Function: **custom-set-faces** *&rest args*
This function installs the face customizations specified by args. Each argument in args should have the form
```
(face spec [now [comment]])
```
face is a face name (a symbol), and spec is the customized face specification for that face (see [Defining Faces](defining-faces)).
The now and comment entries are for internal use only, and may be omitted. now, if non-`nil`, means to install the face specification now, even if the `defface` form has not been evaluated. comment is a string describing the customization.
elisp None #### The Overlay Arrow
The *overlay arrow* is useful for directing the user’s attention to a particular line in a buffer. For example, in the modes used for interface to debuggers, the overlay arrow indicates the line of code about to be executed. This feature has nothing to do with *overlays* (see [Overlays](overlays)).
Variable: **overlay-arrow-string**
This variable holds the string to display to call attention to a particular line, or `nil` if the arrow feature is not in use. On a graphical display the contents of the string are ignored if the left fringe is shown; instead a glyph is displayed in the fringe area to the left of the display area.
Variable: **overlay-arrow-position**
This variable holds a marker that indicates where to display the overlay arrow. It should point at the beginning of a line. On a non-graphical display, or when the left fringe is not shown, the arrow text appears at the beginning of that line, overlaying any text that would otherwise appear. Since the arrow is usually short, and the line usually begins with indentation, normally nothing significant is overwritten.
The overlay-arrow string is displayed in any given buffer if the value of `overlay-arrow-position` in that buffer points into that buffer. Thus, it is possible to display multiple overlay arrow strings by creating buffer-local bindings of `overlay-arrow-position`. However, it is usually cleaner to use `overlay-arrow-variable-list` to achieve this result.
You can do a similar job by creating an overlay with a `before-string` property. See [Overlay Properties](overlay-properties).
You can define multiple overlay arrows via the variable `overlay-arrow-variable-list`.
Variable: **overlay-arrow-variable-list**
This variable’s value is a list of variables, each of which specifies the position of an overlay arrow. The variable `overlay-arrow-position` has its normal meaning because it is on this list.
Each variable on this list can have properties `overlay-arrow-string` and `overlay-arrow-bitmap` that specify an overlay arrow string (for text terminals or graphical terminals without the left fringe shown) or fringe bitmap (for graphical terminals with a left fringe) to display at the corresponding overlay arrow position. If either property is not set, the default `overlay-arrow-string` or `overlay-arrow` fringe indicator is used.
elisp None ### Mouse Window Auto-selection
The following option allows to automatically select the window under the mouse pointer. This accomplishes a policy similar to that of window managers that give focus to a frame (and thus trigger its subsequent selection) whenever the mouse pointer enters its window-system window (see [Input Focus](input-focus)).
User Option: **mouse-autoselect-window**
If this variable is non-`nil`, Emacs will try to automatically select the window under the mouse pointer. The following values are meaningful:
A positive number
This specifies a delay in seconds after which auto-selection triggers. The window under the mouse pointer is selected after the mouse has remained in it for the entire duration of the delay.
A negative number
A negative number has a similar effect as a positive number, but selects the window under the mouse pointer only after the mouse pointer has remained in it for the entire duration of the absolute value of that number and in addition has stopped moving.
Other value Any other non-`nil` value means to select a window instantaneously as soon as the mouse pointer enters it.
In either case, the mouse pointer must enter the text area of a window in order to trigger its selection. Dragging the scroll bar slider or the mode line of a window conceptually should not cause its auto-selection.
Mouse auto-selection selects the minibuffer window only if it is active, and never deselects the active minibuffer window.
Mouse auto-selection can be used to emulate a focus follows mouse policy for child frames (see [Child Frames](child-frames)) which usually are not tracked by the window manager. This requires to set the value of `focus-follows-mouse` (see [Input Focus](input-focus)) to a non-`nil` value. If the value of `focus-follows-mouse` is `auto-raise`, entering a child frame with the mouse will raise it automatically above all other child frames of that frame’s parent frame.
elisp None #### How to Signal an Error
*Signaling* an error means beginning error processing. Error processing normally aborts all or part of the running program and returns to a point that is set up to handle the error (see [Processing of Errors](processing-of-errors)). Here we describe how to signal an error.
Most errors are signaled automatically within Lisp primitives which you call for other purposes, such as if you try to take the CAR of an integer or move forward a character at the end of the buffer. You can also signal errors explicitly with the functions `error` and `signal`.
Quitting, which happens when the user types `C-g`, is not considered an error, but it is handled almost like an error. See [Quitting](quitting).
Every error specifies an error message, one way or another. The message should state what is wrong (“File does not exist”), not how things ought to be (“File must exist”). The convention in Emacs Lisp is that error messages should start with a capital letter, but should not end with any sort of punctuation.
Function: **error** *format-string &rest args*
This function signals an error with an error message constructed by applying `format-message` (see [Formatting Strings](formatting-strings)) to format-string and args.
These examples show typical uses of `error`:
```
(error "That is an error -- try something else")
error→ That is an error -- try something else
```
```
(error "Invalid name `%s'" "A%%B")
error→ Invalid name ‘A%%B’
```
`error` works by calling `signal` with two arguments: the error symbol `error`, and a list containing the string returned by `format-message`.
Typically grave accent and apostrophe in the format translate to matching curved quotes, e.g., "Missing `%s'" might result in "Missing ‘foo’". See [Text Quoting Style](text-quoting-style), for how to influence or inhibit this translation.
**Warning:** If you want to use your own string as an error message verbatim, don’t just write `(error string)`. If string string contains ‘`%`’, ‘```’, or ‘`'`’ it may be reformatted, with undesirable results. Instead, use `(error "%s"
string)`.
Function: **signal** *error-symbol data*
This function signals an error named by error-symbol. The argument data is a list of additional Lisp objects relevant to the circumstances of the error.
The argument error-symbol must be an *error symbol*—a symbol defined with `define-error`. This is how Emacs Lisp classifies different sorts of errors. See [Error Symbols](error-symbols), for a description of error symbols, error conditions and condition names.
If the error is not handled, the two arguments are used in printing the error message. Normally, this error message is provided by the `error-message` property of error-symbol. If data is non-`nil`, this is followed by a colon and a comma separated list of the unevaluated elements of data. For `error`, the error message is the CAR of data (that must be a string). Subcategories of `file-error` are handled specially.
The number and significance of the objects in data depends on error-symbol. For example, with a `wrong-type-argument` error, there should be two objects in the list: a predicate that describes the type that was expected, and the object that failed to fit that type.
Both error-symbol and data are available to any error handlers that handle the error: `condition-case` binds a local variable to a list of the form `(error-symbol .
data)` (see [Handling Errors](handling-errors)).
The function `signal` never returns.
```
(signal 'wrong-number-of-arguments '(x y))
error→ Wrong number of arguments: x, y
```
```
(signal 'no-such-error '("My unknown error condition"))
error→ peculiar error: "My unknown error condition"
```
Function: **user-error** *format-string &rest args*
This function behaves exactly like `error`, except that it uses the error symbol `user-error` rather than `error`. As the name suggests, this is intended to report errors on the part of the user, rather than errors in the code itself. For example, if you try to use the command `Info-history-back` (`l`) to move back beyond the start of your Info browsing history, Emacs signals a `user-error`. Such errors do not cause entry to the debugger, even when `debug-on-error` is non-`nil`. See [Error Debugging](error-debugging).
> **Common Lisp note:** Emacs Lisp has nothing like the Common Lisp concept of continuable errors.
>
>
>
elisp None ### Backward Compatibility
Code compiled with older versions of `cl-defstruct` that doesn’t use records may run into problems when used in a new Emacs. To alleviate this, Emacs detects when an old `cl-defstruct` is used, and enables a mode in which `type-of` handles old struct objects as if they were records.
Function: **cl-old-struct-compat-mode** *arg*
If arg is positive, enable backward compatibility with old-style structs.
elisp None #### Image Descriptors
An *image descriptor* is a list which specifies the underlying data for an image, and how to display it. It is typically used as the value of a `display` overlay or text property (see [Other Display Specs](other-display-specs)); but See [Showing Images](showing-images), for convenient helper functions to insert images into buffers.
Each image descriptor has the form `(image . props)`, where props is a property list of alternating keyword symbols and values, including at least the pair `:type type` that specifies the image type.
Image descriptors which define image dimensions, `:width`, `:height`, `:max-width` and `:max-height`, may take either an integer, which represents the dimension in pixels, or a pair `(value . em)`, where value is the dimension’s length in *ems*[27](#FOOT27). One em is equivalent to the height of the font and value may be an integer or a float.
The following is a list of properties that are meaningful for all image types (there are also properties which are meaningful only for certain image types, as documented in the following subsections):
`:type type`
The image type. See [Image Formats](image-formats). Every image descriptor must include this property.
`:file file`
This says to load the image from file file. If file is not an absolute file name, it is expanded relative to the `images` subdirectory of `data-directory`, and failing that, relative to the directories listed by `x-bitmap-file-path` (see [Face Attributes](face-attributes)).
`:data data`
This specifies the raw image data. Each image descriptor must have either `:data` or `:file`, but not both.
For most image types, the value of a `:data` property should be a string containing the image data. Some image types do not support `:data`; for some others, `:data` alone is not enough, so you need to use other image properties along with `:data`. See the following subsections for details.
`:margin margin`
This specifies how many pixels to add as an extra margin around the image. The value, margin, must be a non-negative number, or a pair `(x . y)` of such numbers. If it is a pair, x specifies how many pixels to add horizontally, and y specifies how many pixels to add vertically. If `:margin` is not specified, the default is zero.
`:ascent ascent`
This specifies the amount of the image’s height to use for its ascent—that is, the part above the baseline. The value, ascent, must be a number in the range 0 to 100, or the symbol `center`.
If ascent is a number, that percentage of the image’s height is used for its ascent.
If ascent is `center`, the image is vertically centered around a centerline which would be the vertical centerline of text drawn at the position of the image, in the manner specified by the text properties and overlays that apply to the image.
If this property is omitted, it defaults to 50.
`:relief relief`
This adds a shadow rectangle around the image. The value, relief, specifies the width of the shadow lines, in pixels. If relief is negative, shadows are drawn so that the image appears as a pressed button; otherwise, it appears as an unpressed button.
`:width width, :height height`
The `:width` and `:height` keywords are used for scaling the image. If only one of them is specified, the other one will be calculated so as to preserve the aspect ratio. If both are specified, aspect ratio may not be preserved.
`:max-width max-width, :max-height max-height`
The `:max-width` and `:max-height` keywords are used for scaling if the size of the image exceeds these values. If `:width` is set, it will have precedence over `max-width`, and if `:height` is set, it will have precedence over `max-height`, but you can otherwise mix these keywords as you wish.
If both `:max-width` and `:height` are specified, but `:width` is not, preserving the aspect ratio might require that width exceeds `:max-width`. If this happens, scaling will use a smaller value for the height so as to preserve the aspect ratio while not exceeding `:max-width`. Similarly when both `:max-height` and `:width` are specified, but `:height` is not. For example, if you have a 200x100 image and specify that `:width` should be 400 and `:max-height` should be 150, you’ll end up with an image that is 300x150: Preserving the aspect ratio and not exceeding the “max” setting. This combination of parameters is a useful way of saying “display this image as large as possible, but no larger than the available display area”.
`:scale scale`
This should be a number, where values higher than 1 means to increase the size, and lower means to decrease the size, by multiplying both the width and height. For instance, a value of 0.25 will make the image a quarter size of what it originally was. If the scaling makes the image larger than specified by `:max-width` or `:max-height`, the resulting size will not exceed those two values. If both `:scale` and `:height`/`:width` are specified, the height/width will be adjusted by the specified scaling factor.
`:rotation angle`
Specifies a rotation angle in degrees. Only multiples of 90 degrees are supported, unless the image type is `imagemagick`. Positive values rotate clockwise, negative values counter-clockwise. Rotation is performed after scaling and cropping.
`:transform-smoothing smooth`
If this is `t`, any image transform will have smoothing applied; if `nil`, no smoothing will be applied. The exact algorithm used is platform dependent, but should be equivalent to bilinear filtering. Disabling smoothing will use the nearest neighbor algorithm.
If this property is not specified, `create-image` will use the `image-transform-smoothing` user option to say whether scaling should be done or not. This option can be `nil` (no smoothing), `t` (use smoothing) or a predicate function that’s called with the image object as the only parameter, and should return either `nil` or `t`. The default is for down-scaling to apply smoothing, and for large up-scaling to not apply smoothing.
`:index frame`
See [Multi-Frame Images](multi_002dframe-images).
`:conversion algorithm`
This specifies a conversion algorithm that should be applied to the image before it is displayed; the value, algorithm, specifies which algorithm.
`laplace` `emboss`
Specifies the Laplace edge detection algorithm, which blurs out small differences in color while highlighting larger differences. People sometimes consider this useful for displaying the image for a disabled button.
`(edge-detection :matrix matrix :color-adjust adjust)`
Specifies a general edge-detection algorithm. matrix must be either a nine-element list or a nine-element vector of numbers. A pixel at position *x/y* in the transformed image is computed from original pixels around that position. matrix specifies, for each pixel in the neighborhood of *x/y*, a factor with which that pixel will influence the transformed pixel; element *0* specifies the factor for the pixel at *x-1/y-1*, element *1* the factor for the pixel at *x/y-1* etc., as shown below:
```
(x-1/y-1 x/y-1 x+1/y-1
x-1/y x/y x+1/y
x-1/y+1 x/y+1 x+1/y+1)
```
The resulting pixel is computed from the color intensity of the color resulting from summing up the RGB values of surrounding pixels, multiplied by the specified factors, and dividing that sum by the sum of the factors’ absolute values.
Laplace edge-detection currently uses a matrix of
```
(1 0 0
0 0 0
0 0 -1)
```
Emboss edge-detection uses a matrix of
```
( 2 -1 0
-1 0 1
0 1 -2)
```
`disabled` Specifies transforming the image so that it looks disabled.
`:mask mask`
If mask is `heuristic` or `(heuristic bg)`, build a clipping mask for the image, so that the background of a frame is visible behind the image. If bg is not specified, or if bg is `t`, determine the background color of the image by looking at the four corners of the image, assuming the most frequently occurring color from the corners is the background color of the image. Otherwise, bg must be a list `(red green blue)` specifying the color to assume for the background of the image.
If mask is `nil`, remove a mask from the image, if it has one. Images in some formats include a mask which can be removed by specifying `:mask nil`.
`:pointer shape`
This specifies the pointer shape when the mouse pointer is over this image. See [Pointer Shape](pointer-shape), for available pointer shapes.
`:map map`
This associates an image map of *hot spots* with this image.
An image map is an alist where each element has the format `(area id plist)`. An area is specified as either a rectangle, a circle, or a polygon.
A rectangle is a cons `(rect . ((x0 . y0) . (x1 . y1)))` which specifies the pixel coordinates of the upper left and bottom right corners of the rectangle area.
A circle is a cons `(circle . ((x0 . y0) . r))` which specifies the center and the radius of the circle; r may be a float or integer.
A polygon is a cons `(poly . [x0 y0 x1 y1 ...])` where each pair in the vector describes one corner in the polygon.
When the mouse pointer lies on a hot-spot area of an image, the plist of that hot-spot is consulted; if it contains a `help-echo` property, that defines a tool-tip for the hot-spot, and if it contains a `pointer` property, that defines the shape of the mouse cursor when it is on the hot-spot. See [Pointer Shape](pointer-shape), for available pointer shapes.
When you click the mouse when the mouse pointer is over a hot-spot, an event is composed by combining the id of the hot-spot with the mouse event; for instance, `[area4 mouse-1]` if the hot-spot’s id is `area4`.
Function: **image-mask-p** *spec &optional frame*
This function returns `t` if image spec has a mask bitmap. frame is the frame on which the image will be displayed. frame `nil` or omitted means to use the selected frame (see [Input Focus](input-focus)).
Function: **image-transforms-p** *&optional frame*
This function returns non-`nil` if frame supports image scaling and rotation. frame `nil` or omitted means to use the selected frame (see [Input Focus](input-focus)). The returned list includes symbols that indicate which image transform operations are supported:
`scale` Image scaling is supported by frame via the `:scale`, `:width`, `:height`, `:max-width`, and `:max-height` properties.
`rotate90` Image rotation is supported by frame if the rotation angle is an integral multiple of 90 degrees.
If image transforms are not supported, `:rotation`, `:crop`, `:width`, `:height`, `:scale`, `:max-width` and `:max-height` will only be usable through ImageMagick, if available (see [ImageMagick Images](imagemagick-images)).
| programming_docs |
elisp None #### Display Specs That Replace The Text
Some kinds of display specifications specify something to display instead of the text that has the property. These are called *replacing* display specifications. Emacs does not allow the user to interactively move point into the middle of buffer text that is replaced in this way.
If a list of display specifications includes more than one replacing display specification, the first overrides the rest. Replacing display specifications make most other display specifications irrelevant, since those don’t apply to the replacement.
For replacing display specifications, *the text that has the property* means all the consecutive characters that have the same Lisp object as their `display` property; these characters are replaced as a single unit. If two characters have different Lisp objects as their `display` properties (i.e., objects which are not `eq`), they are handled separately.
Here is an example which illustrates this point. A string serves as a replacing display specification, which replaces the text that has the property with the specified string (see [Other Display Specs](other-display-specs)). Consider the following function:
```
(defun foo ()
(dotimes (i 5)
(let ((string (concat "A"))
(start (+ i i (point-min))))
(put-text-property start (1+ start) 'display string)
(put-text-property start (+ 2 start) 'display string))))
```
This function gives each of the first ten characters in the buffer a `display` property which is a string `"A"`, but they don’t all get the same string object. The first two characters get the same string object, so they are replaced with one ‘`A`’; the fact that the display property was assigned in two separate calls to `put-text-property` is irrelevant. Similarly, the next two characters get a second string (`concat` creates a new string object), so they are replaced with one ‘`A`’; and so on. Thus, the ten characters appear as five A’s.
elisp None ### The Buffer Gap
Emacs buffers are implemented using an invisible *gap* to make insertion and deletion faster. Insertion works by filling in part of the gap, and deletion adds to the gap. Of course, this means that the gap must first be moved to the locus of the insertion or deletion. Emacs moves the gap only when you try to insert or delete. This is why your first editing command in one part of a large buffer, after previously editing in another far-away part, sometimes involves a noticeable delay.
This mechanism works invisibly, and Lisp code should never be affected by the gap’s current location, but these functions are available for getting information about the gap status.
Function: **gap-position**
This function returns the current gap position in the current buffer.
Function: **gap-size**
This function returns the current gap size of the current buffer.
elisp None ### Creating Buffers
This section describes the two primitives for creating buffers. `get-buffer-create` creates a buffer if it finds no existing buffer with the specified name; `generate-new-buffer` always creates a new buffer and gives it a unique name.
Both functions accept an optional argument inhibit-buffer-hooks. If it is non-`nil`, the buffer they create does not run the hooks `kill-buffer-hook`, `kill-buffer-query-functions` (see [Killing Buffers](killing-buffers)), and `buffer-list-update-hook` (see [Buffer List](buffer-list)). This avoids slowing down internal or temporary buffers that are never presented to users or passed on to other applications.
Other functions you can use to create buffers include `with-output-to-temp-buffer` (see [Temporary Displays](temporary-displays)) and `create-file-buffer` (see [Visiting Files](visiting-files)). Starting a subprocess can also create a buffer (see [Processes](processes)).
Function: **get-buffer-create** *buffer-or-name &optional inhibit-buffer-hooks*
This function returns a buffer named buffer-or-name. The buffer returned does not become the current buffer—this function does not change which buffer is current.
buffer-or-name must be either a string or an existing buffer. If it is a string and a live buffer with that name already exists, `get-buffer-create` returns that buffer. If no such buffer exists, it creates a new buffer. If buffer-or-name is a buffer instead of a string, it is returned as given, even if it is dead.
```
(get-buffer-create "foo")
⇒ #<buffer foo>
```
The major mode for a newly created buffer is set to Fundamental mode. (The default value of the variable `major-mode` is handled at a higher level; see [Auto Major Mode](auto-major-mode).) If the name begins with a space, the buffer initially disables undo information recording (see [Undo](undo)).
Function: **generate-new-buffer** *name &optional inhibit-buffer-hooks*
This function returns a newly created, empty buffer, but does not make it current. The name of the buffer is generated by passing name to the function `generate-new-buffer-name` (see [Buffer Names](buffer-names)). Thus, if there is no buffer named name, then that is the name of the new buffer; if that name is in use, a suffix of the form ‘`<n>`’, where n is an integer, is appended to name.
An error is signaled if name is not a string.
```
(generate-new-buffer "bar")
⇒ #<buffer bar>
```
```
(generate-new-buffer "bar")
⇒ #<buffer bar<2>>
```
```
(generate-new-buffer "bar")
⇒ #<buffer bar<3>>
```
The major mode for the new buffer is set to Fundamental mode. The default value of the variable `major-mode` is handled at a higher level. See [Auto Major Mode](auto-major-mode).
elisp None Lisp Data Types
---------------
A Lisp *object* is a piece of data used and manipulated by Lisp programs. For our purposes, a *type* or *data type* is a set of possible objects.
Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for *the* type of an object.
A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called *primitive types*. Each object belongs to one and only one primitive type. These types include *integer*, *float*, *cons*, *symbol*, *string*, *vector*, *hash-table*, *subr*, *byte-code function*, and *record*, plus several special types, such as *buffer*, that are related to editing. (See [Editing Types](editing-types).)
Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type.
Lisp is unlike many other languages in that its objects are *self-typing*: the primitive type of each object is implicit in the object itself. For example, if an object is a vector, nothing can treat it as a number; Lisp knows it is a vector, not a number.
In most languages, the programmer must declare the data type of each variable, and the type is known by the compiler but not represented in the data. Such type declarations do not exist in Emacs Lisp. A Lisp variable can have any type of value, and it remembers whatever value you store in it, type and all. (Actually, a small number of Emacs Lisp variables can only take on values of a certain type. See [Variables with Restricted Values](variables-with-restricted-values).)
This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters.
| | | |
| --- | --- | --- |
| • [Printed Representation](printed-representation) | | How Lisp objects are represented as text. |
| • [Special Read Syntax](special-read-syntax) | | An overview of all the special sequences. |
| • [Comments](comments) | | Comments and their formatting conventions. |
| • [Programming Types](programming-types) | | Types found in all Lisp systems. |
| • [Editing Types](editing-types) | | Types specific to Emacs. |
| • [Circular Objects](circular-objects) | | Read syntax for circular structure. |
| • [Type Predicates](type-predicates) | | Tests related to types. |
| • [Equality Predicates](equality-predicates) | | Tests of equality between any two objects. |
| • [Mutability](mutability) | | Some objects should not be modified. |
elisp None #### Evaluating Macro Arguments in Expansion
Another problem can happen if the macro definition itself evaluates any of the macro argument expressions, such as by calling `eval` (see [Eval](eval)). You have to take into account that macro expansion may take place long before the code is executed, when the context of the caller (where the macro expansion will be evaluated) is not yet accessible.
Also, if your macro definition does not use `lexical-binding`, its formal arguments may hide the user’s variables of the same name. Inside the macro body, the macro argument binding is the most local binding of such variable, so any references inside the form being evaluated do refer to it. Here is an example:
```
(defmacro foo (a)
(list 'setq (eval a) t))
```
```
(setq x 'b)
(foo x) → (setq b t)
⇒ t ; and `b` has been set.
;; but
(setq a 'c)
(foo a) → (setq a t)
⇒ t ; but this set `a`, not `c`.
```
It makes a difference whether the user’s variable is named `a` or `x`, because `a` conflicts with the macro argument variable `a`.
Also, the expansion of `(foo x)` above will return something different or signal an error when the code is compiled, since in that case `(foo x)` is expanded during compilation, whereas the execution of `(setq x 'b)` will only take place later when the code is executed.
To avoid these problems, **don’t evaluate an argument expression while computing the macro expansion**. Instead, substitute the expression into the macro expansion, so that its value will be computed as part of executing the expansion. This is how the other examples in this chapter work.
elisp None #### Overview
Quoting from the [spec](https://www.jsonrpc.org/), JSONRPC "is transport agnostic in that the concepts can be used within the same process, over sockets, over http, or in many various message passing environments."
To model this agnosticism, the `jsonrpc` library uses objects of a `jsonrpc-connection` class, which represent a connection to a remote JSON endpoint (for details on Emacs’s object system, see [EIEIO](https://www.gnu.org/software/emacs/manual/html_node/eieio/index.html#Top) in EIEIO). In modern object-oriented parlance, this class is “abstract”, i.e. the actual class of a useful connection object is always a subclass of `jsonrpc-connection`. Nevertheless, we can define two distinct APIs around the `jsonrpc-connection` class:
1. A user interface for building JSONRPC applications In this scenario, the JSONRPC application selects a concrete subclass of `jsonrpc-connection`, and proceeds to create objects of that subclass using `make-instance`. To initiate a contact to the remote endpoint, the JSONRPC application passes this object to the functions `jsonrpc-notify`, `jsonrpc-request`, and/or `jsonrpc-async-request`. For handling remotely initiated contacts, which generally come in asynchronously, the instantiation should include `:request-dispatcher` and `:notification-dispatcher` initargs, which are both functions of 3 arguments: the connection object; a symbol naming the JSONRPC method invoked remotely; and a JSONRPC `params` object.
The function passed as `:request-dispatcher` is responsible for handling the remote endpoint’s requests, which expect a reply from the local endpoint (in this case, the program you’re building). Inside that function, you may either return locally (a normal return) or non-locally (an error return). A local return value must be a Lisp object that can be serialized as JSON (see [Parsing JSON](parsing-json)). This determines a success response, and the object is forwarded to the server as the JSONRPC `result` object. A non-local return, achieved by calling the function `jsonrpc-error`, causes an error response to be sent to the server. The details of the accompanying JSONRPC `error` are filled out with whatever was passed to `jsonrpc-error`. A non-local return triggered by an unexpected error of any other type also causes an error response to be sent (unless you have set `debug-on-error`, in which case this calls the Lisp debugger, see [Error Debugging](error-debugging)).
2. A inheritance interface for building JSONRPC transport implementations In this scenario, `jsonrpc-connection` is subclassed to implement a different underlying transport strategy (for details on how to subclass, see [(eieio)Inheritance](https://www.gnu.org/software/emacs/manual/html_node/eieio/Inheritance.html#Inheritance).). Users of the application-building interface can then instantiate objects of this concrete class (using the `make-instance` function) and connect to JSONRPC endpoints using that strategy.
This API has mandatory and optional parts.
To allow its users to initiate JSONRPC contacts (notifications or requests) or reply to endpoint requests, the subclass must have an implementation of the `jsonrpc-connection-send` method.
Likewise, for handling the three types of remote contacts (requests, notifications, and responses to local requests), the transport implementation must arrange for the function `jsonrpc-connection-receive` to be called after noticing a new JSONRPC message on the wire (whatever that "wire" may be).
Finally, and optionally, the `jsonrpc-connection` subclass should implement the `jsonrpc-shutdown` and `jsonrpc-running-p` methods if these concepts apply to the transport. If they do, then any system resources (e.g. processes, timers, etc.) used to listen for messages on the wire should be released in `jsonrpc-shutdown`, i.e. they should only be needed while `jsonrpc-running-p` is non-nil.
elisp None #### Face Remapping
The variable `face-remapping-alist` is used for buffer-local or global changes in the appearance of a face. For instance, it is used to implement the `text-scale-adjust` command (see [Text Scale](https://www.gnu.org/software/emacs/manual/html_node/emacs/Text-Scale.html#Text-Scale) in The GNU Emacs Manual).
Variable: **face-remapping-alist**
The value of this variable is an alist whose elements have the form `(face . remapping)`. This causes Emacs to display any text having the face face with remapping, rather than the ordinary definition of face.
remapping may be any face spec suitable for a `face` text property: either a face (i.e., a face name or a property list of attribute/value pairs), or a list of faces. For details, see the description of the `face` text property in [Special Properties](special-properties). remapping serves as the complete specification for the remapped face—it replaces the normal definition of face, instead of modifying it.
If `face-remapping-alist` is buffer-local, its local value takes effect only within that buffer. If `face-remapping-alist` includes faces applicable only to certain windows, by using the `(:filtered (:window param val) spec)`, that face takes effect only in windows that match the filter conditions (see [Special Properties](special-properties)). To turn off face filtering temporarily, bind `face-filters-always-match` to a non-`nil` value, then all face filters will match any window.
Note: face remapping is non-recursive. If remapping references the same face name face, either directly or via the `:inherit` attribute of some other face in remapping, that reference uses the normal definition of face. For instance, if the `mode-line` face is remapped using this entry in `face-remapping-alist`:
```
(mode-line italic mode-line)
```
then the new definition of the `mode-line` face inherits from the `italic` face, and the *normal* (non-remapped) definition of `mode-line` face.
The following functions implement a higher-level interface to `face-remapping-alist`. Most Lisp code should use these functions instead of setting `face-remapping-alist` directly, to avoid trampling on remappings applied elsewhere. These functions are intended for buffer-local remappings, so they all make `face-remapping-alist` buffer-local as a side-effect. They manage `face-remapping-alist` entries of the form
```
(face relative-spec-1 relative-spec-2 ... base-spec)
```
where, as explained above, each of the relative-spec-N and base-spec is either a face name, or a property list of attribute/value pairs. Each of the *relative remapping* entries, relative-spec-N, is managed by the `face-remap-add-relative` and `face-remap-remove-relative` functions; these are intended for simple modifications like changing the text size. The *base remapping* entry, base-spec, has the lowest priority and is managed by the `face-remap-set-base` and `face-remap-reset-base` functions; it is intended for major modes to remap faces in the buffers they control.
Function: **face-remap-add-relative** *face &rest specs*
This function adds specs as relative remappings for face face in the current buffer. specs should be a list where each element is either a face name, or a property list of attribute/value pairs.
The return value is a Lisp object that serves as a cookie; you can pass this object as an argument to `face-remap-remove-relative` if you need to remove the remapping later.
```
;; Remap the 'escape-glyph' face into a combination
;; of the 'highlight' and 'italic' faces:
(face-remap-add-relative 'escape-glyph 'highlight 'italic)
;; Increase the size of the 'default' face by 50%:
(face-remap-add-relative 'default :height 1.5)
```
Function: **face-remap-remove-relative** *cookie*
This function removes a relative remapping previously added by `face-remap-add-relative`. cookie should be the Lisp object returned by `face-remap-add-relative` when the remapping was added.
Function: **face-remap-set-base** *face &rest specs*
This function sets the base remapping of face in the current buffer to specs. If specs is empty, the default base remapping is restored, similar to calling `face-remap-reset-base` (see below); note that this is different from specs containing a single value `nil`, which has the opposite result (the global definition of face is ignored).
This overwrites the default base-spec, which inherits the global face definition, so it is up to the caller to add such inheritance if so desired.
Function: **face-remap-reset-base** *face*
This function sets the base remapping of face to its default value, which inherits from face’s global definition.
elisp None Files
-----
This chapter describes the Emacs Lisp functions and variables to find, create, view, save, and otherwise work with files and directories. A few other file-related functions are described in [Buffers](buffers), and those related to backups and auto-saving are described in [Backups and Auto-Saving](backups-and-auto_002dsaving).
Many of the file functions take one or more arguments that are file names. A file name is a string. Most of these functions expand file name arguments using the function `expand-file-name`, so that `~` is handled correctly, as are relative file names (including `../` and the empty string). See [File Name Expansion](file-name-expansion).
In addition, certain *magic* file names are handled specially. For example, when a remote file name is specified, Emacs accesses the file over the network via an appropriate protocol. See [Remote Files](https://www.gnu.org/software/emacs/manual/html_node/emacs/Remote-Files.html#Remote-Files) in The GNU Emacs Manual. This handling is done at a very low level, so you may assume that all the functions described in this chapter accept magic file names as file name arguments, except where noted. See [Magic File Names](magic-file-names), for details.
When file I/O functions signal Lisp errors, they usually use the condition `file-error` (see [Handling Errors](handling-errors)). The error message is in most cases obtained from the operating system, according to locale `system-messages-locale`, and decoded using coding system `locale-coding-system` (see [Locales](locales)).
| | | |
| --- | --- | --- |
| • [Visiting Files](visiting-files) | | Reading files into Emacs buffers for editing. |
| • [Saving Buffers](saving-buffers) | | Writing changed buffers back into files. |
| • [Reading from Files](reading-from-files) | | Reading files into buffers without visiting. |
| • [Writing to Files](writing-to-files) | | Writing new files from parts of buffers. |
| • [File Locks](file-locks) | | Locking and unlocking files, to prevent simultaneous editing by two people. |
| • [Information about Files](information-about-files) | | Testing existence, accessibility, size of files. |
| • [Changing Files](changing-files) | | Renaming files, changing permissions, etc. |
| • [Files and Storage](files-and-storage) | | Surviving power and media failures |
| • [File Names](file-names) | | Decomposing and expanding file names. |
| • [Contents of Directories](contents-of-directories) | | Getting a list of the files in a directory. |
| • [Create/Delete Dirs](create_002fdelete-dirs) | | Creating and Deleting Directories. |
| • [Magic File Names](magic-file-names) | | Special handling for certain file names. |
| • [Format Conversion](format-conversion) | | Conversion to and from various file formats. |
| programming_docs |
elisp None #### Functions for Killing
`kill-region` is the usual subroutine for killing text. Any command that calls this function is a kill command (and should probably have ‘`kill`’ in its name). `kill-region` puts the newly killed text in a new element at the beginning of the kill ring or adds it to the most recent element. It determines automatically (using `last-command`) whether the previous command was a kill command, and if so appends the killed text to the most recent entry.
The commands described below can filter the killed text before they save it in the kill ring. They call `filter-buffer-substring` (see [Buffer Contents](buffer-contents)) to perform the filtering. By default, there’s no filtering, but major and minor modes and hook functions can set up filtering, so that text saved in the kill ring is different from what was in the buffer.
Command: **kill-region** *start end &optional region*
This function kills the stretch of text between start and end; but if the optional argument region is non-`nil`, it ignores start and end, and kills the text in the current region instead. The text is deleted but saved in the kill ring, along with its text properties. The value is always `nil`.
In an interactive call, start and end are point and the mark, and region is always non-`nil`, so the command always kills the text in the current region.
If the buffer or text is read-only, `kill-region` modifies the kill ring just the same, then signals an error without modifying the buffer. This is convenient because it lets the user use a series of kill commands to copy text from a read-only buffer into the kill ring.
User Option: **kill-read-only-ok**
If this option is non-`nil`, `kill-region` does not signal an error if the buffer or text is read-only. Instead, it simply returns, updating the kill ring but not changing the buffer.
Command: **copy-region-as-kill** *start end &optional region*
This function saves the stretch of text between start and end on the kill ring (including text properties), but does not delete the text from the buffer. However, if the optional argument region is non-`nil`, the function ignores start and end, and saves the current region instead. It always returns `nil`.
In an interactive call, start and end are point and the mark, and region is always non-`nil`, so the command always saves the text in the current region.
The command does not set `this-command` to `kill-region`, so a subsequent kill command does not append to the same kill ring entry.
elisp None ### Dialog Boxes
A dialog box is a variant of a pop-up menu—it looks a little different, it always appears in the center of a frame, and it has just one level and one or more buttons. The main use of dialog boxes is for asking questions that the user can answer with “yes”, “no”, and a few other alternatives. With a single button, they can also force the user to acknowledge important information. The functions `y-or-n-p` and `yes-or-no-p` use dialog boxes instead of the keyboard, when called from commands invoked by mouse clicks.
Function: **x-popup-dialog** *position contents &optional header*
This function displays a pop-up dialog box and returns an indication of what selection the user makes. The argument contents specifies the alternatives to offer; it has this format:
```
(title (string . value)…)
```
which looks like the list that specifies a single pane for `x-popup-menu`.
The return value is value from the chosen alternative.
As for `x-popup-menu`, an element of the list may be just a string instead of a cons cell `(string . value)`. That makes a box that cannot be selected.
If `nil` appears in the list, it separates the left-hand items from the right-hand items; items that precede the `nil` appear on the left, and items that follow the `nil` appear on the right. If you don’t include a `nil` in the list, then approximately half the items appear on each side.
Dialog boxes always appear in the center of a frame; the argument position specifies which frame. The possible values are as in `x-popup-menu`, but the precise coordinates or the individual window don’t matter; only the frame matters.
If header is non-`nil`, the frame title for the box is ‘`Information`’, otherwise it is ‘`Question`’. The former is used for `message-box` (see [message-box](displaying-messages#message_002dbox)). (On text terminals, the box title is not displayed.)
In some configurations, Emacs cannot display a real dialog box; so instead it displays the same items in a pop-up menu in the center of the frame.
If the user gets rid of the dialog box without making a valid choice, for instance using the window manager, then this produces a quit and `x-popup-dialog` does not return.
elisp None ### Modifying List Variables
These functions, and one macro, provide convenient ways to modify a list which is stored in a variable.
Macro: **push** *element listname*
This macro creates a new list whose CAR is element and whose CDR is the list specified by listname, and saves that list in listname. In the simplest case, listname is an unquoted symbol naming a list, and this macro is equivalent to `(setq listname (cons element listname))`.
```
(setq l '(a b))
⇒ (a b)
(push 'c l)
⇒ (c a b)
l
⇒ (c a b)
```
More generally, `listname` can be a generalized variable. In that case, this macro does the equivalent of `(setf listname (cons element listname))`. See [Generalized Variables](generalized-variables).
For the `pop` macro, which removes the first element from a list, See [List Elements](list-elements).
Two functions modify lists that are the values of variables.
Function: **add-to-list** *symbol element &optional append compare-fn*
This function sets the variable symbol by consing element onto the old value, if element is not already a member of that value. It returns the resulting list, whether updated or not. The value of symbol had better be a list already before the call. `add-to-list` uses compare-fn to compare element against existing list members; if compare-fn is `nil`, it uses `equal`.
Normally, if element is added, it is added to the front of symbol, but if the optional argument append is non-`nil`, it is added at the end.
The argument symbol is not implicitly quoted; `add-to-list` is an ordinary function, like `set` and unlike `setq`. Quote the argument yourself if that is what you want.
Do not use this function when symbol refers to a lexical variable.
Here’s a scenario showing how to use `add-to-list`:
```
(setq foo '(a b))
⇒ (a b)
(add-to-list 'foo 'c) ;; Add `c`.
⇒ (c a b)
(add-to-list 'foo 'b) ;; No effect.
⇒ (c a b)
foo ;; `foo` was changed.
⇒ (c a b)
```
An equivalent expression for `(add-to-list 'var
value)` is this:
```
(if (member value var)
var
(setq var (cons value var)))
```
Function: **add-to-ordered-list** *symbol element &optional order*
This function sets the variable symbol by inserting element into the old value, which must be a list, at the position specified by order. If element is already a member of the list, its position in the list is adjusted according to order. Membership is tested using `eq`. This function returns the resulting list, whether updated or not.
The order is typically a number (integer or float), and the elements of the list are sorted in non-decreasing numerical order.
order may also be omitted or `nil`. Then the numeric order of element stays unchanged if it already has one; otherwise, element has no numeric order. Elements without a numeric list order are placed at the end of the list, in no particular order.
Any other value for order removes the numeric order of element if it already has one; otherwise, it is equivalent to `nil`.
The argument symbol is not implicitly quoted; `add-to-ordered-list` is an ordinary function, like `set` and unlike `setq`. Quote the argument yourself if necessary.
The ordering information is stored in a hash table on symbol’s `list-order` property. symbol cannot refer to a lexical variable.
Here’s a scenario showing how to use `add-to-ordered-list`:
```
(setq foo '())
⇒ nil
(add-to-ordered-list 'foo 'a 1) ;; Add `a`.
⇒ (a)
(add-to-ordered-list 'foo 'c 3) ;; Add `c`.
⇒ (a c)
(add-to-ordered-list 'foo 'b 2) ;; Add `b`.
⇒ (a b c)
(add-to-ordered-list 'foo 'b 4) ;; Move `b`.
⇒ (a c b)
(add-to-ordered-list 'foo 'd) ;; Append `d`.
⇒ (a c b d)
(add-to-ordered-list 'foo 'e) ;; Add `e`.
⇒ (a c b e d)
foo ;; `foo` was changed.
⇒ (a c b e d)
```
elisp None ### Determining whether a Function is Safe to Call
Some major modes, such as SES, call functions that are stored in user files. (See [(ses)Simple Emacs Spreadsheet](https://www.gnu.org/software/emacs/manual/html_node/ses/index.html#Top), for more information on SES.) User files sometimes have poor pedigrees—you can get a spreadsheet from someone you’ve just met, or you can get one through email from someone you’ve never met. So it is risky to call a function whose source code is stored in a user file until you have determined that it is safe.
Function: **unsafep** *form &optional unsafep-vars*
Returns `nil` if form is a *safe* Lisp expression, or returns a list that describes why it might be unsafe. The argument unsafep-vars is a list of symbols known to have temporary bindings at this point; it is mainly used for internal recursive calls. The current buffer is an implicit argument, which provides a list of buffer-local bindings.
Being quick and simple, `unsafep` does a very light analysis and rejects many Lisp expressions that are actually safe. There are no known cases where `unsafep` returns `nil` for an unsafe expression. However, a safe Lisp expression can return a string with a `display` property, containing an associated Lisp expression to be executed after the string is inserted into a buffer. This associated expression can be a virus. In order to be safe, you must delete properties from all strings calculated by user code before inserting them into buffers.
elisp None #### Backslash Constructs in Regular Expressions
For the most part, ‘`\`’ followed by any character matches only that character. However, there are several exceptions: certain sequences starting with ‘`\`’ that have special meanings. Here is a table of the special ‘`\`’ constructs.
‘`\|`’
specifies an alternative. Two regular expressions a and b with ‘`\|`’ in between form an expression that matches anything that either a or b matches.
Thus, ‘`foo\|bar`’ matches either ‘`foo`’ or ‘`bar`’ but no other string.
‘`\|`’ applies to the largest possible surrounding expressions. Only a surrounding ‘`\( … \)`’ grouping can limit the grouping power of ‘`\|`’.
If you need full backtracking capability to handle multiple uses of ‘`\|`’, use the POSIX regular expression functions (see [POSIX Regexps](posix-regexps)).
‘`\{m\}`’
is a postfix operator that repeats the previous pattern exactly m times. Thus, ‘`x\{5\}`’ matches the string ‘`xxxxx`’ and nothing else. ‘`c[ad]\{3\}r`’ matches string such as ‘`caaar`’, ‘`cdddr`’, ‘`cadar`’, and so on.
‘`\{m,n\}`’
is a more general postfix operator that specifies repetition with a minimum of m repeats and a maximum of n repeats. If m is omitted, the minimum is 0; if n is omitted, there is no maximum. For both forms, m and n, if specified, may be no larger than 2\*\*16 - 1 .
For example, ‘`c[ad]\{1,2\}r`’ matches the strings ‘`car`’, ‘`cdr`’, ‘`caar`’, ‘`cadr`’, ‘`cdar`’, and ‘`cddr`’, and nothing else. ‘`\{0,1\}`’ or ‘`\{,1\}`’ is equivalent to ‘`?`’. ‘`\{0,\}`’ or ‘`\{,\}`’ is equivalent to ‘`\*`’. ‘`\{1,\}`’ is equivalent to ‘`+`’.
‘`\( … \)`’
is a grouping construct that serves three purposes:
1. To enclose a set of ‘`\|`’ alternatives for other operations. Thus, the regular expression ‘`\(foo\|bar\)x`’ matches either ‘`foox`’ or ‘`barx`’.
2. To enclose a complicated expression for the postfix operators ‘`\*`’, ‘`+`’ and ‘`?`’ to operate on. Thus, ‘`ba\(na\)\*`’ matches ‘`ba`’, ‘`bana`’, ‘`banana`’, ‘`bananana`’, etc., with any number (zero or more) of ‘`na`’ strings.
3. To record a matched substring for future reference with ‘`\digit`’ (see below).
This last application is not a consequence of the idea of a parenthetical grouping; it is a separate feature that was assigned as a second meaning to the same ‘`\( … \)`’ construct because, in practice, there was usually no conflict between the two meanings. But occasionally there is a conflict, and that led to the introduction of shy groups.
‘`\(?: … \)`’
is the *shy group* construct. A shy group serves the first two purposes of an ordinary group (controlling the nesting of other operators), but it does not get a number, so you cannot refer back to its value with ‘`\digit`’. Shy groups are particularly useful for mechanically-constructed regular expressions, because they can be added automatically without altering the numbering of ordinary, non-shy groups.
Shy groups are also called *non-capturing* or *unnumbered groups*.
‘`\(?num: … \)`’
is the *explicitly numbered group* construct. Normal groups get their number implicitly, based on their position, which can be inconvenient. This construct allows you to force a particular group number. There is no particular restriction on the numbering, e.g., you can have several groups with the same number in which case the last one to match (i.e., the rightmost match) will win. Implicitly numbered groups always get the smallest integer larger than the one of any previous group.
‘`\digit`’
matches the same text that matched the digitth occurrence of a grouping (‘`\( … \)`’) construct.
In other words, after the end of a group, the matcher remembers the beginning and end of the text matched by that group. Later on in the regular expression you can use ‘`\`’ followed by digit to match that same text, whatever it may have been.
The strings matching the first nine grouping constructs appearing in the entire regular expression passed to a search or matching function are assigned numbers 1 through 9 in the order that the open parentheses appear in the regular expression. So you can use ‘`\1`’ through ‘`\9`’ to refer to the text matched by the corresponding grouping constructs.
For example, ‘`\(.\*\)\1`’ matches any newline-free string that is composed of two identical halves. The ‘`\(.\*\)`’ matches the first half, which may be anything, but the ‘`\1`’ that follows must match the same exact text.
If a ‘`\( … \)`’ construct matches more than once (which can happen, for instance, if it is followed by ‘`\*`’), only the last match is recorded.
If a particular grouping construct in the regular expression was never matched—for instance, if it appears inside of an alternative that wasn’t used, or inside of a repetition that repeated zero times—then the corresponding ‘`\digit`’ construct never matches anything. To use an artificial example, ‘`\(foo\(b\*\)\|lose\)\2`’ cannot match ‘`lose`’: the second alternative inside the larger group matches it, but then ‘`\2`’ is undefined and can’t match anything. But it can match ‘`foobb`’, because the first alternative matches ‘`foob`’ and ‘`\2`’ matches ‘`b`’.
‘`\w`’
matches any word-constituent character. The editor syntax table determines which characters these are. See [Syntax Tables](syntax-tables).
‘`\W`’
matches any character that is not a word constituent.
‘`\scode`’
matches any character whose syntax is code. Here code is a character that represents a syntax code: thus, ‘`w`’ for word constituent, ‘`-`’ for whitespace, ‘`(`’ for open parenthesis, etc. To represent whitespace syntax, use either ‘`-`’ or a space character. See [Syntax Class Table](syntax-class-table), for a list of syntax codes and the characters that stand for them.
‘`\Scode`’
matches any character whose syntax is not code.
‘`\cc`’
matches any character whose category is c. Here c is a character that represents a category: thus, ‘`c`’ for Chinese characters or ‘`g`’ for Greek characters in the standard category table. You can see the list of all the currently defined categories with `M-x describe-categories RET`. You can also define your own categories in addition to the standard ones using the `define-category` function (see [Categories](categories)).
‘`\Cc`’ matches any character whose category is not c.
The following regular expression constructs match the empty string—that is, they don’t use up any characters—but whether they match depends on the context. For all, the beginning and end of the accessible portion of the buffer are treated as if they were the actual beginning and end of the buffer.
‘`\``’
matches the empty string, but only at the beginning of the buffer or string being matched against.
‘`\'`’
matches the empty string, but only at the end of the buffer or string being matched against.
‘`\=`’
matches the empty string, but only at point. (This construct is not defined when matching against a string.)
‘`\b`’
matches the empty string, but only at the beginning or end of a word. Thus, ‘`\bfoo\b`’ matches any occurrence of ‘`foo`’ as a separate word. ‘`\bballs?\b`’ matches ‘`ball`’ or ‘`balls`’ as a separate word.
‘`\b`’ matches at the beginning or end of the buffer (or string) regardless of what text appears next to it.
‘`\B`’
matches the empty string, but *not* at the beginning or end of a word, nor at the beginning or end of the buffer (or string).
‘`\<`’
matches the empty string, but only at the beginning of a word. ‘`\<`’ matches at the beginning of the buffer (or string) only if a word-constituent character follows.
‘`\>`’
matches the empty string, but only at the end of a word. ‘`\>`’ matches at the end of the buffer (or string) only if the contents end with a word-constituent character.
‘`\\_<`’
matches the empty string, but only at the beginning of a symbol. A symbol is a sequence of one or more word or symbol constituent characters. ‘`\\_<`’ matches at the beginning of the buffer (or string) only if a symbol-constituent character follows.
‘`\\_>`’
matches the empty string, but only at the end of a symbol. ‘`\\_>`’ matches at the end of the buffer (or string) only if the contents end with a symbol-constituent character.
Not every string is a valid regular expression. For example, a string that ends inside a character alternative without a terminating ‘`]`’ is invalid, and so is a string that ends with a single ‘`\`’. If an invalid regular expression is passed to any of the search functions, an `invalid-regexp` error is signaled.
elisp None ### Hooks for Loading
You can ask for code to be executed each time Emacs loads a library, by using the variable `after-load-functions`:
Variable: **after-load-functions**
This abnormal hook is run after loading a file. Each function in the hook is called with a single argument, the absolute filename of the file that was just loaded.
If you want code to be executed when a *particular* library is loaded, use the macro `with-eval-after-load`:
Macro: **with-eval-after-load** *library body…*
This macro arranges to evaluate body at the end of loading the file library, each time library is loaded. If library is already loaded, it evaluates body right away.
You don’t need to give a directory or extension in the file name library. Normally, you just give a bare file name, like this:
```
(with-eval-after-load "js" (define-key js-mode-map "\C-c\C-c" 'js-eval))
```
To restrict which files can trigger the evaluation, include a directory or an extension or both in library. Only a file whose absolute true name (i.e., the name with all symbolic links chased out) matches all the given name components will match. In the following example, `my\_inst.elc` or `my\_inst.elc.gz` in some directory `..../foo/bar` will trigger the evaluation, but not `my\_inst.el`:
```
(with-eval-after-load "foo/bar/my_inst.elc" …)
```
library can also be a feature (i.e., a symbol), in which case body is evaluated at the end of any file where `(provide library)` is called.
An error in body does not undo the load, but does prevent execution of the rest of body.
Normally, well-designed Lisp programs should not use `with-eval-after-load`. If you need to examine and set the variables defined in another library (those meant for outside use), you can do it immediately—there is no need to wait until the library is loaded. If you need to call functions defined by that library, you should load the library, preferably with `require` (see [Named Features](named-features)).
| programming_docs |
elisp None #### Sequence Types
A *sequence* is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp: *lists* and *arrays*.
Lists are the most commonly-used sequences. A list can hold elements of any type, and its length can be easily changed by adding or removing elements. See the next subsection for more about lists.
Arrays are fixed-length sequences. They are further subdivided into strings, vectors, char-tables and bool-vectors. Vectors can hold elements of any type, whereas string elements must be characters, and bool-vector elements must be `t` or `nil`. Char-tables are like vectors except that they are indexed by any valid character code. The characters in a string can have text properties like characters in a buffer (see [Text Properties](text-properties)), but vectors do not support text properties, even when their elements happen to be characters.
Lists, strings and the other array types also share important similarities. For example, all have a length l, and all have elements which can be indexed from zero to l minus one. Several functions, called sequence functions, accept any kind of sequence. For example, the function `length` reports the length of any kind of sequence. See [Sequences Arrays Vectors](sequences-arrays-vectors).
It is generally impossible to read the same sequence twice, since sequences are always created anew upon reading. If you read the read syntax for a sequence twice, you get two sequences with equal contents. There is one exception: the empty list `()` always stands for the same object, `nil`.
elisp None #### Warning Variables
Programs can customize how their warnings appear by binding the variables described in this section.
Variable: **warning-levels**
This list defines the meaning and severity order of the warning severity levels. Each element defines one severity level, and they are arranged in order of decreasing severity.
Each element has the form `(level string
function)`, where level is the severity level it defines. string specifies the textual description of this level. string should use ‘`%s`’ to specify where to put the warning type information, or it can omit the ‘`%s`’ so as not to include that information.
The optional function, if non-`nil`, is a function to call with no arguments, to get the user’s attention.
Normally you should not change the value of this variable.
Variable: **warning-prefix-function**
If non-`nil`, the value is a function to generate prefix text for warnings. Programs can bind the variable to a suitable function. `display-warning` calls this function with the warnings buffer current, and the function can insert text in it. That text becomes the beginning of the warning message.
The function is called with two arguments, the severity level and its entry in `warning-levels`. It should return a list to use as the entry (this value need not be an actual member of `warning-levels`). By constructing this value, the function can change the severity of the warning, or specify different handling for a given severity level.
If the variable’s value is `nil` then there is no function to call.
Variable: **warning-series**
Programs can bind this variable to `t` to say that the next warning should begin a series. When several warnings form a series, that means to leave point on the first warning of the series, rather than keep moving it for each warning so that it appears on the last one. The series ends when the local binding is unbound and `warning-series` becomes `nil` again.
The value can also be a symbol with a function definition. That is equivalent to `t`, except that the next warning will also call the function with no arguments with the warnings buffer current. The function can insert text which will serve as a header for the series of warnings.
Once a series has begun, the value is a marker which points to the buffer position in the warnings buffer of the start of the series.
The variable’s normal value is `nil`, which means to handle each warning separately.
Variable: **warning-fill-prefix**
When this variable is non-`nil`, it specifies a fill prefix to use for filling each warning’s text.
Variable: **warning-fill-column**
The column at which to fill warnings.
Variable: **warning-type-format**
This variable specifies the format for displaying the warning type in the warning message. The result of formatting the type this way gets included in the message under the control of the string in the entry in `warning-levels`. The default value is `" (%s)"`. If you bind it to `""` then the warning type won’t appear at all.
elisp None Macros
------
*Macros* enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the *expansion* of the macro.
Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.
If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See [Inline Functions](inline-functions).
| | | |
| --- | --- | --- |
| • [Simple Macro](simple-macro) | | A basic example. |
| • [Expansion](expansion) | | How, when and why macros are expanded. |
| • [Compiling Macros](compiling-macros) | | How macros are expanded by the compiler. |
| • [Defining Macros](defining-macros) | | How to write a macro definition. |
| • [Problems with Macros](problems-with-macros) | | Don’t evaluate the macro arguments too many times. Don’t hide the user’s variables. |
| • [Indenting Macros](indenting-macros) | | Specifying how to indent macro calls. |
elisp None ### Reading from Files
To copy the contents of a file into a buffer, use the function `insert-file-contents`. (Don’t use the command `insert-file` in a Lisp program, as that sets the mark.)
Function: **insert-file-contents** *filename &optional visit beg end replace*
This function inserts the contents of file filename into the current buffer after point. It returns a list of the absolute file name and the length of the data inserted. An error is signaled if filename is not the name of a file that can be read.
This function checks the file contents against the defined file formats, and converts the file contents if appropriate and also calls the functions in the list `after-insert-file-functions`. See [Format Conversion](format-conversion). Normally, one of the functions in the `after-insert-file-functions` list determines the coding system (see [Coding Systems](coding-systems)) used for decoding the file’s contents, including end-of-line conversion. However, if the file contains null bytes, it is by default visited without any code conversions. See [inhibit-null-byte-detection](lisp-and-coding-systems).
If visit is non-`nil`, this function additionally marks the buffer as unmodified and sets up various fields in the buffer so that it is visiting the file filename: these include the buffer’s visited file name and its last save file modtime. This feature is used by `find-file-noselect` and you probably should not use it yourself.
If beg and end are non-`nil`, they should be numbers that are byte offsets specifying the portion of the file to insert. In this case, visit must be `nil`. For example,
```
(insert-file-contents filename nil 0 500)
```
inserts the characters coded by the first 500 bytes of a file.
If beg or end happens to be in the middle of a character’s multibyte sequence, Emacs’s character code conversion will insert one or more eight-bit characters (a.k.a. “raw bytes”) (see [Character Sets](character-sets)) into the buffer. If you want to read part of a file this way, we recommend to bind `coding-system-for-read` to a suitable value around the call to this function (see [Specifying Coding Systems](specifying-coding-systems)), and to write Lisp code which will check for raw bytes at the boundaries, read the entire sequence of these bytes, and convert them back to valid characters.
If the argument replace is non-`nil`, it means to replace the contents of the buffer (actually, just the accessible portion) with the contents of the file. This is better than simply deleting the buffer contents and inserting the whole file, because (1) it preserves some marker positions and (2) it puts less data in the undo list.
It is possible to read a special file (such as a FIFO or an I/O device) with `insert-file-contents`, as long as replace and visit are `nil`.
Function: **insert-file-contents-literally** *filename &optional visit beg end replace*
This function works like `insert-file-contents` except that each byte in the file is handled separately, being converted into an eight-bit character if needed. It does not run `after-insert-file-functions`, and does not do format decoding, character code conversion, automatic uncompression, and so on.
If you want to pass a file name to another process so that another program can read the file, use the function `file-local-copy`; see [Magic File Names](magic-file-names).
elisp None ### Functions that Operate on Arrays
In this section, we describe the functions that accept all types of arrays.
Function: **arrayp** *object*
This function returns `t` if object is an array (i.e., a vector, a string, a bool-vector or a char-table).
```
(arrayp [a])
⇒ t
(arrayp "asdf")
⇒ t
(arrayp (syntax-table)) ;; A char-table.
⇒ t
```
Function: **aref** *arr index*
This function returns the indexth element of the array or record arr. The first element is at index zero.
```
(setq primes [2 3 5 7 11 13])
⇒ [2 3 5 7 11 13]
(aref primes 4)
⇒ 11
```
```
(aref "abcdefg" 1)
⇒ 98 ; ‘`b`’ is ASCII code 98.
```
See also the function `elt`, in [Sequence Functions](sequence-functions).
Function: **aset** *array index object*
This function sets the indexth element of array to be object. It returns object.
```
(setq w (vector 'foo 'bar 'baz))
⇒ [foo bar baz]
(aset w 0 'fu)
⇒ fu
w
⇒ [fu bar baz]
```
```
;; `copy-sequence` copies the string to be modified later.
(setq x (copy-sequence "asdfasfd"))
⇒ "asdfasfd"
(aset x 3 ?Z)
⇒ 90
x
⇒ "asdZasfd"
```
The array should be mutable. See [Mutability](mutability).
If array is a string and object is not a character, a `wrong-type-argument` error results. The function converts a unibyte string to multibyte if necessary to insert a character.
Function: **fillarray** *array object*
This function fills the array array with object, so that each element of array is object. It returns array.
```
(setq a (copy-sequence [a b c d e f g]))
⇒ [a b c d e f g]
(fillarray a 0)
⇒ [0 0 0 0 0 0 0]
a
⇒ [0 0 0 0 0 0 0]
```
```
(setq s (copy-sequence "When in the course"))
⇒ "When in the course"
(fillarray s ?-)
⇒ "------------------"
```
If array is a string and object is not a character, a `wrong-type-argument` error results.
The general sequence functions `copy-sequence` and `length` are often useful for objects known to be arrays. See [Sequence Functions](sequence-functions).
elisp None Lists
-----
A *list* represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list.
| | | |
| --- | --- | --- |
| • [Cons Cells](cons-cells) | | How lists are made out of cons cells. |
| • [List-related Predicates](list_002drelated-predicates) | | Is this object a list? Comparing two lists. |
| • [List Elements](list-elements) | | Extracting the pieces of a list. |
| • [Building Lists](building-lists) | | Creating list structure. |
| • [List Variables](list-variables) | | Modifying lists stored in variables. |
| • [Modifying Lists](modifying-lists) | | Storing new pieces into an existing list. |
| • [Sets And Lists](sets-and-lists) | | A list can represent a finite mathematical set. |
| • [Association Lists](association-lists) | | A list can represent a finite relation or mapping. |
| • [Property Lists](property-lists) | | A list of paired elements. |
elisp None #### Font Type
A *font* specifies how to display text on a graphical terminal. There are actually three separate font types—*font objects*, *font specs*, and *font entities*—each of which has slightly different properties. None of them have a read syntax; their print syntax looks like ‘`#<font-object>`’, ‘`#<font-spec>`’, and ‘`#<font-entity>`’ respectively. See [Low-Level Font](low_002dlevel-font), for a description of these Lisp objects.
elisp None #### Window Type
A *window* describes the portion of the screen that Emacs uses to display buffers. Every live window (see [Basic Windows](basic-windows)) has one associated buffer, whose contents appear in that window. By contrast, a given buffer may appear in one window, no window, or several windows. Windows are grouped on the screen into frames; each window belongs to one and only one frame. See [Frame Type](frame-type).
Though many windows may exist simultaneously, at any time one window is designated the *selected window* (see [Selecting Windows](selecting-windows)). This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer (see [Current Buffer](current-buffer)), but this is not necessarily the case.
Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.
```
(selected-window)
⇒ #<window 1 on objects.texi>
```
See [Windows](windows), for a description of the functions that work on windows.
elisp None ### Searching the Active Keymaps
Here is a pseudo-Lisp summary of how Emacs searches the active keymaps:
```
(or (if overriding-terminal-local-map
(find-in overriding-terminal-local-map))
(if overriding-local-map
(find-in overriding-local-map)
(or (find-in (get-char-property (point) 'keymap))
(find-in-any emulation-mode-map-alists)
(find-in-any minor-mode-overriding-map-alist)
(find-in-any minor-mode-map-alist)
(if (get-text-property (point) 'local-map)
(find-in (get-char-property (point) 'local-map))
(find-in (current-local-map)))))
(find-in (current-global-map)))
```
Here, find-in and find-in-any are pseudo functions that search in one keymap and in an alist of keymaps, respectively. Note that the `set-transient-map` function works by setting `overriding-terminal-local-map` (see [Controlling Active Maps](controlling-active-maps)).
In the above pseudo-code, if a key sequence starts with a mouse event (see [Mouse Events](mouse-events)), that event’s position is used instead of point, and the event’s buffer is used instead of the current buffer. In particular, this affects how the `keymap` and `local-map` properties are looked up. If a mouse event occurs on a string embedded with a `display`, `before-string`, or `after-string` property (see [Special Properties](special-properties)), and the string has a non-`nil` `keymap` or `local-map` property, that overrides the corresponding property in the underlying buffer text (i.e., the property specified by the underlying text is ignored).
When a key binding is found in one of the active keymaps, and that binding is a command, the search is over—the command is executed. However, if the binding is a symbol with a value or a string, Emacs replaces the input key sequences with the variable’s value or the string, and restarts the search of the active keymaps. See [Key Lookup](key-lookup).
The command which is finally found might also be remapped. See [Remapping Commands](remapping-commands).
elisp None #### Property Lists and Association Lists
Association lists (see [Association Lists](association-lists)) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant, since the property names must be distinct.
Property lists are better than association lists for attaching information to various Lisp function names or variables. If your program keeps all such information in one association list, it will typically need to search that entire list each time it checks for an association for a particular Lisp function name or variable, which could be slow. By contrast, if you keep the same information in the property lists of the function names or variables themselves, each search will scan only the length of one property list, which is usually short. This is why the documentation for a variable is recorded in a property named `variable-documentation`. The byte compiler likewise uses properties to record those functions needing special treatment.
However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to choose property names that are probably unique, such as by beginning the property name with the program’s usual name-prefix for variables and functions.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list.
elisp None ### Coordinates and Windows
This section describes functions that report positions of and within a window. Most of these functions report positions relative to an origin at the native position of the window’s frame (see [Frame Geometry](frame-geometry)). Some functions report positions relative to the origin of the display of the window’s frame. In any case, the origin has the coordinates (0, 0) and X and Y coordinates increase rightward and downward respectively.
For the following functions, X and Y coordinates are reported in integer character units, i.e., numbers of lines and columns respectively. On a graphical display, each “line” and “column” corresponds to the height and width of the default character specified by the frame’s default font (see [Frame Font](frame-font)).
Function: **window-edges** *&optional window body absolute pixelwise*
This function returns a list of the edge coordinates of window. If window is omitted or `nil`, it defaults to the selected window.
The return value has the form `(left top right
bottom)`. These list elements are, respectively, the X coordinate of the leftmost column occupied by the window, the Y coordinate of the topmost row, the X coordinate one column to the right of the rightmost column, and the Y coordinate one row down from the bottommost row.
Note that these are the actual outer edges of the window, including any of its decorations. On a text terminal, if the window has a neighbor on its right, its right edge includes the separator line between the window and its neighbor.
If the optional argument body is `nil`, this means to return the edges corresponding to the total size of window. body non-`nil` means to return the edges of window’s body. If body is non-`nil`, window must specify a live window.
If the optional argument absolute is `nil`, this means to return edges relative to the native position of window’s frame. absolute non-`nil` means to return coordinates relative to the origin (0, 0) of window’s display. On non-graphical systems this argument has no effect.
If the optional argument pixelwise is `nil`, this means to return the coordinates in terms of the default character width and height of window’s frame (see [Frame Font](frame-font)), rounded if necessary. pixelwise non-`nil` means to return the coordinates in pixels. Note that the pixel specified by right and bottom is immediately outside of these edges. If absolute is non-`nil`, pixelwise is implicitly non-`nil` too.
Function: **window-body-edges** *&optional window*
This function returns the edges of window’s body (see [Window Sizes](window-sizes)). Calling `(window-body-edges window)` is equivalent to calling `(window-edges window t)`, see above.
The following functions can be used to relate a set of frame-relative coordinates to a window:
Function: **window-at** *x y &optional frame*
This function returns the live window at the coordinates x and y given in default character sizes (see [Frame Font](frame-font)) relative to the native position of frame (see [Frame Geometry](frame-geometry)).
If there is no window at that position, the return value is `nil`. If frame is omitted or `nil`, it defaults to the selected frame.
Function: **coordinates-in-window-p** *coordinates window*
This function checks whether a window window occupies the frame relative coordinates coordinates, and if so, which part of the window that is. window should be a live window.
coordinates should be a cons cell of the form `(x
. y)`, where x and y are given in default character sizes (see [Frame Font](frame-font)) relative to the native position of window’s frame (see [Frame Geometry](frame-geometry)).
If there is no window at the specified position, the return value is `nil` . Otherwise, the return value is one of the following:
`(relx . rely)`
The coordinates are inside window. The numbers relx and rely are the equivalent window-relative coordinates for the specified position, counting from 0 at the top left corner of the window.
`mode-line`
The coordinates are in the mode line of window.
`header-line`
The coordinates are in the header line of window.
`tab-line`
The coordinates are in the tab line of window.
`right-divider`
The coordinates are in the divider separating window from a window on the right.
`bottom-divider`
The coordinates are in the divider separating window from a window beneath.
`vertical-line`
The coordinates are in the vertical line between window and its neighbor to the right. This value occurs only if the window doesn’t have a scroll bar; positions in a scroll bar are considered outside the window for these purposes.
`left-fringe` `right-fringe`
The coordinates are in the left or right fringe of the window.
`left-margin` `right-margin`
The coordinates are in the left or right margin of the window.
`nil` The coordinates are not in any part of window.
The function `coordinates-in-window-p` does not require a frame as argument because it always uses the frame that window is on.
The following functions return window positions in pixels, rather than character units. Though mostly useful on graphical displays, they can also be called on text terminals, where the screen area of each text character is taken to be one pixel.
Function: **window-pixel-edges** *&optional window*
This function returns a list of pixel coordinates for the edges of window. Calling `(window-pixel-edges window)` is equivalent to calling `(window-edges window nil nil t)`, see above.
Function: **window-body-pixel-edges** *&optional window*
This function returns the pixel edges of window’s body. Calling `(window-body-pixel-edges window)` is equivalent to calling `(window-edges window t nil t)`, see above.
The following functions return window positions in pixels, relative to the origin of the display screen rather than that of the frame:
Function: **window-absolute-pixel-edges** *&optional window*
This function returns the pixel coordinates of window relative to an origin at (0, 0) of the display of window’s frame. Calling `(window-absolute-pixel-edges)` is equivalent to calling `(window-edges window nil t t)`, see above.
Function: **window-absolute-body-pixel-edges** *&optional window*
This function returns the pixel coordinates of window’s body relative to an origin at (0, 0) of the display of window’s frame. Calling `(window-absolute-body-pixel-edges window)` is equivalent to calling `(window-edges window t t t)`, see above.
Combined with `set-mouse-absolute-pixel-position`, this function can be used to move the mouse pointer to an arbitrary buffer position visible in some window:
```
(let ((edges (window-absolute-body-pixel-edges))
(position (pos-visible-in-window-p nil nil t)))
(set-mouse-absolute-pixel-position
(+ (nth 0 edges) (nth 0 position))
(+ (nth 1 edges) (nth 1 position))))
```
On a graphical terminal this form “warps” the mouse cursor to the upper left corner of the glyph at the selected window’s point. A position calculated this way can be also used to show a tooltip window there.
The following function returns the screen coordinates of a buffer position visible in a window:
Function: **window-absolute-pixel-position** *&optional position window*
If the buffer position position is visible in window window, this function returns the display coordinates of the upper/left corner of the glyph at position. The return value is a cons of the X- and Y-coordinates of that corner, relative to an origin at (0, 0) of window’s display. It returns `nil` if position is not visible in window.
window must be a live window and defaults to the selected window. position defaults to the value of `window-point` of window.
This means that in order to move the mouse pointer to the position of point in the selected window, it’s sufficient to write:
```
(let ((position (window-absolute-pixel-position)))
(set-mouse-absolute-pixel-position
(car position) (cdr position)))
```
The following function returns the largest rectangle that can be inscribed in a window without covering text displayed in that window.
Function: **window-largest-empty-rectangle** *&optional window count min-width min-height positions left*
This function calculates the dimensions of the largest empty rectangle that can be inscribed in the specified window’s text area. window must be a live window and defaults to the selected one.
The return value is a triple of the width and the start and end y-coordinates of the largest rectangle that can be inscribed into the empty space (space not displaying any text) of the text area of window. No x-coordinates are returned by this function—any such rectangle is assumed to end at the right edge of window’s text area. If no empty space can be found, the return value is `nil`.
The optional argument count, if non-`nil`, specifies a maximum number of rectangles to return. This means that the return value is a list of triples specifying rectangles with the largest rectangle first. count can be also a cons cell whose car specifies the number of rectangles to return and whose CDR, if non-`nil`, states that all rectangles returned must be disjoint.
The optional arguments min-width and min-height, if non-`nil`, specify the minimum width and height of any rectangle returned.
The optional argument positions, if non-`nil`, is a cons cell whose CAR specifies the uppermost and whose CDR specifies the lowermost pixel position that must be covered by any rectangle returned. These positions measure from the start of the text area of window.
The optional argument left, if non-`nil`, means to return values suitable for buffers displaying right to left text. In that case, any rectangle returned is assumed to start at the left edge of window’s text area.
Note that this function has to retrieve the dimensions of each line of window’s glyph matrix via `window-lines-pixel-dimensions` (see [Size of Displayed Text](size-of-displayed-text)). Hence, this function may also return `nil` when the current glyph matrix of window is not up-to-date.
| programming_docs |
elisp None ### Managing a Fixed-Size Ring of Objects
A *ring* is a fixed-size data structure that supports insertion, deletion, rotation, and modulo-indexed reference and traversal. An efficient ring data structure is implemented by the `ring` package. It provides the functions listed in this section.
Note that several rings in Emacs, like the kill ring and the mark ring, are actually implemented as simple lists, *not* using the `ring` package; thus the following functions won’t work on them.
Function: **make-ring** *size*
This returns a new ring capable of holding size objects. size should be an integer.
Function: **ring-p** *object*
This returns `t` if object is a ring, `nil` otherwise.
Function: **ring-size** *ring*
This returns the maximum capacity of the ring.
Function: **ring-length** *ring*
This returns the number of objects that ring currently contains. The value will never exceed that returned by `ring-size`.
Function: **ring-elements** *ring*
This returns a list of the objects in ring, in order, newest first.
Function: **ring-copy** *ring*
This returns a new ring which is a copy of ring. The new ring contains the same (`eq`) objects as ring.
Function: **ring-empty-p** *ring*
This returns `t` if ring is empty, `nil` otherwise.
The newest element in the ring always has index 0. Higher indices correspond to older elements. Indices are computed modulo the ring length. Index -1 corresponds to the oldest element, -2 to the next-oldest, and so forth.
Function: **ring-ref** *ring index*
This returns the object in ring found at index index. index may be negative or greater than the ring length. If ring is empty, `ring-ref` signals an error.
Function: **ring-insert** *ring object*
This inserts object into ring, making it the newest element, and returns object.
If the ring is full, insertion removes the oldest element to make room for the new element.
Function: **ring-remove** *ring &optional index*
Remove an object from ring, and return that object. The argument index specifies which item to remove; if it is `nil`, that means to remove the oldest item. If ring is empty, `ring-remove` signals an error.
Function: **ring-insert-at-beginning** *ring object*
This inserts object into ring, treating it as the oldest element. The return value is not significant.
If the ring is full, this function removes the newest element to make room for the inserted element.
Function: **ring-resize** *ring size*
Set the size of ring to size. If the new size is smaller, then the oldest items in the ring are discarded.
If you are careful not to exceed the ring size, you can use the ring as a first-in-first-out queue. For example:
```
(let ((fifo (make-ring 5)))
(mapc (lambda (obj) (ring-insert fifo obj))
'(0 one "two"))
(list (ring-remove fifo) t
(ring-remove fifo) t
(ring-remove fifo)))
⇒ (0 t one t "two")
```
elisp None ### Using Lists as Sets
A list can represent an unordered mathematical set—simply consider a value an element of a set if it appears in the list, and ignore the order of the list. To form the union of two sets, use `append` (as long as you don’t mind having duplicate elements). You can remove `equal` duplicates using `delete-dups` or `seq-uniq`. Other useful functions for sets include `memq` and `delq`, and their `equal` versions, `member` and `delete`.
> **Common Lisp note:** Common Lisp has functions `union` (which avoids duplicate elements) and `intersection` for set operations. In Emacs Lisp, variants of these facilities are provided by the `cl-lib` library. See [Lists as Sets](https://www.gnu.org/software/emacs/manual/html_node/cl/Lists-as-Sets.html#Lists-as-Sets) in Common Lisp Extensions.
>
>
>
Function: **memq** *object list*
This function tests to see whether object is a member of list. If it is, `memq` returns a list starting with the first occurrence of object. Otherwise, it returns `nil`. The letter ‘`q`’ in `memq` says that it uses `eq` to compare object against the elements of the list. For example:
```
(memq 'b '(a b c b a))
⇒ (b c b a)
```
```
(memq '(2) '((1) (2))) ; The two `(2)`s need not be `eq`.
⇒ Unspecified; might be `nil` or `((2))`.
```
Function: **delq** *object list*
This function destructively removes all elements `eq` to object from list, and returns the resulting list. The letter ‘`q`’ in `delq` says that it uses `eq` to compare object against the elements of the list, like `memq` and `remq`.
Typically, when you invoke `delq`, you should use the return value by assigning it to the variable which held the original list. The reason for this is explained below.
The `delq` function deletes elements from the front of the list by simply advancing down the list, and returning a sublist that starts after those elements. For example:
```
(delq 'a '(a b c)) ≡ (cdr '(a b c))
```
When an element to be deleted appears in the middle of the list, removing it involves changing the CDRs (see [Setcdr](setcdr)).
```
(setq sample-list (list 'a 'b 'c '(4)))
⇒ (a b c (4))
```
```
(delq 'a sample-list)
⇒ (b c (4))
```
```
sample-list
⇒ (a b c (4))
```
```
(delq 'c sample-list)
⇒ (a b (4))
```
```
sample-list
⇒ (a b (4))
```
Note that `(delq 'c sample-list)` modifies `sample-list` to splice out the third element, but `(delq 'a sample-list)` does not splice anything—it just returns a shorter list. Don’t assume that a variable which formerly held the argument list now has fewer elements, or that it still holds the original list! Instead, save the result of `delq` and use that. Most often we store the result back into the variable that held the original list:
```
(setq flowers (delq 'rose flowers))
```
In the following example, the `(list 4)` that `delq` attempts to match and the `(4)` in the `sample-list` are `equal` but not `eq`:
```
(delq (list 4) sample-list)
⇒ (a c (4))
```
If you want to delete elements that are `equal` to a given value, use `delete` (see below).
Function: **remq** *object list*
This function returns a copy of list, with all elements removed which are `eq` to object. The letter ‘`q`’ in `remq` says that it uses `eq` to compare object against the elements of `list`.
```
(setq sample-list (list 'a 'b 'c 'a 'b 'c))
⇒ (a b c a b c)
```
```
(remq 'a sample-list)
⇒ (b c b c)
```
```
sample-list
⇒ (a b c a b c)
```
Function: **memql** *object list*
The function `memql` tests to see whether object is a member of list, comparing members with object using `eql`, so floating-point elements are compared by value. If object is a member, `memql` returns a list starting with its first occurrence in list. Otherwise, it returns `nil`.
Compare this with `memq`:
```
(memql 1.2 '(1.1 1.2 1.3)) ; `1.2` and `1.2` are `eql`.
⇒ (1.2 1.3)
```
```
(memq 1.2 '(1.1 1.2 1.3)) ; The two `1.2`s need not be `eq`.
⇒ Unspecified; might be `nil` or `(1.2 1.3)`.
```
The following three functions are like `memq`, `delq` and `remq`, but use `equal` rather than `eq` to compare elements. See [Equality Predicates](equality-predicates).
Function: **member** *object list*
The function `member` tests to see whether object is a member of list, comparing members with object using `equal`. If object is a member, `member` returns a list starting with its first occurrence in list. Otherwise, it returns `nil`.
Compare this with `memq`:
```
(member '(2) '((1) (2))) ; `(2)` and `(2)` are `equal`.
⇒ ((2))
```
```
(memq '(2) '((1) (2))) ; The two `(2)`s need not be `eq`.
⇒ Unspecified; might be `nil` or `(2)`.
```
```
;; Two strings with the same contents are `equal`.
(member "foo" '("foo" "bar"))
⇒ ("foo" "bar")
```
Function: **delete** *object sequence*
This function removes all elements `equal` to object from sequence, and returns the resulting sequence.
If sequence is a list, `delete` is to `delq` as `member` is to `memq`: it uses `equal` to compare elements with object, like `member`; when it finds an element that matches, it cuts the element out just as `delq` would. As with `delq`, you should typically use the return value by assigning it to the variable which held the original list.
If `sequence` is a vector or string, `delete` returns a copy of `sequence` with all elements `equal` to `object` removed.
For example:
```
(setq l (list '(2) '(1) '(2)))
(delete '(2) l)
⇒ ((1))
l
⇒ ((2) (1))
;; If you want to change `l` reliably,
;; write `(setq l (delete '(2) l))`.
```
```
(setq l (list '(2) '(1) '(2)))
(delete '(1) l)
⇒ ((2) (2))
l
⇒ ((2) (2))
;; In this case, it makes no difference whether you set `l`,
;; but you should do so for the sake of the other case.
```
```
(delete '(2) [(2) (1) (2)])
⇒ [(1)]
```
Function: **remove** *object sequence*
This function is the non-destructive counterpart of `delete`. It returns a copy of `sequence`, a list, vector, or string, with elements `equal` to `object` removed. For example:
```
(remove '(2) '((2) (1) (2)))
⇒ ((1))
```
```
(remove '(2) [(2) (1) (2)])
⇒ [(1)]
```
> **Common Lisp note:** The functions `member`, `delete` and `remove` in GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not use `equal` to compare elements.
>
>
>
Function: **member-ignore-case** *object list*
This function is like `member`, except that object should be a string and that it ignores differences in letter-case and text representation: upper-case and lower-case letters are treated as equal, and unibyte strings are converted to multibyte prior to comparison.
Function: **delete-dups** *list*
This function destructively removes all `equal` duplicates from list, stores the result in list and returns it. Of several `equal` occurrences of an element in list, `delete-dups` keeps the first one. See `seq-uniq` for non-destructive operation (see [Sequence Functions](sequence-functions)).
See also the function `add-to-list`, in [List Variables](list-variables), for a way to add an element to a list stored in a variable and used as a set.
elisp None #### Property Lists Outside Symbols
The following functions can be used to manipulate property lists. They all compare property names using `eq`.
Function: **plist-get** *plist property*
This returns the value of the property property stored in the property list plist. It accepts a malformed plist argument. If property is not found in the plist, it returns `nil`. For example,
```
(plist-get '(foo 4) 'foo)
⇒ 4
(plist-get '(foo 4 bad) 'foo)
⇒ 4
(plist-get '(foo 4 bad) 'bad)
⇒ nil
(plist-get '(foo 4 bad) 'bar)
⇒ nil
```
Function: **plist-put** *plist property value*
This stores value as the value of the property property in the property list plist. It may modify plist destructively, or it may construct a new list structure without altering the old. The function returns the modified property list, so you can store that back in the place where you got plist. For example,
```
(setq my-plist (list 'bar t 'foo 4))
⇒ (bar t foo 4)
(setq my-plist (plist-put my-plist 'foo 69))
⇒ (bar t foo 69)
(setq my-plist (plist-put my-plist 'quux '(a)))
⇒ (bar t foo 69 quux (a))
```
Function: **lax-plist-get** *plist property*
Like `plist-get` except that it compares properties using `equal` instead of `eq`.
Function: **lax-plist-put** *plist property value*
Like `plist-put` except that it compares properties using `equal` instead of `eq`.
Function: **plist-member** *plist property*
This returns non-`nil` if plist contains the given property. Unlike `plist-get`, this allows you to distinguish between a missing property and a property with the value `nil`. The value is actually the tail of plist whose `car` is property.
elisp None ### Disabling Multibyte Characters
By default, Emacs starts in multibyte mode: it stores the contents of buffers and strings using an internal encoding that represents non-ASCII characters using multi-byte sequences. Multibyte mode allows you to use all the supported languages and scripts without limitations.
Under very special circumstances, you may want to disable multibyte character support, for a specific buffer. When multibyte characters are disabled in a buffer, we call that *unibyte mode*. In unibyte mode, each character in the buffer has a character code ranging from 0 through 255 (0377 octal); 0 through 127 (0177 octal) represent ASCII characters, and 128 (0200 octal) through 255 (0377 octal) represent non-ASCII characters.
To edit a particular file in unibyte representation, visit it using `find-file-literally`. See [Visiting Functions](visiting-functions). You can convert a multibyte buffer to unibyte by saving it to a file, killing the buffer, and visiting the file again with `find-file-literally`. Alternatively, you can use `C-x RET c` (`universal-coding-system-argument`) and specify ‘`raw-text`’ as the coding system with which to visit or save a file. See [Specifying a Coding System for File Text](https://www.gnu.org/software/emacs/manual/html_node/emacs/Text-Coding.html#Text-Coding) in GNU Emacs Manual. Unlike `find-file-literally`, finding a file as ‘`raw-text`’ doesn’t disable format conversion, uncompression, or auto mode selection.
The buffer-local variable `enable-multibyte-characters` is non-`nil` in multibyte buffers, and `nil` in unibyte ones. The mode line also indicates whether a buffer is multibyte or not. With a graphical display, in a multibyte buffer, the portion of the mode line that indicates the character set has a tooltip that (amongst other things) says that the buffer is multibyte. In a unibyte buffer, the character set indicator is absent. Thus, in a unibyte buffer (when using a graphical display) there is normally nothing before the indication of the visited file’s end-of-line convention (colon, backslash, etc.), unless you are using an input method.
You can turn off multibyte support in a specific buffer by invoking the command `toggle-enable-multibyte-characters` in that buffer.
elisp None ### Creating and Maintaining Package Archives
Via the Package Menu, users may download packages from *package archives*. Such archives are specified by the variable `package-archives`, whose default value lists the archives hosted on [GNU ELPA](https://elpa.gnu.org) and [non-GNU ELPA](https://elpa.nongnu.org). This section describes how to set up and maintain a package archive.
User Option: **package-archives**
The value of this variable is an alist of package archives recognized by the Emacs package manager.
Each alist element corresponds to one archive, and should have the form `(id . location)`, where id is the name of the archive (a string) and location is its *base location* (a string).
If the base location starts with ‘`http:`’ or ‘`https:`’, it is treated as an HTTP(S) URL, and packages are downloaded from this archive via HTTP(S) (as is the case for the default GNU archive).
Otherwise, the base location should be a directory name. In this case, Emacs retrieves packages from this archive via ordinary file access. Such local archives are mainly useful for testing.
A package archive is simply a directory in which the package files, and associated files, are stored. If you want the archive to be reachable via HTTP, this directory must be accessible to a web server; See [Archive Web Server](archive-web-server).
A convenient way to set up and update a package archive is via the `package-x` library. This is included with Emacs, but not loaded by default; type `M-x load-library RET package-x RET` to load it, or add `(require 'package-x)` to your init file. See [Lisp Libraries](https://www.gnu.org/software/emacs/manual/html_node/emacs/Lisp-Libraries.html#Lisp-Libraries) in The GNU Emacs Manual.
After you create an archive, remember that it is not accessible in the Package Menu interface unless it is in `package-archives`.
Maintaining a public package archive entails a degree of responsibility. When Emacs users install packages from your archive, those packages can cause Emacs to run arbitrary code with the permissions of the installing user. (This is true for Emacs code in general, not just for packages.) So you should ensure that your archive is well-maintained and keep the hosting system secure.
One way to increase the security of your packages is to *sign* them using a cryptographic key. If you have generated a private/public gpg key pair, you can use gpg to sign the package like this:
```
gpg -ba -o file.sig file
```
For a single-file package, file is the package Lisp file; for a multi-file package, it is the package tar file. You can also sign the archive’s contents file in the same way. Make the `.sig` files available in the same location as the packages. You should also make your public key available for people to download; e.g., by uploading it to a key server such as <https://pgp.mit.edu/>. When people install packages from your archive, they can use your public key to verify the signatures.
A full explanation of these matters is outside the scope of this manual. For more information on cryptographic keys and signing, see [GnuPG](https://www.gnupg.org/documentation/manuals/gnupg/index.html#Top) in The GNU Privacy Guard Manual. Emacs comes with an interface to GNU Privacy Guard, see [EasyPG](https://www.gnu.org/software/emacs/manual/html_node/epa/index.html#Top) in Emacs EasyPG Assistant Manual.
elisp None #### Completion and the Minibuffer
This section describes the basic interface for reading from the minibuffer with completion.
Function: **completing-read** *prompt collection &optional predicate require-match initial history default inherit-input-method*
This function reads a string in the minibuffer, assisting the user by providing completion. It activates the minibuffer with prompt prompt, which must be a string.
The actual completion is done by passing the completion table collection and the completion predicate predicate to the function `try-completion` (see [Basic Completion](basic-completion)). This happens in certain commands bound in the local keymaps used for completion. Some of these commands also call `test-completion`. Thus, if predicate is non-`nil`, it should be compatible with collection and `completion-ignore-case`. See [Definition of test-completion](basic-completion#Definition-of-test_002dcompletion).
See [Programmed Completion](programmed-completion), for detailed requirements when collection is a function.
The value of the optional argument require-match determines how the user may exit the minibuffer:
* If `nil`, the usual minibuffer exit commands work regardless of the input in the minibuffer.
* If `t`, the usual minibuffer exit commands won’t exit unless the input completes to an element of collection.
* If `confirm`, the user can exit with any input, but is asked for confirmation if the input is not an element of collection.
* If `confirm-after-completion`, the user can exit with any input, but is asked for confirmation if the preceding command was a completion command (i.e., one of the commands in `minibuffer-confirm-exit-commands`) and the resulting input is not an element of collection. See [Completion Commands](completion-commands).
* Any other value of require-match behaves like `t`, except that the exit commands won’t exit if it performs completion.
However, empty input is always permitted, regardless of the value of require-match; in that case, `completing-read` returns the first element of default, if it is a list; `""`, if default is `nil`; or default. The string or strings in default are also available to the user through the history commands.
The function `completing-read` uses `minibuffer-local-completion-map` as the keymap if require-match is `nil`, and uses `minibuffer-local-must-match-map` if require-match is non-`nil`. See [Completion Commands](completion-commands).
The argument history specifies which history list variable to use for saving the input and for minibuffer history commands. It defaults to `minibuffer-history`. If history is the symbol `t`, history is not recorded. See [Minibuffer History](minibuffer-history).
The argument initial is mostly deprecated; we recommend using a non-`nil` value only in conjunction with specifying a cons cell for history. See [Initial Input](initial-input). For default input, use default instead.
If the argument inherit-input-method is non-`nil`, then the minibuffer inherits the current input method (see [Input Methods](input-methods)) and the setting of `enable-multibyte-characters` (see [Text Representations](text-representations)) from whichever buffer was current before entering the minibuffer.
If the variable `completion-ignore-case` is non-`nil`, completion ignores case when comparing the input against the possible matches. See [Basic Completion](basic-completion). In this mode of operation, predicate must also ignore case, or you will get surprising results.
Here’s an example of using `completing-read`:
```
(completing-read
"Complete a foo: "
'(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
nil t "fo")
```
```
;; After evaluation of the preceding expression,
;; the following appears in the minibuffer:
---------- Buffer: Minibuffer ----------
Complete a foo: fo∗
---------- Buffer: Minibuffer ----------
```
If the user then types `DEL DEL b RET`, `completing-read` returns `barfoo`.
The `completing-read` function binds variables to pass information to the commands that actually do completion. They are described in the following section.
Variable: **completing-read-function**
The value of this variable must be a function, which is called by `completing-read` to actually do its work. It should accept the same arguments as `completing-read`. This can be bound to a different function to completely override the normal behavior of `completing-read`.
| programming_docs |
elisp None Frames
------
A *frame* is a screen object that contains one or more Emacs windows (see [Windows](windows)). It is the kind of object called a “window” in the terminology of graphical environments; but we can’t call it a “window” here, because Emacs uses that word in a different way. In Emacs Lisp, a *frame object* is a Lisp object that represents a frame on the screen. See [Frame Type](frame-type).
A frame initially contains a single main window and/or a minibuffer window; you can subdivide the main window vertically or horizontally into smaller windows. See [Splitting Windows](splitting-windows).
A *terminal* is a display device capable of displaying one or more Emacs frames. In Emacs Lisp, a *terminal object* is a Lisp object that represents a terminal. See [Terminal Type](terminal-type).
There are two classes of terminals: *text terminals* and *graphical terminals*. Text terminals are non-graphics-capable displays, including `xterm` and other terminal emulators. On a text terminal, each Emacs frame occupies the terminal’s entire screen; although you can create additional frames and switch between them, the terminal only shows one frame at a time. Graphical terminals, on the other hand, are managed by graphical display systems such as the X Window System, which allow Emacs to show multiple frames simultaneously on the same display.
On GNU and Unix systems, you can create additional frames on any available terminal, within a single Emacs session, regardless of whether Emacs was started on a text or graphical terminal. Emacs can display on both graphical and text terminals simultaneously. This comes in handy, for instance, when you connect to the same session from several remote locations. See [Multiple Terminals](multiple-terminals).
Function: **framep** *object*
This predicate returns a non-`nil` value if object is a frame, and `nil` otherwise. For a frame, the value indicates which kind of display the frame uses:
`t` The frame is displayed on a text terminal.
`x` The frame is displayed on an X graphical terminal.
`w32` The frame is displayed on a MS-Windows graphical terminal.
`ns` The frame is displayed on a GNUstep or Macintosh Cocoa graphical terminal.
`pc` The frame is displayed on an MS-DOS terminal.
Function: **frame-terminal** *&optional frame*
This function returns the terminal object that displays frame. If frame is `nil` or unspecified, it defaults to the selected frame.
Function: **terminal-live-p** *object*
This predicate returns a non-`nil` value if object is a terminal that is live (i.e., not deleted), and `nil` otherwise. For live terminals, the return value indicates what kind of frames are displayed on that terminal; the list of possible values is the same as for `framep` above.
On a graphical terminal we distinguish two types of frames: A normal *top-level frame* is a frame whose window-system window is a child of the window-system’s root window for that terminal. A child frame is a frame whose window-system window is the child of the window-system window of another Emacs frame. See [Child Frames](child-frames).
| | | |
| --- | --- | --- |
| • [Creating Frames](creating-frames) | | Creating additional frames. |
| • [Multiple Terminals](multiple-terminals) | | Displaying on several different devices. |
| • [Frame Geometry](frame-geometry) | | Geometric properties of frames. |
| • [Frame Parameters](frame-parameters) | | Controlling frame size, position, font, etc. |
| • [Terminal Parameters](terminal-parameters) | | Parameters common for all frames on terminal. |
| • [Frame Titles](frame-titles) | | Automatic updating of frame titles. |
| • [Deleting Frames](deleting-frames) | | Frames last until explicitly deleted. |
| • [Finding All Frames](finding-all-frames) | | How to examine all existing frames. |
| • [Minibuffers and Frames](minibuffers-and-frames) | | How a frame finds the minibuffer to use. |
| • [Input Focus](input-focus) | | Specifying the selected frame. |
| • [Visibility of Frames](visibility-of-frames) | | Frames may be visible or invisible, or icons. |
| • [Raising and Lowering](raising-and-lowering) | | Raising, Lowering and Restacking Frames. |
| • [Frame Configurations](frame-configurations) | | Saving the state of all frames. |
| • [Child Frames](child-frames) | | Making a frame the child of another. |
| • [Mouse Tracking](mouse-tracking) | | Getting events that say when the mouse moves. |
| • [Mouse Position](mouse-position) | | Asking where the mouse is, or moving it. |
| • [Pop-Up Menus](pop_002dup-menus) | | Displaying a menu for the user to select from. |
| • [Dialog Boxes](dialog-boxes) | | Displaying a box to ask yes or no. |
| • [Pointer Shape](pointer-shape) | | Specifying the shape of the mouse pointer. |
| • [Window System Selections](window-system-selections) | | Transferring text to and from other X clients. |
| • [Drag and Drop](drag-and-drop) | | Internals of Drag-and-Drop implementation. |
| • [Color Names](color-names) | | Getting the definitions of color names. |
| • [Text Terminal Colors](text-terminal-colors) | | Defining colors for text terminals. |
| • [Resources](resources) | | Getting resource values from the server. |
| • [Display Feature Testing](display-feature-testing) | | Determining the features of a terminal. |
elisp None #### Describing Data Layout
To control unpacking and packing, you write a *data layout specification*, also called a *Bindat type expression*. This can be a *base type* or a *composite type* made of several fields, where the specification controls the length of each field to be processed, and how to pack or unpack it. We normally keep bindat type values in variables whose names end in `-bindat-spec`; that kind of name is automatically recognized as risky (see [File Local Variables](file-local-variables)).
Macro: **bindat-type** *&rest type*
Creates a Bindat type *value* object according to the Bindat type *expression* type.
A field’s *type* describes the size (in bytes) of the object that the field represents and, in the case of multibyte fields, how the bytes are ordered within the field. The two possible orderings are *big endian* (also known as “network byte ordering”) and *little endian*. For instance, the number `#x23cd` (decimal 9165) in big endian would be the two bytes `#x23` `#xcd`; and in little endian, `#xcd` `#x23`. Here are the possible type values:
`u8` `byte`
Unsigned byte, with length 1.
`uint bitlen`
Unsigned integer in network byte order, with bitlen bits. bitlen has to be a multiple of 8.
`uintr bitlen`
Unsigned integer in little endian order, with bitlen bits. bitlen has to be a multiple of 8.
`str len`
String of bytes of length len.
`strz &optional len`
Zero-terminated string of bytes, can be of arbitrary length or in a fixed-size field with length len.
`vec len [type]`
Vector of len elements. The type of the elements is given by type, defaulting to bytes. The type can be any Bindat type expression.
`repeat len [type]`
Like `vec`, but it unpacks to and packs from lists, whereas `vec` unpacks to vectors.
`bits len`
List of bits that are set to 1 in len bytes. The bytes are taken in big-endian order, and the bits are numbered starting with `8 * len - 1` and ending with zero. For example: `bits 2` unpacks `#x28` `#x1c` to `(2 3 4 11 13)` and `#x1c` `#x28` to `(3 5 10 11 12)`.
`fill len`
len bytes used as a mere filler. In packing, these bytes are are left unchanged, which normally means they remain zero. When unpacking, this just returns nil.
`align len`
Same as `fill` except the number of bytes is that needed to skip to the next multiple of len bytes.
`type exp`
This lets you refer to a type indirectly: exp is a Lisp expression which should return a Bindat type *value*.
`unit exp`
This is a trivial type which uses up 0 bits of space. exp describes the value returned when we try to “unpack” such a field.
`struct fields...` Composite type made of several fields. Every field is of the form `(name type)` where type can be any Bindat type expression. name can be `_` when the field’s value does not deserve to be named, as is often the case for `align` and `fill` fields. When the context makes it clear that this is a Bindat type expression, the symbol `struct` can be omitted.
In the types above, len and bitlen are given as an integer specifying the number of bytes (or bits) in the field. When the length of a field is not fixed, it typically depends on the value of preceding fields. For this reason, the length len does not have to be a constant but can be any Lisp expression and it can refer to the value of previous fields via their name.
For example, the specification of a data layout where a leading byte gives the size of a subsequent vector of 16 bit integers could be:
```
(bindat-type
(len u8)
(payload vec (1+ len) uint 16))
```
elisp None Standard Errors
----------------
Here is a list of the more important error symbols in standard Emacs, grouped by concept. The list includes each symbol’s message and a cross reference to a description of how the error can occur.
Each error symbol has a set of parent error conditions that is a list of symbols. Normally this list includes the error symbol itself and the symbol `error`. Occasionally it includes additional symbols, which are intermediate classifications, narrower than `error` but broader than a single error symbol. For example, all the errors in accessing files have the condition `file-error`. If we do not say here that a certain error symbol has additional error conditions, that means it has none.
As a special exception, the error symbols `quit` and `minibuffer-quit` don’t have the condition `error`, because quitting is not considered an error.
Most of these error symbols are defined in C (mainly `data.c`), but some are defined in Lisp. For example, the file `userlock.el` defines the `file-locked` and `file-supersession` errors. Several of the specialized Lisp libraries distributed with Emacs define their own error symbols. We do not attempt to list of all those here.
See [Errors](errors), for an explanation of how errors are generated and handled.
`error`
The message is ‘`error`’. See [Errors](errors).
`quit`
The message is ‘`Quit`’. See [Quitting](quitting).
`minibuffer-quit`
The message is ‘`Quit`’. This is a subcategory of `quit`. See [Quitting](quitting).
`args-out-of-range`
The message is ‘`Args out of range`’. This happens when trying to access an element beyond the range of a sequence, buffer, or other container-like object. See [Sequences Arrays Vectors](sequences-arrays-vectors), and see [Text](text).
`arith-error`
The message is ‘`Arithmetic error`’. This occurs when trying to perform integer division by zero. See [Numeric Conversions](numeric-conversions), and see [Arithmetic Operations](arithmetic-operations).
`beginning-of-buffer`
The message is ‘`Beginning of buffer`’. See [Character Motion](character-motion).
`buffer-read-only`
The message is ‘`Buffer is read-only`’. See [Read Only Buffers](read-only-buffers).
`circular-list`
The message is ‘`List contains a loop`’. This happens when a circular structure is encountered. See [Circular Objects](circular-objects).
`cl-assertion-failed`
The message is ‘`Assertion failed`’. This happens when the `cl-assert` macro fails a test. See [Assertions](https://www.gnu.org/software/emacs/manual/html_node/cl/Assertions.html#Assertions) in Common Lisp Extensions.
`coding-system-error`
The message is ‘`Invalid coding system`’. See [Lisp and Coding Systems](lisp-and-coding-systems).
`cyclic-function-indirection`
The message is ‘`Symbol's chain of function indirections contains a loop`’. See [Function Indirection](function-indirection).
`cyclic-variable-indirection`
The message is ‘`Symbol's chain of variable indirections contains a loop`’. See [Variable Aliases](variable-aliases).
`dbus-error`
The message is ‘`D-Bus error`’. See [Errors and Events](https://www.gnu.org/software/emacs/manual/html_node/dbus/Errors-and-Events.html#Errors-and-Events) in D-Bus integration in Emacs.
`end-of-buffer`
The message is ‘`End of buffer`’. See [Character Motion](character-motion).
`end-of-file`
The message is ‘`End of file during parsing`’. Note that this is not a subcategory of `file-error`, because it pertains to the Lisp reader, not to file I/O. See [Input Functions](input-functions).
`file-already-exists`
This is a subcategory of `file-error`. See [Writing to Files](writing-to-files).
`file-date-error`
This is a subcategory of `file-error`. It occurs when `copy-file` tries and fails to set the last-modification time of the output file. See [Changing Files](changing-files).
`file-error`
We do not list the error-strings of this error and its subcategories, because the error message is normally constructed from the data items alone when the error condition `file-error` is present. Thus, the error-strings are not very relevant. However, these error symbols do have `error-message` properties, and if no data is provided, the `error-message` property *is* used. See [Files](files).
`file-missing`
This is a subcategory of `file-error`. It occurs when an operation attempts to act on a file that is missing. See [Changing Files](changing-files).
`compression-error`
This is a subcategory of `file-error`, which results from problems handling a compressed file. See [How Programs Do Loading](how-programs-do-loading).
`file-locked`
This is a subcategory of `file-error`. See [File Locks](file-locks).
`file-supersession`
This is a subcategory of `file-error`. See [Modification Time](modification-time).
`file-notify-error`
This is a subcategory of `file-error`. It happens, when a file could not be watched for changes. See [File Notifications](file-notifications).
`remote-file-error`
This is a subcategory of `file-error`, which results from problems in accessing a remote file. See [Remote Files](https://www.gnu.org/software/emacs/manual/html_node/emacs/Remote-Files.html#Remote-Files) in The GNU Emacs Manual. Often, this error appears when timers, process filters, process sentinels or special events in general try to access a remote file, and collide with another remote file operation. In general it is a good idea to write a bug report. See [Bugs](https://www.gnu.org/software/emacs/manual/html_node/emacs/Bugs.html#Bugs) in The GNU Emacs Manual.
`ftp-error`
This is a subcategory of `remote-file-error`, which results from problems in accessing a remote file using ftp. See [Remote Files](https://www.gnu.org/software/emacs/manual/html_node/emacs/Remote-Files.html#Remote-Files) in The GNU Emacs Manual.
`invalid-function`
The message is ‘`Invalid function`’. See [Function Indirection](function-indirection).
`invalid-read-syntax`
The message is usually ‘`Invalid read syntax`’. See [Printed Representation](printed-representation). This error can also be raised by commands like `eval-expression` when there’s text following an expression. In that case, the message is ‘`Trailing garbage following expression`’.
`invalid-regexp`
The message is ‘`Invalid regexp`’. See [Regular Expressions](regular-expressions).
`mark-inactive`
The message is ‘`The mark is not active now`’. See [The Mark](the-mark).
`no-catch`
The message is ‘`No catch for tag`’. See [Catch and Throw](catch-and-throw).
`range-error`
The message is `Arithmetic range error`.
`overflow-error`
The message is ‘`Arithmetic overflow error`’. This is a subcategory of `range-error`. This can happen with integers exceeding the `integer-width` limit. See [Integer Basics](integer-basics).
`scan-error`
The message is ‘`Scan error`’. This happens when certain syntax-parsing functions find invalid syntax or mismatched parentheses. Conventionally raised with three argument: a human-readable error message, the start of the obstacle that cannot be moved over, and the end of the obstacle. See [List Motion](list-motion), and see [Parsing Expressions](parsing-expressions).
`search-failed`
The message is ‘`Search failed`’. See [Searching and Matching](searching-and-matching).
`setting-constant`
The message is ‘`Attempt to set a constant symbol`’. This happens when attempting to assign values to `nil`, `t`, `most-positive-fixnum`, `most-negative-fixnum`, and keyword symbols. It also happens when attempting to assign values to `enable-multibyte-characters` and some other symbols whose direct assignment is not allowed for some reason. See [Constant Variables](constant-variables).
`text-read-only`
The message is ‘`Text is read-only`’. This is a subcategory of `buffer-read-only`. See [Special Properties](special-properties).
`undefined-color`
The message is ‘`Undefined color`’. See [Color Names](color-names).
`user-error`
The message is the empty string. See [Signaling Errors](signaling-errors).
`user-search-failed`
This is like ‘`search-failed`’, but doesn’t trigger the debugger, like ‘`user-error`’. See [Signaling Errors](signaling-errors), and see [Searching and Matching](searching-and-matching). This is used for searching in Info files, see [Search Text](https://www.gnu.org/software/emacs/manual/html_node/info/Search-Text.html#Search-Text) in Info.
`void-function`
The message is ‘`Symbol's function definition is void`’. See [Function Cells](function-cells).
`void-variable`
The message is ‘`Symbol's value as variable is void`’. See [Accessing Variables](accessing-variables).
`wrong-number-of-arguments`
The message is ‘`Wrong number of arguments`’. See [Argument List](argument-list).
`wrong-type-argument`
The message is ‘`Wrong type argument`’. See [Type Predicates](type-predicates).
`unknown-image-type`
The message is ‘`Cannot determine image type`’. See [Images](images).
`inhibited-interaction` The message is ‘`User interaction while inhibited`’. This error is signalled when `inhibit-interaction` is non-`nil` and a user interaction function (like `read-from-minibuffer`) is called.
elisp None ### Emacs Dynamic Modules
A *dynamic Emacs module* is a shared library that provides additional functionality for use in Emacs Lisp programs, just like a package written in Emacs Lisp would.
Functions that load Emacs Lisp packages can also load dynamic modules. They recognize dynamic modules by looking at their file-name extension, a.k.a. “suffix”. This suffix is platform-dependent.
Variable: **module-file-suffix**
This variable holds the system-dependent value of the file-name extension of the module files. Its value is `.so` on POSIX hosts, `.dylib` on macOS, and `.dll` on MS-Windows.
On macOS, dynamic modules can also have the suffix `.so` in addition to `.dylib`.
Every dynamic module should export a C-callable function named `emacs_module_init`, which Emacs will call as part of the call to `load` or `require` which loads the module. It should also export a symbol named `plugin_is_GPL_compatible` to indicate that its code is released under the GPL or compatible license; Emacs will signal an error if your program tries to load modules that don’t export such a symbol.
If a module needs to call Emacs functions, it should do so through the API (Application Programming Interface) defined and documented in the header file `emacs-module.h` that is part of the Emacs distribution. See [Writing Dynamic Modules](writing-dynamic-modules), for details of using that API when writing your own modules.
Modules can create `user-ptr` Lisp objects that embed pointers to C struct’s defined by the module. This is useful for keeping around complex data structures created by a module, to be passed back to the module’s functions. User-ptr objects can also have associated *finalizers* – functions to be run when the object is GC’ed; this is useful for freeing any resources allocated for the underlying data structure, such as memory, open file descriptors, etc. See [Module Values](module-values).
Function: **user-ptrp** *object*
This function returns `t` if its argument is a `user-ptr` object.
Function: **module-load** *file*
Emacs calls this low-level primitive to load a module from the specified file and perform the necessary initialization of the module. This is the primitive which makes sure the module exports the `plugin_is_GPL_compatible` symbol, calls the module’s `emacs_module_init` function, and signals an error if that function returns an error indication, or if the use typed `C-g` during the initialization. If the initialization succeeds, `module-load` returns `t`. Note that file must already have the proper file-name extension, as this function doesn’t try looking for files with known extensions, unlike `load`.
Unlike `load`, `module-load` doesn’t record the module in `load-history`, doesn’t print any messages, and doesn’t protect against recursive loads. Most users should therefore use `load`, `load-file`, `load-library`, or `require` instead of `module-load`.
Loadable modules in Emacs are enabled by using the `--with-modules` option at configure time.
---
| programming_docs |
elisp None #### Emulating Mode Line Formatting
You can use the function `format-mode-line` to compute the text that would appear in a mode line or header line based on a certain mode line construct.
Function: **format-mode-line** *format &optional face window buffer*
This function formats a line of text according to format as if it were generating the mode line for window, but it also returns the text as a string. The argument window defaults to the selected window. If buffer is non-`nil`, all the information used is taken from buffer; by default, it comes from window’s buffer.
The value string normally has text properties that correspond to the faces, keymaps, etc., that the mode line would have. Any character for which no `face` property is specified by format gets a default value determined by face. If face is `t`, that stands for either `mode-line` if window is selected, otherwise `mode-line-inactive`. If face is `nil` or omitted, that stands for the default face. If face is an integer, the value returned by this function will have no text properties.
You can also specify other valid faces as the value of face. If specified, that face provides the `face` property for characters whose face is not specified by format.
Note that using `mode-line`, `mode-line-inactive`, or `header-line` as face will actually redisplay the mode line or the header line, respectively, using the current definitions of the corresponding face, in addition to returning the formatted string. (Other faces do not cause redisplay.)
For example, `(format-mode-line header-line-format)` returns the text that would appear in the selected window’s header line (`""` if it has no header line). `(format-mode-line header-line-format
'header-line)` returns the same text, with each character carrying the face that it will have in the header line itself, and also redraws the header line.
elisp None #### Abstract Display Example
Here is a simple example using functions of the ewoc package to implement a *color components* display, an area in a buffer that represents a vector of three integers (itself representing a 24-bit RGB value) in various ways.
```
(setq colorcomp-ewoc nil
colorcomp-data nil
colorcomp-mode-map nil
colorcomp-labels ["Red" "Green" "Blue"])
(defun colorcomp-pp (data)
(if data
(let ((comp (aref colorcomp-data data)))
(insert (aref colorcomp-labels data) "\t: #x"
(format "%02X" comp) " "
(make-string (ash comp -2) ?#) "\n"))
(let ((cstr (format "#%02X%02X%02X"
(aref colorcomp-data 0)
(aref colorcomp-data 1)
(aref colorcomp-data 2)))
(samp " (sample text) "))
(insert "Color\t: "
(propertize samp 'face
`(foreground-color . ,cstr))
(propertize samp 'face
`(background-color . ,cstr))
"\n"))))
(defun colorcomp (color)
"Allow fiddling with COLOR in a new buffer.
The buffer is in Color Components mode."
(interactive "sColor (name or #RGB or #RRGGBB): ")
(when (string= "" color)
(setq color "green"))
(unless (color-values color)
(error "No such color: %S" color))
(switch-to-buffer
(generate-new-buffer (format "originally: %s" color)))
(kill-all-local-variables)
(setq major-mode 'colorcomp-mode
mode-name "Color Components")
(use-local-map colorcomp-mode-map)
(erase-buffer)
(buffer-disable-undo)
(let ((data (apply 'vector (mapcar (lambda (n) (ash n -8))
(color-values color))))
(ewoc (ewoc-create 'colorcomp-pp
"\nColor Components\n\n"
(substitute-command-keys
"\n\\{colorcomp-mode-map}"))))
(set (make-local-variable 'colorcomp-data) data)
(set (make-local-variable 'colorcomp-ewoc) ewoc)
(ewoc-enter-last ewoc 0)
(ewoc-enter-last ewoc 1)
(ewoc-enter-last ewoc 2)
(ewoc-enter-last ewoc nil)))
```
This example can be extended to be a color selection widget (in other words, the “controller” part of the “model–view–controller” design paradigm) by defining commands to modify `colorcomp-data` and to finish the selection process, and a keymap to tie it all together conveniently.
```
(defun colorcomp-mod (index limit delta)
(let ((cur (aref colorcomp-data index)))
(unless (= limit cur)
(aset colorcomp-data index (+ cur delta)))
(ewoc-invalidate
colorcomp-ewoc
(ewoc-nth colorcomp-ewoc index)
(ewoc-nth colorcomp-ewoc -1))))
(defun colorcomp-R-more () (interactive) (colorcomp-mod 0 255 1))
(defun colorcomp-G-more () (interactive) (colorcomp-mod 1 255 1))
(defun colorcomp-B-more () (interactive) (colorcomp-mod 2 255 1))
(defun colorcomp-R-less () (interactive) (colorcomp-mod 0 0 -1))
(defun colorcomp-G-less () (interactive) (colorcomp-mod 1 0 -1))
(defun colorcomp-B-less () (interactive) (colorcomp-mod 2 0 -1))
(defun colorcomp-copy-as-kill-and-exit ()
"Copy the color components into the kill ring and kill the buffer.
The string is formatted #RRGGBB (hash followed by six hex digits)."
(interactive)
(kill-new (format "#%02X%02X%02X"
(aref colorcomp-data 0)
(aref colorcomp-data 1)
(aref colorcomp-data 2)))
(kill-buffer nil))
(setq colorcomp-mode-map
(let ((m (make-sparse-keymap)))
(suppress-keymap m)
(define-key m "i" 'colorcomp-R-less)
(define-key m "o" 'colorcomp-R-more)
(define-key m "k" 'colorcomp-G-less)
(define-key m "l" 'colorcomp-G-more)
(define-key m "," 'colorcomp-B-less)
(define-key m "." 'colorcomp-B-more)
(define-key m " " 'colorcomp-copy-as-kill-and-exit)
m))
```
Note that we never modify the data in each node, which is fixed when the ewoc is created to be either `nil` or an index into the vector `colorcomp-data`, the actual color components.
elisp None ### Character Display
This section describes how characters are actually displayed by Emacs. Typically, a character is displayed as a *glyph* (a graphical symbol which occupies one character position on the screen), whose appearance corresponds to the character itself. For example, the character ‘`a`’ (character code 97) is displayed as ‘`a`’. Some characters, however, are displayed specially. For example, the formfeed character (character code 12) is usually displayed as a sequence of two glyphs, ‘`^L`’, while the newline character (character code 10) starts a new screen line.
You can modify how each character is displayed by defining a *display table*, which maps each character code into a sequence of glyphs. See [Display Tables](display-tables).
| | | |
| --- | --- | --- |
| • [Usual Display](usual-display) | | The usual conventions for displaying characters. |
| • [Display Tables](display-tables) | | What a display table consists of. |
| • [Active Display Table](active-display-table) | | How Emacs selects a display table to use. |
| • [Glyphs](glyphs) | | How to define a glyph, and what glyphs mean. |
| • [Glyphless Chars](glyphless-chars) | | How glyphless characters are drawn. |
elisp None #### Backquote-Style Patterns
This subsection describes *backquote-style patterns*, a set of builtin patterns that eases structural matching. For background, see [Pattern-Matching Conditional](pattern_002dmatching-conditional).
Backquote-style patterns are a powerful set of `pcase` pattern extensions (created using `pcase-defmacro`) that make it easy to match expval against specifications of its *structure*.
For example, to match expval that must be a list of two elements whose first element is a specific string and the second element is any value, you can write a core pattern:
```
(and (pred listp)
ls
```
```
(guard (= 2 (length ls)))
(guard (string= "first" (car ls)))
(let second-elem (cadr ls)))
```
or you can write the equivalent backquote-style pattern:
```
`("first" ,second-elem)
```
The backquote-style pattern is more concise, resembles the structure of expval, and avoids binding `ls`.
A backquote-style pattern has the form ``qpat` where qpat can have the following forms:
`(qpat1 . qpat2)`
Matches if expval is a cons cell whose `car` matches qpat1 and whose `cdr` matches qpat2. This readily generalizes to lists as in `(qpat1 qpat2 …)`.
`[qpat1 qpat2 … qpatm]`
Matches if expval is a vector of length m whose `0`..`(m-1)`th elements match qpat1, qpat2 … qpatm, respectively.
`symbol` `keyword` `number` `string`
Matches if the corresponding element of expval is `equal` to the specified literal object.
`,pattern` Matches if the corresponding element of expval matches pattern. Note that pattern is any kind that `pcase` supports. (In the example above, `second-elem` is a symbol core pattern; it therefore matches anything, and let-binds `second-elem`.)
The *corresponding element* is the portion of expval that is in the same structural position as the structural position of qpat in the backquote-style pattern. (In the example above, the corresponding element of `second-elem` is the second element of expval.)
Here is an example of using `pcase` to implement a simple interpreter for a little expression language (note that this requires lexical binding for the lambda expression in the `fn` clause to properly capture `body` and `arg` (see [Lexical Binding](lexical-binding)):
```
(defun evaluate (form env)
(pcase form
(`(add ,x ,y) (+ (evaluate x env)
(evaluate y env)))
```
```
(`(call ,fun ,arg) (funcall (evaluate fun env)
(evaluate arg env)))
(`(fn ,arg ,body) (lambda (val)
(evaluate body (cons (cons arg val)
env))))
```
```
((pred numberp) form)
((pred symbolp) (cdr (assq form env)))
(_ (error "Syntax error: %S" form))))
```
The first three clauses use backquote-style patterns. ``(add ,x ,y)` is a pattern that checks that `form` is a three-element list starting with the literal symbol `add`, then extracts the second and third elements and binds them to symbols `x` and `y`, respectively. The clause body evaluates `x` and `y` and adds the results. Similarly, the `call` clause implements a function call, and the `fn` clause implements an anonymous function definition.
The remaining clauses use core patterns. `(pred numberp)` matches if `form` is a number. On match, the body evaluates it. `(pred symbolp)` matches if `form` is a symbol. On match, the body looks up the symbol in `env` and returns its association. Finally, `_` is the catch-all pattern that matches anything, so it’s suitable for reporting syntax errors.
Here are some sample programs in this small language, including their evaluation results:
```
(evaluate '(add 1 2) nil) ⇒ 3
(evaluate '(add x y) '((x . 1) (y . 2))) ⇒ 3
(evaluate '(call (fn x (add 1 x)) 2) nil) ⇒ 3
(evaluate '(sub 1 2) nil) ⇒ error
```
elisp None #### Error Symbols and Condition Names
When you signal an error, you specify an *error symbol* to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language.
These narrow classifications are grouped into a hierarchy of wider classes called *error conditions*, identified by *condition names*. The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name `error` which takes in all kinds of errors (but not `quit`). Thus, each error has one or more condition names: `error`, the error symbol if that is distinct from `error`, and perhaps some intermediate classifications.
Function: **define-error** *name message &optional parent*
In order for a symbol to be an error symbol, it must be defined with `define-error` which takes a parent condition (defaults to `error`). This parent defines the conditions that this kind of error belongs to. The transitive set of parents always includes the error symbol itself, and the symbol `error`. Because quitting is not considered an error, the set of parents of `quit` is just `(quit)`.
In addition to its parents, the error symbol has a message which is a string to be printed when that error is signaled but not handled. If that message is not valid, the error message ‘`peculiar error`’ is used. See [Definition of signal](signaling-errors#Definition-of-signal).
Internally, the set of parents is stored in the `error-conditions` property of the error symbol and the message is stored in the `error-message` property of the error symbol.
Here is how we define a new error symbol, `new-error`:
```
(define-error 'new-error "A new error" 'my-own-errors)
```
This error has several condition names: `new-error`, the narrowest classification; `my-own-errors`, which we imagine is a wider classification; and all the conditions of `my-own-errors` which should include `error`, which is the widest of all.
The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs.
Naturally, Emacs will never signal `new-error` on its own; only an explicit call to `signal` (see [Definition of signal](signaling-errors#Definition-of-signal)) in your code can do this:
```
(signal 'new-error '(x y))
error→ A new error: x, y
```
This error can be handled through any of its condition names. This example handles `new-error` and any other errors in the class `my-own-errors`:
```
(condition-case foo
(bar nil t)
(my-own-errors nil))
```
The significant way that errors are classified is by their condition names—the names used to match errors with handlers. An error symbol serves only as a convenient way to specify the intended error message and list of condition names. It would be cumbersome to give `signal` a list of condition names rather than one error symbol.
By contrast, using only error symbols without condition names would seriously decrease the power of `condition-case`. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification.
See [Standard Errors](standard-errors), for a list of the main error symbols and their conditions.
elisp None #### The setf Macro
The `setf` macro is the most basic way to operate on generalized variables. The `setf` form is like `setq`, except that it accepts arbitrary place forms on the left side rather than just symbols. For example, `(setf (car a) b)` sets the car of `a` to `b`, doing the same operation as `(setcar a b)`, but without you having to use two separate functions for setting and accessing this type of place.
Macro: **setf** *[place form]…*
This macro evaluates form and stores it in place, which must be a valid generalized variable form. If there are several place and form pairs, the assignments are done sequentially just as with `setq`. `setf` returns the value of the last form.
The following Lisp forms are the forms in Emacs that will work as generalized variables, and so may appear in the place argument of `setf`:
* A symbol. In other words, `(setf x y)` is exactly equivalent to `(setq x y)`, and `setq` itself is strictly speaking redundant given that `setf` exists. Most programmers will continue to prefer `setq` for setting simple variables, though, for stylistic and historical reasons. The macro `(setf x y)` actually expands to `(setq x y)`, so there is no performance penalty for using it in compiled code.
* A call to any of the following standard Lisp functions:
```
aref cddr symbol-function
car elt symbol-plist
caar get symbol-value
cadr gethash
cdr nth
cdar nthcdr
```
* A call to any of the following Emacs-specific functions:
```
alist-get process-get
frame-parameter process-sentinel
terminal-parameter window-buffer
keymap-parent window-display-table
match-data window-dedicated-p
overlay-get window-hscroll
overlay-start window-parameter
overlay-end window-point
process-buffer window-start
process-filter default-value
```
`setf` signals an error if you pass a place form that it does not know how to handle.
Note that for `nthcdr`, the list argument of the function must itself be a valid place form. For example, `(setf (nthcdr
0 foo) 7)` will set `foo` itself to 7.
The macros `push` (see [List Variables](list-variables)) and `pop` (see [List Elements](list-elements)) can manipulate generalized variables, not just lists. `(pop place)` removes and returns the first element of the list stored in place. It is analogous to `(prog1 (car place) (setf place (cdr place)))`, except that it takes care to evaluate all subforms only once. `(push x place)` inserts x at the front of the list stored in place. It is analogous to `(setf
place (cons x place))`, except for evaluation of the subforms. Note that `push` and `pop` on an `nthcdr` place can be used to insert or delete at any position in a list.
The `cl-lib` library defines various extensions for generalized variables, including additional `setf` places. See [Generalized Variables](https://www.gnu.org/software/emacs/manual/html_node/cl/Generalized-Variables.html#Generalized-Variables) in Common Lisp Extensions.
elisp None ### Writing Dynamically-Loaded Modules
This section describes the Emacs module API and how to use it as part of writing extension modules for Emacs. The module API is defined in the C programming language, therefore the description and the examples in this section assume the module is written in C. For other programming languages, you will need to use the appropriate bindings, interfaces and facilities for calling C code. Emacs C code requires a C99 or later compiler (see [C Dialect](c-dialect)), and so the code examples in this section also follow that standard.
Writing a module and integrating it into Emacs comprises the following tasks:
* Writing initialization code for the module.
* Writing one or more module functions.
* Communicating values and objects between Emacs and your module functions.
* Handling of error conditions and nonlocal exits.
The following subsections describe these tasks and the API itself in more detail.
Once your module is written, compile it to produce a shared library, according to the conventions of the underlying platform. Then place the shared library in a directory mentioned in `load-path` (see [Library Search](library-search)), where Emacs will find it.
If you wish to verify the conformance of a module to the Emacs dynamic module API, invoke Emacs with the `--module-assertions` option. See [Initial Options](https://www.gnu.org/software/emacs/manual/html_node/emacs/Initial-Options.html#Initial-Options) in The GNU Emacs Manual.
| | | |
| --- | --- | --- |
| • [Module Initialization](module-initialization) | | |
| • [Module Functions](module-functions) | | |
| • [Module Values](module-values) | | |
| • [Module Misc](module-misc) | | |
| • [Module Nonlocal](module-nonlocal) | | |
elisp None ### Input Methods
*Input methods* provide convenient ways of entering non-ASCII characters from the keyboard. Unlike coding systems, which translate non-ASCII characters to and from encodings meant to be read by programs, input methods provide human-friendly commands. (See [Input Methods](https://www.gnu.org/software/emacs/manual/html_node/emacs/Input-Methods.html#Input-Methods) in The GNU Emacs Manual, for information on how users use input methods to enter text.) How to define input methods is not yet documented in this manual, but here we describe how to use them.
Each input method has a name, which is currently a string; in the future, symbols may also be usable as input method names.
Variable: **current-input-method**
This variable holds the name of the input method now active in the current buffer. (It automatically becomes local in each buffer when set in any fashion.) It is `nil` if no input method is active in the buffer now.
User Option: **default-input-method**
This variable holds the default input method for commands that choose an input method. Unlike `current-input-method`, this variable is normally global.
Command: **set-input-method** *input-method*
This command activates input method input-method for the current buffer. It also sets `default-input-method` to input-method. If input-method is `nil`, this command deactivates any input method for the current buffer.
Function: **read-input-method-name** *prompt &optional default inhibit-null*
This function reads an input method name with the minibuffer, prompting with prompt. If default is non-`nil`, that is returned by default, if the user enters empty input. However, if inhibit-null is non-`nil`, empty input signals an error.
The returned value is a string.
Variable: **input-method-alist**
This variable defines all the supported input methods. Each element defines one input method, and should have the form:
```
(input-method language-env activate-func
title description args...)
```
Here input-method is the input method name, a string; language-env is another string, the name of the language environment this input method is recommended for. (That serves only for documentation purposes.)
activate-func is a function to call to activate this method. The args, if any, are passed as arguments to activate-func. All told, the arguments to activate-func are input-method and the args.
title is a string to display in the mode line while this method is active. description is a string describing this method and what it is good for.
The fundamental interface to input methods is through the variable `input-method-function`. See [Reading One Event](reading-one-event), and [Invoking the Input Method](invoking-the-input-method).
| programming_docs |
elisp None #### Frame Position
On graphical systems, the position of a normal top-level frame is specified as the absolute position of its outer frame (see [Frame Geometry](frame-geometry)). The position of a child frame (see [Child Frames](child-frames)) is specified via pixel offsets of its outer edges relative to the native position of its parent frame.
You can access or change the position of a frame using the frame parameters `left` and `top` (see [Position Parameters](position-parameters)). Here are two additional functions for working with the positions of an existing, visible frame. For both functions, the argument frame must denote a live frame and defaults to the selected frame.
Function: **frame-position** *&optional frame*
For a normal, non-child frame this function returns a cons of the pixel coordinates of its outer position (see [Frame Layout](frame-layout)) with respect to the origin `(0, 0)` of its display. For a child frame (see [Child Frames](child-frames)) this function returns the pixel coordinates of its outer position with respect to an origin `(0, 0)` at the native position of frame’s parent.
Negative values never indicate an offset from the right or bottom edge of frame’s display or parent frame. Rather, they mean that frame’s outer position is on the left and/or above the origin of its display or the native position of its parent frame. This usually means that frame is only partially visible (or completely invisible). However, on systems where the display’s origin does not coincide with its top-left corner, the frame may be visible on a secondary monitor.
On a text terminal frame both values are zero.
Function: **set-frame-position** *frame x y*
This function sets the outer frame position of frame to (x, y). The latter arguments specify pixels and normally count from the origin at the position (0, 0) of frame’s display. For child frames, they count from the native position of frame’s parent frame.
Negative parameter values position the right edge of the outer frame by -x pixels left from the right edge of the screen (or the parent frame’s native rectangle) and the bottom edge by -y pixels up from the bottom edge of the screen (or the parent frame’s native rectangle).
Note that negative values do not permit to align the right or bottom edge of frame exactly at the right or bottom edge of its display or parent frame. Neither do they allow to specify a position that does not lie within the edges of the display or parent frame. The frame parameters `left` and `top` (see [Position Parameters](position-parameters)) allow to do that, but may still fail to provide good results for the initial or a new frame.
This function has no effect on text terminal frames.
Variable: **move-frame-functions**
This hook specifies the functions that are run when an Emacs frame is moved (assigned a new position) by the window-system or window manager. The functions are run with one argument, the frame that moved. For a child frame (see [Child Frames](child-frames)), the functions are run only when the position of the frame changes in relation to that of its parent frame.
elisp None ### Sending Input to Processes
Asynchronous subprocesses receive input when it is sent to them by Emacs, which is done with the functions in this section. You must specify the process to send input to, and the input data to send. If the subprocess runs a program, the data appears on the standard input of that program; for connections, the data is sent to the connected device or program.
Some operating systems have limited space for buffered input in a pty. On these systems, Emacs sends an EOF periodically amidst the other characters, to force them through. For most programs, these EOFs do no harm.
Subprocess input is normally encoded using a coding system before the subprocess receives it, much like text written into a file. You can use `set-process-coding-system` to specify which coding system to use (see [Process Information](process-information)). Otherwise, the coding system comes from `coding-system-for-write`, if that is non-`nil`; or else from the defaulting mechanism (see [Default Coding Systems](default-coding-systems)).
Sometimes the system is unable to accept input for that process, because the input buffer is full. When this happens, the send functions wait a short while, accepting output from subprocesses, and then try again. This gives the subprocess a chance to read more of its pending input and make space in the buffer. It also allows filters (including the one currently running), sentinels and timers to run—so take account of that in writing your code.
In these functions, the process argument can be a process or the name of a process, or a buffer or buffer name (which stands for a process via `get-buffer-process`). `nil` means the current buffer’s process.
Function: **process-send-string** *process string*
This function sends process the contents of string as standard input. It returns `nil`. For example, to make a Shell buffer list files:
```
(process-send-string "shell<1>" "ls\n")
⇒ nil
```
Function: **process-send-region** *process start end*
This function sends the text in the region defined by start and end as standard input to process.
An error is signaled unless both start and end are integers or markers that indicate positions in the current buffer. (It is unimportant which number is larger.)
Function: **process-send-eof** *&optional process*
This function makes process see an end-of-file in its input. The EOF comes after any text already sent to it. The function returns process.
```
(process-send-eof "shell")
⇒ "shell"
```
Function: **process-running-child-p** *&optional process*
This function will tell you whether a process, which must not be a connection but a real subprocess, has given control of its terminal to a child process of its own. If this is true, the function returns the numeric ID of the foreground process group of process; it returns `nil` if Emacs can be certain that this is not so. The value is `t` if Emacs cannot tell whether this is true. This function signals an error if process is a network, serial, or pipe connection, or if the subprocess is not active.
elisp None ### The Case Table
You can customize case conversion by installing a special *case table*. A case table specifies the mapping between upper case and lower case letters. It affects both the case conversion functions for Lisp objects (see the previous section) and those that apply to text in the buffer (see [Case Changes](case-changes)). Each buffer has a case table; there is also a standard case table which is used to initialize the case table of new buffers.
A case table is a char-table (see [Char-Tables](char_002dtables)) whose subtype is `case-table`. This char-table maps each character into the corresponding lower case character. It has three extra slots, which hold related tables:
upcase The upcase table maps each character into the corresponding upper case character.
canonicalize The canonicalize table maps all of a set of case-related characters into a particular member of that set.
equivalences The equivalences table maps each one of a set of case-related characters into the next character in that set.
In simple cases, all you need to specify is the mapping to lower-case; the three related tables will be calculated automatically from that one.
For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both lower case and upper case.
The extra table canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character. For example, since ‘`a`’ and ‘`A`’ are related by case-conversion, they should have the same canonical equivalent character (which should be either ‘`a`’ for both of them, or ‘`A`’ for both of them).
The extra table equivalences is a map that cyclically permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map ‘`a`’ into ‘`A`’ and ‘`A`’ into ‘`a`’, and likewise for each set of equivalent characters.)
When constructing a case table, you can provide `nil` for canonicalize; then Emacs fills in this slot from the lower case and upper case mappings. You can also provide `nil` for equivalences; then Emacs fills in this slot from canonicalize. In a case table that is actually in use, those components are non-`nil`. Do not try to specify equivalences without also specifying canonicalize.
Here are the functions for working with case tables:
Function: **case-table-p** *object*
This predicate returns non-`nil` if object is a valid case table.
Function: **set-standard-case-table** *table*
This function makes table the standard case table, so that it will be used in any buffers created subsequently.
Function: **standard-case-table**
This returns the standard case table.
Function: **current-case-table**
This function returns the current buffer’s case table.
Function: **set-case-table** *table*
This sets the current buffer’s case table to table.
Macro: **with-case-table** *table body…*
The `with-case-table` macro saves the current case table, makes table the current case table, evaluates the body forms, and finally restores the case table. The return value is the value of the last form in body. The case table is restored even in case of an abnormal exit via `throw` or error (see [Nonlocal Exits](nonlocal-exits)).
Some language environments modify the case conversions of ASCII characters; for example, in the Turkish language environment, the ASCII capital I is downcased into a Turkish dotless i (‘`ı`’). This can interfere with code that requires ordinary ASCII case conversion, such as implementations of ASCII-based network protocols. In that case, use the `with-case-table` macro with the variable ascii-case-table, which stores the unmodified case table for the ASCII character set.
Variable: **ascii-case-table**
The case table for the ASCII character set. This should not be modified by any language environment settings.
The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify the specified case table case-table; they also modify the standard syntax table. See [Syntax Tables](syntax-tables). Normally you would use these functions to change the standard case table.
Function: **set-case-syntax-pair** *uc lc case-table*
This function specifies a pair of corresponding letters, one upper case and one lower case.
Function: **set-case-syntax-delims** *l r case-table*
This function makes characters l and r a matching pair of case-invariant delimiters.
Function: **set-case-syntax** *char syntax case-table*
This function makes char case-invariant, with syntax syntax.
Command: **describe-buffer-case-table**
This command displays a description of the contents of the current buffer’s case table.
elisp None ### Defining Functions
We usually give a name to a function when it is first created. This is called *defining a function*, and we usually do it with the `defun` macro. This section also describes other ways to define a function.
Macro: **defun** *name args [doc] [declare] [interactive] body…*
`defun` is the usual way to define new Lisp functions. It defines the symbol name as a function with argument list args (see [Argument List](argument-list)) and body forms given by body. Neither name nor args should be quoted.
doc, if present, should be a string specifying the function’s documentation string (see [Function Documentation](function-documentation)). declare, if present, should be a `declare` form specifying function metadata (see [Declare Form](declare-form)). interactive, if present, should be an `interactive` form specifying how the function is to be called interactively (see [Interactive Call](interactive-call)).
The return value of `defun` is undefined.
Here are some examples:
```
(defun foo () 5)
(foo)
⇒ 5
```
```
(defun bar (a &optional b &rest c)
(list a b c))
(bar 1 2 3 4 5)
⇒ (1 2 (3 4 5))
```
```
(bar 1)
⇒ (1 nil nil)
```
```
(bar)
error→ Wrong number of arguments.
```
```
(defun capitalize-backwards ()
"Upcase the last letter of the word at point."
(interactive)
(backward-word 1)
(forward-word 1)
(backward-char 1)
(capitalize-word 1))
```
Be careful not to redefine existing functions unintentionally. `defun` redefines even primitive functions such as `car` without any hesitation or notification. Emacs does not prevent you from doing this, because redefining a function is sometimes done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition.
Function: **defalias** *name definition &optional doc*
This function defines the symbol name as a function, with definition definition (which can be any valid Lisp function). Its return value is *undefined*.
If doc is non-`nil`, it becomes the function documentation of name. Otherwise, any documentation provided by definition is used.
Internally, `defalias` normally uses `fset` to set the definition. If name has a `defalias-fset-function` property, however, the associated value is used as a function to call in place of `fset`.
The proper place to use `defalias` is where a specific function name is being defined—especially where that name appears explicitly in the source file being loaded. This is because `defalias` records which file defined the function, just like `defun` (see [Unloading](unloading)).
By contrast, in programs that manipulate function definitions for other purposes, it is better to use `fset`, which does not keep such records. See [Function Cells](function-cells).
You cannot create a new primitive function with `defun` or `defalias`, but you can use them to change the function definition of any symbol, even one such as `car` or `x-popup-menu` whose normal definition is a primitive. However, this is risky: for instance, it is next to impossible to redefine `car` without breaking Lisp completely. Redefining an obscure function such as `x-popup-menu` is less dangerous, but it still may not work as you expect. If there are calls to the primitive from C code, they call the primitive’s C definition directly, so changing the symbol’s definition will have no effect on them.
See also `defsubst`, which defines a function like `defun` and tells the Lisp compiler to perform inline expansion on it. See [Inline Functions](inline-functions).
To undefine a function name, use `fmakunbound`. See [Function Cells](function-cells).
elisp None Preparing Lisp code for distribution
------------------------------------
Emacs provides a standard way to distribute Emacs Lisp code to users. A *package* is a collection of one or more files, formatted and bundled in such a way that users can easily download, install, uninstall, and upgrade it.
The following sections describe how to create a package, and how to put it in a *package archive* for others to download. See [Packages](https://www.gnu.org/software/emacs/manual/html_node/emacs/Packages.html#Packages) in The GNU Emacs Manual, for a description of user-level features of the packaging system.
These sections are mostly directed towards package archive maintainers—much of this information is not relevant for package authors (i.e., people who write code that will be distributed via these archives).
| | | |
| --- | --- | --- |
| • [Packaging Basics](packaging-basics) | | The basic concepts of Emacs Lisp packages. |
| • [Simple Packages](simple-packages) | | How to package a single .el file. |
| • [Multi-file Packages](multi_002dfile-packages) | | How to package multiple files. |
| • [Package Archives](package-archives) | | Maintaining package archives. |
| • [Archive Web Server](archive-web-server) | | Interfacing to an archive web server. |
elisp None #### Reading One Event
The lowest level functions for command input are `read-event`, `read-char`, and `read-char-exclusive`.
If you need a function to read a character using the minibuffer, use `read-char-from-minibuffer` (see [Multiple Queries](multiple-queries)).
Function: **read-event** *&optional prompt inherit-input-method seconds*
This function reads and returns the next event of command input, waiting if necessary until an event is available.
The returned event may come directly from the user, or from a keyboard macro. It is not decoded by the keyboard’s input coding system (see [Terminal I/O Encoding](terminal-i_002fo-encoding)).
If the optional argument prompt is non-`nil`, it should be a string to display in the echo area as a prompt. If prompt is `nil` or the string ‘`""`’, `read-event` does not display any message to indicate it is waiting for input; instead, it prompts by echoing: it displays descriptions of the events that led to or were read by the current command. See [The Echo Area](the-echo-area).
If inherit-input-method is non-`nil`, then the current input method (if any) is employed to make it possible to enter a non-ASCII character. Otherwise, input method handling is disabled for reading this event.
If `cursor-in-echo-area` is non-`nil`, then `read-event` moves the cursor temporarily to the echo area, to the end of any message displayed there. Otherwise `read-event` does not move the cursor.
If seconds is non-`nil`, it should be a number specifying the maximum time to wait for input, in seconds. If no input arrives within that time, `read-event` stops waiting and returns `nil`. A floating point seconds means to wait for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, seconds is rounded down. If seconds is `nil`, `read-event` waits as long as necessary for input to arrive.
If seconds is `nil`, Emacs is considered idle while waiting for user input to arrive. Idle timers—those created with `run-with-idle-timer` (see [Idle Timers](idle-timers))—can run during this period. However, if seconds is non-`nil`, the state of idleness remains unchanged. If Emacs is non-idle when `read-event` is called, it remains non-idle throughout the operation of `read-event`; if Emacs is idle (which can happen if the call happens inside an idle timer), it remains idle.
If `read-event` gets an event that is defined as a help character, then in some cases `read-event` processes the event directly without returning. See [Help Functions](help-functions). Certain other events, called *special events*, are also processed directly within `read-event` (see [Special Events](special-events)).
Here is what happens if you call `read-event` and then press the right-arrow function key:
```
(read-event)
⇒ right
```
Function: **read-char** *&optional prompt inherit-input-method seconds*
This function reads and returns a character input event. If the user generates an event which is not a character (i.e., a mouse click or function key event), `read-char` signals an error. The arguments work as in `read-event`.
If the event has modifiers, Emacs attempts to resolve them and return the code of the corresponding character. For example, if the user types `C-a`, the function returns 1, which is the ASCII code of the ‘`C-a`’ character. If some of the modifiers cannot be reflected in the character code, `read-char` leaves the unresolved modifier bits set in the returned event. For example, if the user types `C-M-a`, the function returns 134217729, 8000001 in hex, i.e. ‘`C-a`’ with the Meta modifier bit set. This value is not a valid character code: it fails the `characterp` test (see [Character Codes](character-codes)). Use `event-basic-type` (see [Classifying Events](classifying-events)) to recover the character code with the modifier bits removed; use `event-modifiers` to test for modifiers in the character event returned by `read-char`.
In the first example below, the user types the character `1` (ASCII code 49). The second example shows a keyboard macro definition that calls `read-char` from the minibuffer using `eval-expression`. `read-char` reads the keyboard macro’s very next character, which is `1`. Then `eval-expression` displays its return value in the echo area.
```
(read-char)
⇒ 49
```
```
;; We assume here you use `M-:` to evaluate this.
(symbol-function 'foo)
⇒ "^[:(read-char)^M1"
```
```
(execute-kbd-macro 'foo)
-| 49
⇒ nil
```
Function: **read-char-exclusive** *&optional prompt inherit-input-method seconds*
This function reads and returns a character input event. If the user generates an event which is not a character event, `read-char-exclusive` ignores it and reads another event, until it gets a character. The arguments work as in `read-event`. The returned value may include modifier bits, as with `read-char`.
None of the above functions suppress quitting.
Variable: **num-nonmacro-input-events**
This variable holds the total number of input events received so far from the terminal—not counting those generated by keyboard macros.
We emphasize that, unlike `read-key-sequence`, the functions `read-event`, `read-char`, and `read-char-exclusive` do not perform the translations described in [Translation Keymaps](translation-keymaps). If you wish to read a single key taking these translations into account (for example, to read [Function Keys](function-keys) in a terminal or [Mouse Events](mouse-events) from `xterm-mouse-mode`), use the function `read-key`:
Function: **read-key** *&optional prompt disable-fallbacks*
This function reads a single key. It is intermediate between `read-key-sequence` and `read-event`. Unlike the former, it reads a single key, not a key sequence. Unlike the latter, it does not return a raw event, but decodes and translates the user input according to `input-decode-map`, `local-function-key-map`, and `key-translation-map` (see [Translation Keymaps](translation-keymaps)).
The argument prompt is either a string to be displayed in the echo area as a prompt, or `nil`, meaning not to display a prompt.
If argument disable-fallbacks is non-`nil` then the usual fallback logic for unbound keys in `read-key-sequence` is not applied. This means that mouse button-down and multi-click events will not be discarded and `local-function-key-map` and `key-translation-map` will not get applied. If `nil` or unspecified, the only fallback disabled is downcasing of the last event.
Function: **read-char-choice** *prompt chars &optional inhibit-quit*
This function uses `read-key` to read and return a single character. It ignores any input that is not a member of chars, a list of accepted characters. Optionally, it will also ignore keyboard-quit events while it is waiting for valid input. If you bind `help-form` (see [Help Functions](help-functions)) to a non-`nil` value while calling `read-char-choice`, then pressing `help-char` causes it to evaluate `help-form` and display the result. It then continues to wait for a valid input character, or keyboard-quit.
Function: **read-multiple-choice** *prompt choices &optional help-string*
Ask user a multiple choice question. prompt should be a string that will be displayed as the prompt.
choices is an alist where the first element in each entry is a character to be entered, the second element is a short name for the entry to be displayed while prompting (if there’s room, it might be shortened), and the third, optional entry is a longer explanation that will be displayed in a help buffer if the user requests more help.
If optional argument help-string is non-`nil`, it should be a string with a more detailed description of all choices. It will be displayed in a help buffer instead of the default auto-generated description when the user types `?`.
The return value is the matching value from choices.
```
(read-multiple-choice
"Continue connecting?"
'((?a "always" "Accept certificate for this and future sessions.")
(?s "session only" "Accept certificate this session only.")
(?n "no" "Refuse to use certificate, close connection.")))
```
The `read-multiple-choice-face` face is used to highlight the matching characters in the name string on graphical terminals.
| programming_docs |
elisp None ### Vectors
A *vector* is a general-purpose array whose elements can be any Lisp objects. (By contrast, the elements of a string can only be characters. See [Strings and Characters](strings-and-characters).) Vectors are used in Emacs for many purposes: as key sequences (see [Key Sequences](key-sequences)), as symbol-lookup tables (see [Creating Symbols](creating-symbols)), as part of the representation of a byte-compiled function (see [Byte Compilation](byte-compilation)), and more.
Like other arrays, vectors use zero-origin indexing: the first element has index 0.
Vectors are printed with square brackets surrounding the elements. Thus, a vector whose elements are the symbols `a`, `b` and `a` is printed as `[a b a]`. You can write vectors in the same way in Lisp input.
A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. This does not evaluate or even examine the elements of the vector. See [Self-Evaluating Forms](self_002devaluating-forms). Vectors written with square brackets should not be modified via `aset` or other destructive operations. See [Mutability](mutability).
Here are examples illustrating these principles:
```
(setq avector [1 two '(three) "four" [five]])
⇒ [1 two '(three) "four" [five]]
(eval avector)
⇒ [1 two '(three) "four" [five]]
(eq avector (eval avector))
⇒ t
```
elisp None ### Batch Mode
The command-line option ‘`-batch`’ causes Emacs to run noninteractively. In this mode, Emacs does not read commands from the terminal, it does not alter the terminal modes, and it does not expect to be outputting to an erasable screen. The idea is that you specify Lisp programs to run; when they are finished, Emacs should exit. The way to specify the programs to run is with ‘`-l file`’, which loads the library named file, or ‘`-f function`’, which calls function with no arguments, or ‘`--eval=form`’.
Any Lisp program output that would normally go to the echo area, either using `message`, or using `prin1`, etc., with `t` as the stream (see [Output Streams](output-streams)), goes instead to Emacs’s standard descriptors when in batch mode: `message` writes to the standard error descriptor, while `prin1` and other print functions write to the standard output. Similarly, input that would normally come from the minibuffer is read from the standard input descriptor. Thus, Emacs behaves much like a noninteractive application program. (The echo area output that Emacs itself normally generates, such as command echoing, is suppressed entirely.)
Non-ASCII text written to the standard output or error descriptors is by default encoded using `locale-coding-system` (see [Locales](locales)) if it is non-`nil`; this can be overridden by binding `coding-system-for-write` to a coding system of you choice (see [Explicit Encoding](explicit-encoding)).
Variable: **noninteractive**
This variable is non-`nil` when Emacs is running in batch mode.
If Emacs exits due to signaling an error in batch mode, the exit status of the Emacs command is non-zero:
```
$ emacs -Q --batch --eval '(error "foo")'; echo $?
foo
255
```
elisp None ### File Names
Files are generally referred to by their names, in Emacs as elsewhere. File names in Emacs are represented as strings. The functions that operate on a file all expect a file name argument.
In addition to operating on files themselves, Emacs Lisp programs often need to operate on file names; i.e., to take them apart and to use part of a name to construct related file names. This section describes how to manipulate file names.
The functions in this section do not actually access files, so they can operate on file names that do not refer to an existing file or directory.
On MS-DOS and MS-Windows, these functions (like the function that actually operate on files) accept MS-DOS or MS-Windows file-name syntax, where backslashes separate the components, as well as POSIX syntax; but they always return POSIX syntax. This enables Lisp programs to specify file names in POSIX syntax and work properly on all systems without change.[18](#FOOT18)
| | | |
| --- | --- | --- |
| • [File Name Components](file-name-components) | | The directory part of a file name, and the rest. |
| • [Relative File Names](relative-file-names) | | Some file names are relative to a current directory. |
| • [Directory Names](directory-names) | | A directory’s name as a directory is different from its name as a file. |
| • [File Name Expansion](file-name-expansion) | | Converting relative file names to absolute ones. |
| • [Unique File Names](unique-file-names) | | Generating names for temporary files. |
| • [File Name Completion](file-name-completion) | | Finding the completions for a given file name. |
| • [Standard File Names](standard-file-names) | | If your package uses a fixed file name, how to handle various operating systems simply. |
elisp None ### What Is a Function?
In a general sense, a function is a rule for carrying out a computation given input values called *arguments*. The result of the computation is called the *value* or *return value* of the function. The computation can also have side effects, such as lasting changes in the values of variables or the contents of data structures (see [Definition of side effect](intro-eval#Definition-of-side-effect)). A *pure function* is a function which, in addition to having no side effects, always returns the same value for the same combination of arguments, regardless of external factors such as machine type or system state.
In most computer languages, every function has a name. But in Lisp, a function in the strictest sense has no name: it is an object which can *optionally* be associated with a symbol (e.g., `car`) that serves as the function name. See [Function Names](function-names). When a function has been given a name, we usually also refer to that symbol as a “function” (e.g., we refer to “the function `car`”). In this manual, the distinction between a function name and the function object itself is usually unimportant, but we will take note wherever it is relevant.
Certain function-like objects, called *special forms* and *macros*, also accept arguments to carry out computations. However, as explained below, these are not considered functions in Emacs Lisp.
Here are important terms for functions and function-like objects:
*lambda expression*
A function (in the strict sense, i.e., a function object) which is written in Lisp. These are described in the following section. See [Lambda Expressions](lambda-expressions).
*primitive*
A function which is callable from Lisp but is actually written in C. Primitives are also called *built-in functions*, or *subrs*. Examples include functions like `car` and `append`. In addition, all special forms (see below) are also considered primitives.
Usually, a function is implemented as a primitive because it is a fundamental part of Lisp (e.g., `car`), or because it provides a low-level interface to operating system services, or because it needs to run fast. Unlike functions defined in Lisp, primitives can be modified or added only by changing the C sources and recompiling Emacs. See [Writing Emacs Primitives](writing-emacs-primitives).
*special form*
A primitive that is like a function but does not evaluate all of its arguments in the usual way. It may evaluate only some of the arguments, or may evaluate them in an unusual order, or several times. Examples include `if`, `and`, and `while`. See [Special Forms](special-forms).
*macro*
A construct defined in Lisp, which differs from a function in that it translates a Lisp expression into another expression which is to be evaluated instead of the original expression. Macros enable Lisp programmers to do the sorts of things that special forms can do. See [Macros](macros).
*command*
An object which can be invoked via the `command-execute` primitive, usually due to the user typing in a key sequence *bound* to that command. See [Interactive Call](interactive-call). A command is usually a function; if the function is written in Lisp, it is made into a command by an `interactive` form in the function definition (see [Defining Commands](defining-commands)). Commands that are functions can also be called from Lisp expressions, just like other functions.
Keyboard macros (strings and vectors) are commands also, even though they are not functions. See [Keyboard Macros](keyboard-macros). We say that a symbol is a command if its function cell contains a command (see [Symbol Components](symbol-components)); such a *named command* can be invoked with `M-x`.
*closure*
A function object that is much like a lambda expression, except that it also encloses an environment of lexical variable bindings. See [Closures](closures).
*byte-code function*
A function that has been compiled by the byte compiler. See [Byte-Code Type](byte_002dcode-type).
*autoload object*
A place-holder for a real function. If the autoload object is called, Emacs loads the file containing the definition of the real function, and then calls the real function. See [Autoload](autoload).
You can use the function `functionp` to test if an object is a function:
Function: **functionp** *object*
This function returns `t` if object is any kind of function, i.e., can be passed to `funcall`. Note that `functionp` returns `t` for symbols that are function names, and returns `nil` for special forms.
It is also possible to find out how many arguments an arbitrary function expects:
Function: **func-arity** *function*
This function provides information about the argument list of the specified function. The returned value is a cons cell of the form `(min . max)`, where min is the minimum number of arguments, and max is either the maximum number of arguments, or the symbol `many` for functions with `&rest` arguments, or the symbol `unevalled` if function is a special form.
Note that this function might return inaccurate results in some situations, such as the following:
* - Functions defined using `apply-partially` (see [apply-partially](calling-functions)).
* - Functions that are advised using `advice-add` (see [Advising Named Functions](advising-named-functions)).
* - Functions that determine the argument list dynamically, as part of their code.
Unlike `functionp`, the next three functions do *not* treat a symbol as its function definition.
Function: **subrp** *object*
This function returns `t` if object is a built-in function (i.e., a Lisp primitive).
```
(subrp 'message) ; `message` is a symbol,
⇒ nil ; not a subr object.
```
```
(subrp (symbol-function 'message))
⇒ t
```
Function: **byte-code-function-p** *object*
This function returns `t` if object is a byte-code function. For example:
```
(byte-code-function-p (symbol-function 'next-line))
⇒ t
```
Function: **subr-arity** *subr*
This works like `func-arity`, but only for built-in functions and without symbol indirection. It signals an error for non-built-in functions. We recommend to use `func-arity` instead.
elisp None ### Character Properties
A *character property* is a named attribute of a character that specifies how the character behaves and how it should be handled during text processing and display. Thus, character properties are an important part of specifying the character’s semantics.
On the whole, Emacs follows the Unicode Standard in its implementation of character properties. In particular, Emacs supports the [Unicode Character Property Model](https://www.unicode.org/reports/tr23/), and the Emacs character property database is derived from the Unicode Character Database (UCD). See the [Character Properties chapter of the Unicode Standard](https://www.unicode.org/versions/Unicode14.0.0/ch04.pdf), for a detailed description of Unicode character properties and their meaning. This section assumes you are already familiar with that chapter of the Unicode Standard, and want to apply that knowledge to Emacs Lisp programs.
In Emacs, each property has a name, which is a symbol, and a set of possible values, whose types depend on the property; if a character does not have a certain property, the value is `nil`. As a general rule, the names of character properties in Emacs are produced from the corresponding Unicode properties by downcasing them and replacing each ‘`\_`’ character with a dash ‘`-`’. For example, `Canonical_Combining_Class` becomes `canonical-combining-class`. However, sometimes we shorten the names to make their use easier.
Some codepoints are left *unassigned* by the UCD—they don’t correspond to any character. The Unicode Standard defines default values of properties for such codepoints; they are mentioned below for each property.
Here is the full list of value types for all the character properties that Emacs knows about:
`name`
Corresponds to the `Name` Unicode property. The value is a string consisting of upper-case Latin letters A to Z, digits, spaces, and hyphen ‘`-`’ characters. For unassigned codepoints, the value is `nil`.
`general-category`
Corresponds to the `General_Category` Unicode property. The value is a symbol whose name is a 2-letter abbreviation of the character’s classification. For unassigned codepoints, the value is `Cn`.
`canonical-combining-class`
Corresponds to the `Canonical_Combining_Class` Unicode property. The value is an integer. For unassigned codepoints, the value is zero.
`bidi-class`
Corresponds to the Unicode `Bidi_Class` property. The value is a symbol whose name is the Unicode *directional type* of the character. Emacs uses this property when it reorders bidirectional text for display (see [Bidirectional Display](bidirectional-display)). For unassigned codepoints, the value depends on the code blocks to which the codepoint belongs: most unassigned codepoints get the value of `L` (strong L), but some get values of `AL` (Arabic letter) or `R` (strong R).
`decomposition`
Corresponds to the Unicode properties `Decomposition_Type` and `Decomposition_Value`. The value is a list, whose first element may be a symbol representing a compatibility formatting tag, such as `small`[21](#FOOT21); the other elements are characters that give the compatibility decomposition sequence of this character. For characters that don’t have decomposition sequences, and for unassigned codepoints, the value is a list with a single member, the character itself.
`decimal-digit-value`
Corresponds to the Unicode `Numeric_Value` property for characters whose `Numeric_Type` is ‘`Decimal`’. The value is an integer, or `nil` if the character has no decimal digit value. For unassigned codepoints, the value is `nil`, which means NaN, or “not a number”.
`digit-value`
Corresponds to the Unicode `Numeric_Value` property for characters whose `Numeric_Type` is ‘`Digit`’. The value is an integer. Examples of such characters include compatibility subscript and superscript digits, for which the value is the corresponding number. For characters that don’t have any numeric value, and for unassigned codepoints, the value is `nil`, which means NaN.
`numeric-value`
Corresponds to the Unicode `Numeric_Value` property for characters whose `Numeric_Type` is ‘`Numeric`’. The value of this property is a number. Examples of characters that have this property include fractions, subscripts, superscripts, Roman numerals, currency numerators, and encircled numbers. For example, the value of this property for the character U+2155 VULGAR FRACTION ONE FIFTH is `0.2`. For characters that don’t have any numeric value, and for unassigned codepoints, the value is `nil`, which means NaN.
`mirrored`
Corresponds to the Unicode `Bidi_Mirrored` property. The value of this property is a symbol, either `Y` or `N`. For unassigned codepoints, the value is `N`.
`mirroring`
Corresponds to the Unicode `Bidi_Mirroring_Glyph` property. The value of this property is a character whose glyph represents the mirror image of the character’s glyph, or `nil` if there’s no defined mirroring glyph. All the characters whose `mirrored` property is `N` have `nil` as their `mirroring` property; however, some characters whose `mirrored` property is `Y` also have `nil` for `mirroring`, because no appropriate characters exist with mirrored glyphs. Emacs uses this property to display mirror images of characters when appropriate (see [Bidirectional Display](bidirectional-display)). For unassigned codepoints, the value is `nil`.
`paired-bracket`
Corresponds to the Unicode `Bidi_Paired_Bracket` property. The value of this property is the codepoint of a character’s *paired bracket*, or `nil` if the character is not a bracket character. This establishes a mapping between characters that are treated as bracket pairs by the Unicode Bidirectional Algorithm; Emacs uses this property when it decides how to reorder for display parentheses, braces, and other similar characters (see [Bidirectional Display](bidirectional-display)).
`bracket-type`
Corresponds to the Unicode `Bidi_Paired_Bracket_Type` property. For characters whose `paired-bracket` property is non-`nil`, the value of this property is a symbol, either `o` (for opening bracket characters) or `c` (for closing bracket characters). For characters whose `paired-bracket` property is `nil`, the value is the symbol `n` (None). Like `paired-bracket`, this property is used for bidirectional display.
`old-name`
Corresponds to the Unicode `Unicode_1_Name` property. The value is a string. For unassigned codepoints, and characters that have no value for this property, the value is `nil`.
`iso-10646-comment`
Corresponds to the Unicode `ISO_Comment` property. The value is either a string or `nil`. For unassigned codepoints, the value is `nil`.
`uppercase`
Corresponds to the Unicode `Simple_Uppercase_Mapping` property. The value of this property is a single character. For unassigned codepoints, the value is `nil`, which means the character itself.
`lowercase`
Corresponds to the Unicode `Simple_Lowercase_Mapping` property. The value of this property is a single character. For unassigned codepoints, the value is `nil`, which means the character itself.
`titlecase`
Corresponds to the Unicode `Simple_Titlecase_Mapping` property. *Title case* is a special form of a character used when the first character of a word needs to be capitalized. The value of this property is a single character. For unassigned codepoints, the value is `nil`, which means the character itself.
`special-uppercase`
Corresponds to Unicode language- and context-independent special upper-casing rules. The value of this property is a string (which may be empty). For example mapping for U+00DF LATIN SMALL LETTER SHARP S is `"SS"`. For characters with no special mapping, the value is `nil` which means `uppercase` property needs to be consulted instead.
`special-lowercase`
Corresponds to Unicode language- and context-independent special lower-casing rules. The value of this property is a string (which may be empty). For example mapping for U+0130 LATIN CAPITAL LETTER I WITH DOT ABOVE the value is `"i\u0307"` (i.e. 2-character string consisting of LATIN SMALL LETTER I followed by U+0307 COMBINING DOT ABOVE). For characters with no special mapping, the value is `nil` which means `lowercase` property needs to be consulted instead.
`special-titlecase` Corresponds to Unicode unconditional special title-casing rules. The value of this property is a string (which may be empty). For example mapping for U+FB01 LATIN SMALL LIGATURE FI the value is `"Fi"`. For characters with no special mapping, the value is `nil` which means `titlecase` property needs to be consulted instead.
Function: **get-char-code-property** *char propname*
This function returns the value of char’s propname property.
```
(get-char-code-property ?\s 'general-category)
⇒ Zs
```
```
(get-char-code-property ?1 'general-category)
⇒ Nd
```
```
;; U+2084
(get-char-code-property ?\N{SUBSCRIPT FOUR}
'digit-value)
⇒ 4
```
```
;; U+2155
(get-char-code-property ?\N{VULGAR FRACTION ONE FIFTH}
'numeric-value)
⇒ 0.2
```
```
;; U+2163
(get-char-code-property ?\N{ROMAN NUMERAL FOUR}
'numeric-value)
⇒ 4
```
```
(get-char-code-property ?\( 'paired-bracket)
⇒ 41 ; closing parenthesis
```
```
(get-char-code-property ?\) 'bracket-type)
⇒ c
```
Function: **char-code-property-description** *prop value*
This function returns the description string of property prop’s value, or `nil` if value has no description.
```
(char-code-property-description 'general-category 'Zs)
⇒ "Separator, Space"
```
```
(char-code-property-description 'general-category 'Nd)
⇒ "Number, Decimal Digit"
```
```
(char-code-property-description 'numeric-value '1/5)
⇒ nil
```
Function: **put-char-code-property** *char propname value*
This function stores value as the value of the property propname for the character char.
Variable: **unicode-category-table**
The value of this variable is a char-table (see [Char-Tables](char_002dtables)) that specifies, for each character, its Unicode `General_Category` property as a symbol.
Variable: **char-script-table**
The value of this variable is a char-table that specifies, for each character, a symbol whose name is the script to which the character belongs, according to the Unicode Standard classification of the Unicode code space into script-specific blocks. This char-table has a single extra slot whose value is the list of all script symbols. Note that Emacs’ classification of characters into scripts is not a 1-for-1 reflection of the Unicode standard, e.g. there is no ‘`symbol`’ script in Unicode.
Variable: **char-width-table**
The value of this variable is a char-table that specifies the width of each character in columns that it will occupy on the screen.
Variable: **printable-chars**
The value of this variable is a char-table that specifies, for each character, whether it is printable or not. That is, if evaluating `(aref printable-chars char)` results in `t`, the character is printable, and if it results in `nil`, it is not.
| programming_docs |
elisp None #### Suspending Emacs
On text terminals, it is possible to *suspend Emacs*, which means stopping Emacs temporarily and returning control to its superior process, which is usually the shell. This allows you to resume editing later in the same Emacs process, with the same buffers, the same kill ring, the same undo history, and so on. To resume Emacs, use the appropriate command in the parent shell—most likely `fg`.
Suspending works only on a terminal device from which the Emacs session was started. We call that device the *controlling terminal* of the session. Suspending is not allowed if the controlling terminal is a graphical terminal. Suspending is usually not relevant in graphical environments, since you can simply switch to another application without doing anything special to Emacs.
Some operating systems (those without `SIGTSTP`, or MS-DOS) do not support suspension of jobs; on these systems, suspension actually creates a new shell temporarily as a subprocess of Emacs. Then you would exit the shell to return to Emacs.
Command: **suspend-emacs** *&optional string*
This function stops Emacs and returns control to the superior process. If and when the superior process resumes Emacs, `suspend-emacs` returns `nil` to its caller in Lisp.
This function works only on the controlling terminal of the Emacs session; to relinquish control of other tty devices, use `suspend-tty` (see below). If the Emacs session uses more than one terminal, you must delete the frames on all the other terminals before suspending Emacs, or this function signals an error. See [Multiple Terminals](multiple-terminals).
If string is non-`nil`, its characters are sent to Emacs’s superior shell, to be read as terminal input. The characters in string are not echoed by the superior shell; only the results appear.
Before suspending, `suspend-emacs` runs the normal hook `suspend-hook`. After the user resumes Emacs, `suspend-emacs` runs the normal hook `suspend-resume-hook`. See [Hooks](hooks).
The next redisplay after resumption will redraw the entire screen, unless the variable `no-redraw-on-reenter` is non-`nil`. See [Refresh Screen](refresh-screen).
Here is an example of how you could use these hooks:
```
(add-hook 'suspend-hook
(lambda () (or (y-or-n-p "Really suspend?")
(error "Suspend canceled"))))
```
```
(add-hook 'suspend-resume-hook (lambda () (message "Resumed!")
(sit-for 2)))
```
Here is what you would see upon evaluating `(suspend-emacs "pwd")`:
```
---------- Buffer: Minibuffer ----------
Really suspend? y
---------- Buffer: Minibuffer ----------
```
```
---------- Parent Shell ----------
bash$ /home/username
bash$ fg
```
```
---------- Echo Area ----------
Resumed!
```
Note that ‘`pwd`’ is not echoed after Emacs is suspended. But it is read and executed by the shell.
Variable: **suspend-hook**
This variable is a normal hook that Emacs runs before suspending.
Variable: **suspend-resume-hook**
This variable is a normal hook that Emacs runs on resuming after a suspension.
Function: **suspend-tty** *&optional tty*
If tty specifies a terminal device used by Emacs, this function relinquishes the device and restores it to its prior state. Frames that used the device continue to exist, but are not updated and Emacs doesn’t read input from them. tty can be a terminal object, a frame (meaning the terminal for that frame), or `nil` (meaning the terminal for the selected frame). See [Multiple Terminals](multiple-terminals).
If tty is already suspended, this function does nothing.
This function runs the hook `suspend-tty-functions`, passing the terminal object as an argument to each function.
Function: **resume-tty** *&optional tty*
This function resumes the previously suspended terminal device tty; where tty has the same possible values as it does for `suspend-tty`.
This function reopens the terminal device, re-initializes it, and redraws it with that terminal’s selected frame. It then runs the hook `resume-tty-functions`, passing the terminal object as an argument to each function.
If the same device is already used by another Emacs terminal, this function signals an error. If tty is not suspended, this function does nothing.
Function: **controlling-tty-p** *&optional tty*
This function returns non-`nil` if tty is the controlling terminal of the Emacs session; tty can be a terminal object, a frame (meaning the terminal for that frame), or `nil` (meaning the terminal for the selected frame).
Command: **suspend-frame**
This command *suspends* a frame. For GUI frames, it calls `iconify-frame` (see [Visibility of Frames](visibility-of-frames)); for frames on text terminals, it calls either `suspend-emacs` or `suspend-tty`, depending on whether the frame is displayed on the controlling terminal device or not.
elisp None #### Lazy Computation of Text Properties
Instead of computing text properties for all the text in the buffer, you can arrange to compute the text properties for parts of the text when and if something depends on them.
The primitive that extracts text from the buffer along with its properties is `buffer-substring`. Before examining the properties, this function runs the abnormal hook `buffer-access-fontify-functions`.
Variable: **buffer-access-fontify-functions**
This variable holds a list of functions for computing text properties. Before `buffer-substring` copies the text and text properties for a portion of the buffer, it calls all the functions in this list. Each of the functions receives two arguments that specify the range of the buffer being accessed. (The buffer itself is always the current buffer.)
The function `buffer-substring-no-properties` does not call these functions, since it ignores text properties anyway.
In order to prevent the hook functions from being called more than once for the same part of the buffer, you can use the variable `buffer-access-fontified-property`.
Variable: **buffer-access-fontified-property**
If this variable’s value is non-`nil`, it is a symbol which is used as a text property name. A non-`nil` value for that text property means the other text properties for this character have already been computed.
If all the characters in the range specified for `buffer-substring` have a non-`nil` value for this property, `buffer-substring` does not call the `buffer-access-fontify-functions` functions. It assumes these characters already have the right text properties, and just copies the properties they already have.
The normal way to use this feature is that the `buffer-access-fontify-functions` functions add this property, as well as others, to the characters they operate on. That way, they avoid being called over and over for the same text.
elisp None #### Thread Type
A *thread* in Emacs represents a separate thread of Emacs Lisp execution. It runs its own Lisp program, has its own current buffer, and can have subprocesses locked to it, i.e. subprocesses whose output only this thread can accept. See [Threads](threads).
Thread objects have no read syntax. They print in hash notation, giving the name of the thread (if it has been given a name) or its address in core:
```
(all-threads)
⇒ (#<thread 0176fc40>)
```
elisp None #### Checking Whether to Stop
Whenever Edebug is entered, it needs to save and restore certain data before even deciding whether to make trace information or stop the program.
* `max-lisp-eval-depth` (see [Eval](eval)) and `max-specpdl-size` (see [Local Variables](local-variables)) are both increased to reduce Edebug’s impact on the stack. You could, however, still run out of stack space when using Edebug. You can also enlarge the value of `edebug-max-depth` if Edebug reaches the limit of recursion depth instrumenting code that contains very large quoted lists.
* The state of keyboard macro execution is saved and restored. While Edebug is active, `executing-kbd-macro` is bound to `nil` unless `edebug-continue-kbd-macro` is non-`nil`.
elisp None #### Instrumenting Macro Calls
When Edebug instruments an expression that calls a Lisp macro, it needs additional information about the macro to do the job properly. This is because there is no a-priori way to tell which subexpressions of the macro call are forms to be evaluated. (Evaluation may occur explicitly in the macro body, or when the resulting expansion is evaluated, or any time later.)
Therefore, you must define an Edebug specification for each macro that Edebug will encounter, to explain the format of calls to that macro. To do this, add a `debug` declaration to the macro definition. Here is a simple example that shows the specification for the `for` example macro (see [Argument Evaluation](argument-evaluation)).
```
(defmacro for (var from init to final do &rest body)
"Execute a simple \"for\" loop.
For example, (for i from 1 to 10 do (print i))."
(declare (debug (symbolp "from" form "to" form "do" &rest form)))
...)
```
The Edebug specification says which parts of a call to the macro are forms to be evaluated. For simple macros, the specification often looks very similar to the formal argument list of the macro definition, but specifications are much more general than macro arguments. See [Defining Macros](defining-macros), for more explanation of the `declare` form.
Take care to ensure that the specifications are known to Edebug when you instrument code. If you are instrumenting a function which uses a macro defined in another file, you may first need to either evaluate the `require` forms in the file containing your function, or explicitly load the file containing the macro. If the definition of a macro is wrapped by `eval-when-compile`, you may need to evaluate it.
You can also define an edebug specification for a macro separately from the macro definition with `def-edebug-spec`. Adding `debug` declarations is preferred, and more convenient, for macro definitions in Lisp, but `def-edebug-spec` makes it possible to define Edebug specifications for special forms implemented in C.
Macro: **def-edebug-spec** *macro specification*
Specify which expressions of a call to macro macro are forms to be evaluated. specification should be the Edebug specification. Neither argument is evaluated.
The macro argument can actually be any symbol, not just a macro name.
Here is a table of the possibilities for specification and how each directs processing of arguments.
`t`
All arguments are instrumented for evaluation. This is short for `(body)`.
a symbol
The symbol must have an Edebug specification, which is used instead. This indirection is repeated until another kind of specification is found. This allows you to inherit the specification from another macro.
a list The elements of the list describe the types of the arguments of a calling form. The possible elements of a specification list are described in the following sections.
If a macro has no Edebug specification, neither through a `debug` declaration nor through a `def-edebug-spec` call, the variable `edebug-eval-macro-args` comes into play.
User Option: **edebug-eval-macro-args**
This controls the way Edebug treats macro arguments with no explicit Edebug specification. If it is `nil` (the default), none of the arguments is instrumented for evaluation. Otherwise, all arguments are instrumented.
elisp None ### Scanning for Character Sets
Sometimes it is useful to find out which character set a particular character belongs to. One use for this is in determining which coding systems (see [Coding Systems](coding-systems)) are capable of representing all of the text in question; another is to determine the font(s) for displaying that text.
Function: **charset-after** *&optional pos*
This function returns the charset of highest priority containing the character at position pos in the current buffer. If pos is omitted or `nil`, it defaults to the current value of point. If pos is out of range, the value is `nil`.
Function: **find-charset-region** *beg end &optional translation*
This function returns a list of the character sets of highest priority that contain characters in the current buffer between positions beg and end.
The optional argument translation specifies a translation table to use for scanning the text (see [Translation of Characters](translation-of-characters)). If it is non-`nil`, then each character in the region is translated through this table, and the value returned describes the translated characters instead of the characters actually in the buffer.
Function: **find-charset-string** *string &optional translation*
This function returns a list of character sets of highest priority that contain characters in string. It is just like `find-charset-region`, except that it applies to the contents of string instead of part of the current buffer.
elisp None ### The Window Start and End Positions
Each window maintains a marker used to keep track of a buffer position that specifies where in the buffer display should start. This position is called the *display-start* position of the window (or just the *start*). The character after this position is the one that appears at the upper left corner of the window. It is usually, but not inevitably, at the beginning of a text line.
After switching windows or buffers, and in some other cases, if the window start is in the middle of a line, Emacs adjusts the window start to the start of a line. This prevents certain operations from leaving the window start at a meaningless point within a line. This feature may interfere with testing some Lisp code by executing it using the commands of Lisp mode, because they trigger this readjustment. To test such code, put it into a command and bind the command to a key.
Function: **window-start** *&optional window*
This function returns the display-start position of window window. If window is `nil`, the selected window is used.
When you create a window, or display a different buffer in it, the display-start position is set to a display-start position recently used for the same buffer, or to `point-min` if the buffer doesn’t have any.
Redisplay updates the window-start position (if you have not specified it explicitly since the previous redisplay)—to make sure point appears on the screen. Nothing except redisplay automatically changes the window-start position; if you move point, do not expect the window-start position to change in response until after the next redisplay.
Function: **window-group-start** *&optional window*
This function is like `window-start`, except that when window is a part of a group of windows (see [Window Group](selecting-windows#Window-Group)), `window-group-start` returns the start position of the entire group. This condition holds when the buffer local variable `window-group-start-function` is set to a function. In this case, `window-group-start` calls the function with the single argument window, then returns its result.
Function: **window-end** *&optional window update*
This function returns the position where display of its buffer ends in window. The default for window is the selected window.
Simply changing the buffer text or moving point does not update the value that `window-end` returns. The value is updated only when Emacs redisplays and redisplay completes without being preempted.
If the last redisplay of window was preempted, and did not finish, Emacs does not know the position of the end of display in that window. In that case, this function returns `nil`.
If update is non-`nil`, `window-end` always returns an up-to-date value for where display ends, based on the current `window-start` value. If a previously saved value of that position is still valid, `window-end` returns that value; otherwise it computes the correct value by scanning the buffer text.
Even if update is non-`nil`, `window-end` does not attempt to scroll the display if point has moved off the screen, the way real redisplay would do. It does not alter the `window-start` value. In effect, it reports where the displayed text will end if scrolling is not required. Note that the position it returns might be only partially visible.
Function: **window-group-end** *&optional window update*
This function is like `window-end`, except that when window is a part of a group of windows (see [Window Group](selecting-windows#Window-Group)), `window-group-end` returns the end position of the entire group. This condition holds when the buffer local variable `window-group-end-function` is set to a function. In this case, `window-group-end` calls the function with the two arguments window and update, then returns its result. The argument update has the same meaning as in `window-end`.
Function: **set-window-start** *window position &optional noforce*
This function sets the display-start position of window to position in window’s buffer. It returns position.
The display routines insist that the position of point be visible when a buffer is displayed. Normally, they select the display-start position according to their internal logic (and scroll the window if necessary) to make point visible. However, if you specify the start position with this function using `nil` for noforce, it means you want display to start at position even if that would put the location of point off the screen. If this does place point off screen, the display routines attempt to move point to the left margin on the middle line in the window.
For example, if point is 1 and you set the start of the window to 37, the start of the next line, point will be above the top of the window. The display routines will automatically move point if it is still 1 when redisplay occurs. Here is an example:
```
;; Here is what ‘`foo`’ looks like before executing
;; the `set-window-start` expression.
```
```
---------- Buffer: foo ----------
∗This is the contents of buffer foo.
2
3
4
5
6
---------- Buffer: foo ----------
```
```
(set-window-start
(selected-window)
(save-excursion
(goto-char 1)
(forward-line 1)
(point)))
⇒ 37
```
```
;; Here is what ‘`foo`’ looks like after executing
;; the `set-window-start` expression.
---------- Buffer: foo ----------
2
3
∗4
5
6
---------- Buffer: foo ----------
```
If the attempt to make point visible (i.e., in a fully-visible screen line) fails, the display routines will disregard the requested window-start position and compute a new one anyway. Thus, for reliable results Lisp programs that call this function should always move point to be inside the window whose display starts at position.
If noforce is non-`nil`, and position would place point off screen at the next redisplay, then redisplay computes a new window-start position that works well with point, and thus position is not used.
Function: **set-window-group-start** *window position &optional noforce*
This function is like `set-window-start`, except that when window is a part of a group of windows (see [Window Group](selecting-windows#Window-Group)), `set-window-group-start` sets the start position of the entire group. This condition holds when the buffer local variable `set-window-group-start-function` is set to a function. In this case, `set-window-group-start` calls the function with the three arguments window, position, and noforce, then returns its result. The arguments position and noforce in this function have the same meaning as in `set-window-start`.
Function: **pos-visible-in-window-p** *&optional position window partially*
This function returns non-`nil` if position is within the range of text currently visible on the screen in window. It returns `nil` if position is scrolled vertically out of view. Locations that are partially obscured are not considered visible unless partially is non-`nil`. The argument position defaults to the current position of point in window; window defaults to the selected window. If position is `t`, that means to check either the first visible position of the last screen line in window, or the end-of-buffer position, whichever comes first.
This function considers only vertical scrolling. If position is out of view only because window has been scrolled horizontally, `pos-visible-in-window-p` returns non-`nil` anyway. See [Horizontal Scrolling](horizontal-scrolling).
If position is visible, `pos-visible-in-window-p` returns `t` if partially is `nil`; if partially is non-`nil`, and the character following position is fully visible, it returns a list of the form `(x y)`, where x and y are the pixel coordinates relative to the top left corner of the window; otherwise it returns an extended list of the form `(x y rtop rbot rowh vpos)`, where rtop and rbot specify the number of off-window pixels at the top and bottom of the row at position, rowh specifies the visible height of that row, and vpos specifies the vertical position (zero-based row number) of that row.
Here is an example:
```
;; If point is off the screen now, recenter it now.
(or (pos-visible-in-window-p
(point) (selected-window))
(recenter 0))
```
Function: **pos-visible-in-window-group-p** *&optional position window partially*
This function is like `pos-visible-in-window-p`, except that when window is a part of a group of windows (see [Window Group](selecting-windows#Window-Group)), `pos-visible-in-window-group-p` tests the visibility of pos in the entire group, not just in the single window. This condition holds when the buffer local variable `pos-visible-in-window-group-p-function` is set to a function. In this case `pos-visible-in-window-group-p` calls the function with the three arguments position, window, and partially, then returns its result. The arguments position and partially have the same meaning as in `pos-visible-in-window-p`.
Function: **window-line-height** *&optional line window*
This function returns the height of text line line in window. If line is one of `header-line` or `mode-line`, `window-line-height` returns information about the corresponding line of the window. Otherwise, line is a text line number starting from 0. A negative number counts from the end of the window. The default for line is the current line in window; the default for window is the selected window.
If the display is not up to date, `window-line-height` returns `nil`. In that case, `pos-visible-in-window-p` may be used to obtain related information.
If there is no line corresponding to the specified line, `window-line-height` returns `nil`. Otherwise, it returns a list `(height vpos ypos offbot)`, where height is the height in pixels of the visible part of the line, vpos and ypos are the vertical position in lines and pixels of the line relative to the top of the first text line, and offbot is the number of off-window pixels at the bottom of the text line. If there are off-window pixels at the top of the (first) text line, ypos is negative.
| programming_docs |
elisp None #### Functions to Unpack and Pack Bytes
In the following documentation, type refers to a Bindat type value as returned from `bindat-type`, raw to a byte array, and struct to an alist representing unpacked field data.
Function: **bindat-unpack** *type raw &optional idx*
This function unpacks data from the unibyte string or byte array raw according to type. Normally, this starts unpacking at the beginning of the byte array, but if idx is non-`nil`, it specifies a zero-based starting position to use instead.
The value is an alist or nested alist in which each element describes one unpacked field.
Function: **bindat-get-field** *struct &rest name*
This function selects a field’s data from the nested alist struct. Usually struct was returned by `bindat-unpack`. If name corresponds to just one argument, that means to extract a top-level field value. Multiple name arguments specify repeated lookup of sub-structures. An integer name acts as an array index.
For example, `(bindat-get-field struct a b 2 c)` means to find field `c` in the third element of subfield `b` of field `a`. (This corresponds to `struct.a.b[2].c` in the C programming language syntax.)
Although packing and unpacking operations change the organization of data (in memory), they preserve the data’s *total length*, which is the sum of all the fields’ lengths, in bytes. This value is not generally inherent in either the specification or alist alone; instead, both pieces of information contribute to its calculation. Likewise, the length of a string or array being unpacked may be longer than the data’s total length as described by the specification.
Function: **bindat-length** *type struct*
This function returns the total length of the data in struct, according to type.
Function: **bindat-pack** *type struct &optional raw idx*
This function returns a byte array packed according to type from the data in the alist struct. It normally creates and fills a new byte array starting at the beginning. However, if raw is non-`nil`, it specifies a pre-allocated unibyte string or vector to pack into. If idx is non-`nil`, it specifies the starting offset for packing into raw.
When pre-allocating, you should make sure `(length raw)` meets or exceeds the total length to avoid an out-of-range error.
Function: **bindat-ip-to-string** *ip*
Convert the Internet address vector ip to a string in the usual dotted notation.
```
(bindat-ip-to-string [127 0 0 1])
⇒ "127.0.0.1"
```
elisp None ### Time Conversion
These functions convert time values (see [Time of Day](time-of-day)) to Lisp timestamps, or into calendrical information and vice versa.
Many 32-bit operating systems are limited to system times containing 32 bits of information in their seconds component; these systems typically handle only the times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 Universal Time. However, 64-bit and some 32-bit operating systems have larger seconds components, and can represent times far in the past or future.
Calendrical conversion functions always use the Gregorian calendar, even for dates before the Gregorian calendar was introduced. Year numbers count the number of years since the year 1 BC, and do not skip zero as traditional Gregorian years do; for example, the year number -37 represents the Gregorian year 38 BC.
Function: **time-convert** *time &optional form*
This function converts a time value into a Lisp timestamp.
The optional form argument specifies the timestamp form to be returned. If form is the symbol `integer`, this function returns an integer count of seconds. If form is a positive integer, it specifies a clock frequency and this function returns an integer-pair timestamp `(ticks
. form)`.[29](#FOOT29) If form is `t`, this function treats it as a positive integer suitable for representing the timestamp; for example, it is treated as 1000000000 if time is nil and the platform timestamp has nanosecond resolution. If form is `list`, this function returns an integer list `(high low micro pico)`. Although an omitted or `nil` form currently acts like `list`, this is planned to change in a future Emacs version, so callers requiring list timestamps should pass `list` explicitly.
If time is infinite or a NaN, this function signals an error. Otherwise, if time cannot be represented exactly, conversion truncates it toward minus infinity. When form is `t`, conversion is always exact so no truncation occurs, and the returned clock resolution is no less than that of time. By way of contrast, `float-time` can convert any Lisp time value without signaling an error, although the result might not be exact. See [Time of Day](time-of-day).
For efficiency this function might return a value that is `eq` to time, or that otherwise shares structure with time.
Although `(time-convert nil nil)` is equivalent to `(current-time)`, the latter may be a bit faster.
```
(setq a (time-convert nil t))
⇒ (1564826753904873156 . 1000000000)
```
```
(time-convert a 100000)
⇒ (156482675390487 . 100000)
```
```
(time-convert a 'integer)
⇒ 1564826753
```
```
(time-convert a 'list)
⇒ (23877 23681 904873 156000)
```
Function: **decode-time** *&optional time zone form*
This function converts a time value into calendrical information. If you don’t specify time, it decodes the current time, and similarly zone defaults to the current time zone rule. See [Time Zone Rules](time-zone-rules). The operating system limits the range of time and zone values.
The form argument controls the form of the returned seconds element, as described below. The return value is a list of nine elements, as follows:
```
(seconds minutes hour day month year dow dst utcoff)
```
Here is what the elements mean:
seconds The number of seconds past the minute, with form described below.
minutes The number of minutes past the hour, as an integer between 0 and 59.
hour The hour of the day, as an integer between 0 and 23.
day The day of the month, as an integer between 1 and 31.
month The month of the year, as an integer between 1 and 12.
year The year, an integer typically greater than 1900.
dow The day of week, as an integer between 0 and 6, where 0 stands for Sunday.
dst `t` if daylight saving time is effect, `nil` if it is not in effect, and -1 if this information is not available.
utcoff An integer indicating the Universal Time offset in seconds, i.e., the number of seconds east of Greenwich.
The seconds element is a Lisp timestamp that is nonnegative and less than 61; it is less than 60 except during positive leap seconds (assuming the operating system supports leap seconds). If the optional form argument is `t`, seconds uses the same precision as time; if form is `integer`, seconds is truncated to an integer. For example, if time is the timestamp `(1566009571321 . 1000)`, which represents 2019-08-17 02:39:31.321 UTC on typical systems that lack leap seconds, then `(decode-time time t t)` returns `((31321 . 1000)
39 2 17 8 2019 6 nil 0)`, whereas `(decode-time time t
'integer)` returns `(31 39 2 17 8 2019 6 nil 0)`. If form is omitted or `nil`, it currently defaults to `integer` but this default may change in future Emacs releases, so callers requiring a particular form should specify form.
**Common Lisp Note:** Common Lisp has different meanings for dow and utcoff, and its second is an integer between 0 and 59 inclusive.
To access (or alter) the elements in the time value, the `decoded-time-second`, `decoded-time-minute`, `decoded-time-hour`, `decoded-time-day`, `decoded-time-month`, `decoded-time-year`, `decoded-time-weekday`, `decoded-time-dst` and `decoded-time-zone` accessors can be used.
For instance, to increase the year in a decoded time, you could say:
```
(setf (decoded-time-year decoded-time)
(+ (decoded-time-year decoded-time) 4))
```
Also see the following function.
Function: **decoded-time-add** *time delta*
This function takes a decoded time structure and adds delta (also a decoded time structure) to it. Elements in delta that are `nil` are ignored.
For instance, if you want “same time next month”, you could say:
```
(let ((time (decode-time nil nil t))
(delta (make-decoded-time :month 2)))
(encode-time (decoded-time-add time delta)))
```
If this date doesn’t exist (if you’re running this on January 31st, for instance), then the date will be shifted back until you get a valid date (which will be February 28th or 29th, depending).
Fields are added in a most to least significant order, so if the adjustment described above happens, it happens before adding days, hours, minutes or seconds.
The values in delta can be negative to subtract values instead.
The return value is a decoded time structure.
Function: **make-decoded-time** *&key second minute hour day month year dst zone*
Return a decoded time structure with only the given keywords filled out, leaving the rest `nil`. For instance, to get a structure that represents “two months”, you could say:
```
(make-decoded-time :month 2)
```
Function: **encode-time** *time &rest obsolescent-arguments*
This function converts time to a Lisp timestamp. It can act as the inverse of `decode-time`.
Ordinarily the first argument is a list `(second minute hour day month
year ignored dst zone)` that specifies a decoded time in the style of `decode-time`, so that `(encode-time (decode-time ...))` works. For the meanings of these list members, see the table under `decode-time`.
As an obsolescent calling convention, this function can be given six or more arguments. The first six arguments second, minute, hour, day, month, and year specify most of the components of a decoded time. If there are more than six arguments the *last* argument is used as zone and any other extra arguments are ignored, so that `(apply
#'encode-time (decode-time ...))` works. In this obsolescent convention, zone defaults to the current time zone rule (see [Time Zone Rules](time-zone-rules)), and dst is treated as if it was -1.
Year numbers less than 100 are not treated specially. If you want them to stand for years above 1900, or years above 2000, you must alter them yourself before you call `encode-time`. The operating system limits the range of time and zone values.
The `encode-time` function acts as a rough inverse to `decode-time`. For example, you can pass the output of the latter to the former as follows:
```
(encode-time (decode-time …))
```
You can perform simple date arithmetic by using out-of-range values for seconds, minutes, hour, day, and month; for example, day 0 means the day preceding the given month.
elisp None #### Command-Line Arguments
You can use command-line arguments to request various actions when you start Emacs. Note that the recommended way of using Emacs is to start it just once, after logging in, and then do all editing in the same Emacs session (see [Entering Emacs](https://www.gnu.org/software/emacs/manual/html_node/emacs/Entering-Emacs.html#Entering-Emacs) in The GNU Emacs Manual). For this reason, you might not use command-line arguments very often; nonetheless, they can be useful when invoking Emacs from session scripts or debugging Emacs. This section describes how Emacs processes command-line arguments.
Function: **command-line**
This function parses the command line that Emacs was called with, processes it, and (amongst other things) loads the user’s init file and displays the startup messages.
Variable: **command-line-processed**
The value of this variable is `t` once the command line has been processed.
If you redump Emacs by calling `dump-emacs` (see [Building Emacs](building-emacs)), you may wish to set this variable to `nil` first in order to cause the new dumped Emacs to process its new command-line arguments.
Variable: **command-switch-alist**
This variable is an alist of user-defined command-line options and associated handler functions. By default it is empty, but you can add elements if you wish.
A *command-line option* is an argument on the command line, which has the form:
```
-option
```
The elements of the `command-switch-alist` look like this:
```
(option . handler-function)
```
The CAR, option, is a string, the name of a command-line option (including the initial hyphen). The handler-function is called to handle option, and receives the option name as its sole argument.
In some cases, the option is followed in the command line by an argument. In these cases, the handler-function can find all the remaining command-line arguments in the variable `command-line-args-left` (see below). (The entire list of command-line arguments is in `command-line-args`.)
Note that the handling of `command-switch-alist` doesn’t treat equals signs in option specially. That is, if there’s an option like `--name=value` on the command line, then only a `command-switch-alist` member whose `car` is literally `--name=value` will match this option. If you want to parse such options, you need to use `command-line-functions` instead (see below).
The command-line arguments are parsed by the `command-line-1` function in the `startup.el` file. See also [Command Line Arguments for Emacs Invocation](https://www.gnu.org/software/emacs/manual/html_node/emacs/Emacs-Invocation.html#Emacs-Invocation) in The GNU Emacs Manual.
Variable: **command-line-args**
The value of this variable is the list of command-line arguments passed to Emacs.
Variable: **command-line-args-left**
The value of this variable is the list of command-line arguments that have not yet been processed.
Variable: **command-line-functions**
This variable’s value is a list of functions for handling an unrecognized command-line argument. Each time the next argument to be processed has no special meaning, the functions in this list are called, in order of appearance, until one of them returns a non-`nil` value.
These functions are called with no arguments. They can access the command-line argument under consideration through the variable `argi`, which is bound temporarily at this point. The remaining arguments (not including the current one) are in the variable `command-line-args-left`.
When a function recognizes and processes the argument in `argi`, it should return a non-`nil` value to say it has dealt with that argument. If it has also dealt with some of the following arguments, it can indicate that by deleting them from `command-line-args-left`.
If all of these functions return `nil`, then the argument is treated as a file name to visit.
elisp None ### Key Lookup
*Key lookup* is the process of finding the binding of a key sequence from a given keymap. The execution or use of the binding is not part of key lookup.
Key lookup uses just the event type of each event in the key sequence; the rest of the event is ignored. In fact, a key sequence used for key lookup may designate a mouse event with just its types (a symbol) instead of the entire event (a list). See [Input Events](input-events). Such a key sequence is insufficient for `command-execute` to run, but it is sufficient for looking up or rebinding a key.
When the key sequence consists of multiple events, key lookup processes the events sequentially: the binding of the first event is found, and must be a keymap; then the second event’s binding is found in that keymap, and so on until all the events in the key sequence are used up. (The binding thus found for the last event may or may not be a keymap.) Thus, the process of key lookup is defined in terms of a simpler process for looking up a single event in a keymap. How that is done depends on the type of object associated with the event in that keymap.
Let’s use the term *keymap entry* to describe the value found by looking up an event type in a keymap. (This doesn’t include the item string and other extra elements in a keymap element for a menu item, because `lookup-key` and other key lookup functions don’t include them in the returned value.) While any Lisp object may be stored in a keymap as a keymap entry, not all make sense for key lookup. Here is a table of the meaningful types of keymap entries:
`nil`
`nil` means that the events used so far in the lookup form an undefined key. When a keymap fails to mention an event type at all, and has no default binding, that is equivalent to a binding of `nil` for that event type.
command
The events used so far in the lookup form a complete key, and command is its binding. See [What Is a Function](what-is-a-function).
array
The array (either a string or a vector) is a keyboard macro. The events used so far in the lookup form a complete key, and the array is its binding. See [Keyboard Macros](keyboard-macros), for more information.
keymap
The events used so far in the lookup form a prefix key. The next event of the key sequence is looked up in keymap.
list
The meaning of a list depends on what it contains:
* If the CAR of list is the symbol `keymap`, then the list is a keymap, and is treated as a keymap (see above).
* If the CAR of list is `lambda`, then the list is a lambda expression. This is presumed to be a function, and is treated as such (see above). In order to execute properly as a key binding, this function must be a command—it must have an `interactive` specification. See [Defining Commands](defining-commands).
symbol
The function definition of symbol is used in place of symbol. If that too is a symbol, then this process is repeated, any number of times. Ultimately this should lead to an object that is a keymap, a command, or a keyboard macro.
Note that keymaps and keyboard macros (strings and vectors) are not valid functions, so a symbol with a keymap, string, or vector as its function definition is invalid as a function. It is, however, valid as a key binding. If the definition is a keyboard macro, then the symbol is also valid as an argument to `command-execute` (see [Interactive Call](interactive-call)).
The symbol `undefined` is worth special mention: it means to treat the key as undefined. Strictly speaking, the key is defined, and its binding is the command `undefined`; but that command does the same thing that is done automatically for an undefined key: it rings the bell (by calling `ding`) but does not signal an error.
`undefined` is used in local keymaps to override a global key binding and make the key undefined locally. A local binding of `nil` would fail to do this because it would not override the global binding.
anything else If any other type of object is found, the events used so far in the lookup form a complete key, and the object is its binding, but the binding is not executable as a command.
In short, a keymap entry may be a keymap, a command, a keyboard macro, a symbol that leads to one of them, or `nil`.
elisp None ### Case Changes
The case change commands described here work on text in the current buffer. See [Case Conversion](case-conversion), for case conversion functions that work on strings and characters. See [Case Tables](case-tables), for how to customize which characters are upper or lower case and how to convert them.
Command: **capitalize-region** *start end*
This function capitalizes all words in the region defined by start and end. To capitalize means to convert each word’s first character to upper case and convert the rest of each word to lower case. The function returns `nil`.
If one end of the region is in the middle of a word, the part of the word within the region is treated as an entire word.
When `capitalize-region` is called interactively, start and end are point and the mark, with the smallest first.
```
---------- Buffer: foo ----------
This is the contents of the 5th foo.
---------- Buffer: foo ----------
```
```
(capitalize-region 1 37)
⇒ nil
---------- Buffer: foo ----------
This Is The Contents Of The 5th Foo.
---------- Buffer: foo ----------
```
Command: **downcase-region** *start end*
This function converts all of the letters in the region defined by start and end to lower case. The function returns `nil`.
When `downcase-region` is called interactively, start and end are point and the mark, with the smallest first.
Command: **upcase-region** *start end*
This function converts all of the letters in the region defined by start and end to upper case. The function returns `nil`.
When `upcase-region` is called interactively, start and end are point and the mark, with the smallest first.
Command: **capitalize-word** *count*
This function capitalizes count words after point, moving point over as it does. To capitalize means to convert each word’s first character to upper case and convert the rest of each word to lower case. If count is negative, the function capitalizes the -count previous words but does not move point. The value is `nil`.
If point is in the middle of a word, the part of the word before point is ignored when moving forward. The rest is treated as an entire word.
When `capitalize-word` is called interactively, count is set to the numeric prefix argument.
Command: **downcase-word** *count*
This function converts the count words after point to all lower case, moving point over as it does. If count is negative, it converts the -count previous words but does not move point. The value is `nil`.
When `downcase-word` is called interactively, count is set to the numeric prefix argument.
Command: **upcase-word** *count*
This function converts the count words after point to all upper case, moving point over as it does. If count is negative, it converts the -count previous words but does not move point. The value is `nil`.
When `upcase-word` is called interactively, count is set to the numeric prefix argument.
| programming_docs |
elisp None ### JSONRPC communication
The `jsonrpc` library implements the JSONRPC specification, version 2.0, as it is described in <https://www.jsonrpc.org/>. As the name suggests, JSONRPC is a generic *Remote Procedure Call* protocol designed around JSON objects, which you can convert to and from Lisp objects (see [Parsing JSON](parsing-json)).
| | | |
| --- | --- | --- |
| • [JSONRPC Overview](jsonrpc-overview) | | |
| • [Process-based JSONRPC connections](process_002dbased-jsonrpc-connections) | | |
| • [JSONRPC JSON object format](jsonrpc-json-object-format) | | |
| • [JSONRPC deferred requests](jsonrpc-deferred-requests) | | |
elisp None #### Invoking the Input Method
The event-reading functions invoke the current input method, if any (see [Input Methods](input-methods)). If the value of `input-method-function` is non-`nil`, it should be a function; when `read-event` reads a printing character (including SPC) with no modifier bits, it calls that function, passing the character as an argument.
Variable: **input-method-function**
If this is non-`nil`, its value specifies the current input method function.
**Warning:** don’t bind this variable with `let`. It is often buffer-local, and if you bind it around reading input (which is exactly when you *would* bind it), switching buffers asynchronously while Emacs is waiting will cause the value to be restored in the wrong buffer.
The input method function should return a list of events which should be used as input. (If the list is `nil`, that means there is no input, so `read-event` waits for another event.) These events are processed before the events in `unread-command-events` (see [Event Input Misc](event-input-misc)). Events returned by the input method function are not passed to the input method function again, even if they are printing characters with no modifier bits.
If the input method function calls `read-event` or `read-key-sequence`, it should bind `input-method-function` to `nil` first, to prevent recursion.
The input method function is not called when reading the second and subsequent events of a key sequence. Thus, these characters are not subject to input method processing. The input method function should test the values of `overriding-local-map` and `overriding-terminal-local-map`; if either of these variables is non-`nil`, the input method should put its argument into a list and return that list with no further processing.
elisp None #### Format of GnuTLS Cryptography Inputs
The inputs to GnuTLS cryptographic functions can be specified in several ways, both as primitive Emacs Lisp types or as lists.
The list form is currently similar to how `md5` and `secure-hash` operate.
`buffer`
Simply passing a buffer as input means the whole buffer should be used.
`string`
A string as input will be used directly. It may be modified by the function (unlike most other Emacs Lisp functions) to reduce the chance of exposing sensitive data after the function does its work.
`(buffer-or-string start end coding-system noerror)`
This specifies a buffer or a string as described above, but an optional range can be specified with start and end.
In addition an optional coding-system can be specified if needed.
The last optional item, noerror, overrides the normal error when the text can’t be encoded using the specified or chosen coding system. When noerror is non-`nil`, this function silently uses `raw-text` coding instead.
`(`iv-auto` length)`
This generates a random IV (Initialization Vector) of the specified length and passes it to the function. This ensures that the IV is unpredictable and unlikely to be reused in the same session.
elisp None ### The Kill Ring
*Kill functions* delete text like the deletion functions, but save it so that the user can reinsert it by *yanking*. Most of these functions have ‘`kill-`’ in their name. By contrast, the functions whose names start with ‘`delete-`’ normally do not save text for yanking (though they can still be undone); these are deletion functions.
Most of the kill commands are primarily for interactive use, and are not described here. What we do describe are the functions provided for use in writing such commands. You can use these functions to write commands for killing text. When you need to delete text for internal purposes within a Lisp function, you should normally use deletion functions, so as not to disturb the kill ring contents. See [Deletion](deletion).
Killed text is saved for later yanking in the *kill ring*. This is a list that holds a number of recent kills, not just the last text kill. We call this a “ring” because yanking treats it as having elements in a cyclic order. The list is kept in the variable `kill-ring`, and can be operated on with the usual functions for lists; there are also specialized functions, described in this section, that treat it as a ring.
Some people think this use of the word “kill” is unfortunate, since it refers to operations that specifically *do not* destroy the entities killed. This is in sharp contrast to ordinary life, in which death is permanent and killed entities do not come back to life. Therefore, other metaphors have been proposed. For example, the term “cut ring” makes sense to people who, in pre-computer days, used scissors and paste to cut up and rearrange manuscripts. However, it would be difficult to change the terminology now.
| | | |
| --- | --- | --- |
| • [Kill Ring Concepts](kill-ring-concepts) | | What text looks like in the kill ring. |
| • [Kill Functions](kill-functions) | | Functions that kill text. |
| • [Yanking](yanking) | | How yanking is done. |
| • [Yank Commands](yank-commands) | | Commands that access the kill ring. |
| • [Low-Level Kill Ring](low_002dlevel-kill-ring) | | Functions and variables for kill ring access. |
| • [Internals of Kill Ring](internals-of-kill-ring) | | Variables that hold kill ring data. |
elisp None ### GnuTLS Cryptography
If compiled with GnuTLS, Emacs offers built-in cryptographic support. Following the GnuTLS API terminology, the available tools are digests, MACs, symmetric ciphers, and AEAD ciphers.
The terms used herein, such as IV (Initialization Vector), require some familiarity with cryptography and will not be defined in detail. Please consult <https://www.gnutls.org/> for specific documentation which may help you understand the terminology and structure of the GnuTLS library.
| | | |
| --- | --- | --- |
| • [Format of GnuTLS Cryptography Inputs](format-of-gnutls-cryptography-inputs) | | |
| • [GnuTLS Cryptographic Functions](gnutls-cryptographic-functions) | | |
elisp None ### Conditionals
Conditional control structures choose among alternatives. Emacs Lisp has five conditional forms: `if`, which is much the same as in other languages; `when` and `unless`, which are variants of `if`; `cond`, which is a generalized case statement; and `pcase`, which is a generalization of `cond` (see [Pattern-Matching Conditional](pattern_002dmatching-conditional)).
Special Form: **if** *condition then-form else-forms…*
`if` chooses between the then-form and the else-forms based on the value of condition. If the evaluated condition is non-`nil`, then-form is evaluated and the result returned. Otherwise, the else-forms are evaluated in textual order, and the value of the last one is returned. (The else part of `if` is an example of an implicit `progn`. See [Sequencing](sequencing).)
If condition has the value `nil`, and no else-forms are given, `if` returns `nil`.
`if` is a special form because the branch that is not selected is never evaluated—it is ignored. Thus, in this example, `true` is not printed because `print` is never called:
```
(if nil
(print 'true)
'very-false)
⇒ very-false
```
Macro: **when** *condition then-forms…*
This is a variant of `if` where there are no else-forms, and possibly several then-forms. In particular,
```
(when condition a b c)
```
is entirely equivalent to
```
(if condition (progn a b c) nil)
```
Macro: **unless** *condition forms…*
This is a variant of `if` where there is no then-form:
```
(unless condition a b c)
```
is entirely equivalent to
```
(if condition nil
a b c)
```
Special Form: **cond** *clause…*
`cond` chooses among an arbitrary number of alternatives. Each clause in the `cond` must be a list. The CAR of this list is the condition; the remaining elements, if any, the body-forms. Thus, a clause looks like this:
```
(condition body-forms…)
```
`cond` tries the clauses in textual order, by evaluating the condition of each clause. If the value of condition is non-`nil`, the clause succeeds; then `cond` evaluates its body-forms, and returns the value of the last of body-forms. Any remaining clauses are ignored.
If the value of condition is `nil`, the clause fails, so the `cond` moves on to the following clause, trying its condition.
A clause may also look like this:
```
(condition)
```
Then, if condition is non-`nil` when tested, the `cond` form returns the value of condition.
If every condition evaluates to `nil`, so that every clause fails, `cond` returns `nil`.
The following example has four clauses, which test for the cases where the value of `x` is a number, string, buffer and symbol, respectively:
```
(cond ((numberp x) x)
((stringp x) x)
((bufferp x)
(setq temporary-hack x) ; multiple body-forms
(buffer-name x)) ; in one clause
((symbolp x) (symbol-value x)))
```
Often we want to execute the last clause whenever none of the previous clauses was successful. To do this, we use `t` as the condition of the last clause, like this: `(t
body-forms)`. The form `t` evaluates to `t`, which is never `nil`, so this clause never fails, provided the `cond` gets to it at all. For example:
```
(setq a 5)
(cond ((eq a 'hack) 'foo)
(t "default"))
⇒ "default"
```
This `cond` expression returns `foo` if the value of `a` is `hack`, and returns the string `"default"` otherwise.
Any conditional construct can be expressed with `cond` or with `if`. Therefore, the choice between them is a matter of style. For example:
```
(if a b c)
≡
(cond (a b) (t c))
```
elisp None ### Creating an Asynchronous Process
In this section, we describe how to create an *asynchronous process*. After an asynchronous process is created, it runs in parallel with Emacs, and Emacs can communicate with it using the functions described in the following sections (see [Input to Processes](input-to-processes), and see [Output from Processes](output-from-processes)). Note that process communication is only partially asynchronous: Emacs sends and receives data to and from a process only when those functions are called.
An asynchronous process is controlled either via a *pty* (pseudo-terminal) or a *pipe*. The choice of pty or pipe is made when creating the process, by default based on the value of the variable `process-connection-type` (see below). If available, ptys are usually preferable for processes visible to the user, as in Shell mode, because they allow for job control (`C-c`, `C-z`, etc.) between the process and its children, and because interactive programs treat ptys as terminal devices, whereas pipes don’t support these features. However, for subprocesses used by Lisp programs for internal purposes (i.e., no user interaction with the subprocess is required), where significant amounts of data need to be exchanged between the subprocess and the Lisp program, it is often better to use a pipe, because pipes are more efficient. Also, the total number of ptys is limited on many systems, and it is good not to waste them unnecessarily.
Function: **make-process** *&rest args*
This function is the basic low-level primitive for starting asynchronous subprocesses. It returns a process object representing the subprocess. Compared to the more high-level `start-process`, described below, it takes keyword arguments, is more flexible, and allows to specify process filters and sentinels in a single call.
The arguments args are a list of keyword/argument pairs. Omitting a keyword is always equivalent to specifying it with value `nil`. Here are the meaningful keywords:
:name name
Use the string name as the process name; if a process with this name already exists, then name is modified (by appending ‘`<1>`’, etc.) to be unique.
:buffer buffer
Use buffer as the process buffer. If the value is `nil`, the subprocess is not associated with any buffer.
:command command
Use command as the command line of the process. The value should be a list starting with the program’s executable file name, followed by strings to give to the program as its arguments. If the first element of the list is `nil`, Emacs opens a new pseudoterminal (pty) and associates its input and output with buffer, without actually running any program; the rest of the list elements are ignored in that case.
:coding coding
If coding is a symbol, it specifies the coding system to be used for both reading and writing of data from and to the connection. If coding is a cons cell `(decoding . encoding)`, then decoding will be used for reading and encoding for writing. The coding system used for encoding the data written to the program is also used for encoding the command-line arguments (but not the program itself, whose file name is encoded as any other file name; see [file-name-coding-system](encoding-and-i_002fo)).
If coding is `nil`, the default rules for finding the coding system will apply. See [Default Coding Systems](default-coding-systems).
:connection-type type
Initialize the type of device used to communicate with the subprocess. Possible values are `pty` to use a pty, `pipe` to use a pipe, or `nil` to use the default derived from the value of the `process-connection-type` variable. This parameter and the value of `process-connection-type` are ignored if a non-`nil` value is specified for the `:stderr` parameter; in that case, the type will always be `pipe`. On systems where ptys are not available (MS-Windows), this parameter is likewise ignored, and pipes are used unconditionally.
:noquery query-flag
Initialize the process query flag to query-flag. See [Query Before Exit](query-before-exit).
:stop stopped
If provided, stopped must be `nil`; it is an error to use any non-`nil` value. The `:stop` key is ignored otherwise and is retained for compatibility with other process types such as pipe processes. Asynchronous subprocesses never start in the stopped state.
:filter filter
Initialize the process filter to filter. If not specified, a default filter will be provided, which can be overridden later. See [Filter Functions](filter-functions).
:sentinel sentinel
Initialize the process sentinel to sentinel. If not specified, a default sentinel will be used, which can be overridden later. See [Sentinels](sentinels).
:stderr stderr
Associate stderr with the standard error of the process. A non-`nil` value should be either a buffer or a pipe process created with `make-pipe-process`, described below. If stderr is `nil`, standard error is mixed with standard output, and both are sent to buffer or filter.
If stderr is a buffer, Emacs will create a pipe process, the *standard error process*. This process will have the default filter (see [Filter Functions](filter-functions)), sentinel (see [Sentinels](sentinels)), and coding systems (see [Default Coding Systems](default-coding-systems)). On the other hand, it will use query-flag as its query-on-exit flag (see [Query Before Exit](query-before-exit)). It will be associated with the stderr buffer (see [Process Buffers](process-buffers)) and send its output (which is the standard error of the main process) there. To get the process object for the standard error process, pass the stderr buffer to `get-buffer-process`.
If stderr is a pipe process, Emacs will use it as standard error process for the new process.
:file-handler file-handler
If file-handler is non-`nil`, then look for a file name handler for the current buffer’s `default-directory`, and invoke that file name handler to make the process. If there is no such handler, proceed as if file-handler were `nil`.
The original argument list, modified with the actual connection information, is available via the `process-contact` function.
The current working directory of the subprocess is set to the current buffer’s value of `default-directory` if that is local (as determined by `unhandled-file-name-directory`), or `~` otherwise. If you want to run a process in a remote directory, pass `:file-handler t` to `make-process`. In that case, the current working directory is the local name component of `default-directory` (as determined by `file-local-name`).
Depending on the implementation of the file name handler, it might not be possible to apply filter or sentinel to the resulting process object. The `:stderr` argument cannot be a pipe process, file name handlers do not support pipe processes for this. A buffer as `:stderr` argument is accepted, its contents is shown without the use of pipe processes. See [Filter Functions](filter-functions), [Sentinels](sentinels), and [Accepting Output](accepting-output).
Some file name handlers may not support `make-process`. In such cases, this function does nothing and returns `nil`.
Function: **make-pipe-process** *&rest args*
This function creates a bidirectional pipe which can be attached to a child process. This is useful with the `:stderr` keyword of `make-process`. The function returns a process object.
The arguments args are a list of keyword/argument pairs. Omitting a keyword is always equivalent to specifying it with value `nil`.
Here are the meaningful keywords:
:name name
Use the string name as the process name. As with `make-process`, it is modified if necessary to make it unique.
:buffer buffer
Use buffer as the process buffer.
:coding coding
If coding is a symbol, it specifies the coding system to be used for both reading and writing of data from and to the connection. If coding is a cons cell `(decoding . encoding)`, then decoding will be used for reading and encoding for writing.
If coding is `nil`, the default rules for finding the coding system will apply. See [Default Coding Systems](default-coding-systems).
:noquery query-flag
Initialize the process query flag to query-flag. See [Query Before Exit](query-before-exit).
:stop stopped
If stopped is non-`nil`, start the process in the stopped state. In the stopped state, a pipe process does not accept incoming data, but you can send outgoing data. The stopped state is set by `stop-process` and cleared by `continue-process` (see [Signals to Processes](signals-to-processes)).
:filter filter
Initialize the process filter to filter. If not specified, a default filter will be provided, which can be changed later. See [Filter Functions](filter-functions).
:sentinel sentinel
Initialize the process sentinel to sentinel. If not specified, a default sentinel will be used, which can be changed later. See [Sentinels](sentinels).
The original argument list, modified with the actual connection information, is available via the `process-contact` function.
Function: **start-process** *name buffer-or-name program &rest args*
This function is a higher-level wrapper around `make-process`, exposing an interface that is similar to `call-process`. It creates a new asynchronous subprocess and starts the specified program running in it. It returns a process object that stands for the new subprocess in Lisp. The argument name specifies the name for the process object; as with `make-process`, it is modified if necessary to make it unique. The buffer buffer-or-name is the buffer to associate with the process.
If program is `nil`, Emacs opens a new pseudoterminal (pty) and associates its input and output with buffer-or-name, without creating a subprocess. In that case, the remaining arguments args are ignored.
The rest of args are strings that specify command line arguments for the subprocess.
In the example below, the first process is started and runs (rather, sleeps) for 100 seconds (the output buffer ‘`foo`’ is created immediately). Meanwhile, the second process is started, and given the name ‘`my-process<1>`’ for the sake of uniqueness. It inserts the directory listing at the end of the buffer ‘`foo`’, before the first process finishes. Then it finishes, and a message to that effect is inserted in the buffer. Much later, the first process finishes, and another message is inserted in the buffer for it.
```
(start-process "my-process" "foo" "sleep" "100")
⇒ #<process my-process>
```
```
(start-process "my-process" "foo" "ls" "-l" "/bin")
⇒ #<process my-process<1>>
---------- Buffer: foo ----------
total 8336
-rwxr-xr-x 1 root root 971384 Mar 30 10:14 bash
-rwxr-xr-x 1 root root 146920 Jul 5 2011 bsd-csh
…
-rwxr-xr-x 1 root root 696880 Feb 28 15:55 zsh4
Process my-process<1> finished
Process my-process finished
---------- Buffer: foo ----------
```
Function: **start-file-process** *name buffer-or-name program &rest args*
Like `start-process`, this function starts a new asynchronous subprocess running program in it, and returns its process object.
The difference from `start-process` is that this function may invoke a file name handler based on the value of `default-directory`. This handler ought to run program, perhaps on the local host, perhaps on a remote host that corresponds to `default-directory`. In the latter case, the local part of `default-directory` becomes the working directory of the process.
This function does not try to invoke file name handlers for program or for the rest of args. For that reason, if program or any of args use the remote-file syntax (see [Magic File Names](magic-file-names)), they must be converted either to file names relative to `default-directory`, or to names that identify the files locally on the remote host, by running them through `file-local-name`.
Depending on the implementation of the file name handler, it might not be possible to apply `process-filter` or `process-sentinel` to the resulting process object. See [Filter Functions](filter-functions), and [Sentinels](sentinels).
Some file name handlers may not support `start-file-process` (for example the function `ange-ftp-hook-function`). In such cases, this function does nothing and returns `nil`.
Function: **start-process-shell-command** *name buffer-or-name command*
This function is like `start-process`, except that it uses a shell to execute the specified command. The argument command is a shell command string. The variable `shell-file-name` specifies which shell to use.
The point of running a program through the shell, rather than directly with `make-process` or `start-process`, is so that you can employ shell features such as wildcards in the arguments. It follows that if you include any arbitrary user-specified arguments in the command, you should quote them with `shell-quote-argument` first, so that any special shell characters do *not* have their special shell meanings. See [Shell Arguments](shell-arguments). Of course, when executing commands based on user input you should also consider the security implications.
Function: **start-file-process-shell-command** *name buffer-or-name command*
This function is like `start-process-shell-command`, but uses `start-file-process` internally. Because of this, command can also be executed on remote hosts, depending on `default-directory`.
Variable: **process-connection-type**
This variable controls the type of device used to communicate with asynchronous subprocesses. If it is non-`nil`, then ptys are used, when available. Otherwise, pipes are used.
The value of `process-connection-type` takes effect when `make-process` or `start-process` is called. So you can specify how to communicate with one subprocess by binding the variable around the call to these functions.
Note that the value of this variable is ignored when `make-process` is called with a non-`nil` value of the `:stderr` parameter; in that case, Emacs will communicate with the process using pipes. It is also ignored if ptys are unavailable (MS-Windows).
```
(let ((process-connection-type nil)) ; use a pipe
(start-process …))
```
To determine whether a given subprocess actually got a pipe or a pty, use the function `process-tty-name` (see [Process Information](process-information)).
| programming_docs |
elisp None #### Saving and Restoring the Match Data
When you call a function that may search, you may need to save and restore the match data around that call, if you want to preserve the match data from an earlier search for later use. Here is an example that shows the problem that arises if you fail to save the match data:
```
(re-search-forward "The \\(cat \\)")
⇒ 48
(foo) ; `foo` does more searching.
(match-end 0)
⇒ 61 ; Unexpected result—not 48!
```
You can save and restore the match data with `save-match-data`:
Macro: **save-match-data** *body…*
This macro executes body, saving and restoring the match data around it. The return value is the value of the last form in body.
You could use `set-match-data` together with `match-data` to imitate the effect of the special form `save-match-data`. Here is how:
```
(let ((data (match-data)))
(unwind-protect
… ; Ok to change the original match data.
(set-match-data data)))
```
Emacs automatically saves and restores the match data when it runs process filter functions (see [Filter Functions](filter-functions)) and process sentinels (see [Sentinels](sentinels)).
elisp None ### Parsing and Formatting Times
These functions convert time values to text in a string, and vice versa. Time values include `nil`, numbers, and Lisp timestamps (see [Time of Day](time-of-day)).
Function: **date-to-time** *string*
This function parses the time-string string and returns the corresponding Lisp timestamp. The argument string should represent a date-time, and should be in one of the forms recognized by `parse-time-string` (see below). This function assumes Universal Time if string lacks explicit time zone information. The operating system limits the range of time and zone values.
Function: **parse-time-string** *string*
This function parses the time-string string into a list of the following form:
```
(sec min hour day mon year dow dst tz)
```
The format of this list is the same as what `decode-time` accepts (see [Time Conversion](time-conversion)), and is described in more detail there. Any `dst` element that cannot be determined from the input is set to -1, and any other unknown element is set to `nil`. The argument string should resemble an RFC 822 (or later) or ISO 8601 string, like “Fri, 25 Mar 2016 16:24:56 +0100” or “1998-09-12T12:21:54-0200”, but this function will attempt to parse less well-formed time strings as well.
Function: **iso8601-parse** *string*
For a more strict function (that will error out upon invalid input), this function can be used instead. It can parse all variants of the ISO 8601 standard, so in addition to the formats mentioned above, it also parses things like “1998W45-3” (week number) and “1998-245” (ordinal day number). To parse durations, there’s `iso8601-parse-duration`, and to parse intervals, there’s `iso8601-parse-interval`. All these functions return decoded time structures, except the final one, which returns three of them (the start, the end, and the duration).
Function: **format-time-string** *format-string &optional time zone*
This function converts time (or the current time, if time is omitted or `nil`) to a string according to format-string. The conversion uses the time zone rule zone, which defaults to the current time zone rule. See [Time Zone Rules](time-zone-rules). The argument format-string may contain ‘`%`’-sequences which say to substitute parts of the time. Here is a table of what the ‘`%`’-sequences mean:
‘`%a`’ This stands for the abbreviated name of the day of week.
‘`%A`’ This stands for the full name of the day of week.
‘`%b`’ This stands for the abbreviated name of the month.
‘`%B`’ This stands for the full name of the month.
‘`%c`’ This is a synonym for ‘`%x %X`’.
‘`%C`’ This stands for the century, that is, the year divided by 100, truncated toward zero. The default field width is 2.
‘`%d`’ This stands for the day of month, zero-padded.
‘`%D`’ This is a synonym for ‘`%m/%d/%y`’.
‘`%e`’ This stands for the day of month, blank-padded.
‘`%F`’ This stands for the ISO 8601 date format, which is like ‘`%+4Y-%m-%d`’ except that any flags or field width override the ‘`+`’ and (after subtracting 6) the ‘`4`’.
‘`%g`’
This stands for the year without century (00–99) corresponding to the current *ISO week* number. ISO weeks start on Monday and end on Sunday. If an ISO week begins in one year and ends in another, the rules regarding which year ‘`%g`’ will produce are complex and will not be described here; however, in general, if most of the week’s days are in the ending year, ‘`%g`’ will produce that year.
‘`%G`’ This stands for the year with century corresponding to the current ISO week number.
‘`%h`’ This is a synonym for ‘`%b`’.
‘`%H`’ This stands for the hour (00–23).
‘`%I`’ This stands for the hour (01–12).
‘`%j`’ This stands for the day of the year (001–366).
‘`%k`’ This stands for the hour (0–23), blank padded.
‘`%l`’ This stands for the hour (1–12), blank padded.
‘`%m`’ This stands for the month (01–12).
‘`%M`’ This stands for the minute (00–59).
‘`%n`’ This stands for a newline.
‘`%N`’ This stands for the nanoseconds (000000000–999999999). To ask for fewer digits, use ‘`%3N`’ for milliseconds, ‘`%6N`’ for microseconds, etc. Any excess digits are discarded, without rounding.
‘`%p`’ This stands for ‘`AM`’ or ‘`PM`’, as appropriate.
‘`%q`’ This stands for the calendar quarter (1–4).
‘`%r`’ This is a synonym for ‘`%I:%M:%S %p`’.
‘`%R`’ This is a synonym for ‘`%H:%M`’.
‘`%s`’ This stands for the integer number of seconds since the epoch.
‘`%S`’ This stands for the second (00–59, or 00–60 on platforms that support leap seconds).
‘`%t`’ This stands for a tab character.
‘`%T`’ This is a synonym for ‘`%H:%M:%S`’.
‘`%u`’ This stands for the numeric day of week (1–7). Monday is day 1.
‘`%U`’ This stands for the week of the year (01–52), assuming that weeks start on Sunday.
‘`%V`’ This stands for the week of the year according to ISO 8601.
‘`%w`’ This stands for the numeric day of week (0–6). Sunday is day 0.
‘`%W`’ This stands for the week of the year (01–52), assuming that weeks start on Monday.
‘`%x`’ This has a locale-specific meaning. In the default locale (named ‘`C`’), it is equivalent to ‘`%D`’.
‘`%X`’ This has a locale-specific meaning. In the default locale (named ‘`C`’), it is equivalent to ‘`%T`’.
‘`%y`’ This stands for the year without century (00–99).
‘`%Y`’ This stands for the year with century.
‘`%Z`’ This stands for the time zone abbreviation (e.g., ‘`EST`’).
‘`%z`’ This stands for the time zone numerical offset. The ‘`z`’ can be preceded by one, two, or three colons; if plain ‘`%z`’ stands for ‘`-0500`’, then ‘`%:z`’ stands for ‘`-05:00`’, ‘`%::z`’ stands for ‘`-05:00:00`’, and ‘`%:::z`’ is like ‘`%::z`’ except it suppresses trailing instances of ‘`:00`’ so it stands for ‘`-05`’ in the same example.
‘`%%`’ This stands for a single ‘`%`’.
One or more flag characters can appear immediately after the ‘`%`’. ‘`0`’ pads with zeros, ‘`+`’ pads with zeros and also puts ‘`+`’ before nonnegative year numbers with more than four digits, ‘`\_`’ pads with blanks, ‘`-`’ suppresses padding, ‘`^`’ upper-cases letters, and ‘`#`’ reverses the case of letters.
You can also specify the field width and type of padding for any of these ‘`%`’-sequences. This works as in `printf`: you write the field width as digits in a ‘`%`’-sequence, after any flags. For example, ‘`%S`’ specifies the number of seconds since the minute; ‘`%03S`’ means to pad this with zeros to 3 positions, ‘`%\_3S`’ to pad with spaces to 3 positions. Plain ‘`%3S`’ pads with zeros, because that is how ‘`%S`’ normally pads to two positions.
The characters ‘`E`’ and ‘`O`’ act as modifiers when used after any flags and field widths in a ‘`%`’-sequence. ‘`E`’ specifies using the current locale’s alternative version of the date and time. In a Japanese locale, for example, `%Ex` might yield a date format based on the Japanese Emperors’ reigns. ‘`E`’ is allowed in ‘`%Ec`’, ‘`%EC`’, ‘`%Ex`’, ‘`%EX`’, ‘`%Ey`’, and ‘`%EY`’.
‘`O`’ means to use the current locale’s alternative representation of numbers, instead of the ordinary decimal digits. This is allowed with most letters, all the ones that output numbers.
To help debug programs, unrecognized ‘`%`’-sequences stand for themselves and are output as-is. Programs should not rely on this behavior, as future versions of Emacs may recognize new ‘`%`’-sequences as extensions.
This function uses the C library function `strftime` (see [Formatting Calendar Time](https://www.gnu.org/software/libc/manual/html_node/Formatting-Calendar-Time.html#Formatting-Calendar-Time) in The GNU C Library Reference Manual) to do most of the work. In order to communicate with that function, it first converts time and zone to internal form; the operating system limits the range of time and zone values. This function also encodes format-string using the coding system specified by `locale-coding-system` (see [Locales](locales)); after `strftime` returns the resulting string, this function decodes the string using that same coding system.
Function: **format-seconds** *format-string seconds*
This function converts its argument seconds into a string of years, days, hours, etc., according to format-string. The argument format-string may contain ‘`%`’-sequences which control the conversion. Here is a table of what the ‘`%`’-sequences mean:
‘`%y`’ ‘`%Y`’ The integer number of 365-day years.
‘`%d`’ ‘`%D`’ The integer number of days.
‘`%h`’ ‘`%H`’ The integer number of hours.
‘`%m`’ ‘`%M`’ The integer number of minutes.
‘`%s`’ ‘`%S`’ The number of seconds. If the optional ‘`,`’ parameter is used, it’s a floating point number, and the number after the ‘`,`’ specifies how many decimals to be used. ‘`%,2s`’ means “use two decimals”.
‘`%z`’ Non-printing control flag. When it is used, other specifiers must be given in the order of decreasing size, i.e., years before days, hours before minutes, etc. Nothing will be produced in the result string to the left of ‘`%z`’ until the first non-zero conversion is encountered. For example, the default format used by `emacs-uptime` (see [emacs-uptime](processor-run-time)) `"%Y, %D, %H, %M, %z%S"` means that the number of seconds will always be produced, but years, days, hours, and minutes will only be shown if they are non-zero.
‘`%%`’ Produces a literal ‘`%`’.
Upper-case format sequences produce the units in addition to the numbers, lower-case formats produce only the numbers.
You can also specify the field width by following the ‘`%`’ with a number; shorter numbers will be padded with blanks. An optional period before the width requests zero-padding instead. For example, `"%.3Y"` might produce `"004 years"`.
elisp None #### Indentation Primitives
This section describes the primitive functions used to count and insert indentation. The functions in the following sections use these primitives. See [Size of Displayed Text](size-of-displayed-text), for related functions.
Function: **current-indentation**
This function returns the indentation of the current line, which is the horizontal position of the first nonblank character. If the contents are entirely blank, then this is the horizontal position of the end of the line.
This function considers invisible text as having zero width, unless `buffer-invisibility-spec` specifies that invisible text should be displayed as ellipsis. See [Invisible Text](invisible-text).
Command: **indent-to** *column &optional minimum*
This function indents from point with tabs and spaces until column is reached. If minimum is specified and non-`nil`, then at least that many spaces are inserted even if this requires going beyond column. Otherwise the function does nothing if point is already beyond column. The value is the column at which the inserted indentation ends.
The inserted whitespace characters inherit text properties from the surrounding text (usually, from the preceding text only). See [Sticky Properties](sticky-properties).
User Option: **indent-tabs-mode**
If this variable is non-`nil`, indentation functions can insert tabs as well as spaces. Otherwise, they insert only spaces. Setting this variable automatically makes it buffer-local in the current buffer.
elisp None ### Counting Columns
The column functions convert between a character position (counting characters from the beginning of the buffer) and a column position (counting screen characters from the beginning of a line).
These functions count each character according to the number of columns it occupies on the screen. This means control characters count as occupying 2 or 4 columns, depending upon the value of `ctl-arrow`, and tabs count as occupying a number of columns that depends on the value of `tab-width` and on the column where the tab begins. See [Usual Display](usual-display).
Column number computations ignore the width of the window and the amount of horizontal scrolling. Consequently, a column value can be arbitrarily high. The first (or leftmost) column is numbered 0. They also ignore overlays and text properties, aside from invisibility. Invisible text is considered as having zero width, unless `buffer-invisibility-spec` specifies that invisible text should be displayed as ellipsis (see [Invisible Text](invisible-text)).
Function: **current-column**
This function returns the horizontal position of point, measured in columns, counting from 0 at the left margin. The column position is the sum of the widths of all the displayed representations of the characters between the start of the current line and point.
Command: **move-to-column** *column &optional force*
This function moves point to column in the current line. The calculation of column takes into account the widths of the displayed representations of the characters between the start of the line and point.
When called interactively, column is the value of prefix numeric argument. If column is not an integer, an error is signaled.
If it is impossible to move to column column because that is in the middle of a multicolumn character such as a tab, point moves to the end of that character. However, if force is non-`nil`, and column is in the middle of a tab, then `move-to-column` either converts the tab into spaces (when `indent-tabs-mode` is `nil`), or inserts enough spaces before it (otherwise), so that point can move precisely to column column. Other multicolumn characters can cause anomalies despite force, since there is no way to split them.
The argument force also has an effect if the line isn’t long enough to reach column column; if it is `t`, that means to add whitespace at the end of the line to reach that column.
The return value is the column number actually moved to.
elisp None Syntax Tables
-------------
A *syntax table* specifies the syntactic role of each character in a buffer. It can be used to determine where words, symbols, and other syntactic constructs begin and end. This information is used by many Emacs facilities, including Font Lock mode (see [Font Lock Mode](font-lock-mode)) and the various complex movement commands (see [Motion](motion)).
| | | |
| --- | --- | --- |
| • [Basics](syntax-basics) | | Basic concepts of syntax tables. |
| • [Syntax Descriptors](syntax-descriptors) | | How characters are classified. |
| • [Syntax Table Functions](syntax-table-functions) | | How to create, examine and alter syntax tables. |
| • [Syntax Properties](syntax-properties) | | Overriding syntax with text properties. |
| • [Motion and Syntax](motion-and-syntax) | | Moving over characters with certain syntaxes. |
| • [Parsing Expressions](parsing-expressions) | | Parsing balanced expressions using the syntax table. |
| • [Syntax Table Internals](syntax-table-internals) | | How syntax table information is stored. |
| • [Categories](categories) | | Another way of classifying character syntax. |
elisp None ### Narrowing
*Narrowing* means limiting the text addressable by Emacs editing commands to a limited range of characters in a buffer. The text that remains addressable is called the *accessible portion* of the buffer.
Narrowing is specified with two buffer positions, which become the beginning and end of the accessible portion. For most editing commands and primitives, these positions replace the values of the beginning and end of the buffer. While narrowing is in effect, no text outside the accessible portion is displayed, and point cannot move outside the accessible portion. Note that narrowing does not alter actual buffer positions (see [Point](point)); it only determines which positions are considered the accessible portion of the buffer. Most functions refuse to operate on text that is outside the accessible portion.
Commands for saving buffers are unaffected by narrowing; they save the entire buffer regardless of any narrowing.
If you need to display in a single buffer several very different types of text, consider using an alternative facility described in [Swapping Text](swapping-text).
Command: **narrow-to-region** *start end*
This function sets the accessible portion of the current buffer to start at start and end at end. Both arguments should be character positions.
In an interactive call, start and end are set to the bounds of the current region (point and the mark, with the smallest first).
Command: **narrow-to-page** *&optional move-count*
This function sets the accessible portion of the current buffer to include just the current page. An optional first argument move-count non-`nil` means to move forward or backward by move-count pages and then narrow to one page. The variable `page-delimiter` specifies where pages start and end (see [Standard Regexps](standard-regexps)).
In an interactive call, move-count is set to the numeric prefix argument.
Command: **widen**
This function cancels any narrowing in the current buffer, so that the entire contents are accessible. This is called *widening*. It is equivalent to the following expression:
```
(narrow-to-region 1 (1+ (buffer-size)))
```
Function: **buffer-narrowed-p**
This function returns non-`nil` if the buffer is narrowed, and `nil` otherwise.
Special Form: **save-restriction** *body…*
This special form saves the current bounds of the accessible portion, evaluates the body forms, and finally restores the saved bounds, thus restoring the same state of narrowing (or absence thereof) formerly in effect. The state of narrowing is restored even in the event of an abnormal exit via `throw` or error (see [Nonlocal Exits](nonlocal-exits)). Therefore, this construct is a clean way to narrow a buffer temporarily.
The value returned by `save-restriction` is that returned by the last form in body, or `nil` if no body forms were given.
**Caution:** it is easy to make a mistake when using the `save-restriction` construct. Read the entire description here before you try it.
If body changes the current buffer, `save-restriction` still restores the restrictions on the original buffer (the buffer whose restrictions it saved from), but it does not restore the identity of the current buffer.
`save-restriction` does *not* restore point; use `save-excursion` for that. If you use both `save-restriction` and `save-excursion` together, `save-excursion` should come first (on the outside). Otherwise, the old point value would be restored with temporary narrowing still in effect. If the old point value were outside the limits of the temporary narrowing, this would fail to restore it accurately.
Here is a simple example of correct use of `save-restriction`:
```
---------- Buffer: foo ----------
This is the contents of foo
This is the contents of foo
This is the contents of foo∗
---------- Buffer: foo ----------
```
```
(save-excursion
(save-restriction
(goto-char 1)
(forward-line 2)
(narrow-to-region 1 (point))
(goto-char (point-min))
(replace-string "foo" "bar")))
---------- Buffer: foo ----------
This is the contents of bar
This is the contents of bar
This is the contents of foo∗
---------- Buffer: foo ----------
```
| programming_docs |
elisp None ### Window System Selections
In window systems, such as X, data can be transferred between different applications by means of *selections*. X defines an arbitrary number of *selection types*, each of which can store its own data; however, only three are commonly used: the *clipboard*, *primary selection*, and *secondary selection*. Other window systems support only the clipboard. See [Cut and Paste](https://www.gnu.org/software/emacs/manual/html_node/emacs/Cut-and-Paste.html#Cut-and-Paste) in The GNU Emacs Manual, for Emacs commands that make use of these selections. This section documents the low-level functions for reading and setting window-system selections.
Command: **gui-set-selection** *type data*
This function sets a window-system selection. It takes two arguments: a selection type type, and the value to assign to it, data.
type should be a symbol; it is usually one of `PRIMARY`, `SECONDARY` or `CLIPBOARD`. These are symbols with upper-case names, in accord with X Window System conventions. If type is `nil`, that stands for `PRIMARY`.
If data is `nil`, it means to clear out the selection. Otherwise, data may be a string, a symbol, an integer (or a cons of two integers or list of two integers), an overlay, or a cons of two markers pointing to the same buffer. An overlay or a pair of markers stands for text in the overlay or between the markers. The argument data may also be a vector of valid non-vector selection values.
This function returns data.
Function: **gui-get-selection** *&optional type data-type*
This function accesses selections set up by Emacs or by other programs. It takes two optional arguments, type and data-type. The default for type, the selection type, is `PRIMARY`.
The data-type argument specifies the form of data conversion to use, to convert the raw data obtained from another program into Lisp data. Meaningful values include `TEXT`, `STRING`, `UTF8_STRING`, `TARGETS`, `LENGTH`, `DELETE`, `FILE_NAME`, `CHARACTER_POSITION`, `NAME`, `LINE_NUMBER`, `COLUMN_NUMBER`, `OWNER_OS`, `HOST_NAME`, `USER`, `CLASS`, `ATOM`, and `INTEGER`. (These are symbols with upper-case names in accord with X conventions.) The default for data-type is `STRING`. Window systems other than X usually support only a small subset of these types, in addition to `STRING`.
User Option: **selection-coding-system**
This variable specifies the coding system to use when reading and writing selections or the clipboard. See [Coding Systems](coding-systems). The default is `compound-text-with-extensions`, which converts to the text representation that X11 normally uses.
When Emacs runs on MS-Windows, it does not implement X selections in general, but it does support the clipboard. `gui-get-selection` and `gui-set-selection` on MS-Windows support the text data type only; if the clipboard holds other types of data, Emacs treats the clipboard as empty. The supported data type is `STRING`.
For backward compatibility, there are obsolete aliases `x-get-selection` and `x-set-selection`, which were the names of `gui-get-selection` and `gui-set-selection` before Emacs 25.1.
elisp None #### Mouse Dragging Parameters
The parameters described below provide support for resizing a frame by dragging its internal borders with the mouse. They also allow moving a frame with the mouse by dragging the header or tab line of its topmost or the mode line of its bottommost window.
These parameters are mostly useful for child frames (see [Child Frames](child-frames)) that come without window manager decorations. If necessary, they can be used for undecorated top-level frames as well.
`drag-internal-border`
If non-`nil`, the frame can be resized by dragging its internal borders, if present, with the mouse.
`drag-with-header-line`
If non-`nil`, the frame can be moved with the mouse by dragging the header line of its topmost window.
`drag-with-tab-line`
If non-`nil`, the frame can be moved with the mouse by dragging the tab line of its topmost window.
`drag-with-mode-line`
If non-`nil`, the frame can be moved with the mouse by dragging the mode line of its bottommost window. Note that such a frame is not allowed to have its own minibuffer window.
`snap-width`
A frame that is moved with the mouse will “snap” at the border(s) of the display or its parent frame whenever it is dragged as near to such an edge as the number of pixels specified by this parameter.
`top-visible`
If this parameter is a number, the top edge of the frame never appears above the top edge of its display or parent frame. Moreover, as many pixels of the frame as specified by that number will remain visible when the frame is moved against any of the remaining edges of its display or parent frame. Setting this parameter is useful to guard against dragging a child frame with a non-`nil` `drag-with-header-line` parameter completely out of the area of its parent frame.
`bottom-visible` If this parameter is a number, the bottom edge of the frame never appears below the bottom edge of its display or parent frame. Moreover, as many pixels of the frame as specified by that number will remain visible when the frame is moved against any of the remaining edges of its display or parent frame. Setting this parameter is useful to guard against dragging a child frame with a non-`nil` `drag-with-mode-line` parameter completely out of the area of its parent frame.
elisp None ### Syntax Table Functions
In this section we describe functions for creating, accessing and altering syntax tables.
Function: **make-syntax-table** *&optional table*
This function creates a new syntax table. If table is non-`nil`, the parent of the new syntax table is table; otherwise, the parent is the standard syntax table.
In the new syntax table, all characters are initially given the “inherit” (‘`@`’) syntax class, i.e., their syntax is inherited from the parent table (see [Syntax Class Table](syntax-class-table)).
Function: **copy-syntax-table** *&optional table*
This function constructs a copy of table and returns it. If table is omitted or `nil`, it returns a copy of the standard syntax table. Otherwise, an error is signaled if table is not a syntax table.
Command: **modify-syntax-entry** *char syntax-descriptor &optional table*
This function sets the syntax entry for char according to syntax-descriptor. char must be a character, or a cons cell of the form `(min . max)`; in the latter case, the function sets the syntax entries for all characters in the range between min and max, inclusive.
The syntax is changed only for table, which defaults to the current buffer’s syntax table, and not in any other syntax table.
The argument syntax-descriptor is a syntax descriptor, i.e., a string whose first character is a syntax class designator and whose second and subsequent characters optionally specify a matching character and syntax flags. See [Syntax Descriptors](syntax-descriptors). An error is signaled if syntax-descriptor is not a valid syntax descriptor.
This function always returns `nil`. The old syntax information in the table for this character is discarded.
```
Examples:
```
```
;; Put the space character in class whitespace.
(modify-syntax-entry ?\s " ")
⇒ nil
```
```
;; Make ‘`$`’ an open parenthesis character,
;; with ‘`^`’ as its matching close.
(modify-syntax-entry ?$ "(^")
⇒ nil
```
```
;; Make ‘`^`’ a close parenthesis character,
;; with ‘`$`’ as its matching open.
(modify-syntax-entry ?^ ")$")
⇒ nil
```
```
;; Make ‘`/`’ a punctuation character,
;; the first character of a start-comment sequence,
;; and the second character of an end-comment sequence.
;; This is used in C mode.
(modify-syntax-entry ?/ ". 14")
⇒ nil
```
Function: **char-syntax** *character*
This function returns the syntax class of character, represented by its designator character (see [Syntax Class Table](syntax-class-table)). This returns *only* the class, not its matching character or syntax flags.
The following examples apply to C mode. (We use `string` to make it easier to see the character returned by `char-syntax`.)
```
;; Space characters have whitespace syntax class.
(string (char-syntax ?\s))
⇒ " "
```
```
;; Forward slash characters have punctuation syntax.
;; Note that this char-syntax call does not reveal
;; that it is also part of comment-start and -end sequences.
(string (char-syntax ?/))
⇒ "."
```
```
;; Open parenthesis characters have open parenthesis syntax.
;; Note that this char-syntax call does not reveal that
;; it has a matching character, ‘)’.
(string (char-syntax ?\())
⇒ "("
```
Function: **set-syntax-table** *table*
This function makes table the syntax table for the current buffer. It returns table.
Function: **syntax-table**
This function returns the current syntax table, which is the table for the current buffer.
Command: **describe-syntax** *&optional buffer*
This command displays the contents of the syntax table of buffer (by default, the current buffer) in a help buffer.
Macro: **with-syntax-table** *table body…*
This macro executes body using table as the current syntax table. It returns the value of the last form in body, after restoring the old current syntax table.
Since each buffer has its own current syntax table, we should make that more precise: `with-syntax-table` temporarily alters the current syntax table of whichever buffer is current at the time the macro execution starts. Other buffers are not affected.
elisp None #### Defining new rx forms
The `rx` notation can be extended by defining new symbols and parameterized forms in terms of other `rx` expressions. This is handy for sharing parts between several regexps, and for making complex ones easier to build and understand by putting them together from smaller pieces.
For example, you could define `name` to mean `(one-or-more letter)`, and `(quoted x)` to mean `(seq ?' x ?')` for any x. These forms could then be used in `rx` expressions like any other: `(rx (quoted name))` would match a nonempty sequence of letters inside single quotes.
The Lisp macros below provide different ways of binding names to definitions. Common to all of them are the following rules:
* Built-in `rx` forms, like `digit` and `group`, cannot be redefined.
* The definitions live in a name space of their own, separate from that of Lisp variables. There is thus no need to attach a suffix like `-regexp` to names; they cannot collide with anything else.
* Definitions cannot refer to themselves recursively, directly or indirectly. If you find yourself needing this, you want a parser, not a regular expression.
* Definitions are only ever expanded in calls to `rx` or `rx-to-string`, not merely by their presence in definition macros. This means that the order of definitions doesn’t matter, even when they refer to each other, and that syntax errors only show up when they are used, not when they are defined.
* User-defined forms are allowed wherever arbitrary `rx` expressions are expected; for example, in the body of a `zero-or-one` form, but not inside `any` or `category` forms. They are also allowed inside `not` and `intersection` forms.
Macro: **rx-define** *name [arglist] rx-form*
Define name globally in all subsequent calls to `rx` and `rx-to-string`. If arglist is absent, then name is defined as a plain symbol to be replaced with rx-form. Example:
```
(rx-define haskell-comment (seq "--" (zero-or-more nonl)))
(rx haskell-comment)
⇒ "--.*"
```
If arglist is present, it must be a list of zero or more argument names, and name is then defined as a parameterized form. When used in an `rx` expression as `(name arg…)`, each arg will replace the corresponding argument name inside rx-form.
arglist may end in `&rest` and one final argument name, denoting a rest parameter. The rest parameter will expand to all extra actual argument values not matched by any other parameter in arglist, spliced into rx-form where it occurs. Example:
```
(rx-define moan (x y &rest r) (seq x (one-or-more y) r "!"))
(rx (moan "MOO" "A" "MEE" "OW"))
⇒ "MOOA+MEEOW!"
```
Since the definition is global, it is recommended to give name a package prefix to avoid name clashes with definitions elsewhere, as is usual when naming non-local variables and functions.
Forms defined this way only perform simple template substitution. For arbitrary computations, use them together with the `rx` forms `eval`, `regexp` or `literal`. Example:
```
(defun n-tuple-rx (n element)
`(seq "<"
(group-n 1 ,element)
,@(mapcar (lambda (i) `(seq ?, (group-n ,i ,element)))
(number-sequence 2 n))
">"))
(rx-define n-tuple (n element) (eval (n-tuple-rx n 'element)))
(rx (n-tuple 3 (+ (in "0-9"))))
⇒ "<\\(?1:[0-9]+\\),\\(?2:[0-9]+\\),\\(?3:[0-9]+\\)>"
```
Macro: **rx-let** *(bindings…) body…*
Make the `rx` definitions in bindings available locally for `rx` macro invocations in body, which is then evaluated.
Each element of bindings is on the form `(name [arglist] rx-form)`, where the parts have the same meaning as in `rx-define` above. Example:
```
(rx-let ((comma-separated (item) (seq item (0+ "," item)))
(number (1+ digit))
(numbers (comma-separated number)))
(re-search-forward (rx "(" numbers ")")))
```
The definitions are only available during the macro-expansion of body, and are thus not present during execution of compiled code.
`rx-let` can be used not only inside a function, but also at top level to include global variable and function definitions that need to share a common set of `rx` forms. Since the names are local inside body, there is no need for any package prefixes. Example:
```
(rx-let ((phone-number (seq (opt ?+) (1+ (any digit ?-)))))
(defun find-next-phone-number ()
(re-search-forward (rx phone-number)))
(defun phone-number-p (string)
(string-match-p (rx bos phone-number eos) string)))
```
The scope of the `rx-let` bindings is lexical, which means that they are not visible outside body itself, even in functions called from body.
Macro: **rx-let-eval** *bindings body…*
Evaluate bindings to a list of bindings as in `rx-let`, and evaluate body with those bindings in effect for calls to `rx-to-string`.
This macro is similar to `rx-let`, except that the bindings argument is evaluated (and thus needs to be quoted if it is a list literal), and the definitions are substituted at run time, which is required for `rx-to-string` to work. Example:
```
(rx-let-eval
'((ponder (x) (seq "Where have all the " x " gone?")))
(looking-at (rx-to-string
'(ponder (or "flowers" "young girls"
"left socks")))))
```
Another difference from `rx-let` is that the bindings are dynamically scoped, and thus also available in functions called from body. However, they are not visible inside functions defined in body.
elisp None Processes
---------
In the terminology of operating systems, a *process* is a space in which a program can execute. Emacs runs in a process. Emacs Lisp programs can invoke other programs in processes of their own. These are called *subprocesses* or *child processes* of the Emacs process, which is their *parent process*.
A subprocess of Emacs may be *synchronous* or *asynchronous*, depending on how it is created. When you create a synchronous subprocess, the Lisp program waits for the subprocess to terminate before continuing execution. When you create an asynchronous subprocess, it can run in parallel with the Lisp program. This kind of subprocess is represented within Emacs by a Lisp object which is also called a “process”. Lisp programs can use this object to communicate with the subprocess or to control it. For example, you can send signals, obtain status information, receive output from the process, or send input to it.
In addition to processes that run programs, Lisp programs can open connections of several types to devices or processes running on the same machine or on other machines. The supported connection types are: TCP and UDP network connections, serial port connections, and pipe connections. Each such connection is also represented by a process object.
Function: **processp** *object*
This function returns `t` if object represents an Emacs process object, `nil` otherwise. The process object can represent a subprocess running a program or a connection of any supported type.
In addition to subprocesses of the current Emacs session, you can also access other processes running on your machine. See [System Processes](system-processes).
| | | |
| --- | --- | --- |
| • [Subprocess Creation](subprocess-creation) | | Functions that start subprocesses. |
| • [Shell Arguments](shell-arguments) | | Quoting an argument to pass it to a shell. |
| • [Synchronous Processes](synchronous-processes) | | Details of using synchronous subprocesses. |
| • [Asynchronous Processes](asynchronous-processes) | | Starting up an asynchronous subprocess. |
| • [Deleting Processes](deleting-processes) | | Eliminating an asynchronous subprocess. |
| • [Process Information](process-information) | | Accessing run-status and other attributes. |
| • [Input to Processes](input-to-processes) | | Sending input to an asynchronous subprocess. |
| • [Signals to Processes](signals-to-processes) | | Stopping, continuing or interrupting an asynchronous subprocess. |
| • [Output from Processes](output-from-processes) | | Collecting output from an asynchronous subprocess. |
| • [Sentinels](sentinels) | | Sentinels run when process run-status changes. |
| • [Query Before Exit](query-before-exit) | | Whether to query if exiting will kill a process. |
| • [System Processes](system-processes) | | Accessing other processes running on your system. |
| • [Transaction Queues](transaction-queues) | | Transaction-based communication with subprocesses. |
| • [Network](network) | | Opening network connections. |
| • [Network Servers](network-servers) | | Network servers let Emacs accept net connections. |
| • [Datagrams](datagrams) | | UDP network connections. |
| • [Low-Level Network](low_002dlevel-network) | | Lower-level but more general function to create connections and servers. |
| • [Misc Network](misc-network) | | Additional relevant functions for net connections. |
| • [Serial Ports](serial-ports) | | Communicating with serial ports. |
| • [Byte Packing](byte-packing) | | Using bindat to pack and unpack binary data. |
elisp None #### Buffer Parameters
These frame parameters, meaningful on all kinds of terminals, deal with which buffers have been, or should, be displayed in the frame.
`minibuffer`
Whether this frame has its own minibuffer. The value `t` means yes, `nil` means no, `only` means this frame is just a minibuffer. If the value is a minibuffer window (in some other frame), the frame uses that minibuffer.
This parameter takes effect when the frame is created. If specified as `nil`, Emacs will try to set it to the minibuffer window of `default-minibuffer-frame` (see [Minibuffers and Frames](minibuffers-and-frames)). For an existing frame, this parameter can be used exclusively to specify another minibuffer window. It is not allowed to change it from a minibuffer window to `t` and vice-versa, or from `t` to `nil`. If the parameter specifies a minibuffer window already, setting it to `nil` has no effect.
The special value `child-frame` means to make a minibuffer-only child frame (see [Child Frames](child-frames)) whose parent becomes the frame created. As if specified as `nil`, Emacs will set this parameter to the minibuffer window of the child frame but will not select the child frame after its creation.
`buffer-predicate`
The buffer-predicate function for this frame. The function `other-buffer` uses this predicate (from the selected frame) to decide which buffers it should consider, if the predicate is not `nil`. It calls the predicate with one argument, a buffer, once for each buffer; if the predicate returns a non-`nil` value, it considers that buffer.
`buffer-list`
A list of buffers that have been selected in this frame, ordered most-recently-selected first.
`unsplittable` If non-`nil`, this frame’s window is never split automatically.
| programming_docs |
elisp None #### SVG Images
SVG (Scalable Vector Graphics) is an XML format for specifying images. SVG images support the following additional image descriptor properties:
`:foreground foreground`
foreground, if non-`nil`, should be a string specifying a color, which is used as the image’s foreground color. If the value is `nil`, it defaults to the current face’s foreground color.
`:background background`
background, if non-`nil`, should be a string specifying a color, which is used as the image’s background color if the image supports transparency. If the value is `nil`, it defaults to the current face’s background color.
`:css css` css, if non-`nil`, should be a string specifying the CSS to override the default CSS used when generating the image.
#### SVG library
If your Emacs build has SVG support, you can create and manipulate these images with the following functions from the `svg.el` library.
Function: **svg-create** *width height &rest args*
Create a new, empty SVG image with the specified dimensions. args is an argument plist with you can specify following:
`:stroke-width`
The default width (in pixels) of any lines created.
`:stroke` The default stroke color on any lines created.
This function returns an *SVG object*, a Lisp data structure that specifies an SVG image, and all the following functions work on that structure. The argument svg in the following functions specifies such an SVG object.
Function: **svg-gradient** *svg id type stops*
Create a gradient in svg with identifier id. type specifies the gradient type, and can be either `linear` or `radial`. stops is a list of percentage/color pairs.
The following will create a linear gradient that goes from red at the start, to green 25% of the way, to blue at the end:
```
(svg-gradient svg "gradient1" 'linear
'((0 . "red") (25 . "green") (100 . "blue")))
```
The gradient created (and inserted into the SVG object) can later be used by all functions that create shapes.
All the following functions take an optional list of keyword parameters that alter the various attributes from their default values. Valid attributes include:
`:stroke-width`
The width (in pixels) of lines drawn, and outlines around solid shapes.
`:stroke-color`
The color of lines drawn, and outlines around solid shapes.
`:fill-color`
The color used for solid shapes.
`:id`
The identified of the shape.
`:gradient`
If given, this should be the identifier of a previously defined gradient object.
`:clip-path` Identifier of a clip path.
Function: **svg-rectangle** *svg x y width height &rest args*
Add to svg a rectangle whose upper left corner is at position x/y and whose size is width/height.
```
(svg-rectangle svg 100 100 500 500 :gradient "gradient1")
```
Function: **svg-circle** *svg x y radius &rest args*
Add to svg a circle whose center is at x/y and whose radius is radius.
Function: **svg-ellipse** *svg x y x-radius y-radius &rest args*
Add to svg an ellipse whose center is at x/y, and whose horizontal radius is x-radius and the vertical radius is y-radius.
Function: **svg-line** *svg x1 y1 x2 y2 &rest args*
Add to svg a line that starts at x1/y1 and extends to x2/y2.
Function: **svg-polyline** *svg points &rest args*
Add to svg a multiple-segment line (a.k.a. “polyline”) that goes through points, which is a list of X/Y position pairs.
```
(svg-polyline svg '((200 . 100) (500 . 450) (80 . 100))
:stroke-color "green")
```
Function: **svg-polygon** *svg points &rest args*
Add a polygon to svg where points is a list of X/Y pairs that describe the outer circumference of the polygon.
```
(svg-polygon svg '((100 . 100) (200 . 150) (150 . 90))
:stroke-color "blue" :fill-color "red")
```
Function: **svg-path** *svg commands &rest args*
Add the outline of a shape to svg according to commands, see [SVG Path Commands](#SVG-Path-Commands).
Coordinates by default are absolute. To use coordinates relative to the last position, or – initially – to the origin, set the attribute :relative to `t`. This attribute can be specified for the function or for individual commands. If specified for the function, then all commands use relative coordinates by default. To make an individual command use absolute coordinates, set :relative to `nil`.
```
(svg-path svg
'((moveto ((100 . 100)))
(lineto ((200 . 0) (0 . 200) (-200 . 0)))
(lineto ((100 . 100)) :relative nil))
:stroke-color "blue"
:fill-color "lightblue"
:relative t)
```
Function: **svg-text** *svg text &rest args*
Add the specified text to svg.
```
(svg-text
svg "This is a text"
:font-size "40"
:font-weight "bold"
:stroke "black"
:fill "white"
:font-family "impact"
:letter-spacing "4pt"
:x 300
:y 400
:stroke-width 1)
```
Function: **svg-embed** *svg image image-type datap &rest args*
Add an embedded (raster) image to svg. If datap is `nil`, image should be a file name; otherwise it should be a string containing the image data as raw bytes. image-type should be a MIME image type, for instance `"image/jpeg"`.
```
(svg-embed svg "~/rms.jpg" "image/jpeg" nil
:width "100px" :height "100px"
:x "50px" :y "75px")
```
Function: **svg-embed-base-uri-image** *svg relative-filename &rest args*
To svg add an embedded (raster) image placed at relative-filename. relative-filename is searched inside `file-name-directory` of the `:base-uri` svg image property. `:base-uri` specifies a (possibly non-existing) file name of the svg image to be created, thus all the embedded files are searched relatively to the `:base-uri` filename’s directory. If `:base-uri` is omitted, then filename from where svg image is loaded is used. Using `:base-uri` improves the performance of embedding large images, comparing to `svg-embed`, because all the work is done directly by librsvg.
```
;; Embeding /tmp/subdir/rms.jpg and /tmp/another/rms.jpg
(svg-embed-base-uri-image svg "subdir/rms.jpg"
:width "100px" :height "100px"
:x "50px" :y "75px")
(svg-embed-base-uri-image svg "another/rms.jpg"
:width "100px" :height "100px"
:x "75px" :y "50px")
(svg-image svg :scale 1.0
:base-uri "/tmp/dummy"
:width 175 :height 175)
```
Function: **svg-clip-path** *svg &rest args*
Add a clipping path to svg. If applied to a shape via the :clip-path property, parts of that shape which lie outside of the clipping path are not drawn.
```
(let ((clip-path (svg-clip-path svg :id "foo")))
(svg-circle clip-path 200 200 175))
(svg-rectangle svg 50 50 300 300
:fill-color "red"
:clip-path "url(#foo)")
```
Function: **svg-node** *svg tag &rest args*
Add the custom node tag to svg.
```
(svg-node svg
'rect
:width 300 :height 200 :x 50 :y 100 :fill-color "green")
```
Function: **svg-remove** *svg id*
Remove the element with identifier `id` from the `svg`.
Function: **svg-image** *svg*
Finally, the `svg-image` takes an SVG object as its argument and returns an image object suitable for use in functions like `insert-image`.
Here’s a complete example that creates and inserts an image with a circle:
```
(let ((svg (svg-create 400 400 :stroke-width 10)))
(svg-gradient svg "gradient1" 'linear '((0 . "red") (100 . "blue")))
(svg-circle svg 200 200 100 :gradient "gradient1"
:stroke-color "green")
(insert-image (svg-image svg)))
```
#### SVG Path Commands
*SVG paths* allow creation of complex images by combining lines, curves, arcs, and other basic shapes. The functions described below allow invoking SVG path commands from a Lisp program.
Command: **moveto** *points*
Move the pen to the first point in points. Additional points are connected with lines. points is a list of X/Y coordinate pairs. Subsequent `moveto` commands represent the start of a new *subpath*.
```
(svg-path svg '((moveto ((200 . 100) (100 . 200) (0 . 100))))
:fill "white" :stroke "black")
```
Command: **closepath**
End the current subpath by connecting it back to its initial point. A line is drawn along the connection.
```
(svg-path svg '((moveto ((200 . 100) (100 . 200) (0 . 100)))
(closepath)
(moveto ((75 . 125) (100 . 150) (125 . 125)))
(closepath))
:fill "red" :stroke "black")
```
Command: **lineto** *points*
Draw a line from the current point to the first element in points, a list of X/Y position pairs. If more than one point is specified, draw a polyline.
```
(svg-path svg '((moveto ((200 . 100)))
(lineto ((100 . 200) (0 . 100))))
:fill "yellow" :stroke "red")
```
Command: **horizontal-lineto** *x-coordinates*
Draw a horizontal line from the current point to the first element in x-coordinates. Specifying multiple coordinates is possible, although usually this doesn’t make sense.
```
(svg-path svg '((moveto ((100 . 200)))
(horizontal-lineto (300)))
:stroke "green")
```
Command: **vertical-lineto** *y-coordinates*
Draw vertical lines.
```
(svg-path svg '((moveto ((200 . 100)))
(vertical-lineto (300)))
:stroke "green")
```
Command: **curveto** *coordinate-sets*
Using the first element in coordinate-sets, draw a cubic Bézier curve from the current point. If there are multiple coordinate sets, draw a polybezier. Each coordinate set is a list of the form `(x1 y1 x2 y2 x y)`, where (x, y) is the curve’s end point. (x1, y1) and (x2, y2) are control points at the beginning and at the end, respectively.
```
(svg-path svg '((moveto ((100 . 100)))
(curveto ((200 100 100 200 200 200)
(300 200 0 100 100 100))))
:fill "transparent" :stroke "red")
```
Command: **smooth-curveto** *coordinate-sets*
Using the first element in coordinate-sets, draw a cubic Bézier curve from the current point. If there are multiple coordinate sets, draw a polybezier. Each coordinate set is a list of the form `(x2 y2 x y)`, where (x, y) is the curve’s end point and (x2, y2) is the corresponding control point. The first control point is the reflection of the second control point of the previous command relative to the current point, if that command was `curveto` or `smooth-curveto`. Otherwise the first control point coincides with the current point.
```
(svg-path svg '((moveto ((100 . 100)))
(curveto ((200 100 100 200 200 200)))
(smooth-curveto ((0 100 100 100))))
:fill "transparent" :stroke "blue")
```
Command: **quadratic-bezier-curveto** *coordinate-sets*
Using the first element in coordinate-sets, draw a quadratic Bézier curve from the current point. If there are multiple coordinate sets, draw a polybezier. Each coordinate set is a list of the form `(x1 y1 x y)`, where (x, y) is the curve’s end point and (x1, y1) is the control point.
```
(svg-path svg '((moveto ((200 . 100)))
(quadratic-bezier-curveto ((300 100 300 200)))
(quadratic-bezier-curveto ((300 300 200 300)))
(quadratic-bezier-curveto ((100 300 100 200)))
(quadratic-bezier-curveto ((100 100 200 100))))
:fill "transparent" :stroke "pink")
```
Command: **smooth-quadratic-bezier-curveto** *coordinate-sets*
Using the first element in coordinate-sets, draw a quadratic Bézier curve from the current point. If there are multiple coordinate sets, draw a polybezier. Each coordinate set is a list of the form `(x y)`, where (x, y) is the curve’s end point. The control point is the reflection of the control point of the previous command relative to the current point, if that command was `quadratic-bezier-curveto` or `smooth-quadratic-bezier-curveto`. Otherwise the control point coincides with the current point.
```
(svg-path svg '((moveto ((200 . 100)))
(quadratic-bezier-curveto ((300 100 300 200)))
(smooth-quadratic-bezier-curveto ((200 300)))
(smooth-quadratic-bezier-curveto ((100 200)))
(smooth-quadratic-bezier-curveto ((200 100))))
:fill "transparent" :stroke "lightblue")
```
Command: **elliptical-arc** *coordinate-sets*
Using the first element in coordinate-sets, draw an elliptical arc from the current point. If there are multiple coordinate sets, draw a sequence of elliptical arcs. Each coordinate set is a list of the form `(rx ry x y)`, where (x, y) is the end point of the ellipse, and (rx, ry) are its radii. Attributes may be appended to the list:
`:x-axis-rotation`
The angle in degrees by which the x-axis of the ellipse is rotated relative to the x-axis of the current coordinate system.
`:large-arc`
If set to `t`, draw an arc sweep greater than or equal to 180 degrees. Otherwise, draw an arc sweep smaller than or equal to 180 degrees.
`:sweep` If set to `t`, draw an arc in *positive angle direction*. Otherwise, draw it in *negative angle direction*.
```
(svg-path svg '((moveto ((200 . 250)))
(elliptical-arc ((75 75 200 350))))
:fill "transparent" :stroke "red")
(svg-path svg '((moveto ((200 . 250)))
(elliptical-arc ((75 75 200 350 :large-arc t))))
:fill "transparent" :stroke "green")
(svg-path svg '((moveto ((200 . 250)))
(elliptical-arc ((75 75 200 350 :sweep t))))
:fill "transparent" :stroke "blue")
(svg-path svg '((moveto ((200 . 250)))
(elliptical-arc ((75 75 200 350 :large-arc t
:sweep t))))
:fill "transparent" :stroke "gray")
(svg-path svg '((moveto ((160 . 100)))
(elliptical-arc ((40 100 80 0)))
(elliptical-arc ((40 100 -40 -70
:x-axis-rotation -120)))
(elliptical-arc ((40 100 -40 70
:x-axis-rotation -240))))
:stroke "pink" :fill "lightblue"
:relative t)
```
elisp None ### Other Hash Table Functions
Here are some other functions for working with hash tables.
Function: **hash-table-p** *table*
This returns non-`nil` if table is a hash table object.
Function: **copy-hash-table** *table*
This function creates and returns a copy of table. Only the table itself is copied—the keys and values are shared.
Function: **hash-table-count** *table*
This function returns the actual number of entries in table.
Function: **hash-table-test** *table*
This returns the test value that was given when table was created, to specify how to hash and compare keys. See `make-hash-table` (see [Creating Hash](creating-hash)).
Function: **hash-table-weakness** *table*
This function returns the weak value that was specified for hash table table.
Function: **hash-table-rehash-size** *table*
This returns the rehash size of table.
Function: **hash-table-rehash-threshold** *table*
This returns the rehash threshold of table.
Function: **hash-table-size** *table*
This returns the current nominal size of table.
elisp None ### Distinguish Interactive Calls
Sometimes a command should display additional visual feedback (such as an informative message in the echo area) for interactive calls only. There are three ways to do this. The recommended way to test whether the function was called using `call-interactively` is to give it an optional argument `print-message` and use the `interactive` spec to make it non-`nil` in interactive calls. Here’s an example:
```
(defun foo (&optional print-message)
(interactive "p")
(when print-message
(message "foo")))
```
We use `"p"` because the numeric prefix argument is never `nil`. Defined in this way, the function does display the message when called from a keyboard macro.
The above method with the additional argument is usually best, because it allows callers to say “treat this call as interactive”. But you can also do the job by testing `called-interactively-p`.
Function: **called-interactively-p** *kind*
This function returns `t` when the calling function was called using `call-interactively`.
The argument kind should be either the symbol `interactive` or the symbol `any`. If it is `interactive`, then `called-interactively-p` returns `t` only if the call was made directly by the user—e.g., if the user typed a key sequence bound to the calling function, but *not* if the user ran a keyboard macro that called the function (see [Keyboard Macros](keyboard-macros)). If kind is `any`, `called-interactively-p` returns `t` for any kind of interactive call, including keyboard macros.
If in doubt, use `any`; the only known proper use of `interactive` is if you need to decide whether to display a helpful message while a function is running.
A function is never considered to be called interactively if it was called via Lisp evaluation (or with `apply` or `funcall`).
Here is an example of using `called-interactively-p`:
```
(defun foo ()
(interactive)
(when (called-interactively-p 'any)
(message "Interactive!")
'foo-called-interactively))
```
```
;; Type `M-x foo`.
-| Interactive!
```
```
(foo)
⇒ nil
```
Here is another example that contrasts direct and indirect calls to `called-interactively-p`.
```
(defun bar ()
(interactive)
(message "%s" (list (foo) (called-interactively-p 'any))))
```
```
;; Type `M-x bar`.
-| (nil t)
```
elisp None ### Floating-Point Basics
Floating-point numbers are useful for representing numbers that are not integral. The range of floating-point numbers is the same as the range of the C data type `double` on the machine you are using. On all computers supported by Emacs, this is IEEE binary64 floating point format, which is standardized by [IEEE Std 754-2019](https://standards.ieee.org/standard/754-2019.html) and is discussed further in David Goldberg’s paper “[What Every Computer Scientist Should Know About Floating-Point Arithmetic](https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html)”. On modern platforms, floating-point operations follow the IEEE-754 standard closely; however, results are not always rounded correctly on some obsolescent platforms, notably 32-bit x86.
The read syntax for floating-point numbers requires either a decimal point, an exponent, or both. Optional signs (‘`+`’ or ‘`-`’) precede the number and its exponent. For example, ‘`1500.0`’, ‘`+15e2`’, ‘`15.0e+2`’, ‘`+1500000e-3`’, and ‘`.15e4`’ are five ways of writing a floating-point number whose value is 1500. They are all equivalent. Like Common Lisp, Emacs Lisp requires at least one digit after a decimal point in a floating-point number that does not have an exponent; ‘`1500.`’ is an integer, not a floating-point number.
Emacs Lisp treats `-0.0` as numerically equal to ordinary zero with respect to numeric comparisons like `=`. This follows the IEEE floating-point standard, which says `-0.0` and `0.0` are numerically equal even though other operations can distinguish them.
The IEEE floating-point standard supports positive infinity and negative infinity as floating-point values. It also provides for a class of values called NaN, or “not a number”; numerical functions return such values in cases where there is no correct answer. For example, `(/ 0.0 0.0)` returns a NaN. A NaN is never numerically equal to any value, not even to itself. NaNs carry a sign and a significand, and non-numeric functions treat two NaNs as equal when their signs and significands agree. Significands of NaNs are machine-dependent, as are the digits in their string representation.
When NaNs and signed zeros are involved, non-numeric functions like `eql`, `equal`, `sxhash-eql`, `sxhash-equal` and `gethash` determine whether values are indistinguishable, not whether they are numerically equal. For example, when x and y are the same NaN, `(equal x y)` returns `t` whereas `(= x y)` uses numeric comparison and returns `nil`; conversely, `(equal 0.0 -0.0)` returns `nil` whereas `(= 0.0 -0.0)` returns `t`.
Here are read syntaxes for these special floating-point values:
infinity ‘`1.0e+INF`’ and ‘`-1.0e+INF`’
not-a-number ‘`0.0e+NaN`’ and ‘`-0.0e+NaN`’
The following functions are specialized for handling floating-point numbers:
Function: **isnan** *x*
This predicate returns `t` if its floating-point argument is a NaN, `nil` otherwise.
Function: **frexp** *x*
This function returns a cons cell `(s . e)`, where s and e are respectively the significand and exponent of the floating-point number x.
If x is finite, then s is a floating-point number between 0.5 (inclusive) and 1.0 (exclusive), e is an integer, and x = s \* 2\*\*e. If x is zero or infinity, then s is the same as x. If x is a NaN, then s is also a NaN. If x is zero, then e is 0.
Function: **ldexp** *s e*
Given a numeric significand s and an integer exponent e, this function returns the floating point number s \* 2\*\*e.
Function: **copysign** *x1 x2*
This function copies the sign of x2 to the value of x1, and returns the result. x1 and x2 must be floating point.
Function: **logb** *x*
This function returns the binary exponent of x. More precisely, if x is finite and nonzero, the value is the logarithm base 2 of *|x|*, rounded down to an integer. If x is zero or infinite, the value is infinity; if x is a NaN, the value is a NaN.
```
(logb 10)
⇒ 3
(logb 10.0e20)
⇒ 69
(logb 0)
⇒ -1.0e+INF
```
| programming_docs |
elisp None ### Stack-allocated Objects
The garbage collector described above is used to manage data visible from Lisp programs, as well as most of the data internally used by the Lisp interpreter. Sometimes it may be useful to allocate temporary internal objects using the C stack of the interpreter. This can help performance, as stack allocation is typically faster than using heap memory to allocate and the garbage collector to free. The downside is that using such objects after they are freed results in undefined behavior, so uses should be well thought out and carefully debugged by using the `GC_CHECK_MARKED_OBJECTS` feature (see `src/alloc.c`). In particular, stack-allocated objects should never be made visible to user Lisp code.
Currently, cons cells and strings can be allocated this way. This is implemented by C macros like `AUTO_CONS` and `AUTO_STRING` that define a named `Lisp_Object` with block lifetime. These objects are not freed by the garbage collector; instead, they have automatic storage duration, i.e., they are allocated like local variables and are automatically freed at the end of execution of the C block that defined the object.
For performance reasons, stack-allocated strings are limited to ASCII characters, and many of these strings are immutable, i.e., calling `ASET` on them produces undefined behavior.
elisp None ### Buffer Modification Time
Suppose that you visit a file and make changes in its buffer, and meanwhile the file itself is changed on disk. At this point, saving the buffer would overwrite the changes in the file. Occasionally this may be what you want, but usually it would lose valuable information. Emacs therefore checks the file’s modification time using the functions described below before saving the file. (See [File Attributes](file-attributes), for how to examine a file’s modification time.)
Function: **verify-visited-file-modtime** *&optional buffer*
This function compares what buffer (by default, the current-buffer) has recorded for the modification time of its visited file against the actual modification time of the file as recorded by the operating system. The two should be the same unless some other process has written the file since Emacs visited or saved it.
The function returns `t` if the last actual modification time and Emacs’s recorded modification time are the same, `nil` otherwise. It also returns `t` if the buffer has no recorded last modification time, that is if `visited-file-modtime` would return zero.
It always returns `t` for buffers that are not visiting a file, even if `visited-file-modtime` returns a non-zero value. For instance, it always returns `t` for dired buffers. It returns `t` for buffers that are visiting a file that does not exist and never existed, but `nil` for file-visiting buffers whose file has been deleted.
Function: **clear-visited-file-modtime**
This function clears out the record of the last modification time of the file being visited by the current buffer. As a result, the next attempt to save this buffer will not complain of a discrepancy in file modification times.
This function is called in `set-visited-file-name` and other exceptional places where the usual test to avoid overwriting a changed file should not be done.
Function: **visited-file-modtime**
This function returns the current buffer’s recorded last file modification time, as a Lisp timestamp (see [Time of Day](time-of-day)).
If the buffer has no recorded last modification time, this function returns zero. This case occurs, for instance, if the buffer is not visiting a file or if the time has been explicitly cleared by `clear-visited-file-modtime`. Note, however, that `visited-file-modtime` returns a timestamp for some non-file buffers too. For instance, in a Dired buffer listing a directory, it returns the last modification time of that directory, as recorded by Dired.
If the buffer is visiting a file that doesn’t exist, this function returns -1.
Function: **set-visited-file-modtime** *&optional time*
This function updates the buffer’s record of the last modification time of the visited file, to the value specified by time if time is not `nil`, and otherwise to the last modification time of the visited file.
If time is neither `nil` nor an integer flag returned by `visited-file-modtime`, it should be a Lisp time value (see [Time of Day](time-of-day)).
This function is useful if the buffer was not read from the file normally, or if the file itself has been changed for some known benign reason.
Function: **ask-user-about-supersession-threat** *filename*
This function is used to ask a user how to proceed after an attempt to modify a buffer visiting file filename when the file is newer than the buffer text. Emacs detects this because the modification time of the file on disk is newer than the last save-time and its contents have changed. This means some other program has probably altered the file.
Depending on the user’s answer, the function may return normally, in which case the modification of the buffer proceeds, or it may signal a `file-supersession` error with data `(filename)`, in which case the proposed buffer modification is not allowed.
This function is called automatically by Emacs on the proper occasions. It exists so you can customize Emacs by redefining it. See the file `userlock.el` for the standard definition.
See also the file locking mechanism in [File Locks](file-locks).
elisp None #### File Attributes
This section describes the functions for getting detailed information about a file, including the owner and group numbers, the number of names, the inode number, the size, and the times of access and modification.
Function: **file-newer-than-file-p** *filename1 filename2*
This function returns `t` if the file filename1 is newer than file filename2. If filename1 does not exist, it returns `nil`. If filename1 does exist, but filename2 does not, it returns `t`.
In the following example, assume that the file `aug-19` was written on the 19th, `aug-20` was written on the 20th, and the file `no-file` doesn’t exist at all.
```
(file-newer-than-file-p "aug-19" "aug-20")
⇒ nil
```
```
(file-newer-than-file-p "aug-20" "aug-19")
⇒ t
```
```
(file-newer-than-file-p "aug-19" "no-file")
⇒ t
```
```
(file-newer-than-file-p "no-file" "aug-19")
⇒ nil
```
Function: **file-attributes** *filename &optional id-format*
This function returns a list of attributes of file filename. If the specified file does not exist, it returns `nil`. This function does not follow symbolic links. The optional parameter id-format specifies the preferred format of attributes UID and GID (see below)—the valid values are `'string` and `'integer`. The latter is the default, but we plan to change that, so you should specify a non-`nil` value for id-format if you use the returned UID or GID.
On GNU platforms when operating on a local file, this function is atomic: if the filesystem is simultaneously being changed by some other process, this function returns the file’s attributes either before or after the change. Otherwise this function is not atomic, and might return `nil` if it detects the race condition, or might return a hodgepodge of the previous and current file attributes.
Accessor functions are provided to access the elements in this list. The accessors are mentioned along with the descriptions of the elements below.
The elements of the list, in order, are:
0. `t` for a directory, a string for a symbolic link (the name linked to), or `nil` for a text file (`file-attribute-type`).
1. The number of names the file has (`file-attribute-link-number`). Alternate names, also known as hard links, can be created by using the `add-name-to-file` function (see [Changing Files](changing-files)).
2. The file’s UID, normally as a string (`file-attribute-user-id`). However, if it does not correspond to a named user, the value is an integer.
3. The file’s GID, likewise (`file-attribute-group-id`).
4. The time of last access as a Lisp timestamp (`file-attribute-access-time`). The timestamp is in the style of `current-time` (see [Time of Day](time-of-day)) and is truncated to that of the filesystem’s timestamp resolution; for example, on some FAT-based filesystems, only the date of last access is recorded, so this time will always hold the midnight of the day of the last access.
5. The time of last modification as a Lisp timestamp (`file-attribute-modification-time`). This is the last time when the file’s contents were modified.
6. The time of last status change as a Lisp timestamp (`file-attribute-status-change-time`). This is the time of the last change to the file’s access mode bits, its owner and group, and other information recorded in the filesystem for the file, beyond the file’s contents.
7. The size of the file in bytes (`file-attribute-size`).
8. The file’s modes, as a string of ten letters or dashes, as in ‘`ls -l`’ (`file-attribute-modes`).
9. An unspecified value, present for backward compatibility.
10. The file’s inode number (`file-attribute-inode-number`), a nonnegative integer.
11. The filesystem number of the device that the file is on `file-attribute-device-number`), an integer. This element and the file’s inode number together give enough information to distinguish any two files on the system—no two files can have the same values for both of these numbers.
For example, here are the file attributes for `files.texi`:
```
(file-attributes "files.texi" 'string)
⇒ (nil 1 "lh" "users"
(20614 64019 50040 152000)
(20000 23 0 0)
(20614 64555 902289 872000)
122295 "-rw-rw-rw-"
t 6473924464520138
1014478468)
```
and here is how the result is interpreted:
`nil`
is neither a directory nor a symbolic link.
`1`
has only one name (the name `files.texi` in the current default directory).
`"lh"`
is owned by the user with name ‘`lh`’.
`"users"`
is in the group with name ‘`users`’.
`(20614 64019 50040 152000)`
was last accessed on October 23, 2012, at 20:12:03.050040152 UTC.
`(20000 23 0 0)`
was last modified on July 15, 2001, at 08:53:43 UTC.
`(20614 64555 902289 872000)`
last had its status changed on October 23, 2012, at 20:20:59.902289872 UTC.
`122295`
is 122295 bytes long. (It may not contain 122295 characters, though, if some of the bytes belong to multibyte sequences, and also if the end-of-line format is CR-LF.)
`"-rw-rw-rw-"`
has a mode of read and write access for the owner, group, and world.
`t`
is merely a placeholder; it carries no information.
`6473924464520138`
has an inode number of 6473924464520138.
`1014478468` is on the file-system device whose number is 1014478468.
Function: **file-nlinks** *filename*
This function returns the number of names (i.e., hard links) that file filename has. If the file does not exist, this function returns `nil`. Note that symbolic links have no effect on this function, because they are not considered to be names of the files they link to. This function does not follow symbolic links.
```
$ ls -l foo*
-rw-rw-rw- 2 rms rms 4 Aug 19 01:27 foo
-rw-rw-rw- 2 rms rms 4 Aug 19 01:27 foo1
```
```
(file-nlinks "foo")
⇒ 2
```
```
(file-nlinks "doesnt-exist")
⇒ nil
```
elisp None ### Hash Table Access
This section describes the functions for accessing and storing associations in a hash table. In general, any Lisp object can be used as a hash key, unless the comparison method imposes limits. Any Lisp object can also be used as the value.
Function: **gethash** *key table &optional default*
This function looks up key in table, and returns its associated value—or default, if key has no association in table.
Function: **puthash** *key value table*
This function enters an association for key in table, with value value. If key already has an association in table, value replaces the old associated value. This function always returns value.
Function: **remhash** *key table*
This function removes the association for key from table, if there is one. If key has no association, `remhash` does nothing.
**Common Lisp note:** In Common Lisp, `remhash` returns non-`nil` if it actually removed an association and `nil` otherwise. In Emacs Lisp, `remhash` always returns `nil`.
Function: **clrhash** *table*
This function removes all the associations from hash table table, so that it becomes empty. This is also called *clearing* the hash table. `clrhash` returns the empty table.
Function: **maphash** *function table*
This function calls function once for each of the associations in table. The function function should accept two arguments—a key listed in table, and its associated value. `maphash` returns `nil`.
elisp None ### Waiting for Elapsed Time or Input
The wait functions are designed to wait for a certain amount of time to pass or until there is input. For example, you may wish to pause in the middle of a computation to allow the user time to view the display. `sit-for` pauses and updates the screen, and returns immediately if input comes in, while `sleep-for` pauses without updating the screen.
Function: **sit-for** *seconds &optional nodisp*
This function performs redisplay (provided there is no pending input from the user), then waits seconds seconds, or until input is available. The usual purpose of `sit-for` is to give the user time to read text that you display. The value is `t` if `sit-for` waited the full time with no input arriving (see [Event Input Misc](event-input-misc)). Otherwise, the value is `nil`.
The argument seconds need not be an integer. If it is floating point, `sit-for` waits for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, seconds is rounded down.
The expression `(sit-for 0)` is equivalent to `(redisplay)`, i.e., it requests a redisplay, without any delay, if there is no pending input. See [Forcing Redisplay](forcing-redisplay).
If nodisp is non-`nil`, then `sit-for` does not redisplay, but it still returns as soon as input is available (or when the timeout elapses).
In batch mode (see [Batch Mode](batch-mode)), `sit-for` cannot be interrupted, even by input from the standard input descriptor. It is thus equivalent to `sleep-for`, which is described below.
It is also possible to call `sit-for` with three arguments, as `(sit-for seconds millisec nodisp)`, but that is considered obsolete.
Function: **sleep-for** *seconds &optional millisec*
This function simply pauses for seconds seconds without updating the display. It pays no attention to available input. It returns `nil`.
The argument seconds need not be an integer. If it is floating point, `sleep-for` waits for a fractional number of seconds. Some systems support only a whole number of seconds; on these systems, seconds is rounded down.
The optional argument millisec specifies an additional waiting period measured in milliseconds. This adds to the period specified by seconds. If the system doesn’t support waiting fractions of a second, you get an error if you specify nonzero millisec.
Use `sleep-for` when you wish to guarantee a delay.
See [Time of Day](time-of-day), for functions to get the current time.
elisp None #### Other Image Types
For PBM images, specify image type `pbm`. Color, gray-scale and monochromatic images are supported. For mono PBM images, two additional image properties are supported.
`:foreground foreground`
The value, foreground, should be a string specifying the image foreground color, or `nil` for the default color. This color is used for each pixel in the PBM that is 1. The default is the frame’s foreground color.
`:background background` The value, background, should be a string specifying the image background color, or `nil` for the default color. This color is used for each pixel in the PBM that is 0. The default is the frame’s background color.
The remaining image types that Emacs can support are:
GIF
Image type `gif`. Supports the `:index` property. See [Multi-Frame Images](multi_002dframe-images).
JPEG
Image type `jpeg`.
PNG
Image type `png`.
TIFF Image type `tiff`. Supports the `:index` property. See [Multi-Frame Images](multi_002dframe-images).
elisp None #### Frame Interaction Parameters
These parameters supply forms of interactions between different frames.
`parent-frame`
If non-`nil`, this means that this frame is a child frame (see [Child Frames](child-frames)), and this parameter specifies its parent frame. If `nil`, this means that this frame is a normal, top-level frame.
`delete-before`
If non-`nil`, this parameter specifies another frame whose deletion will automatically trigger the deletion of this frame. See [Deleting Frames](deleting-frames).
`mouse-wheel-frame`
If non-`nil`, this parameter specifies the frame whose windows will be scrolled whenever the mouse wheel is scrolled with the mouse pointer hovering over this frame, see [Mouse Commands](https://www.gnu.org/software/emacs/manual/html_node/emacs/Mouse-Commands.html#Mouse-Commands) in The GNU Emacs Manual.
`no-other-frame`
If this is non-`nil`, then this frame is not eligible as candidate for the functions `next-frame`, `previous-frame` (see [Finding All Frames](finding-all-frames)) and `other-frame`, see [Frame Commands](https://www.gnu.org/software/emacs/manual/html_node/emacs/Frame-Commands.html#Frame-Commands) in The GNU Emacs Manual.
`auto-hide-function`
When this parameter specifies a function, that function will be called instead of the function specified by the variable `frame-auto-hide-function` when quitting the frame’s only window (see [Quitting Windows](quitting-windows)) and there are other frames left.
`minibuffer-exit`
When this parameter is non-`nil`, Emacs will by default make this frame invisible whenever the minibuffer (see [Minibuffers](minibuffers)) is exited. Alternatively, it can specify the functions `iconify-frame` and `delete-frame`. This parameter is useful to make a child frame disappear automatically (similar to how Emacs deals with a window) when exiting the minibuffer.
`keep-ratio`
This parameter is currently meaningful for child frames (see [Child Frames](child-frames)) only. If it is non-`nil`, then Emacs will try to keep the frame’s size (width and height) ratios (see [Size Parameters](size-parameters)) as well as its left and right position ratios (see [Position Parameters](position-parameters)) unaltered whenever its parent frame is resized.
If the value of this parameter is `nil`, the frame’s position and size remain unaltered when the parent frame is resized, so the position and size ratios may change. If the value of this parameter is `t`, Emacs will try to preserve the frame’s size and position ratios, hence the frame’s size and position relative to its parent frame may change.
More individual control is possible by using a cons cell: In that case the frame’s width ratio is preserved if the CAR of the cell is either `t` or `width-only`. The height ratio is preserved if the CAR of the cell is either `t` or `height-only`. The left position ratio is preserved if the CDR of the cell is either `t` or `left-only`. The top position ratio is preserved if the CDR of the cell is either `t` or `top-only`.
elisp None ### Dynamic Loading of Individual Functions
When you compile a file, you can optionally enable the *dynamic function loading* feature (also known as *lazy loading*). With dynamic function loading, loading the file doesn’t fully read the function definitions in the file. Instead, each function definition contains a place-holder which refers to the file. The first time each function is called, it reads the full definition from the file, to replace the place-holder.
The advantage of dynamic function loading is that loading the file should become faster. This is a good thing for a file which contains many separate user-callable functions, if using one of them does not imply you will probably also use the rest. A specialized mode which provides many keyboard commands often has that usage pattern: a user may invoke the mode, but use only a few of the commands it provides.
The dynamic loading feature has certain disadvantages:
* If you delete or move the compiled file after loading it, Emacs can no longer load the remaining function definitions not already loaded.
* If you alter the compiled file (such as by compiling a new version), then trying to load any function not already loaded will usually yield nonsense results.
These problems will never happen in normal circumstances with installed Emacs files. But they are quite likely to happen with Lisp files that you are changing. The easiest way to prevent these problems is to reload the new compiled file immediately after each recompilation.
*Experience shows that using dynamic function loading provides benefits that are hardly measurable, so this feature is deprecated since Emacs 27.1.*
The byte compiler uses the dynamic function loading feature if the variable `byte-compile-dynamic` is non-`nil` at compilation time. Do not set this variable globally, since dynamic loading is desirable only for certain files. Instead, enable the feature for specific source files with file-local variable bindings. For example, you could do it by writing this text in the source file’s first line:
```
-*-byte-compile-dynamic: t;-*-
```
Variable: **byte-compile-dynamic**
If this is non-`nil`, the byte compiler generates compiled files that are set up for dynamic function loading.
Function: **fetch-bytecode** *function*
If function is a byte-code function object, this immediately finishes loading the byte code of function from its byte-compiled file, if it is not fully loaded already. Otherwise, it does nothing. It always returns function.
| programming_docs |
elisp None Strings and Characters
----------------------
A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files; to send messages to users; to hold text being copied between buffers; and for many other purposes. Because strings are so important, Emacs Lisp has many functions expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters.
See [Strings of Events](strings-of-events), for special considerations for strings of keyboard character events.
| | | |
| --- | --- | --- |
| • [Basics](string-basics) | | Basic properties of strings and characters. |
| • [Predicates for Strings](predicates-for-strings) | | Testing whether an object is a string or char. |
| • [Creating Strings](creating-strings) | | Functions to allocate new strings. |
| • [Modifying Strings](modifying-strings) | | Altering the contents of an existing string. |
| • [Text Comparison](text-comparison) | | Comparing characters or strings. |
| • [String Conversion](string-conversion) | | Converting to and from characters and strings. |
| • [Formatting Strings](formatting-strings) | | `format`: Emacs’s analogue of `printf`. |
| • [Custom Format Strings](custom-format-strings) | | Formatting custom `format` specifications. |
| • [Case Conversion](case-conversion) | | Case conversion functions. |
| • [Case Tables](case-tables) | | Customizing case conversion. |
elisp None #### Making Buttons
Buttons are associated with a region of text, using an overlay or text properties to hold button-specific information, all of which are initialized from the button’s type (which defaults to the built-in button type `button`). Like all Emacs text, the appearance of the button is governed by the `face` property; by default (via the `face` property inherited from the `button` button-type) this is a simple underline, like a typical web-page link.
For convenience, there are two sorts of button-creation functions, those that add button properties to an existing region of a buffer, called `make-...button`, and those that also insert the button text, called `insert-...button`.
The button-creation functions all take the `&rest` argument properties, which should be a sequence of property value pairs, specifying properties to add to the button; see [Button Properties](button-properties). In addition, the keyword argument `:type` may be used to specify a button-type from which to inherit other properties; see [Button Types](button-types). Any properties not explicitly specified during creation will be inherited from the button’s type (if the type defines such a property).
The following functions add a button using an overlay (see [Overlays](overlays)) to hold the button properties:
Function: **make-button** *beg end &rest properties*
This makes a button from beg to end in the current buffer, and returns it.
Function: **insert-button** *label &rest properties*
This inserts a button with the label label at point, and returns it.
The following functions are similar, but using text properties (see [Text Properties](text-properties)) to hold the button properties. Such buttons do not add markers to the buffer, so editing in the buffer does not slow down if there is an extremely large numbers of buttons. However, if there is an existing face text property on the text (e.g., a face assigned by Font Lock mode), the button face may not be visible. Both of these functions return the starting position of the new button.
Function: **make-text-button** *beg end &rest properties*
This makes a button from beg to end in the current buffer, using text properties.
Function: **insert-text-button** *label &rest properties*
This inserts a button with the label label at point, using text properties.
Function: **button-buttonize** *string callback &optional data*
Sometimes it’s more convenient to make a string into a button without inserting it into a buffer immediately, for instance when creating data structures that may then, later, be inserted into a buffer. This function makes string into such a string, and callback will be called when the user clicks on the button. The optional data parameter will be used as the parameter when callback is called. If `nil`, the button is used as the parameter instead.
elisp None Searching and Matching
----------------------
GNU Emacs provides two ways to search through a buffer for specified text: exact string searches and regular expression searches. After a regular expression search, you can examine the *match data* to determine which text matched the whole regular expression or various portions of it.
| | | |
| --- | --- | --- |
| • [String Search](string-search) | | Search for an exact match. |
| • [Searching and Case](searching-and-case) | | Case-independent or case-significant searching. |
| • [Regular Expressions](regular-expressions) | | Describing classes of strings. |
| • [Regexp Search](regexp-search) | | Searching for a match for a regexp. |
| • [POSIX Regexps](posix-regexps) | | Searching POSIX-style for the longest match. |
| • [Match Data](match-data) | | Finding out which part of the text matched, after a string or regexp search. |
| • [Search and Replace](search-and-replace) | | Commands that loop, searching and replacing. |
| • [Standard Regexps](standard-regexps) | | Useful regexps for finding sentences, pages,... |
The ‘`skip-chars…`’ functions also perform a kind of searching. See [Skipping Characters](skipping-characters). To search for changes in character properties, see [Property Search](property-search).
elisp None #### String Type
A *string* is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants: evaluation of a string returns the same string.
See [Strings and Characters](strings-and-characters), for functions that operate on strings.
| | | |
| --- | --- | --- |
| • [Syntax for Strings](syntax-for-strings) | | How to specify Lisp strings. |
| • [Non-ASCII in Strings](non_002dascii-in-strings) | | International characters in strings. |
| • [Nonprinting Characters](nonprinting-characters) | | Literal unprintable characters in strings. |
| • [Text Props and Strings](text-props-and-strings) | | Strings with text properties. |
elisp None Emacs Display
-------------
This chapter describes a number of features related to the display that Emacs presents to the user.
| | | |
| --- | --- | --- |
| • [Refresh Screen](refresh-screen) | | Clearing the screen and redrawing everything on it. |
| • [Forcing Redisplay](forcing-redisplay) | | Forcing redisplay. |
| • [Truncation](truncation) | | Folding or wrapping long text lines. |
| • [The Echo Area](the-echo-area) | | Displaying messages at the bottom of the screen. |
| • [Warnings](warnings) | | Displaying warning messages for the user. |
| • [Invisible Text](invisible-text) | | Hiding part of the buffer text. |
| • [Selective Display](selective-display) | | Hiding part of the buffer text (the old way). |
| • [Temporary Displays](temporary-displays) | | Displays that go away automatically. |
| • [Overlays](overlays) | | Use overlays to highlight parts of the buffer. |
| • [Size of Displayed Text](size-of-displayed-text) | | How large displayed text is. |
| • [Line Height](line-height) | | Controlling the height of lines. |
| • [Faces](faces) | | A face defines a graphics style for text characters: font, colors, etc. |
| • [Fringes](fringes) | | Controlling window fringes. |
| • [Scroll Bars](scroll-bars) | | Controlling scroll bars. |
| • [Window Dividers](window-dividers) | | Separating windows visually. |
| • [Display Property](display-property) | | Images, margins, text size, etc. |
| • [Images](images) | | Displaying images in Emacs buffers. |
| • [Xwidgets](xwidgets) | | Displaying native widgets in Emacs buffers. |
| • [Buttons](buttons) | | Adding clickable buttons to Emacs buffers. |
| • [Abstract Display](abstract-display) | | Emacs’s Widget for Object Collections. |
| • [Blinking](blinking) | | How Emacs shows the matching open parenthesis. |
| • [Character Display](character-display) | | How Emacs displays individual characters. |
| • [Beeping](beeping) | | Audible signal to the user. |
| • [Window Systems](window-systems) | | Which window system is being used. |
| • [Tooltips](tooltips) | | Tooltip display in Emacs. |
| • [Bidirectional Display](bidirectional-display) | | Display of bidirectional scripts, such as Arabic and Farsi. |
elisp None ### How Programs Do Loading
Emacs Lisp has several interfaces for loading. For example, `autoload` creates a placeholder object for a function defined in a file; trying to call the autoloading function loads the file to get the function’s real definition (see [Autoload](autoload)). `require` loads a file if it isn’t already loaded (see [Named Features](named-features)). Ultimately, all these facilities call the `load` function to do the work.
Function: **load** *filename &optional missing-ok nomessage nosuffix must-suffix*
This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file.
To find the file, `load` first looks for a file named `filename.elc`, that is, for a file whose name is filename with the extension ‘`.elc`’ appended. If such a file exists, and Emacs was compiled with native-compilation support (see [Native Compilation](native-compilation)), `load` attempts to find a corresponding ‘`.eln`’ file, and if found, loads it instead of `filename.elc`. Otherwise, it loads `filename.elc`. If there is no file by that name, then `load` looks for a file named `filename.el`. If that file exists, it is loaded. If Emacs was compiled with support for dynamic modules (see [Dynamic Modules](dynamic-modules)), `load` next looks for a file named `filename.ext`, where ext is a system-dependent file-name extension of shared libraries. Finally, if neither of those names is found, `load` looks for a file named filename with nothing appended, and loads it if it exists. (The `load` function is not clever about looking at filename. In the perverse case of a file named `foo.el.el`, evaluation of `(load "foo.el")` will indeed find it.)
If Auto Compression mode is enabled, as it is by default, then if `load` can not find a file, it searches for a compressed version of the file before trying other file names. It decompresses and loads it if it exists. It looks for compressed versions by appending each of the suffixes in `jka-compr-load-suffixes` to the file name. The value of this variable must be a list of strings. Its standard value is `(".gz")`.
If the optional argument nosuffix is non-`nil`, then `load` does not try the suffixes ‘`.elc`’ and ‘`.el`’. In this case, you must specify the precise file name you want, except that, if Auto Compression mode is enabled, `load` will still use `jka-compr-load-suffixes` to find compressed versions. By specifying the precise file name and using `t` for nosuffix, you can prevent file names like `foo.el.el` from being tried.
If the optional argument must-suffix is non-`nil`, then `load` insists that the file name used must end in either ‘`.el`’ or ‘`.elc`’ (possibly extended with a compression suffix) or the shared-library extension, unless it contains an explicit directory name.
If the option `load-prefer-newer` is non-`nil`, then when searching suffixes, `load` selects whichever version of a file (‘`.elc`’, ‘`.el`’, etc.) has been modified most recently. In this case, `load` doesn’t load the ‘`.eln`’ natively-compiled file even if it exists.
If filename is a relative file name, such as `foo` or `baz/foo.bar`, `load` searches for the file using the variable `load-path`. It appends filename to each of the directories listed in `load-path`, and loads the first file it finds whose name matches. The current default directory is tried only if it is specified in `load-path`, where `nil` stands for the default directory. `load` tries all three possible suffixes in the first directory in `load-path`, then all three suffixes in the second directory, and so on. See [Library Search](library-search).
Whatever the name under which the file is eventually found, and the directory where Emacs found it, Emacs sets the value of the variable `load-file-name` to that file’s name.
If you get a warning that `foo.elc` is older than `foo.el`, it means you should consider recompiling `foo.el`. See [Byte Compilation](byte-compilation).
When loading a source file (not compiled), `load` performs character set translation just as Emacs would do when visiting the file. See [Coding Systems](coding-systems).
When loading an uncompiled file, Emacs tries to expand any macros that the file contains (see [Macros](macros)). We refer to this as *eager macro expansion*. Doing this (rather than deferring the expansion until the relevant code runs) can significantly speed up the execution of uncompiled code. Sometimes, this macro expansion cannot be done, owing to a cyclic dependency. In the simplest example of this, the file you are loading refers to a macro defined in another file, and that file in turn requires the file you are loading. This is generally harmless. Emacs prints a warning (‘`Eager macro-expansion skipped due to cycle…`’) giving details of the problem, but it still loads the file, just leaving the macro unexpanded for now. You may wish to restructure your code so that this does not happen. Loading a compiled file does not cause macroexpansion, because this should already have happened during compilation. See [Compiling Macros](compiling-macros).
Messages like ‘`Loading foo...`’ and ‘`Loading foo...done`’ appear in the echo area during loading unless nomessage is non-`nil`. If a natively-compiled ‘`.eln`’ file is loaded, the message says so.
Any unhandled errors while loading a file terminate loading. If the load was done for the sake of `autoload`, any function definitions made during the loading are undone.
If `load` can’t find the file to load, then normally it signals a `file-error` (with ‘`Cannot open load file filename`’). But if missing-ok is non-`nil`, then `load` just returns `nil`.
You can use the variable `load-read-function` to specify a function for `load` to use instead of `read` for reading expressions. See below.
`load` returns `t` if the file loads successfully.
Command: **load-file** *filename*
This command loads the file filename. If filename is a relative file name, then the current default directory is assumed. This command does not use `load-path`, and does not append suffixes. However, it does look for compressed versions (if Auto Compression Mode is enabled). Use this command if you wish to specify precisely the file name to load.
Command: **load-library** *library*
This command loads the library named library. It is equivalent to `load`, except for the way it reads its argument interactively. See [Lisp Libraries](https://www.gnu.org/software/emacs/manual/html_node/emacs/Lisp-Libraries.html#Lisp-Libraries) in The GNU Emacs Manual.
Variable: **load-in-progress**
This variable is non-`nil` if Emacs is in the process of loading a file, and it is `nil` otherwise.
Variable: **load-file-name**
When Emacs is in the process of loading a file, this variable’s value is the name of that file, as Emacs found it during the search described earlier in this section.
Variable: **load-read-function**
This variable specifies an alternate expression-reading function for `load` and `eval-region` to use instead of `read`. The function should accept one argument, just as `read` does.
By default, this variable’s value is `read`. See [Input Functions](input-functions).
Instead of using this variable, it is cleaner to use another, newer feature: to pass the function as the read-function argument to `eval-region`. See [Eval](eval#Definition-of-eval_002dregion).
For information about how `load` is used in building Emacs, see [Building Emacs](building-emacs).
elisp None #### Motion by Characters
These functions move point based on a count of characters. `goto-char` is the fundamental primitive; the other functions use that.
Command: **goto-char** *position*
This function sets point in the current buffer to the value position.
If narrowing is in effect, position still counts from the beginning of the buffer, but point cannot go outside the accessible portion. If position is out of range, `goto-char` moves point to the beginning or the end of the accessible portion.
When this function is called interactively, position is the numeric prefix argument, if provided; otherwise it is read from the minibuffer.
`goto-char` returns position.
Command: **forward-char** *&optional count*
This function moves point count characters forward, towards the end of the buffer (or backward, towards the beginning of the buffer, if count is negative). If count is `nil`, the default is 1.
If this attempts to move past the beginning or end of the buffer (or the limits of the accessible portion, when narrowing is in effect), it signals an error with error symbol `beginning-of-buffer` or `end-of-buffer`.
In an interactive call, count is the numeric prefix argument.
Command: **backward-char** *&optional count*
This is just like `forward-char` except that it moves in the opposite direction.
elisp None #### Button-Down Events
Click and drag events happen when the user releases a mouse button. They cannot happen earlier, because there is no way to distinguish a click from a drag until the button is released.
If you want to take action as soon as a button is pressed, you need to handle *button-down* events.[16](#FOOT16) These occur as soon as a button is pressed. They are represented by lists that look exactly like click events (see [Click Events](click-events)), except that the event-type symbol name contains the prefix ‘`down-`’. The ‘`down-`’ prefix follows modifier key prefixes such as ‘`C-`’ and ‘`M-`’.
The function `read-key-sequence` ignores any button-down events that don’t have command bindings; therefore, the Emacs command loop ignores them too. This means that you need not worry about defining button-down events unless you want them to do something. The usual reason to define a button-down event is so that you can track mouse motion (by reading motion events) until the button is released. See [Motion Events](motion-events).
elisp None ### Parsing Expressions
This section describes functions for parsing and scanning balanced expressions. We will refer to such expressions as *sexps*, following the terminology of Lisp, even though these functions can act on languages other than Lisp. Basically, a sexp is either a balanced parenthetical grouping, a string, or a symbol (i.e., a sequence of characters whose syntax is either word constituent or symbol constituent). However, characters in the expression prefix syntax class (see [Syntax Class Table](syntax-class-table)) are treated as part of the sexp if they appear next to it.
The syntax table controls the interpretation of characters, so these functions can be used for Lisp expressions when in Lisp mode and for C expressions when in C mode. See [List Motion](list-motion), for convenient higher-level functions for moving over balanced expressions.
A character’s syntax controls how it changes the state of the parser, rather than describing the state itself. For example, a string delimiter character toggles the parser state between in-string and in-code, but the syntax of characters does not directly say whether they are inside a string. For example (note that 15 is the syntax code for generic string delimiters),
```
(put-text-property 1 9 'syntax-table '(15 . nil))
```
does not tell Emacs that the first eight chars of the current buffer are a string, but rather that they are all string delimiters. As a result, Emacs treats them as four consecutive empty string constants.
| | | |
| --- | --- | --- |
| • [Motion via Parsing](motion-via-parsing) | | Motion functions that work by parsing. |
| • [Position Parse](position-parse) | | Determining the syntactic state of a position. |
| • [Parser State](parser-state) | | How Emacs represents a syntactic state. |
| • [Low-Level Parsing](low_002dlevel-parsing) | | Parsing across a specified region. |
| • [Control Parsing](control-parsing) | | Parameters that affect parsing. |
| programming_docs |
elisp None #### Repeat Events
If you press the same mouse button more than once in quick succession without moving the mouse, Emacs generates special *repeat* mouse events for the second and subsequent presses.
The most common repeat events are *double-click* events. Emacs generates a double-click event when you click a button twice; the event happens when you release the button (as is normal for all click events).
The event type of a double-click event contains the prefix ‘`double-`’. Thus, a double click on the second mouse button with meta held down comes to the Lisp program as `M-double-mouse-2`. If a double-click event has no binding, the binding of the corresponding ordinary click event is used to execute it. Thus, you need not pay attention to the double click feature unless you really want to.
When the user performs a double click, Emacs generates first an ordinary click event, and then a double-click event. Therefore, you must design the command binding of the double click event to assume that the single-click command has already run. It must produce the desired results of a double click, starting from the results of a single click.
This is convenient, if the meaning of a double click somehow builds on the meaning of a single click—which is recommended user interface design practice for double clicks.
If you click a button, then press it down again and start moving the mouse with the button held down, then you get a *double-drag* event when you ultimately release the button. Its event type contains ‘`double-drag`’ instead of just ‘`drag`’. If a double-drag event has no binding, Emacs looks for an alternate binding as if the event were an ordinary drag.
Before the double-click or double-drag event, Emacs generates a *double-down* event when the user presses the button down for the second time. Its event type contains ‘`double-down`’ instead of just ‘`down`’. If a double-down event has no binding, Emacs looks for an alternate binding as if the event were an ordinary button-down event. If it finds no binding that way either, the double-down event is ignored.
To summarize, when you click a button and then press it again right away, Emacs generates a down event and a click event for the first click, a double-down event when you press the button again, and finally either a double-click or a double-drag event.
If you click a button twice and then press it again, all in quick succession, Emacs generates a *triple-down* event, followed by either a *triple-click* or a *triple-drag*. The event types of these events contain ‘`triple`’ instead of ‘`double`’. If any triple event has no binding, Emacs uses the binding that it would use for the corresponding double event.
If you click a button three or more times and then press it again, the events for the presses beyond the third are all triple events. Emacs does not have separate event types for quadruple, quintuple, etc. events. However, you can look at the event list to find out precisely how many times the button was pressed.
Function: **event-click-count** *event*
This function returns the number of consecutive button presses that led up to event. If event is a double-down, double-click or double-drag event, the value is 2. If event is a triple event, the value is 3 or greater. If event is an ordinary mouse event (not a repeat event), the value is 1.
User Option: **double-click-fuzz**
To generate repeat events, successive mouse button presses must be at approximately the same screen position. The value of `double-click-fuzz` specifies the maximum number of pixels the mouse may be moved (horizontally or vertically) between two successive clicks to make a double-click.
This variable is also the threshold for motion of the mouse to count as a drag.
User Option: **double-click-time**
To generate repeat events, the number of milliseconds between successive button presses must be less than the value of `double-click-time`. Setting `double-click-time` to `nil` disables multi-click detection entirely. Setting it to `t` removes the time limit; Emacs then detects multi-clicks by position only.
elisp None #### Introduction to Buffer-Local Variables
A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it.
The variable’s ordinary binding, which is not associated with any specific buffer, is called the *default binding*. In most cases, this is the global binding.
A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don’t have their own bindings for the variable. (This includes all newly-created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding, so the new value is visible in all the buffers that see the default binding.
The most common use of buffer-local bindings is for major modes to change variables that control the behavior of commands. For example, C mode and Lisp mode both set the variable `paragraph-start` to specify that only blank lines separate paragraphs. They do this by making the variable buffer-local in the buffer that is being put into C mode or Lisp mode, and then setting it to the new value for that mode. See [Major Modes](major-modes).
The usual way to make a buffer-local binding is with `make-local-variable`, which is what major mode commands typically use. This affects just the current buffer; all other buffers (including those yet to be created) will continue to share the default value unless they are explicitly given their own buffer-local bindings.
A more powerful operation is to mark the variable as *automatically buffer-local* by calling `make-variable-buffer-local`. You can think of this as making the variable local in all buffers, even those yet to be created. More precisely, the effect is that setting the variable automatically makes the variable local to the current buffer if it is not already so. All buffers start out by sharing the default value of the variable as usual, but setting the variable creates a buffer-local binding for the current buffer. The new value is stored in the buffer-local binding, leaving the default binding untouched. This means that the default value cannot be changed with `setq` in any buffer; the only way to change it is with `setq-default`.
**Warning:** When a variable has buffer-local bindings in one or more buffers, `let` rebinds the binding that’s currently in effect. For instance, if the current buffer has a buffer-local value, `let` temporarily rebinds that. If no buffer-local bindings are in effect, `let` rebinds the default value. If inside the `let` you then change to a different current buffer in which a different binding is in effect, you won’t see the `let` binding any more. And if you exit the `let` while still in the other buffer, you won’t see the unbinding occur (though it will occur properly). Here is an example to illustrate:
```
(setq foo 'g)
(set-buffer "a")
(make-local-variable 'foo)
```
```
(setq foo 'a)
(let ((foo 'temp))
;; foo ⇒ 'temp ; let binding in buffer ‘`a`’
(set-buffer "b")
;; foo ⇒ 'g ; the global value since foo is not local in ‘`b`’
body…)
```
```
foo ⇒ 'g ; exiting restored the local value in buffer ‘`a`’,
; but we don’t see that in buffer ‘`b`’
```
```
(set-buffer "a") ; verify the local value was restored
foo ⇒ 'a
```
Note that references to `foo` in body access the buffer-local binding of buffer ‘`b`’.
When a file specifies local variable values, these become buffer-local values when you visit the file. See [File Variables](https://www.gnu.org/software/emacs/manual/html_node/emacs/File-Variables.html#File-Variables) in The GNU Emacs Manual.
A buffer-local variable cannot be made terminal-local (see [Multiple Terminals](multiple-terminals)).
elisp None #### Locating Files in Standard Places
This section explains how to search for a file in a list of directories (a *path*), or for an executable file in the standard list of executable file directories.
To search for a user-specific configuration file, See [Standard File Names](standard-file-names), for the `locate-user-emacs-file` function.
Function: **locate-file** *filename path &optional suffixes predicate*
This function searches for a file whose name is filename in a list of directories given by path, trying the suffixes in suffixes. If it finds such a file, it returns the file’s absolute file name (see [Relative File Names](relative-file-names)); otherwise it returns `nil`.
The optional argument suffixes gives the list of file-name suffixes to append to filename when searching. `locate-file` tries each possible directory with each of these suffixes. If suffixes is `nil`, or `("")`, then there are no suffixes, and filename is used only as-is. Typical values of suffixes are `exec-suffixes` (see [Subprocess Creation](subprocess-creation)), `load-suffixes`, `load-file-rep-suffixes` and the return value of the function `get-load-suffixes` (see [Load Suffixes](load-suffixes)).
Typical values for path are `exec-path` (see [Subprocess Creation](subprocess-creation)) when looking for executable programs, or `load-path` (see [Library Search](library-search)) when looking for Lisp files. If filename is absolute, path has no effect, but the suffixes in suffixes are still tried.
The optional argument predicate, if non-`nil`, specifies a predicate function for testing whether a candidate file is suitable. The predicate is passed the candidate file name as its single argument. If predicate is `nil` or omitted, `locate-file` uses `file-readable-p` as the predicate. See [Kinds of Files](kinds-of-files), for other useful predicates, e.g., `file-executable-p` and `file-directory-p`.
This function will normally skip directories, so if you want it to find directories, make sure the predicate function returns `dir-ok` for them. For example:
```
(locate-file "html" '("/var/www" "/srv") nil
(lambda (f) (if (file-directory-p f) 'dir-ok)))
```
For compatibility, predicate can also be one of the symbols `executable`, `readable`, `writable`, `exists`, or a list of one or more of these symbols.
Function: **executable-find** *program &optional remote*
This function searches for the executable file of the named program and returns the absolute file name of the executable, including its file-name extensions, if any. It returns `nil` if the file is not found. The function searches in all the directories in `exec-path`, and tries all the file-name extensions in `exec-suffixes` (see [Subprocess Creation](subprocess-creation)).
If remote is non-`nil`, and `default-directory` is a remote directory, program is searched on the respective remote host.
elisp None #### Edebug Breakpoints
While using Edebug, you can specify *breakpoints* in the program you are testing: these are places where execution should stop. You can set a breakpoint at any stop point, as defined in [Using Edebug](using-edebug). For setting and unsetting breakpoints, the stop point that is affected is the first one at or after point in the source code buffer. Here are the Edebug commands for breakpoints:
`b`
Set a breakpoint at the stop point at or after point (`edebug-set-breakpoint`). If you use a prefix argument, the breakpoint is temporary—it turns off the first time it stops the program. An overlay with the `edebug-enabled-breakpoint` or `edebug-disabled-breakpoint` faces is put at the breakpoint.
`u`
Unset the breakpoint (if any) at the stop point at or after point (`edebug-unset-breakpoint`).
`U`
Unset any breakpoints in the current form (`edebug-unset-breakpoints`).
`D`
Toggle whether to disable the breakpoint near point (`edebug-toggle-disable-breakpoint`). This command is mostly useful if the breakpoint is conditional and it would take some work to recreate the condition.
`x condition RET`
Set a conditional breakpoint which stops the program only if evaluating condition produces a non-`nil` value (`edebug-set-conditional-breakpoint`). With a prefix argument, the breakpoint is temporary.
`B` Move point to the next breakpoint in the current definition (`edebug-next-breakpoint`).
While in Edebug, you can set a breakpoint with `b` and unset one with `u`. First move point to the Edebug stop point of your choice, then type `b` or `u` to set or unset a breakpoint there. Unsetting a breakpoint where none has been set has no effect.
Re-evaluating or reinstrumenting a definition removes all of its previous breakpoints.
A *conditional breakpoint* tests a condition each time the program gets there. Any errors that occur as a result of evaluating the condition are ignored, as if the result were `nil`. To set a conditional breakpoint, use `x`, and specify the condition expression in the minibuffer. Setting a conditional breakpoint at a stop point that has a previously established conditional breakpoint puts the previous condition expression in the minibuffer so you can edit it.
You can make a conditional or unconditional breakpoint *temporary* by using a prefix argument with the command to set the breakpoint. When a temporary breakpoint stops the program, it is automatically unset.
Edebug always stops or pauses at a breakpoint, except when the Edebug mode is Go-nonstop. In that mode, it ignores breakpoints entirely.
To find out where your breakpoints are, use the `B` command, which moves point to the next breakpoint following point, within the same function, or to the first breakpoint if there are no following breakpoints. This command does not continue execution—it just moves point in the buffer.
elisp None #### Button Properties
Each button has an associated list of properties defining its appearance and behavior, and other arbitrary properties may be used for application specific purposes. The following properties have special meaning to the Button package:
`action`
The function to call when the user invokes the button, which is passed the single argument button. By default this is `ignore`, which does nothing.
`mouse-action`
This is similar to `action`, and when present, will be used instead of `action` for button invocations resulting from mouse-clicks (instead of the user hitting RET). If not present, mouse-clicks use `action` instead.
`face`
This is an Emacs face controlling how buttons of this type are displayed; by default this is the `button` face.
`mouse-face`
This is an additional face which controls appearance during mouse-overs (merged with the usual button face); by default this is the usual Emacs `highlight` face.
`keymap`
The button’s keymap, defining bindings active within the button region. By default this is the usual button region keymap, stored in the variable `button-map`, which defines RET and mouse-2 to invoke the button.
`type`
The button type. See [Button Types](button-types).
`help-echo`
A string displayed by the Emacs tooltip help system; by default, `"mouse-2, RET: Push this button"`. Alternatively, a function that returns, or a form that evaluates to, a string to be displayed or `nil`. For details see [Text help-echo](special-properties#Text-help_002decho).
The function is called with three arguments, window, object, and pos. The second argument, object, is either the overlay that had the property (for overlay buttons), or the buffer containing the button (for text property buttons). The other arguments have the same meaning as for the special text property `help-echo`.
`follow-link`
The `follow-link` property, defining how a mouse-1 click behaves on this button, See [Clickable Text](clickable-text).
`button`
All buttons have a non-`nil` `button` property, which may be useful in finding regions of text that comprise buttons (which is what the standard button functions do).
There are other properties defined for the regions of text in a button, but these are not generally interesting for typical uses.
elisp None #### How Many Times is the Macro Expanded?
Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded.
Therefore, you should avoid side effects in computation of the macro expansion, unless you really know what you are doing.
One special kind of side effect can’t be avoided: constructing Lisp objects. Almost all macro expansions include constructed lists; that is the whole point of most macros. This is usually safe; there is just one case where you must be careful: when the object you construct is part of a quoted constant in the macro expansion.
If the macro is expanded just once, in compilation, then the object is constructed just once, during compilation. But in interpreted execution, the macro is expanded each time the macro call runs, and this means a new object is constructed each time.
In most clean Lisp code, this difference won’t matter. It can matter only if you perform side-effects on the objects constructed by the macro definition. Thus, to avoid trouble, **avoid side effects on objects constructed by macro definitions**. Here is an example of how such side effects can get you into trouble:
```
(defmacro empty-object ()
(list 'quote (cons nil nil)))
```
```
(defun initialize (condition)
(let ((object (empty-object)))
(if condition
(setcar object condition))
object))
```
If `initialize` is interpreted, a new list `(nil)` is constructed each time `initialize` is called. Thus, no side effect survives between calls. If `initialize` is compiled, then the macro `empty-object` is expanded during compilation, producing a single constant `(nil)` that is reused and altered each time `initialize` is called.
One way to avoid pathological cases like this is to think of `empty-object` as a funny kind of constant, not as a memory allocation construct. You wouldn’t use `setcar` on a constant such as `'(nil)`, so naturally you won’t use it on `(empty-object)` either.
elisp None #### nil and t
In Emacs Lisp, the symbol `nil` has three separate meanings: it is a symbol with the name ‘`nil`’; it is the logical truth value false; and it is the empty list—the list of zero elements. When used as a variable, `nil` always has the value `nil`.
As far as the Lisp reader is concerned, ‘`()`’ and ‘`nil`’ are identical: they stand for the same object, the symbol `nil`. The different ways of writing the symbol are intended entirely for human readers. After the Lisp reader has read either ‘`()`’ or ‘`nil`’, there is no way to determine which representation was actually written by the programmer.
In this manual, we write `()` when we wish to emphasize that it means the empty list, and we write `nil` when we wish to emphasize that it means the truth value false. That is a good convention to use in Lisp programs also.
```
(cons 'foo ()) ; Emphasize the empty list
(setq foo-flag nil) ; Emphasize the truth value false
```
In contexts where a truth value is expected, any non-`nil` value is considered to be true. However, `t` is the preferred way to represent the truth value true. When you need to choose a value that represents true, and there is no other basis for choosing, use `t`. The symbol `t` always has the value `t`.
In Emacs Lisp, `nil` and `t` are special symbols that always evaluate to themselves. This is so that you do not need to quote them to use them as constants in a program. An attempt to change their values results in a `setting-constant` error. See [Constant Variables](constant-variables).
Function: **booleanp** *object*
Return non-`nil` if object is one of the two canonical boolean values: `t` or `nil`.
| programming_docs |
elisp None ### Regular Expression Searching
In GNU Emacs, you can search for the next match for a regular expression (see [Syntax of Regexps](syntax-of-regexps)) either incrementally or not. For incremental search commands, see [Regular Expression Search](https://www.gnu.org/software/emacs/manual/html_node/emacs/Regexp-Search.html#Regexp-Search) in The GNU Emacs Manual. Here we describe only the search functions useful in programs. The principal one is `re-search-forward`.
These search functions convert the regular expression to multibyte if the buffer is multibyte; they convert the regular expression to unibyte if the buffer is unibyte. See [Text Representations](text-representations).
Command: **re-search-forward** *regexp &optional limit noerror count*
This function searches forward in the current buffer for a string of text that is matched by the regular expression regexp. The function skips over any amount of text that is not matched by regexp, and leaves point at the end of the first match found. It returns the new value of point.
If limit is non-`nil`, it must be a position in the current buffer. It specifies the upper bound to the search. No match extending after that position is accepted. If limit is omitted or `nil`, it defaults to the end of the accessible portion of the buffer.
What `re-search-forward` does when the search fails depends on the value of noerror:
`nil` Signal a `search-failed` error.
`t` Do nothing and return `nil`.
anything else Move point to limit (or the end of the accessible portion of the buffer) and return `nil`.
The argument noerror only affects valid searches which fail to find a match. Invalid arguments cause errors regardless of noerror.
If count is a positive number n, the search is done n times; each successive search starts at the end of the previous match. If all these successive searches succeed, the function call succeeds, moving point and returning its new value. Otherwise the function call fails, with results depending on the value of noerror, as described above. If count is a negative number -n, the search is done n times in the opposite (backward) direction.
In the following example, point is initially before the ‘`T`’. Evaluating the search call moves point to the end of that line (between the ‘`t`’ of ‘`hat`’ and the newline).
```
---------- Buffer: foo ----------
I read "∗The cat in the hat
comes back" twice.
---------- Buffer: foo ----------
```
```
(re-search-forward "[a-z]+" nil t 5)
⇒ 27
---------- Buffer: foo ----------
I read "The cat in the hat∗
comes back" twice.
---------- Buffer: foo ----------
```
Command: **re-search-backward** *regexp &optional limit noerror count*
This function searches backward in the current buffer for a string of text that is matched by the regular expression regexp, leaving point at the beginning of the first text found.
This function is analogous to `re-search-forward`, but they are not simple mirror images. `re-search-forward` finds the match whose beginning is as close as possible to the starting point. If `re-search-backward` were a perfect mirror image, it would find the match whose end is as close as possible. However, in fact it finds the match whose beginning is as close as possible (and yet ends before the starting point). The reason for this is that matching a regular expression at a given spot always works from beginning to end, and starts at a specified beginning position.
A true mirror-image of `re-search-forward` would require a special feature for matching regular expressions from end to beginning. It’s not worth the trouble of implementing that.
Function: **string-match** *regexp string &optional start*
This function returns the index of the start of the first match for the regular expression regexp in string, or `nil` if there is no match. If start is non-`nil`, the search starts at that index in string.
For example,
```
(string-match
"quick" "The quick brown fox jumped quickly.")
⇒ 4
```
```
(string-match
"quick" "The quick brown fox jumped quickly." 8)
⇒ 27
```
The index of the first character of the string is 0, the index of the second character is 1, and so on.
If this function finds a match, the index of the first character beyond the match is available as `(match-end 0)`. See [Match Data](match-data).
```
(string-match
"quick" "The quick brown fox jumped quickly." 8)
⇒ 27
```
```
(match-end 0)
⇒ 32
```
Function: **string-match-p** *regexp string &optional start*
This predicate function does what `string-match` does, but it avoids modifying the match data.
Function: **looking-at** *regexp*
This function determines whether the text in the current buffer directly following point matches the regular expression regexp. “Directly following” means precisely that: the search is “anchored” and it can succeed only starting with the first character following point. The result is `t` if so, `nil` otherwise.
This function does not move point, but it does update the match data. See [Match Data](match-data). If you need to test for a match without modifying the match data, use `looking-at-p`, described below.
In this example, point is located directly before the ‘`T`’. If it were anywhere else, the result would be `nil`.
```
---------- Buffer: foo ----------
I read "∗The cat in the hat
comes back" twice.
---------- Buffer: foo ----------
(looking-at "The cat in the hat$")
⇒ t
```
Function: **looking-back** *regexp limit &optional greedy*
This function returns `t` if regexp matches the text immediately before point (i.e., ending at point), and `nil` otherwise.
Because regular expression matching works only going forward, this is implemented by searching backwards from point for a match that ends at point. That can be quite slow if it has to search a long distance. You can bound the time required by specifying a non-`nil` value for limit, which says not to search before limit. In this case, the match that is found must begin at or after limit. Here’s an example:
```
---------- Buffer: foo ----------
I read "∗The cat in the hat
comes back" twice.
---------- Buffer: foo ----------
(looking-back "read \"" 3)
⇒ t
(looking-back "read \"" 4)
⇒ nil
```
If greedy is non-`nil`, this function extends the match backwards as far as possible, stopping when a single additional previous character cannot be part of a match for regexp. When the match is extended, its starting position is allowed to occur before limit.
As a general recommendation, try to avoid using `looking-back` wherever possible, since it is slow. For this reason, there are no plans to add a `looking-back-p` function.
Function: **looking-at-p** *regexp*
This predicate function works like `looking-at`, but without updating the match data.
Variable: **search-spaces-regexp**
If this variable is non-`nil`, it should be a regular expression that says how to search for whitespace. In that case, any group of spaces in a regular expression being searched for stands for use of this regular expression. However, spaces inside of constructs such as ‘`[…]`’ and ‘`\*`’, ‘`+`’, ‘`?`’ are not affected by `search-spaces-regexp`.
Since this variable affects all regular expression search and match constructs, you should bind it temporarily for as small as possible a part of the code.
elisp None #### Defining and Using Fields
A field is a range of consecutive characters in the buffer that are identified by having the same value (comparing with `eq`) of the `field` property (either a text-property or an overlay property). This section describes special functions that are available for operating on fields.
You specify a field with a buffer position, pos. We think of each field as containing a range of buffer positions, so the position you specify stands for the field containing that position.
When the characters before and after pos are part of the same field, there is no doubt which field contains pos: the one those characters both belong to. When pos is at a boundary between fields, which field it belongs to depends on the stickiness of the `field` properties of the two surrounding characters (see [Sticky Properties](sticky-properties)). The field whose property would be inherited by text inserted at pos is the field that contains pos.
There is an anomalous case where newly inserted text at pos would not inherit the `field` property from either side. This happens if the previous character’s `field` property is not rear-sticky, and the following character’s `field` property is not front-sticky. In this case, pos belongs to neither the preceding field nor the following field; the field functions treat it as belonging to an empty field whose beginning and end are both at pos.
In all of these functions, if pos is omitted or `nil`, the value of point is used by default. If narrowing is in effect, then pos should fall within the accessible portion. See [Narrowing](narrowing).
Function: **field-beginning** *&optional pos escape-from-edge limit*
This function returns the beginning of the field specified by pos.
If pos is at the beginning of its field, and escape-from-edge is non-`nil`, then the return value is always the beginning of the preceding field that *ends* at pos, regardless of the stickiness of the `field` properties around pos.
If limit is non-`nil`, it is a buffer position; if the beginning of the field is before limit, then limit will be returned instead.
Function: **field-end** *&optional pos escape-from-edge limit*
This function returns the end of the field specified by pos.
If pos is at the end of its field, and escape-from-edge is non-`nil`, then the return value is always the end of the following field that *begins* at pos, regardless of the stickiness of the `field` properties around pos.
If limit is non-`nil`, it is a buffer position; if the end of the field is after limit, then limit will be returned instead.
Function: **field-string** *&optional pos*
This function returns the contents of the field specified by pos, as a string.
Function: **field-string-no-properties** *&optional pos*
This function returns the contents of the field specified by pos, as a string, discarding text properties.
Function: **delete-field** *&optional pos*
This function deletes the text of the field specified by pos.
Function: **constrain-to-field** *new-pos old-pos &optional escape-from-edge only-in-line inhibit-capture-property*
This function constrains new-pos to the field that old-pos belongs to—in other words, it returns the position closest to new-pos that is in the same field as old-pos.
If new-pos is `nil`, then `constrain-to-field` uses the value of point instead, and moves point to the resulting position in addition to returning that position.
If old-pos is at the boundary of two fields, then the acceptable final positions depend on the argument escape-from-edge. If escape-from-edge is `nil`, then new-pos must be in the field whose `field` property equals what new characters inserted at old-pos would inherit. (This depends on the stickiness of the `field` property for the characters before and after old-pos.) If escape-from-edge is non-`nil`, new-pos can be anywhere in the two adjacent fields. Additionally, if two fields are separated by another field with the special value `boundary`, then any point within this special field is also considered to be on the boundary.
Commands like `C-a` with no argument, that normally move backward to a specific kind of location and stay there once there, probably should specify `nil` for escape-from-edge. Other motion commands that check fields should probably pass `t`.
If the optional argument only-in-line is non-`nil`, and constraining new-pos in the usual way would move it to a different line, new-pos is returned unconstrained. This used in commands that move by line, such as `next-line` and `beginning-of-line`, so that they respect field boundaries only in the case where they can still move to the right line.
If the optional argument inhibit-capture-property is non-`nil`, and old-pos has a non-`nil` property of that name, then any field boundaries are ignored.
You can cause `constrain-to-field` to ignore all field boundaries (and so never constrain anything) by binding the variable `inhibit-field-text-motion` to a non-`nil` value.
elisp None ### X Resources
This section describes some of the functions and variables for querying and using X resources, or their equivalent on your operating system. See [X Resources](https://www.gnu.org/software/emacs/manual/html_node/emacs/X-Resources.html#X-Resources) in The GNU Emacs Manual, for more information about X resources.
Function: **x-get-resource** *attribute class &optional component subclass*
The function `x-get-resource` retrieves a resource value from the X Window defaults database.
Resources are indexed by a combination of a *key* and a *class*. This function searches using a key of the form ‘`instance.attribute`’ (where instance is the name under which Emacs was invoked), and using ‘`Emacs.class`’ as the class.
The optional arguments component and subclass add to the key and the class, respectively. You must specify both of them or neither. If you specify them, the key is ‘`instance.component.attribute`’, and the class is ‘`Emacs.class.subclass`’.
Variable: **x-resource-class**
This variable specifies the application name that `x-get-resource` should look up. The default value is `"Emacs"`. You can examine X resources for other application names by binding this variable to some other string, around a call to `x-get-resource`.
Variable: **x-resource-name**
This variable specifies the instance name that `x-get-resource` should look up. The default value is the name Emacs was invoked with, or the value specified with the ‘`-name`’ or ‘`-rn`’ switches.
To illustrate some of the above, suppose that you have the line:
```
xterm.vt100.background: yellow
```
in your X resources file (whose name is usually `~/.Xdefaults` or `~/.Xresources`). Then:
```
(let ((x-resource-class "XTerm") (x-resource-name "xterm"))
(x-get-resource "vt100.background" "VT100.Background"))
⇒ "yellow"
```
```
(let ((x-resource-class "XTerm") (x-resource-name "xterm"))
(x-get-resource "background" "VT100" "vt100" "Background"))
⇒ "yellow"
```
Variable: **inhibit-x-resources**
If this variable is non-`nil`, Emacs does not look up X resources, and X resources do not have any effect when creating new frames.
elisp None ### Buffer File Name
The *buffer file name* is the name of the file that is visited in that buffer. When a buffer is not visiting a file, its buffer file name is `nil`. Most of the time, the buffer name is the same as the nondirectory part of the buffer file name, but the buffer file name and the buffer name are distinct and can be set independently. See [Visiting Files](visiting-files).
Function: **buffer-file-name** *&optional buffer*
This function returns the absolute file name of the file that buffer is visiting. If buffer is not visiting any file, `buffer-file-name` returns `nil`. If buffer is not supplied, it defaults to the current buffer.
```
(buffer-file-name (other-buffer))
⇒ "/usr/user/lewis/manual/files.texi"
```
Variable: **buffer-file-name**
This buffer-local variable contains the name of the file being visited in the current buffer, or `nil` if it is not visiting a file. It is a permanent local variable, unaffected by `kill-all-local-variables`.
```
buffer-file-name
⇒ "/usr/user/lewis/manual/buffers.texi"
```
It is risky to change this variable’s value without doing various other things. Normally it is better to use `set-visited-file-name` (see below); some of the things done there, such as changing the buffer name, are not strictly necessary, but others are essential to avoid confusing Emacs.
Variable: **buffer-file-truename**
This buffer-local variable holds the abbreviated truename of the file visited in the current buffer, or `nil` if no file is visited. It is a permanent local, unaffected by `kill-all-local-variables`. See [Truenames](truenames), and [abbreviate-file-name](directory-names#abbreviate_002dfile_002dname).
Variable: **buffer-file-number**
This buffer-local variable holds the file number and directory device number of the file visited in the current buffer, or `nil` if no file or a nonexistent file is visited. It is a permanent local, unaffected by `kill-all-local-variables`.
The value is normally a list of the form `(filenum
devnum)`. This pair of numbers uniquely identifies the file among all files accessible on the system. See the function `file-attributes`, in [File Attributes](file-attributes), for more information about them.
If `buffer-file-name` is the name of a symbolic link, then both numbers refer to the recursive target.
Function: **get-file-buffer** *filename*
This function returns the buffer visiting file filename. If there is no such buffer, it returns `nil`. The argument filename, which must be a string, is expanded (see [File Name Expansion](file-name-expansion)), then compared against the visited file names of all live buffers. Note that the buffer’s `buffer-file-name` must match the expansion of filename exactly. This function will not recognize other names for the same file.
```
(get-file-buffer "buffers.texi")
⇒ #<buffer buffers.texi>
```
In unusual circumstances, there can be more than one buffer visiting the same file name. In such cases, this function returns the first such buffer in the buffer list.
Function: **find-buffer-visiting** *filename &optional predicate*
This is like `get-file-buffer`, except that it can return any buffer visiting the file *possibly under a different name*. That is, the buffer’s `buffer-file-name` does not need to match the expansion of filename exactly, it only needs to refer to the same file. If predicate is non-`nil`, it should be a function of one argument, a buffer visiting filename. The buffer is only considered a suitable return value if predicate returns non-`nil`. If it can not find a suitable buffer to return, `find-buffer-visiting` returns `nil`.
Command: **set-visited-file-name** *filename &optional no-query along-with-file*
If filename is a non-empty string, this function changes the name of the file visited in the current buffer to filename. (If the buffer had no visited file, this gives it one.) The *next time* the buffer is saved it will go in the newly-specified file.
This command marks the buffer as modified, since it does not (as far as Emacs knows) match the contents of filename, even if it matched the former visited file. It also renames the buffer to correspond to the new file name, unless the new name is already in use.
If filename is `nil` or the empty string, that stands for “no visited file”. In this case, `set-visited-file-name` marks the buffer as having no visited file, without changing the buffer’s modified flag.
Normally, this function asks the user for confirmation if there already is a buffer visiting filename. If no-query is non-`nil`, that prevents asking this question. If there already is a buffer visiting filename, and the user confirms or no-query is non-`nil`, this function makes the new buffer name unique by appending a number inside of ‘`<…>`’ to filename.
If along-with-file is non-`nil`, that means to assume that the former visited file has been renamed to filename. In this case, the command does not change the buffer’s modified flag, nor the buffer’s recorded last file modification time as reported by `visited-file-modtime` (see [Modification Time](modification-time)). If along-with-file is `nil`, this function clears the recorded last file modification time, after which `visited-file-modtime` returns zero.
When the function `set-visited-file-name` is called interactively, it prompts for filename in the minibuffer.
Variable: **list-buffers-directory**
This buffer-local variable specifies a string to display in a buffer listing where the visited file name would go, for buffers that don’t have a visited file name. Dired buffers use this variable.
| programming_docs |
elisp None #### Menus and the Mouse
The usual way to make a menu keymap produce a menu is to make it the definition of a prefix key. (A Lisp program can explicitly pop up a menu and receive the user’s choice—see [Pop-Up Menus](pop_002dup-menus).)
If the prefix key ends with a mouse event, Emacs handles the menu keymap by popping up a visible menu, so that the user can select a choice with the mouse. When the user clicks on a menu item, the event generated is whatever character or symbol has the binding that brought about that menu item. (A menu item may generate a series of events if the menu has multiple levels or comes from the menu bar.)
It’s often best to use a button-down event to trigger the menu. Then the user can select a menu item by releasing the button.
If the menu keymap contains a binding to a nested keymap, the nested keymap specifies a *submenu*. There will be a menu item, labeled by the nested keymap’s item string, and clicking on this item automatically pops up the specified submenu. As a special exception, if the menu keymap contains a single nested keymap and no other menu items, the menu shows the contents of the nested keymap directly, not as a submenu.
However, if Emacs is compiled without X toolkit support, or on text terminals, submenus are not supported. Each nested keymap is shown as a menu item, but clicking on it does not automatically pop up the submenu. If you wish to imitate the effect of submenus, you can do that by giving a nested keymap an item string which starts with ‘`@`’. This causes Emacs to display the nested keymap using a separate *menu pane*; the rest of the item string after the ‘`@`’ is the pane label. If Emacs is compiled without X toolkit support, or if a menu is displayed on a text terminal, menu panes are not used; in that case, a ‘`@`’ at the beginning of an item string is omitted when the menu label is displayed, and has no other effect.
elisp None ### Resizing Windows
This section describes functions for resizing a window without changing the size of its frame. Because live windows do not overlap, these functions are meaningful only on frames that contain two or more windows: resizing a window also changes the size of at least one other window. If there is just one window on a frame, its size cannot be changed except by resizing the frame (see [Frame Size](frame-size)).
Except where noted, these functions also accept internal windows as arguments. Resizing an internal window causes its child windows to be resized to fit the same space.
Function: **window-resizable** *window delta &optional horizontal ignore pixelwise*
This function returns delta if the size of window can be changed vertically by delta lines. If the optional argument horizontal is non-`nil`, it instead returns delta if window can be resized horizontally by delta columns. It does not actually change the window size.
If window is `nil`, it defaults to the selected window.
A positive value of delta means to check whether the window can be enlarged by that number of lines or columns; a negative value of delta means to check whether the window can be shrunk by that many lines or columns. If delta is non-zero, a return value of 0 means that the window cannot be resized.
Normally, the variables `window-min-height` and `window-min-width` specify the smallest allowable window size (see [Window Sizes](window-sizes)). However, if the optional argument ignore is non-`nil`, this function ignores `window-min-height` and `window-min-width`, as well as `window-size-fixed`. Instead, it considers the minimum height of a window as the sum of its top and bottom decorations plus the text of one line; and its minimum width as the sum of its left and right decorations plus text that takes two columns.
If the optional argument pixelwise is non-`nil`, delta is interpreted as pixels.
Function: **window-resize** *window delta &optional horizontal ignore pixelwise*
This function resizes window by delta increments. If horizontal is `nil`, it changes the height by delta lines; otherwise, it changes the width by delta columns. A positive delta means to enlarge the window, and a negative delta means to shrink it.
If window is `nil`, it defaults to the selected window. If the window cannot be resized as demanded, an error is signaled.
The optional argument ignore has the same meaning as for the function `window-resizable` above.
If the optional argument pixelwise is non-`nil`, delta will be interpreted as pixels.
The choice of which window edges this function alters depends on the values of the option `window-combination-resize` and the combination limits of the involved windows; in some cases, it may alter both edges. See [Recombining Windows](recombining-windows). To resize by moving only the bottom or right edge of a window, use the function `adjust-window-trailing-edge`.
Function: **adjust-window-trailing-edge** *window delta &optional horizontal pixelwise*
This function moves window’s bottom edge by delta lines. If optional argument horizontal is non-`nil`, it instead moves the right edge by delta columns. If window is `nil`, it defaults to the selected window.
If the optional argument pixelwise is non-`nil`, delta is interpreted as pixels.
A positive delta moves the edge downwards or to the right; a negative delta moves it upwards or to the left. If the edge cannot be moved as far as specified by delta, this function moves it as far as possible but does not signal an error.
This function tries to resize windows adjacent to the edge that is moved. If this is not possible for some reason (e.g., if that adjacent window is fixed-size), it may resize other windows.
User Option: **window-resize-pixelwise**
If the value of this option is non-`nil`, Emacs resizes windows in units of pixels. This currently affects functions like `split-window` (see [Splitting Windows](splitting-windows)), `maximize-window`, `minimize-window`, `fit-window-to-buffer`, `fit-frame-to-buffer` and `shrink-window-if-larger-than-buffer` (all listed below).
Note that when a frame’s pixel size is not a multiple of its character size, at least one window may get resized pixelwise even if this option is `nil`. The default value is `nil`.
The following commands resize windows in more specific ways. When called interactively, they act on the selected window.
Command: **fit-window-to-buffer** *&optional window max-height min-height max-width min-width preserve-size*
This command adjusts the height or width of window to fit the text in it. It returns non-`nil` if it was able to resize window, and `nil` otherwise. If window is omitted or `nil`, it defaults to the selected window. Otherwise, it should be a live window.
If window is part of a vertical combination, this function adjusts window’s height. The new height is calculated from the actual height of the accessible portion of its buffer. The optional argument max-height, if non-`nil`, specifies the maximum total height that this function can give window. The optional argument min-height, if non-`nil`, specifies the minimum total height that it can give, which overrides the variable `window-min-height`. Both max-height and min-height are specified in lines and include any top or bottom decorations of window.
If window is part of a horizontal combination and the value of the option `fit-window-to-buffer-horizontally` (see below) is non-`nil`, this function adjusts window’s width. The new width of window is calculated from the maximum length of its buffer’s lines that follow the current start position of window. The optional argument max-width specifies a maximum width and defaults to the width of window’s frame. The optional argument min-width specifies a minimum width and defaults to `window-min-width`. Both max-width and min-width are specified in columns and include any left or right decorations of window.
The optional argument preserve-size, if non-`nil`, will install a parameter to preserve the size of window during future resize operations (see [Preserving Window Sizes](preserving-window-sizes)).
If the option `fit-frame-to-buffer` (see below) is non-`nil`, this function will try to resize the frame of window to fit its contents by calling `fit-frame-to-buffer` (see below).
User Option: **fit-window-to-buffer-horizontally**
If this is non-`nil`, `fit-window-to-buffer` can resize windows horizontally. If this is `nil` (the default) `fit-window-to-buffer` never resizes windows horizontally. If this is `only`, it can resize windows horizontally only. Any other value means `fit-window-to-buffer` can resize windows in both dimensions.
User Option: **fit-frame-to-buffer**
If this option is non-`nil`, `fit-window-to-buffer` can fit a frame to its buffer. A frame is fit if and only if its root window is a live window and this option is non-`nil`. If this is `horizontally`, frames are fit horizontally only. If this is `vertically`, frames are fit vertically only. Any other non-`nil` value means frames can be resized in both dimensions.
If you have a frame that displays only one window, you can fit that frame to its buffer using the command `fit-frame-to-buffer`.
Command: **fit-frame-to-buffer** *&optional frame max-height min-height max-width min-width only*
This command adjusts the size of frame to display the contents of its buffer exactly. frame can be any live frame and defaults to the selected one. Fitting is done only if frame’s root window is live. The arguments max-height, min-height, max-width and min-width specify bounds on the new total size of frame’s root window. min-height and min-width default to the values of `window-min-height` and `window-min-width` respectively.
If the optional argument only is `vertically`, this function may resize the frame vertically only. If only is `horizontally`, it may resize the frame horizontally only.
The behavior of `fit-frame-to-buffer` can be controlled with the help of the two options listed next.
User Option: **fit-frame-to-buffer-margins**
This option can be used to specify margins around frames to be fit by `fit-frame-to-buffer`. Such margins can be useful to avoid, for example, that the resized frame overlaps the taskbar or parts of its parent frame.
It specifies the numbers of pixels to be left free on the left, above, the right, and below a frame that shall be fit. The default specifies `nil` for each which means to use no margins. The value specified here can be overridden for a specific frame by that frame’s `fit-frame-to-buffer-margins` parameter, if present.
User Option: **fit-frame-to-buffer-sizes**
This option specifies size boundaries for `fit-frame-to-buffer`. It specifies the total maximum and minimum lines and maximum and minimum columns of the root window of any frame that shall be fit to its buffer. If any of these values is non-`nil`, it overrides the corresponding argument of `fit-frame-to-buffer`.
Command: **shrink-window-if-larger-than-buffer** *&optional window*
This command attempts to reduce window’s height as much as possible while still showing its full buffer, but no less than `window-min-height` lines. The return value is non-`nil` if the window was resized, and `nil` otherwise. If window is omitted or `nil`, it defaults to the selected window. Otherwise, it should be a live window.
This command does nothing if the window is already too short to display all of its buffer, or if any of the buffer is scrolled off-screen, or if the window is the only live window in its frame.
This command calls `fit-window-to-buffer` (see above) to do its work.
Command: **balance-windows** *&optional window-or-frame*
This function balances windows in a way that gives more space to full-width and/or full-height windows. If window-or-frame specifies a frame, it balances all windows on that frame. If window-or-frame specifies a window, it balances only that window and its siblings (see [Windows and Frames](windows-and-frames)).
Command: **balance-windows-area**
This function attempts to give all windows on the selected frame approximately the same share of the screen area. Full-width or full-height windows are not given more space than other windows.
Command: **maximize-window** *&optional window*
This function attempts to make window as large as possible, in both dimensions, without resizing its frame or deleting other windows. If window is omitted or `nil`, it defaults to the selected window.
Command: **minimize-window** *&optional window*
This function attempts to make window as small as possible, in both dimensions, without deleting it or resizing its frame. If window is omitted or `nil`, it defaults to the selected window.
elisp None #### Summary: Sequence of Actions at Startup
When Emacs is started up, it performs the following operations (see `normal-top-level` in `startup.el`):
1. It adds subdirectories to `load-path`, by running the file named `subdirs.el` in each directory in the list. Normally, this file adds the directory’s subdirectories to the list, and those are scanned in their turn. The files `subdirs.el` are normally generated automatically when Emacs is installed.
2. It loads any `leim-list.el` that it finds in the `load-path` directories. This file is intended for registering input methods. The search is only for any personal `leim-list.el` files that you may have created; it skips the directories containing the standard Emacs libraries (these should contain only a single `leim-list.el` file, which is compiled into the Emacs executable).
3. It sets the variable `before-init-time` to the value of `current-time` (see [Time of Day](time-of-day)). It also sets `after-init-time` to `nil`, which signals to Lisp programs that Emacs is being initialized.
4. It sets the language environment and the terminal coding system, if requested by environment variables such as `LANG`.
5. It does some basic parsing of the command-line arguments.
6. It loads your early init file (see [Early Init File](https://www.gnu.org/software/emacs/manual/html_node/emacs/Early-Init-File.html#Early-Init-File) in The GNU Emacs Manual). This is not done if the options ‘`-q`’, ‘`-Q`’, or ‘`--batch`’ were specified. If the ‘`-u`’ option was specified, Emacs looks for the init file in that user’s home directory instead.
7. It calls the function `package-activate-all` to activate any optional Emacs Lisp package that has been installed. See [Packaging Basics](packaging-basics). However, Emacs doesn’t activate the packages when `package-enable-at-startup` is `nil` or when it’s started with one of the options ‘`-q`’, ‘`-Q`’, or ‘`--batch`’. To activate the packages in the latter case, `package-activate-all` should be called explicitly (e.g., via the ‘`--funcall`’ option).
8. If not running in batch mode, it initializes the window system that the variable `initial-window-system` specifies (see [initial-window-system](window-systems)). The initialization function, `window-system-initialization`, is a *generic function* (see [Generic Functions](generic-functions)) whose actual implementation is different for each supported window system. If the value of `initial-window-system` is windowsystem, then the appropriate implementation of the initialization function is defined in the file `term/windowsystem-win.el`. This file should have been compiled into the Emacs executable when it was built.
9. It runs the normal hook `before-init-hook`.
10. If appropriate, it creates a graphical frame. As part of creating the graphical frame, it initializes the window system specified by `initial-frame-alist` and `default-frame-alist` (see [Initial Parameters](initial-parameters)) for the graphical frame, by calling the `window-system-initialization` function for that window system. This is not done in batch (noninteractive) or daemon mode.
11. It initializes the initial frame’s faces, and sets up the menu bar and tool bar if needed. If graphical frames are supported, it sets up the tool bar even if the current frame is not a graphical one, since a graphical frame may be created later on.
12. It use `custom-reevaluate-setting` to re-initialize the members of the list `custom-delayed-init-variables`. These are any pre-loaded user options whose default value depends on the run-time, rather than build-time, context. See [custom-initialize-delay](building-emacs).
13. It loads the library `site-start`, if it exists. This is not done if the options ‘`-Q`’ or ‘`--no-site-file`’ were specified.
14. It loads your init file (see [Init File](init-file)). This is not done if the options ‘`-q`’, ‘`-Q`’, or ‘`--batch`’ were specified. If the ‘`-u`’ option was specified, Emacs looks for the init file in that user’s home directory instead.
15. It loads the library `default`, if it exists. This is not done if `inhibit-default-init` is non-`nil`, nor if the options ‘`-q`’, ‘`-Q`’, or ‘`--batch`’ were specified.
16. It loads your abbrevs from the file specified by `abbrev-file-name`, if that file exists and can be read (see [abbrev-file-name](abbrev-files)). This is not done if the option ‘`--batch`’ was specified.
17. It sets the variable `after-init-time` to the value of `current-time`. This variable was set to `nil` earlier; setting it to the current time signals that the initialization phase is over, and, together with `before-init-time`, provides the measurement of how long it took.
18. It runs the normal hook `after-init-hook`.
19. If the buffer `\*scratch\*` exists and is still in Fundamental mode (as it should be by default), it sets its major mode according to `initial-major-mode`.
20. If started on a text terminal, it loads the terminal-specific Lisp library (see [Terminal-Specific](terminal_002dspecific)), and runs the hook `tty-setup-hook`. This is not done in `--batch` mode, nor if `term-file-prefix` is `nil`.
21. It displays the initial echo area message, unless you have suppressed that with `inhibit-startup-echo-area-message`.
22. It processes any command-line options that were not handled earlier.
23. It now exits if the option `--batch` was specified.
24. If the `\*scratch\*` buffer exists and is empty, it inserts `(substitute-command-keys initial-scratch-message)` into that buffer.
25. If `initial-buffer-choice` is a string, it visits the file (or directory) with that name. If it is a function, it calls the function with no arguments and selects the buffer that it returns. If one file is given as a command line argument, that file is visited and its buffer displayed alongside `initial-buffer-choice`. If more than one file is given, all of the files are visited and the `\*Buffer List\*` buffer is displayed alongside `initial-buffer-choice`.
26. It runs `emacs-startup-hook`.
27. It calls `frame-notice-user-settings`, which modifies the parameters of the selected frame according to whatever the init files specify.
28. It runs `window-setup-hook`. The only difference between this hook and `emacs-startup-hook` is that this one runs after the previously mentioned modifications to the frame parameters.
29. It displays the *startup screen*, which is a special buffer that contains information about copyleft and basic Emacs usage. This is not done if `inhibit-startup-screen` or `initial-buffer-choice` are non-`nil`, or if the ‘`--no-splash`’ or ‘`-Q`’ command-line options were specified.
30. If a daemon was requested, it calls `server-start`. (On POSIX systems, if a background daemon was requested, it then detaches from the controlling terminal.) See [Emacs Server](https://www.gnu.org/software/emacs/manual/html_node/emacs/Emacs-Server.html#Emacs-Server) in The GNU Emacs Manual.
31. If started by the X session manager, it calls `emacs-session-restore` passing it as argument the ID of the previous session. See [Session Management](session-management).
The following options affect some aspects of the startup sequence.
User Option: **inhibit-startup-screen**
This variable, if non-`nil`, inhibits the startup screen. In that case, Emacs typically displays the `\*scratch\*` buffer; but see `initial-buffer-choice`, below.
Do not set this variable in the init file of a new user, or in a way that affects more than one user, as that would prevent new users from receiving information about copyleft and basic Emacs usage.
`inhibit-startup-message` and `inhibit-splash-screen` are aliases for this variable.
User Option: **initial-buffer-choice**
If non-`nil`, this variable is a string that specifies a file or directory for Emacs to display after starting up, instead of the startup screen. If its value is a function, Emacs calls that function which must return a buffer which is then displayed. If its value is `t`, Emacs displays the `\*scratch\*` buffer.
User Option: **inhibit-startup-echo-area-message**
This variable controls the display of the startup echo area message. You can suppress the startup echo area message by adding text with this form to your init file:
```
(setq inhibit-startup-echo-area-message
"your-login-name")
```
Emacs explicitly checks for an expression as shown above in your init file; your login name must appear in the expression as a Lisp string constant. You can also use the Customize interface. Other methods of setting `inhibit-startup-echo-area-message` to the same value do not inhibit the startup message. This way, you can easily inhibit the message for yourself if you wish, but thoughtless copying of your init file will not inhibit the message for someone else.
User Option: **initial-scratch-message**
This variable, if non-`nil`, should be a string, which is treated as documentation to be inserted into the `\*scratch\*` buffer when Emacs starts up. If it is `nil`, the `\*scratch\*` buffer is empty.
The following command-line options affect some aspects of the startup sequence. See [Initial Options](https://www.gnu.org/software/emacs/manual/html_node/emacs/Initial-Options.html#Initial-Options) in The GNU Emacs Manual.
`--no-splash`
Do not display a splash screen.
`--batch`
Run without an interactive terminal. See [Batch Mode](batch-mode).
`--daemon` `--bg-daemon` `--fg-daemon`
Do not initialize any display; just start a server. (A “background” daemon automatically runs in the background.)
`--no-init-file` `-q`
Do not load either the init file, or the `default` library.
`--no-site-file`
Do not load the `site-start` library.
`--quick` `-Q` Equivalent to ‘`-q --no-site-file --no-splash`’.
| programming_docs |
elisp None #### Parameters to Control Parsing
Variable: **multibyte-syntax-as-symbol**
If this variable is non-`nil`, `scan-sexps` treats all non-ASCII characters as symbol constituents regardless of what the syntax table says about them. (However, `syntax-table`text properties can still override the syntax.)
User Option: **parse-sexp-ignore-comments**
If the value is non-`nil`, then comments are treated as whitespace by the functions in this section and by `forward-sexp`, `scan-lists` and `scan-sexps`.
The behavior of `parse-partial-sexp` is also affected by `parse-sexp-lookup-properties` (see [Syntax Properties](syntax-properties)).
Variable: **comment-end-can-be-escaped**
If this buffer local variable is non-`nil`, a single character which usually terminates a comment doesn’t do so when that character is escaped. This is used in C and C++ Modes, where line comments starting with ‘`//`’ can be continued onto the next line by escaping the newline with ‘`\`’.
You can use `forward-comment` to move forward or backward over one comment or several comments.
elisp None ### Read-Only Buffers
If a buffer is *read-only*, then you cannot change its contents, although you may change your view of the contents by scrolling and narrowing.
Read-only buffers are used in two kinds of situations:
* A buffer visiting a write-protected file is normally read-only. Here, the purpose is to inform the user that editing the buffer with the aim of saving it in the file may be futile or undesirable. The user who wants to change the buffer text despite this can do so after clearing the read-only flag with `C-x C-q`.
* Modes such as Dired and Rmail make buffers read-only when altering the contents with the usual editing commands would probably be a mistake. The special commands of these modes bind `buffer-read-only` to `nil` (with `let`) or bind `inhibit-read-only` to `t` around the places where they themselves change the text.
Variable: **buffer-read-only**
This buffer-local variable specifies whether the buffer is read-only. The buffer is read-only if this variable is non-`nil`. However, characters that have the `inhibit-read-only` text property can still be modified. See [inhibit-read-only](special-properties).
Variable: **inhibit-read-only**
If this variable is non-`nil`, then read-only buffers and, depending on the actual value, some or all read-only characters may be modified. Read-only characters in a buffer are those that have a non-`nil` `read-only` text property. See [Special Properties](special-properties), for more information about text properties.
If `inhibit-read-only` is `t`, all `read-only` character properties have no effect. If `inhibit-read-only` is a list, then `read-only` character properties have no effect if they are members of the list (comparison is done with `eq`).
Command: **read-only-mode** *&optional arg*
This is the mode command for Read Only minor mode, a buffer-local minor mode. When the mode is enabled, `buffer-read-only` is non-`nil` in the buffer; when disabled, `buffer-read-only` is `nil` in the buffer. The calling convention is the same as for other minor mode commands (see [Minor Mode Conventions](minor-mode-conventions)).
This minor mode mainly serves as a wrapper for `buffer-read-only`; unlike most minor modes, there is no separate `read-only-mode` variable. Even when Read Only mode is disabled, characters with non-`nil` `read-only` text properties remain read-only. To temporarily ignore all read-only states, bind `inhibit-read-only`, as described above.
When enabling Read Only mode, this mode command also enables View mode if the option `view-read-only` is non-`nil`. See [Miscellaneous Buffer Operations](https://www.gnu.org/software/emacs/manual/html_node/emacs/Misc-Buffer.html#Misc-Buffer) in The GNU Emacs Manual. When disabling Read Only mode, it disables View mode if View mode was enabled.
Function: **barf-if-buffer-read-only** *&optional position*
This function signals a `buffer-read-only` error if the current buffer is read-only. If the text at position (which defaults to point) has the `inhibit-read-only` text property set, the error will not be raised.
See [Using Interactive](using-interactive), for another way to signal an error if the current buffer is read-only.
elisp None #### Stream Type
A *stream* is an object that can be used as a source or sink for characters—either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a `\*Help\*` buffer, or to the echo area.
The object `nil`, in addition to its other meanings, may be used as a stream. It stands for the value of the variable `standard-input` or `standard-output`. Also, the object `t` as a stream specifies input using the minibuffer (see [Minibuffers](minibuffers)) or output in the echo area (see [The Echo Area](the-echo-area)).
Streams have no special printed representation or read syntax, and print as whatever primitive type they are.
See [Read and Print](read-and-print), for a description of functions related to streams, including parsing and printing functions.
elisp None #### Face Attributes
*Face attributes* determine the visual appearance of a face. The following table lists all the face attributes, their possible values, and their effects.
Apart from the values given below, each face attribute can have the value `unspecified`. This special value means that the face doesn’t specify that attribute directly. An `unspecified` attribute tells Emacs to refer instead to a parent face (see the description `:inherit` attribute below); or, failing that, to an underlying face (see [Displaying Faces](displaying-faces)). The `default` face must specify all attributes.
Some of these attributes are meaningful only on certain kinds of displays. If your display cannot handle a certain attribute, the attribute is ignored.
`:family`
Font family name (a string). See [Fonts](https://www.gnu.org/software/emacs/manual/html_node/emacs/Fonts.html#Fonts) in The GNU Emacs Manual, for more information about font families. The function `font-family-list` (see below) returns a list of available family names.
`:foundry`
The name of the *font foundry* for the font family specified by the `:family` attribute (a string). See [Fonts](https://www.gnu.org/software/emacs/manual/html_node/emacs/Fonts.html#Fonts) in The GNU Emacs Manual.
`:width`
Relative character width. This should be one of the symbols `ultra-condensed`, `extra-condensed`, `condensed`, `semi-condensed`, `normal`, `semi-expanded`, `expanded`, `extra-expanded`, or `ultra-expanded`.
`:height`
The height of the font. In the simplest case, this is an integer in units of 1/10 point.
The value can also be floating point or a function, which specifies the height relative to an *underlying face* (see [Displaying Faces](displaying-faces)). A floating-point value specifies the amount by which to scale the height of the underlying face. A function value is called with one argument, the height of the underlying face, and returns the height of the new face. If the function is passed an integer argument, it must return an integer.
The height of the default face must be specified using an integer; floating point and function values are not allowed.
`:weight`
Font weight—one of the symbols (from densest to faintest) `ultra-bold`, `extra-bold`, `bold`, `semi-bold`, `normal`, `semi-light`, `light`, `extra-light`, or `ultra-light`. On text terminals which support variable-brightness text, any weight greater than normal is displayed as extra bright, and any weight less than normal is displayed as half-bright.
`:slant`
Font slant—one of the symbols `italic`, `oblique`, `normal`, `reverse-italic`, or `reverse-oblique`. On text terminals that support variable-brightness text, slanted text is displayed as half-bright.
`:foreground`
Foreground color, a string. The value can be a system-defined color name, or a hexadecimal color specification. See [Color Names](color-names). On black-and-white displays, certain shades of gray are implemented by stipple patterns.
`:distant-foreground`
Alternative foreground color, a string. This is like `:foreground` but the color is only used as a foreground when the background color is near to the foreground that would have been used. This is useful for example when marking text (i.e., the region face). If the text has a foreground that is visible with the region face, that foreground is used. If the foreground is near the region face background, `:distant-foreground` is used instead so the text is readable.
`:background`
Background color, a string. The value can be a system-defined color name, or a hexadecimal color specification. See [Color Names](color-names).
`:underline`
Whether or not characters should be underlined, and in what way. The possible values of the `:underline` attribute are:
`nil`
Don’t underline.
`t`
Underline with the foreground color of the face.
color
Underline in color color, a string specifying a color.
`(:color color :style style)` color is either a string, or the symbol `foreground-color`, meaning the foreground color of the face. Omitting the attribute `:color` means to use the foreground color of the face. style should be a symbol `line` or `wave`, meaning to use a straight or wavy line. Omitting the attribute `:style` means to use a straight line.
`:overline`
Whether or not characters should be overlined, and in what color. If the value is `t`, overlining uses the foreground color of the face. If the value is a string, overlining uses that color. The value `nil` means do not overline.
`:strike-through`
Whether or not characters should be strike-through, and in what color. The value is used like that of `:overline`.
`:box`
Whether or not a box should be drawn around characters, its color, the width of the box lines, and 3D appearance. Here are the possible values of the `:box` attribute, and what they mean:
`nil`
Don’t draw a box.
`t`
Draw a box with lines of width 1, in the foreground color.
color
Draw a box with lines of width 1, in color color.
`(:line-width (vwidth . hwidth) :color color :style style)`
This way you can explicitly specify all aspects of the box. The values vwidth and hwidth specifies respectively the width of the vertical and horizontal lines to draw; they default to (1 . 1). A negative horizontal or vertical width -n means to draw a line of width n that occupies the space of the underlying text, thus avoiding any increase in the character height or width. For simplification the width could be specified with only a single number n instead of a list, such case is equivalent to `((abs n) . n)`.
The value style specifies whether to draw a 3D box. If it is `released-button`, the box looks like a 3D button that is not being pressed. If it is `pressed-button`, the box looks like a 3D button that is being pressed. If it is `nil`, `flat-button` or omitted, a plain 2D box is used.
The value color specifies the color to draw with. The default is the background color of the face for 3D boxes and `flat-button`, and the foreground color of the face for other boxes.
`:inverse-video`
Whether or not characters should be displayed in inverse video. The value should be `t` (yes) or `nil` (no).
`:stipple`
The background stipple, a bitmap.
The value can be a string; that should be the name of a file containing external-format X bitmap data. The file is found in the directories listed in the variable `x-bitmap-file-path`.
Alternatively, the value can specify the bitmap directly, with a list of the form `(width height data)`. Here, width and height specify the size in pixels, and data is a string containing the raw bits of the bitmap, row by row. Each row occupies *(width + 7) / 8* consecutive bytes in the string (which should be a unibyte string for best results). This means that each row always occupies at least one whole byte.
If the value is `nil`, that means use no stipple pattern.
Normally you do not need to set the stipple attribute, because it is used automatically to handle certain shades of gray.
`:font`
The font used to display the face. Its value should be a font object or a fontset. If it is a font object, it specifies the font to be used by the face for displaying ASCII characters. See [Low-Level Font](low_002dlevel-font), for information about font objects, font specs, and font entities. See [Fontsets](fontsets), for information about fontsets.
When specifying this attribute using `set-face-attribute` or `set-face-font` (see [Attribute Functions](attribute-functions)), you may also supply a font spec, a font entity, or a string. Emacs converts such values to an appropriate font object, and stores that font object as the actual attribute value. If you specify a string, the contents of the string should be a font name (see [Fonts](https://www.gnu.org/software/emacs/manual/html_node/emacs/Fonts.html#Fonts) in The GNU Emacs Manual); if the font name is an XLFD containing wildcards, Emacs chooses the first font matching those wildcards. Specifying this attribute also changes the values of the `:family`, `:foundry`, `:width`, `:height`, `:weight`, and `:slant` attributes.
`:inherit`
The name of a face from which to inherit attributes, or a list of face names. Attributes from inherited faces are merged into the face like an underlying face would be, with higher priority than underlying faces (see [Displaying Faces](displaying-faces)). If the face to inherit from is `unspecified`, it is treated the same as `nil`, since Emacs never merges `:inherit` attributes. If a list of faces is used, attributes from faces earlier in the list override those from later faces.
`:extend`
Whether or not this face will be extended beyond end of line and will affect the display of the empty space between the end of line and the edge of the window. The value should be `t` to display the empty space between end of line and edge of the window using this face, or `nil` to not use this face for the space between the end of the line and the edge of the window. When Emacs merges several faces for displaying the empty space beyond end of line, only those faces with `:extend` non-`nil` will be merged. By default, only a small number of faces, notably, `region`, have this attribute set. This attribute is different from the others in that when a theme doesn’t specify an explicit value for a face, the value from the original face definition by `defface` is inherited (see [Defining Faces](defining-faces)).
Some modes, like `hl-line-mode`, use a face with an `:extend` property to mark the entire current line. Note, however, that Emacs will always allow you to move point after the final character in a buffer, and if the buffer ends with a newline character, point can be placed on what is seemingly a line at the end of the buffer—but Emacs can’t highlight that “line”, because it doesn’t really exist.
Function: **font-family-list** *&optional frame*
This function returns a list of available font family names. The optional argument frame specifies the frame on which the text is to be displayed; if it is `nil`, the selected frame is used.
User Option: **underline-minimum-offset**
This variable specifies the minimum distance between the baseline and the underline, in pixels, when displaying underlined text.
User Option: **x-bitmap-file-path**
This variable specifies a list of directories for searching for bitmap files, for the `:stipple` attribute.
Function: **bitmap-spec-p** *object*
This returns `t` if object is a valid bitmap specification, suitable for use with `:stipple` (see above). It returns `nil` otherwise.
elisp None #### Fringe Cursors
When a line is exactly as wide as the window, Emacs displays the cursor in the right fringe instead of using two lines. Different bitmaps are used to represent the cursor in the fringe depending on the current buffer’s cursor type.
User Option: **overflow-newline-into-fringe**
If this is non-`nil`, lines exactly as wide as the window (not counting the final newline character) are not continued. Instead, when point is at the end of the line, the cursor appears in the right fringe.
Variable: **fringe-cursor-alist**
This variable specifies the mapping from logical cursor type to the actual fringe bitmaps displayed in the right fringe. The value is an alist where each element has the form `(cursor-type
. bitmap)`, which means to use the fringe bitmap bitmap to display cursors of type cursor-type.
Each cursor-type should be one of `box`, `hollow`, `bar`, `hbar`, or `hollow-small`. The first four have the same meanings as in the `cursor-type` frame parameter (see [Cursor Parameters](cursor-parameters)). The `hollow-small` type is used instead of `hollow` when the normal `hollow-rectangle` bitmap is too tall to fit on a specific display line.
Each bitmap should be a symbol specifying the fringe bitmap to be displayed for that logical cursor type. See [Fringe Bitmaps](fringe-bitmaps).
When `fringe-cursor-alist` has a buffer-local value, and there is no bitmap defined for a cursor type, the corresponding value from the default value of `fringes-indicator-alist` is used.
elisp None ### Variable Aliases
It is sometimes useful to make two variables synonyms, so that both variables always have the same value, and changing either one also changes the other. Whenever you change the name of a variable—either because you realize its old name was not well chosen, or because its meaning has partly changed—it can be useful to keep the old name as an *alias* of the new one for compatibility. You can do this with `defvaralias`.
Function: **defvaralias** *new-alias base-variable &optional docstring*
This function defines the symbol new-alias as a variable alias for symbol base-variable. This means that retrieving the value of new-alias returns the value of base-variable, and changing the value of new-alias changes the value of base-variable. The two aliased variable names always share the same value and the same bindings.
If the docstring argument is non-`nil`, it specifies the documentation for new-alias; otherwise, the alias gets the same documentation as base-variable has, if any, unless base-variable is itself an alias, in which case new-alias gets the documentation of the variable at the end of the chain of aliases.
This function returns base-variable.
Variable aliases are convenient for replacing an old name for a variable with a new name. `make-obsolete-variable` declares that the old name is obsolete and therefore that it may be removed at some stage in the future.
Function: **make-obsolete-variable** *obsolete-name current-name when &optional access-type*
This function makes the byte compiler warn that the variable obsolete-name is obsolete. If current-name is a symbol, it is the variable’s new name; then the warning message says to use current-name instead of obsolete-name. If current-name is a string, this is the message and there is no replacement variable. when should be a string indicating when the variable was first made obsolete (usually a version number string).
The optional argument access-type, if non-`nil`, should specify the kind of access that will trigger obsolescence warnings; it can be either `get` or `set`.
You can make two variables synonyms and declare one obsolete at the same time using the macro `define-obsolete-variable-alias`.
Macro: **define-obsolete-variable-alias** *obsolete-name current-name &optional when docstring*
This macro marks the variable obsolete-name as obsolete and also makes it an alias for the variable current-name. It is equivalent to the following:
```
(defvaralias obsolete-name current-name docstring)
(make-obsolete-variable obsolete-name current-name when)
```
This macro evaluates all its parameters, and both obsolete-name and current-name should be symbols, so a typical usage would look like:
```
(define-obsolete-variable-alias 'foo-thing 'bar-thing "27.1")
```
Function: **indirect-variable** *variable*
This function returns the variable at the end of the chain of aliases of variable. If variable is not a symbol, or if variable is not defined as an alias, the function returns variable.
This function signals a `cyclic-variable-indirection` error if there is a loop in the chain of symbols.
```
(defvaralias 'foo 'bar)
(indirect-variable 'foo)
⇒ bar
(indirect-variable 'bar)
⇒ bar
(setq bar 2)
bar
⇒ 2
```
```
foo
⇒ 2
```
```
(setq foo 0)
bar
⇒ 0
foo
⇒ 0
```
| programming_docs |
elisp None ### Functions that Create Markers
When you create a new marker, you can make it point nowhere, or point to the present position of point, or to the beginning or end of the accessible portion of the buffer, or to the same place as another given marker.
The next four functions all return markers with insertion type `nil`. See [Marker Insertion Types](marker-insertion-types).
Function: **make-marker**
This function returns a newly created marker that does not point anywhere.
```
(make-marker)
⇒ #<marker in no buffer>
```
Function: **point-marker**
This function returns a new marker that points to the present position of point in the current buffer. See [Point](point). For an example, see `copy-marker`, below.
Function: **point-min-marker**
This function returns a new marker that points to the beginning of the accessible portion of the buffer. This will be the beginning of the buffer unless narrowing is in effect. See [Narrowing](narrowing).
Function: **point-max-marker**
This function returns a new marker that points to the end of the accessible portion of the buffer. This will be the end of the buffer unless narrowing is in effect. See [Narrowing](narrowing).
Here are examples of this function and `point-min-marker`, shown in a buffer containing a version of the source file for the text of this chapter.
```
(point-min-marker)
⇒ #<marker at 1 in markers.texi>
(point-max-marker)
⇒ #<marker at 24080 in markers.texi>
```
```
(narrow-to-region 100 200)
⇒ nil
```
```
(point-min-marker)
⇒ #<marker at 100 in markers.texi>
```
```
(point-max-marker)
⇒ #<marker at 200 in markers.texi>
```
Function: **copy-marker** *&optional marker-or-integer insertion-type*
If passed a marker as its argument, `copy-marker` returns a new marker that points to the same place and the same buffer as does marker-or-integer. If passed an integer as its argument, `copy-marker` returns a new marker that points to position marker-or-integer in the current buffer.
The new marker’s insertion type is specified by the argument insertion-type. See [Marker Insertion Types](marker-insertion-types).
```
(copy-marker 0)
⇒ #<marker at 1 in markers.texi>
```
```
(copy-marker 90000)
⇒ #<marker at 24080 in markers.texi>
```
An error is signaled if marker is neither a marker nor an integer.
Two distinct markers are considered `equal` (even though not `eq`) to each other if they have the same position and buffer, or if they both point nowhere.
```
(setq p (point-marker))
⇒ #<marker at 2139 in markers.texi>
```
```
(setq q (copy-marker p))
⇒ #<marker at 2139 in markers.texi>
```
```
(eq p q)
⇒ nil
```
```
(equal p q)
⇒ t
```
elisp None ### Sound Output
To play sound using Emacs, use the function `play-sound`. Only certain systems are supported; if you call `play-sound` on a system which cannot really do the job, it gives an error.
The sound must be stored as a file in RIFF-WAVE format (‘`.wav`’) or Sun Audio format (‘`.au`’).
Function: **play-sound** *sound*
This function plays a specified sound. The argument, sound, has the form `(sound properties...)`, where the properties consist of alternating keywords (particular symbols recognized specially) and values corresponding to them.
Here is a table of the keywords that are currently meaningful in sound, and their meanings:
`:file file`
This specifies the file containing the sound to play. If the file name is not absolute, it is expanded against the directory `data-directory`.
`:data data`
This specifies the sound to play without need to refer to a file. The value, data, should be a string containing the same bytes as a sound file. We recommend using a unibyte string.
`:volume volume`
This specifies how loud to play the sound. It should be a number in the range of 0 to 1. The default is to use whatever volume has been specified before.
`:device device` This specifies the system device on which to play the sound, as a string. The default device is system-dependent.
Before actually playing the sound, `play-sound` calls the functions in the list `play-sound-functions`. Each function is called with one argument, sound.
Command: **play-sound-file** *file &optional volume device*
This function is an alternative interface to playing a sound file specifying an optional volume and device.
Variable: **play-sound-functions**
A list of functions to be called before playing a sound. Each function is called with one argument, a property list that describes the sound.
elisp None Threads
-------
Emacs Lisp provides a limited form of concurrency, called *threads*. All the threads in a given instance of Emacs share the same memory. Concurrency in Emacs Lisp is “mostly cooperative”, meaning that Emacs will only switch execution between threads at well-defined times. However, the Emacs thread support has been designed in a way to later allow more fine-grained concurrency, and correct programs should not rely on cooperative threading.
Currently, thread switching will occur upon explicit request via `thread-yield`, when waiting for keyboard input or for process output from asynchronous processes (e.g., during `accept-process-output`), or during blocking operations relating to threads, such as mutex locking or `thread-join`.
Emacs Lisp provides primitives to create and control threads, and also to create and control mutexes and condition variables, useful for thread synchronization.
While global variables are shared among all Emacs Lisp threads, local variables are not—a dynamic `let` binding is local. Each thread also has its own current buffer (see [Current Buffer](current-buffer)) and its own match data (see [Match Data](match-data)).
Note that `let` bindings are treated specially by the Emacs Lisp implementation. There is no way to duplicate this unwinding and rewinding behavior other than by using `let`. For example, a manual implementation of `let` written using `unwind-protect` cannot arrange for variable values to be thread-specific.
In the case of lexical bindings (see [Variable Scoping](variable-scoping)), a closure is an object like any other in Emacs Lisp, and bindings in a closure are shared by any threads invoking the closure.
| | | |
| --- | --- | --- |
| • [Basic Thread Functions](basic-thread-functions) | | Basic thread functions. |
| • [Mutexes](mutexes) | | Mutexes allow exclusive access to data. |
| • [Condition Variables](condition-variables) | | Inter-thread events. |
| • [The Thread List](the-thread-list) | | Show the active threads. |
elisp None Numbers
-------
GNU Emacs supports two numeric data types: *integers* and *floating-point numbers*. Integers are whole numbers such as -3, 0, 7, 13, and 511. Floating-point numbers are numbers with fractional parts, such as -4.5, 0.0, and 2.71828. They can also be expressed in exponential notation: ‘`1.5e2`’ is the same as ‘`150.0`’; here, ‘`e2`’ stands for ten to the second power, and that is multiplied by 1.5. Integer computations are exact. Floating-point computations often involve rounding errors, as the numbers have a fixed amount of precision.
| | | |
| --- | --- | --- |
| • [Integer Basics](integer-basics) | | Representation and range of integers. |
| • [Float Basics](float-basics) | | Representation and range of floating point. |
| • [Predicates on Numbers](predicates-on-numbers) | | Testing for numbers. |
| • [Comparison of Numbers](comparison-of-numbers) | | Equality and inequality predicates. |
| • [Numeric Conversions](numeric-conversions) | | Converting float to integer and vice versa. |
| • [Arithmetic Operations](arithmetic-operations) | | How to add, subtract, multiply and divide. |
| • [Rounding Operations](rounding-operations) | | Explicitly rounding floating-point numbers. |
| • [Bitwise Operations](bitwise-operations) | | Logical and, or, not, shifting. |
| • [Math Functions](math-functions) | | Trig, exponential and logarithmic functions. |
| • [Random Numbers](random-numbers) | | Obtaining random integers, predictable or not. |
elisp None #### Faces for Font Lock
Font Lock mode can highlight using any face, but Emacs defines several faces specifically for Font Lock to use to highlight text. These *Font Lock faces* are listed below. They can also be used by major modes for syntactic highlighting outside of Font Lock mode (see [Major Mode Conventions](major-mode-conventions)).
Each of these symbols is both a face name, and a variable whose default value is the symbol itself. Thus, the default value of `font-lock-comment-face` is `font-lock-comment-face`.
The faces are listed with descriptions of their typical usage, and in order of greater to lesser prominence. If a mode’s syntactic categories do not fit well with the usage descriptions, the faces can be assigned using the ordering as a guide.
`font-lock-warning-face`
for a construct that is peculiar (e.g., an unescaped confusable quote in an Emacs Lisp symbol like ‘`‘foo`’), or that greatly changes the meaning of other text, like ‘`;;;###autoload`’ in Emacs Lisp and ‘`#error`’ in C.
`font-lock-function-name-face`
for the name of a function being defined or declared.
`font-lock-variable-name-face`
for the name of a variable being defined or declared.
`font-lock-keyword-face`
for a keyword with special syntactic significance, like ‘`for`’ and ‘`if`’ in C.
`font-lock-comment-face`
for comments.
`font-lock-comment-delimiter-face`
for comments delimiters, like ‘`/\*`’ and ‘`\*/`’ in C. On most terminals, this inherits from `font-lock-comment-face`.
`font-lock-type-face`
for the names of user-defined data types.
`font-lock-constant-face`
for the names of constants, like ‘`NULL`’ in C.
`font-lock-builtin-face`
for the names of built-in functions.
`font-lock-preprocessor-face`
for preprocessor commands. This inherits, by default, from `font-lock-builtin-face`.
`font-lock-string-face`
for string constants.
`font-lock-doc-face`
for documentation embedded in program code inside specially-formed comments or strings. This face inherits, by default, from `font-lock-string-face`.
`font-lock-doc-markup-face`
for mark-up elements in text using `font-lock-doc-face`. It is typically used for the mark-up constructs in documentation embedded in program code, following conventions such as Haddock, Javadoc or Doxygen. This face inherits, by default, from `font-lock-constant-face`.
`font-lock-negation-char-face`
for easily-overlooked negation characters.
elisp None ### Advising Emacs Lisp Functions
When you need to modify a function defined in another library, or when you need to modify a hook like `foo-function`, a process filter, or basically any variable or object field which holds a function value, you can use the appropriate setter function, such as `fset` or `defun` for named functions, `setq` for hook variables, or `set-process-filter` for process filters, but those are often too blunt, completely throwing away the previous value.
The *advice* feature lets you add to the existing definition of a function, by *advising the function*. This is a cleaner method than redefining the whole function.
Emacs’s advice system provides two sets of primitives for that: the core set, for function values held in variables and object fields (with the corresponding primitives being `add-function` and `remove-function`) and another set layered on top of it for named functions (with the main primitives being `advice-add` and `advice-remove`).
As a trivial example, here’s how to add advice that’ll modify the return value of a function every time it’s called:
```
(defun my-double (x)
(* x 2))
(defun my-increase (x)
(+ x 1))
(advice-add 'my-double :filter-return #'my-increase)
```
After adding this advice, if you call `my-double` with ‘`3`’, the return value will be ‘`7`’. To remove this advice, say
```
(advice-remove 'my-double #'my-increase)
```
A more advanced example would be to trace the calls to the process filter of a process proc:
```
(defun my-tracing-function (proc string)
(message "Proc %S received %S" proc string))
(add-function :before (process-filter proc) #'my-tracing-function)
```
This will cause the process’s output to be passed to `my-tracing-function` before being passed to the original process filter. `my-tracing-function` receives the same arguments as the original function. When you’re done with it, you can revert to the untraced behavior with:
```
(remove-function (process-filter proc) #'my-tracing-function)
```
Similarly, if you want to trace the execution of the function named `display-buffer`, you could use:
```
(defun his-tracing-function (orig-fun &rest args)
(message "display-buffer called with args %S" args)
(let ((res (apply orig-fun args)))
(message "display-buffer returned %S" res)
res))
(advice-add 'display-buffer :around #'his-tracing-function)
```
Here, `his-tracing-function` is called instead of the original function and receives the original function (additionally to that function’s arguments) as argument, so it can call it if and when it needs to. When you’re tired of seeing this output, you can revert to the untraced behavior with:
```
(advice-remove 'display-buffer #'his-tracing-function)
```
The arguments `:before` and `:around` used in the above examples specify how the two functions are composed, since there are many different ways to do it. The added function is also called a piece of *advice*.
| | | |
| --- | --- | --- |
| • [Core Advising Primitives](core-advising-primitives) | | Primitives to manipulate advice. |
| • [Advising Named Functions](advising-named-functions) | | Advising named functions. |
| • [Advice Combinators](advice-combinators) | | Ways to compose advice. |
| • [Porting Old Advice](porting-old-advice) | | Adapting code using the old defadvice. |
elisp None #### Killing Emacs
Killing Emacs means ending the execution of the Emacs process. If you started Emacs from a terminal, the parent process normally resumes control. The low-level primitive for killing Emacs is `kill-emacs`.
Command: **kill-emacs** *&optional exit-data*
This command calls the hook `kill-emacs-hook`, then exits the Emacs process and kills it.
If exit-data is an integer, that is used as the exit status of the Emacs process. (This is useful primarily in batch operation; see [Batch Mode](batch-mode).)
If exit-data is a string, its contents are stuffed into the terminal input buffer so that the shell (or whatever program next reads input) can read them.
If exit-data is neither an integer nor a string, or is omitted, that means to use the (system-specific) exit status which indicates successful program termination.
The `kill-emacs` function is normally called via the higher-level command `C-x C-c` (`save-buffers-kill-terminal`). See [Exiting](https://www.gnu.org/software/emacs/manual/html_node/emacs/Exiting.html#Exiting) in The GNU Emacs Manual. It is also called automatically if Emacs receives a `SIGTERM` or `SIGHUP` operating system signal (e.g., when the controlling terminal is disconnected), or if it receives a `SIGINT` signal while running in batch mode (see [Batch Mode](batch-mode)).
Variable: **kill-emacs-hook**
This normal hook is run by `kill-emacs`, before it kills Emacs.
Because `kill-emacs` can be called in situations where user interaction is impossible (e.g., when the terminal is disconnected), functions on this hook should not attempt to interact with the user. If you want to interact with the user when Emacs is shutting down, use `kill-emacs-query-functions`, described below.
When Emacs is killed, all the information in the Emacs process, aside from files that have been saved, is lost. Because killing Emacs inadvertently can lose a lot of work, the `save-buffers-kill-terminal` command queries for confirmation if you have buffers that need saving or subprocesses that are running. It also runs the abnormal hook `kill-emacs-query-functions`:
User Option: **kill-emacs-query-functions**
When `save-buffers-kill-terminal` is killing Emacs, it calls the functions in this hook, after asking the standard questions and before calling `kill-emacs`. The functions are called in order of appearance, with no arguments. Each function can ask for additional confirmation from the user. If any of them returns `nil`, `save-buffers-kill-emacs` does not kill Emacs, and does not run the remaining functions in this hook. Calling `kill-emacs` directly does not run this hook.
elisp None ### Scoping Rules for Variable Bindings
When you create a local binding for a variable, that binding takes effect only within a limited portion of the program (see [Local Variables](local-variables)). This section describes exactly what this means.
Each local binding has a certain *scope* and *extent*. *Scope* refers to *where* in the textual source code the binding can be accessed. *Extent* refers to *when*, as the program is executing, the binding exists.
By default, the local bindings that Emacs creates are *dynamic bindings*. Such a binding has *dynamic scope*, meaning that any part of the program can potentially access the variable binding. It also has *dynamic extent*, meaning that the binding lasts only while the binding construct (such as the body of a `let` form) is being executed.
Emacs can optionally create *lexical bindings*. A lexical binding has *lexical scope*, meaning that any reference to the variable must be located textually within the binding construct[10](#FOOT10). It also has *indefinite extent*, meaning that under some circumstances the binding can live on even after the binding construct has finished executing, by means of special objects called *closures*.
The following subsections describe dynamic binding and lexical binding in greater detail, and how to enable lexical binding in Emacs Lisp programs.
| | | |
| --- | --- | --- |
| • [Dynamic Binding](dynamic-binding) | | The default for binding local variables in Emacs. |
| • [Dynamic Binding Tips](https://www.gnu.org/software/emacs/manual/html_node/elisp/Dynamic-Binding-Tips.html) | | Avoiding problems with dynamic binding. |
| • [Lexical Binding](lexical-binding) | | A different type of local variable binding. |
| • [Using Lexical Binding](using-lexical-binding) | | How to enable lexical binding. |
| • [Converting to Lexical Binding](converting-to-lexical-binding) | | Convert existing code to lexical binding. |
elisp None #### Destructuring with pcase Patterns
Pcase patterns not only express a condition on the form of the objects they can match, but they can also extract sub-fields of those objects. For example we can extract 2 elements from a list that is the value of the variable `my-list` with the following code:
```
(pcase my-list
(`(add ,x ,y) (message "Contains %S and %S" x y)))
```
This will not only extract `x` and `y` but will additionally test that `my-list` is a list containing exactly 3 elements and whose first element is the symbol `add`. If any of those tests fail, `pcase` will immediately return `nil` without calling `message`.
Extraction of multiple values stored in an object is known as *destructuring*. Using `pcase` patterns allows to perform *destructuring binding*, which is similar to a local binding (see [Local Variables](local-variables)), but gives values to multiple elements of a variable by extracting those values from an object of compatible structure.
The macros described in this section use `pcase` patterns to perform destructuring binding. The condition of the object to be of compatible structure means that the object must match the pattern, because only then the object’s subfields can be extracted. For example:
```
(pcase-let ((`(add ,x ,y) my-list))
(message "Contains %S and %S" x y))
```
does the same as the previous example, except that it directly tries to extract `x` and `y` from `my-list` without first verifying if `my-list` is a list which has the right number of elements and has `add` as its first element. The precise behavior when the object does not actually match the pattern is undefined, although the body will not be silently skipped: either an error is signaled or the body is run with some of the variables potentially bound to arbitrary values like `nil`.
The pcase patterns that are useful for destructuring bindings are generally those described in [Backquote Patterns](backquote-patterns), since they express a specification of the structure of objects that will match.
For an alternative facility for destructuring binding, see [seq-let](sequence-functions#seq_002dlet).
Macro: **pcase-let** *bindings body…*
Perform destructuring binding of variables according to bindings, and then evaluate body.
bindings is a list of bindings of the form `(pattern exp)`, where exp is an expression to evaluate and pattern is a `pcase` pattern.
All exps are evaluated first, after which they are matched against their respective pattern, introducing new variable bindings that can then be used inside body. The variable bindings are produced by destructuring binding of elements of pattern to the values of the corresponding elements of the evaluated exp.
Macro: **pcase-let\*** *bindings body…*
Perform destructuring binding of variables according to bindings, and then evaluate body.
bindings is a list of bindings of the form `(pattern
exp)`, where exp is an expression to evaluate and pattern is a `pcase` pattern. The variable bindings are produced by destructuring binding of elements of pattern to the values of the corresponding elements of the evaluated exp.
Unlike `pcase-let`, but similarly to `let*`, each exp is matched against its corresponding pattern before processing the next element of bindings, so the variable bindings introduced in each one of the bindings are available in the exps of the bindings that follow it, additionally to being available in body.
Macro: **pcase-dolist** *(pattern list) body…*
Execute body once for each element of list, on each iteration performing a destructuring binding of variables in pattern to the values of the corresponding subfields of the element of list. The bindings are performed as if by `pcase-let`. When pattern is a simple variable, this ends up being equivalent to `dolist` (see [Iteration](iteration)).
Macro: **pcase-setq** *pattern value…*
Assign values to variables in a `setq` form, destructuring each value according to its respective pattern.
| programming_docs |
elisp None #### Absolute and Relative File Names
All the directories in the file system form a tree starting at the root directory. A file name can specify all the directory names starting from the root of the tree; then it is called an *absolute* file name. Or it can specify the position of the file in the tree relative to a default directory; then it is called a *relative* file name. On GNU and other POSIX-like systems, after any leading ‘`~`’ has been expanded, an absolute file name starts with a ‘`/`’ (see [abbreviate-file-name](directory-names#abbreviate_002dfile_002dname)), and a relative one does not. On MS-DOS and MS-Windows, an absolute file name starts with a slash or a backslash, or with a drive specification ‘`x:/`’, where x is the *drive letter*.
Function: **file-name-absolute-p** *filename*
This function returns `t` if file filename is an absolute file name, `nil` otherwise. A file name is considered to be absolute if its first component is ‘`~`’, or is ‘`~user`’ where user is a valid login name. In the following examples, assume that there is a user named ‘`rms`’ but no user named ‘`nosuchuser`’.
```
(file-name-absolute-p "~rms/foo")
⇒ t
```
```
(file-name-absolute-p "~nosuchuser/foo")
⇒ nil
```
```
(file-name-absolute-p "rms/foo")
⇒ nil
```
```
(file-name-absolute-p "/user/rms/foo")
⇒ t
```
Given a possibly relative file name, you can expand any leading ‘`~`’ and convert the result to an absolute name using `expand-file-name` (see [File Name Expansion](file-name-expansion)). This function converts absolute file names to relative names:
Function: **file-relative-name** *filename &optional directory*
This function tries to return a relative name that is equivalent to filename, assuming the result will be interpreted relative to directory (an absolute directory name or directory file name). If directory is omitted or `nil`, it defaults to the current buffer’s default directory.
On some operating systems, an absolute file name begins with a device name. On such systems, filename has no relative equivalent based on directory if they start with two different device names. In this case, `file-relative-name` returns filename in absolute form.
```
(file-relative-name "/foo/bar" "/foo/")
⇒ "bar"
(file-relative-name "/foo/bar" "/hack/")
⇒ "../foo/bar"
```
elisp None ### Directory Local Variables
A directory can specify local variable values common to all files in that directory; Emacs uses these to create buffer-local bindings for those variables in buffers visiting any file in that directory. This is useful when the files in the directory belong to some *project* and therefore share the same local variables.
There are two different methods for specifying directory local variables: by putting them in a special file, or by defining a *project class* for that directory.
Constant: **dir-locals-file**
This constant is the name of the file where Emacs expects to find the directory-local variables. The name of the file is `.dir-locals.el`[11](#FOOT11). A file by that name in a directory causes Emacs to apply its settings to any file in that directory or any of its subdirectories (optionally, you can exclude subdirectories; see below). If some of the subdirectories have their own `.dir-locals.el` files, Emacs uses the settings from the deepest file it finds starting from the file’s directory and moving up the directory tree. This constant is also used to derive the name of a second dir-locals file `.dir-locals-2.el`. If this second dir-locals file is present, then that is loaded in addition to `.dir-locals.el`. This is useful when `.dir-locals.el` is under version control in a shared repository and cannot be used for personal customizations. The file specifies local variables as a specially formatted list; see [Per-directory Local Variables](https://www.gnu.org/software/emacs/manual/html_node/emacs/Directory-Variables.html#Directory-Variables) in The GNU Emacs Manual, for more details.
Function: **hack-dir-local-variables**
This function reads the `.dir-locals.el` file and stores the directory-local variables in `file-local-variables-alist` that is local to the buffer visiting any file in the directory, without applying them. It also stores the directory-local settings in `dir-locals-class-alist`, where it defines a special class for the directory in which `.dir-locals.el` file was found. This function works by calling `dir-locals-set-class-variables` and `dir-locals-set-directory-class`, described below.
Function: **hack-dir-local-variables-non-file-buffer**
This function looks for directory-local variables, and immediately applies them in the current buffer. It is intended to be called in the mode commands for non-file buffers, such as Dired buffers, to let them obey directory-local variable settings. For non-file buffers, Emacs looks for directory-local variables in `default-directory` and its parent directories.
Function: **dir-locals-set-class-variables** *class variables*
This function defines a set of variable settings for the named class, which is a symbol. You can later assign the class to one or more directories, and Emacs will apply those variable settings to all files in those directories. The list in variables can be of one of the two forms: `(major-mode . alist)` or `(directory . list)`. With the first form, if the file’s buffer turns on a mode that is derived from major-mode, then all the variables in the associated alist are applied; alist should be of the form `(name . value)`. A special value `nil` for major-mode means the settings are applicable to any mode. In alist, you can use a special name: `subdirs`. If the associated value is `nil`, the alist is only applied to files in the relevant directory, not to those in any subdirectories.
With the second form of variables, if directory is the initial substring of the file’s directory, then list is applied recursively by following the above rules; list should be of one of the two forms accepted by this function in variables.
Function: **dir-locals-set-directory-class** *directory class &optional mtime*
This function assigns class to all the files in `directory` and its subdirectories. Thereafter, all the variable settings specified for class will be applied to any visited file in directory and its children. class must have been already defined by `dir-locals-set-class-variables`.
Emacs uses this function internally when it loads directory variables from a `.dir-locals.el` file. In that case, the optional argument mtime holds the file modification time (as returned by `file-attributes`). Emacs uses this time to check stored local variables are still valid. If you are assigning a class directly, not via a file, this argument should be `nil`.
Variable: **dir-locals-class-alist**
This alist holds the class symbols and the associated variable settings. It is updated by `dir-locals-set-class-variables`.
Variable: **dir-locals-directory-cache**
This alist holds directory names, their assigned class names, and modification times of the associated directory local variables file (if there is one). The function `dir-locals-set-directory-class` updates this list.
Variable: **enable-dir-local-variables**
If `nil`, directory-local variables are ignored. This variable may be useful for modes that want to ignore directory-locals while still respecting file-local variables (see [File Local Variables](file-local-variables)).
elisp None ### Prefix Keys
A *prefix key* is a key sequence whose binding is a keymap. The keymap defines what to do with key sequences that extend the prefix key. For example, `C-x` is a prefix key, and it uses a keymap that is also stored in the variable `ctl-x-map`. This keymap defines bindings for key sequences starting with `C-x`.
Some of the standard Emacs prefix keys use keymaps that are also found in Lisp variables:
* `esc-map` is the global keymap for the ESC prefix key. Thus, the global definitions of all meta characters are actually found here. This map is also the function definition of `ESC-prefix`.
* `help-map` is the global keymap for the `C-h` prefix key.
* `mode-specific-map` is the global keymap for the prefix key `C-c`. This map is actually global, not mode-specific, but its name provides useful information about `C-c` in the output of `C-h b` (`display-bindings`), since the main use of this prefix key is for mode-specific bindings.
* `ctl-x-map` is the global keymap used for the `C-x` prefix key. This map is found via the function cell of the symbol `Control-X-prefix`.
* `mule-keymap` is the global keymap used for the `C-x RET` prefix key.
* `ctl-x-4-map` is the global keymap used for the `C-x 4` prefix key.
* `ctl-x-5-map` is the global keymap used for the `C-x 5` prefix key.
* `2C-mode-map` is the global keymap used for the `C-x 6` prefix key.
* `tab-prefix-map` is the global keymap used for the `C-x t` prefix key.
* `vc-prefix-map` is the global keymap used for the `C-x v` prefix key.
* `goto-map` is the global keymap used for the `M-g` prefix key.
* `search-map` is the global keymap used for the `M-s` prefix key.
* The other Emacs prefix keys are `C-x @`, `C-x a i`, `C-x ESC` and `ESC ESC`. They use keymaps that have no special names.
The keymap binding of a prefix key is used for looking up the event that follows the prefix key. (It may instead be a symbol whose function definition is a keymap. The effect is the same, but the symbol serves as a name for the prefix key.) Thus, the binding of `C-x` is the symbol `Control-X-prefix`, whose function cell holds the keymap for `C-x` commands. (The same keymap is also the value of `ctl-x-map`.)
Prefix key definitions can appear in any active keymap. The definitions of `C-c`, `C-x`, `C-h` and ESC as prefix keys appear in the global map, so these prefix keys are always available. Major and minor modes can redefine a key as a prefix by putting a prefix key definition for it in the local map or the minor mode’s map. See [Active Keymaps](active-keymaps).
If a key is defined as a prefix in more than one active map, then its various definitions are in effect merged: the commands defined in the minor mode keymaps come first, followed by those in the local map’s prefix definition, and then by those from the global map.
In the following example, we make `C-p` a prefix key in the local keymap, in such a way that `C-p` is identical to `C-x`. Then the binding for `C-p C-f` is the function `find-file`, just like `C-x C-f`. The key sequence `C-p 6` is not found in any active keymap.
```
(use-local-map (make-sparse-keymap))
⇒ nil
```
```
(local-set-key "\C-p" ctl-x-map)
⇒ nil
```
```
(key-binding "\C-p\C-f")
⇒ find-file
```
```
(key-binding "\C-p6")
⇒ nil
```
Function: **define-prefix-command** *symbol &optional mapvar prompt*
This function prepares symbol for use as a prefix key’s binding: it creates a sparse keymap and stores it as symbol’s function definition. Subsequently binding a key sequence to symbol will make that key sequence into a prefix key. The return value is `symbol`.
This function also sets symbol as a variable, with the keymap as its value. But if mapvar is non-`nil`, it sets mapvar as a variable instead.
If prompt is non-`nil`, that becomes the overall prompt string for the keymap. The prompt string should be given for menu keymaps (see [Defining Menus](defining-menus)).
elisp None ### Looking Up and Expanding Abbreviations
Abbrevs are usually expanded by certain interactive commands, including `self-insert-command`. This section describes the subroutines used in writing such commands, as well as the variables they use for communication.
Function: **abbrev-symbol** *abbrev &optional table*
This function returns the symbol representing the abbrev named abbrev. It returns `nil` if that abbrev is not defined. The optional second argument table is the abbrev table in which to look it up. If table is `nil`, this function tries first the current buffer’s local abbrev table, and second the global abbrev table.
Function: **abbrev-expansion** *abbrev &optional table*
This function returns the string that abbrev would expand into (as defined by the abbrev tables used for the current buffer). It returns `nil` if abbrev is not a valid abbrev. The optional argument table specifies the abbrev table to use, as in `abbrev-symbol`.
Command: **expand-abbrev**
This command expands the abbrev before point, if any. If point does not follow an abbrev, this command does nothing. To do the expansion, it calls the function that is the value of the `abbrev-expand-function` variable, with no arguments, and returns whatever that function does.
The default expansion function returns the abbrev symbol if it did expansion, and `nil` otherwise. If the abbrev symbol has a hook function that is a symbol whose `no-self-insert` property is non-`nil`, and if the hook function returns `nil` as its value, then the default expansion function returns `nil`, even though expansion did occur.
Function: **abbrev-insert** *abbrev &optional name start end*
This function inserts the abbrev expansion of `abbrev`, replacing the text between `start` and `end`. If `start` is omitted, it defaults to point. `name`, if non-`nil`, should be the name by which this abbrev was found (a string); it is used to figure out whether to adjust the capitalization of the expansion. The function returns `abbrev` if the abbrev was successfully inserted, otherwise it returns `nil`.
Command: **abbrev-prefix-mark** *&optional arg*
This command marks the current location of point as the beginning of an abbrev. The next call to `expand-abbrev` will use the text from here to point (where it is then) as the abbrev to expand, rather than using the previous word as usual.
First, this command expands any abbrev before point, unless arg is non-`nil`. (Interactively, arg is the prefix argument.) Then it inserts a hyphen before point, to indicate the start of the next abbrev to be expanded. The actual expansion removes the hyphen.
User Option: **abbrev-all-caps**
When this is set non-`nil`, an abbrev entered entirely in upper case is expanded using all upper case. Otherwise, an abbrev entered entirely in upper case is expanded by capitalizing each word of the expansion.
Variable: **abbrev-start-location**
The value of this variable is a buffer position (an integer or a marker) for `expand-abbrev` to use as the start of the next abbrev to be expanded. The value can also be `nil`, which means to use the word before point instead. `abbrev-start-location` is set to `nil` each time `expand-abbrev` is called. This variable is also set by `abbrev-prefix-mark`.
Variable: **abbrev-start-location-buffer**
The value of this variable is the buffer for which `abbrev-start-location` has been set. Trying to expand an abbrev in any other buffer clears `abbrev-start-location`. This variable is set by `abbrev-prefix-mark`.
Variable: **last-abbrev**
This is the `abbrev-symbol` of the most recent abbrev expanded. This information is left by `expand-abbrev` for the sake of the `unexpand-abbrev` command (see [Expanding Abbrevs](https://www.gnu.org/software/emacs/manual/html_node/emacs/Expanding-Abbrevs.html#Expanding-Abbrevs) in The GNU Emacs Manual).
Variable: **last-abbrev-location**
This is the location of the most recent abbrev expanded. This contains information left by `expand-abbrev` for the sake of the `unexpand-abbrev` command.
Variable: **last-abbrev-text**
This is the exact expansion text of the most recent abbrev expanded, after case conversion (if any). Its value is `nil` if the abbrev has already been unexpanded. This contains information left by `expand-abbrev` for the sake of the `unexpand-abbrev` command.
Variable: **abbrev-expand-function**
The value of this variable is a function that `expand-abbrev` will call with no arguments to do the expansion. The function can do anything it wants before and after performing the expansion. It should return the abbrev symbol if expansion took place.
The following sample code shows a simple use of `abbrev-expand-function`. It assumes that `foo-mode` is a mode for editing certain files in which lines that start with ‘`#`’ are comments. You want to use Text mode abbrevs for those lines. The regular local abbrev table, `foo-mode-abbrev-table` is appropriate for all other lines. See [Standard Abbrev Tables](standard-abbrev-tables), for the definitions of `local-abbrev-table` and `text-mode-abbrev-table`. See [Advising Functions](advising-functions), for details of `add-function`.
```
(defun foo-mode-abbrev-expand-function (expand)
(if (not (save-excursion (forward-line 0) (eq (char-after) ?#)))
;; Performs normal expansion.
(funcall expand)
;; We're inside a comment: use the text-mode abbrevs.
(let ((local-abbrev-table text-mode-abbrev-table))
(funcall expand))))
(add-hook 'foo-mode-hook
(lambda ()
(add-function :around (local 'abbrev-expand-function)
#'foo-mode-abbrev-expand-function)))
```
elisp None ### Quoting
The special form `quote` returns its single argument, as written, without evaluating it. This provides a way to include constant symbols and lists, which are not self-evaluating objects, in a program. (It is not necessary to quote self-evaluating objects such as numbers, strings, and vectors.)
Special Form: **quote** *object*
This special form returns object, without evaluating it. The returned value might be shared and should not be modified. See [Self-Evaluating Forms](self_002devaluating-forms).
Because `quote` is used so often in programs, Lisp provides a convenient read syntax for it. An apostrophe character (‘`'`’) followed by a Lisp object (in read syntax) expands to a list whose first element is `quote`, and whose second element is the object. Thus, the read syntax `'x` is an abbreviation for `(quote x)`.
Here are some examples of expressions that use `quote`:
```
(quote (+ 1 2))
⇒ (+ 1 2)
```
```
(quote foo)
⇒ foo
```
```
'foo
⇒ foo
```
```
''foo
⇒ 'foo
```
```
'(quote foo)
⇒ 'foo
```
```
['foo]
⇒ ['foo]
```
Although the expressions `(list '+ 1 2)` and `'(+ 1 2)` both yield lists equal to `(+ 1 2)`, the former yields a freshly-minted mutable list whereas the latter yields a list built from conses that might be shared and should not be modified. See [Self-Evaluating Forms](self_002devaluating-forms).
Other quoting constructs include `function` (see [Anonymous Functions](anonymous-functions)), which causes an anonymous lambda expression written in Lisp to be compiled, and ‘```’ (see [Backquote](backquote)), which is used to quote only part of a list, while computing and substituting other parts.
elisp None #### Buffer Internals
Two structures (see `buffer.h`) are used to represent buffers in C. The `buffer_text` structure contains fields describing the text of a buffer; the `buffer` structure holds other fields. In the case of indirect buffers, two or more `buffer` structures reference the same `buffer_text` structure.
Here are some of the fields in `struct buffer_text`:
`beg`
The address of the buffer contents. The buffer contents is a linear C array of `char`, with the gap somewhere in its midst.
`gpt` `gpt_byte`
The character and byte positions of the buffer gap. See [Buffer Gap](buffer-gap).
`z` `z_byte`
The character and byte positions of the end of the buffer text.
`gap_size`
The size of buffer’s gap. See [Buffer Gap](buffer-gap).
`modiff` `save_modiff` `chars_modiff` `overlay_modiff`
These fields count the number of buffer-modification events performed in this buffer. `modiff` is incremented after each buffer-modification event, and is never otherwise changed; `save_modiff` contains the value of `modiff` the last time the buffer was visited or saved; `chars_modiff` counts only modifications to the characters in the buffer, ignoring all other kinds of changes (such as text properties); and `overlay_modiff` counts only modifications to the buffer’s overlays.
`beg_unchanged` `end_unchanged`
The number of characters at the start and end of the text that are known to be unchanged since the last complete redisplay.
`unchanged_modified` `overlay_unchanged_modified`
The values of `modiff` and `overlay_modiff`, respectively, after the last complete redisplay. If their current values match `modiff` or `overlay_modiff`, that means `beg_unchanged` and `end_unchanged` contain no useful information.
`markers`
The markers that refer to this buffer. This is actually a single marker, and successive elements in its marker *chain* (a linked list) are the other markers referring to this buffer text.
`intervals` The interval tree which records the text properties of this buffer.
Some of the fields of `struct buffer` are:
`header`
A header of type `union vectorlike_header` is common to all vectorlike objects.
`own_text`
A `struct buffer_text` structure that ordinarily holds the buffer contents. In indirect buffers, this field is not used.
`text`
A pointer to the `buffer_text` structure for this buffer. In an ordinary buffer, this is the `own_text` field above. In an indirect buffer, this is the `own_text` field of the base buffer.
`next`
A pointer to the next buffer, in the chain of all buffers, including killed buffers. This chain is used only for allocation and garbage collection, in order to collect killed buffers properly.
`pt` `pt_byte`
The character and byte positions of point in a buffer.
`begv` `begv_byte`
The character and byte positions of the beginning of the accessible range of text in the buffer.
`zv` `zv_byte`
The character and byte positions of the end of the accessible range of text in the buffer.
`base_buffer`
In an indirect buffer, this points to the base buffer. In an ordinary buffer, it is null.
`local_flags`
This field contains flags indicating that certain variables are local in this buffer. Such variables are declared in the C code using `DEFVAR_PER_BUFFER`, and their buffer-local bindings are stored in fields in the buffer structure itself. (Some of these fields are described in this table.)
`modtime`
The modification time of the visited file. It is set when the file is written or read. Before writing the buffer into a file, this field is compared to the modification time of the file to see if the file has changed on disk. See [Buffer Modification](buffer-modification).
`auto_save_modified`
The time when the buffer was last auto-saved.
`last_window_start`
The `window-start` position in the buffer as of the last time the buffer was displayed in a window.
`clip_changed`
This flag indicates that narrowing has changed in the buffer. See [Narrowing](narrowing).
`prevent_redisplay_optimizations_p`
This flag indicates that redisplay optimizations should not be used to display this buffer.
`inhibit_buffer_hooks`
This flag indicates that the buffer should not run the hooks `kill-buffer-hook`, `kill-buffer-query-functions` (see [Killing Buffers](killing-buffers)), and `buffer-list-update-hook` (see [Buffer List](buffer-list)). It is set at buffer creation (see [Creating Buffers](creating-buffers)), and avoids slowing down internal or temporary buffers, such as those created by `with-temp-buffer` (see [Current Buffer](current-buffer#Definition-of-with_002dtemp_002dbuffer)).
`overlay_center`
This field holds the current overlay center position. See [Managing Overlays](managing-overlays).
`overlays_before` `overlays_after`
These fields hold, respectively, a list of overlays that end at or before the current overlay center, and a list of overlays that end after the current overlay center. See [Managing Overlays](managing-overlays). `overlays_before` is sorted in order of decreasing end position, and `overlays_after` is sorted in order of increasing beginning position.
`name`
A Lisp string that names the buffer. It is guaranteed to be unique. See [Buffer Names](buffer-names). This and the following fields have their names in the C struct definition end in a `_` to indicate that they should not be accessed directly, but via the `BVAR` macro, like this:
```
Lisp_Object buf_name = BVAR (buffer, name);
```
`save_length`
The length of the file this buffer is visiting, when last read or saved. It can have 2 special values: -1 means auto-saving was turned off in this buffer, and -2 means don’t turn off auto-saving if buffer text shrinks a lot. This and other fields concerned with saving are not kept in the `buffer_text` structure because indirect buffers are never saved.
`directory`
The directory for expanding relative file names. This is the value of the buffer-local variable `default-directory` (see [File Name Expansion](file-name-expansion)).
`filename`
The name of the file visited in this buffer, or `nil`. This is the value of the buffer-local variable `buffer-file-name` (see [Buffer File Name](buffer-file-name)).
`undo_list` `backed_up` `auto_save_file_name` `auto_save_file_format` `read_only` `file_format` `file_truename` `invisibility_spec` `display_count` `display_time`
These fields store the values of Lisp variables that are automatically buffer-local (see [Buffer-Local Variables](buffer_002dlocal-variables)), whose corresponding variable names have the additional prefix `buffer-` and have underscores replaced with dashes. For instance, `undo_list` stores the value of `buffer-undo-list`.
`mark`
The mark for the buffer. The mark is a marker, hence it is also included on the list `markers`. See [The Mark](the-mark).
`local_var_alist`
The association list describing the buffer-local variable bindings of this buffer, not including the built-in buffer-local bindings that have special slots in the buffer object. (Those slots are omitted from this table.) See [Buffer-Local Variables](buffer_002dlocal-variables).
`major_mode`
Symbol naming the major mode of this buffer, e.g., `lisp-mode`.
`mode_name`
Pretty name of the major mode, e.g., `"Lisp"`.
`keymap` `abbrev_table` `syntax_table` `category_table` `display_table`
These fields store the buffer’s local keymap (see [Keymaps](keymaps)), abbrev table (see [Abbrev Tables](abbrev-tables)), syntax table (see [Syntax Tables](syntax-tables)), category table (see [Categories](categories)), and display table (see [Display Tables](display-tables)).
`downcase_table` `upcase_table` `case_canon_table`
These fields store the conversion tables for converting text to lower case, upper case, and for canonicalizing text for case-fold search. See [Case Tables](case-tables).
`minor_modes`
An alist of the minor modes of this buffer.
`pt_marker` `begv_marker` `zv_marker`
These fields are only used in an indirect buffer, or in a buffer that is the base of an indirect buffer. Each holds a marker that records `pt`, `begv`, and `zv` respectively, for this buffer when the buffer is not current.
`mode_line_format` `header_line_format` `case_fold_search` `tab_width` `fill_column` `left_margin` `auto_fill_function` `truncate_lines` `word_wrap` `ctl_arrow` `bidi_display_reordering` `bidi_paragraph_direction` `selective_display` `selective_display_ellipses` `overwrite_mode` `abbrev_mode` `mark_active` `enable_multibyte_characters` `buffer_file_coding_system` `cache_long_line_scans` `point_before_scroll` `left_fringe_width` `right_fringe_width` `fringes_outside_margins` `scroll_bar_width` `indicate_empty_lines` `indicate_buffer_boundaries` `fringe_indicator_alist` `fringe_cursor_alist` `scroll_up_aggressively` `scroll_down_aggressively` `cursor_type` `cursor_in_non_selected_windows`
These fields store the values of Lisp variables that are automatically buffer-local (see [Buffer-Local Variables](buffer_002dlocal-variables)), whose corresponding variable names have underscores replaced with dashes. For instance, `mode_line_format` stores the value of `mode-line-format`.
`last_selected_window` This is the last window that was selected with this buffer in it, or `nil` if that window no longer displays this buffer.
| programming_docs |
elisp None ### Keyboard Macros
A *keyboard macro* is a canned sequence of input events that can be considered a command and made the definition of a key. The Lisp representation of a keyboard macro is a string or vector containing the events. Don’t confuse keyboard macros with Lisp macros (see [Macros](macros)).
Function: **execute-kbd-macro** *kbdmacro &optional count loopfunc*
This function executes kbdmacro as a sequence of events. If kbdmacro is a string or vector, then the events in it are executed exactly as if they had been input by the user. The sequence is *not* expected to be a single key sequence; normally a keyboard macro definition consists of several key sequences concatenated.
If kbdmacro is a symbol, then its function definition is used in place of kbdmacro. If that is another symbol, this process repeats. Eventually the result should be a string or vector. If the result is not a symbol, string, or vector, an error is signaled.
The argument count is a repeat count; kbdmacro is executed that many times. If count is omitted or `nil`, kbdmacro is executed once. If it is 0, kbdmacro is executed over and over until it encounters an error or a failing search.
If loopfunc is non-`nil`, it is a function that is called, without arguments, prior to each iteration of the macro. If loopfunc returns `nil`, then this stops execution of the macro.
See [Reading One Event](reading-one-event), for an example of using `execute-kbd-macro`.
Variable: **executing-kbd-macro**
This variable contains the string or vector that defines the keyboard macro that is currently executing. It is `nil` if no macro is currently executing. A command can test this variable so as to behave differently when run from an executing macro. Do not set this variable yourself.
Variable: **defining-kbd-macro**
This variable is non-`nil` if and only if a keyboard macro is being defined. A command can test this variable so as to behave differently while a macro is being defined. The value is `append` while appending to the definition of an existing macro. The commands `start-kbd-macro`, `kmacro-start-macro` and `end-kbd-macro` set this variable—do not set it yourself.
The variable is always local to the current terminal and cannot be buffer-local. See [Multiple Terminals](multiple-terminals).
Variable: **last-kbd-macro**
This variable is the definition of the most recently defined keyboard macro. Its value is a string or vector, or `nil`.
The variable is always local to the current terminal and cannot be buffer-local. See [Multiple Terminals](multiple-terminals).
Variable: **kbd-macro-termination-hook**
This normal hook is run when a keyboard macro terminates, regardless of what caused it to terminate (reaching the macro end or an error which ended the macro prematurely).
elisp None ### Vertical Fractional Scrolling
*Vertical fractional scrolling* means shifting text in a window up or down by a specified multiple or fraction of a line. Emacs uses it, for example, on images and screen lines which are taller than the window. Each window has a *vertical scroll position*, which is a number, never less than zero. It specifies how far to raise the contents of the window when displaying them. Raising the window contents generally makes all or part of some lines disappear off the top, and all or part of some other lines appear at the bottom. The usual value is zero.
The vertical scroll position is measured in units of the normal line height, which is the height of the default font. Thus, if the value is .5, that means the window contents will be scrolled up half the normal line height. If it is 3.3, that means the window contents are scrolled up somewhat over three times the normal line height.
What fraction of a line the vertical scrolling covers, or how many lines, depends on what the lines contain. A value of .5 could scroll a line whose height is very short off the screen, while a value of 3.3 could scroll just part of the way through a tall line or an image.
Function: **window-vscroll** *&optional window pixels-p*
This function returns the current vertical scroll position of window. The default for window is the selected window. If pixels-p is non-`nil`, the return value is measured in pixels, rather than in units of the normal line height.
```
(window-vscroll)
⇒ 0
```
Function: **set-window-vscroll** *window lines &optional pixels-p*
This function sets window’s vertical scroll position to lines. If window is `nil`, the selected window is used. The argument lines should be zero or positive; if not, it is taken as zero.
The actual vertical scroll position must always correspond to an integral number of pixels, so the value you specify is rounded accordingly.
The return value is the result of this rounding.
```
(set-window-vscroll (selected-window) 1.2)
⇒ 1.13
```
If pixels-p is non-`nil`, lines specifies a number of pixels. In this case, the return value is lines.
Variable: **auto-window-vscroll**
If this variable is non-`nil`, the `line-move`, `scroll-up`, and `scroll-down` functions will automatically modify the vertical scroll position to scroll through display rows that are taller than the height of the window, for example in the presence of large images.
elisp None #### Syntactic Font Lock
Syntactic fontification uses a syntax table (see [Syntax Tables](syntax-tables)) to find and highlight syntactically relevant text. If enabled, it runs prior to search-based fontification. The variable `font-lock-syntactic-face-function`, documented below, determines which syntactic constructs to highlight. There are several variables that affect syntactic fontification; you should set them by means of `font-lock-defaults` (see [Font Lock Basics](font-lock-basics)).
Whenever Font Lock mode performs syntactic fontification on a stretch of text, it first calls the function specified by `syntax-propertize-function`. Major modes can use this to apply `syntax-table` text properties to override the buffer’s syntax table in special cases. See [Syntax Properties](syntax-properties).
Variable: **font-lock-keywords-only**
If the value of this variable is non-`nil`, Font Lock does not do syntactic fontification, only search-based fontification based on `font-lock-keywords`. It is normally set by Font Lock mode based on the keywords-only element in `font-lock-defaults`. If the value is `nil`, Font Lock will call `jit-lock-register` (see [Other Font Lock Variables](other-font-lock-variables)) to set up for automatic refontification of buffer text following a modified line to reflect the new syntactic context due to the change.
Variable: **font-lock-syntax-table**
This variable holds the syntax table to use for fontification of comments and strings. It is normally set by Font Lock mode based on the syntax-alist element in `font-lock-defaults`. If this value is `nil`, syntactic fontification uses the buffer’s syntax table (the value returned by the function `syntax-table`; see [Syntax Table Functions](syntax-table-functions)).
Variable: **font-lock-syntactic-face-function**
If this variable is non-`nil`, it should be a function to determine which face to use for a given syntactic element (a string or a comment).
The function is called with one argument, the parse state at point returned by `parse-partial-sexp`, and should return a face. The default value returns `font-lock-comment-face` for comments and `font-lock-string-face` for strings (see [Faces for Font Lock](faces-for-font-lock)).
This variable is normally set through the “other” elements in `font-lock-defaults`:
```
(setq-local font-lock-defaults
`(,python-font-lock-keywords
nil nil nil nil
(font-lock-syntactic-face-function
. python-font-lock-syntactic-face-function)))
```
elisp None ### Major Modes
Major modes specialize Emacs for editing or interacting with particular kinds of text. Each buffer has exactly one major mode at a time. Every major mode is associated with a *major mode command*, whose name should end in ‘`-mode`’. This command takes care of switching to that mode in the current buffer, by setting various buffer-local variables such as a local keymap. See [Major Mode Conventions](major-mode-conventions). Note that unlike minor modes there is no way to “turn off” a major mode, instead the buffer must be switched to a different one. However, you can temporarily *suspend* a major mode and later *restore* the suspended mode, see below.
The least specialized major mode is called *Fundamental mode*, which has no mode-specific definitions or variable settings.
Command: **fundamental-mode**
This is the major mode command for Fundamental mode. Unlike other mode commands, it does *not* run any mode hooks (see [Major Mode Conventions](major-mode-conventions)), since you are not supposed to customize this mode.
Function: **major-mode-suspend**
This function works like `fundamental-mode`, in that it kills all buffer-local variables, but it also records the major mode in effect, so that it could subsequently be restored. This function and `major-mode-restore` (described next) are useful when you need to put a buffer under some specialized mode other than the one Emacs chooses for it automatically (see [Auto Major Mode](auto-major-mode)), but would also like to be able to switch back to the original mode later.
Function: **major-mode-restore** *&optional avoided-modes*
This function restores the major mode recorded by `major-mode-suspend`. If no major mode was recorded, this function calls `normal-mode` (see [normal-mode](auto-major-mode)), but tries to force it not to choose any modes in avoided-modes, if that argument is non-`nil`.
The easiest way to write a major mode is to use the macro `define-derived-mode`, which sets up the new mode as a variant of an existing major mode. See [Derived Modes](derived-modes). We recommend using `define-derived-mode` even if the new mode is not an obvious derivative of another mode, as it automatically enforces many coding conventions for you. See [Basic Major Modes](basic-major-modes), for common modes to derive from.
The standard GNU Emacs Lisp directory tree contains the code for several major modes, in files such as `text-mode.el`, `texinfo.el`, `lisp-mode.el`, and `rmail.el`. You can study these libraries to see how modes are written.
User Option: **major-mode**
The buffer-local value of this variable holds the symbol for the current major mode. Its default value holds the default major mode for new buffers. The standard default value is `fundamental-mode`.
If the default value is `nil`, then whenever Emacs creates a new buffer via a command such as `C-x b` (`switch-to-buffer`), the new buffer is put in the major mode of the previously current buffer. As an exception, if the major mode of the previous buffer has a `mode-class` symbol property with value `special`, the new buffer is put in Fundamental mode (see [Major Mode Conventions](major-mode-conventions)).
| | | |
| --- | --- | --- |
| • [Major Mode Conventions](major-mode-conventions) | | Coding conventions for keymaps, etc. |
| • [Auto Major Mode](auto-major-mode) | | How Emacs chooses the major mode automatically. |
| • [Mode Help](mode-help) | | Finding out how to use a mode. |
| • [Derived Modes](derived-modes) | | Defining a new major mode based on another major mode. |
| • [Basic Major Modes](basic-major-modes) | | Modes that other modes are often derived from. |
| • [Mode Hooks](mode-hooks) | | Hooks run at the end of major mode functions. |
| • [Tabulated List Mode](tabulated-list-mode) | | Parent mode for buffers containing tabulated data. |
| • [Generic Modes](generic-modes) | | Defining a simple major mode that supports comment syntax and Font Lock mode. |
| • [Example Major Modes](example-major-modes) | | Text mode and Lisp modes. |
elisp None ### Predicates on Markers
You can test an object to see whether it is a marker, or whether it is either an integer or a marker. The latter test is useful in connection with the arithmetic functions that work with both markers and integers.
Function: **markerp** *object*
This function returns `t` if object is a marker, `nil` otherwise. Note that integers are not markers, even though many functions will accept either a marker or an integer.
Function: **integer-or-marker-p** *object*
This function returns `t` if object is an integer or a marker, `nil` otherwise.
Function: **number-or-marker-p** *object*
This function returns `t` if object is a number (either integer or floating point) or a marker, `nil` otherwise.
elisp None ### Information about Files
This section describes the functions for retrieving various types of information about files (or directories or symbolic links), such as whether a file is readable or writable, and its size. These functions all take arguments which are file names. Except where noted, these arguments need to specify existing files, or an error is signaled.
Be careful with file names that end in spaces. On some filesystems (notably, MS-Windows), trailing whitespace characters in file names are silently and automatically ignored.
| | | |
| --- | --- | --- |
| • [Testing Accessibility](testing-accessibility) | | Is a given file readable? Writable? |
| • [Kinds of Files](kinds-of-files) | | Is it a directory? A symbolic link? |
| • [Truenames](truenames) | | Eliminating symbolic links from a file name. |
| • [File Attributes](file-attributes) | | File sizes, modification times, etc. |
| • [Extended Attributes](extended-attributes) | | Extended file attributes for access control. |
| • [Locating Files](locating-files) | | How to find a file in standard places. |
elisp None #### Multiline Font Lock Constructs
Normally, elements of `font-lock-keywords` should not match across multiple lines; that doesn’t work reliably, because Font Lock usually scans just part of the buffer, and it can miss a multi-line construct that crosses the line boundary where the scan starts. (The scan normally starts at the beginning of a line.)
Making elements that match multiline constructs work properly has two aspects: correct *identification* and correct *rehighlighting*. The first means that Font Lock finds all multiline constructs. The second means that Font Lock will correctly rehighlight all the relevant text when a multiline construct is changed—for example, if some of the text that was previously part of a multiline construct ceases to be part of it. The two aspects are closely related, and often getting one of them to work will appear to make the other also work. However, for reliable results you must attend explicitly to both aspects.
There are three ways to ensure correct identification of multiline constructs:
* Add a function to `font-lock-extend-region-functions` that does the *identification* and extends the scan so that the scanned text never starts or ends in the middle of a multiline construct.
* Use the `font-lock-fontify-region-function` hook similarly to extend the scan so that the scanned text never starts or ends in the middle of a multiline construct.
* Somehow identify the multiline construct right when it gets inserted into the buffer (or at any point after that but before font-lock tries to highlight it), and mark it with a `font-lock-multiline` which will instruct font-lock not to start or end the scan in the middle of the construct.
There are several ways to do rehighlighting of multiline constructs:
* Place a `font-lock-multiline` property on the construct. This will rehighlight the whole construct if any part of it is changed. In some cases you can do this automatically by setting the `font-lock-multiline` variable, which see.
* Make sure `jit-lock-contextually` is set and rely on it doing its job. This will only rehighlight the part of the construct that follows the actual change, and will do it after a short delay. This only works if the highlighting of the various parts of your multiline construct never depends on text in subsequent lines. Since `jit-lock-contextually` is activated by default, this can be an attractive solution.
* Place a `jit-lock-defer-multiline` property on the construct. This works only if `jit-lock-contextually` is used, and with the same delay before rehighlighting, but like `font-lock-multiline`, it also handles the case where highlighting depends on subsequent lines.
* If parsing the *syntax* of a construct depends on it being parsed in one single chunk, you can add the `syntax-multiline` text property over the construct in question. The most common use for this is when the syntax property to apply to ‘`FOO`’ depend on some later text ‘`BAR`’: By placing this text property over the whole of ‘`FOO...BAR`’, you make sure that any change of ‘`BAR`’ will also cause the syntax property of ‘`FOO`’ to be recomputed. Note: For this to work, the mode needs to add `syntax-propertize-multiline` to `syntax-propertize-extend-region-functions`.
| | | |
| --- | --- | --- |
| • [Font Lock Multiline](font-lock-multiline) | | Marking multiline chunks with a text property. |
| • [Region to Refontify](region-to-refontify) | | Controlling which region gets refontified after a buffer change. |
elisp None #### Managing Overlays
This section describes the functions to create, delete and move overlays, and to examine their contents. Overlay changes are not recorded in the buffer’s undo list, since the overlays are not part of the buffer’s contents.
Function: **overlayp** *object*
This function returns `t` if object is an overlay.
Function: **make-overlay** *start end &optional buffer front-advance rear-advance*
This function creates and returns an overlay that belongs to buffer and ranges from start to end. Both start and end must specify buffer positions; they may be integers or markers. If buffer is omitted, the overlay is created in the current buffer.
An overlay whose start and end specify the same buffer position is known as *empty*. A non-empty overlay can become empty if the text between its start and end is deleted. When that happens, the overlay is by default not deleted, but you can cause it to be deleted by giving it the ‘`evaporate`’ property (see [evaporate property](overlay-properties)).
The arguments front-advance and rear-advance specify the marker insertion type for the start of the overlay and for the end of the overlay, respectively. See [Marker Insertion Types](marker-insertion-types). If they are both `nil`, the default, then the overlay extends to include any text inserted at the beginning, but not text inserted at the end. If front-advance is non-`nil`, text inserted at the beginning of the overlay is excluded from the overlay. If rear-advance is non-`nil`, text inserted at the end of the overlay is included in the overlay.
Function: **overlay-start** *overlay*
This function returns the position at which overlay starts, as an integer.
Function: **overlay-end** *overlay*
This function returns the position at which overlay ends, as an integer.
Function: **overlay-buffer** *overlay*
This function returns the buffer that overlay belongs to. It returns `nil` if overlay has been deleted.
Function: **delete-overlay** *overlay*
This function deletes overlay. The overlay continues to exist as a Lisp object, and its property list is unchanged, but it ceases to be attached to the buffer it belonged to, and ceases to have any effect on display.
A deleted overlay is not permanently disconnected. You can give it a position in a buffer again by calling `move-overlay`.
Function: **move-overlay** *overlay start end &optional buffer*
This function moves overlay to buffer, and places its bounds at start and end. Both arguments start and end must specify buffer positions; they may be integers or markers.
If buffer is omitted, overlay stays in the same buffer it was already associated with; if overlay was deleted, it goes into the current buffer.
The return value is overlay.
This is the only valid way to change the endpoints of an overlay. Do not try modifying the markers in the overlay by hand, as that fails to update other vital data structures and can cause some overlays to be lost.
Function: **remove-overlays** *&optional start end name value*
This function removes all the overlays between start and end whose property name has the value value. It can move the endpoints of the overlays in the region, or split them.
If name is omitted or `nil`, it means to delete all overlays in the specified region. If start and/or end are omitted or `nil`, that means the beginning and end of the buffer respectively. Therefore, `(remove-overlays)` removes all the overlays in the current buffer.
Function: **copy-overlay** *overlay*
This function returns a copy of overlay. The copy has the same endpoints and properties as overlay. However, the marker insertion type for the start of the overlay and for the end of the overlay are set to their default values (see [Marker Insertion Types](marker-insertion-types)).
Here are some examples:
```
;; Create an overlay.
(setq foo (make-overlay 1 10))
⇒ #<overlay from 1 to 10 in display.texi>
(overlay-start foo)
⇒ 1
(overlay-end foo)
⇒ 10
(overlay-buffer foo)
⇒ #<buffer display.texi>
;; Give it a property we can check later.
(overlay-put foo 'happy t)
⇒ t
;; Verify the property is present.
(overlay-get foo 'happy)
⇒ t
;; Move the overlay.
(move-overlay foo 5 20)
⇒ #<overlay from 5 to 20 in display.texi>
(overlay-start foo)
⇒ 5
(overlay-end foo)
⇒ 20
;; Delete the overlay.
(delete-overlay foo)
⇒ nil
;; Verify it is deleted.
foo
⇒ #<overlay in no buffer>
;; A deleted overlay has no position.
(overlay-start foo)
⇒ nil
(overlay-end foo)
⇒ nil
(overlay-buffer foo)
⇒ nil
;; Undelete the overlay.
(move-overlay foo 1 20)
⇒ #<overlay from 1 to 20 in display.texi>
;; Verify the results.
(overlay-start foo)
⇒ 1
(overlay-end foo)
⇒ 20
(overlay-buffer foo)
⇒ #<buffer display.texi>
;; Moving and deleting the overlay does not change its properties.
(overlay-get foo 'happy)
⇒ t
```
Emacs stores the overlays of each buffer in two lists, divided around an arbitrary center position. One list extends backwards through the buffer from that center position, and the other extends forwards from that center position. The center position can be anywhere in the buffer.
Function: **overlay-recenter** *pos*
This function recenters the overlays of the current buffer around position pos. That makes overlay lookup faster for positions near pos, but slower for positions far away from pos.
A loop that scans the buffer forwards, creating overlays, can run faster if you do `(overlay-recenter (point-max))` first.
| programming_docs |
elisp None #### Fringe Size and Position
The following buffer-local variables control the position and width of fringes in windows showing that buffer.
Variable: **fringes-outside-margins**
The fringes normally appear between the display margins and the window text. If the value is non-`nil`, they appear outside the display margins. See [Display Margins](display-margins).
Variable: **left-fringe-width**
This variable, if non-`nil`, specifies the width of the left fringe in pixels. A value of `nil` means to use the left fringe width from the window’s frame.
Variable: **right-fringe-width**
This variable, if non-`nil`, specifies the width of the right fringe in pixels. A value of `nil` means to use the right fringe width from the window’s frame.
Any buffer which does not specify values for these variables uses the values specified by the `left-fringe` and `right-fringe` frame parameters (see [Layout Parameters](layout-parameters)).
The above variables actually take effect via the function `set-window-buffer` (see [Buffers and Windows](buffers-and-windows)), which calls `set-window-fringes` as a subroutine. If you change one of these variables, the fringe display is not updated in existing windows showing the buffer, unless you call `set-window-buffer` again in each affected window. You can also use `set-window-fringes` to control the fringe display in individual windows.
Function: **set-window-fringes** *window left &optional right outside-margins persistent*
This function sets the fringe widths of window window. If window is `nil`, the selected window is used.
The argument left specifies the width in pixels of the left fringe, and likewise right for the right fringe. A value of `nil` for either one stands for the default width. If outside-margins is non-`nil`, that specifies that fringes should appear outside of the display margins.
If window is not large enough to accommodate fringes of the desired width, this leaves the fringes of window unchanged.
The values specified here may be later overridden by invoking `set-window-buffer` (see [Buffers and Windows](buffers-and-windows)) on window with its keep-margins argument `nil` or omitted. However, if the optional fifth argument persistent is non-`nil` and the other arguments are processed successfully, the values specified here unconditionally survive subsequent invocations of `set-window-buffer`. This can be used to permanently turn off fringes in the minibuffer window, consult the description of `set-window-scroll-bars` for an example (see [Scroll Bars](scroll-bars)).
Function: **window-fringes** *&optional window*
This function returns information about the fringes of a window window. If window is omitted or `nil`, the selected window is used. The value has the form `(left-width
right-width outside-margins persistent)`.
elisp None ### Special Read Syntax
Emacs Lisp represents many special objects and constructs via special hash notations.
‘`#<…>`’
Objects that have no read syntax are presented like this (see [Printed Representation](printed-representation)).
‘`##`’
The printed representation of an interned symbol whose name is an empty string (see [Symbol Type](symbol-type)).
‘`#'`’
This is a shortcut for `function`, see [Anonymous Functions](anonymous-functions).
‘`#:`’
The printed representation of an uninterned symbol whose name is foo is ‘`#:foo`’ (see [Symbol Type](symbol-type)).
‘`#N`’
When printing circular structures, this construct is used to represent where the structure loops back onto itself, and ‘`N`’ is the starting list count:
```
(let ((a (list 1)))
(setcdr a a))
=> (1 . #0)
```
‘`#N=`’ ‘`#N#`’
‘`#N=`’ gives the name to an object, and ‘`#N#`’ represents that object, so when reading back the object, they will be the same object instead of copies (see [Circular Objects](circular-objects)).
‘`#xN`’
‘`N`’ represented as a hexadecimal number (‘`#x2a`’).
‘`#oN`’
‘`N`’ represented as an octal number (‘`#o52`’).
‘`#bN`’
‘`N`’ represented as a binary number (‘`#b101010`’).
‘`#(…)`’
String text properties (see [Text Props and Strings](text-props-and-strings)).
‘`#^`’
A char table (see [Char-Table Type](char_002dtable-type)).
‘`#s(hash-table …)`’
A hash table (see [Hash Table Type](hash-table-type)).
‘`?C`’
A character (see [Basic Char Syntax](basic-char-syntax)).
‘`#$`’
The current file name in byte-compiled files (see [Docs and Compilation](docs-and-compilation)). This is not meant to be used in Emacs Lisp source files.
‘`#@N`’
Skip the next ‘`N`’ characters (see [Comments](comments)). This is used in byte-compiled files, and is not meant to be used in Emacs Lisp source files.
‘`#f`’ Indicates that the following form isn’t readable by the Emacs Lisp reader. This is only in text for display purposes (when that would look prettier than alternative ways of indicating an unreadable form) and will never appear in any Lisp file.
elisp None #### Character Type
A *character* in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character `A` is represented as the integer 65.
Individual characters are used occasionally in programs, but it is more common to work with *strings*, which are sequences composed of characters. See [String Type](string-type).
Characters in strings and buffers are currently limited to the range of 0 to 4194303—twenty two bits (see [Character Codes](character-codes)). Codes 0 through 127 are ASCII codes; the rest are non-ASCII (see [Non-ASCII Characters](non_002dascii-characters)). Characters that represent keyboard input have a much wider range, to encode modifier keys such as Control, Meta and Shift.
There are special functions for producing a human-readable textual description of a character for the sake of messages. See [Describing Characters](describing-characters).
| | | |
| --- | --- | --- |
| • [Basic Char Syntax](basic-char-syntax) | | Syntax for regular characters. |
| • [General Escape Syntax](general-escape-syntax) | | How to specify characters by their codes. |
| • [Ctl-Char Syntax](ctl_002dchar-syntax) | | Syntax for control characters. |
| • [Meta-Char Syntax](meta_002dchar-syntax) | | Syntax for meta-characters. |
| • [Other Char Bits](other-char-bits) | | Syntax for hyper-, super-, and alt-characters. |
elisp None #### Modifying Menus
When you insert a new item in an existing menu, you probably want to put it in a particular place among the menu’s existing items. If you use `define-key` to add the item, it normally goes at the front of the menu. To put it elsewhere in the menu, use `define-key-after`:
Function: **define-key-after** *map key binding &optional after*
Define a binding in map for key, with value binding, just like `define-key`, but position the binding in map after the binding for the event after. The argument key should be of length one—a vector or string with just one element. But after should be a single event type—a symbol or a character, not a sequence. The new binding goes after the binding for after. If after is `t` or is omitted, then the new binding goes last, at the end of the keymap. However, new bindings are added before any inherited keymap.
Here is an example:
```
(define-key-after my-menu [drink]
'("Drink" . drink-command) 'eat)
```
makes a binding for the fake function key DRINK and puts it right after the binding for EAT.
Here is how to insert an item called ‘`Work`’ in the ‘`Signals`’ menu of Shell mode, after the item `break`:
```
(define-key-after shell-mode-map [menu-bar signals work]
'("Work" . work-command) 'break)
```
elisp None #### Kill Ring Concepts
The kill ring records killed text as strings in a list, most recent first. A short kill ring, for example, might look like this:
```
("some text" "a different piece of text" "even older text")
```
When the list reaches `kill-ring-max` entries in length, adding a new entry automatically deletes the last entry.
When kill commands are interwoven with other commands, each kill command makes a new entry in the kill ring. Multiple kill commands in succession build up a single kill ring entry, which would be yanked as a unit; the second and subsequent consecutive kill commands add text to the entry made by the first one.
For yanking, one entry in the kill ring is designated the front of the ring. Some yank commands rotate the ring by designating a different element as the front. But this virtual rotation doesn’t change the list itself—the most recent entry always comes first in the list.
elisp None #### Subroutines of Visiting
The `find-file-noselect` function uses two important subroutines which are sometimes useful in user Lisp code: `create-file-buffer` and `after-find-file`. This section explains how to use them.
Function: **create-file-buffer** *filename*
This function creates a suitably named buffer for visiting filename, and returns it. It uses filename (sans directory) as the name if that name is free; otherwise, it appends a string such as ‘`<2>`’ to get an unused name. See also [Creating Buffers](creating-buffers). Note that the `uniquify` library affects the result of this function. See [Uniquify](https://www.gnu.org/software/emacs/manual/html_node/emacs/Uniquify.html#Uniquify) in The GNU Emacs Manual.
**Please note:** `create-file-buffer` does *not* associate the new buffer with a file and does not select the buffer. It also does not use the default major mode.
```
(create-file-buffer "foo")
⇒ #<buffer foo>
```
```
(create-file-buffer "foo")
⇒ #<buffer foo<2>>
```
```
(create-file-buffer "foo")
⇒ #<buffer foo<3>>
```
This function is used by `find-file-noselect`. It uses `generate-new-buffer` (see [Creating Buffers](creating-buffers)).
Function: **after-find-file** *&optional error warn noauto after-find-file-from-revert-buffer nomodes*
This function sets the buffer major mode, and parses local variables (see [Auto Major Mode](auto-major-mode)). It is called by `find-file-noselect` and by the default revert function (see [Reverting](reverting)).
If reading the file got an error because the file does not exist, but its directory does exist, the caller should pass a non-`nil` value for error. In that case, `after-find-file` issues a warning: ‘`(New file)`’. For more serious errors, the caller should usually not call `after-find-file`.
If warn is non-`nil`, then this function issues a warning if an auto-save file exists and is more recent than the visited file.
If noauto is non-`nil`, that says not to enable or disable Auto-Save mode. The mode remains enabled if it was enabled before.
If after-find-file-from-revert-buffer is non-`nil`, that means this call was from `revert-buffer`. This has no direct effect, but some mode functions and hook functions check the value of this variable.
If nomodes is non-`nil`, that means don’t alter the buffer’s major mode, don’t process local variables specifications in the file, and don’t run `find-file-hook`. This feature is used by `revert-buffer` in some cases.
The last thing `after-find-file` does is call all the functions in the list `find-file-hook`.
elisp None Standard Hooks
---------------
The following is a list of some hook variables that let you provide functions to be called from within Emacs on suitable occasions.
Most of these variables have names ending with ‘`-hook`’. They are *normal hooks*, run by means of `run-hooks`. The value of such a hook is a list of functions; the functions are called with no arguments and their values are completely ignored. The recommended way to put a new function on such a hook is to call `add-hook`. See [Hooks](hooks), for more information about using hooks.
The variables whose names end in ‘`-functions`’ are usually *abnormal hooks* (some old code may also use the deprecated ‘`-hooks`’ suffix). Their values are lists of functions, but these functions are called in a special way: they are either passed arguments, or their return values are used in some way. The variables whose names end in ‘`-function`’ have single functions as their values.
This is not an exhaustive list, it only covers the more general hooks. For example, every major mode defines a hook named ‘`modename-mode-hook`’. The major mode command runs this normal hook with `run-mode-hooks` as the very last thing it does. See [Mode Hooks](mode-hooks). Most minor modes have mode hooks too.
A special feature allows you to specify expressions to evaluate if and when a file is loaded (see [Hooks for Loading](hooks-for-loading)). That feature is not exactly a hook, but does a similar job.
`activate-mark-hook` `deactivate-mark-hook`
See [The Mark](the-mark).
`after-change-functions` `before-change-functions` `first-change-hook`
See [Change Hooks](change-hooks).
`after-change-major-mode-hook` `change-major-mode-after-body-hook`
See [Mode Hooks](mode-hooks).
`after-init-hook` `before-init-hook` `emacs-startup-hook` `window-setup-hook`
See [Init File](init-file).
`after-insert-file-functions` `write-region-annotate-functions` `write-region-post-annotation-function`
See [Format Conversion](format-conversion).
`after-make-frame-functions` `before-make-frame-hook` `server-after-make-frame-hook`
See [Creating Frames](creating-frames).
`after-save-hook` `before-save-hook` `write-contents-functions` `write-file-functions`
See [Saving Buffers](saving-buffers).
`after-setting-font-hook`
Hook run after a frame’s font changes.
`auto-save-hook`
See [Auto-Saving](auto_002dsaving).
`before-hack-local-variables-hook` `hack-local-variables-hook`
See [File Local Variables](file-local-variables).
`buffer-access-fontify-functions`
See [Lazy Properties](lazy-properties).
`buffer-list-update-hook`
Hook run when the buffer list changes (see [Buffer List](buffer-list)).
`buffer-quit-function`
Function to call to quit the current buffer.
`change-major-mode-hook`
See [Creating Buffer-Local](creating-buffer_002dlocal).
`comint-password-function`
This abnormal hook permits a derived mode to supply a password for the underlying command interpreter without prompting the user.
`command-line-functions`
See [Command-Line Arguments](command_002dline-arguments).
`delayed-warnings-hook`
The command loop runs this soon after `post-command-hook` (q.v.).
`focus-in-hook` `focus-out-hook`
See [Input Focus](input-focus).
`delete-frame-functions` `after-delete-frame-functions`
See [Deleting Frames](deleting-frames).
`delete-terminal-functions`
See [Multiple Terminals](multiple-terminals).
`pop-up-frame-function` `split-window-preferred-function`
See [Choosing Window Options](choosing-window-options).
`echo-area-clear-hook`
See [Echo Area Customization](echo-area-customization).
`find-file-hook` `find-file-not-found-functions`
See [Visiting Functions](visiting-functions).
`font-lock-extend-after-change-region-function`
See [Region to Refontify](region-to-refontify).
`font-lock-extend-region-functions`
See [Multiline Font Lock](multiline-font-lock).
`font-lock-fontify-buffer-function` `font-lock-fontify-region-function` `font-lock-mark-block-function` `font-lock-unfontify-buffer-function` `font-lock-unfontify-region-function`
See [Other Font Lock Variables](other-font-lock-variables).
`fontification-functions`
See [Automatic Face Assignment](auto-faces).
`frame-auto-hide-function`
See [Quitting Windows](quitting-windows).
`quit-window-hook`
See [Quitting Windows](quitting-windows).
`kill-buffer-hook` `kill-buffer-query-functions`
See [Killing Buffers](killing-buffers).
`kill-emacs-hook` `kill-emacs-query-functions`
See [Killing Emacs](killing-emacs).
`menu-bar-update-hook`
See [Menu Bar](menu-bar).
`minibuffer-setup-hook` `minibuffer-exit-hook`
See [Minibuffer Misc](minibuffer-misc).
`mouse-leave-buffer-hook`
Hook run when the user mouse-clicks in a window.
`mouse-position-function`
See [Mouse Position](mouse-position).
`prefix-command-echo-keystrokes-functions`
An abnormal hook run by prefix commands (such as `C-u`) which should return a string describing the current prefix state. For example, `C-u` produces ‘`C-u-`’ and ‘`C-u 1 2 3-`’. Each hook function is called with no arguments and should return a string describing the current prefix state, or `nil` if there’s no prefix state. See [Prefix Command Arguments](prefix-command-arguments).
`prefix-command-preserve-state-hook`
Hook run when a prefix command needs to preserve the prefix by passing the current prefix command state to the next command. For example, `C-u` needs to pass the state to the next command when the user types `C-u -` or follows `C-u` with a digit.
`pre-redisplay-functions`
Hook run in each window just before redisplaying it. See [Forcing Redisplay](forcing-redisplay).
`post-command-hook` `pre-command-hook`
See [Command Overview](command-overview).
`post-gc-hook`
See [Garbage Collection](garbage-collection).
`post-self-insert-hook`
See [Keymaps and Minor Modes](keymaps-and-minor-modes).
`suspend-hook` `suspend-resume-hook` `suspend-tty-functions` `resume-tty-functions`
See [Suspending Emacs](suspending-emacs).
`syntax-begin-function` `syntax-propertize-extend-region-functions` `syntax-propertize-function` `font-lock-syntactic-face-function`
See [Syntactic Font Lock](syntactic-font-lock). See [Syntax Properties](syntax-properties).
`temp-buffer-setup-hook` `temp-buffer-show-function` `temp-buffer-show-hook`
See [Temporary Displays](temporary-displays).
`tty-setup-hook`
See [Terminal-Specific](terminal_002dspecific).
`window-configuration-change-hook` `window-scroll-functions` `window-size-change-functions` See [Window Hooks](window-hooks).
elisp None ### Recursive Minibuffers
These functions and variables deal with recursive minibuffers (see [Recursive Editing](recursive-editing)):
Function: **minibuffer-depth**
This function returns the current depth of activations of the minibuffer, a nonnegative integer. If no minibuffers are active, it returns zero.
User Option: **enable-recursive-minibuffers**
If this variable is non-`nil`, you can invoke commands (such as `find-file`) that use minibuffers even while the minibuffer is active. Such invocation produces a recursive editing level for a new minibuffer. By default, the outer-level minibuffer is invisible while you are editing the inner one. If you have `minibuffer-follows-selected-frame` set to `nil`, you can have minibuffers visible on several frames at the same time. See [(emacs)Basic Minibuffer](https://www.gnu.org/software/emacs/manual/html_node/emacs/Basic-Minibuffer.html#Basic-Minibuffer).
If this variable is `nil`, you cannot invoke minibuffer commands when the minibuffer is active, not even if you switch to another window to do it.
If a command name has a property `enable-recursive-minibuffers` that is non-`nil`, then the command can use the minibuffer to read arguments even if it is invoked from the minibuffer. A command can also achieve this by binding `enable-recursive-minibuffers` to `t` in the interactive declaration (see [Using Interactive](using-interactive)). The minibuffer command `next-matching-history-element` (normally `M-s` in the minibuffer) does the latter.
elisp None ### Creating and Interning Symbols
To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same sequence of characters in the same context. Failure to do so would cause complete confusion.
When the Lisp reader encounters a name that references a symbol in the source code, it reads all the characters of that name. Then it looks up that name in a table called an *obarray* to find the symbol that the programmer meant. The technique used in this lookup is called “hashing”, an efficient method of looking something up by converting a sequence of characters to a number, known as a “hash code”. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J’s and go from there. That is a simple version of hashing. Each element of the obarray is a *bucket* which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name’s hash code. (The same idea is used for general Emacs hash tables, but they are a different data type; see [Hash Tables](hash-tables).)
When looking up names, the Lisp reader also considers “shorthands”. If the programmer supplied them, this allows the reader to find a symbol even if its name isn’t present in its full form in the source code. Of course, the reader needs to be aware of some pre-established context about such shorthands, much as one needs context to be to able to refer uniquely to Jan Jones by just the name “Jan”: it’s probably fine when amongst the Joneses, or when Jan has been mentioned recently, but very ambiguous in any other situation. See [Shorthands](shorthands).
If a symbol with the desired name is found, the reader uses that symbol. If the obarray does not contain a symbol with that name, the reader makes a new symbol and adds it to the obarray. Finding or adding a symbol with a certain name is called *interning* it, and the symbol is then called an *interned symbol*.
Interning ensures that each obarray has just one symbol with any particular name. Other like-named symbols may exist, but not in the same obarray. Thus, the reader gets the same symbols for the same names, as long as you keep reading with the same obarray.
Interning usually happens automatically in the reader, but sometimes other programs may want to do it. For example, after the `M-x` command obtains the command name as a string using the minibuffer, it then interns the string, to get the interned symbol with that name. As another example, a hypothetical telephone book program could intern the name of each looked up person’s name as a symbol, even if the obarray did not contain it, so that it could attach information to that new symbol, such as the last time someone looked it up.
No obarray contains all symbols; in fact, some symbols are not in any obarray. They are called *uninterned symbols*. An uninterned symbol has the same four cells as other symbols; however, the only way to gain access to it is by finding it in some other object or as the value of a variable. Uninterned symbols are sometimes useful in generating Lisp code, see below.
In Emacs Lisp, an obarray is actually a vector. Each element of the vector is a bucket; its value is either an interned symbol whose name hashes to that bucket, or 0 if the bucket is empty. Each interned symbol has an internal link (invisible to the user) to the next symbol in the bucket. Because these links are invisible, there is no way to find all the symbols in an obarray except using `mapatoms` (below). The order of symbols in a bucket is not significant.
In an empty obarray, every element is 0, so you can create an obarray with `(make-vector length 0)`. **This is the only valid way to create an obarray.** Prime numbers as lengths tend to result in good hashing; lengths one less than a power of two are also good.
**Do not try to put symbols in an obarray yourself.** This does not work—only `intern` can enter a symbol in an obarray properly.
> **Common Lisp note:** Unlike Common Lisp, Emacs Lisp does not provide for interning the same name in several different “packages”, thus creating multiple symbols with the same name but different packages. Emacs Lisp provides a different namespacing system called “shorthands” (see [Shorthands](shorthands)).
>
>
>
Most of the functions below take a name and sometimes an obarray as arguments. A `wrong-type-argument` error is signaled if the name is not a string, or if the obarray is not a vector.
Function: **symbol-name** *symbol*
This function returns the string that is symbol’s name. For example:
```
(symbol-name 'foo)
⇒ "foo"
```
**Warning:** Changing the string by substituting characters does change the name of the symbol, but fails to update the obarray, so don’t do it!
Creating an uninterned symbol is useful in generating Lisp code, because an uninterned symbol used as a variable in the code you generate cannot clash with any variables used in other Lisp programs.
Function: **make-symbol** *name*
This function returns a newly-allocated, uninterned symbol whose name is name (which must be a string). Its value and function definition are void, and its property list is `nil`. In the example below, the value of `sym` is not `eq` to `foo` because it is a distinct uninterned symbol whose name is also ‘`foo`’.
```
(setq sym (make-symbol "foo"))
⇒ foo
(eq sym 'foo)
⇒ nil
```
Function: **gensym** *&optional prefix*
This function returns a symbol using `make-symbol`, whose name is made by appending `gensym-counter` to prefix and incrementing that counter, guaranteeing that no two calls to this function will generate a symbol with the same name. The prefix defaults to `"g"`.
To avoid problems when accidentally interning printed representation of generated code (see [Printed Representation](printed-representation)), it is recommended to use `gensym` instead of `make-symbol`.
Function: **intern** *name &optional obarray*
This function returns the interned symbol whose name is name. If there is no such symbol in the obarray obarray, `intern` creates a new one, adds it to the obarray, and returns it. If obarray is omitted, the value of the global variable `obarray` is used.
```
(setq sym (intern "foo"))
⇒ foo
(eq sym 'foo)
⇒ t
(setq sym1 (intern "foo" other-obarray))
⇒ foo
(eq sym1 'foo)
⇒ nil
```
> **Common Lisp note:** In Common Lisp, you can intern an existing symbol in an obarray. In Emacs Lisp, you cannot do this, because the argument to `intern` must be a string, not a symbol.
>
>
>
Function: **intern-soft** *name &optional obarray*
This function returns the symbol in obarray whose name is name, or `nil` if obarray has no symbol with that name. Therefore, you can use `intern-soft` to test whether a symbol with a given name is already interned. If obarray is omitted, the value of the global variable `obarray` is used.
The argument name may also be a symbol; in that case, the function returns name if name is interned in the specified obarray, and otherwise `nil`.
```
(intern-soft "frazzle") ; No such symbol exists.
⇒ nil
(make-symbol "frazzle") ; Create an uninterned one.
⇒ frazzle
```
```
(intern-soft "frazzle") ; That one cannot be found.
⇒ nil
```
```
(setq sym (intern "frazzle")) ; Create an interned one.
⇒ frazzle
```
```
(intern-soft "frazzle") ; That one can be found!
⇒ frazzle
```
```
(eq sym 'frazzle) ; And it is the same one.
⇒ t
```
Variable: **obarray**
This variable is the standard obarray for use by `intern` and `read`.
Function: **mapatoms** *function &optional obarray*
This function calls function once with each symbol in the obarray obarray. Then it returns `nil`. If obarray is omitted, it defaults to the value of `obarray`, the standard obarray for ordinary symbols.
```
(setq count 0)
⇒ 0
(defun count-syms (s)
(setq count (1+ count)))
⇒ count-syms
(mapatoms 'count-syms)
⇒ nil
count
⇒ 1871
```
See `documentation` in [Accessing Documentation](accessing-documentation), for another example using `mapatoms`.
Function: **unintern** *symbol obarray*
This function deletes symbol from the obarray obarray. If `symbol` is not actually in the obarray, `unintern` does nothing. If obarray is `nil`, the current obarray is used.
If you provide a string instead of a symbol as symbol, it stands for a symbol name. Then `unintern` deletes the symbol (if any) in the obarray which has that name. If there is no such symbol, `unintern` does nothing.
If `unintern` does delete a symbol, it returns `t`. Otherwise it returns `nil`.
| programming_docs |
elisp None #### Window Configuration Type
A *window configuration* stores information about the positions, sizes, and contents of the windows in a frame, so you can recreate the same arrangement of windows later.
Window configurations do not have a read syntax; their print syntax looks like ‘`#<window-configuration>`’. See [Window Configurations](window-configurations), for a description of several functions related to window configurations.
elisp None #### Frame Font
Each frame has a *default font* which specifies the default character size for that frame. This size is meant when retrieving or changing the size of a frame in terms of columns or lines (see [Size Parameters](size-parameters)). It is also used when resizing (see [Window Sizes](window-sizes)) or splitting (see [Splitting Windows](splitting-windows)) windows.
The terms *line height* and *canonical character height* are sometimes used instead of “default character height”. Similarly, the terms *column width* and *canonical character width* are used instead of “default character width”.
Function: **frame-char-height** *&optional frame*
Function: **frame-char-width** *&optional frame*
These functions return the default height and width of a character in frame, measured in pixels. Together, these values establish the size of the default font on frame. The values depend on the choice of font for frame, see [Font and Color Parameters](font-and-color-parameters).
The default font can be also set directly with the following function:
Command: **set-frame-font** *font &optional keep-size frames*
This sets the default font to font. When called interactively, it prompts for the name of a font, and uses that font on the selected frame. When called from Lisp, font should be a font name (a string), a font object, font entity, or a font spec.
If the optional argument keep-size is `nil`, this keeps the number of frame lines and columns fixed. (If non-`nil`, the option `frame-inhibit-implied-resize` described in the next section will override this.) If keep-size is non-`nil` (or with a prefix argument), it tries to keep the size of the display area of the current frame fixed by adjusting the number of lines and columns.
If the optional argument frames is `nil`, this applies the font to the selected frame only. If frames is non-`nil`, it should be a list of frames to act upon, or `t` meaning all existing and all future graphical frames.
elisp None #### Displaying Buffers in Side Windows
The following action function for `display-buffer` (see [Buffer Display Action Functions](buffer-display-action-functions)) creates or reuses a side window for displaying the specified buffer.
Function: **display-buffer-in-side-window** *buffer alist*
This function displays buffer in a side window of the selected frame. It returns the window used for displaying buffer, `nil` if no such window can be found or created.
alist is an association list of symbols and values as for `display-buffer`. The following symbols in alist are special for this function:
`side`
Denotes the side of the frame where the window shall be located. Valid values are `left`, `top`, `right` and `bottom`. If unspecified, the window is located at the bottom of the frame.
`slot`
Denotes a slot at the specified side where to locate the window. A value of zero means to preferably position the window in the middle of the specified side. A negative value means to use a slot preceding (that is, above or on the left of) the middle slot. A positive value means to use a slot following (that is, below or on the right of) the middle slot. Hence, all windows on a specific side are ordered by their `slot` value. If unspecified, the window is located in the middle of the specified side.
`dedicated`
The dedicated flag (see [Dedicated Windows](dedicated-windows)) has a slightly different meaning for side windows. When a side window is created, that flag is set to the value `side` to prevent `display-buffer` to use the window in other action functions. Its value persists across invocations of `quit-window`, `kill-buffer`, `previous-buffer` and `next-buffer`.
In particular, these commands will refrain from showing, in a side window, buffers that have not been displayed in that window before. They will also refrain from having a normal, non-side window show a buffer that has been already displayed in a side window. A notable exception to the latter rule occurs when an application, after displaying a buffer, resets that buffer’s local variables. To override these rules and always delete a side window with `quit-window` or `kill-buffer`, and eventually prevent the use of `previous-buffer` and `next-buffer`, set this value to `t` or specify a value via `display-buffer-mark-dedicated`.
If you specify the same slot on the same side for two or more different buffers, the buffer displayed last is shown in the corresponding window. Hence, slots can be used for sharing the same side window between buffers.
This function installs the `window-side` and `window-slot` parameters (see [Window Parameters](window-parameters)) and makes them persistent. It does not install any other window parameters unless they have been explicitly provided via a `window-parameters` entry in alist.
By default, side windows cannot be split via `split-window` (see [Splitting Windows](splitting-windows)). Also, a side window is not reused or split by any buffer display action (see [Buffer Display Action Functions](buffer-display-action-functions)) unless it is explicitly specified as target of that action. Note also that `delete-other-windows` cannot make a side window the only window on its frame (see [Deleting Windows](deleting-windows)).
elisp None ### Margins for Filling
User Option: **fill-prefix**
This buffer-local variable, if non-`nil`, specifies a string of text that appears at the beginning of normal text lines and should be disregarded when filling them. Any line that fails to start with the fill prefix is considered the start of a paragraph; so is any line that starts with the fill prefix followed by additional whitespace. Lines that start with the fill prefix but no additional whitespace are ordinary text lines that can be filled together. The resulting filled lines also start with the fill prefix.
The fill prefix follows the left margin whitespace, if any.
User Option: **fill-column**
This buffer-local variable specifies the maximum width of filled lines. Its value should be an integer, which is a number of columns. All the filling, justification, and centering commands are affected by this variable, including Auto Fill mode (see [Auto Filling](auto-filling)).
As a practical matter, if you are writing text for other people to read, you should set `fill-column` to no more than 70. Otherwise the line will be too long for people to read comfortably, and this can make the text seem clumsy.
The default value for `fill-column` is 70. To disable Auto Fill mode in a specific mode, you could say something like:
```
(add-hook 'foo-mode-hook (lambda () (auto-fill-mode -1))
```
Command: **set-left-margin** *from to margin*
This sets the `left-margin` property on the text from from to to to the value margin. If Auto Fill mode is enabled, this command also refills the region to fit the new margin.
Command: **set-right-margin** *from to margin*
This sets the `right-margin` property on the text from from to to to the value margin. If Auto Fill mode is enabled, this command also refills the region to fit the new margin.
Function: **current-left-margin**
This function returns the proper left margin value to use for filling the text around point. The value is the sum of the `left-margin` property of the character at the start of the current line (or zero if none), and the value of the variable `left-margin`.
Function: **current-fill-column**
This function returns the proper fill column value to use for filling the text around point. The value is the value of the `fill-column` variable, minus the value of the `right-margin` property of the character after point.
Command: **move-to-left-margin** *&optional n force*
This function moves point to the left margin of the current line. The column moved to is determined by calling the function `current-left-margin`. If the argument n is non-`nil`, `move-to-left-margin` moves forward n-1 lines first.
If force is non-`nil`, that says to fix the line’s indentation if that doesn’t match the left margin value.
Function: **delete-to-left-margin** *&optional from to*
This function removes left margin indentation from the text between from and to. The amount of indentation to delete is determined by calling `current-left-margin`. In no case does this function delete non-whitespace. If from and to are omitted, they default to the whole buffer.
Function: **indent-to-left-margin**
This function adjusts the indentation at the beginning of the current line to the value specified by the variable `left-margin`. (That may involve either inserting or deleting whitespace.) This function is value of `indent-line-function` in Paragraph-Indent Text mode.
User Option: **left-margin**
This variable specifies the base left margin column. In Fundamental mode, RET indents to this column. This variable automatically becomes buffer-local when set in any fashion.
User Option: **fill-nobreak-predicate**
This variable gives major modes a way to specify not to break a line at certain places. Its value should be a list of functions. Whenever filling considers breaking the line at a certain place in the buffer, it calls each of these functions with no arguments and with point located at that place. If any of the functions returns non-`nil`, then the line won’t be broken there.
elisp None ### Substituting Key Bindings in Documentation
When documentation strings refer to key sequences, they should use the current, actual key bindings. They can do so using certain special text sequences described below. Accessing documentation strings in the usual way substitutes current key binding information for these special sequences. This works by calling `substitute-command-keys`. You can also call that function yourself.
Here is a list of the special sequences and what they mean:
`\[command]`
stands for a key sequence that will invoke command, or ‘`M-x command`’ if command has no key bindings.
`\{mapvar}`
stands for a summary of the keymap which is the value of the variable mapvar. The summary is made using `describe-bindings`.
`\<mapvar>`
stands for no text itself. It is used only for a side effect: it specifies mapvar’s value as the keymap for any following ‘`\[command]`’ sequences in this documentation string.
```
(grave accent) stands for a left quote. This generates a left single quotation mark, an apostrophe, or a grave accent depending on the value of `text-quoting-style`. See [Text Quoting Style](text-quoting-style).
`'`
(apostrophe) stands for a right quote. This generates a right single quotation mark or an apostrophe depending on the value of `text-quoting-style`.
`\=` quotes the following character and is discarded; thus, ‘`\=``’ puts ‘```’ into the output, ‘`\=\[`’ puts ‘`\[`’ into the output, and ‘`\=\=`’ puts ‘`\=`’ into the output.
**Please note:** Each ‘`\`’ must be doubled when written in a string in Emacs Lisp.
User Option: **text-quoting-style**
The value of this variable is a symbol that specifies the style Emacs should use for single quotes in the wording of help and messages. If the variable’s value is `curve`, the style is ‘like this’ with curved single quotes. If the value is `straight`, the style is 'like this' with straight apostrophes. If the value is `grave`, quotes are not translated and the style is `like this' with grave accent and apostrophe, the standard style before Emacs version 25. The default value `nil` acts like `curve` if curved single quotes seem to be displayable, and like `grave` otherwise.
This option is useful on platforms that have problems with curved quotes. You can customize it freely according to your personal preference.
Function: **substitute-command-keys** *string &optional no-face*
This function scans string for the above special sequences and replaces them by what they stand for, returning the result as a string. This permits display of documentation that refers accurately to the user’s own customized key bindings. By default, the key bindings are given a special face `help-key-binding`, but if the optional argument no-face is non-`nil`, the function doesn’t add this face to the produced string.
If a command has multiple bindings, this function normally uses the first one it finds. You can specify one particular key binding by assigning an `:advertised-binding` symbol property to the command, like this:
```
(put 'undo :advertised-binding [?\C-/])
```
The `:advertised-binding` property also affects the binding shown in menu items (see [Menu Bar](menu-bar)). The property is ignored if it specifies a key binding that the command does not actually have.
Here are examples of the special sequences:
```
(substitute-command-keys
"To abort recursive edit, type `\\[abort-recursive-edit]'.")
⇒ "To abort recursive edit, type ‘C-]’."
```
```
(substitute-command-keys
"The keys that are defined for the minibuffer here are:
\\{minibuffer-local-must-match-map}")
⇒ "The keys that are defined for the minibuffer here are:
```
```
? minibuffer-completion-help
SPC minibuffer-complete-word
TAB minibuffer-complete
C-j minibuffer-complete-and-exit
RET minibuffer-complete-and-exit
C-g abort-recursive-edit
"
```
```
(substitute-command-keys
"To abort a recursive edit from the minibuffer, type \
`\\<minibuffer-local-must-match-map>\\[abort-recursive-edit]'.")
⇒ "To abort a recursive edit from the minibuffer, type ‘C-g’."
```
There are other special conventions for the text in documentation strings—for instance, you can refer to functions, variables, and sections of this manual. See [Documentation Tips](https://www.gnu.org/software/emacs/manual/html_node/elisp/Documentation-Tips.html), for details.
elisp None ### Desktop Notifications
Emacs is able to send *notifications* on systems that support the freedesktop.org Desktop Notifications Specification and on MS-Windows. In order to use this functionality on POSIX hosts, Emacs must have been compiled with D-Bus support, and the `notifications` library must be loaded. See [D-Bus](https://www.gnu.org/software/emacs/manual/html_node/dbus/index.html#Top) in D-Bus integration in Emacs. The following function is supported when D-Bus support is available:
Function: **notifications-notify** *&rest params*
This function sends a notification to the desktop via D-Bus, consisting of the parameters specified by the params arguments. These arguments should consist of alternating keyword and value pairs. The supported keywords and values are as follows:
`:bus bus`
The D-Bus bus. This argument is needed only if a bus other than `:session` shall be used.
`:title title`
The notification title.
`:body text`
The notification body text. Depending on the implementation of the notification server, the text could contain HTML markups, like ‘`"<b>bold text</b>"`’, hyperlinks, or images. Special HTML characters must be encoded, as ‘`"Contact <postmaster@localhost>!"`’.
`:app-name name`
The name of the application sending the notification. The default is `notifications-application-name`.
`:replaces-id id`
The notification id that this notification replaces. id must be the result of a previous `notifications-notify` call.
`:app-icon icon-file`
The file name of the notification icon. If set to `nil`, no icon is displayed. The default is `notifications-application-icon`.
`:actions (key title key title ...)`
A list of actions to be applied. key and title are both strings. The default action (usually invoked by clicking the notification) should have a key named ‘`"default"`’. The title can be anything, though implementations are free not to display it.
`:timeout timeout`
The timeout time in milliseconds since the display of the notification at which the notification should automatically close. If -1, the notification’s expiration time is dependent on the notification server’s settings, and may vary for the type of notification. If 0, the notification never expires. Default value is -1.
`:urgency urgency`
The urgency level. It can be `low`, `normal`, or `critical`.
`:action-items`
When this keyword is given, the title string of the actions is interpreted as icon name.
`:category category`
The type of notification this is, a string. See the [Desktop Notifications Specification](https://developer.gnome.org/notification-spec/#categories) for a list of standard categories.
`:desktop-entry filename`
This specifies the name of the desktop filename representing the calling program, like ‘`"emacs"`’.
`:image-data (width height rowstride has-alpha bits channels data)`
This is a raw data image format that describes the width, height, rowstride, whether there is an alpha channel, bits per sample, channels and image data, respectively.
`:image-path path`
This is represented either as a URI (‘`file://`’ is the only URI schema supported right now) or a name in a freedesktop.org-compliant icon theme from ‘`$XDG\_DATA\_DIRS/icons`’.
`:sound-file filename`
The path to a sound file to play when the notification pops up.
`:sound-name name`
A themable named sound from the freedesktop.org sound naming specification from ‘`$XDG\_DATA\_DIRS/sounds`’, to play when the notification pops up. Similar to the icon name, only for sounds. An example would be ‘`"message-new-instant"`’.
`:suppress-sound`
Causes the server to suppress playing any sounds, if it has that ability.
`:resident`
When set the server will not automatically remove the notification when an action has been invoked. The notification will remain resident in the server until it is explicitly removed by the user or by the sender. This hint is likely only useful when the server has the `:persistence` capability.
`:transient`
When set the server will treat the notification as transient and by-pass the server’s persistence capability, if it should exist.
`:x position` `:y position`
Specifies the X, Y location on the screen that the notification should point to. Both arguments must be used together.
`:on-action function`
Function to call when an action is invoked. The notification id and the key of the action are passed as arguments to the function.
`:on-close function`
Function to call when the notification has been closed by timeout or by the user. The function receive the notification id and the closing reason as arguments:
* `expired` if the notification has expired
* `dismissed` if the notification was dismissed by the user
* `close-notification` if the notification was closed by a call to `notifications-close-notification`
* `undefined` if the notification server hasn’t provided a reason
Which parameters are accepted by the notification server can be checked via `notifications-get-capabilities`.
This function returns a notification id, an integer, which can be used to manipulate the notification item with `notifications-close-notification` or the `:replaces-id` argument of another `notifications-notify` call. For example:
```
(defun my-on-action-function (id key)
(message "Message %d, key \"%s\" pressed" id key))
⇒ my-on-action-function
```
```
(defun my-on-close-function (id reason)
(message "Message %d, closed due to \"%s\"" id reason))
⇒ my-on-close-function
```
```
(notifications-notify
:title "Title"
:body "This is <b>important</b>."
:actions '("Confirm" "I agree" "Refuse" "I disagree")
:on-action 'my-on-action-function
:on-close 'my-on-close-function)
⇒ 22
```
```
A message window opens on the desktop. Press ``I agree''.
⇒ Message 22, key "Confirm" pressed
Message 22, closed due to "dismissed"
```
Function: **notifications-close-notification** *id &optional bus*
This function closes a notification with identifier id. bus can be a string denoting a D-Bus connection, the default is `:session`.
Function: **notifications-get-capabilities** *&optional bus*
Returns the capabilities of the notification server, a list of symbols. bus can be a string denoting a D-Bus connection, the default is `:session`. The following capabilities can be expected:
`:actions`
The server will provide the specified actions to the user.
`:body`
Supports body text.
`:body-hyperlinks`
The server supports hyperlinks in the notifications.
`:body-images`
The server supports images in the notifications.
`:body-markup`
Supports markup in the body text.
`:icon-multi`
The server will render an animation of all the frames in a given image array.
`:icon-static`
Supports display of exactly 1 frame of any given image array. This value is mutually exclusive with `:icon-multi`.
`:persistence`
The server supports persistence of notifications.
`:sound` The server supports sounds on notifications.
Further vendor-specific caps start with `:x-vendor`, like `:x-gnome-foo-cap`.
Function: **notifications-get-server-information** *&optional bus*
Return information on the notification server, a list of strings. bus can be a string denoting a D-Bus connection, the default is `:session`. The returned list is `(name vendor
version spec-version)`.
name
The product name of the server.
vendor
The vendor name. For example, ‘`"KDE"`’, ‘`"GNOME"`’.
version
The server’s version number.
spec-version The specification version the server is compliant with.
If spec\_version is `nil`, the server supports a specification prior to ‘`"1.0"`’.
When Emacs runs on MS-Windows as a GUI session, it supports a small subset of the D-Bus notifications functionality via a native primitive:
Function: **w32-notification-notify** *&rest params*
This function displays an MS-Windows tray notification as specified by params. MS-Windows tray notifications are displayed in a balloon from an icon in the notification area of the taskbar.
Value is the integer unique ID of the notification that can be used to remove the notification using `w32-notification-close`, described below. If the function fails, the return value is `nil`.
The arguments params are specified as keyword/value pairs. All the parameters are optional, but if no parameters are specified, the function will do nothing and return `nil`.
The following parameters are supported:
`:icon icon`
Display icon in the system tray. If icon is a string, it should specify a file name from which to load the icon; the specified file should be a `.ico` Windows icon file. If icon is not a string, or if this parameter is not specified, the standard Emacs icon will be used.
`:tip tip`
Use tip as the tooltip for the notification. If tip is a string, this is the text of a tooltip that will be shown when the mouse pointer hovers over the tray icon added by the notification. If tip is not a string, or if this parameter is not specified, the default tooltip text is ‘`Emacs notification`’. The tooltip text can be up to 127 characters long (63 on Windows versions before W2K). Longer strings will be truncated.
`:level level`
Notification severity level, one of `info`, `warning`, or `error`. If given, the value determines the icon displayed to the left of the notification title, but only if the `:title` parameter (see below) is also specified and is a string.
`:title title`
The title of the notification. If title is a string, it is displayed in a larger font immediately above the body text. The title text can be up to 63 characters long; longer text will be truncated.
`:body body` The body of the notification. If body is a string, it specifies the text of the notification message. Use embedded newlines to control how the text is broken into lines. The body text can be up to 255 characters long, and will be truncated if it’s longer. Unlike with D-Bus, the body text should be plain text, with no markup.
Note that versions of Windows before W2K support only `:icon` and `:tip`. The other parameters can be passed, but they will be ignored on those old systems.
There can be at most one active notification at any given time. An active notification must be removed by calling `w32-notification-close` before a new one can be shown.
To remove the notification and its icon from the taskbar, use the following function:
Function: **w32-notification-close** *id*
This function removes the tray notification given by its unique id.
| programming_docs |
elisp None ### Deferred and Lazy Evaluation
Sometimes it is useful to delay the evaluation of an expression, for example if you want to avoid performing a time-consuming calculation if it turns out that the result is not needed in the future of the program. The `thunk` library provides the following functions and macros to support such *deferred evaluation*:
Macro: **thunk-delay** *forms…*
Return a *thunk* for evaluating the forms. A thunk is a closure (see [Closures](closures)) that inherits the lexical environment of the `thunk-delay` call. Using this macro requires `lexical-binding`.
Function: **thunk-force** *thunk*
Force thunk to perform the evaluation of the forms specified in the `thunk-delay` that created the thunk. The result of the evaluation of the last form is returned. The thunk also “remembers” that it has been forced: Any further calls of `thunk-force` with the same thunk will just return the same result without evaluating the forms again.
Macro: **thunk-let** *(bindings…) forms…*
This macro is analogous to `let` but creates “lazy” variable bindings. Any binding has the form `(symbol value-form)`. Unlike `let`, the evaluation of any value-form is deferred until the binding of the according symbol is used for the first time when evaluating the forms. Any value-form is evaluated at most once. Using this macro requires `lexical-binding`.
Example:
```
(defun f (number)
(thunk-let ((derived-number
(progn (message "Calculating 1 plus 2 times %d" number)
(1+ (* 2 number)))))
(if (> number 10)
derived-number
number)))
```
```
(f 5)
⇒ 5
```
```
(f 12)
-| Calculating 1 plus 2 times 12
⇒ 25
```
Because of the special nature of lazily bound variables, it is an error to set them (e.g. with `setq`).
Macro: **thunk-let\*** *(bindings…) forms…*
This is like `thunk-let` but any expression in bindings is allowed to refer to preceding bindings in this `thunk-let*` form. Using this macro requires `lexical-binding`.
```
(thunk-let* ((x (prog2 (message "Calculating x...")
(+ 1 1)
(message "Finished calculating x")))
(y (prog2 (message "Calculating y...")
(+ x 1)
(message "Finished calculating y")))
(z (prog2 (message "Calculating z...")
(+ y 1)
(message "Finished calculating z")))
(a (prog2 (message "Calculating a...")
(+ z 1)
(message "Finished calculating a"))))
(* z x))
-| Calculating z...
-| Calculating y...
-| Calculating x...
-| Finished calculating x
-| Finished calculating y
-| Finished calculating z
⇒ 8
```
`thunk-let` and `thunk-let*` use thunks implicitly: their expansion creates helper symbols and binds them to thunks wrapping the binding expressions. All references to the original variables in the body forms are then replaced by an expression that calls `thunk-force` with the according helper variable as the argument. So, any code using `thunk-let` or `thunk-let*` could be rewritten to use thunks, but in many cases using these macros results in nicer code than using thunks explicitly.
elisp None #### Evaluation
While within Edebug, you can evaluate expressions as if Edebug were not running. Edebug tries to be invisible to the expression’s evaluation and printing. Evaluation of expressions that cause side effects will work as expected, except for changes to data that Edebug explicitly saves and restores. See [The Outside Context](the-outside-context), for details on this process.
`e exp RET`
Evaluate expression exp in the context outside of Edebug (`edebug-eval-expression`). That is, Edebug tries to minimize its interference with the evaluation.
`M-: exp RET`
Evaluate expression exp in the context of Edebug itself (`eval-expression`).
`C-x C-e` Evaluate the expression before point, in the context outside of Edebug (`edebug-eval-last-sexp`). With the prefix argument of zero (`C-u 0 C-x C-e`), don’t shorten long items (like strings and lists).
Edebug supports evaluation of expressions containing references to lexically bound symbols created by the following constructs in `cl.el`: `lexical-let`, `macrolet`, and `symbol-macrolet`.
elisp None ### Rounding Operations
The functions `ffloor`, `fceiling`, `fround`, and `ftruncate` take a floating-point argument and return a floating-point result whose value is a nearby integer. `ffloor` returns the nearest integer below; `fceiling`, the nearest integer above; `ftruncate`, the nearest integer in the direction towards zero; `fround`, the nearest integer.
Function: **ffloor** *float*
This function rounds float to the next lower integral value, and returns that value as a floating-point number.
Function: **fceiling** *float*
This function rounds float to the next higher integral value, and returns that value as a floating-point number.
Function: **ftruncate** *float*
This function rounds float towards zero to an integral value, and returns that value as a floating-point number.
Function: **fround** *float*
This function rounds float to the nearest integral value, and returns that value as a floating-point number. Rounding a value equidistant between two integers returns the even integer.
elisp None ### C Dialect
The C part of Emacs is portable to C99 or later: C11-specific features such as ‘`<stdalign.h>`’ and ‘`\_Noreturn`’ are not used without a check, typically at configuration time, and the Emacs build procedure provides a substitute implementation if necessary. Some C11 features, such as anonymous structures and unions, are too difficult to emulate, so they are avoided entirely.
At some point in the future the base C dialect will no doubt change to C11.
elisp None ### Customization Types
When you define a user option with `defcustom`, you must specify its *customization type*. That is a Lisp object which describes (1) which values are legitimate and (2) how to display the value in the customization buffer for editing.
You specify the customization type in `defcustom` with the `:type` keyword. The argument of `:type` is evaluated, but only once when the `defcustom` is executed, so it isn’t useful for the value to vary. Normally we use a quoted constant. For example:
```
(defcustom diff-command "diff"
"The command to use to run diff."
:type '(string)
:group 'diff)
```
In general, a customization type is a list whose first element is a symbol, one of the customization type names defined in the following sections. After this symbol come a number of arguments, depending on the symbol. Between the type symbol and its arguments, you can optionally write keyword-value pairs (see [Type Keywords](type-keywords)).
Some type symbols do not use any arguments; those are called *simple types*. For a simple type, if you do not use any keyword-value pairs, you can omit the parentheses around the type symbol. For example just `string` as a customization type is equivalent to `(string)`.
All customization types are implemented as widgets; see [Introduction](https://www.gnu.org/software/emacs/manual/html_node/widget/index.html#Top) in The Emacs Widget Library, for details.
| | | |
| --- | --- | --- |
| • [Simple Types](simple-types) | | Simple customization types: sexp, integer, etc. |
| • [Composite Types](composite-types) | | Build new types from other types or data. |
| • [Splicing into Lists](splicing-into-lists) | | Splice elements into list with `:inline`. |
| • [Type Keywords](type-keywords) | | Keyword-argument pairs in a customization type. |
| • [Defining New Types](defining-new-types) | | Give your type a name. |
elisp None #### Making and Deleting Numbered Backup Files
If a file’s name is `foo`, the names of its numbered backup versions are `foo.~v~`, for various integers v, like this: `foo.~1~`, `foo.~2~`, `foo.~3~`, …, `foo.~259~`, and so on.
User Option: **version-control**
This variable controls whether to make a single non-numbered backup file or multiple numbered backups.
`nil`
Make numbered backups if the visited file already has numbered backups; otherwise, do not. This is the default.
`never`
Do not make numbered backups.
anything else Make numbered backups.
The use of numbered backups ultimately leads to a large number of backup versions, which must then be deleted. Emacs can do this automatically or it can ask the user whether to delete them.
User Option: **kept-new-versions**
The value of this variable is the number of newest versions to keep when a new numbered backup is made. The newly made backup is included in the count. The default value is 2.
User Option: **kept-old-versions**
The value of this variable is the number of oldest versions to keep when a new numbered backup is made. The default value is 2.
If there are backups numbered 1, 2, 3, 5, and 7, and both of these variables have the value 2, then the backups numbered 1 and 2 are kept as old versions and those numbered 5 and 7 are kept as new versions; backup version 3 is excess. The function `find-backup-file-name` (see [Backup Names](backup-names)) is responsible for determining which backup versions to delete, but does not delete them itself.
User Option: **delete-old-versions**
If this variable is `t`, then saving a file deletes excess backup versions silently. If it is `nil`, that means to ask for confirmation before deleting excess backups. Otherwise, they are not deleted at all.
User Option: **dired-kept-versions**
This variable specifies how many of the newest backup versions to keep in the Dired command `.` (`dired-clean-directory`). That’s the same thing `kept-new-versions` specifies when you make a new backup file. The default is 2.
elisp None #### Helper Functions for Indentation Rules
SMIE provides various functions designed specifically for use in the indentation rules function (several of those functions break if used in another context). These functions all start with the prefix `smie-rule-`.
Function: **smie-rule-bolp**
Return non-`nil` if the current token is the first on the line.
Function: **smie-rule-hanging-p**
Return non-`nil` if the current token is *hanging*. A token is *hanging* if it is the last token on the line and if it is preceded by other tokens: a lone token on a line is not hanging.
Function: **smie-rule-next-p** *&rest tokens*
Return non-`nil` if the next token is among tokens.
Function: **smie-rule-prev-p** *&rest tokens*
Return non-`nil` if the previous token is among tokens.
Function: **smie-rule-parent-p** *&rest parents*
Return non-`nil` if the current token’s parent is among parents.
Function: **smie-rule-sibling-p**
Return non-`nil` if the current token’s parent is actually a sibling. This is the case for example when the parent of a `","` is just the previous `","`.
Function: **smie-rule-parent** *&optional offset*
Return the proper offset to align the current token with the parent. If non-`nil`, offset should be an integer giving an additional offset to apply.
Function: **smie-rule-separator** *method*
Indent current token as a *separator*.
By *separator*, we mean here a token whose sole purpose is to separate various elements within some enclosing syntactic construct, and which does not have any semantic significance in itself (i.e., it would typically not exist as a node in an abstract syntax tree).
Such a token is expected to have an associative syntax and be closely tied to its syntactic parent. Typical examples are `","` in lists of arguments (enclosed inside parentheses), or `";"` in sequences of instructions (enclosed in a `{...}` or `begin...end` block).
method should be the method name that was passed to `smie-rules-function`.
elisp None #### Using Lexical Binding
When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical binding is enabled if the buffer-local variable `lexical-binding` is non-`nil`:
Variable: **lexical-binding**
If this buffer-local variable is non-`nil`, Emacs Lisp files and buffers are evaluated using lexical binding instead of dynamic binding. (However, special variables are still dynamically bound; see below.) If `nil`, dynamic binding is used for all local variables. This variable is typically set for a whole Emacs Lisp file, as a file local variable (see [File Local Variables](file-local-variables)). Note that unlike other such variables, this one must be set in the first line of a file.
When evaluating Emacs Lisp code directly using an `eval` call, lexical binding is enabled if the lexical argument to `eval` is non-`nil`. See [Eval](eval).
Lexical binding is also enabled in Lisp Interaction and IELM mode, used in the `\*scratch\*` and `\*ielm\*` buffers, and also when evaluating expressions via `M-:` (`eval-expression`) and when processing the `--eval` command-line options of Emacs (see [Action Arguments](https://www.gnu.org/software/emacs/manual/html_node/emacs/Action-Arguments.html#Action-Arguments) in The GNU Emacs Manual) and `emacsclient` (see [emacsclient Options](https://www.gnu.org/software/emacs/manual/html_node/emacs/emacsclient-Options.html#emacsclient-Options) in The GNU Emacs Manual).
Even when lexical binding is enabled, certain variables will continue to be dynamically bound. These are called *special variables*. Every variable that has been defined with `defvar`, `defcustom` or `defconst` is a special variable (see [Defining Variables](defining-variables)). All other variables are subject to lexical binding.
Using `defvar` without a value, it is possible to bind a variable dynamically just in one file, or in just one part of a file while still binding it lexically elsewhere. For example:
```
(let (_)
(defvar x) ; Let-bindings of `x` will be dynamic within this let.
(let ((x -99)) ; This is a dynamic binding of `x`.
(defun get-dynamic-x ()
x)))
(let ((x 'lexical)) ; This is a lexical binding of `x`.
(defun get-lexical-x ()
x))
(let (_)
(defvar x)
(let ((x 'dynamic))
(list (get-lexical-x)
(get-dynamic-x))))
⇒ (lexical dynamic)
```
Function: **special-variable-p** *symbol*
This function returns non-`nil` if symbol is a special variable (i.e., it has a `defvar`, `defcustom`, or `defconst` variable definition). Otherwise, the return value is `nil`.
Note that since this is a function, it can only return non-`nil` for variables which are permanently special, but not for those that are only special in the current lexical scope.
The use of a special variable as a formal argument in a function is not supported.
elisp None #### Type Descriptors
A *type descriptor* is a `record` which holds information about a type. Slot 1 in the record must be a symbol naming the type, and `type-of` relies on this to return the type of `record` objects. No other type descriptor slot is used by Emacs; they are free for use by Lisp extensions.
An example of a type descriptor is any instance of `cl-structure-class`.
elisp None #### Warning Basics
Every warning has a textual message, which explains the problem for the user, and a *severity level* which is a symbol. Here are the possible severity levels, in order of decreasing severity, and their meanings:
`:emergency` A problem that will seriously impair Emacs operation soon if you do not attend to it promptly.
`:error` A report of data or circumstances that are inherently wrong.
`:warning` A report of data or circumstances that are not inherently wrong, but raise suspicion of a possible problem.
`:debug` A report of information that may be useful if you are debugging.
When your program encounters invalid input data, it can either signal a Lisp error by calling `error` or `signal` or report a warning with severity `:error`. Signaling a Lisp error is the easiest thing to do, but it means the program cannot continue processing. If you want to take the trouble to implement a way to continue processing despite the bad data, then reporting a warning of severity `:error` is the right way to inform the user of the problem. For instance, the Emacs Lisp byte compiler can report an error that way and continue compiling other functions. (If the program signals a Lisp error and then handles it with `condition-case`, the user won’t see the error message; it could show the message to the user by reporting it as a warning.)
Each warning has a *warning type* to classify it. The type is a list of symbols. The first symbol should be the custom group that you use for the program’s user options. For example, byte compiler warnings use the warning type `(bytecomp)`. You can also subcategorize the warnings, if you wish, by using more symbols in the list.
Function: **display-warning** *type message &optional level buffer-name*
This function reports a warning, using message as the message and type as the warning type. level should be the severity level, with `:warning` being the default.
buffer-name, if non-`nil`, specifies the name of the buffer for logging the warning. By default, it is `\*Warnings\*`.
Function: **lwarn** *type level message &rest args*
This function reports a warning using the value of `(format-message
message args...)` as the message in the `\*Warnings\*` buffer. In other respects it is equivalent to `display-warning`.
Function: **warn** *message &rest args*
This function reports a warning using the value of `(format-message
message args...)` as the message, `(emacs)` as the type, and `:warning` as the severity level. It exists for compatibility only; we recommend not using it, because you should specify a specific warning type.
elisp None #### Automatic Face Assignment
This hook is used for automatically assigning faces to text in the buffer. It is part of the implementation of Jit-Lock mode, used by Font-Lock.
Variable: **fontification-functions**
This variable holds a list of functions that are called by Emacs redisplay as needed, just before doing redisplay. They are called even when Font Lock Mode isn’t enabled. When Font Lock Mode is enabled, this variable usually holds just one function, `jit-lock-function`.
The functions are called in the order listed, with one argument, a buffer position pos. Collectively they should attempt to assign faces to the text in the current buffer starting at pos.
The functions should record the faces they assign by setting the `face` property. They should also add a non-`nil` `fontified` property to all the text they have assigned faces to. That property tells redisplay that faces have been assigned to that text already.
It is probably a good idea for the functions to do nothing if the character after pos already has a non-`nil` `fontified` property, but this is not required. If one function overrides the assignments made by a previous one, the properties after the last function finishes are the ones that really matter.
For efficiency, we recommend writing these functions so that they usually assign faces to around 400 to 600 characters at each call.
elisp None ### Images
To display an image in an Emacs buffer, you must first create an image descriptor, then use it as a display specifier in the `display` property of text that is displayed (see [Display Property](display-property)).
Emacs is usually able to display images when it is run on a graphical terminal. Images cannot be displayed in a text terminal, on certain graphical terminals that lack the support for this, or if Emacs is compiled without image support. You can use the function `display-images-p` to determine if images can in principle be displayed (see [Display Feature Testing](display-feature-testing)).
| | | |
| --- | --- | --- |
| • [Image Formats](image-formats) | | Supported image formats. |
| • [Image Descriptors](image-descriptors) | | How to specify an image for use in `:display`. |
| • [XBM Images](xbm-images) | | Special features for XBM format. |
| • [XPM Images](xpm-images) | | Special features for XPM format. |
| • [ImageMagick Images](imagemagick-images) | | Special features available through ImageMagick. |
| • [SVG Images](svg-images) | | Creating and manipulating SVG images. |
| • [Other Image Types](other-image-types) | | Various other formats are supported. |
| • [Defining Images](defining-images) | | Convenient ways to define an image for later use. |
| • [Showing Images](showing-images) | | Convenient ways to display an image once it is defined. |
| • [Multi-Frame Images](multi_002dframe-images) | | Some images contain more than one frame. |
| • [Image Cache](image-cache) | | Internal mechanisms of image display. |
| programming_docs |
elisp None #### Variables Used in the Mode Line
This section describes variables incorporated by the standard value of `mode-line-format` into the text of the mode line. There is nothing inherently special about these variables; any other variables could have the same effects on the mode line if the value of `mode-line-format` is changed to use them. However, various parts of Emacs set these variables on the understanding that they will control parts of the mode line; therefore, practically speaking, it is essential for the mode line to use them. Also see [Optional Mode Line](https://www.gnu.org/software/emacs/manual/html_node/emacs/Optional-Mode-Line.html#Optional-Mode-Line) in The GNU Emacs Manual.
Variable: **mode-line-mule-info**
This variable holds the value of the mode line construct that displays information about the language environment, buffer coding system, and current input method. See [Non-ASCII Characters](non_002dascii-characters).
Variable: **mode-line-modified**
This variable holds the value of the mode line construct that displays whether the current buffer is modified. Its default value displays ‘`\*\*`’ if the buffer is modified, ‘`--`’ if the buffer is not modified, ‘`%%`’ if the buffer is read only, and ‘`%\*`’ if the buffer is read only and modified.
Changing this variable does not force an update of the mode line.
Variable: **mode-line-frame-identification**
This variable identifies the current frame. Its default value displays `" "` if you are using a window system which can show multiple frames, or `"-%F "` on an ordinary terminal which shows only one frame at a time.
Variable: **mode-line-buffer-identification**
This variable identifies the buffer being displayed in the window. Its default value displays the buffer name, padded with spaces to at least 12 columns.
Variable: **mode-line-position**
This variable indicates the position in the buffer. Its default value displays the buffer percentage and, optionally, the buffer size, the line number and the column number.
User Option: **mode-line-percent-position**
This option is used in `mode-line-position`. Its value specifies both the buffer percentage to display (one of `nil`, `"%o"`, `"%p"`, `"%P"` or `"%q"`, see [%-Constructs](_0025_002dconstructs)) and a width to space-fill or truncate to. You are recommended to set this option with the `customize-variable` facility.
Variable: **vc-mode**
The variable `vc-mode`, buffer-local in each buffer, records whether the buffer’s visited file is maintained with version control, and, if so, which kind. Its value is a string that appears in the mode line, or `nil` for no version control.
Variable: **mode-line-modes**
This variable displays the buffer’s major and minor modes. Its default value also displays the recursive editing level, information on the process status, and whether narrowing is in effect.
Variable: **mode-line-remote**
This variable is used to show whether `default-directory` for the current buffer is remote.
Variable: **mode-line-client**
This variable is used to identify `emacsclient` frames.
The following three variables are used in `mode-line-modes`:
Variable: **mode-name**
This buffer-local variable holds the “pretty” name of the current buffer’s major mode. Each major mode should set this variable so that the mode name will appear in the mode line. The value does not have to be a string, but can use any of the data types valid in a mode-line construct (see [Mode Line Data](mode-line-data)). To compute the string that will identify the mode name in the mode line, use `format-mode-line` (see [Emulating Mode Line](emulating-mode-line)).
Variable: **mode-line-process**
This buffer-local variable contains the mode line information on process status in modes used for communicating with subprocesses. It is displayed immediately following the major mode name, with no intervening space. For example, its value in the `\*shell\*` buffer is `(":%s")`, which allows the shell to display its status along with the major mode as: ‘`(Shell:run)`’. Normally this variable is `nil`.
Variable: **mode-line-front-space**
This variable is displayed at the front of the mode line. By default, this construct is displayed right at the beginning of the mode line, except that if there is a memory-full message, it is displayed first.
Variable: **mode-line-end-spaces**
This variable is displayed at the end of the mode line.
Variable: **mode-line-misc-info**
Mode line construct for miscellaneous information. By default, this shows the information specified by `global-mode-string`.
Variable: **mode-line-position-line-format**
The format used to display line numbers when `line-number-mode` (see [Optional Mode Line](https://www.gnu.org/software/emacs/manual/html_node/emacs/Optional-Mode-Line.html#Optional-Mode-Line) in The GNU Emacs Manual) is switched on. ‘`%l`’ in the format will be replaced with the line number.
Variable: **mode-line-position-column-format**
The format used to display column numbers when `column-number-mode` (see [Optional Mode Line](https://www.gnu.org/software/emacs/manual/html_node/emacs/Optional-Mode-Line.html#Optional-Mode-Line) in The GNU Emacs Manual) is switched on. ‘`%c`’ in the format will be replaced with a zero-based column number, and ‘`%C`’ will be replaced with a one-based column number.
Variable: **mode-line-position-column-line-format**
The format used to display column numbers when both `line-number-mode` and `column-number-mode` are switched on. See the previous two variables for the meaning of the ‘`%l`’, ‘`%c`’ and ‘`%C`’ format specs.
Variable: **minor-mode-alist**
This variable holds an association list whose elements specify how the mode line should indicate that a minor mode is active. Each element of the `minor-mode-alist` should be a two-element list:
```
(minor-mode-variable mode-line-string)
```
More generally, mode-line-string can be any mode line construct. It appears in the mode line when the value of minor-mode-variable is non-`nil`, and not otherwise. These strings should begin with spaces so that they don’t run together. Conventionally, the minor-mode-variable for a specific mode is set to a non-`nil` value when that minor mode is activated.
`minor-mode-alist` itself is not buffer-local. Each variable mentioned in the alist should be buffer-local if its minor mode can be enabled separately in each buffer.
Variable: **global-mode-string**
This variable holds a mode line construct that, by default, appears in the mode line just after the `which-function-mode` minor mode if set, else after `mode-line-modes`. Elements that are added to this construct should normally end in a space (to ensure that consecutive `global-mode-string` elements display properly). For instance, the command `display-time` sets `global-mode-string` to refer to the variable `display-time-string`, which holds a string containing the time and load information.
The ‘`%M`’ construct substitutes the value of `global-mode-string`, but that is obsolete, since the variable is included in the mode line from `mode-line-format`.
Here is a simplified version of the default value of `mode-line-format`. The real default value also specifies addition of text properties.
```
("-"
mode-line-mule-info
mode-line-modified
mode-line-frame-identification
mode-line-buffer-identification
```
```
" "
mode-line-position
(vc-mode vc-mode)
" "
```
```
mode-line-modes
(which-function-mode ("" which-func-format "--"))
(global-mode-string ("--" global-mode-string))
"-%-")
```
elisp None ### Multiple Terminals
Emacs represents each terminal as a *terminal object* data type (see [Terminal Type](terminal-type)). On GNU and Unix systems, Emacs can use multiple terminals simultaneously in each session. On other systems, it can only use a single terminal. Each terminal object has the following attributes:
* The name of the device used by the terminal (e.g., ‘`:0.0`’ or `/dev/tty`).
* The terminal and keyboard coding systems used on the terminal. See [Terminal I/O Encoding](terminal-i_002fo-encoding).
* The kind of display associated with the terminal. This is the symbol returned by the function `terminal-live-p` (i.e., `x`, `t`, `w32`, `ns`, or `pc`). See [Frames](frames).
* A list of terminal parameters. See [Terminal Parameters](terminal-parameters).
There is no primitive for creating terminal objects. Emacs creates them as needed, such as when you call `make-frame-on-display` (described below).
Function: **terminal-name** *&optional terminal*
This function returns the file name of the device used by terminal. If terminal is omitted or `nil`, it defaults to the selected frame’s terminal. terminal can also be a frame, meaning that frame’s terminal.
Function: **terminal-list**
This function returns a list of all live terminal objects.
Function: **get-device-terminal** *device*
This function returns a terminal whose device name is given by device. If device is a string, it can be either the file name of a terminal device, or the name of an X display of the form ‘`host:server.screen`’. If device is a frame, this function returns that frame’s terminal; `nil` means the selected frame. Finally, if device is a terminal object that represents a live terminal, that terminal is returned. The function signals an error if its argument is none of the above.
Function: **delete-terminal** *&optional terminal force*
This function deletes all frames on terminal and frees the resources used by it. It runs the abnormal hook `delete-terminal-functions`, passing terminal as the argument to each function.
If terminal is omitted or `nil`, it defaults to the selected frame’s terminal. terminal can also be a frame, meaning that frame’s terminal.
Normally, this function signals an error if you attempt to delete the sole active terminal, but if force is non-`nil`, you are allowed to do so. Emacs automatically calls this function when the last frame on a terminal is deleted (see [Deleting Frames](deleting-frames)).
Variable: **delete-terminal-functions**
An abnormal hook run by `delete-terminal`. Each function receives one argument, the terminal argument passed to `delete-terminal`. Due to technical details, the functions may be called either just before the terminal is deleted, or just afterwards.
A few Lisp variables are *terminal-local*; that is, they have a separate binding for each terminal. The binding in effect at any time is the one for the terminal that the currently selected frame belongs to. These variables include `default-minibuffer-frame`, `defining-kbd-macro`, `last-kbd-macro`, and `system-key-alist`. They are always terminal-local, and can never be buffer-local (see [Buffer-Local Variables](buffer_002dlocal-variables)).
On GNU and Unix systems, each X display is a separate graphical terminal. When Emacs is started from within the X window system, it uses the X display specified by the `DISPLAY` environment variable, or by the ‘`--display`’ option (see [Initial Options](https://www.gnu.org/software/emacs/manual/html_node/emacs/Initial-Options.html#Initial-Options) in The GNU Emacs Manual). Emacs can connect to other X displays via the command `make-frame-on-display`. Each X display has its own selected frame and its own minibuffer windows; however, only one of those frames is *the* selected frame at any given moment (see [Input Focus](input-focus)). Emacs can even connect to other text terminals, by interacting with the `emacsclient` program. See [Emacs Server](https://www.gnu.org/software/emacs/manual/html_node/emacs/Emacs-Server.html#Emacs-Server) in The GNU Emacs Manual.
A single X server can handle more than one display. Each X display has a three-part name, ‘`hostname:displaynumber.screennumber`’. The first part, hostname, specifies the name of the machine to which the display is physically connected. The second part, displaynumber, is a zero-based number that identifies one or more monitors connected to that machine that share a common keyboard and pointing device (mouse, tablet, etc.). The third part, screennumber, identifies a zero-based screen number (a separate monitor) that is part of a single monitor collection on that X server. When you use two or more screens belonging to one server, Emacs knows by the similarity in their names that they share a single keyboard.
Systems that don’t use the X window system, such as MS-Windows, don’t support the notion of X displays, and have only one display on each host. The display name on these systems doesn’t follow the above 3-part format; for example, the display name on MS-Windows systems is a constant string ‘`w32`’, and exists for compatibility, so that you could pass it to functions that expect a display name.
Command: **make-frame-on-display** *display &optional parameters*
This function creates and returns a new frame on display, taking the other frame parameters from the alist parameters. display should be the name of an X display (a string).
Before creating the frame, this function ensures that Emacs is set up to display graphics. For instance, if Emacs has not processed X resources (e.g., if it was started on a text terminal), it does so at this time. In all other respects, this function behaves like `make-frame` (see [Creating Frames](creating-frames)).
Function: **x-display-list**
This function returns a list that indicates which X displays Emacs has a connection to. The elements of the list are strings, and each one is a display name.
Function: **x-open-connection** *display &optional xrm-string must-succeed*
This function opens a connection to the X display display, without creating a frame on that display. Normally, Emacs Lisp programs need not call this function, as `make-frame-on-display` calls it automatically. The only reason for calling it is to check whether communication can be established with a given X display.
The optional argument xrm-string, if not `nil`, is a string of resource names and values, in the same format used in the `.Xresources` file. See [X Resources](https://www.gnu.org/software/emacs/manual/html_node/emacs/X-Resources.html#X-Resources) in The GNU Emacs Manual. These values apply to all Emacs frames created on this display, overriding the resource values recorded in the X server. Here’s an example of what this string might look like:
```
"*BorderWidth: 3\n*InternalBorder: 2\n"
```
If must-succeed is non-`nil`, failure to open the connection terminates Emacs. Otherwise, it is an ordinary Lisp error.
Function: **x-close-connection** *display*
This function closes the connection to display display. Before you can do this, you must first delete all the frames that were open on that display (see [Deleting Frames](deleting-frames)).
On some multi-monitor setups, a single X display outputs to more than one physical monitor. You can use the functions `display-monitor-attributes-list` and `frame-monitor-attributes` to obtain information about such setups.
Function: **display-monitor-attributes-list** *&optional display*
This function returns a list of physical monitor attributes on display, which can be a display name (a string), a terminal, or a frame; if omitted or `nil`, it defaults to the selected frame’s display. Each element of the list is an association list, representing the attributes of a physical monitor. The first element corresponds to the primary monitor. The attribute keys and values are:
‘`geometry`’
Position of the top-left corner of the monitor’s screen and its size, in pixels, as ‘`(x y width height)`’. Note that, if the monitor is not the primary monitor, some of the coordinates might be negative.
‘`workarea`’
Position of the top-left corner and size of the work area (usable space) in pixels as ‘`(x y width height)`’. This may be different from ‘`geometry`’ in that space occupied by various window manager features (docks, taskbars, etc.) may be excluded from the work area. Whether or not such features actually subtract from the work area depends on the platform and environment. Again, if the monitor is not the primary monitor, some of the coordinates might be negative.
‘`mm-size`’
Width and height in millimeters as ‘`(width height)`’
‘`frames`’
List of frames that this physical monitor dominates (see below).
‘`name`’
Name of the physical monitor as string.
‘`source`’ Source of the multi-monitor information as string; e.g., ‘`XRandr`’ or ‘`Xinerama`’.
x, y, width, and height are integers. ‘`name`’ and ‘`source`’ may be absent.
A frame is *dominated* by a physical monitor when either the largest area of the frame resides in that monitor, or (if the frame does not intersect any physical monitors) that monitor is the closest to the frame. Every (non-tooltip) frame (whether visible or not) in a graphical display is dominated by exactly one physical monitor at a time, though the frame can span multiple (or no) physical monitors.
Here’s an example of the data produced by this function on a 2-monitor display:
```
(display-monitor-attributes-list)
⇒
(((geometry 0 0 1920 1080) ;; Left-hand, primary monitor
(workarea 0 0 1920 1050) ;; A taskbar occupies some of the height
(mm-size 677 381)
(name . "DISPLAY1")
(frames #<frame emacs@host *Messages* 0x11578c0>
#<frame emacs@host *scratch* 0x114b838>))
((geometry 1920 0 1680 1050) ;; Right-hand monitor
(workarea 1920 0 1680 1050) ;; Whole screen can be used
(mm-size 593 370)
(name . "DISPLAY2")
(frames)))
```
Function: **frame-monitor-attributes** *&optional frame*
This function returns the attributes of the physical monitor dominating (see above) frame, which defaults to the selected frame.
On multi-monitor displays it is possible to use the command `make-frame-on-monitor` to make frames on the specified monitor.
Command: **make-frame-on-monitor** *monitor &optional display parameters*
This function creates and returns a new frame on monitor located on display, taking the other frame parameters from the alist parameters. monitor should be the name of the physical monitor, the same string as returned by the function `display-monitor-attributes-list` in the attribute `name`. display should be the name of an X display (a string).
elisp None #### Accepting Output from Processes
Output from asynchronous subprocesses normally arrives only while Emacs is waiting for some sort of external event, such as elapsed time or terminal input. Occasionally it is useful in a Lisp program to explicitly permit output to arrive at a specific point, or even to wait until output arrives from a process.
Function: **accept-process-output** *&optional process seconds millisec just-this-one*
This function allows Emacs to read pending output from processes. The output is given to their filter functions. If process is non-`nil` then this function does not return until some output has been received from process or process has closed the connection.
The arguments seconds and millisec let you specify timeout periods. The former specifies a period measured in seconds and the latter specifies one measured in milliseconds. The two time periods thus specified are added together, and `accept-process-output` returns after that much time, even if there is no subprocess output.
The argument millisec is obsolete (and should not be used), because seconds can be floating point to specify waiting a fractional number of seconds. If seconds is 0, the function accepts whatever output is pending but does not wait.
If process is a process, and the argument just-this-one is non-`nil`, only output from that process is handled, suspending output from other processes until some output has been received from that process or the timeout expires. If just-this-one is an integer, also inhibit running timers. This feature is generally not recommended, but may be necessary for specific applications, such as speech synthesis.
The function `accept-process-output` returns non-`nil` if it got output from process, or from any process if process is `nil`; this can occur even after a process has exited if the corresponding connection contains buffered data. The function returns `nil` if the timeout expired or the connection was closed before output arrived.
If a connection from a process contains buffered data, `accept-process-output` can return non-`nil` even after the process has exited. Therefore, although the following loop:
```
;; This loop contains a bug.
(while (process-live-p process)
(accept-process-output process))
```
will often read all output from process, it has a race condition and can miss some output if `process-live-p` returns `nil` while the connection still contains data. Better is to write the loop like this:
```
(while (accept-process-output process))
```
If you have passed a non-`nil` stderr to `make-process`, it will have a standard error process. See [Asynchronous Processes](asynchronous-processes). In that case, waiting for process output from the main process doesn’t wait for output from the standard error process. To make sure you have received both all of standard output and all of standard error from a process, use the following code:
```
(while (accept-process-output process))
(while (accept-process-output stderr-process))
```
If you passed a buffer to the stderr argument of `make-process`, you still have to wait for the standard error process, like so:
```
(let* ((stdout (generate-new-buffer "stdout"))
(stderr (generate-new-buffer "stderr"))
(process (make-process :name "test"
:command '("my-program")
:buffer stdout
:stderr stderr))
(stderr-process (get-buffer-process stderr)))
(unless (and process stderr-process)
(error "Process unexpectedly nil"))
(while (accept-process-output process))
(while (accept-process-output stderr-process)))
```
Only when both `accept-process-output` forms return `nil`, you can be sure that the process has exited and Emacs has read all its output.
Reading pending standard error from a process running on a remote host is not possible this way.
| programming_docs |
elisp None Minibuffers
-----------
A *minibuffer* is a special buffer that Emacs commands use to read arguments more complicated than the single numeric prefix argument. These arguments include file names, buffer names, and command names (as in `M-x`). The minibuffer is displayed on the bottom line of the frame, in the same place as the echo area (see [The Echo Area](the-echo-area)), but only while it is in use for reading an argument.
| | | |
| --- | --- | --- |
| • [Intro to Minibuffers](intro-to-minibuffers) | | Basic information about minibuffers. |
| • [Text from Minibuffer](text-from-minibuffer) | | How to read a straight text string. |
| • [Object from Minibuffer](object-from-minibuffer) | | How to read a Lisp object or expression. |
| • [Minibuffer History](minibuffer-history) | | Recording previous minibuffer inputs so the user can reuse them. |
| • [Initial Input](initial-input) | | Specifying initial contents for the minibuffer. |
| • [Completion](completion) | | How to invoke and customize completion. |
| • [Yes-or-No Queries](yes_002dor_002dno-queries) | | Asking a question with a simple answer. |
| • [Multiple Queries](multiple-queries) | | Asking complex questions. |
| • [Reading a Password](reading-a-password) | | Reading a password from the terminal. |
| • [Minibuffer Commands](minibuffer-commands) | | Commands used as key bindings in minibuffers. |
| • [Minibuffer Windows](minibuffer-windows) | | Operating on the special minibuffer windows. |
| • [Minibuffer Contents](minibuffer-contents) | | How such commands access the minibuffer text. |
| • [Recursive Mini](recursive-mini) | | Whether recursive entry to minibuffer is allowed. |
| • [Inhibiting Interaction](inhibiting-interaction) | | Running Emacs when no interaction is possible. |
| • [Minibuffer Misc](minibuffer-misc) | | Various customization hooks and variables. |
elisp None #### Menu Separators
A menu separator is a kind of menu item that doesn’t display any text—instead, it divides the menu into subparts with a horizontal line. A separator looks like this in the menu keymap:
```
(menu-item separator-type)
```
where separator-type is a string starting with two or more dashes.
In the simplest case, separator-type consists of only dashes. That specifies the default kind of separator. (For compatibility, `""` and `-` also count as separators.)
Certain other values of separator-type specify a different style of separator. Here is a table of them:
`"--no-line"` `"--space"`
An extra vertical space, with no actual line.
`"--single-line"`
A single line in the menu’s foreground color.
`"--double-line"`
A double line in the menu’s foreground color.
`"--single-dashed-line"`
A single dashed line in the menu’s foreground color.
`"--double-dashed-line"`
A double dashed line in the menu’s foreground color.
`"--shadow-etched-in"`
A single line with a 3D sunken appearance. This is the default, used separators consisting of dashes only.
`"--shadow-etched-out"`
A single line with a 3D raised appearance.
`"--shadow-etched-in-dash"`
A single dashed line with a 3D sunken appearance.
`"--shadow-etched-out-dash"`
A single dashed line with a 3D raised appearance.
`"--shadow-double-etched-in"`
Two lines with a 3D sunken appearance.
`"--shadow-double-etched-out"`
Two lines with a 3D raised appearance.
`"--shadow-double-etched-in-dash"`
Two dashed lines with a 3D sunken appearance.
`"--shadow-double-etched-out-dash"` Two dashed lines with a 3D raised appearance.
You can also give these names in another style, adding a colon after the double-dash and replacing each single dash with capitalization of the following word. Thus, `"--:singleLine"`, is equivalent to `"--single-line"`.
You can use a longer form to specify keywords such as `:enable` and `:visible` for a menu separator:
`(menu-item separator-type nil . item-property-list)`
For example:
```
(menu-item "--" nil :visible (boundp 'foo))
```
Some systems and display toolkits don’t really handle all of these separator types. If you use a type that isn’t supported, the menu displays a similar kind of separator that is supported.
elisp None #### Dynamic Binding
By default, the local variable bindings made by Emacs are dynamic bindings. When a variable is dynamically bound, its current binding at any point in the execution of the Lisp program is simply the most recently-created dynamic local binding for that symbol, or the global binding if there is no such local binding.
Dynamic bindings have dynamic scope and extent, as shown by the following example:
```
(defvar x -99) ; `x` receives an initial value of -99.
(defun getx ()
x) ; `x` is used free in this function.
(let ((x 1)) ; `x` is dynamically bound.
(getx))
⇒ 1
;; After the `let` form finishes, `x` reverts to its
;; previous value, which is -99.
(getx)
⇒ -99
```
The function `getx` refers to `x`. This is a *free* reference, in the sense that there is no binding for `x` within that `defun` construct itself. When we call `getx` from within a `let` form in which `x` is (dynamically) bound, it retrieves the local value (i.e., 1). But when we call `getx` outside the `let` form, it retrieves the global value (i.e., -99).
Here is another example, which illustrates setting a dynamically bound variable using `setq`:
```
(defvar x -99) ; `x` receives an initial value of -99.
(defun addx ()
(setq x (1+ x))) ; Add 1 to `x` and return its new value.
(let ((x 1))
(addx)
(addx))
⇒ 3 ; The two `addx` calls add to `x` twice.
;; After the `let` form finishes, `x` reverts to its
;; previous value, which is -99.
(addx)
⇒ -98
```
Dynamic binding is implemented in Emacs Lisp in a simple way. Each symbol has a value cell, which specifies its current dynamic value (or absence of value). See [Symbol Components](symbol-components). When a symbol is given a dynamic local binding, Emacs records the contents of the value cell (or absence thereof) in a stack, and stores the new local value in the value cell. When the binding construct finishes executing, Emacs pops the old value off the stack, and puts it in the value cell.
Note that when code using Dynamic Binding is native compiled the native compiler will not perform any Lisp specific optimization.
elisp None ### Defining Commands
The special form `interactive` turns a Lisp function into a command. The `interactive` form must be located at top-level in the function body, usually as the first form in the body; this applies to both lambda expressions (see [Lambda Expressions](lambda-expressions)) and `defun` forms (see [Defining Functions](defining-functions)). This form does nothing during the actual execution of the function; its presence serves as a flag, telling the Emacs command loop that the function can be called interactively. The argument of the `interactive` form specifies how the arguments for an interactive call should be read.
Alternatively, an `interactive` form may be specified in a function symbol’s `interactive-form` property. A non-`nil` value for this property takes precedence over any `interactive` form in the function body itself. This feature is seldom used.
Sometimes, a function is only intended to be called interactively, never directly from Lisp. In that case, give the function a non-`nil` `interactive-only` property, either directly or via `declare` (see [Declare Form](declare-form)). This causes the byte compiler to warn if the command is called from Lisp. The output of `describe-function` will include similar information. The value of the property can be: a string, which the byte-compiler will use directly in its warning (it should end with a period, and not start with a capital, e.g., `"use (system-name) instead."`); `t`; any other symbol, which should be an alternative function to use in Lisp code.
Generic functions (see [Generic Functions](generic-functions)) cannot be turned into commands by adding the `interactive` form to them.
| | | |
| --- | --- | --- |
| • [Using Interactive](using-interactive) | | General rules for `interactive`. |
| • [Interactive Codes](interactive-codes) | | The standard letter-codes for reading arguments in various ways. |
| • [Interactive Examples](interactive-examples) | | Examples of how to read interactive arguments. |
| • [Command Modes](command-modes) | | Specifying that commands are for a specific mode. |
| • [Generic Commands](generic-commands) | | Select among command alternatives. |
elisp None ### Making Certain File Names “Magic”
You can implement special handling for certain file names. This is called making those names *magic*. The principal use for this feature is in implementing access to remote files (see [Remote Files](https://www.gnu.org/software/emacs/manual/html_node/emacs/Remote-Files.html#Remote-Files) in The GNU Emacs Manual).
To define a kind of magic file name, you must supply a regular expression to define the class of names (all those that match the regular expression), plus a handler that implements all the primitive Emacs file operations for file names that match.
The variable `file-name-handler-alist` holds a list of handlers, together with regular expressions that determine when to apply each handler. Each element has this form:
```
(regexp . handler)
```
All the Emacs primitives for file access and file name transformation check the given file name against `file-name-handler-alist`. If the file name matches regexp, the primitives handle that file by calling handler.
The first argument given to handler is the name of the primitive, as a symbol; the remaining arguments are the arguments that were passed to that primitive. (The first of these arguments is most often the file name itself.) For example, if you do this:
```
(file-exists-p filename)
```
and filename has handler handler, then handler is called like this:
```
(funcall handler 'file-exists-p filename)
```
When a function takes two or more arguments that must be file names, it checks each of those names for a handler. For example, if you do this:
```
(expand-file-name filename dirname)
```
then it checks for a handler for filename and then for a handler for dirname. In either case, the handler is called like this:
```
(funcall handler 'expand-file-name filename dirname)
```
The handler then needs to figure out whether to handle filename or dirname.
If the specified file name matches more than one handler, the one whose match starts last in the file name gets precedence. This rule is chosen so that handlers for jobs such as uncompression are handled first, before handlers for jobs such as remote file access.
Here are the operations that a magic file name handler gets to handle:
`access-file`, `add-name-to-file`, `byte-compiler-base-file-name`, `copy-directory`, `copy-file`, `delete-directory`, `delete-file`, `diff-latest-backup-file`, `directory-file-name`, `directory-files`, `directory-files-and-attributes`, `dired-compress-file`, `dired-uncache`, `exec-path`, `expand-file-name`, `file-accessible-directory-p`, `file-acl`, `file-attributes`, `file-directory-p`, `file-equal-p`, `file-executable-p`, `file-exists-p`, `file-in-directory-p`, `file-local-copy`, `file-locked-p`, `file-modes`, `file-name-all-completions`, `file-name-as-directory`, `file-name-case-insensitive-p`, `file-name-completion`, `file-name-directory`, `file-name-nondirectory`, `file-name-sans-versions`, `file-newer-than-file-p`, `file-notify-add-watch`, `file-notify-rm-watch`, `file-notify-valid-p`, `file-ownership-preserved-p`, `file-readable-p`, `file-regular-p`, `file-remote-p`, `file-selinux-context`, `file-symlink-p`, `file-system-info`, `file-truename`, `file-writable-p`, `find-backup-file-name`, `get-file-buffer`, `insert-directory`, `insert-file-contents`, `load`, `lock-file`, `make-auto-save-file-name`, `make-directory`, `make-directory-internal`, `make-lock-file-name`, `make-nearby-temp-file`, `make-process`, `make-symbolic-link`, `process-file`, `rename-file`, `set-file-acl`, `set-file-modes`, `set-file-selinux-context`, `set-file-times`, `set-visited-file-modtime`, `shell-command`, `start-file-process`, `substitute-in-file-name`, `temporary-file-directory`, `unhandled-file-name-directory`, `unlock-file`, `vc-registered`, `verify-visited-file-modtime`, `write-region`.
Handlers for `insert-file-contents` typically need to clear the buffer’s modified flag, with `(set-buffer-modified-p nil)`, if the visit argument is non-`nil`. This also has the effect of unlocking the buffer if it is locked.
The handler function must handle all of the above operations, and possibly others to be added in the future. It need not implement all these operations itself—when it has nothing special to do for a certain operation, it can reinvoke the primitive, to handle the operation in the usual way. It should always reinvoke the primitive for an operation it does not recognize. Here’s one way to do this:
```
(defun my-file-handler (operation &rest args)
;; First check for the specific operations
;; that we have special handling for.
(cond ((eq operation 'insert-file-contents) …)
((eq operation 'write-region) …)
…
;; Handle any operation we don’t know about.
(t (let ((inhibit-file-name-handlers
(cons 'my-file-handler
(and (eq inhibit-file-name-operation operation)
inhibit-file-name-handlers)))
(inhibit-file-name-operation operation))
(apply operation args)))))
```
When a handler function decides to call the ordinary Emacs primitive for the operation at hand, it needs to prevent the primitive from calling the same handler once again, thus leading to an infinite recursion. The example above shows how to do this, with the variables `inhibit-file-name-handlers` and `inhibit-file-name-operation`. Be careful to use them exactly as shown above; the details are crucial for proper behavior in the case of multiple handlers, and for operations that have two file names that may each have handlers.
Handlers that don’t really do anything special for actual access to the file—such as the ones that implement completion of host names for remote file names—should have a non-`nil` `safe-magic` property. For instance, Emacs normally protects directory names it finds in `PATH` from becoming magic, if they look like magic file names, by prefixing them with ‘`/:`’. But if the handler that would be used for them has a non-`nil` `safe-magic` property, the ‘`/:`’ is not added.
A file name handler can have an `operations` property to declare which operations it handles in a nontrivial way. If this property has a non-`nil` value, it should be a list of operations; then only those operations will call the handler. This avoids inefficiency, but its main purpose is for autoloaded handler functions, so that they won’t be loaded except when they have real work to do.
Simply deferring all operations to the usual primitives does not work. For instance, if the file name handler applies to `file-exists-p`, then it must handle `load` itself, because the usual `load` code won’t work properly in that case. However, if the handler uses the `operations` property to say it doesn’t handle `file-exists-p`, then it need not handle `load` nontrivially.
Variable: **inhibit-file-name-handlers**
This variable holds a list of handlers whose use is presently inhibited for a certain operation.
Variable: **inhibit-file-name-operation**
The operation for which certain handlers are presently inhibited.
Function: **find-file-name-handler** *file operation*
This function returns the handler function for file name file, or `nil` if there is none. The argument operation should be the operation to be performed on the file—the value you will pass to the handler as its first argument when you call it. If operation equals `inhibit-file-name-operation`, or if it is not found in the `operations` property of the handler, this function returns `nil`.
Function: **file-local-copy** *filename*
This function copies file filename to an ordinary non-magic file on the local machine, if it isn’t on the local machine already. Magic file names should handle the `file-local-copy` operation if they refer to files on other machines. A magic file name that is used for other purposes than remote file access should not handle `file-local-copy`; then this function will treat the file as local.
If filename is local, whether magic or not, this function does nothing and returns `nil`. Otherwise it returns the file name of the local copy file.
Function: **file-remote-p** *filename &optional identification connected*
This function tests whether filename is a remote file. If filename is local (not remote), the return value is `nil`. If filename is indeed remote, the return value is a string that identifies the remote system.
This identifier string can include a host name and a user name, as well as characters designating the method used to access the remote system. For example, the remote identifier string for the file name `/sudo::/some/file` is `/sudo:root@localhost:`.
If `file-remote-p` returns the same identifier for two different file names, that means they are stored on the same file system and can be accessed locally with respect to each other. This means, for example, that it is possible to start a remote process accessing both files at the same time. Implementers of file name handlers need to ensure this principle is valid.
identification specifies which part of the identifier shall be returned as string. identification can be the symbol `method`, `user` or `host`; any other value is handled like `nil` and means to return the complete identifier string. In the example above, the remote `user` identifier string would be `root`.
If connected is non-`nil`, this function returns `nil` even if filename is remote, if Emacs has no network connection to its host. This is useful when you want to avoid the delay of making connections when they don’t exist.
Function: **unhandled-file-name-directory** *filename*
This function returns the name of a directory that is not magic. For a non-magic filename it returns the corresponding directory name (see [Directory Names](directory-names)). For a magic filename, it invokes the file name handler, which therefore decides what value to return. If filename is not accessible from a local process, then the file name handler should indicate that by returning `nil`.
This is useful for running a subprocess; every subprocess must have a non-magic directory to serve as its current directory, and this function is a good way to come up with one.
Function: **file-local-name** *filename*
This function returns the *local part* of filename. This is the part of the file’s name that identifies it on the remote host, and is typically obtained by removing from the remote file name the parts that specify the remote host and the method of accessing it. For example:
```
(file-local-name "/ssh:user@host:/foo/bar")
⇒ "/foo/bar"
```
For a remote filename, this function returns a file name which could be used directly as an argument of a remote process (see [Asynchronous Processes](asynchronous-processes), and see [Synchronous Processes](synchronous-processes)), and as the program to run on the remote host. If filename is local, this function returns it unchanged.
User Option: **remote-file-name-inhibit-cache**
The attributes of remote files can be cached for better performance. If they are changed outside of Emacs’s control, the cached values become invalid, and must be reread.
When this variable is set to `nil`, cached values are never expired. Use this setting with caution, only if you are sure nothing other than Emacs ever changes the remote files. If it is set to `t`, cached values are never used. This is the safest value, but could result in performance degradation.
A compromise is to set it to a positive number. This means that cached values are used for that amount of seconds since they were cached. If a remote file is checked regularly, it might be a good idea to let-bind this variable to a value less than the time period between consecutive checks. For example:
```
(defun display-time-file-nonempty-p (file)
(let ((remote-file-name-inhibit-cache
(- display-time-interval 5)))
(and (file-exists-p file)
(< 0 (file-attribute-size
(file-attributes
(file-chase-links file)))))))
```
| programming_docs |
elisp None #### Window Frame Parameters
Just what parameters a frame has depends on what display mechanism it uses. This section describes the parameters that have special meanings on some or all kinds of terminals. Of these, `name`, `title`, `height`, `width`, `buffer-list` and `buffer-predicate` provide meaningful information in terminal frames, and `tty-color-mode` is meaningful only for frames on text terminals.
| | | |
| --- | --- | --- |
| • [Basic Parameters](basic-parameters) | | Parameters that are fundamental. |
| • [Position Parameters](position-parameters) | | The position of the frame on the screen. |
| • [Size Parameters](size-parameters) | | Frame’s size. |
| • [Layout Parameters](layout-parameters) | | Size of parts of the frame, and enabling or disabling some parts. |
| • [Buffer Parameters](buffer-parameters) | | Which buffers have been or should be shown. |
| • [Frame Interaction Parameters](frame-interaction-parameters) | | Parameters for interacting with other frames. |
| • [Mouse Dragging Parameters](mouse-dragging-parameters) | | Parameters for resizing and moving frames with the mouse. |
| • [Management Parameters](management-parameters) | | Communicating with the window manager. |
| • [Cursor Parameters](cursor-parameters) | | Controlling the cursor appearance. |
| • [Font and Color Parameters](font-and-color-parameters) | | Fonts and colors for the frame text. |
elisp None ### Type Predicates
The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do so, since function arguments in Lisp do not have declared data types, as they do in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that the function can use.
All built-in functions do check the types of their actual arguments when appropriate, and signal a `wrong-type-argument` error if an argument is of the wrong type. For example, here is what happens if you pass an argument to `+` that it cannot handle:
```
(+ 2 'a)
error→ Wrong type argument: number-or-marker-p, a
```
If you want your program to handle different types differently, you must do explicit type checking. The most common way to check the type of an object is to call a *type predicate* function. Emacs has a type predicate for each type, as well as some predicates for combinations of types.
A type predicate function takes one argument; it returns `t` if the argument belongs to the appropriate type, and `nil` otherwise. Following a general Lisp convention for predicate functions, most type predicates’ names end with ‘`p`’.
Here is an example which uses the predicates `listp` to check for a list and `symbolp` to check for a symbol.
```
(defun add-on (x)
(cond ((symbolp x)
;; If X is a symbol, put it on LIST.
(setq list (cons x list)))
((listp x)
;; If X is a list, add its elements to LIST.
(setq list (append x list)))
(t
;; We handle only symbols and lists.
(error "Invalid argument %s in add-on" x))))
```
Here is a table of predefined type predicates, in alphabetical order, with references to further information.
`atom`
See [atom](list_002drelated-predicates).
`arrayp`
See [arrayp](array-functions).
`bignump`
See [floatp](predicates-on-numbers).
`bool-vector-p`
See [bool-vector-p](bool_002dvectors).
`booleanp`
See [booleanp](nil-and-t).
`bufferp`
See [bufferp](buffer-basics).
`byte-code-function-p`
See [byte-code-function-p](byte_002dcode-type).
`case-table-p`
See [case-table-p](case-tables).
`char-or-string-p`
See [char-or-string-p](predicates-for-strings).
`char-table-p`
See [char-table-p](char_002dtables).
`commandp`
See [commandp](interactive-call).
`condition-variable-p`
See [condition-variable-p](condition-variables).
`consp`
See [consp](list_002drelated-predicates).
`custom-variable-p`
See [custom-variable-p](variable-definitions).
`fixnump`
See [floatp](predicates-on-numbers).
`floatp`
See [floatp](predicates-on-numbers).
`fontp`
See [Low-Level Font](low_002dlevel-font).
`frame-configuration-p`
See [frame-configuration-p](frame-configurations).
`frame-live-p`
See [frame-live-p](deleting-frames).
`framep`
See [framep](frames).
`functionp`
See [functionp](functions).
`hash-table-p`
See [hash-table-p](other-hash).
`integer-or-marker-p`
See [integer-or-marker-p](predicates-on-markers).
`integerp`
See [integerp](predicates-on-numbers).
`keymapp`
See [keymapp](creating-keymaps).
`keywordp`
See [Constant Variables](constant-variables).
`listp`
See [listp](list_002drelated-predicates).
`markerp`
See [markerp](predicates-on-markers).
`mutexp`
See [mutexp](mutexes).
`nlistp`
See [nlistp](list_002drelated-predicates).
`number-or-marker-p`
See [number-or-marker-p](predicates-on-markers).
`numberp`
See [numberp](predicates-on-numbers).
`overlayp`
See [overlayp](overlays).
`processp`
See [processp](processes).
`recordp`
See [recordp](record-type).
`sequencep`
See [sequencep](sequence-functions).
`string-or-null-p`
See [string-or-null-p](predicates-for-strings).
`stringp`
See [stringp](predicates-for-strings).
`subrp`
See [subrp](function-cells).
`symbolp`
See [symbolp](symbols).
`syntax-table-p`
See [syntax-table-p](syntax-tables).
`threadp`
See [threadp](basic-thread-functions).
`vectorp`
See [vectorp](vectors).
`wholenump`
See [wholenump](predicates-on-numbers).
`window-configuration-p`
See [window-configuration-p](window-configurations).
`window-live-p`
See [window-live-p](deleting-windows).
`windowp` See [windowp](basic-windows).
The most general way to check the type of an object is to call the function `type-of`. Recall that each object belongs to one and only one primitive type; `type-of` tells you which one (see [Lisp Data Types](lisp-data-types)). But `type-of` knows nothing about non-primitive types. In most cases, it is more convenient to use type predicates than `type-of`.
Function: **type-of** *object*
This function returns a symbol naming the primitive type of object. The value is one of the symbols `bool-vector`, `buffer`, `char-table`, `compiled-function`, `condition-variable`, `cons`, `finalizer`, `float`, `font-entity`, `font-object`, `font-spec`, `frame`, `hash-table`, `integer`, `marker`, `mutex`, `overlay`, `process`, `string`, `subr`, `symbol`, `thread`, `vector`, `window`, or `window-configuration`. However, if object is a record, the type specified by its first slot is returned; [Records](records).
```
(type-of 1)
⇒ integer
```
```
(type-of 'nil)
⇒ symbol
(type-of '()) ; `()` is `nil`.
⇒ symbol
(type-of '(x))
⇒ cons
(type-of (record 'foo))
⇒ foo
```
elisp None ### Format of Keymaps
Each keymap is a list whose CAR is the symbol `keymap`. The remaining elements of the list define the key bindings of the keymap. A symbol whose function definition is a keymap is also a keymap. Use the function `keymapp` (see below) to test whether an object is a keymap.
Several kinds of elements may appear in a keymap, after the symbol `keymap` that begins it:
`(type . binding)`
This specifies one binding, for events of type type. Each ordinary binding applies to events of a particular *event type*, which is always a character or a symbol. See [Classifying Events](classifying-events). In this kind of binding, binding is a command.
`(type item-name . binding)`
This specifies a binding which is also a simple menu item that displays as item-name in the menu. See [Simple Menu Items](simple-menu-items).
`(type item-name help-string . binding)`
This is a simple menu item with help string help-string.
`(type menu-item . details)`
This specifies a binding which is also an extended menu item. This allows use of other features. See [Extended Menu Items](extended-menu-items).
`(t . binding)`
This specifies a *default key binding*; any event not bound by other elements of the keymap is given binding as its binding. Default bindings allow a keymap to bind all possible event types without having to enumerate all of them. A keymap that has a default binding completely masks any lower-precedence keymap, except for events explicitly bound to `nil` (see below).
`char-table`
If an element of a keymap is a char-table, it counts as holding bindings for all character events with no modifier bits (see [modifier bits](other-char-bits#modifier-bits)): the element whose index is c is the binding for the character c. This is a compact way to record lots of bindings. A keymap with such a char-table is called a *full keymap*. Other keymaps are called *sparse keymaps*.
`vector`
This kind of element is similar to a char-table: the element whose index is c is the binding for the character c. Since the range of characters that can be bound this way is limited by the vector size, and vector creation allocates space for all character codes from 0 up, this format should not be used except for creating menu keymaps (see [Menu Keymaps](menu-keymaps)), where the bindings themselves don’t matter.
`string`
Aside from elements that specify bindings for keys, a keymap can also have a string as an element. This is called the *overall prompt string* and makes it possible to use the keymap as a menu. See [Defining Menus](defining-menus).
`(keymap …)` If an element of a keymap is itself a keymap, it counts as if this inner keymap were inlined in the outer keymap. This is used for multiple-inheritance, such as in `make-composed-keymap`.
When the binding is `nil`, it doesn’t constitute a definition but it does take precedence over a default binding or a binding in the parent keymap. On the other hand, a binding of `nil` does *not* override lower-precedence keymaps; thus, if the local map gives a binding of `nil`, Emacs uses the binding from the global map.
Keymaps do not directly record bindings for the meta characters. Instead, meta characters are regarded for purposes of key lookup as sequences of two characters, the first of which is ESC (or whatever is currently the value of `meta-prefix-char`). Thus, the key `M-a` is internally represented as `ESC a`, and its global binding is found at the slot for `a` in `esc-map` (see [Prefix Keys](prefix-keys)).
This conversion applies only to characters, not to function keys or other input events; thus, `M-end` has nothing to do with `ESC end`.
Here as an example is the local keymap for Lisp mode, a sparse keymap. It defines bindings for DEL, `C-c C-z`, `C-M-q`, and `C-M-x` (the actual value also contains a menu binding, which is omitted here for the sake of brevity).
```
lisp-mode-map
⇒
```
```
(keymap
(3 keymap
;; C-c C-z
(26 . run-lisp))
```
```
(27 keymap
;; `C-M-x`, treated as `ESC C-x`
(24 . lisp-send-defun))
```
```
;; This part is inherited from `lisp-mode-shared-map`.
keymap
;; DEL
(127 . backward-delete-char-untabify)
```
```
(27 keymap
;; `C-M-q`, treated as `ESC C-q`
(17 . indent-sexp)))
```
Function: **keymapp** *object*
This function returns `t` if object is a keymap, `nil` otherwise. More precisely, this function tests for a list whose CAR is `keymap`, or for a symbol whose function definition satisfies `keymapp`.
```
(keymapp '(keymap))
⇒ t
```
```
(fset 'foo '(keymap))
(keymapp 'foo)
⇒ t
```
```
(keymapp (current-global-map))
⇒ t
```
elisp None #### Key Sequence Input
The command loop reads input a key sequence at a time, by calling `read-key-sequence`. Lisp programs can also call this function; for example, `describe-key` uses it to read the key to describe.
Function: **read-key-sequence** *prompt &optional continue-echo dont-downcase-last switch-frame-ok command-loop*
This function reads a key sequence and returns it as a string or vector. It keeps reading events until it has accumulated a complete key sequence; that is, enough to specify a non-prefix command using the currently active keymaps. (Remember that a key sequence that starts with a mouse event is read using the keymaps of the buffer in the window that the mouse was in, not the current buffer.)
If the events are all characters and all can fit in a string, then `read-key-sequence` returns a string (see [Strings of Events](strings-of-events)). Otherwise, it returns a vector, since a vector can hold all kinds of events—characters, symbols, and lists. The elements of the string or vector are the events in the key sequence.
Reading a key sequence includes translating the events in various ways. See [Translation Keymaps](translation-keymaps).
The argument prompt is either a string to be displayed in the echo area as a prompt, or `nil`, meaning not to display a prompt. The argument continue-echo, if non-`nil`, means to echo this key as a continuation of the previous key.
Normally any upper case event is converted to lower case if the original event is undefined and the lower case equivalent is defined. The argument dont-downcase-last, if non-`nil`, means do not convert the last event to lower case. This is appropriate for reading a key sequence to be defined.
The argument switch-frame-ok, if non-`nil`, means that this function should process a `switch-frame` event if the user switches frames before typing anything. If the user switches frames in the middle of a key sequence, or at the start of the sequence but switch-frame-ok is `nil`, then the event will be put off until after the current key sequence.
The argument command-loop, if non-`nil`, means that this key sequence is being read by something that will read commands one after another. It should be `nil` if the caller will read just one key sequence.
In the following example, Emacs displays the prompt ‘`?`’ in the echo area, and then the user types `C-x C-f`.
```
(read-key-sequence "?")
```
```
---------- Echo Area ----------
?C-x C-f
---------- Echo Area ----------
⇒ "^X^F"
```
The function `read-key-sequence` suppresses quitting: `C-g` typed while reading with this function works like any other character, and does not set `quit-flag`. See [Quitting](quitting).
Function: **read-key-sequence-vector** *prompt &optional continue-echo dont-downcase-last switch-frame-ok command-loop*
This is like `read-key-sequence` except that it always returns the key sequence as a vector, never as a string. See [Strings of Events](strings-of-events).
If an input character is upper-case (or has the shift modifier) and has no key binding, but its lower-case equivalent has one, then `read-key-sequence` converts the character to lower case. Note that `lookup-key` does not perform case conversion in this way.
When reading input results in such a *shift-translation*, Emacs sets the variable `this-command-keys-shift-translated` to a non-`nil` value. Lisp programs can examine this variable if they need to modify their behavior when invoked by shift-translated keys. For example, the function `handle-shift-selection` examines the value of this variable to determine how to activate or deactivate the region (see [handle-shift-selection](the-mark)).
The function `read-key-sequence` also transforms some mouse events. It converts unbound drag events into click events, and discards unbound button-down events entirely. It also reshuffles focus events and miscellaneous window events so that they never appear in a key sequence with any other events.
When mouse events occur in special parts of a window or frame, such as a mode line or a scroll bar, the event type shows nothing special—it is the same symbol that would normally represent that combination of mouse button and modifier keys. The information about the window part is kept elsewhere in the event—in the coordinates. But `read-key-sequence` translates this information into imaginary prefix keys, all of which are symbols: `tab-line`, `header-line`, `horizontal-scroll-bar`, `menu-bar`, `tab-bar`, `mode-line`, `vertical-line`, `vertical-scroll-bar`, `left-margin`, `right-margin`, `left-fringe`, `right-fringe`, `right-divider`, and `bottom-divider`. You can define meanings for mouse clicks in special window parts by defining key sequences using these imaginary prefix keys.
For example, if you call `read-key-sequence` and then click the mouse on the window’s mode line, you get two events, like this:
```
(read-key-sequence "Click on the mode line: ")
⇒ [mode-line
(mouse-1
(#<window 6 on NEWS> mode-line
(40 . 63) 5959987))]
```
Variable: **num-input-keys**
This variable’s value is the number of key sequences processed so far in this Emacs session. This includes key sequences read from the terminal and key sequences read from keyboard macros being executed.
elisp None ### Acknowledgments
This manual was originally written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert J. Chassell helped to review and edit the manual, with the support of the Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by Warren A. Hunt, Jr. of Computational Logic, Inc. Additional sections have since been written by Miles Bader, Lars Brinkhoff, Chong Yidong, Kenichi Handa, Lute Kamstra, Juri Linkov, Glenn Morris, Thien-Thi Nguyen, Dan Nicolaescu, Martin Rudalics, Kim F. Storm, Luc Teirlinck, and Eli Zaretskii, and others.
Corrections were supplied by Drew Adams, Juanma Barranquero, Karl Berry, Jim Blandy, Bard Bloom, Stephane Boucher, David Boyes, Alan Carroll, Richard Davis, Lawrence R. Dodd, Peter Doornbosch, David A. Duff, Chris Eich, Beverly Erlebacher, David Eckelkamp, Ralf Fassel, Eirik Fuller, Stephen Gildea, Bob Glickstein, Eric Hanchrow, Jesper Harder, George Hartzell, Nathan Hess, Masayuki Ida, Dan Jacobson, Jak Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, Glenn M. Lewis, K. Richard Magill, Brian Marick, Roland McGrath, Stefan Monnier, Skip Montanaro, John Gardiner Myers, Thomas A. Peterson, Francesco Potortì, Friedrich Pukelsheim, Arnold D. Robbins, Raul Rockwell, Jason Rumney, Per Starbäck, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Rickard Westman, Jean White, Eduard Wiebe, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn.
For a more complete list of contributors, please see the relevant change log entries in the Emacs source repository.
elisp None #### Terminal-Specific Initialization
Each terminal type can have its own Lisp library that Emacs loads when run on that type of terminal. The library’s name is constructed by concatenating the value of the variable `term-file-prefix` and the terminal type (specified by the environment variable `TERM`). Normally, `term-file-prefix` has the value `"term/"`; changing this is not recommended. If there is an entry matching `TERM` in the `term-file-aliases` association list, Emacs uses the associated value in place of `TERM`. Emacs finds the file in the normal manner, by searching the `load-path` directories, and trying the ‘`.elc`’ and ‘`.el`’ suffixes.
The usual role of a terminal-specific library is to enable special keys to send sequences that Emacs can recognize. It may also need to set or add to `input-decode-map` if the Termcap or Terminfo entry does not specify all the terminal’s function keys. See [Terminal Input](terminal-input).
When the name of the terminal type contains a hyphen or underscore, and no library is found whose name is identical to the terminal’s name, Emacs strips from the terminal’s name the last hyphen or underscore and everything that follows it, and tries again. This process is repeated until Emacs finds a matching library, or until there are no more hyphens or underscores in the name (i.e., there is no terminal-specific library). For example, if the terminal name is ‘`xterm-256color`’ and there is no `term/xterm-256color.el` library, Emacs tries to load `term/xterm.el`. If necessary, the terminal library can evaluate `(getenv "TERM")` to find the full name of the terminal type.
Your init file can prevent the loading of the terminal-specific library by setting the variable `term-file-prefix` to `nil`.
You can also arrange to override some of the actions of the terminal-specific library by using `tty-setup-hook`. This is a normal hook that Emacs runs after initializing a new text terminal. You could use this hook to define initializations for terminals that do not have their own libraries. See [Hooks](hooks).
User Option: **term-file-prefix**
If the value of this variable is non-`nil`, Emacs loads a terminal-specific initialization file as follows:
```
(load (concat term-file-prefix (getenv "TERM")))
```
You may set the `term-file-prefix` variable to `nil` in your init file if you do not wish to load the terminal-initialization file.
On MS-DOS, Emacs sets the `TERM` environment variable to ‘`internal`’.
User Option: **term-file-aliases**
This variable is an association list mapping terminal types to their aliases. For example, an element of the form `("vt102"
. "vt100")` means to treat a terminal of type ‘`vt102`’ like one of type ‘`vt100`’.
Variable: **tty-setup-hook**
This variable is a normal hook that Emacs runs after initializing a new text terminal. (This applies when Emacs starts up in non-windowed mode, and when making a tty `emacsclient` connection.) The hook runs after loading your init file (if applicable) and the terminal-specific Lisp file, so you can use it to adjust the definitions made by that file.
For a related feature, see [window-setup-hook](init-file).
| programming_docs |
elisp None ### Mode Line Format
Each Emacs window (aside from minibuffer windows) typically has a mode line at the bottom, which displays status information about the buffer displayed in the window. The mode line contains information about the buffer, such as its name, associated file, depth of recursive editing, and major and minor modes. A window can also have a *header line*, which is much like the mode line but appears at the top of the window.
This section describes how to control the contents of the mode line and header line. We include it in this chapter because much of the information displayed in the mode line relates to the enabled major and minor modes.
| | | |
| --- | --- | --- |
| • [Base](mode-line-basics) | | Basic ideas of mode line control. |
| • [Data](mode-line-data) | | The data structure that controls the mode line. |
| • [Top](mode-line-top) | | The top level variable, mode-line-format. |
| • [Mode Line Variables](mode-line-variables) | | Variables used in that data structure. |
| • [%-Constructs](_0025_002dconstructs) | | Putting information into a mode line. |
| • [Properties in Mode](properties-in-mode) | | Using text properties in the mode line. |
| • [Header Lines](header-lines) | | Like a mode line, but at the top. |
| • [Emulating Mode Line](emulating-mode-line) | | Formatting text as the mode line would. |
elisp None ### Timers for Delayed Execution
You can set up a *timer* to call a function at a specified future time or after a certain length of idleness. A timer is a special object that stores the information about the next invocation times and the function to invoke.
Function: **timerp** *object*
This predicate function returns non-`nil` if `object` is a timer.
Emacs cannot run timers at any arbitrary point in a Lisp program; it can run them only when Emacs could accept output from a subprocess: namely, while waiting or inside certain primitive functions such as `sit-for` or `read-event` which *can* wait. Therefore, a timer’s execution may be delayed if Emacs is busy. However, the time of execution is very precise if Emacs is idle.
Emacs binds `inhibit-quit` to `t` before calling the timer function, because quitting out of many timer functions can leave things in an inconsistent state. This is normally unproblematical because most timer functions don’t do a lot of work. Indeed, for a timer to call a function that takes substantial time to run is likely to be annoying. If a timer function needs to allow quitting, it should use `with-local-quit` (see [Quitting](quitting)). For example, if a timer function calls `accept-process-output` to receive output from an external process, that call should be wrapped inside `with-local-quit`, to ensure that `C-g` works if the external process hangs.
It is usually a bad idea for timer functions to alter buffer contents. When they do, they usually should call `undo-boundary` both before and after changing the buffer, to separate the timer’s changes from user commands’ changes and prevent a single undo entry from growing to be quite large.
Timer functions should also avoid calling functions that cause Emacs to wait, such as `sit-for` (see [Waiting](waiting)). This can lead to unpredictable effects, since other timers (or even the same timer) can run while waiting. If a timer function needs to perform an action after a certain time has elapsed, it can do this by scheduling a new timer.
If a timer function performs a remote file operation, it can be in conflict with an already running remote file operation of the same connection. Such conflicts are detected, and they result in a `remote-file-error` error (see [Standard Errors](standard-errors)). This should be protected by wrapping the timer function body with
```
(ignore-error 'remote-file-error
…)
```
If a timer function calls functions that can change the match data, it should save and restore the match data. See [Saving Match Data](saving-match-data).
Command: **run-at-time** *time repeat function &rest args*
This sets up a timer that calls the function function with arguments args at time time. If repeat is a number (integer or floating point), the timer is scheduled to run again every repeat seconds after time. If repeat is `nil`, the timer runs only once.
time may specify an absolute or a relative time.
Absolute times may be specified using a string with a limited variety of formats, and are taken to be times *today*, even if already in the past. The recognized forms are ‘`xxxx`’, ‘`x:xx`’, or ‘`xx:xx`’ (military time), and ‘`xxam`’, ‘`xxAM`’, ‘`xxpm`’, ‘`xxPM`’, ‘`xx:xxam`’, ‘`xx:xxAM`’, ‘`xx:xxpm`’, or ‘`xx:xxPM`’. A period can be used instead of a colon to separate the hour and minute parts.
To specify a relative time as a string, use numbers followed by units. For example:
‘`1 min`’ denotes 1 minute from now.
‘`1 min 5 sec`’ denotes 65 seconds from now.
‘`1 min 2 sec 3 hour 4 day 5 week 6 fortnight 7 month 8 year`’ denotes exactly 103 months, 123 days, and 10862 seconds from now.
For relative time values, Emacs considers a month to be exactly thirty days, and a year to be exactly 365.25 days.
Not all convenient formats are strings. If time is a number (integer or floating point), that specifies a relative time measured in seconds. The result of `encode-time` can also be used to specify an absolute value for time.
In most cases, repeat has no effect on when *first* call takes place—time alone specifies that. There is one exception: if time is `t`, then the timer runs whenever the time is a multiple of repeat seconds after the epoch. This is useful for functions like `display-time`.
If Emacs didn’t get any CPU time when the timer would have run (for example if the system was busy running another process or if the computer was sleeping or in a suspended state), the timer will run as soon as Emacs resumes and is idle.
The function `run-at-time` returns a timer value that identifies the particular scheduled future action. You can use this value to call `cancel-timer` (see below).
Command: **run-with-timer** *secs repeat function &rest args*
This is exactly the same as `run-at-time` (so see that definition for an explanation of the parameters; secs is passed as time to that function), but is meant to be used when the delay is specified in seconds.
A repeating timer nominally ought to run every repeat seconds, but remember that any invocation of a timer can be late. Lateness of one repetition has no effect on the scheduled time of the next repetition. For instance, if Emacs is busy computing for long enough to cover three scheduled repetitions of the timer, and then starts to wait, it will immediately call the timer function three times in immediate succession (presuming no other timers trigger before or between them). If you want a timer to run again no less than n seconds after the last invocation, don’t use the repeat argument. Instead, the timer function should explicitly reschedule the timer.
User Option: **timer-max-repeats**
This variable’s value specifies the maximum number of times to repeat calling a timer function in a row, when many previously scheduled calls were unavoidably delayed.
Macro: **with-timeout** *(seconds timeout-forms…) body…*
Execute body, but give up after seconds seconds. If body finishes before the time is up, `with-timeout` returns the value of the last form in body. If, however, the execution of body is cut short by the timeout, then `with-timeout` executes all the timeout-forms and returns the value of the last of them.
This macro works by setting a timer to run after seconds seconds. If body finishes before that time, it cancels the timer. If the timer actually runs, it terminates execution of body, then executes timeout-forms.
Since timers can run within a Lisp program only when the program calls a primitive that can wait, `with-timeout` cannot stop executing body while it is in the midst of a computation—only when it calls one of those primitives. So use `with-timeout` only with a body that waits for input, not one that does a long computation.
The function `y-or-n-p-with-timeout` provides a simple way to use a timer to avoid waiting too long for an answer. See [Yes-or-No Queries](yes_002dor_002dno-queries).
Function: **cancel-timer** *timer*
This cancels the requested action for timer, which should be a timer—usually, one previously returned by `run-at-time` or `run-with-idle-timer`. This cancels the effect of that call to one of these functions; the arrival of the specified time will not cause anything special to happen.
The `list-timers` command lists all the currently active timers. The command `c` (`timer-list-cancel`) will cancel the timer on the line under point. You can sort the list by column using the command `S` (`tabulated-list-sort`).
elisp None #### Low-Level Font Representation
Normally, it is not necessary to manipulate fonts directly. In case you need to do so, this section explains how.
In Emacs Lisp, fonts are represented using three different Lisp object types: *font objects*, *font specs*, and *font entities*.
Function: **fontp** *object &optional type*
Return `t` if object is a font object, font spec, or font entity. Otherwise, return `nil`.
The optional argument type, if non-`nil`, determines the exact type of Lisp object to check for. In that case, type should be one of `font-object`, `font-spec`, or `font-entity`.
A font object is a Lisp object that represents a font that Emacs has *opened*. Font objects cannot be modified in Lisp, but they can be inspected.
Function: **font-at** *position &optional window string*
Return the font object that is being used to display the character at position position in the window window. If window is `nil`, it defaults to the selected window. If string is `nil`, position specifies a position in the current buffer; otherwise, string should be a string, and position specifies a position in that string.
A font spec is a Lisp object that contains a set of specifications that can be used to find a font. More than one font may match the specifications in a font spec.
Function: **font-spec** *&rest arguments*
Return a new font spec using the specifications in arguments, which should come in `property`-`value` pairs. The possible specifications are as follows:
`:name`
The font name (a string), in either XLFD, Fontconfig, or GTK+ format. See [Fonts](https://www.gnu.org/software/emacs/manual/html_node/emacs/Fonts.html#Fonts) in The GNU Emacs Manual.
`:family` `:foundry` `:weight` `:slant` `:width`
These have the same meanings as the face attributes of the same name. See [Face Attributes](face-attributes). `:family` and `:foundry` are strings, while the other three are symbols. As example values, `:slant` may be `italic`, `:weight` may be `bold` and `:width` may be `normal`.
`:size`
The font size—either a non-negative integer that specifies the pixel size, or a floating-point number that specifies the point size.
`:adstyle`
Additional typographic style information for the font, such as ‘`sans`’. The value should be a string or a symbol.
`:registry`
The charset registry and encoding of the font, such as ‘`iso8859-1`’. The value should be a string or a symbol.
`:dpi`
The resolution in dots per inch for which the font is designed. The value must be a non-negative number.
`:spacing`
The spacing of the font: proportional, dual, mono, or charcell. The value should be either an integer (0 for proportional, 90 for dual, 100 for mono, 110 for charcell) or a one-letter symbol (one of `P`, `D`, `M`, or `C`).
`:avgwidth`
The average width of the font in 1/10 pixel units. The value should be a non-negative number.
`:script`
The script that the font must support (a symbol).
`:lang`
The language that the font should support. The value should be a symbol whose name is a two-letter ISO-639 language name. On X, the value is matched against the “Additional Style” field of the XLFD name of a font, if it is non-empty. On MS-Windows, fonts matching the spec are required to support codepages needed for the language. Currently, only a small set of CJK languages is supported with this property: ‘`ja`’, ‘`ko`’, and ‘`zh`’.
`:otf`
The font must be an OpenType font that supports these OpenType features, provided Emacs is compiled with a library, such as ‘`libotf`’ on GNU/Linux, that supports complex text layout for scripts which need that. The value must be a list of the form
```
(script-tag langsys-tag gsub gpos)
```
where script-tag is the OpenType script tag symbol; langsys-tag is the OpenType language system tag symbol, or `nil` to use the default language system; `gsub` is a list of OpenType GSUB feature tag symbols, or `nil` if none is required; and `gpos` is a list of OpenType GPOS feature tag symbols, or `nil` if none is required. If `gsub` or `gpos` is a list, a `nil` element in that list means that the font must not match any of the remaining tag symbols. The `gpos` element may be omitted. For the list of OpenType script, language, and feature tags, see [the list of registered OTF tags](https://docs.microsoft.com/en-us/typography/opentype/spec/ttoreg).
`:type`
The symbol that specifies the *font backend* used to draw the characters. The possible values depend on the platform and on how Emacs was configured at build time. Typical values include `ftcrhb` and `xfthb` on X, `harfbuzz` on MS-Windows, `ns` on GNUstep, etc. It can also be `nil` if left unspecified, typically in a font-spec.
Function: **font-put** *font-spec property value*
Set the font property property in the font-spec font-spec to value. The property can any of the ones described above.
A font entity is a reference to a font that need not be open. Its properties are intermediate between a font object and a font spec: like a font object, and unlike a font spec, it refers to a single, specific font. Unlike a font object, creating a font entity does not load the contents of that font into computer memory. Emacs may open multiple font objects of different sizes from a single font entity referring to a scalable font.
Function: **find-font** *font-spec &optional frame*
This function returns a font entity that best matches the font spec font-spec on frame frame. If frame is `nil`, it defaults to the selected frame.
Function: **list-fonts** *font-spec &optional frame num prefer*
This function returns a list of all font entities that match the font spec font-spec.
The optional argument frame, if non-`nil`, specifies the frame on which the fonts are to be displayed. The optional argument num, if non-`nil`, should be an integer that specifies the maximum length of the returned list. The optional argument prefer, if non-`nil`, should be another font spec, which is used to control the order of the returned list; the returned font entities are sorted in order of decreasing closeness to that font spec.
If you call `set-face-attribute` and pass a font spec, font entity, or font name string as the value of the `:font` attribute, Emacs opens the best matching font that is available for display. It then stores the corresponding font object as the actual value of the `:font` attribute for that face.
The following functions can be used to obtain information about a font. For these functions, the font argument can be a font object, a font entity, or a font spec.
Function: **font-get** *font property*
This function returns the value of the font property property for font. The property can any of the ones that `font-spec` supports.
If font is a font spec and the font spec does not specify property, the return value is `nil`. If font is a font object or font entity, the value for the :script property may be a list of scripts supported by the font, and the value of the `:otf` property is a cons of the form `(gsub . gpos)`, where gsub and gpos are lists representing OpenType features supported by the font, of the form
```
((script-tag (langsys-tag feature…) …) …)
```
where script-tag, langsys-tag, and feature are symbols representing OpenType layout tags.
If font is a font object, the special property `:combining-capability` is non-`nil` if the font backend of font supports rendering of combining characters for non-OpenType fonts.
Function: **font-face-attributes** *font &optional frame*
This function returns a list of face attributes corresponding to font. The optional argument frame specifies the frame on which the font is to be displayed. If it is `nil`, the selected frame is used. The return value has the form
```
(:family family :height height :weight weight
:slant slant :width width)
```
where the values of family, height, weight, slant, and width are face attribute values. Some of these key-attribute pairs may be omitted from the list if they are not specified by font.
Function: **font-xlfd-name** *font &optional fold-wildcards*
This function returns the XLFD (X Logical Font Descriptor), a string, matching font. See [Fonts](https://www.gnu.org/software/emacs/manual/html_node/emacs/Fonts.html#Fonts) in The GNU Emacs Manual, for information about XLFDs. If the name is too long for an XLFD (which can contain at most 255 characters), the function returns `nil`.
If the optional argument fold-wildcards is non-`nil`, consecutive wildcards in the XLFD are folded into one.
The following two functions return important information about a font.
Function: **font-info** *name &optional frame*
This function returns information about a font specified by its name, a string, as it is used on frame. If frame is omitted or `nil`, it defaults to the selected frame.
The value returned by the function is a vector of the form `[opened-name full-name size height
baseline-offset relative-compose default-ascent
max-width ascent descent space-width
average-width filename capability]`. Here’s the description of each components of this vector:
opened-name
The name used to open the font, a string.
full-name
The full name of the font, a string.
size
The pixel size of the font.
height
The height of the font in pixels.
baseline-offset
The offset in pixels from the ASCII baseline, positive upward.
relative-compose default-ascent
Numbers controlling how to compose characters.
max-width
The maximum advance width of the font.
ascent descent
The ascent and descent of this font. The sum of these two numbers should be equal to the value of height above.
space-width
The width, in pixels, of the font’s space character.
average-width
The average width of the font characters. If this is zero, Emacs uses the value of space-width instead, when it calculates text layout on display.
filename
The file name of the font as a string. This can be `nil` if the font back-end does not provide a way to find out the font’s file name.
capability A list whose first element is a symbol representing the font type, one of `x`, `opentype`, `truetype`, `type1`, `pcf`, or `bdf`. For OpenType fonts, the list includes 2 additional elements describing the GSUB and GPOS features supported by the font. Each of these elements is a list of the form `((script (langsys feature …) …)
…)`, where script is a symbol representing an OpenType script tag, langsys is a symbol representing an OpenType langsys tag (or `nil`, which stands for the default langsys), and each feature is a symbol representing an OpenType feature tag.
Function: **query-font** *font-object*
This function returns information about a font-object. (This is in contrast to `font-info`, which takes the font name, a string, as its argument.)
The value returned by the function is a vector of the form `[name filename pixel-size max-width
ascent descent space-width average-width
capability]`. Here’s the description of each components of this vector:
name
The font name, a string.
filename
The file name of the font as a string. This can be `nil` if the font back-end does not provide a way to find out the font’s file name.
pixel-size
The pixel size of the font used to open the font.
max-width
The maximum advance width of the font.
ascent descent
The ascent and descent of this font. The sum of these two numbers gives the font height.
space-width
The width, in pixels, of the font’s space character.
average-width
The average width of the font characters. If this is zero, Emacs uses the value of space-width instead, when it calculates text layout on display.
capability A list whose first element is a symbol representing the font type, one of `x`, `opentype`, `truetype`, `type1`, `pcf`, or `bdf`. For OpenType fonts, the list includes 2 additional elements describing the GSUB and GPOS features supported by the font. Each of these elements is a list of the form `((script (langsys feature …) …)
…)`, where script is a symbol representing an OpenType script tag, langsys is a symbol representing an OpenType langsys tag (or `nil`, which stands for the default langsys), and each feature is a symbol representing an OpenType feature tag.
The following four functions return size information about fonts used by various faces, allowing various layout considerations in Lisp programs. These functions take face remapping into consideration, returning information about the remapped face, if the face in question was remapped. See [Face Remapping](face-remapping).
Function: **default-font-width**
This function returns the average width in pixels of the font used by the current buffer’s default face, as that face is defined for the selected frame.
Function: **default-font-height**
This function returns the height in pixels of the font used by the current buffer’s default face, as that face is defined for the selected frame.
Function: **window-font-width** *&optional window face*
This function returns the average width in pixels for the font used by face in window. The specified window must be a live window. If `nil` or omitted, window defaults to the selected window, and face defaults to the default face in window.
Function: **window-font-height** *&optional window face*
This function returns the height in pixels for the font used by face in window. The specified window must be a live window. If `nil` or omitted, window defaults to the selected window, and face defaults to the default face in window.
| programming_docs |
elisp None ### Commands for Binding Keys
This section describes some convenient interactive interfaces for changing key bindings. They work by calling `define-key`.
People often use `global-set-key` in their init files (see [Init File](init-file)) for simple customization. For example,
```
(global-set-key (kbd "C-x C-\\") 'next-line)
```
or
```
(global-set-key [?\C-x ?\C-\\] 'next-line)
```
or
```
(global-set-key [(control ?x) (control ?\\)] 'next-line)
```
redefines `C-x C-\` to move down a line.
```
(global-set-key [M-mouse-1] 'mouse-set-point)
```
redefines the first (leftmost) mouse button, entered with the Meta key, to set point where you click.
Be careful when using non-ASCII text characters in Lisp specifications of keys to bind. If these are read as multibyte text, as they usually will be in a Lisp file (see [Loading Non-ASCII](loading-non_002dascii)), you must type the keys as multibyte too. For instance, if you use this:
```
(global-set-key "ö" 'my-function) ; bind o-umlaut
```
or
```
(global-set-key ?ö 'my-function) ; bind o-umlaut
```
and your language environment is multibyte Latin-1, these commands actually bind the multibyte character with code 246, not the byte code 246 (`M-v`) sent by a Latin-1 terminal. In order to use this binding, you need to teach Emacs how to decode the keyboard by using an appropriate input method (see [Input Methods](https://www.gnu.org/software/emacs/manual/html_node/emacs/Input-Methods.html#Input-Methods) in The GNU Emacs Manual).
Command: **global-set-key** *key binding*
This function sets the binding of key in the current global map to binding.
```
(global-set-key key binding)
≡
(define-key (current-global-map) key binding)
```
Command: **global-unset-key** *key*
This function removes the binding of key from the current global map.
One use of this function is in preparation for defining a longer key that uses key as a prefix—which would not be allowed if key has a non-prefix binding. For example:
```
(global-unset-key "\C-l")
⇒ nil
```
```
(global-set-key "\C-l\C-l" 'redraw-display)
⇒ nil
```
This function is equivalent to using `define-key` as follows:
```
(global-unset-key key)
≡
(define-key (current-global-map) key nil)
```
Command: **local-set-key** *key binding*
This function sets the binding of key in the current local keymap to binding.
```
(local-set-key key binding)
≡
(define-key (current-local-map) key binding)
```
Command: **local-unset-key** *key*
This function removes the binding of key from the current local map.
```
(local-unset-key key)
≡
(define-key (current-local-map) key nil)
```
elisp None ### Time Zone Rules
The default time zone is determined by the `TZ` environment variable. See [System Environment](system-environment). For example, you can tell Emacs to default to Universal Time with `(setenv "TZ" "UTC0")`. If `TZ` is not in the environment, Emacs uses system wall clock time, which is a platform-dependent default time zone.
The set of supported `TZ` strings is system-dependent. GNU and many other systems support the tzdata database, e.g., ‘`"America/New\_York"`’ specifies the time zone and daylight saving time history for locations near New York City. GNU and most other systems support POSIX-style `TZ` strings, e.g., ‘`"EST+5EDT,M4.1.0/2,M10.5.0/2"`’ specifies the rules used in New York from 1987 through 2006. All systems support the string ‘`"UTC0"`’ meaning Universal Time.
Functions that convert to and from local time accept an optional *time zone rule* argument, which specifies the conversion’s time zone and daylight saving time history. If the time zone rule is omitted or `nil`, the conversion uses Emacs’s default time zone. If it is `t`, the conversion uses Universal Time. If it is `wall`, the conversion uses the system wall clock time. If it is a string, the conversion uses the time zone rule equivalent to setting `TZ` to that string. If it is a list (offset abbr), where offset is an integer number of seconds east of Universal Time and abbr is a string, the conversion uses a fixed time zone with the given offset and abbreviation. An integer offset is treated as if it were (offset abbr), where abbr is a numeric abbreviation on POSIX-compatible platforms and is unspecified on MS-Windows.
Function: **current-time-zone** *&optional time zone*
This function returns a list describing the time zone that the user is in.
The value has the form `(offset abbr)`. Here offset is an integer giving the number of seconds ahead of Universal Time (east of Greenwich). A negative value means west of Greenwich. The second element, abbr, is a string giving an abbreviation for the time zone, e.g., ‘`"CST"`’ for China Standard Time or for U.S. Central Standard Time. Both elements can change when daylight saving time begins or ends; if the user has specified a time zone that does not use a seasonal time adjustment, then the value is constant through time.
If the operating system doesn’t supply all the information necessary to compute the value, the unknown elements of the list are `nil`.
The argument time, if given, specifies a time value to analyze instead of the current time. The optional argument zone defaults to the current time zone rule. The operating system limits the range of time and zone values.
elisp None ### Terminal Input
This section describes functions and variables for recording or manipulating terminal input. See [Display](display), for related functions.
| | | |
| --- | --- | --- |
| • [Input Modes](input-modes) | | Options for how input is processed. |
| • [Recording Input](recording-input) | | Saving histories of recent or all input events. |
elisp None #### Glyphs
A *glyph* is a graphical symbol which occupies a single character position on the screen. Each glyph is represented in Lisp as a *glyph code*, which specifies a character and optionally a face to display it in (see [Faces](faces)). The main use of glyph codes is as the entries of display tables (see [Display Tables](display-tables)). The following functions are used to manipulate glyph codes:
Function: **make-glyph-code** *char &optional face*
This function returns a glyph code representing char char with face face. If face is omitted or `nil`, the glyph uses the default face; in that case, the glyph code is an integer. If face is non-`nil`, the glyph code is not necessarily an integer object.
Function: **glyph-char** *glyph*
This function returns the character of glyph code glyph.
Function: **glyph-face** *glyph*
This function returns face of glyph code glyph, or `nil` if glyph uses the default face.
You can set up a *glyph table* to change how glyph codes are actually displayed on text terminals. This feature is semi-obsolete; use `glyphless-char-display` instead (see [Glyphless Chars](glyphless-chars)).
Variable: **glyph-table**
The value of this variable, if non-`nil`, is the current glyph table. It takes effect only on character terminals; on graphical displays, all glyphs are displayed literally. The glyph table should be a vector whose gth element specifies how to display glyph code g, where g is the glyph code for a glyph whose face is unspecified. Each element should be one of the following:
`nil`
Display this glyph literally.
a string
Display this glyph by sending the specified string to the terminal.
a glyph code Display the specified glyph code instead.
Any integer glyph code greater than or equal to the length of the glyph table is displayed literally.
elisp None ### Saving Abbrevs in Files
A file of saved abbrev definitions is actually a file of Lisp code. The abbrevs are saved in the form of a Lisp program to define the same abbrev tables with the same contents. Therefore, you can load the file with `load` (see [How Programs Do Loading](how-programs-do-loading)). However, the function `quietly-read-abbrev-file` is provided as a more convenient interface. Emacs automatically calls this function at startup.
User-level facilities such as `save-some-buffers` can save abbrevs in a file automatically, under the control of variables described here.
User Option: **abbrev-file-name**
This is the default file name for reading and saving abbrevs. By default, Emacs will look for `~/.emacs.d/abbrev\_defs`, and, if not found, for `~/.abbrev\_defs`; if neither file exists, Emacs will create `~/.emacs.d/abbrev\_defs`.
Function: **quietly-read-abbrev-file** *&optional filename*
This function reads abbrev definitions from a file named filename, previously written with `write-abbrev-file`. If filename is omitted or `nil`, the file specified in `abbrev-file-name` is used.
As the name implies, this function does not display any messages.
User Option: **save-abbrevs**
A non-`nil` value for `save-abbrevs` means that Emacs should offer to save abbrevs (if any have changed) when files are saved. If the value is `silently`, Emacs saves the abbrevs without asking the user. `abbrev-file-name` specifies the file to save the abbrevs in. The default value is `t`.
Variable: **abbrevs-changed**
This variable is set non-`nil` by defining or altering any abbrevs (except system abbrevs). This serves as a flag for various Emacs commands to offer to save your abbrevs.
Command: **write-abbrev-file** *&optional filename*
Save all abbrev definitions (except system abbrevs), for all abbrev tables listed in `abbrev-table-name-list`, in the file filename, in the form of a Lisp program that when loaded will define the same abbrevs. Tables that do not have any abbrevs to save are omitted. If filename is `nil` or omitted, `abbrev-file-name` is used. This function returns `nil`.
elisp None ### Hooks for Window Scrolling and Changes
This section describes how Lisp programs can take action after a window has been scrolled or other window modifications occurred. We first consider the case where a window shows a different part of its buffer.
Variable: **window-scroll-functions**
This variable holds a list of functions that Emacs should call before redisplaying a window with scrolling. Displaying a different buffer in a window and making a new window also call these functions.
This variable is not a normal hook, because each function is called with two arguments: the window, and its new display-start position. At the time of the call, the display-start position of the argument window is already set to its new value, and the buffer to be displayed in the window is set as the current buffer.
These functions must take care when using `window-end` (see [Window Start and End](window-start-and-end)); if you need an up-to-date value, you must use the update argument to ensure you get it.
**Warning:** don’t use this feature to alter the way the window is scrolled. It’s not designed for that, and such use probably won’t work.
In addition, you can use `jit-lock-register` to register a Font Lock fontification function, which will be called whenever parts of a buffer are (re)fontified because a window was scrolled or its size changed. See [Other Font Lock Variables](other-font-lock-variables).
The remainder of this section covers six hooks that are called during redisplay provided a significant, non-scrolling change of a window has been detected. For simplicity, these hooks and the functions they call will be collectively referred to as *window change functions*. As any hook, these hooks can be set either globally of buffer-locally via the local argument of `add-hook` (see [Setting Hooks](setting-hooks)) when the hook is installed.
The first of these hooks is run after a *window buffer change* is detected, which means that a window was created, deleted or assigned another buffer.
Variable: **window-buffer-change-functions**
This variable specifies functions called during redisplay when window buffers have changed. The value should be a list of functions that take one argument.
Functions specified buffer-locally are called for any window showing the corresponding buffer if that window has been created or assigned that buffer since the last time window change functions were run. In this case the window is passed as argument.
Functions specified by the default value are called for a frame if at least one window on that frame has been added, deleted or assigned another buffer since the last time window change functions were run. In this case the frame is passed as argument.
The second of these hooks is run when a *window size change* has been detected which means that a window was created, assigned another buffer, or changed its total size or that of its text area.
Variable: **window-size-change-functions**
This variable specifies functions called during redisplay when a window size change occurred. The value should be a list of functions that take one argument.
Functions specified buffer-locally are called for any window showing the corresponding buffer if that window has been added or assigned another buffer or changed its total or body size since the last time window change functions were run. In this case the window is passed as argument.
Functions specified by the default value are called for a frame if at least one window on that frame has been added or assigned another buffer or changed its total or body size since the last time window change functions were run. In this case the frame is passed as argument.
The third of these hooks is run when a *window selection change* has selected another window since the last redisplay.
Variable: **window-selection-change-functions**
This variable specifies functions called during redisplay when the selected window or a frame’s selected window has changed. The value should be a list of functions that take one argument.
Functions specified buffer-locally are called for any window showing the corresponding buffer if that window has been selected or deselected (among all windows or among all windows on its frame) since the last time window change functions were run. In this case the window is passed as argument.
Functions specified by the default value are called for a frame if that frame has been selected or deselected or the frame’s selected window has changed since the last time window change functions were run. In this case the frame is passed as argument.
The fourth of these hooks is run when a *window state change* has been detected, which means that at least one of the three preceding window changes has occurred.
Variable: **window-state-change-functions**
This variable specifies functions called during redisplay when a window buffer or size change occurred or the selected window or a frame’s selected window has changed. The value should be a list of functions that take one argument.
Functions specified buffer-locally are called for any window showing the corresponding buffer if that window has been added or assigned another buffer, changed its total or body size or has been selected or deselected (among all windows or among all windows on its frame) since the last time window change functions were run. In this case the window is passed as argument.
Functions specified by the default value are called for a frame if at least one window on that frame has been added, deleted or assigned another buffer, changed its total or body size or that frame has been selected or deselected or the frame’s selected window has changed since the last time window change functions were run. In this case the frame is passed as argument.
Functions specified by the default value are also run for a frame when that frame’s window state change flag (see below) has been set since last redisplay.
The fifth of these hooks is run when a *window configuration change* has been detected which means that either the buffer or the size of a window changed. It differs from the four preceding hooks in the way it is run.
Variable: **window-configuration-change-hook**
This variable specifies functions called during redisplay when either the buffer or the size of a window has changed. The value should be a list of functions that take no argument.
Functions specified buffer-locally are called for any window showing the corresponding buffer if at least one window on that frame has been added, deleted or assigned another buffer or changed its total or body size since the last time window change functions were run. Each call is performed with the window showing the buffer temporarily selected and its buffer current.
Functions specified by the default value are called for each frame if at least one window on that frame has been added, deleted or assigned another buffer or changed its total or body size since the last time window change functions were run. Each call is performed with the frame temporarily selected and the selected window’s buffer current.
Finally, Emacs runs a normal hook that generalizes the behavior of `window-state-change-functions`.
Variable: **window-state-change-hook**
The default value of this variable specifies functions called during redisplay when a window state change has been detected or the window state change flag has been set on at least one frame. The value should be a list of functions that take no argument.
Applications should put a function on this hook only if they want to react to changes that happened on (or have been signaled for) two or more frames since last redisplay. In every other case, putting the function on `window-state-change-functions` should be preferred.
Window change functions are called during redisplay for each frame as follows: First, any buffer-local window buffer change function, window size change function, selected window change and window state change functions are called in this order. Next, the default values for these functions are called in the same order. Then any buffer-local window configuration change functions are called followed by functions specified by the default value of those functions. Finally, functions on `window-state-change-hook` are run.
Window change functions are run for a specific frame only if a corresponding change was registered for that frame earlier. Such changes include the creation or deletion of a window or the assignment of another buffer or size to a window. Note that even when such a change has been registered, this does not mean that any of the hooks described above is run. If, for example, a change was registered within the scope of a window excursion (see [Window Configurations](window-configurations)), this will trigger a call of window change functions only if that excursion still persists at the time change functions are run. If it is exited earlier, hooks will be run only if registered by a change outside the scope of that excursion.
The *window state change flag* of a frame, if set, will cause the default values of `window-state-change-functions` (for that frame) and `window-state-change-hook` to be run during next redisplay regardless of whether a window state change actually occurred for that frame or not. After running any functions on these hooks, the flag is reset for each frame. Applications can set that flag and inspect its value using the following functions.
Function: **set-frame-window-state-change** *&optional frame arg*
This function sets frame’s window state change flag if arg is non-`nil` and resets it otherwise. frame must be a live frame and defaults to the selected one.
Function: **frame-window-state-change** *&optional frame*
This functions returns `t` if frame’s window state change flag is set and `nil` otherwise. frame must be a live frame and defaults to the selected one.
While window change functions are run, the functions described next can be called to get more insight into what has changed for a specific window or frame since the last redisplay. All these functions take a live window as single, optional argument, defaulting to the selected window.
Function: **window-old-buffer** *&optional window*
This function returns the buffer shown in window at the last time window change functions were run for window’s frame. If it returns `nil`, window has been created after that. If it returns `t`, window was not shown at that time but has been restored from a previously saved window configuration afterwards. Otherwise, the return value is the buffer shown by `window` at that time.
Function: **window-old-pixel-width** *&optional window*
This function returns the total pixel width of window the last time window change functions found `window` live on its frame. It is zero if `window` was created after that.
Function: **window-old-pixel-height** *&optional window*
This function returns the total pixel height of window the last time window change functions found `window` live on its frame. It is zero if `window` was created after that.
Function: **window-old-body-pixel-width** *&optional window*
This function returns the pixel width of window’s text area the last time window change functions found `window` live on its frame. It is zero if `window` was created after that.
Function: **window-old-body-pixel-height** *&optional window*
This function returns the pixel height of window’s text area the last time window change functions found `window` live on its frame. It is zero if `window` was created after that.
In order to find out which window or frame was selected the last time window change functions were run, the following functions can be used:
Function: **frame-old-selected-window** *&optional frame*
This function returns the selected window of frame at the last time window change functions were run. If omitted or `nil` frame defaults to the selected frame.
Function: **old-selected-window**
This function returns the selected window at the last time window change functions were run.
Function: **old-selected-frame**
This function returns the selected frame at the last time window change functions were run.
Note that window change functions provide no information about which windows have been deleted since the last time they were run. If necessary, applications should remember any window showing a specific buffer in a local variable of that buffer and update it in a function run by the default values of any of the hooks that are run when a window buffer change was detected.
The following caveats should be considered when adding a function to window change functions:
* Some operations will not trigger a call of window change functions. These include showing another buffer in a minibuffer window or any change of a tooltip window.
* Window change functions should not create or delete windows or change the buffer, size or selection status of any window because there is no guarantee that the information about such a change will be propagated to other window change functions. If at all, any such change should be executed only by the last function listed by the default value of `window-state-change-hook`.
* Macros like `save-window-excursion`, `with-selected-window` or `with-current-buffer` can be used when running window change functions.
* Running window change functions does not save and restore match data. Unless running `window-configuration-change-hook` it does not save or restore the selected window or frame or the current buffer either.
* Any redisplay triggering the run of window change functions may be aborted. If the abort occurs before window change functions have run to their completion, they will be run again with the previous values, that is, as if redisplay had not been performed. If aborted later, they will be run with the new values, that is, as if redisplay had been actually performed.
| programming_docs |
elisp None #### Function Type
Lisp functions are executable code, just like functions in other programming languages. In Lisp, unlike most languages, functions are also Lisp objects. A non-compiled function in Lisp is a lambda expression: that is, a list whose first element is the symbol `lambda` (see [Lambda Expressions](lambda-expressions)).
In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression can be called as a function even though it has no name; to emphasize this, we also call it an *anonymous function* (see [Anonymous Functions](anonymous-functions)). A named function in Lisp is just a symbol with a valid function in its function cell (see [Defining Functions](defining-functions)).
Most of the time, functions are called when their names are written in Lisp expressions in Lisp programs. However, you can construct or obtain a function object at run time and then call it with the primitive functions `funcall` and `apply`. See [Calling Functions](calling-functions).
elisp None #### The Zen of Buffer Display
In its most simplistic form, a frame accommodates always one single window that can be used for displaying a buffer. As a consequence, it is always the latest call of `display-buffer` that will have succeeded in placing its buffer there.
Since working with such a frame is not very practical, Emacs by default allows for more complex frame layouts controlled by the default values of the frame size and the `split-height-threshold` and `split-width-threshold` options. Displaying a buffer not yet shown on a frame then either splits the single window on that frame or (re-)uses one of its two windows.
The default behavior is abandoned as soon as the user customizes one of these thresholds or manually changes the frame’s layout. The default behavior is also abandoned when calling `display-buffer` with a non-`nil` action argument or the user customizes one of the options mentioned in the previous subsections. Mastering `display-buffer` soon may become a frustrating experience due to the plethora of applicable display actions and the resulting frame layouts.
However, refraining from using buffer display functions and falling back on a split & delete windows metaphor is not a good idea either. Buffer display functions give Lisp programs and users a framework to reconcile their different needs; no comparable framework exists for splitting and deleting windows. Buffer display functions also allow to at least partially restore the layout of a frame when removing a buffer from it later (see [Quitting Windows](quitting-windows)).
Below we will give a number of guidelines to redeem the frustration mentioned above and thus to avoid literally losing buffers in-between the windows of a frame.
Write display actions without stress
Writing display actions can be a pain because one has to lump together action functions and action alists in one huge list. (Historical reasons prevented us from having `display-buffer` support separate arguments for these.) It might help to memorize some basic forms like the ones listed below:
```
'(nil (inhibit-same-window . t))
```
specifies an action alist entry only and no action function. Its sole purpose is to inhibit a `display-buffer-same-window` function specified elsewhere from showing the buffer in the same window, see also the last example of the preceding subsection.
```
'(display-buffer-below-selected)
```
on the other hand, specifies one action function and an empty action alist. To combine the effects of the above two specifications one would write the form
```
'(display-buffer-below-selected (inhibit-same-window . t))
```
to add another action function one would write
```
'((display-buffer-below-selected display-buffer-at-bottom)
(inhibit-same-window . t))
```
and to add another alist entry one would write
```
'((display-buffer-below-selected display-buffer-at-bottom)
(inhibit-same-window . t)
(window-height . fit-window-to-buffer))
```
That last form can be used as action argument of `display-buffer` in the following way:
```
(display-buffer
(get-buffer-create "*foo*")
'((display-buffer-below-selected display-buffer-at-bottom)
(inhibit-same-window . t)
(window-height . fit-window-to-buffer)))
```
In a customization of `display-buffer-alist` it would be used as follows:
```
(customize-set-variable
'display-buffer-alist
'(("\\*foo\\*"
(display-buffer-below-selected display-buffer-at-bottom)
(inhibit-same-window . t)
(window-height . fit-window-to-buffer))))
```
To add a customization for a second buffer one would then write:
```
(customize-set-variable
'display-buffer-alist
'(("\\*foo\\*"
(display-buffer-below-selected display-buffer-at-bottom)
(inhibit-same-window . t)
(window-height . fit-window-to-buffer))
("\\*bar\\*"
(display-buffer-reuse-window display-buffer-pop-up-frame)
(reusable-frames . visible))))
```
Treat each other with respect
`display-buffer-alist` and `display-buffer-base-action` are user options—Lisp programs must never set or rebind them. `display-buffer-overriding-action`, on the other hand, is reserved for applications—who seldom use that option and if they use it, then with utmost care.
Older implementations of `display-buffer` frequently caused users and applications to fight over the settings of user options like `pop-up-frames` and `pop-up-windows` (see [Choosing Window Options](choosing-window-options)). This was one major reason for redesigning `display-buffer`—to provide a clear framework specifying what users and applications should be allowed to do.
Lisp programs must be prepared that user customizations may cause buffers to get displayed in an unexpected way. They should never assume in their subsequent behavior, that the buffer has been shown precisely the way they asked for in the action argument of `display-buffer`.
Users should not pose too many and too severe restrictions on how arbitrary buffers get displayed. Otherwise, they will risk to lose the characteristics of showing a buffer for a certain purpose. Suppose a Lisp program has been written to compare different versions of a buffer in two windows side-by-side. If the customization of `display-buffer-alist` prescribes that any such buffer should be always shown in or below the selected window, the program will have a hard time to set up the desired window configuration via `display-buffer`.
To specify a preference for showing an arbitrary buffer, users should customize `display-buffer-base-action`. An example of how users who prefer working with multiple frames would do that was given in the previous subsection. `display-buffer-alist` should be reserved for displaying specific buffers in a specific way.
Consider reusing a window that already shows the buffer
Generally, it’s always a good idea for users and Lisp programmers to be prepared for the case that a window already shows the buffer in question and to reuse that window. In the preceding subsection we have shown that failing to do so properly may cause `display-buffer` to continuously pop up a new frame although a frame showing that buffer existed already. In a few cases only, it might be undesirable to reuse a window, for example, when a different portion of the buffer should be shown in that window.
Hence, `display-buffer-reuse-window` is one action function that should be used as often as possible, both in action arguments and customizations. An `inhibit-same-window` entry in the action argument usually takes care of the most common case where reusing a window showing the buffer should be avoided—that where the window in question is the selected one.
Attract focus to the window chosen
This is a no-brainer for people working with multiple frames—the frame showing the buffer will automatically raise and get focus unless an `inhibit-switch-frame` entry forbids it. For single frame users this task can be considerably more difficult. In particular, `display-buffer-pop-up-window` and `display-buffer-use-some-window` can become obtrusive in this regard. They split or use a seemingly arbitrary (often the largest or least recently used) window, distracting the user’s attention.
Some Lisp programs therefore try to choose a window at the bottom of the frame, for example, in order to display the buffer in vicinity of the minibuffer window where the user is expected to answer a question related to the new window. For non-input related actions `display-buffer-below-selected` might be preferable because the selected window usually already has the user’s attention.
Handle subsequent invocations of `display-buffer`
`display-buffer` is not overly well suited for displaying several buffers in sequence and making sure that all these buffers are shown orderly in the resulting window configuration. Again, the standard action functions `display-buffer-pop-up-window` and `display-buffer-use-some-window` are not very suited for this purpose due to their somewhat chaotic nature in more complex configurations.
To produce a window configuration displaying multiple buffers (or different views of one and the same buffer) in one and the same display cycle, Lisp programmers will unavoidably have to write their own action functions. A few tricks listed below might help in this regard.
* Making windows atomic (see [Atomic Windows](atomic-windows)) avoids breaking an existing window composition when popping up a new window. The new window will pop up outside the composition instead.
* Temporarily dedicating windows to their buffers (see [Dedicated Windows](dedicated-windows)) avoids using a window for displaying a different buffer. A non-dedicated window will be used instead.
* Calling `window-preserve-size` (see [Preserving Window Sizes](preserving-window-sizes)) will try to keep the size of the argument window unchanged when popping up a new window. You have to make sure that another window in the same combination can be shrunk instead, though.
* Side windows (see [Side Windows](side-windows)) can be used for displaying specific buffers always in a window at the same position of a frame. This permits grouping buffers that do not compete for being shown at the same time on a frame and showing any such buffer in the same window without disrupting the display of other buffers.
* Child frames (see [Child Frames](child-frames)) can be used to display a buffer within the screen estate of the selected frame without disrupting that frame’s window configuration and without the overhead associated with full-fledged frames as inflicted by `display-buffer-pop-up-frame`.
elisp None #### Advising Named Functions
A common use of advice is for named functions and macros. You could just use `add-function` as in:
```
(add-function :around (symbol-function 'fun) #'his-tracing-function)
```
But you should use `advice-add` and `advice-remove` for that instead. This separate set of functions to manipulate pieces of advice applied to named functions, offers the following extra features compared to `add-function`: they know how to deal with macros and autoloaded functions, they let `describe-function` preserve the original docstring as well as document the added advice, and they let you add and remove advice before a function is even defined.
`advice-add` can be useful for altering the behavior of existing calls to an existing function without having to redefine the whole function. However, it can be a source of bugs, since existing callers to the function may assume the old behavior, and work incorrectly when the behavior is changed by advice. Advice can also cause confusion in debugging, if the person doing the debugging does not notice or remember that the function has been modified by advice.
For these reasons, advice should be reserved for the cases where you cannot modify a function’s behavior in any other way. If it is possible to do the same thing via a hook, that is preferable (see [Hooks](hooks)). If you simply want to change what a particular key does, it may be better to write a new command, and remap the old command’s key bindings to the new one (see [Remapping Commands](remapping-commands)).
If you are writing code for release, for others to use, try to avoid including advice in it. If the function you want to advise has no hook to do the job, please talk with the Emacs developers about adding a suitable hook. Especially, Emacs’s own source files should not put advice on functions in Emacs. (There are currently a few exceptions to this convention, but we aim to correct them.) It is generally cleaner to create a new hook in `foo`, and make `bar` use the hook, than to have `bar` put advice in `foo`.
Special forms (see [Special Forms](special-forms)) cannot be advised, however macros can be advised, in much the same way as functions. Of course, this will not affect code that has already been macro-expanded, so you need to make sure the advice is installed before the macro is expanded.
It is possible to advise a primitive (see [What Is a Function](what-is-a-function)), but one should typically *not* do so, for two reasons. Firstly, some primitives are used by the advice mechanism, and advising them could cause an infinite recursion. Secondly, many primitives are called directly from C, and such calls ignore advice; hence, one ends up in a confusing situation where some calls (occurring from Lisp code) obey the advice and other calls (from C code) do not.
Macro: **define-advice** *symbol (where lambda-list &optional name depth) &rest body*
This macro defines a piece of advice and adds it to the function named symbol. The advice is an anonymous function if name is `nil` or a function named `symbol@name`. See `advice-add` for explanation of other arguments.
Function: **advice-add** *symbol where function &optional props*
Add the advice function to the named function symbol. where and props have the same meaning as for `add-function` (see [Core Advising Primitives](core-advising-primitives)).
Function: **advice-remove** *symbol function*
Remove the advice function from the named function symbol. function can also be the `name` of a piece of advice.
Function: **advice-member-p** *function symbol*
Return non-`nil` if the advice function is already in the named function symbol. function can also be the `name` of a piece of advice.
Function: **advice-mapc** *function symbol*
Call function for every piece of advice that was added to the named function symbol. function is called with two arguments: the advice function and its properties.
elisp None ### Recursive Editing
The Emacs command loop is entered automatically when Emacs starts up. This top-level invocation of the command loop never exits; it keeps running as long as Emacs does. Lisp programs can also invoke the command loop. Since this makes more than one activation of the command loop, we call it *recursive editing*. A recursive editing level has the effect of suspending whatever command invoked it and permitting the user to do arbitrary editing before resuming that command.
The commands available during recursive editing are the same ones available in the top-level editing loop and defined in the keymaps. Only a few special commands exit the recursive editing level; the others return to the recursive editing level when they finish. (The special commands for exiting are always available, but they do nothing when recursive editing is not in progress.)
All command loops, including recursive ones, set up all-purpose error handlers so that an error in a command run from the command loop will not exit the loop.
Minibuffer input is a special kind of recursive editing. It has a few special wrinkles, such as enabling display of the minibuffer and the minibuffer window, but fewer than you might suppose. Certain keys behave differently in the minibuffer, but that is only because of the minibuffer’s local map; if you switch windows, you get the usual Emacs commands.
To invoke a recursive editing level, call the function `recursive-edit`. This function contains the command loop; it also contains a call to `catch` with tag `exit`, which makes it possible to exit the recursive editing level by throwing to `exit` (see [Catch and Throw](catch-and-throw)). Throwing a `t` value causes `recursive-edit` to quit, so that control returns to the command loop one level up. This is called *aborting*, and is done by `C-]` (`abort-recursive-edit`). Similarly, you can throw a string value to make `recursive-edit` signal an error, printing this string as the message. If you throw a function, `recursive-edit` will call it without arguments before returning. Throwing any other value, will make `recursive-edit` return normally to the function that called it. The command `C-M-c` (`exit-recursive-edit`) does this.
Most applications should not use recursive editing, except as part of using the minibuffer. Usually it is more convenient for the user if you change the major mode of the current buffer temporarily to a special major mode, which should have a command to go back to the previous mode. (The `e` command in Rmail uses this technique.) Or, if you wish to give the user different text to edit recursively, create and select a new buffer in a special mode. In this mode, define a command to complete the processing and go back to the previous buffer. (The `m` command in Rmail does this.)
Recursive edits are useful in debugging. You can insert a call to `debug` into a function definition as a sort of breakpoint, so that you can look around when the function gets there. `debug` invokes a recursive edit but also provides the other features of the debugger.
Recursive editing levels are also used when you type `C-r` in `query-replace` or use `C-x q` (`kbd-macro-query`).
Command: **recursive-edit**
This function invokes the editor command loop. It is called automatically by the initialization of Emacs, to let the user begin editing. When called from a Lisp program, it enters a recursive editing level.
If the current buffer is not the same as the selected window’s buffer, `recursive-edit` saves and restores the current buffer. Otherwise, if you switch buffers, the buffer you switched to is current after `recursive-edit` returns.
In the following example, the function `simple-rec` first advances point one word, then enters a recursive edit, printing out a message in the echo area. The user can then do any editing desired, and then type `C-M-c` to exit and continue executing `simple-rec`.
```
(defun simple-rec ()
(forward-word 1)
(message "Recursive edit in progress")
(recursive-edit)
(forward-word 1))
⇒ simple-rec
(simple-rec)
⇒ nil
```
Command: **exit-recursive-edit**
This function exits from the innermost recursive edit (including minibuffer input). Its definition is effectively `(throw 'exit
nil)`.
Command: **abort-recursive-edit**
This function aborts the command that requested the innermost recursive edit (including minibuffer input), by signaling `quit` after exiting the recursive edit. Its definition is effectively `(throw 'exit t)`. See [Quitting](quitting).
Command: **top-level**
This function exits all recursive editing levels; it does not return a value, as it jumps completely out of any computation directly back to the main command loop.
Function: **recursion-depth**
This function returns the current depth of recursive edits. When no recursive edit is active, it returns 0.
elisp None ### Command History
The command loop keeps a history of the complex commands that have been executed, to make it convenient to repeat these commands. A *complex command* is one for which the interactive argument reading uses the minibuffer. This includes any `M-x` command, any `M-:` command, and any command whose `interactive` specification reads an argument from the minibuffer. Explicit use of the minibuffer during the execution of the command itself does not cause the command to be considered complex.
Variable: **command-history**
This variable’s value is a list of recent complex commands, each represented as a form to evaluate. It continues to accumulate all complex commands for the duration of the editing session, but when it reaches the maximum size (see [Minibuffer History](minibuffer-history)), the oldest elements are deleted as new ones are added.
```
command-history
⇒ ((switch-to-buffer "chistory.texi")
(describe-key "^X^[")
(visit-tags-table "~/emacs/src/")
(find-tag "repeat-complex-command"))
```
This history list is actually a special case of minibuffer history (see [Minibuffer History](minibuffer-history)), with one special twist: the elements are expressions rather than strings.
There are a number of commands devoted to the editing and recall of previous commands. The commands `repeat-complex-command`, and `list-command-history` are described in the user manual (see [Repetition](https://www.gnu.org/software/emacs/manual/html_node/emacs/Repetition.html#Repetition) in The GNU Emacs Manual). Within the minibuffer, the usual minibuffer history commands are available.
| programming_docs |
elisp None ### Property Lists
A *property list* (*plist* for short) is a list of paired elements. Each of the pairs associates a property name (usually a symbol) with a property or value. Here is an example of a property list:
```
(pine cones numbers (1 2 3) color "blue")
```
This property list associates `pine` with `cones`, `numbers` with `(1 2 3)`, and `color` with `"blue"`. The property names and values can be any Lisp objects, but the names are usually symbols (as they are in this example).
Property lists are used in several contexts. For instance, the function `put-text-property` takes an argument which is a property list, specifying text properties and associated values which are to be applied to text in a string or buffer. See [Text Properties](text-properties).
Another prominent use of property lists is for storing symbol properties. Every symbol possesses a list of properties, used to record miscellaneous information about the symbol; these properties are stored in the form of a property list. See [Symbol Properties](symbol-properties).
| | | |
| --- | --- | --- |
| • [Plists and Alists](plists-and-alists) | | Comparison of the advantages of property lists and association lists. |
| • [Plist Access](plist-access) | | Accessing property lists stored elsewhere. |
elisp None ### Generalized Variables
A *generalized variable* or *place form* is one of the many places in Lisp memory where values can be stored using the `setf` macro (see [Setting Generalized Variables](setting-generalized-variables)). The simplest place form is a regular Lisp variable. But the CARs and CDRs of lists, elements of arrays, properties of symbols, and many other locations are also places where Lisp values get stored.
Generalized variables are analogous to lvalues in the C language, where ‘`x = a[i]`’ gets an element from an array and ‘`a[i] = x`’ stores an element using the same notation. Just as certain forms like `a[i]` can be lvalues in C, there is a set of forms that can be generalized variables in Lisp.
| | | |
| --- | --- | --- |
| • [Setting Generalized Variables](setting-generalized-variables) | | The `setf` macro. |
| • [Adding Generalized Variables](adding-generalized-variables) | | Defining new `setf` forms. |
elisp None ### Evaluation During Compilation
These features permit you to write code to be evaluated during compilation of a program.
Macro: **eval-and-compile** *body…*
This form marks body to be evaluated both when you compile the containing code and when you run it (whether compiled or not).
You can get a similar result by putting body in a separate file and referring to that file with `require`. That method is preferable when body is large. Effectively `require` is automatically `eval-and-compile`, the package is loaded both when compiling and executing.
`autoload` is also effectively `eval-and-compile` too. It’s recognized when compiling, so uses of such a function don’t produce “not known to be defined” warnings.
Most uses of `eval-and-compile` are fairly sophisticated.
If a macro has a helper function to build its result, and that macro is used both locally and outside the package, then `eval-and-compile` should be used to get the helper both when compiling and then later when running.
If functions are defined programmatically (with `fset` say), then `eval-and-compile` can be used to have that done at compile-time as well as run-time, so calls to those functions are checked (and warnings about “not known to be defined” suppressed).
Macro: **eval-when-compile** *body…*
This form marks body to be evaluated at compile time but not when the compiled program is loaded. The result of evaluation by the compiler becomes a constant which appears in the compiled program. If you load the source file, rather than compiling it, body is evaluated normally.
If you have a constant that needs some calculation to produce, `eval-when-compile` can do that at compile-time. For example,
```
(defvar my-regexp
(eval-when-compile (regexp-opt '("aaa" "aba" "abb"))))
```
If you’re using another package, but only need macros from it (the byte compiler will expand those), then `eval-when-compile` can be used to load it for compiling, but not executing. For example,
```
(eval-when-compile
(require 'my-macro-package))
```
The same sort of thing goes for macros and `defsubst` functions defined locally and only for use within the file. They are needed for compiling the file, but in most cases they are not needed for execution of the compiled file. For example,
```
(eval-when-compile
(unless (fboundp 'some-new-thing)
(defmacro 'some-new-thing ()
(compatibility code))))
```
This is often good for code that’s only a fallback for compatibility with other versions of Emacs.
**Common Lisp Note:** At top level, `eval-when-compile` is analogous to the Common Lisp idiom `(eval-when (compile eval) …)`. Elsewhere, the Common Lisp ‘`#.`’ reader macro (but not when interpreting) is closer to what `eval-when-compile` does.
elisp None #### ImageMagick Images
If your Emacs build has ImageMagick support, you can use the ImageMagick library to load many image formats (see [File Conveniences](https://www.gnu.org/software/emacs/manual/html_node/emacs/File-Conveniences.html#File-Conveniences) in The GNU Emacs Manual). The image type symbol for images loaded via ImageMagick is `imagemagick`, regardless of the actual underlying image format.
To check for ImageMagick support, use the following:
```
(image-type-available-p 'imagemagick)
```
Function: **imagemagick-types**
This function returns a list of image file extensions supported by the current ImageMagick installation. Each list element is a symbol representing an internal ImageMagick name for an image type, such as `BMP` for `.bmp` images.
User Option: **imagemagick-enabled-types**
The value of this variable is a list of ImageMagick image types which Emacs may attempt to render using ImageMagick. Each list element should be one of the symbols in the list returned by `imagemagick-types`, or an equivalent string. Alternatively, a value of `t` enables ImageMagick for all possible image types. Regardless of the value of this variable, `imagemagick-types-inhibit` (see below) takes precedence.
User Option: **imagemagick-types-inhibit**
The value of this variable lists the ImageMagick image types which should never be rendered using ImageMagick, regardless of the value of `imagemagick-enabled-types`. A value of `t` disables ImageMagick entirely.
Variable: **image-format-suffixes**
This variable is an alist mapping image types to file name extensions. Emacs uses this in conjunction with the `:format` image property (see below) to give a hint to the ImageMagick library as to the type of an image. Each element has the form `(type
extension)`, where type is a symbol specifying an image content-type, and extension is a string that specifies the associated file name extension.
Images loaded with ImageMagick support the following additional image descriptor properties:
`:background background`
background, if non-`nil`, should be a string specifying a color, which is used as the image’s background color if the image supports transparency. If the value is `nil`, it defaults to the frame’s background color.
`:format type`
The value, type, should be a symbol specifying the type of the image data, as found in `image-format-suffixes`. This is used when the image does not have an associated file name, to provide a hint to ImageMagick to help it detect the image type.
`:crop geometry`
The value of geometry should be a list of the form `(width height x y)`. width and height specify the width and height of the cropped image. If x is a positive number it specifies the offset of the cropped area from the left of the original image, and if negative the offset from the right. If y is a positive number it specifies the offset from the top of the original image, and if negative from the bottom. If x or y are `nil` or unspecified the crop area will be centered on the original image.
If the crop area is outside or overlaps the edge of the image it will be reduced to exclude any areas outside of the image. This means it is not possible to use `:crop` to increase the size of the image by entering large width or height values.
Cropping is performed after scaling but before rotation.
elisp None #### Extended File Attributes
On some operating systems, each file can be associated with arbitrary *extended file attributes*. At present, Emacs supports querying and setting two specific sets of extended file attributes: Access Control Lists (ACLs) and SELinux contexts. These extended file attributes are used, on some systems, to impose more sophisticated file access controls than the basic Unix-style permissions discussed in the previous sections.
A detailed explanation of ACLs and SELinux is beyond the scope of this manual. For our purposes, each file can be associated with an *ACL*, which specifies its properties under an ACL-based file control system, and/or an *SELinux context*, which specifies its properties under the SELinux system.
Function: **file-acl** *filename*
This function returns the ACL for the file filename. The exact Lisp representation of the ACL is unspecified (and may change in future Emacs versions), but it is the same as what `set-file-acl` takes for its acl argument (see [Changing Files](changing-files)).
The underlying ACL implementation is platform-specific; on GNU/Linux and BSD, Emacs uses the POSIX ACL interface, while on MS-Windows Emacs emulates the POSIX ACL interface with native file security APIs.
If ACLs are not supported or the file does not exist, then the return value is `nil`.
Function: **file-selinux-context** *filename*
This function returns the SELinux context of the file filename, as a list of the form `(user role type
range)`. The list elements are the context’s user, role, type, and range respectively, as Lisp strings; see the SELinux documentation for details about what these actually mean. The return value has the same form as what `set-file-selinux-context` takes for its context argument (see [Changing Files](changing-files)).
If SELinux is not supported or the file does not exist, then the return value is `(nil nil nil nil)`.
Function: **file-extended-attributes** *filename*
This function returns an alist of the Emacs-recognized extended attributes of file filename. Currently, it serves as a convenient way to retrieve both the ACL and SELinux context; you can then call the function `set-file-extended-attributes`, with the returned alist as its second argument, to apply the same file access attributes to another file (see [Changing Files](changing-files)).
One of the elements is `(acl . acl)`, where acl has the same form returned by `file-acl`.
Another element is `(selinux-context . context)`, where context is the SELinux context, in the same form returned by `file-selinux-context`.
elisp None ### Initial Input
Several of the functions for minibuffer input have an argument called initial. This is a mostly-deprecated feature for specifying that the minibuffer should start out with certain text, instead of empty as usual.
If initial is a string, the minibuffer starts out containing the text of the string, with point at the end, when the user starts to edit the text. If the user simply types RET to exit the minibuffer, it will use the initial input string to determine the value to return.
**We discourage use of a non-`nil` value for initial**, because initial input is an intrusive interface. History lists and default values provide a much more convenient method to offer useful default inputs to the user.
There is just one situation where you should specify a string for an initial argument. This is when you specify a cons cell for the history argument. See [Minibuffer History](minibuffer-history).
initial can also be a cons cell of the form `(string
. position)`. This means to insert string in the minibuffer but put point at position within the string’s text.
As a historical accident, position was implemented inconsistently in different functions. In `completing-read`, position’s value is interpreted as origin-zero; that is, a value of 0 means the beginning of the string, 1 means after the first character, etc. In `read-minibuffer`, and the other non-completion minibuffer input functions that support this argument, 1 means the beginning of the string, 2 means after the first character, etc.
Use of a cons cell as the value for initial arguments is deprecated.
elisp None #### Classifying Events
Every event has an *event type*, which classifies the event for key binding purposes. For a keyboard event, the event type equals the event value; thus, the event type for a character is the character, and the event type for a function key symbol is the symbol itself. For events that are lists, the event type is the symbol in the CAR of the list. Thus, the event type is always a symbol or a character.
Two events of the same type are equivalent where key bindings are concerned; thus, they always run the same command. That does not necessarily mean they do the same things, however, as some commands look at the whole event to decide what to do. For example, some commands use the location of a mouse event to decide where in the buffer to act.
Sometimes broader classifications of events are useful. For example, you might want to ask whether an event involved the META key, regardless of which other key or mouse button was used.
The functions `event-modifiers` and `event-basic-type` are provided to get such information conveniently.
Function: **event-modifiers** *event*
This function returns a list of the modifiers that event has. The modifiers are symbols; they include `shift`, `control`, `meta`, `alt`, `hyper` and `super`. In addition, the modifiers list of a mouse event symbol always contains one of `click`, `drag`, and `down`. For double or triple events, it also contains `double` or `triple`.
The argument event may be an entire event object, or just an event type. If event is a symbol that has never been used in an event that has been read as input in the current Emacs session, then `event-modifiers` can return `nil`, even when event actually has modifiers.
Here are some examples:
```
(event-modifiers ?a)
⇒ nil
(event-modifiers ?A)
⇒ (shift)
(event-modifiers ?\C-a)
⇒ (control)
(event-modifiers ?\C-%)
⇒ (control)
(event-modifiers ?\C-\S-a)
⇒ (control shift)
(event-modifiers 'f5)
⇒ nil
(event-modifiers 's-f5)
⇒ (super)
(event-modifiers 'M-S-f5)
⇒ (meta shift)
(event-modifiers 'mouse-1)
⇒ (click)
(event-modifiers 'down-mouse-1)
⇒ (down)
```
The modifiers list for a click event explicitly contains `click`, but the event symbol name itself does not contain ‘`click`’. Similarly, the modifiers list for an ASCII control character, such as ‘`C-a`’, contains `control`, even though reading such an event via `read-char` will return the value 1 with the control modifier bit removed.
Function: **event-basic-type** *event*
This function returns the key or mouse button that event describes, with all modifiers removed. The event argument is as in `event-modifiers`. For example:
```
(event-basic-type ?a)
⇒ 97
(event-basic-type ?A)
⇒ 97
(event-basic-type ?\C-a)
⇒ 97
(event-basic-type ?\C-\S-a)
⇒ 97
(event-basic-type 'f5)
⇒ f5
(event-basic-type 's-f5)
⇒ f5
(event-basic-type 'M-S-f5)
⇒ f5
(event-basic-type 'down-mouse-1)
⇒ mouse-1
```
Function: **mouse-movement-p** *object*
This function returns non-`nil` if object is a mouse movement event. See [Motion Events](motion-events).
Function: **event-convert-list** *list*
This function converts a list of modifier names and a basic event type to an event type which specifies all of them. The basic event type must be the last element of the list. For example,
```
(event-convert-list '(control ?a))
⇒ 1
(event-convert-list '(control meta ?a))
⇒ -134217727
(event-convert-list '(control super f1))
⇒ C-s-f1
```
elisp None ### Windows and Point
Each window has its own value of point (see [Point](point)), independent of the value of point in other windows displaying the same buffer. This makes it useful to have multiple windows showing one buffer.
* The window point is established when a window is first created; it is initialized from the buffer’s point, or from the window point of another window opened on the buffer if such a window exists.
* Selecting a window sets the value of point in its buffer from the window’s value of point. Conversely, deselecting a window sets the window’s value of point from that of the buffer. Thus, when you switch between windows that display a given buffer, the point value for the selected window is in effect in the buffer, while the point values for the other windows are stored in those windows.
* As long as the selected window displays the current buffer, the window’s point and the buffer’s point always move together; they remain equal.
Emacs displays the cursor, by default as a rectangular block, in each window at the position of that window’s point. When the user switches to another buffer in a window, Emacs moves that window’s cursor to where point is in that buffer. If the exact position of point is hidden behind some display element, such as a display string or an image, Emacs displays the cursor immediately before or after that display element.
Function: **window-point** *&optional window*
This function returns the current position of point in window. For a nonselected window, this is the value point would have (in that window’s buffer) if that window were selected. The default for window is the selected window.
When window is the selected window, the value returned is the value of point in that window’s buffer. Strictly speaking, it would be more correct to return the top-level value of point, outside of any `save-excursion` forms. But that value is hard to find.
Function: **set-window-point** *window position*
This function positions point in window at position position in window’s buffer. It returns position.
If window is selected, this simply does `goto-char` in window’s buffer.
Variable: **window-point-insertion-type**
This variable specifies the marker insertion type (see [Marker Insertion Types](marker-insertion-types)) of `window-point`. The default is `nil`, so `window-point` will stay behind text inserted there.
elisp None ### File Locks
When two users edit the same file at the same time, they are likely to interfere with each other. Emacs tries to prevent this situation from arising by recording a *file lock* when a file is being modified. Emacs can then detect the first attempt to modify a buffer visiting a file that is locked by another Emacs job, and ask the user what to do. The file lock is really a file, a symbolic link with a special name, stored in the same directory as the file you are editing. The name is constructed by prepending `.#` to the file name of the buffer. The target of the symbolic link will be of the form `[email protected]:boot`, where user is replaced with the current username (from `user-login-name`), host with the name of the host where Emacs is running (from `system-name`), pid with Emacs’s process id, and boot with the time since the last reboot. `:boot` is omitted if the boot time is unavailable. (On file systems that do not support symbolic links, a regular file is used instead, with contents of the form `[email protected]:boot`.)
When you access files using NFS, there may be a small probability that you and another user will both lock the same file simultaneously. If this happens, it is possible for the two users to make changes simultaneously, but Emacs will still warn the user who saves second. Also, the detection of modification of a buffer visiting a file changed on disk catches some cases of simultaneous editing; see [Modification Time](modification-time).
Function: **file-locked-p** *filename*
This function returns `nil` if the file filename is not locked. It returns `t` if it is locked by this Emacs process, and it returns the name of the user who has locked it if it is locked by some other job.
```
(file-locked-p "foo")
⇒ nil
```
Function: **lock-buffer** *&optional filename*
This function locks the file filename, if the current buffer is modified. The argument filename defaults to the current buffer’s visited file. Nothing is done if the current buffer is not visiting a file, or is not modified, or if the option `create-lockfiles` is `nil`.
Function: **unlock-buffer**
This function unlocks the file being visited in the current buffer, if the buffer is modified. If the buffer is not modified, then the file should not be locked, so this function does nothing. It also does nothing if the current buffer is not visiting a file, or is not locked. This function handles file system errors by calling `display-warning` and otherwise ignores the error.
User Option: **create-lockfiles**
If this variable is `nil`, Emacs does not lock files.
User Option: **lock-file-name-transforms**
By default, Emacs creates the lock files in the same directory as the files that are being locked. This can be changed by customizing this variable. Is has the same syntax as `auto-save-file-name-transforms` (see [Auto-Saving](auto_002dsaving)). For instance, to make Emacs write all the lock files to `/var/tmp/`, you could say something like:
```
(setq lock-file-name-transforms
'(("\\`/.*/\\([^/]+\\)\\'" "/var/tmp/\\1" t)))
```
Function: **ask-user-about-lock** *file other-user*
This function is called when the user tries to modify file, but it is locked by another user named other-user. The default definition of this function asks the user to say what to do. The value this function returns determines what Emacs does next:
* A value of `t` says to grab the lock on the file. Then this user may edit the file and other-user loses the lock.
* A value of `nil` says to ignore the lock and let this user edit the file anyway.
* This function may instead signal a `file-locked` error, in which case the change that the user was about to make does not take place. The error message for this error looks like this:
```
error→ File is locked: file other-user
```
where `file` is the name of the file and other-user is the name of the user who has locked the file.
If you wish, you can replace the `ask-user-about-lock` function with your own version that makes the decision in another way.
User Option: **remote-file-name-inhibit-locks**
You can prevent the creation of remote lock files by setting the variable `remote-file-name-inhibit-locks` to `t`.
Command: **lock-file-mode**
This command, called interactively, toggles the local value of `create-lockfiles` in the current buffer.
| programming_docs |
elisp None ### Motion and Syntax
This section describes functions for moving across characters that have certain syntax classes.
Function: **skip-syntax-forward** *syntaxes &optional limit*
This function moves point forward across characters having syntax classes mentioned in syntaxes (a string of syntax class characters). It stops when it encounters the end of the buffer, or position limit (if specified), or a character it is not supposed to skip.
If syntaxes starts with ‘`^`’, then the function skips characters whose syntax is *not* in syntaxes.
The return value is the distance traveled, which is a nonnegative integer.
Function: **skip-syntax-backward** *syntaxes &optional limit*
This function moves point backward across characters whose syntax classes are mentioned in syntaxes. It stops when it encounters the beginning of the buffer, or position limit (if specified), or a character it is not supposed to skip.
If syntaxes starts with ‘`^`’, then the function skips characters whose syntax is *not* in syntaxes.
The return value indicates the distance traveled. It is an integer that is zero or less.
Function: **backward-prefix-chars**
This function moves point backward over any number of characters with expression prefix syntax. This includes both characters in the expression prefix syntax class, and characters with the ‘`p`’ flag.
elisp None ### Drag and Drop
When a user drags something from another application over Emacs, that other application expects Emacs to tell it if Emacs can handle the data that is dragged. The variable `x-dnd-test-function` is used by Emacs to determine what to reply. The default value is `x-dnd-default-test-function` which accepts drops if the type of the data to be dropped is present in `x-dnd-known-types`. You can customize `x-dnd-test-function` and/or `x-dnd-known-types` if you want Emacs to accept or reject drops based on some other criteria.
If you want to change the way Emacs handles drop of different types or add a new type, customize `x-dnd-types-alist`. This requires detailed knowledge of what types other applications use for drag and drop.
When an URL is dropped on Emacs it may be a file, but it may also be another URL type (https, etc.). Emacs first checks `dnd-protocol-alist` to determine what to do with the URL. If there is no match there, Emacs looks for a match in `browse-url-handlers` and `browse-url-default-handlers`. If still no match has been found, the text for the URL is inserted. If you want to alter Emacs behavior, you can customize these variables.
elisp None ### Translation of Characters
A *translation table* is a char-table (see [Char-Tables](char_002dtables)) that specifies a mapping of characters into characters. These tables are used in encoding and decoding, and for other purposes. Some coding systems specify their own particular translation tables; there are also default translation tables which apply to all other coding systems.
A translation table has two extra slots. The first is either `nil` or a translation table that performs the reverse translation; the second is the maximum number of characters to look up for translating sequences of characters (see the description of `make-translation-table-from-alist` below).
Function: **make-translation-table** *&rest translations*
This function returns a translation table based on the argument translations. Each element of translations should be a list of elements of the form `(from . to)`; this says to translate the character from into to.
The arguments and the forms in each argument are processed in order, and if a previous form already translates to to some other character, say to-alt, from is also translated to to-alt.
During decoding, the translation table’s translations are applied to the characters that result from ordinary decoding. If a coding system has the property `:decode-translation-table`, that specifies the translation table to use, or a list of translation tables to apply in sequence. (This is a property of the coding system, as returned by `coding-system-get`, not a property of the symbol that is the coding system’s name. See [Basic Concepts of Coding Systems](coding-system-basics).) Finally, if `standard-translation-table-for-decode` is non-`nil`, the resulting characters are translated by that table.
During encoding, the translation table’s translations are applied to the characters in the buffer, and the result of translation is actually encoded. If a coding system has property `:encode-translation-table`, that specifies the translation table to use, or a list of translation tables to apply in sequence. In addition, if the variable `standard-translation-table-for-encode` is non-`nil`, it specifies the translation table to use for translating the result.
Variable: **standard-translation-table-for-decode**
This is the default translation table for decoding. If a coding system specifies its own translation tables, the table that is the value of this variable, if non-`nil`, is applied after them.
Variable: **standard-translation-table-for-encode**
This is the default translation table for encoding. If a coding system specifies its own translation tables, the table that is the value of this variable, if non-`nil`, is applied after them.
Variable: **translation-table-for-input**
Self-inserting characters are translated through this translation table before they are inserted. Search commands also translate their input through this table, so they can compare more reliably with what’s in the buffer.
This variable automatically becomes buffer-local when set.
Function: **make-translation-table-from-vector** *vec*
This function returns a translation table made from vec that is an array of 256 elements to map bytes (values 0 through #xFF) to characters. Elements may be `nil` for untranslated bytes. The returned table has a translation table for reverse mapping in the first extra slot, and the value `1` in the second extra slot.
This function provides an easy way to make a private coding system that maps each byte to a specific character. You can specify the returned table and the reverse translation table using the properties `:decode-translation-table` and `:encode-translation-table` respectively in the props argument to `define-coding-system`.
Function: **make-translation-table-from-alist** *alist*
This function is similar to `make-translation-table` but returns a complex translation table rather than a simple one-to-one mapping. Each element of alist is of the form `(from
. to)`, where from and to are either characters or vectors specifying a sequence of characters. If from is a character, that character is translated to to (i.e., to a character or a character sequence). If from is a vector of characters, that sequence is translated to to. The returned table has a translation table for reverse mapping in the first extra slot, and the maximum length of all the from character sequences in the second extra slot.
elisp None #### Deferred JSONRPC requests
In many RPC situations, synchronization between the two communicating endpoints is a matter of correctly designing the RPC application: when synchronization is needed, requests (which are blocking) should be used; when it isn’t, notifications should suffice. However, when Emacs acts as one of these endpoints, asynchronous events (e.g. timer- or process-related) may be triggered while there is still uncertainty about the state of the remote endpoint. Furthermore, acting on these events may only sometimes demand synchronization, depending on the event’s specific nature.
The `:deferred` keyword argument to `jsonrpc-request` and `jsonrpc-async-request` is designed to let the caller indicate that the specific request needs synchronization and its actual issuance may be delayed to the future, until some condition is satisfied. Specifying `:deferred` for a request doesn’t mean it *will* be delayed, only that it *can* be. If the request isn’t sent immediately, `jsonrpc` will make renewed efforts to send it at certain key times during communication, such as when receiving or sending other messages to the endpoint.
Before any attempt to send the request, the application-specific conditions are checked. Since the `jsonrpc` library can’t know what these conditions are, the program can use the `jsonrpc-connection-ready-p` generic function (see [Generic Functions](generic-functions)) to specify them. The default method for this function returns `t`, but you can add overriding methods that return `nil` in some situations, based on the arguments passed to it, which are the `jsonrpc-connection` object (see [JSONRPC Overview](jsonrpc-overview)) and whichever value you passed as the `:deferred` keyword argument.
elisp None ### Printed Representation and Read Syntax
The *printed representation* of an object is the format of the output generated by the Lisp printer (the function `prin1`) for that object. Every data type has a unique printed representation. The *read syntax* of an object is the format of the input accepted by the Lisp reader (the function `read`) for that object. This is not necessarily unique; many kinds of object have more than one syntax. See [Read and Print](read-and-print).
In most cases, an object’s printed representation is also a read syntax for the object. However, some types have no read syntax, since it does not make sense to enter objects of these types as constants in a Lisp program. These objects are printed in *hash notation*, which consists of the characters ‘`#<`’, a descriptive string (typically the type name followed by the name of the object), and a closing ‘`>`’. For example:
```
(current-buffer)
⇒ #<buffer objects.texi>
```
Hash notation cannot be read at all, so the Lisp reader signals the error `invalid-read-syntax` whenever it encounters ‘`#<`’.
In other languages, an expression is text; it has no other form. In Lisp, an expression is primarily a Lisp object and only secondarily the text that is the object’s read syntax. Often there is no need to emphasize this distinction, but you must keep it in the back of your mind, or you will occasionally be very confused.
When you evaluate an expression interactively, the Lisp interpreter first reads the textual representation of it, producing a Lisp object, and then evaluates that object (see [Evaluation](evaluation)). However, evaluation and reading are separate activities. Reading returns the Lisp object represented by the text that is read; the object may or may not be evaluated later. See [Input Functions](input-functions), for a description of `read`, the basic function for reading objects.
elisp None ### Excursions
It is often useful to move point temporarily within a localized portion of the program. This is called an *excursion*, and it is done with the `save-excursion` special form. This construct remembers the initial identity of the current buffer, and its value of point, and restores them after the excursion completes. It is the standard way to move point within one part of a program and avoid affecting the rest of the program, and is used thousands of times in the Lisp sources of Emacs.
If you only need to save and restore the identity of the current buffer, use `save-current-buffer` or `with-current-buffer` instead (see [Current Buffer](current-buffer)). If you need to save or restore window configurations, see the forms described in [Window Configurations](window-configurations) and in [Frame Configurations](frame-configurations).
Special Form: **save-excursion** *body…*
This special form saves the identity of the current buffer and the value of point in it, evaluates body, and finally restores the buffer and its saved value of point. Both saved values are restored even in case of an abnormal exit via `throw` or error (see [Nonlocal Exits](nonlocal-exits)).
The value returned by `save-excursion` is the result of the last form in body, or `nil` if no body forms were given.
Because `save-excursion` only saves point for the buffer that was current at the start of the excursion, any changes made to point in other buffers, during the excursion, will remain in effect afterward. This frequently leads to unintended consequences, so the byte compiler warns if you call `set-buffer` during an excursion:
```
Warning: Use ‘with-current-buffer’ rather than
save-excursion+set-buffer
```
To avoid such problems, you should call `save-excursion` only after setting the desired current buffer, as in the following example:
```
(defun append-string-to-buffer (string buffer)
"Append STRING to the end of BUFFER."
(with-current-buffer buffer
(save-excursion
(goto-char (point-max))
(insert string))))
```
Likewise, `save-excursion` does not restore window-buffer correspondences altered by functions such as `switch-to-buffer`.
**Warning:** Ordinary insertion of text adjacent to the saved point value relocates the saved value, just as it relocates all markers. More precisely, the saved value is a marker with insertion type `nil`. See [Marker Insertion Types](marker-insertion-types). Therefore, when the saved point value is restored, it normally comes before the inserted text.
Macro: **save-mark-and-excursion** *body…*
This macro is like `save-excursion`, but also saves and restores the mark location and `mark-active`. This macro does what `save-excursion` did before Emacs 25.1.
elisp None #### Displaying Faces
When Emacs displays a given piece of text, the visual appearance of the text may be determined by faces drawn from different sources. If these various sources together specify more than one face for a particular character, Emacs merges the attributes of the various faces. Here is the order in which Emacs merges the faces, from highest to lowest priority:
* If the text consists of a special glyph, the glyph can specify a particular face. See [Glyphs](glyphs).
* If the text lies within an active region, Emacs highlights it using the `region` face. See [Standard Faces](https://www.gnu.org/software/emacs/manual/html_node/emacs/Standard-Faces.html#Standard-Faces) in The GNU Emacs Manual.
* If the text lies within an overlay with a non-`nil` `face` property, Emacs applies the face(s) specified by that property. If the overlay has a `mouse-face` property and the mouse is near enough to the overlay, Emacs applies the face or face attributes specified by the `mouse-face` property instead. See [Overlay Properties](overlay-properties). When multiple overlays cover one character, an overlay with higher priority overrides those with lower priority. See [Overlays](overlays).
* If the text contains a `face` or `mouse-face` property, Emacs applies the specified faces and face attributes. See [Special Properties](special-properties). (This is how Font Lock mode faces are applied. See [Font Lock Mode](font-lock-mode).)
* If the text lies within the mode line of the selected window, Emacs applies the `mode-line` face. For the mode line of a non-selected window, Emacs applies the `mode-line-inactive` face. For a header line, Emacs applies the `header-line` face. For a tab line, Emacs applies the `tab-line` face.
* If the text comes from an overlay string via `before-string` or `after-string` properties (see [Overlay Properties](overlay-properties)), or from a display string (see [Other Display Specs](other-display-specs)), and the string doesn’t contain a `face` or `mouse-face` property, or these properties leave some face attributes undefined, but the buffer text affected by the overlay/display property does define a face or those attributes, Emacs applies the face attributes of the “underlying” buffer text. Note that this is so even if the overlay or display string is displayed in the display margins (see [Display Margins](display-margins)).
* If any given attribute has not been specified during the preceding steps, Emacs applies the attribute of the `default` face.
At each stage, if a face has a valid `:inherit` attribute, Emacs treats any attribute with an `unspecified` value as having the corresponding value drawn from the parent face(s). see [Face Attributes](face-attributes). Note that the parent face(s) may also leave the attribute unspecified; in that case, the attribute remains unspecified at the next level of face merging.
elisp None #### Generating Unique File Names
Some programs need to write temporary files. Here is the usual way to construct a name for such a file:
```
(make-temp-file name-of-application)
```
The job of `make-temp-file` is to prevent two different users or two different jobs from trying to use the exact same file name.
Function: **make-temp-file** *prefix &optional dir-flag suffix text*
This function creates a temporary file and returns its name. Emacs creates the temporary file’s name by adding to prefix some random characters that are different in each Emacs job. The result is guaranteed to be a newly created file, containing text if that’s given as a string and empty otherwise. On MS-DOS, this function can truncate prefix to fit into the 8+3 file-name limits. If prefix is a relative file name, it is expanded against `temporary-file-directory`.
```
(make-temp-file "foo")
⇒ "/tmp/foo232J6v"
```
When `make-temp-file` returns, the file has been created and is empty. At that point, you should write the intended contents into the file.
If dir-flag is non-`nil`, `make-temp-file` creates an empty directory instead of an empty file. It returns the file name, not the directory name, of that directory. See [Directory Names](directory-names).
If suffix is non-`nil`, `make-temp-file` adds it at the end of the file name.
If text is a string, `make-temp-file` inserts it in the file.
To prevent conflicts among different libraries running in the same Emacs, each Lisp program that uses `make-temp-file` should have its own prefix. The number added to the end of prefix distinguishes between the same application running in different Emacs jobs. Additional added characters permit a large number of distinct names even in one Emacs job.
The default directory for temporary files is controlled by the variable `temporary-file-directory`. This variable gives the user a uniform way to specify the directory for all temporary files. Some programs use `small-temporary-file-directory` instead, if that is non-`nil`. To use it, you should expand the prefix against the proper directory before calling `make-temp-file`.
User Option: **temporary-file-directory**
This variable specifies the directory name for creating temporary files. Its value should be a directory name (see [Directory Names](directory-names)), but it is good for Lisp programs to cope if the value is a directory’s file name instead. Using the value as the second argument to `expand-file-name` is a good way to achieve that.
The default value is determined in a reasonable way for your operating system; it is based on the `TMPDIR`, `TMP` and `TEMP` environment variables, with a fall-back to a system-dependent name if none of these variables is defined.
Even if you do not use `make-temp-file` to create the temporary file, you should still use this variable to decide which directory to put the file in. However, if you expect the file to be small, you should use `small-temporary-file-directory` first if that is non-`nil`.
User Option: **small-temporary-file-directory**
This variable specifies the directory name for creating certain temporary files, which are likely to be small.
If you want to write a temporary file which is likely to be small, you should compute the directory like this:
```
(make-temp-file
(expand-file-name prefix
(or small-temporary-file-directory
temporary-file-directory)))
```
Function: **make-temp-name** *base-name*
This function generates a string that might be a unique file name. The name starts with base-name, and has several random characters appended to it, which are different in each Emacs job. It is like `make-temp-file` except that (i) it just constructs a name and does not create a file, (ii) base-name should be an absolute file name that is not magic, and (iii) if the returned file name is magic, it might name an existing file. See [Magic File Names](magic-file-names).
**Warning:** In most cases, you should not use this function; use `make-temp-file` instead! This function is susceptible to a race condition, between the `make-temp-name` call and the creation of the file, which in some cases may cause a security hole.
Sometimes, it is necessary to create a temporary file on a remote host or a mounted directory. The following two functions support this.
Function: **make-nearby-temp-file** *prefix &optional dir-flag suffix*
This function is similar to `make-temp-file`, but it creates a temporary file as close as possible to `default-directory`. If prefix is a relative file name, and `default-directory` is a remote file name or located on a mounted file systems, the temporary file is created in the directory returned by the function `temporary-file-directory`. Otherwise, the function `make-temp-file` is used. prefix, dir-flag and suffix have the same meaning as in `make-temp-file`.
```
(let ((default-directory "/ssh:remotehost:"))
(make-nearby-temp-file "foo"))
⇒ "/ssh:remotehost:/tmp/foo232J6v"
```
Function: **temporary-file-directory**
The directory for writing temporary files via `make-nearby-temp-file`. In case of a remote `default-directory`, this is a directory for temporary files on that remote host. If such a directory does not exist, or `default-directory` ought to be located on a mounted file system (see `mounted-file-systems`), the function returns `default-directory`. For a non-remote and non-mounted `default-directory`, the value of the variable `temporary-file-directory` is returned.
In order to extract the local part of the file’s name of a temporary file, use `file-local-name` (see [Magic File Names](magic-file-names)).
| programming_docs |
elisp None ### Truncation
When a line of text extends beyond the right edge of a window, Emacs can *continue* the line (make it wrap to the next screen line), or *truncate* the line (limit it to one screen line). The additional screen lines used to display a long text line are called *continuation* lines. Continuation is not the same as filling; continuation happens on the screen only, not in the buffer contents, and it breaks a line precisely at the right margin, not at a word boundary. See [Filling](filling).
On a graphical display, tiny arrow images in the window fringes indicate truncated and continued lines (see [Fringes](fringes)). On a text terminal, a ‘`$`’ in the rightmost column of the window indicates truncation; a ‘`\`’ on the rightmost column indicates a line that wraps. (The display table can specify alternate characters to use for this; see [Display Tables](display-tables)).
Since wrapping and truncation of text contradict each other, Emacs turns off line truncation when wrapping is requested, and vice versa.
User Option: **truncate-lines**
If this buffer-local variable is non-`nil`, lines that extend beyond the right edge of the window are truncated; otherwise, they are continued. As a special exception, the variable `truncate-partial-width-windows` takes precedence in *partial-width* windows (i.e., windows that do not occupy the entire frame width).
User Option: **truncate-partial-width-windows**
This variable controls line truncation in *partial-width* windows. A partial-width window is one that does not occupy the entire frame width (see [Splitting Windows](splitting-windows)). If the value is `nil`, line truncation is determined by the variable `truncate-lines` (see above). If the value is an integer n, lines are truncated if the partial-width window has fewer than n columns, regardless of the value of `truncate-lines`; if the partial-width window has n or more columns, line truncation is determined by `truncate-lines`. For any other non-`nil` value, lines are truncated in every partial-width window, regardless of the value of `truncate-lines`.
When horizontal scrolling (see [Horizontal Scrolling](horizontal-scrolling)) is in use in a window, that forces truncation.
Variable: **wrap-prefix**
If this buffer-local variable is non-`nil`, it defines a *wrap prefix* which Emacs displays at the start of every continuation line. (If lines are truncated, `wrap-prefix` is never used.) Its value may be a string or an image (see [Other Display Specs](other-display-specs)), or a stretch of whitespace such as specified by the `:width` or `:align-to` display properties (see [Specified Space](specified-space)). The value is interpreted in the same way as a `display` text property. See [Display Property](display-property).
A wrap prefix may also be specified for regions of text, using the `wrap-prefix` text or overlay property. This takes precedence over the `wrap-prefix` variable. See [Special Properties](special-properties).
Variable: **line-prefix**
If this buffer-local variable is non-`nil`, it defines a *line prefix* which Emacs displays at the start of every non-continuation line. Its value may be a string or an image (see [Other Display Specs](other-display-specs)), or a stretch of whitespace such as specified by the `:width` or `:align-to` display properties (see [Specified Space](specified-space)). The value is interpreted in the same way as a `display` text property. See [Display Property](display-property).
A line prefix may also be specified for regions of text using the `line-prefix` text or overlay property. This takes precedence over the `line-prefix` variable. See [Special Properties](special-properties).
elisp None #### Process Filter Functions
A process *filter function* is a function that receives the standard output from the associated process. *All* output from that process is passed to the filter. The default filter simply outputs directly to the process buffer.
By default, the error output from the process, if any, is also passed to the filter function, unless the destination for the standard error stream of the process was separated from the standard output when the process was created. Emacs will only call the filter function during certain function calls. See [Output from Processes](output-from-processes). Note that if any of those functions are called by the filter, the filter may be called recursively.
A filter function must accept two arguments: the associated process and a string, which is output just received from it. The function is then free to do whatever it chooses with the output.
Quitting is normally inhibited within a filter function—otherwise, the effect of typing `C-g` at command level or to quit a user command would be unpredictable. If you want to permit quitting inside a filter function, bind `inhibit-quit` to `nil`. In most cases, the right way to do this is with the macro `with-local-quit`. See [Quitting](quitting).
If an error happens during execution of a filter function, it is caught automatically, so that it doesn’t stop the execution of whatever program was running when the filter function was started. However, if `debug-on-error` is non-`nil`, errors are not caught. This makes it possible to use the Lisp debugger to debug filter functions. See [Debugger](debugger).
Many filter functions sometimes (or always) insert the output in the process’s buffer, mimicking the actions of the default filter. Such filter functions need to make sure that they save the current buffer, select the correct buffer (if different) before inserting output, and then restore the original buffer. They should also check whether the buffer is still alive, update the process marker, and in some cases update the value of point. Here is how to do these things:
```
(defun ordinary-insertion-filter (proc string)
(when (buffer-live-p (process-buffer proc))
(with-current-buffer (process-buffer proc)
(let ((moving (= (point) (process-mark proc))))
```
```
(save-excursion
;; Insert the text, advancing the process marker.
(goto-char (process-mark proc))
(insert string)
(set-marker (process-mark proc) (point)))
(if moving (goto-char (process-mark proc)))))))
```
To make the filter force the process buffer to be visible whenever new text arrives, you could insert a line like the following just before the `with-current-buffer` construct:
```
(display-buffer (process-buffer proc))
```
To force point to the end of the new output, no matter where it was previously, eliminate the variable `moving` from the example and call `goto-char` unconditionally. Note that this doesn’t necessarily move the window point. The default filter actually uses `insert-before-markers` which moves all markers, including the window point. This may move unrelated markers, so it’s generally better to move the window point explicitly, or set its insertion type to `t` (see [Window Point](window-point)).
Note that Emacs automatically saves and restores the match data while executing filter functions. See [Match Data](match-data).
The output to the filter may come in chunks of any size. A program that produces the same output twice in a row may send it as one batch of 200 characters one time, and five batches of 40 characters the next. If the filter looks for certain text strings in the subprocess output, make sure to handle the case where one of these strings is split across two or more batches of output; one way to do this is to insert the received text into a temporary buffer, which can then be searched.
Function: **set-process-filter** *process filter*
This function gives process the filter function filter. If filter is `nil`, it gives the process the default filter, which inserts the process output into the process buffer. If filter is `t`, Emacs stops accepting output from the process, unless it’s a network server process that listens for incoming connections.
Function: **process-filter** *process*
This function returns the filter function of process.
In case the process’s output needs to be passed to several filters, you can use `add-function` to combine an existing filter with a new one. See [Advising Functions](advising-functions).
Here is an example of the use of a filter function:
```
(defun keep-output (process output)
(setq kept (cons output kept)))
⇒ keep-output
```
```
(setq kept nil)
⇒ nil
```
```
(set-process-filter (get-process "shell") 'keep-output)
⇒ keep-output
```
```
(process-send-string "shell" "ls ~/other\n")
⇒ nil
kept
⇒ ("lewis@slug:$ "
```
```
"FINAL-W87-SHORT.MSS backup.otl kolstad.mss~
address.txt backup.psf kolstad.psf
backup.bib~ david.mss resume-Dec-86.mss~
backup.err david.psf resume-Dec.psf
backup.mss dland syllabus.mss
"
"#backups.mss# backup.mss~ kolstad.mss
")
```
elisp None ### Session Management
Emacs supports the X Session Management Protocol, which is used to suspend and restart applications. In the X Window System, a program called the *session manager* is responsible for keeping track of the applications that are running. When the X server shuts down, the session manager asks applications to save their state, and delays the actual shutdown until they respond. An application can also cancel the shutdown.
When the session manager restarts a suspended session, it directs these applications to individually reload their saved state. It does this by specifying a special command-line argument that says what saved session to restore. For Emacs, this argument is ‘`--smid session`’.
Variable: **emacs-save-session-functions**
Emacs supports saving state via a hook called `emacs-save-session-functions`. Emacs runs this hook when the session manager tells it that the window system is shutting down. The functions are called with no arguments, and with the current buffer set to a temporary buffer. Each function can use `insert` to add Lisp code to this buffer. At the end, Emacs saves the buffer in a file, called the *session file*.
Subsequently, when the session manager restarts Emacs, it loads the session file automatically (see [Loading](loading)). This is performed by a function named `emacs-session-restore`, which is called during startup. See [Startup Summary](startup-summary).
If a function in `emacs-save-session-functions` returns non-`nil`, Emacs tells the session manager to cancel the shutdown.
Here is an example that just inserts some text into `\*scratch\*` when Emacs is restarted by the session manager.
```
(add-hook 'emacs-save-session-functions 'save-yourself-test)
```
```
(defun save-yourself-test ()
(insert
(format "%S" '(with-current-buffer "*scratch*"
(insert "I am restored"))))
nil)
```
elisp None #### Symbol Type
A *symbol* in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary Lisp use, with one single obarray (see [Creating Symbols](creating-symbols)), a symbol’s name is unique—no two symbols have the same name.
A symbol can serve as a variable, as a function name, or to hold a property list. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended. But you can use one symbol in all of these ways, independently.
A symbol whose name starts with a colon (‘`:`’) is called a *keyword symbol*. These symbols automatically act as constants, and are normally used only by comparing an unknown symbol with a few specific alternatives. See [Constant Variables](constant-variables).
A symbol name can contain any characters whatever. Most symbol names are written with letters, digits, and the punctuation characters ‘`-+=\*/`’. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a ‘`\`’ at the beginning of the name to force interpretation as a symbol.) The characters ‘`\_~!@$%^&:<>{}?`’ are less often used but also require no special punctuation. Any other characters may be included in a symbol’s name by escaping them with a backslash. In contrast to its use in strings, however, a backslash in the name of a symbol simply quotes the single character that follows the backslash. For example, in a string, ‘`\t`’ represents a tab character; in the name of a symbol, however, ‘`\t`’ merely quotes the letter ‘`t`’. To have a symbol with a tab character in its name, you must actually use a tab (preceded with a backslash). But it’s rare to do such a thing.
> **Common Lisp note:** In Common Lisp, lower case letters are always folded to upper case, unless they are explicitly escaped. In Emacs Lisp, upper case and lower case letters are distinct.
>
>
>
Here are several examples of symbol names. Note that the ‘`+`’ in the fourth example is escaped to prevent it from being read as a number. This is not necessary in the sixth example because the rest of the name makes it invalid as a number.
```
foo ; A symbol named ‘`foo`’.
FOO ; A symbol named ‘`FOO`’, different from ‘`foo`’.
```
```
1+ ; A symbol named ‘`1+`’
; (not ‘`+1`’, which is an integer).
```
```
\+1 ; A symbol named ‘`+1`’
; (not a very readable name).
```
```
\(*\ 1\ 2\) ; A symbol named ‘`(\* 1 2)`’ (a worse name).
+-*/_~!@$%^&=:<>{} ; A symbol named ‘`+-\*/\_~!@$%^&=:<>{}`’.
; These characters need not be escaped.
```
As an exception to the rule that a symbol’s name serves as its printed representation, ‘`##`’ is the printed representation for an interned symbol whose name is an empty string. Furthermore, ‘`#:foo`’ is the printed representation for an uninterned symbol whose name is foo. (Normally, the Lisp reader interns all symbols; see [Creating Symbols](creating-symbols).)
elisp None #### Entering the Debugger on an Error
The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error.
However, entry to the debugger is not a normal consequence of an error. Many commands signal Lisp errors when invoked inappropriately, and during ordinary editing it would be very inconvenient to enter the debugger each time this happens. So if you want errors to enter the debugger, set the variable `debug-on-error` to non-`nil`. (The command `toggle-debug-on-error` provides an easy way to do this.)
User Option: **debug-on-error**
This variable determines whether the debugger is called when an error is signaled and not handled. If `debug-on-error` is `t`, all kinds of errors call the debugger, except those listed in `debug-ignored-errors` (see below). If it is `nil`, none call the debugger.
The value can also be a list of error conditions (see [Signaling Errors](signaling-errors)). Then the debugger is called only for error conditions in this list (except those also listed in `debug-ignored-errors`). For example, if you set `debug-on-error` to the list `(void-variable)`, the debugger is only called for errors about a variable that has no value.
Note that `eval-expression-debug-on-error` overrides this variable in some cases; see below.
When this variable is non-`nil`, Emacs does not create an error handler around process filter functions and sentinels. Therefore, errors in these functions also invoke the debugger. See [Processes](processes).
User Option: **debug-ignored-errors**
This variable specifies errors which should not enter the debugger, regardless of the value of `debug-on-error`. Its value is a list of error condition symbols and/or regular expressions. If the error has any of those condition symbols, or if the error message matches any of the regular expressions, then that error does not enter the debugger.
The normal value of this variable includes `user-error`, as well as several errors that happen often during editing but rarely result from bugs in Lisp programs. However, “rarely” is not “never”; if your program fails with an error that matches this list, you may try changing this list to debug the error. The easiest way is usually to set `debug-ignored-errors` to `nil`.
User Option: **eval-expression-debug-on-error**
If this variable has a non-`nil` value (the default), running the command `eval-expression` causes `debug-on-error` to be temporarily bound to `t`. See [Evaluating Emacs Lisp Expressions](https://www.gnu.org/software/emacs/manual/html_node/emacs/Lisp-Eval.html#Lisp-Eval) in The GNU Emacs Manual.
If `eval-expression-debug-on-error` is `nil`, then the value of `debug-on-error` is not changed during `eval-expression`.
User Option: **debug-on-signal**
Normally, errors caught by `condition-case` never invoke the debugger. The `condition-case` gets a chance to handle the error before the debugger gets a chance.
If you change `debug-on-signal` to a non-`nil` value, the debugger gets the first chance at every error, regardless of the presence of `condition-case`. (To invoke the debugger, the error must still fulfill the criteria specified by `debug-on-error` and `debug-ignored-errors`.)
For example, setting this variable is useful to get a backtrace from code evaluated by emacsclient’s `--eval` option. If Lisp code evaluated by emacsclient signals an error while this variable is non-`nil`, the backtrace will popup in the running Emacs.
**Warning:** Setting this variable to non-`nil` may have annoying effects. Various parts of Emacs catch errors in the normal course of affairs, and you may not even realize that errors happen there. If you need to debug code wrapped in `condition-case`, consider using `condition-case-unless-debug` (see [Handling Errors](handling-errors)).
User Option: **debug-on-event**
If you set `debug-on-event` to a special event (see [Special Events](special-events)), Emacs will try to enter the debugger as soon as it receives this event, bypassing `special-event-map`. At present, the only supported values correspond to the signals `SIGUSR1` and `SIGUSR2` (this is the default). This can be helpful when `inhibit-quit` is set and Emacs is not otherwise responding.
Variable: **debug-on-message**
If you set `debug-on-message` to a regular expression, Emacs will enter the debugger if it displays a matching message in the echo area. For example, this can be useful when trying to find the cause of a particular message.
To debug an error that happens during loading of the init file, use the option ‘`--debug-init`’. This binds `debug-on-error` to `t` while loading the init file, and bypasses the `condition-case` which normally catches errors in the init file.
elisp None #### Programmed Completion
Sometimes it is not possible or convenient to create an alist or an obarray containing all the intended possible completions ahead of time. In such a case, you can supply your own function to compute the completion of a given string. This is called *programmed completion*. Emacs uses programmed completion when completing file names (see [File Name Completion](file-name-completion)), among many other cases.
To use this feature, pass a function as the collection argument to `completing-read`. The function `completing-read` arranges to pass your completion function along to `try-completion`, `all-completions`, and other basic completion functions, which will then let your function do all the work.
The completion function should accept three arguments:
* The string to be completed.
* A predicate function with which to filter possible matches, or `nil` if none. The function should call the predicate for each possible match, and ignore the match if the predicate returns `nil`.
* A flag specifying the type of completion operation to perform; see [Basic Completion](basic-completion), for the details of those operations. This flag may be one of the following values. `nil`
This specifies a `try-completion` operation. The function should return `nil` if there are no matches; it should return `t` if the specified string is a unique and exact match; and it should return the longest common prefix substring of all matches otherwise.
`t`
This specifies an `all-completions` operation. The function should return a list of all possible completions of the specified string.
`lambda`
This specifies a `test-completion` operation. The function should return `t` if the specified string is an exact match for some completion alternative; `nil` otherwise.
`(boundaries . suffix)`
This specifies a `completion-boundaries` operation. The function should return `(boundaries start . end)`, where start is the position of the beginning boundary in the specified string, and end is the position of the end boundary in suffix.
If a Lisp program returns nontrivial boundaries, it should make sure that the `all-completions` operation is consistent with them. The completions returned by `all-completions` should only pertain to the piece of the prefix and suffix covered by the completion boundaries. See [Basic Completion](basic-completion), for the precise expected semantics of completion boundaries.
`metadata` This specifies a request for information about the state of the current completion. The return value should have the form `(metadata . alist)`, where alist is an alist whose elements are described below.
If the flag has any other value, the completion function should return `nil`.
The following is a list of metadata entries that a completion function may return in response to a `metadata` flag argument:
`category`
The value should be a symbol describing what kind of text the completion function is trying to complete. If the symbol matches one of the keys in `completion-category-overrides`, the usual completion behavior is overridden. See [Completion Variables](completion-variables).
`annotation-function`
The value should be a function for *annotating* completions. The function should take one argument, string, which is a possible completion. It should return a string, which is displayed after the completion string in the `\*Completions\*` buffer. Unless this function puts own face on the annotation suffix string, the `completions-annotations` face is added by default to that string.
`affixation-function`
The value should be a function for adding prefixes and suffixes to completions. The function should take one argument, completions, which is a list of possible completions. It should return such a list of completions where each element contains a list of three elements: a completion, a prefix which is displayed before the completion string in the `\*Completions\*` buffer, and a suffix displayed after the completion string. This function takes priority over `annotation-function`.
`group-function`
The value should be a function for grouping the completion candidates. The function must take two arguments, completion, which is a completion candidate and transform, which is a boolean flag. If transform is `nil`, the function must return the group title of the group to which the candidate belongs. The returned title can also be `nil`. Otherwise the function must return the transformed candidate. The transformation can for example remove a redundant prefix, which is displayed in the group title.
`display-sort-function`
The value should be a function for sorting completions. The function should take one argument, a list of completion strings, and return a sorted list of completion strings. It is allowed to alter the input list destructively.
`cycle-sort-function` The value should be a function for sorting completions, when `completion-cycle-threshold` is non-`nil` and the user is cycling through completion alternatives. See [Completion Options](https://www.gnu.org/software/emacs/manual/html_node/emacs/Completion-Options.html#Completion-Options) in The GNU Emacs Manual. Its argument list and return value are the same as for `display-sort-function`.
Function: **completion-table-dynamic** *function &optional switch-buffer*
This function is a convenient way to write a function that can act as a programmed completion function. The argument function should be a function that takes one argument, a string, and returns a completion table (see [Basic Completion](basic-completion)) containing all the possible completions. The table returned by function can also include elements that don’t match the string argument; they are automatically filtered out by `completion-table-dynamic`. In particular, function can ignore its argument and return a full list of all possible completions. You can think of `completion-table-dynamic` as a transducer between function and the interface for programmed completion functions.
If the optional argument switch-buffer is non-`nil`, and completion is performed in the minibuffer, function will be called with current buffer set to the buffer from which the minibuffer was entered.
The return value of `completion-table-dynamic` is a function that can be used as the 2nd argument to `try-completion` and `all-completions`. Note that this function will always return empty metadata and trivial boundaries.
Function: **completion-table-with-cache** *function &optional ignore-case*
This is a wrapper for `completion-table-dynamic` that saves the last argument-result pair. This means that multiple lookups with the same argument only need to call function once. This can be useful when a slow operation is involved, such as calling an external process.
| programming_docs |
elisp None #### Symbol Forms
When a symbol is evaluated, it is treated as a variable. The result is the variable’s value, if it has one. If the symbol has no value as a variable, the Lisp interpreter signals an error. For more information on the use of variables, see [Variables](variables).
In the following example, we set the value of a symbol with `setq`. Then we evaluate the symbol, and get back the value that `setq` stored.
```
(setq a 123)
⇒ 123
```
```
(eval 'a)
⇒ 123
```
```
a
⇒ 123
```
The symbols `nil` and `t` are treated specially, so that the value of `nil` is always `nil`, and the value of `t` is always `t`; you cannot set or bind them to any other values. Thus, these two symbols act like self-evaluating forms, even though `eval` treats them like any other symbol. A symbol whose name starts with ‘`:`’ also self-evaluates in the same way; likewise, its value ordinarily cannot be changed. See [Constant Variables](constant-variables).
elisp None #### Button Buffer Commands
These are commands and functions for locating and operating on buttons in an Emacs buffer.
`push-button` is the command that a user uses to actually push a button, and is bound by default in the button itself to RET and to mouse-2 using a local keymap in the button’s overlay or text properties. Commands that are useful outside the buttons itself, such as `forward-button` and `backward-button` are additionally available in the keymap stored in `button-buffer-map`; a mode which uses buttons may want to use `button-buffer-map` as a parent keymap for its keymap. Alternatively, the `button-mode` can be switched on for much the same effect: It’s a minor mode that does nothing else than install `button-buffer-map` as a minor mode keymap.
If the button has a non-`nil` `follow-link` property, and `mouse-1-click-follows-link` is set, a quick mouse-1 click will also activate the `push-button` command. See [Clickable Text](clickable-text).
Command: **push-button** *&optional pos use-mouse-action*
Perform the action specified by a button at location pos. pos may be either a buffer position or a mouse-event. If use-mouse-action is non-`nil`, or pos is a mouse-event (see [Mouse Events](mouse-events)), try to invoke the button’s `mouse-action` property instead of `action`; if the button has no `mouse-action` property, use `action` as normal. pos defaults to point, except when `push-button` is invoked interactively as the result of a mouse-event, in which case, the mouse event’s position is used. If there’s no button at pos, do nothing and return `nil`, otherwise return `t`.
Command: **forward-button** *n &optional wrap display-message no-error*
Move to the nth next button, or nth previous button if n is negative. If n is zero, move to the start of any button at point. If wrap is non-`nil`, moving past either end of the buffer continues from the other end. If display-message is non-`nil`, the button’s help-echo string is displayed. Any button with a non-`nil` `skip` property is skipped over. Returns the button found, and signals an error if no buttons can be found. If no-error is non-`nil`, return nil instead of signaling the error.
Command: **backward-button** *n &optional wrap display-message no-error*
Move to the nth previous button, or nth next button if n is negative. If n is zero, move to the start of any button at point. If wrap is non-`nil`, moving past either end of the buffer continues from the other end. If display-message is non-`nil`, the button’s help-echo string is displayed. Any button with a non-`nil` `skip` property is skipped over. Returns the button found, and signals an error if no buttons can be found. If no-error is non-`nil`, return nil instead of signaling the error.
Function: **next-button** *pos &optional count-current*
Function: **previous-button** *pos &optional count-current*
Return the next button after (for `next-button`) or before (for `previous-button`) position pos in the current buffer. If count-current is non-`nil`, count any button at pos in the search, instead of starting at the next button.
elisp None #### Text Properties in Strings
A string can hold properties for the characters it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to copy the text’s properties with no special effort. See [Text Properties](text-properties), for an explanation of what text properties mean. Strings with text properties use a special read and print syntax:
```
#("characters" property-data...)
```
where property-data consists of zero or more elements, in groups of three as follows:
```
beg end plist
```
The elements beg and end are integers, and together specify a range of indices in the string; plist is the property list for that range. For example,
```
#("foo bar" 0 3 (face bold) 3 4 nil 4 7 (face italic))
```
represents a string whose textual contents are ‘`foo bar`’, in which the first three characters have a `face` property with value `bold`, and the last three have a `face` property with value `italic`. (The fourth character has no text properties, so its property list is `nil`. It is not actually necessary to mention ranges with `nil` as the property list, since any characters not mentioned in any range will default to having no properties.)
elisp None ### Buffers and Windows
This section describes low-level functions for examining and setting the contents of windows. See [Switching Buffers](switching-buffers), for higher-level functions for displaying a specific buffer in a window.
Function: **window-buffer** *&optional window*
This function returns the buffer that window is displaying. If window is omitted or `nil` it defaults to the selected window. If window is an internal window, this function returns `nil`.
Function: **set-window-buffer** *window buffer-or-name &optional keep-margins*
This function makes window display buffer-or-name. window should be a live window; if `nil`, it defaults to the selected window. buffer-or-name should be a buffer, or the name of an existing buffer. This function does not change which window is selected, nor does it directly change which buffer is current (see [Current Buffer](current-buffer)). Its return value is `nil`.
If window is *strongly dedicated* to a buffer and buffer-or-name does not specify that buffer, this function signals an error. See [Dedicated Windows](dedicated-windows).
By default, this function resets window’s position, display margins, fringe widths, and scroll bar settings, based on the local variables in the specified buffer. However, if the optional argument keep-margins is non-`nil`, it leaves window’s display margins, fringes and scroll bar settings alone.
When writing an application, you should normally use `display-buffer` (see [Choosing Window](choosing-window)) or the higher-level functions described in [Switching Buffers](switching-buffers), instead of calling `set-window-buffer` directly.
This runs `window-scroll-functions`, followed by `window-configuration-change-hook`. See [Window Hooks](window-hooks).
Variable: **buffer-display-count**
This buffer-local variable records the number of times a buffer has been displayed in a window. It is incremented each time `set-window-buffer` is called for the buffer.
Variable: **buffer-display-time**
This buffer-local variable records the time at which a buffer was last displayed in a window. The value is `nil` if the buffer has never been displayed. It is updated each time `set-window-buffer` is called for the buffer, with the value returned by `current-time` (see [Time of Day](time-of-day)).
Function: **get-buffer-window** *&optional buffer-or-name all-frames*
This function returns the first window displaying buffer-or-name in the cyclic ordering of windows, starting from the selected window (see [Cyclic Window Ordering](cyclic-window-ordering)). If no such window exists, the return value is `nil`.
buffer-or-name should be a buffer or the name of a buffer; if omitted or `nil`, it defaults to the current buffer. The optional argument all-frames specifies which windows to consider:
* `t` means consider windows on all existing frames.
* `visible` means consider windows on all visible frames.
* 0 means consider windows on all visible or iconified frames.
* A frame means consider windows on that frame only.
* Any other value means consider windows on the selected frame.
Note that these meanings differ slightly from those of the all-frames argument to `next-window` (see [Cyclic Window Ordering](cyclic-window-ordering)). This function may be changed in a future version of Emacs to eliminate this discrepancy.
Function: **get-buffer-window-list** *&optional buffer-or-name minibuf all-frames*
This function returns a list of all windows currently displaying buffer-or-name. buffer-or-name should be a buffer or the name of an existing buffer. If omitted or `nil`, it defaults to the current buffer. If the currently selected window displays buffer-or-name, it will be the first in the list returned by this function.
The arguments minibuf and all-frames have the same meanings as in the function `next-window` (see [Cyclic Window Ordering](cyclic-window-ordering)). Note that the all-frames argument does *not* behave exactly like in `get-buffer-window`.
Command: **replace-buffer-in-windows** *&optional buffer-or-name*
This command replaces buffer-or-name with some other buffer, in all windows displaying it. buffer-or-name should be a buffer, or the name of an existing buffer; if omitted or `nil`, it defaults to the current buffer.
The replacement buffer in each window is chosen via `switch-to-prev-buffer` (see [Window History](window-history)). With the exception of side windows (see [Side Windows](side-windows)), any dedicated window displaying buffer-or-name is deleted if possible (see [Dedicated Windows](dedicated-windows)). If such a window is the only window on its frame and there are other frames on the same terminal, the frame is deleted as well. If the dedicated window is the only window on the only frame on its terminal, the buffer is replaced anyway.
elisp None Command Loop
------------
When you run Emacs, it enters the *editor command loop* almost immediately. This loop reads key sequences, executes their definitions, and displays the results. In this chapter, we describe how these things are done, and the subroutines that allow Lisp programs to do them.
| | | |
| --- | --- | --- |
| • [Command Overview](command-overview) | | How the command loop reads commands. |
| • [Defining Commands](defining-commands) | | Specifying how a function should read arguments. |
| • [Interactive Call](interactive-call) | | Calling a command, so that it will read arguments. |
| • [Distinguish Interactive](distinguish-interactive) | | Making a command distinguish interactive calls. |
| • [Command Loop Info](command-loop-info) | | Variables set by the command loop for you to examine. |
| • [Adjusting Point](adjusting-point) | | Adjustment of point after a command. |
| • [Input Events](input-events) | | What input looks like when you read it. |
| • [Reading Input](reading-input) | | How to read input events from the keyboard or mouse. |
| • [Special Events](special-events) | | Events processed immediately and individually. |
| • [Waiting](waiting) | | Waiting for user input or elapsed time. |
| • [Quitting](quitting) | | How `C-g` works. How to catch or defer quitting. |
| • [Prefix Command Arguments](prefix-command-arguments) | | How the commands to set prefix args work. |
| • [Recursive Editing](recursive-editing) | | Entering a recursive edit, and why you usually shouldn’t. |
| • [Disabling Commands](disabling-commands) | | How the command loop handles disabled commands. |
| • [Command History](command-history) | | How the command history is set up, and how accessed. |
| • [Keyboard Macros](keyboard-macros) | | How keyboard macros are implemented. |
elisp None ### Minibuffer Miscellany
Function: **minibufferp** *&optional buffer-or-name live*
This function returns non-`nil` if buffer-or-name is a minibuffer. If buffer-or-name is omitted or `nil`, it tests the current buffer. When live is non-`nil`, the function returns non-`nil` only when buffer-or-name is an active minibuffer.
Variable: **minibuffer-setup-hook**
This is a normal hook that is run whenever a minibuffer is entered. See [Hooks](hooks).
Macro: **minibuffer-with-setup-hook** *function &rest body*
This macro executes body after arranging for the specified function to be called via `minibuffer-setup-hook`. By default, function is called before the other functions in the `minibuffer-setup-hook` list, but if function is of the form `(:append func)`, func will be called *after* the other hook functions.
The body forms should not use the minibuffer more than once. If the minibuffer is re-entered recursively, function will only be called once, for the outermost use of the minibuffer.
Variable: **minibuffer-exit-hook**
This is a normal hook that is run whenever a minibuffer is exited. See [Hooks](hooks).
Variable: **minibuffer-help-form**
The current value of this variable is used to rebind `help-form` locally inside the minibuffer (see [Help Functions](help-functions)).
Variable: **minibuffer-scroll-window**
If the value of this variable is non-`nil`, it should be a window object. When the function `scroll-other-window` is called in the minibuffer, it scrolls this window (see [Textual Scrolling](textual-scrolling)).
Function: **minibuffer-selected-window**
This function returns the window that was selected just before the minibuffer window was selected. If the selected window is not a minibuffer window, it returns `nil`.
Function: **minibuffer-message** *string &rest args*
This function displays string temporarily at the end of the minibuffer text, for a few seconds, or until the next input event arrives, whichever comes first. The variable `minibuffer-message-timeout` specifies the number of seconds to wait in the absence of input. It defaults to 2. If args is non-`nil`, the actual message is obtained by passing string and args through `format-message`. See [Formatting Strings](formatting-strings).
Command: **minibuffer-inactive-mode**
This is the major mode used in inactive minibuffers. It uses keymap `minibuffer-inactive-mode-map`. This can be useful if the minibuffer is in a separate frame. See [Minibuffers and Frames](minibuffers-and-frames).
elisp None ### Replacing Buffer Text
You can use the following function to replace the text of one buffer with the text of another buffer:
Command: **replace-buffer-contents** *source &optional max-secs max-costs*
This function replaces the accessible portion of the current buffer with the accessible portion of the buffer source. source may either be a buffer object or the name of a buffer. When `replace-buffer-contents` succeeds, the text of the accessible portion of the current buffer will be equal to the text of the accessible portion of the source buffer.
This function attempts to keep point, markers, text properties, and overlays in the current buffer intact. One potential case where this behavior is useful is external code formatting programs: they typically write the reformatted text into a temporary buffer or file, and using `delete-region` and `insert-buffer-substring` would destroy these properties. However, the latter combination is typically faster (See [Deletion](deletion), and [Insertion](insertion)).
For its working, `replace-buffer-contents` needs to compare the contents of the original buffer with that of source which is a costly operation if the buffers are huge and there is a high number of differences between them. In order to keep `replace-buffer-contents`’s runtime in bounds, it has two optional arguments.
max-secs defines a hard boundary in terms of seconds. If given and exceeded, it will fall back to `delete-region` and `insert-buffer-substring`.
max-costs defines the quality of the difference computation. If the actual costs exceed this limit, heuristics are used to provide a faster but suboptimal solution. The default value is 1000000.
`replace-buffer-contents` returns t if a non-destructive replacement could be performed. Otherwise, i.e., if max-secs was exceeded, it returns nil.
Function: **replace-region-contents** *beg end replace-fn &optional max-secs max-costs*
This function replaces the region between beg and end using the given replace-fn. The function replace-fn is run in the current buffer narrowed to the specified region and it should return either a string or a buffer replacing the region.
The replacement is performed using `replace-buffer-contents` (see above) which also describes the max-secs and max-costs arguments and the return value.
Note: If the replacement is a string, it will be placed in a temporary buffer so that `replace-buffer-contents` can operate on it. Therefore, if you already have the replacement in a buffer, it makes no sense to convert it to a string using `buffer-substring` or similar.
elisp None ### Packaging Basics
A package is either a *simple package* or a *multi-file package*. A simple package is stored in a package archive as a single Emacs Lisp file, while a multi-file package is stored as a tar file (containing multiple Lisp files, and possibly non-Lisp files such as a manual).
In ordinary usage, the difference between simple packages and multi-file packages is relatively unimportant; the Package Menu interface makes no distinction between them. However, the procedure for creating them differs, as explained in the following sections.
Each package (whether simple or multi-file) has certain *attributes*:
Name
A short word (e.g., ‘`auctex`’). This is usually also the symbol prefix used in the program (see [Coding Conventions](https://www.gnu.org/software/emacs/manual/html_node/elisp/Coding-Conventions.html)).
Version
A version number, in a form that the function `version-to-list` understands (e.g., ‘`11.86`’). Each release of a package should be accompanied by an increase in the version number so that it will be recognized as an upgrade by users querying the package archive.
Brief description
This is shown when the package is listed in the Package Menu. It should occupy a single line, ideally in 36 characters or less.
Long description
This is shown in the buffer created by `C-h P` (`describe-package`), following the package’s brief description and installation status. It normally spans multiple lines, and should fully describe the package’s capabilities and how to begin using it once it is installed.
Dependencies A list of other packages (possibly including minimal acceptable version numbers) on which this package depends. The list may be empty, meaning this package has no dependencies. Otherwise, installing this package also automatically installs its dependencies, recursively; if any dependency cannot be found, the package cannot be installed.
Installing a package, either via the command `package-install-file`, or via the Package Menu, creates a subdirectory of `package-user-dir` named `name-version`, where name is the package’s name and version its version (e.g., `~/.emacs.d/elpa/auctex-11.86/`). We call this the package’s *content directory*. It is where Emacs puts the package’s contents (the single Lisp file for a simple package, or the files extracted from a multi-file package).
Emacs then searches every Lisp file in the content directory for autoload magic comments (see [Autoload](autoload)). These autoload definitions are saved to a file named `name-autoloads.el` in the content directory. They are typically used to autoload the principal user commands defined in the package, but they can also perform other tasks, such as adding an element to `auto-mode-alist` (see [Auto Major Mode](auto-major-mode)). Note that a package typically does *not* autoload every function and variable defined within it—only the handful of commands typically called to begin using the package. Emacs then byte-compiles every Lisp file in the package.
After installation, the installed package is *loaded*: Emacs adds the package’s content directory to `load-path`, and evaluates the autoload definitions in `name-autoloads.el`.
Whenever Emacs starts up, it automatically calls the function `package-activate-all` to make installed packages available to the current session. This is done after loading the early init file, but before loading the regular init file (see [Startup Summary](startup-summary)). Packages are not automatically made available if the user option `package-enable-at-startup` is set to `nil` in the early init file.
Function: **package-activate-all**
This function makes the packages available to the current session. The user option `package-load-list` specifies which packages to make available; by default, all installed packages are made available. See [Package Installation](https://www.gnu.org/software/emacs/manual/html_node/emacs/Package-Installation.html#Package-Installation) in The GNU Emacs Manual.
In most cases, you should not need to call `package-activate-all`, as this is done automatically during startup. Simply make sure to put any code that should run before `package-activate-all` in the early init file, and any code that should run after it in the primary init file (see [Init File](https://www.gnu.org/software/emacs/manual/html_node/emacs/Init-File.html#Init-File) in The GNU Emacs Manual).
Command: **package-initialize** *&optional no-activate*
This function initializes Emacs’ internal record of which packages are installed, and then calls `package-activate-all`.
The optional argument no-activate, if non-`nil`, causes Emacs to update its record of installed packages without actually making them available.
| programming_docs |
elisp None #### Delayed Warnings
Sometimes, you may wish to avoid showing a warning while a command is running, and only show it only after the end of the command. You can use the function `delay-warning` for this.
Function: **delay-warning** *type message &optional level buffer-name*
This function is the delayed counterpart to `display-warning` (see [Warning Basics](warning-basics)), and it is called with the same arguments. The warning message is queued into `delayed-warnings-list`.
Variable: **delayed-warnings-list**
The value of this variable is a list of warnings to be displayed after the current command has finished. Each element must be a list
```
(type message [level [buffer-name]])
```
with the same form, and the same meanings, as the argument list of `display-warning`. Immediately after running `post-command-hook` (see [Command Overview](command-overview)), the Emacs command loop displays all the warnings specified by this variable, then resets it to `nil`.
Programs which need to further customize the delayed warnings mechanism can change the variable `delayed-warnings-hook`:
Variable: **delayed-warnings-hook**
This is a normal hook which is run by the Emacs command loop, after `post-command-hook`, in order to process and display delayed warnings.
Its default value is a list of two functions:
```
(collapse-delayed-warnings display-delayed-warnings)
```
The function `collapse-delayed-warnings` removes repeated entries from `delayed-warnings-list`. The function `display-delayed-warnings` calls `display-warning` on each of the entries in `delayed-warnings-list`, in turn, and then sets `delayed-warnings-list` to `nil`.
elisp None ### Searching and Case
By default, searches in Emacs ignore the case of the text they are searching through; if you specify searching for ‘`FOO`’, then ‘`Foo`’ or ‘`foo`’ is also considered a match. This applies to regular expressions, too; thus, ‘`[aB]`’ would match ‘`a`’ or ‘`A`’ or ‘`b`’ or ‘`B`’.
If you do not want this feature, set the variable `case-fold-search` to `nil`. Then all letters must match exactly, including case. This is a buffer-local variable; altering the variable affects only the current buffer. (See [Intro to Buffer-Local](intro-to-buffer_002dlocal).) Alternatively, you may change the default value. In Lisp code, you will more typically use `let` to bind `case-fold-search` to the desired value.
Note that the user-level incremental search feature handles case distinctions differently. When the search string contains only lower case letters, the search ignores case, but when the search string contains one or more upper case letters, the search becomes case-sensitive. But this has nothing to do with the searching functions used in Lisp code. See [Incremental Search](https://www.gnu.org/software/emacs/manual/html_node/emacs/Incremental-Search.html#Incremental-Search) in The GNU Emacs Manual.
User Option: **case-fold-search**
This buffer-local variable determines whether searches should ignore case. If the variable is `nil` they do not ignore case; otherwise (and by default) they do ignore case.
User Option: **case-replace**
This variable determines whether the higher-level replacement functions should preserve case. If the variable is `nil`, that means to use the replacement text verbatim. A non-`nil` value means to convert the case of the replacement text according to the text being replaced.
This variable is used by passing it as an argument to the function `replace-match`. See [Replacing Match](replacing-match).
elisp None ### Functions that Create Subprocesses
There are three primitives that create a new subprocess in which to run a program. One of them, `make-process`, creates an asynchronous process and returns a process object (see [Asynchronous Processes](asynchronous-processes)). The other two, `call-process` and `call-process-region`, create a synchronous process and do not return a process object (see [Synchronous Processes](synchronous-processes)). There are various higher-level functions that make use of these primitives to run particular types of process.
Synchronous and asynchronous processes are explained in the following sections. Since the three functions are all called in a similar fashion, their common arguments are described here.
In all cases, the functions specify the program to be run. An error is signaled if the file is not found or cannot be executed. If the file name is relative, the variable `exec-path` contains a list of directories to search. Emacs initializes `exec-path` when it starts up, based on the value of the environment variable `PATH`. The standard file name constructs, ‘`~`’, ‘`.`’, and ‘`..`’, are interpreted as usual in `exec-path`, but environment variable substitutions (‘`$HOME`’, etc.) are not recognized; use `substitute-in-file-name` to perform them (see [File Name Expansion](file-name-expansion)). `nil` in this list refers to `default-directory`.
Executing a program can also try adding suffixes to the specified name:
User Option: **exec-suffixes**
This variable is a list of suffixes (strings) to try adding to the specified program file name. The list should include `""` if you want the name to be tried exactly as specified. The default value is system-dependent.
**Please note:** The argument program contains only the name of the program file; it may not contain any command-line arguments. You must use a separate argument, args, to provide those, as described below.
Each of the subprocess-creating functions has a buffer-or-name argument that specifies where the output from the program will go. It should be a buffer or a buffer name; if it is a buffer name, that will create the buffer if it does not already exist. It can also be `nil`, which says to discard the output, unless a custom filter function handles it. (See [Filter Functions](filter-functions), and [Read and Print](read-and-print).) Normally, you should avoid having multiple processes send output to the same buffer because their output would be intermixed randomly. For synchronous processes, you can send the output to a file instead of a buffer (and the corresponding argument is therefore more appropriately called destination). By default, both standard output and standard error streams go to the same destination, but all the 3 primitives allow optionally to direct the standard error stream to a different destination.
All three of the subprocess-creating functions allow to specify command-line arguments for the process to run. For `call-process` and `call-process-region`, these come in the form of a `&rest` argument, args. For `make-process`, both the program to run and its command-line arguments are specified as a list of strings. The command-line arguments must all be strings, and they are supplied to the program as separate argument strings. Wildcard characters and other shell constructs have no special meanings in these strings, since the strings are passed directly to the specified program.
The subprocess inherits its environment from Emacs, but you can specify overrides for it with `process-environment`. See [System Environment](system-environment). The subprocess gets its current directory from the value of `default-directory`.
Variable: **exec-directory**
The value of this variable is a string, the name of a directory that contains programs that come with GNU Emacs and are intended for Emacs to invoke. The program `movemail` is an example of such a program; Rmail uses it to fetch new mail from an inbox.
User Option: **exec-path**
The value of this variable is a list of directories to search for programs to run in subprocesses. Each element is either the name of a directory (i.e., a string), or `nil`, which stands for the default directory (which is the value of `default-directory`). See [executable-find](locating-files), for the details of this search.
The value of `exec-path` is used by `call-process` and `start-process` when the program argument is not an absolute file name.
Generally, you should not modify `exec-path` directly. Instead, ensure that your `PATH` environment variable is set appropriately before starting Emacs. Trying to modify `exec-path` independently of `PATH` can lead to confusing results.
Function: **exec-path**
This function is an extension of the variable `exec-path`. If `default-directory` indicates a remote directory, this function returns a list of directories used for searching programs on the respective remote host. In case of a local `default-directory`, the function returns just the value of the variable `exec-path`.
elisp None ### Beeping
This section describes how to make Emacs ring the bell (or blink the screen) to attract the user’s attention. Be conservative about how often you do this; frequent bells can become irritating. Also be careful not to use just beeping when signaling an error is more appropriate (see [Errors](errors)).
Function: **ding** *&optional do-not-terminate*
This function beeps, or flashes the screen (see `visible-bell` below). It also terminates any keyboard macro currently executing unless do-not-terminate is non-`nil`.
Function: **beep** *&optional do-not-terminate*
This is a synonym for `ding`.
User Option: **visible-bell**
This variable determines whether Emacs should flash the screen to represent a bell. Non-`nil` means yes, `nil` means no. This is effective on graphical displays, and on text terminals provided the terminal’s Termcap entry defines the visible bell capability (‘`vb`’).
User Option: **ring-bell-function**
If this is non-`nil`, it specifies how Emacs should ring the bell. Its value should be a function of no arguments. If this is non-`nil`, it takes precedence over the `visible-bell` variable.
elisp None #### Yanking
Yanking means inserting text from the kill ring, but it does not insert the text blindly. The `yank` command, and related commands, use `insert-for-yank` to perform special processing on the text before it is inserted.
Function: **insert-for-yank** *string*
This function works like `insert`, except that it processes the text in string according to the `yank-handler` text property, as well as the variables `yank-handled-properties` and `yank-excluded-properties` (see below), before inserting the result into the current buffer.
Function: **insert-buffer-substring-as-yank** *buf &optional start end*
This function resembles `insert-buffer-substring`, except that it processes the text according to `yank-handled-properties` and `yank-excluded-properties`. (It does not handle the `yank-handler` property, which does not normally occur in buffer text anyway.)
If you put a `yank-handler` text property on all or part of a string, that alters how `insert-for-yank` inserts the string. If different parts of the string have different `yank-handler` values (comparison being done with `eq`), each substring is handled separately. The property value must be a list of one to four elements, with the following format (where elements after the first may be omitted):
```
(function param noexclude undo)
```
Here is what the elements do:
function
When function is non-`nil`, it is called instead of `insert` to insert the string, with one argument—the string to insert.
param
If param is present and non-`nil`, it replaces string (or the substring of string being processed) as the object passed to function (or `insert`). For example, if function is `yank-rectangle`, param should be a list of strings to insert as a rectangle.
noexclude
If noexclude is present and non-`nil`, that disables the normal action of `yank-handled-properties` and `yank-excluded-properties` on the inserted string.
undo If undo is present and non-`nil`, it is a function that will be called by `yank-pop` to undo the insertion of the current object. It is called with two arguments, the start and end of the current region. function can set `yank-undo-function` to override the undo value.
User Option: **yank-handled-properties**
This variable specifies special text property handling conditions for yanked text. It takes effect after the text has been inserted (either normally, or via the `yank-handler` property), and prior to `yank-excluded-properties` taking effect.
The value should be an alist of elements `(prop
. fun)`. Each alist element is handled in order. The inserted text is scanned for stretches of text having text properties `eq` to prop; for each such stretch, fun is called with three arguments: the value of the property, and the start and end positions of the text.
User Option: **yank-excluded-properties**
The value of this variable is the list of properties to remove from inserted text. Its default value contains properties that might lead to annoying results, such as causing the text to respond to the mouse or specifying key bindings. It takes effect after `yank-handled-properties`.
elisp None ### Syntax Table Concepts
A syntax table is a data structure which can be used to look up the *syntax class* and other syntactic properties of each character. Syntax tables are used by Lisp programs for scanning and moving across text.
Internally, a syntax table is a char-table (see [Char-Tables](char_002dtables)). The element at index c describes the character with code c; its value is a cons cell which specifies the syntax of the character in question. See [Syntax Table Internals](syntax-table-internals), for details. However, instead of using `aset` and `aref` to modify and inspect syntax table contents, you should usually use the higher-level functions `char-syntax` and `modify-syntax-entry`, which are described in [Syntax Table Functions](syntax-table-functions).
Function: **syntax-table-p** *object*
This function returns `t` if object is a syntax table.
Each buffer has its own major mode, and each major mode has its own idea of the syntax class of various characters. For example, in Lisp mode, the character ‘`;`’ begins a comment, but in C mode, it terminates a statement. To support these variations, the syntax table is local to each buffer. Typically, each major mode has its own syntax table, which it installs in all buffers that use that mode. For example, the variable `emacs-lisp-mode-syntax-table` holds the syntax table used by Emacs Lisp mode, and `c-mode-syntax-table` holds the syntax table used by C mode. Changing a major mode’s syntax table alters the syntax in all of that mode’s buffers, as well as in any buffers subsequently put in that mode. Occasionally, several similar modes share one syntax table. See [Example Major Modes](example-major-modes), for an example of how to set up a syntax table.
A syntax table can *inherit* from another syntax table, which is called its *parent syntax table*. A syntax table can leave the syntax class of some characters unspecified, by giving them the “inherit” syntax class; such a character then acquires the syntax class specified by the parent syntax table (see [Syntax Class Table](syntax-class-table)). Emacs defines a *standard syntax table*, which is the default parent syntax table, and is also the syntax table used by Fundamental mode.
Function: **standard-syntax-table**
This function returns the standard syntax table, which is the syntax table used in Fundamental mode.
Syntax tables are not used by the Emacs Lisp reader, which has its own built-in syntactic rules which cannot be changed. (Some Lisp systems provide ways to redefine the read syntax, but we decided to leave this feature out of Emacs Lisp for simplicity.)
elisp None ### Getting Out of Emacs
There are two ways to get out of Emacs: you can kill the Emacs job, which exits permanently, or you can suspend it, which permits you to reenter the Emacs process later. (In a graphical environment, you can of course simply switch to another application without doing anything special to Emacs, then switch back to Emacs when you want.)
| | | |
| --- | --- | --- |
| • [Killing Emacs](killing-emacs) | | Exiting Emacs irreversibly. |
| • [Suspending Emacs](suspending-emacs) | | Exiting Emacs reversibly. |
elisp None #### Active Display Table
Each window can specify a display table, and so can each buffer. The window’s display table, if there is one, takes precedence over the buffer’s display table. If neither exists, Emacs tries to use the standard display table; if that is `nil`, Emacs uses the usual character display conventions (see [Usual Display](usual-display)).
Note that display tables affect how the mode line is displayed, so if you want to force redisplay of the mode line using a new display table, call `force-mode-line-update` (see [Mode Line Format](mode-line-format)).
Function: **window-display-table** *&optional window*
This function returns window’s display table, or `nil` if there is none. The default for window is the selected window.
Function: **set-window-display-table** *window table*
This function sets the display table of window to table. The argument table should be either a display table or `nil`.
Variable: **buffer-display-table**
This variable is automatically buffer-local in all buffers; its value specifies the buffer’s display table. If it is `nil`, there is no buffer display table.
Variable: **standard-display-table**
The value of this variable is the standard display table, which is used when Emacs is displaying a buffer in a window with neither a window display table nor a buffer display table defined, or when Emacs is outputting text to the standard output or error streams. Although its default is typically `nil`, in an interactive session if the terminal cannot display curved quotes, its default maps curved quotes to ASCII approximations. See [Text Quoting Style](text-quoting-style).
The `disp-table` library defines several functions for changing the standard display table.
elisp None #### Specifying Modes For Commands
Many commands in Emacs are general, and not tied to any specific mode. For instance, `M-x kill-region` can be used in pretty much any mode that has editable text, and commands that display information (like `M-x list-buffers`) can be used in pretty much any context.
Many other commands, however, are specifically tied to a mode, and make no sense outside of that context. For instance, `M-x
dired-diff` will just signal an error if used outside of a Dired buffer.
Emacs therefore has a mechanism for specifying what mode (or modes) a command “belongs” to:
```
(defun dired-diff (...)
...
(interactive "p" dired-mode)
...)
```
This will mark the command as applicable to `dired-mode` only (or any modes that are derived from `dired-mode`). Any number of modes can be added to the `interactive` form.
Specifying modes affects command completion in `M-S-x` (`execute-extended-command-for-buffer`, see [Interactive Call](interactive-call)). It may also affect completion in `M-x`, depending on the value of `read-extended-command-predicate`.
For instance, when using the `command-completion-default-include-p` predicate as the value of `read-extended-command-predicate`, `M-x` won’t list commands that have been marked as being applicable to a specific mode (unless you are in a buffer that uses that mode, of course). This goes for both major and minor modes. (By contrast, `M-S-x` always omits inapplicable commands from the completion candidates.)
By default, `read-extended-command-predicate` is `nil`, and completion in `M-x` lists all the commands that match what the user has typed, whether those commands are or aren’t marked as applicable to the current buffer’s mode.
Marking commands to be applicable to a mode will also make `C-h m` list these commands (if they aren’t bound to any keys).
If using this extended `interactive` form isn’t convenient (because the code is supposed to work in older versions of Emacs that don’t support the extended `interactive` form), the following equivalent declaration (see [Declare Form](declare-form)) can be used instead:
```
(declare (modes dired-mode))
```
Which commands to tag with modes is to some degree a matter of taste, but commands that clearly do not work outside of the mode should be tagged. This includes commands that will signal an error if called from somewhere else, but also commands that are destructive when called from an unexpected mode. (This usually includes most of the commands that are written for special (i.e., non-editing) modes.)
Some commands may be harmless, and “work” when called from other modes, but should still be tagged with a mode if they don’t actually make much sense to use elsewhere. For instance, many special modes have commands to exit the buffer bound to `q`, and may not do anything but issue a message like "Goodbye from this mode" and then call `kill-buffer`. This command will “work” from any mode, but it is highly unlikely that anybody would actually want to use the command outside the context of this special mode.
Many modes have a set of different commands that start the mode in different ways (e.g., `eww-open-in-new-buffer` and `eww-open-file`). Commands like that should never be tagged as mode-specific, as they can be issued by the user from pretty much any context.
Note that specifying command modes is not supported in native-compiled functions in Emacs 28.1 (but this is fixed in later Emacs versions). This means that `read-extended-command-predicate` isn’t supported in native-compile builds, either.
| programming_docs |
elisp None #### Fringe Bitmaps
The *fringe bitmaps* are the actual bitmaps which represent the logical fringe indicators for truncated or continued lines, buffer boundaries, overlay arrows, etc. Each bitmap is represented by a symbol. These symbols are referred to by the variable `fringe-indicator-alist`, which maps fringe indicators to bitmaps (see [Fringe Indicators](fringe-indicators)), and the variable `fringe-cursor-alist`, which maps fringe cursors to bitmaps (see [Fringe Cursors](fringe-cursors)).
Lisp programs can also directly display a bitmap in the left or right fringe, by using a `display` property for one of the characters appearing in the line (see [Other Display Specs](other-display-specs)). Such a display specification has the form
```
(fringe bitmap [face])
```
fringe is either the symbol `left-fringe` or `right-fringe`. bitmap is a symbol identifying the bitmap to display. The optional face names a face whose foreground and background colors are to be used to display the bitmap, using the attributes of the `fringe` face for colors that face didn’t specify. If face is omitted, that means to use the attributes of the `default` face for the colors which the `fringe` face didn’t specify. For predictable results that don’t depend on the attributes of the `default` and `fringe` faces, we recommend you never omit face, but always provide a specific face. In particular, if you want the bitmap to be always displayed in the `fringe` face, use `fringe` as face.
For instance, to display an arrow in the left fringe, using the `warning` face, you could say something like:
```
(overlay-put
(make-overlay (point) (point))
'before-string (propertize
"x" 'display
`(left-fringe right-arrow warning)))
```
Here is a list of the standard fringe bitmaps defined in Emacs, and how they are currently used in Emacs (via `fringe-indicator-alist` and `fringe-cursor-alist`):
`left-arrow`, `right-arrow`
Used to indicate truncated lines.
`left-curly-arrow`, `right-curly-arrow`
Used to indicate continued lines.
`right-triangle`, `left-triangle`
The former is used by overlay arrows. The latter is unused.
`up-arrow`, `down-arrow`
`bottom-left-angle`, `bottom-right-angle`
`top-left-angle`, `top-right-angle`
`left-bracket`, `right-bracket`
`empty-line`
Used to indicate buffer boundaries.
`filled-rectangle`, `hollow-rectangle`
`filled-square`, `hollow-square`
`vertical-bar`, `horizontal-bar`
Used for different types of fringe cursors.
`exclamation-mark`, `question-mark`
Not used by core Emacs features.
The next subsection describes how to define your own fringe bitmaps.
Function: **fringe-bitmaps-at-pos** *&optional pos window*
This function returns the fringe bitmaps of the display line containing position pos in window window. The return value has the form `(left right ov)`, where left is the symbol for the fringe bitmap in the left fringe (or `nil` if no bitmap), right is similar for the right fringe, and ov is non-`nil` if there is an overlay arrow in the left fringe.
The value is `nil` if pos is not visible in window. If window is `nil`, that stands for the selected window. If pos is `nil`, that stands for the value of point in window.
elisp None ### Formatting Strings
*Formatting* means constructing a string by substituting computed values at various places in a constant string. This constant string controls how the other values are printed, as well as where they appear; it is called a *format string*.
Formatting is often useful for computing messages to be displayed. In fact, the functions `message` and `error` provide the same formatting feature described here; they differ from `format-message` only in how they use the result of formatting.
Function: **format** *string &rest objects*
This function returns a string equal to string, replacing any format specifications with encodings of the corresponding objects. The arguments objects are the computed values to be formatted.
The characters in string, other than the format specifications, are copied directly into the output, including their text properties, if any. Any text properties of the format specifications are copied to the produced string representations of the argument objects.
The output string need not be newly-allocated. For example, if `x` is the string `"foo"`, the expressions `(eq x
(format x))` and `(eq x (format "%s" x))` might both yield `t`.
Function: **format-message** *string &rest objects*
This function acts like `format`, except it also converts any grave accents (`) and apostrophes (') in string as per the value of `text-quoting-style`.
Typically grave accent and apostrophe in the format translate to matching curved quotes, e.g., "Missing `%s'" might result in "Missing ‘foo’". See [Text Quoting Style](text-quoting-style), for how to influence or inhibit this translation.
A format specification is a sequence of characters beginning with a ‘`%`’. Thus, if there is a ‘`%d`’ in string, the `format` function replaces it with the printed representation of one of the values to be formatted (one of the arguments objects). For example:
```
(format "The value of fill-column is %d." fill-column)
⇒ "The value of fill-column is 72."
```
Since `format` interprets ‘`%`’ characters as format specifications, you should *never* pass an arbitrary string as the first argument. This is particularly true when the string is generated by some Lisp code. Unless the string is *known* to never include any ‘`%`’ characters, pass `"%s"`, described below, as the first argument, and the string as the second, like this:
```
(format "%s" arbitrary-string)
```
Certain format specifications require values of particular types. If you supply a value that doesn’t fit the requirements, an error is signaled.
Here is a table of valid format specifications:
‘`%s`’
Replace the specification with the printed representation of the object, made without quoting (that is, using `princ`, not `prin1`—see [Output Functions](output-functions)). Thus, strings are represented by their contents alone, with no ‘`"`’ characters, and symbols appear without ‘`\`’ characters.
If the object is a string, its text properties are copied into the output. The text properties of the ‘`%s`’ itself are also copied, but those of the object take priority.
‘`%S`’
Replace the specification with the printed representation of the object, made with quoting (that is, using `prin1`—see [Output Functions](output-functions)). Thus, strings are enclosed in ‘`"`’ characters, and ‘`\`’ characters appear where necessary before special characters.
‘`%o`’
Replace the specification with the base-eight representation of an integer. Negative integers are formatted in a platform-dependent way. The object can also be a floating-point number that is formatted as an integer, dropping any fraction.
‘`%d`’
Replace the specification with the base-ten representation of a signed integer. The object can also be a floating-point number that is formatted as an integer, dropping any fraction.
‘`%x`’ ‘`%X`’
Replace the specification with the base-sixteen representation of an integer. Negative integers are formatted in a platform-dependent way. ‘`%x`’ uses lower case and ‘`%X`’ uses upper case. The object can also be a floating-point number that is formatted as an integer, dropping any fraction.
‘`%c`’
Replace the specification with the character which is the value given.
‘`%e`’
Replace the specification with the exponential notation for a floating-point number.
‘`%f`’
Replace the specification with the decimal-point notation for a floating-point number.
‘`%g`’
Replace the specification with notation for a floating-point number, using either exponential notation or decimal-point notation. The exponential notation is used if the exponent would be less than -4 or greater than or equal to the precision (default: 6). By default, trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.
‘`%%`’ Replace the specification with a single ‘`%`’. This format specification is unusual in that its only form is plain ‘`%%`’ and that it does not use a value. For example, `(format "%% %d" 30)` returns `"% 30"`.
Any other format character results in an ‘`Invalid format operation`’ error.
Here are several examples, which assume the typical `text-quoting-style` settings:
```
(format "The octal value of %d is %o,
and the hex value is %x." 18 18 18)
⇒ "The octal value of 18 is 22,
and the hex value is 12."
(format-message
"The name of this buffer is ‘%s’." (buffer-name))
⇒ "The name of this buffer is ‘strings.texi’."
(format-message
"The buffer object prints as `%s'." (current-buffer))
⇒ "The buffer object prints as ‘strings.texi’."
```
By default, format specifications correspond to successive values from objects. Thus, the first format specification in string uses the first such value, the second format specification uses the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause an error. Any extra values to be formatted are ignored.
A format specification can have a *field number*, which is a decimal number immediately after the initial ‘`%`’, followed by a literal dollar sign ‘`$`’. It causes the format specification to convert the argument with the given number instead of the next argument. Field numbers start at 1. A format can contain either numbered or unnumbered format specifications but not both, except that ‘`%%`’ can be mixed with numbered specifications.
```
(format "%2$s, %3$s, %%, %1$s" "x" "y" "z")
⇒ "y, z, %, x"
```
After the ‘`%`’ and any field number, you can put certain *flag characters*.
The flag ‘`+`’ inserts a plus sign before a nonnegative number, so that it always has a sign. A space character as flag inserts a space before a nonnegative number. (Otherwise, nonnegative numbers start with the first digit.) These flags are useful for ensuring that nonnegative and negative numbers use the same number of columns. They are ignored except for ‘`%d`’, ‘`%e`’, ‘`%f`’, ‘`%g`’, and if both flags are used, ‘`+`’ takes precedence.
The flag ‘`#`’ specifies an alternate form which depends on the format in use. For ‘`%o`’, it ensures that the result begins with a ‘`0`’. For ‘`%x`’ and ‘`%X`’, it prefixes nonzero results with ‘`0x`’ or ‘`0X`’. For ‘`%e`’ and ‘`%f`’, the ‘`#`’ flag means include a decimal point even if the precision is zero. For ‘`%g`’, it always includes a decimal point, and also forces any trailing zeros after the decimal point to be left in place where they would otherwise be removed.
The flag ‘`0`’ ensures that the padding consists of ‘`0`’ characters instead of spaces. This flag is ignored for non-numerical specification characters like ‘`%s`’, ‘`%S`’ and ‘`%c`’. These specification characters accept the ‘`0`’ flag, but still pad with *spaces*.
The flag ‘`-`’ causes any padding inserted by the width, if specified, to be inserted on the right rather than the left. If both ‘`-`’ and ‘`0`’ are present, the ‘`0`’ flag is ignored.
```
(format "%06d is padded on the left with zeros" 123)
⇒ "000123 is padded on the left with zeros"
(format "'%-6d' is padded on the right" 123)
⇒ "'123 ' is padded on the right"
(format "The word '%-7s' actually has %d letters in it."
"foo" (length "foo"))
⇒ "The word 'foo ' actually has 3 letters in it."
```
A specification can have a *width*, which is a decimal number that appears after any field number and flags. If the printed representation of the object contains fewer characters than this width, `format` extends it with padding. Any padding introduced by the width normally consists of spaces inserted on the left:
```
(format "%5d is padded on the left with spaces" 123)
⇒ " 123 is padded on the left with spaces"
```
If the width is too small, `format` does not truncate the object’s printed representation. Thus, you can use a width to specify a minimum spacing between columns with no risk of losing information. In the following two examples, ‘`%7s`’ specifies a minimum width of 7. In the first case, the string inserted in place of ‘`%7s`’ has only 3 letters, and needs 4 blank spaces as padding. In the second case, the string `"specification"` is 13 letters wide but is not truncated.
```
(format "The word '%7s' has %d letters in it."
"foo" (length "foo"))
⇒ "The word ' foo' has 3 letters in it."
(format "The word '%7s' has %d letters in it."
"specification" (length "specification"))
⇒ "The word 'specification' has 13 letters in it."
```
All the specification characters allow an optional *precision* after the field number, flags and width, if present. The precision is a decimal-point ‘`.`’ followed by a digit-string. For the floating-point specifications (‘`%e`’ and ‘`%f`’), the precision specifies how many digits following the decimal point to show; if zero, the decimal-point itself is also omitted. For ‘`%g`’, the precision specifies how many significant digits to show (significant digits are the first digit before the decimal point and all the digits after it). If the precision of %g is zero or unspecified, it is treated as 1. For ‘`%s`’ and ‘`%S`’, the precision truncates the string to the given width, so ‘`%.3s`’ shows only the first three characters of the representation for object. For other specification characters, the effect of precision is what the local library functions of the `printf` family produce.
If you plan to use `read` later on the formatted string to retrieve a copy of the formatted value, use a specification that lets `read` reconstruct the value. To format numbers in this reversible way you can use ‘`%s`’ and ‘`%S`’, to format just integers you can also use ‘`%d`’, and to format just nonnegative integers you can also use ‘`#x%x`’ and ‘`#o%o`’. Other formats may be problematic; for example, ‘`%d`’ and ‘`%g`’ can mishandle NaNs and can lose precision and type, and ‘`#x%x`’ and ‘`#o%o`’ can mishandle negative integers. See [Input Functions](input-functions).
The functions described in this section accept a fixed set of specification characters. The next section describes a function `format-spec` which can accept custom specification characters, such as ‘`%a`’ or ‘`%z`’.
elisp None ### Lisp History
Lisp (LISt Processing language) was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it ideal for other purposes as well, such as writing editing commands.
Dozens of Lisp implementations have been built over the years, each with its own idiosyncrasies. Many of them were inspired by Maclisp, which was written in the 1960s at MIT’s Project MAC. Eventually the implementers of the descendants of Maclisp came together and developed a standard for Lisp systems, called Common Lisp. In the meantime, Gerry Sussman and Guy Steele at MIT developed a simplified but very powerful dialect of Lisp, called Scheme.
GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common Lisp. If you know Common Lisp, you will notice many similarities. However, many features of Common Lisp have been omitted or simplified in order to reduce the memory requirements of GNU Emacs. Sometimes the simplifications are so drastic that a Common Lisp user might be very confused. We will occasionally point out how GNU Emacs Lisp differs from Common Lisp. If you don’t know Common Lisp, don’t worry about it; this manual is self-contained.
A certain amount of Common Lisp emulation is available via the `cl-lib` library. See [Overview](https://www.gnu.org/software/emacs/manual/html_node/cl/index.html#Top) in Common Lisp Extensions.
Emacs Lisp is not at all influenced by Scheme; but the GNU project has an implementation of Scheme, called Guile. We use it in all new GNU software that calls for extensibility.
elisp None ### Refreshing the Screen
The function `redraw-frame` clears and redisplays the entire contents of a given frame (see [Frames](frames)). This is useful if the screen is corrupted.
Function: **redraw-frame** *&optional frame*
This function clears and redisplays frame frame. If frame is omitted or `nil`, it redraws the selected frame.
Even more powerful is `redraw-display`:
Command: **redraw-display**
This function clears and redisplays all visible frames.
In Emacs, processing user input takes priority over redisplay. If you call these functions when input is available, they don’t redisplay immediately, but the requested redisplay does happen eventually—after all the input has been processed.
On text terminals, suspending and resuming Emacs normally also refreshes the screen. Some terminal emulators record separate contents for display-oriented programs such as Emacs and for ordinary sequential display. If you are using such a terminal, you might want to inhibit the redisplay on resumption.
User Option: **no-redraw-on-reenter**
This variable controls whether Emacs redraws the entire screen after it has been suspended and resumed. Non-`nil` means there is no need to redraw, `nil` means redrawing is needed. The default is `nil`.
elisp None ### Disabling Commands
*Disabling a command* marks the command as requiring user confirmation before it can be executed. Disabling is used for commands which might be confusing to beginning users, to prevent them from using the commands by accident.
The low-level mechanism for disabling a command is to put a non-`nil` `disabled` property on the Lisp symbol for the command. These properties are normally set up by the user’s init file (see [Init File](init-file)) with Lisp expressions such as this:
```
(put 'upcase-region 'disabled t)
```
For a few commands, these properties are present by default (you can remove them in your init file if you wish).
If the value of the `disabled` property is a string, the message saying the command is disabled includes that string. For example:
```
(put 'delete-region 'disabled
"Text deleted this way cannot be yanked back!\n")
```
See [Disabling](https://www.gnu.org/software/emacs/manual/html_node/emacs/Disabling.html#Disabling) in The GNU Emacs Manual, for the details on what happens when a disabled command is invoked interactively. Disabling a command has no effect on calling it as a function from Lisp programs.
Command: **enable-command** *command*
Allow command (a symbol) to be executed without special confirmation from now on, and alter the user’s init file (see [Init File](init-file)) so that this will apply to future sessions.
Command: **disable-command** *command*
Require special confirmation to execute command from now on, and alter the user’s init file so that this will apply to future sessions.
Variable: **disabled-command-function**
The value of this variable should be a function. When the user invokes a disabled command interactively, this function is called instead of the disabled command. It can use `this-command-keys` to determine what the user typed to run the command, and thus find the command itself.
The value may also be `nil`. Then all commands work normally, even disabled ones.
By default, the value is a function that asks the user whether to proceed.
elisp None ### Writing to Files
You can write the contents of a buffer, or part of a buffer, directly to a file on disk using the `append-to-file` and `write-region` functions. Don’t use these functions to write to files that are being visited; that could cause confusion in the mechanisms for visiting.
Command: **append-to-file** *start end filename*
This function appends the contents of the region delimited by start and end in the current buffer to the end of file filename. If that file does not exist, it is created. This function returns `nil`.
An error is signaled if you cannot write or create filename.
When called from Lisp, this function is completely equivalent to:
```
(write-region start end filename t)
```
Command: **write-region** *start end filename &optional append visit lockname mustbenew*
This function writes the region delimited by start and end in the current buffer into the file specified by filename.
If start is `nil`, then the command writes the entire buffer contents (*not* just the accessible portion) to the file and ignores end.
If start is a string, then `write-region` writes or appends that string, rather than text from the buffer. end is ignored in this case.
If append is non-`nil`, then the specified text is appended to the existing file contents (if any). If append is a number, `write-region` seeks to that byte offset from the start of the file and writes the data from there.
If mustbenew is non-`nil`, then `write-region` asks for confirmation if filename names an existing file. If mustbenew is the symbol `excl`, then `write-region` does not ask for confirmation, but instead it signals an error `file-already-exists` if the file already exists. Although `write-region` normally follows a symbolic link and creates the pointed-to file if the symbolic link is dangling, it does not follow symbolic links if mustbenew is `excl`.
The test for an existing file, when mustbenew is `excl`, uses a special system feature. At least for files on a local disk, there is no chance that some other program could create a file of the same name before Emacs does, without Emacs’s noticing.
If visit is `t`, then Emacs establishes an association between the buffer and the file: the buffer is then visiting that file. It also sets the last file modification time for the current buffer to filename’s modtime, and marks the buffer as not modified. This feature is used by `save-buffer`, but you probably should not use it yourself.
If visit is a string, it specifies the file name to visit. This way, you can write the data to one file (filename) while recording the buffer as visiting another file (visit). The argument visit is used in the echo area message and also for file locking; visit is stored in `buffer-file-name`. This feature is used to implement `file-precious-flag`; don’t use it yourself unless you really know what you’re doing.
The optional argument lockname, if non-`nil`, specifies the file name to use for purposes of locking and unlocking, overriding filename and visit for that purpose.
The function `write-region` converts the data which it writes to the appropriate file formats specified by `buffer-file-format` and also calls the functions in the list `write-region-annotate-functions`. See [Format Conversion](format-conversion).
Normally, `write-region` displays the message ‘`Wrote filename`’ in the echo area. This message is inhibited if visit is neither `t` nor `nil` nor a string, or if Emacs is operating in batch mode (see [Batch Mode](batch-mode)). This feature is useful for programs that use files for internal purposes, files that the user does not need to know about.
Variable: **write-region-inhibit-fsync**
If this variable’s value is `nil`, `write-region` uses the `fsync` system call after writing a file. Although this slows Emacs down, it lessens the risk of data loss after power failure. If the value is `t`, Emacs does not use `fsync`. The default value is `nil` when Emacs is interactive, and `t` when Emacs runs in batch mode. See [Files and Storage](files-and-storage).
Macro: **with-temp-file** *file body…*
The `with-temp-file` macro evaluates the body forms with a temporary buffer as the current buffer; then, at the end, it writes the buffer contents into file file. It kills the temporary buffer when finished, restoring the buffer that was current before the `with-temp-file` form. Then it returns the value of the last form in body.
The current buffer is restored even in case of an abnormal exit via `throw` or error (see [Nonlocal Exits](nonlocal-exits)).
Like `with-temp-buffer` (see [Current Buffer](current-buffer#Definition-of-with_002dtemp_002dbuffer)), the temporary buffer used by this macro does not run the hooks `kill-buffer-hook`, `kill-buffer-query-functions` (see [Killing Buffers](killing-buffers)), and `buffer-list-update-hook` (see [Buffer List](buffer-list)).
| programming_docs |
elisp None #### Searching for Overlays
Function: **overlays-at** *pos &optional sorted*
This function returns a list of all the overlays that cover the character at position pos in the current buffer. If sorted is non-`nil`, the list is in decreasing order of priority, otherwise it is in no particular order. An overlay contains position pos if it begins at or before pos, and ends after pos.
To illustrate usage, here is a Lisp function that returns a list of the overlays that specify property prop for the character at point:
```
(defun find-overlays-specifying (prop)
(let ((overlays (overlays-at (point)))
found)
(while overlays
(let ((overlay (car overlays)))
(if (overlay-get overlay prop)
(setq found (cons overlay found))))
(setq overlays (cdr overlays)))
found))
```
Function: **overlays-in** *beg end*
This function returns a list of the overlays that overlap the region beg through end. An overlay overlaps with a region if it contains one or more characters in the region; empty overlays (see [empty overlay](managing-overlays)) overlap if they are at beg, strictly between beg and end, or at end when end denotes the position at the end of the accessible part of the buffer.
Function: **next-overlay-change** *pos*
This function returns the buffer position of the next beginning or end of an overlay, after pos. If there is none, it returns `(point-max)`.
Function: **previous-overlay-change** *pos*
This function returns the buffer position of the previous beginning or end of an overlay, before pos. If there is none, it returns `(point-min)`.
As an example, here’s a simplified (and inefficient) version of the primitive function `next-single-char-property-change` (see [Property Search](property-search)). It searches forward from position pos for the next position where the value of a given property `prop`, as obtained from either overlays or text properties, changes.
```
(defun next-single-char-property-change (position prop)
(save-excursion
(goto-char position)
(let ((propval (get-char-property (point) prop)))
(while (and (not (eobp))
(eq (get-char-property (point) prop) propval))
(goto-char (min (next-overlay-change (point))
(next-single-property-change (point) prop)))))
(point)))
```
elisp None #### Simple Minded Indentation Engine
SMIE is a package that provides a generic navigation and indentation engine. Based on a very simple parser using an operator precedence grammar, it lets major modes extend the sexp-based navigation of Lisp to non-Lisp languages as well as provide a simple to use but reliable auto-indentation.
Operator precedence grammar is a very primitive technology for parsing compared to some of the more common techniques used in compilers. It has the following characteristics: its parsing power is very limited, and it is largely unable to detect syntax errors, but it has the advantage of being algorithmically efficient and able to parse forward just as well as backward. In practice that means that SMIE can use it for indentation based on backward parsing, that it can provide both `forward-sexp` and `backward-sexp` functionality, and that it will naturally work on syntactically incorrect code without any extra effort. The downside is that it also means that most programming languages cannot be parsed correctly using SMIE, at least not without resorting to some special tricks (see [SMIE Tricks](smie-tricks)).
| | | |
| --- | --- | --- |
| • [SMIE setup](smie-setup) | | SMIE setup and features. |
| • [Operator Precedence Grammars](operator-precedence-grammars) | | A very simple parsing technique. |
| • [SMIE Grammar](smie-grammar) | | Defining the grammar of a language. |
| • [SMIE Lexer](smie-lexer) | | Defining tokens. |
| • [SMIE Tricks](smie-tricks) | | Working around the parser’s limitations. |
| • [SMIE Indentation](smie-indentation) | | Specifying indentation rules. |
| • [SMIE Indentation Helpers](smie-indentation-helpers) | | Helper functions for indentation rules. |
| • [SMIE Indentation Example](smie-indentation-example) | | Sample indentation rules. |
| • [SMIE Customization](smie-customization) | | Customizing indentation. |
elisp None ### String and Character Basics
A character is a Lisp object which represents a single character of text. In Emacs Lisp, characters are simply integers; whether an integer is a character or not is determined only by how it is used. See [Character Codes](character-codes), for details about character representation in Emacs.
A string is a fixed sequence of characters. It is a type of sequence called a *array*, meaning that its length is fixed and cannot be altered once it is created (see [Sequences Arrays Vectors](sequences-arrays-vectors)). Unlike in C, Emacs Lisp strings are *not* terminated by a distinguished character code.
Since strings are arrays, and therefore sequences as well, you can operate on them with the general array and sequence functions documented in [Sequences Arrays Vectors](sequences-arrays-vectors). For example, you can access individual characters in a string using the function `aref` (see [Array Functions](array-functions)).
There are two text representations for non-ASCII characters in Emacs strings (and in buffers): unibyte and multibyte. For most Lisp programming, you don’t need to be concerned with these two representations. See [Text Representations](text-representations), for details.
Sometimes key sequences are represented as unibyte strings. When a unibyte string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are large integers) rather than character codes in the range 128 to 255. Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no other control characters. They do not distinguish case in ASCII control characters. If you want to store such characters in a sequence, such as a key sequence, you must use a vector instead of a string. See [Character Type](character-type), for more information about keyboard input characters.
Strings are useful for holding regular expressions. You can also match regular expressions against strings with `string-match` (see [Regexp Search](regexp-search)). The functions `match-string` (see [Simple Match Data](simple-match-data)) and `replace-match` (see [Replacing Match](replacing-match)) are useful for decomposing and modifying strings after matching regular expressions against them.
Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See [Text Properties](text-properties). All the Lisp primitives that copy text from strings to buffers or other strings also copy the properties of the characters being copied.
See [Text](text), for information about functions that display strings or copy them into buffers. See [Character Type](character-type), and [String Type](string-type), for information about the syntax of characters and strings. See [Non-ASCII Characters](non_002dascii-characters), for functions to convert between text representations and to encode and decode character codes. Also, note that `length` should *not* be used for computing the width of a string on display; use `string-width` (see [Size of Displayed Text](size-of-displayed-text)) instead.
elisp None #### Manipulating Buttons
These are functions for getting and setting properties of buttons. Often these are used by a button’s invocation function to determine what to do.
Where a button parameter is specified, it means an object referring to a specific button, either an overlay (for overlay buttons), or a buffer-position or marker (for text property buttons). Such an object is passed as the first argument to a button’s invocation function when it is invoked.
Function: **button-start** *button*
Return the position at which button starts.
Function: **button-end** *button*
Return the position at which button ends.
Function: **button-get** *button prop*
Get the property of button button named prop.
Function: **button-put** *button prop val*
Set button’s prop property to val.
Function: **button-activate** *button &optional use-mouse-action*
Call button’s `action` property (i.e., invoke the function that is the value of that property, passing it the single argument button). If use-mouse-action is non-`nil`, try to invoke the button’s `mouse-action` property instead of `action`; if the button has no `mouse-action` property, use `action` as normal. If the `button-data` property is present in button, use that as the argument for the `action` function instead of button.
Function: **button-label** *button*
Return button’s text label.
Function: **button-type** *button*
Return button’s button-type.
Function: **button-has-type-p** *button type*
Return `t` if button has button-type type, or one of type’s subtypes.
Function: **button-at** *pos*
Return the button at position pos in the current buffer, or `nil`. If the button at pos is a text property button, the return value is a marker pointing to pos.
Function: **button-type-put** *type prop val*
Set the button-type type’s prop property to val.
Function: **button-type-get** *type prop*
Get the property of button-type type named prop.
Function: **button-type-subtype-p** *type supertype*
Return `t` if button-type type is a subtype of supertype.
elisp None ### Test Coverage
You can do coverage testing for a file of Lisp code by loading the `testcover` library and using the command `M-x testcover-start RET file RET` to instrument the code. Then test your code by calling it one or more times. Then use the command `M-x testcover-mark-all` to display colored highlights on the code to show where coverage is insufficient. The command `M-x testcover-next-mark` will move point forward to the next highlighted spot.
Normally, a red highlight indicates the form was never completely evaluated; a brown highlight means it always evaluated to the same value (meaning there has been little testing of what is done with the result). However, the red highlight is skipped for forms that can’t possibly complete their evaluation, such as `error`. The brown highlight is skipped for forms that are expected to always evaluate to the same value, such as `(setq x 14)`.
For difficult cases, you can add do-nothing macros to your code to give advice to the test coverage tool.
Macro: **1value** *form*
Evaluate form and return its value, but inform coverage testing that form’s value should always be the same.
Macro: **noreturn** *form*
Evaluate form, informing coverage testing that form should never return. If it ever does return, you get a run-time error.
Edebug also has a coverage testing feature (see [Coverage Testing](coverage-testing)). These features partly duplicate each other, and it would be cleaner to combine them.
elisp None #### Altering the CDR of a List
The lowest-level primitive for modifying a CDR is `setcdr`:
Function: **setcdr** *cons object*
This function stores object as the new CDR of cons, replacing its previous CDR. In other words, it changes the CDR slot of cons to refer to object. It returns the value object.
Here is an example of replacing the CDR of a list with a different list. All but the first element of the list are removed in favor of a different sequence of elements. The first element is unchanged, because it resides in the CAR of the list, and is not reached via the CDR.
```
(setq x (list 1 2 3))
⇒ (1 2 3)
```
```
(setcdr x '(4))
⇒ (4)
```
```
x
⇒ (1 4)
```
You can delete elements from the middle of a list by altering the CDRs of the cons cells in the list. For example, here we delete the second element, `b`, from the list `(a b c)`, by changing the CDR of the first cons cell:
```
(setq x1 (list 'a 'b 'c))
⇒ (a b c)
(setcdr x1 (cdr (cdr x1)))
⇒ (c)
x1
⇒ (a c)
```
Here is the result in box notation:
```
--------------------
| |
-------------- | -------------- | --------------
| car | cdr | | | car | cdr | -->| car | cdr |
| a | o----- | b | o-------->| c | nil |
| | | | | | | | |
-------------- -------------- --------------
```
The second cons cell, which previously held the element `b`, still exists and its CAR is still `b`, but it no longer forms part of this list.
It is equally easy to insert a new element by changing CDRs:
```
(setq x1 (list 'a 'b 'c))
⇒ (a b c)
(setcdr x1 (cons 'd (cdr x1)))
⇒ (d b c)
x1
⇒ (a d b c)
```
Here is this result in box notation:
```
-------------- ------------- -------------
| car | cdr | | car | cdr | | car | cdr |
| a | o | -->| b | o------->| c | nil |
| | | | | | | | | | |
--------- | -- | ------------- -------------
| |
----- --------
| |
| --------------- |
| | car | cdr | |
-->| d | o------
| | |
---------------
```
elisp None ### Registers
A register is a sort of variable used in Emacs editing that can hold a variety of different kinds of values. Each register is named by a single character. All ASCII characters and their meta variants (but with the exception of `C-g`) can be used to name registers. Thus, there are 255 possible registers. A register is designated in Emacs Lisp by the character that is its name.
Variable: **register-alist**
This variable is an alist of elements of the form `(name .
contents)`. Normally, there is one element for each Emacs register that has been used.
The object name is a character (an integer) identifying the register.
The contents of a register can have several possible types:
a number
A number stands for itself. If `insert-register` finds a number in the register, it converts the number to decimal.
a marker
A marker represents a buffer position to jump to.
a string
A string is text saved in the register.
a rectangle
A rectangle is represented by a list of strings.
`(window-configuration position)`
This represents a window configuration to restore in one frame, and a position to jump to in the current buffer.
`(frame-configuration position)`
This represents a frame configuration to restore, and a position to jump to in the current buffer.
(file filename)
This represents a file to visit; jumping to this value visits file filename.
(file-query filename position) This represents a file to visit and a position in it; jumping to this value visits file filename and goes to buffer position position. Restoring this type of position asks the user for confirmation first.
The functions in this section return unpredictable values unless otherwise stated.
Function: **get-register** *reg*
This function returns the contents of the register reg, or `nil` if it has no contents.
Function: **set-register** *reg value*
This function sets the contents of register reg to value. A register can be set to any value, but the other register functions expect only certain data types. The return value is value.
Command: **view-register** *reg*
This command displays what is contained in register reg.
Command: **insert-register** *reg &optional beforep*
This command inserts contents of register reg into the current buffer.
Normally, this command puts point before the inserted text, and the mark after it. However, if the optional second argument beforep is non-`nil`, it puts the mark before and point after.
When called interactively, the command defaults to putting point after text, and a prefix argument inverts this behavior.
If the register contains a rectangle, then the rectangle is inserted with its upper left corner at point. This means that text is inserted in the current line and underneath it on successive lines.
If the register contains something other than saved text (a string) or a rectangle (a list), currently useless things happen. This may be changed in the future.
Function: **register-read-with-preview** *prompt*
This function reads and returns a register name, prompting with prompt and possibly showing a preview of the existing registers and their contents. The preview is shown in a temporary window, after the delay specified by the user option `register-preview-delay`, if its value and `register-alist` are both non-`nil`. The preview is also shown if the user requests help (e.g., by typing the help character). We recommend that all interactive commands which read register names use this function.
elisp None #### How Emacs Processes Errors
When an error is signaled, `signal` searches for an active *handler* for the error. A handler is a sequence of Lisp expressions designated to be executed if an error happens in part of the Lisp program. If the error has an applicable handler, the handler is executed, and control resumes following the handler. The handler executes in the environment of the `condition-case` that established it; all functions called within that `condition-case` have already been exited, and the handler cannot return to them.
If there is no applicable handler for the error, it terminates the current command and returns control to the editor command loop. (The command loop has an implicit handler for all kinds of errors.) The command loop’s handler uses the error symbol and associated data to print an error message. You can use the variable `command-error-function` to control how this is done:
Variable: **command-error-function**
This variable, if non-`nil`, specifies a function to use to handle errors that return control to the Emacs command loop. The function should take three arguments: data, a list of the same form that `condition-case` would bind to its variable; context, a string describing the situation in which the error occurred, or (more often) `nil`; and caller, the Lisp function which called the primitive that signaled the error.
An error that has no explicit handler may call the Lisp debugger. The debugger is enabled if the variable `debug-on-error` (see [Error Debugging](error-debugging)) is non-`nil`. Unlike error handlers, the debugger runs in the environment of the error, so that you can examine values of variables precisely as they were at the time of the error.
elisp None #### Process Buffers
A process can (and usually does) have an *associated buffer*, which is an ordinary Emacs buffer that is used for two purposes: storing the output from the process, and deciding when to kill the process. You can also use the buffer to identify a process to operate on, since in normal practice only one process is associated with any given buffer. Many applications of processes also use the buffer for editing input to be sent to the process, but this is not built into Emacs Lisp.
By default, process output is inserted in the associated buffer. (You can change this by defining a custom filter function, see [Filter Functions](filter-functions).) The position to insert the output is determined by the `process-mark`, which is then updated to point to the end of the text just inserted. Usually, but not always, the `process-mark` is at the end of the buffer.
Killing the associated buffer of a process also kills the process. Emacs asks for confirmation first, if the process’s `process-query-on-exit-flag` is non-`nil` (see [Query Before Exit](query-before-exit)). This confirmation is done by the function `process-kill-buffer-query-function`, which is run from `kill-buffer-query-functions` (see [Killing Buffers](killing-buffers)).
Function: **process-buffer** *process*
This function returns the associated buffer of the specified process.
```
(process-buffer (get-process "shell"))
⇒ #<buffer *shell*>
```
Function: **process-mark** *process*
This function returns the process marker for process, which is the marker that says where to insert output from the process.
If process does not have a buffer, `process-mark` returns a marker that points nowhere.
The default filter function uses this marker to decide where to insert process output, and updates it to point after the inserted text. That is why successive batches of output are inserted consecutively.
Custom filter functions normally should use this marker in the same fashion. For an example of a filter function that uses `process-mark`, see [Process Filter Example](filter-functions#Process-Filter-Example).
When the user is expected to enter input in the process buffer for transmission to the process, the process marker separates the new input from previous output.
Function: **set-process-buffer** *process buffer*
This function sets the buffer associated with process to buffer. If buffer is `nil`, the process becomes associated with no buffer; if non-`nil`, the process mark will be set to point to the end of buffer.
Function: **get-buffer-process** *buffer-or-name*
This function returns a nondeleted process associated with the buffer specified by buffer-or-name. If there are several processes associated with it, this function chooses one (currently, the one most recently created, but don’t count on that). Deletion of a process (see `delete-process`) makes it ineligible for this function to return.
It is usually a bad idea to have more than one process associated with the same buffer.
```
(get-buffer-process "*shell*")
⇒ #<process shell>
```
Killing the process’s buffer deletes the process, which kills the subprocess with a `SIGHUP` signal (see [Signals to Processes](signals-to-processes)).
If the process’s buffer is displayed in a window, your Lisp program may wish to tell the process the dimensions of that window, so that the process could adapt its output to those dimensions, much as it adapts to the screen dimensions. The following functions allow communicating this kind of information to processes; however, not all systems support the underlying functionality, so it is best to provide fallbacks, e.g., via command-line arguments or environment variables.
Function: **set-process-window-size** *process height width*
Tell process that its logical window size has dimensions width by height, in character units. If this function succeeds in communicating this information to the process, it returns `t`; otherwise it returns `nil`.
When windows that display buffers associated with process change their dimensions, the affected processes should be told about these changes. By default, when the window configuration changes, Emacs will automatically call `set-process-window-size` on behalf of every process whose buffer is displayed in a window, passing it the smallest dimensions of all the windows displaying the process’s buffer. This works via `window-configuration-change-hook` (see [Window Hooks](window-hooks)), which is told to invoke the function that is the value of the variable `window-adjust-process-window-size-function` for each process whose buffer is displayed in at least one window. You can customize this behavior by setting the value of that variable.
User Option: **window-adjust-process-window-size-function**
The value of this variable should be a function of two arguments: a process and the list of windows displaying the process’s buffer. When the function is called, the process’s buffer is the current buffer. The function should return a cons cell `(width . height)` that describes the dimensions of the logical process window to be passed via a call to `set-process-window-size`. The function can also return `nil`, in which case Emacs will not call `set-process-window-size` for this process.
Emacs supplies two predefined values for this variable: `window-adjust-process-window-size-smallest`, which returns the smallest of all the dimensions of the windows that display a process’s buffer; and `window-adjust-process-window-size-largest`, which returns the largest dimensions. For more complex strategies, write your own function.
This variable can be buffer-local.
If the process has the `adjust-window-size-function` property (see [Process Information](process-information)), its value overrides the global and buffer-local values of `window-adjust-process-window-size-function`.
| programming_docs |
elisp None #### Edebug Options
These options affect the behavior of Edebug:
User Option: **edebug-setup-hook**
Functions to call before Edebug is used. Each time it is set to a new value, Edebug will call those functions once and then reset `edebug-setup-hook` to `nil`. You could use this to load up Edebug specifications associated with a package you are using, but only when you also use Edebug. See [Instrumenting](instrumenting).
User Option: **edebug-all-defs**
If this is non-`nil`, normal evaluation of defining forms such as `defun` and `defmacro` instruments them for Edebug. This applies to `eval-defun`, `eval-region`, `eval-buffer`, and `eval-current-buffer`.
Use the command `M-x edebug-all-defs` to toggle the value of this option. See [Instrumenting](instrumenting).
User Option: **edebug-all-forms**
If this is non-`nil`, the commands `eval-defun`, `eval-region`, `eval-buffer`, and `eval-current-buffer` instrument all forms, even those that don’t define anything. This doesn’t apply to loading or evaluations in the minibuffer.
Use the command `M-x edebug-all-forms` to toggle the value of this option. See [Instrumenting](instrumenting).
User Option: **edebug-eval-macro-args**
When this is non-`nil`, all macro arguments will be instrumented in the generated code. For any macro, the `debug` declaration overrides this option. So to specify exceptions for macros that have some arguments evaluated and some not, use the `debug` declaration specify an Edebug form specification.
User Option: **edebug-save-windows**
If this is non-`nil`, Edebug saves and restores the window configuration. That takes some time, so if your program does not care what happens to the window configurations, it is better to set this variable to `nil`.
If the value is a list, only the listed windows are saved and restored.
You can use the `W` command in Edebug to change this variable interactively. See [Edebug Display Update](edebug-display-update).
User Option: **edebug-save-displayed-buffer-points**
If this is non-`nil`, Edebug saves and restores point in all displayed buffers.
Saving and restoring point in other buffers is necessary if you are debugging code that changes the point of a buffer that is displayed in a non-selected window. If Edebug or the user then selects the window, point in that buffer will move to the window’s value of point.
Saving and restoring point in all buffers is expensive, since it requires selecting each window twice, so enable this only if you need it. See [Edebug Display Update](edebug-display-update).
User Option: **edebug-initial-mode**
If this variable is non-`nil`, it specifies the initial execution mode for Edebug when it is first activated. Possible values are `step`, `next`, `go`, `Go-nonstop`, `trace`, `Trace-fast`, `continue`, and `Continue-fast`.
The default value is `step`. This variable can be set interactively with `C-x C-a C-m` (`edebug-set-initial-mode`). See [Edebug Execution Modes](edebug-execution-modes).
User Option: **edebug-trace**
If this is non-`nil`, trace each function entry and exit. Tracing output is displayed in a buffer named `\*edebug-trace\*`, one function entry or exit per line, indented by the recursion level.
Also see `edebug-tracing`, in [Trace Buffer](trace-buffer).
User Option: **edebug-test-coverage**
If non-`nil`, Edebug tests coverage of all expressions debugged. See [Coverage Testing](coverage-testing).
User Option: **edebug-continue-kbd-macro**
If non-`nil`, continue defining or executing any keyboard macro that is executing outside of Edebug. Use this with caution since it is not debugged. See [Edebug Execution Modes](edebug-execution-modes).
User Option: **edebug-print-length**
If non-`nil`, the default value of `print-length` for printing results in Edebug. See [Output Variables](output-variables).
User Option: **edebug-print-level**
If non-`nil`, the default value of `print-level` for printing results in Edebug. See [Output Variables](output-variables).
User Option: **edebug-print-circle**
If non-`nil`, the default value of `print-circle` for printing results in Edebug. See [Output Variables](output-variables).
User Option: **edebug-unwrap-results**
If non-`nil`, Edebug tries to remove any of its own instrumentation when showing the results of expressions. This is relevant when debugging macros where the results of expressions are themselves instrumented expressions. As a very artificial example, suppose that the example function `fac` has been instrumented, and consider a macro of the form:
```
(defmacro test () "Edebug example."
(if (symbol-function 'fac)
…))
```
If you instrument the `test` macro and step through it, then by default the result of the `symbol-function` call has numerous `edebug-after` and `edebug-before` forms, which can make it difficult to see the actual result. If `edebug-unwrap-results` is non-`nil`, Edebug tries to remove these forms from the result.
User Option: **edebug-on-error**
Edebug binds `debug-on-error` to this value, if `debug-on-error` was previously `nil`. See [Trapping Errors](trapping-errors).
User Option: **edebug-on-quit**
Edebug binds `debug-on-quit` to this value, if `debug-on-quit` was previously `nil`. See [Trapping Errors](trapping-errors).
If you change the values of `edebug-on-error` or `edebug-on-quit` while Edebug is active, their values won’t be used until the *next* time Edebug is invoked via a new command.
User Option: **edebug-global-break-condition**
If non-`nil`, an expression to test for at every stop point. If the result is non-`nil`, then break. Errors are ignored. See [Global Break Condition](global-break-condition).
User Option: **edebug-sit-for-seconds**
Number of seconds to pause when a breakpoint is reached and the execution mode is trace or continue. See [Edebug Execution Modes](edebug-execution-modes).
User Option: **edebug-sit-on-break**
Whether or not to pause for `edebug-sit-for-seconds` on reaching a breakpoint. Set to `nil` to prevent the pause, non-`nil` to allow it.
User Option: **edebug-behavior-alist**
By default, this alist contains one entry with the key `edebug` and a list of three functions, which are the default implementations of the functions inserted in instrumented code: `edebug-enter`, `edebug-before` and `edebug-after`. To change Edebug’s behavior globally, modify the default entry.
Edebug’s behavior may also be changed on a per-definition basis by adding an entry to this alist, with a key of your choice and three functions. Then set the `edebug-behavior` symbol property of an instrumented definition to the key of the new entry, and Edebug will call the new functions in place of its own for that definition.
User Option: **edebug-new-definition-function**
A function run by Edebug after it wraps the body of a definition or closure. After Edebug has initialized its own data, this function is called with one argument, the symbol associated with the definition, which may be the actual symbol defined or one generated by Edebug. This function may be used to set the `edebug-behavior` symbol property of each definition instrumented by Edebug.
User Option: **edebug-after-instrumentation-function**
To inspect or modify Edebug’s instrumentation before it is used, set this variable to a function which takes one argument, an instrumented top-level form, and returns either the same or a replacement form, which Edebug will then use as the final result of instrumentation.
elisp None #### Overlay Properties
Overlay properties are like text properties in that the properties that alter how a character is displayed can come from either source. But in most respects they are different. See [Text Properties](text-properties), for comparison.
Text properties are considered a part of the text; overlays and their properties are specifically considered not to be part of the text. Thus, copying text between various buffers and strings preserves text properties, but does not try to preserve overlays. Changing a buffer’s text properties marks the buffer as modified, while moving an overlay or changing its properties does not. Unlike text property changes, overlay property changes are not recorded in the buffer’s undo list.
Since more than one overlay can specify a property value for the same character, Emacs lets you specify a priority value of each overlay. The priority value is used to decide which of the overlapping overlays will “win”.
These functions read and set the properties of an overlay:
Function: **overlay-get** *overlay prop*
This function returns the value of property prop recorded in overlay, if any. If overlay does not record any value for that property, but it does have a `category` property which is a symbol, that symbol’s prop property is used. Otherwise, the value is `nil`.
Function: **overlay-put** *overlay prop value*
This function sets the value of property prop recorded in overlay to value. It returns value.
Function: **overlay-properties** *overlay*
This returns a copy of the property list of overlay.
See also the function `get-char-property` which checks both overlay properties and text properties for a given character. See [Examining Properties](examining-properties).
Many overlay properties have special meanings; here is a table of them:
`priority`
This property’s value determines the priority of the overlay. If you want to specify a priority value, use either `nil` (or zero), or a positive integer. Any other value has undefined behavior.
The priority matters when two or more overlays cover the same character and both specify the same property; the one whose `priority` value is larger overrides the other. (For the `face` property, the higher priority overlay’s value does not completely override the other value; instead, its face attributes override the face attributes of the lower priority `face` property.) If two overlays have the same priority value, and one is nested in the other, then the inner one will prevail over the outer one. If neither is nested in the other then you should not make assumptions about which overlay will prevail.
Currently, all overlays take priority over text properties.
Note that Emacs sometimes uses non-numeric priority values for some of its internal overlays, so do not try to do arithmetic on the priority of an overlay (unless it is one that you created). In particular, the overlay used for showing the region uses a priority value of the form `(primary . secondary)`, where the primary value is used as described above, and secondary is the fallback value used when primary and the nesting considerations fail to resolve the precedence between overlays. However, you are advised not to design Lisp programs based on this implementation detail; if you need to put overlays in priority order, use the sorted argument of `overlays-at`. See [Finding Overlays](finding-overlays).
`window`
If the `window` property is non-`nil`, then the overlay applies only on that window.
`category`
If an overlay has a `category` property, we call it the *category* of the overlay. It should be a symbol. The properties of the symbol serve as defaults for the properties of the overlay.
`face`
This property controls the appearance of the text (see [Faces](faces)). The value of the property can be the following:
* A face name (a symbol or string).
* An anonymous face: a property list of the form `(keyword
value …)`, where each keyword is a face attribute name and value is a value for that attribute.
* A list of faces. Each list element should be either a face name or an anonymous face. This specifies a face which is an aggregate of the attributes of each of the listed faces. Faces occurring earlier in the list have higher priority.
* A cons cell of the form `(foreground-color . color-name)` or `(background-color . color-name)`. This specifies the foreground or background color, similar to `(:foreground
color-name)` or `(:background color-name)`. This form is supported for backward compatibility only, and should be avoided.
`mouse-face`
This property is used instead of `face` when the mouse is within the range of the overlay. However, Emacs ignores all face attributes from this property that alter the text size (e.g., `:height`, `:weight`, and `:slant`). Those attributes are always the same as in the unhighlighted text.
`display`
This property activates various features that change the way text is displayed. For example, it can make text appear taller or shorter, higher or lower, wider or narrower, or replaced with an image. See [Display Property](display-property).
`help-echo`
If an overlay has a `help-echo` property, then when you move the mouse onto the text in the overlay, Emacs displays a help string in the echo area, or in the tooltip window. For details see [Text help-echo](special-properties#Text-help_002decho).
`field`
Consecutive characters with the same `field` property constitute a *field*. Some motion functions including `forward-word` and `beginning-of-line` stop moving at a field boundary. See [Fields](fields).
`modification-hooks`
This property’s value is a list of functions to be called if any character within the overlay is changed or if text is inserted strictly within the overlay.
The hook functions are called both before and after each change. If the functions save the information they receive, and compare notes between calls, they can determine exactly what change has been made in the buffer text.
When called before a change, each function receives four arguments: the overlay, `nil`, and the beginning and end of the text range to be modified.
When called after a change, each function receives five arguments: the overlay, `t`, the beginning and end of the text range just modified, and the length of the pre-change text replaced by that range. (For an insertion, the pre-change length is zero; for a deletion, that length is the number of characters deleted, and the post-change beginning and end are equal.)
When these functions are called, `inhibit-modification-hooks` is bound to non-`nil`. If the functions modify the buffer, you might want to bind `inhibit-modification-hooks` to `nil`, so as to cause the change hooks to run for these modifications. However, doing this may call your own change hook recursively, so be sure to prepare for that. See [Change Hooks](change-hooks).
Text properties also support the `modification-hooks` property, but the details are somewhat different (see [Special Properties](special-properties)).
`insert-in-front-hooks`
This property’s value is a list of functions to be called before and after inserting text right at the beginning of the overlay. The calling conventions are the same as for the `modification-hooks` functions.
`insert-behind-hooks`
This property’s value is a list of functions to be called before and after inserting text right at the end of the overlay. The calling conventions are the same as for the `modification-hooks` functions.
`invisible`
The `invisible` property can make the text in the overlay invisible, which means that it does not appear on the screen. See [Invisible Text](invisible-text), for details.
`intangible`
The `intangible` property on an overlay works just like the `intangible` text property. It is obsolete. See [Special Properties](special-properties), for details.
`isearch-open-invisible`
This property tells incremental search how to make an invisible overlay visible, permanently, if the final match overlaps it. See [Invisible Text](invisible-text).
`isearch-open-invisible-temporary`
This property tells incremental search how to make an invisible overlay visible, temporarily, during the search. See [Invisible Text](invisible-text).
`before-string`
This property’s value is a string to add to the display at the beginning of the overlay. The string does not appear in the buffer in any sense—only on the screen.
`after-string`
This property’s value is a string to add to the display at the end of the overlay. The string does not appear in the buffer in any sense—only on the screen.
`line-prefix`
This property specifies a display spec to prepend to each non-continuation line at display-time. See [Truncation](truncation).
`wrap-prefix`
This property specifies a display spec to prepend to each continuation line at display-time. See [Truncation](truncation).
`evaporate`
If this property is non-`nil`, the overlay is deleted automatically if it becomes empty (i.e., if its length becomes zero). If you give an empty overlay (see [empty overlay](managing-overlays)) a non-`nil` `evaporate` property, that deletes it immediately. Note that, unless an overlay has this property, it will not be deleted when the text between its starting and ending positions is deleted from the buffer.
`keymap`
If this property is non-`nil`, it specifies a keymap for a portion of the text. This keymap takes precedence over most other keymaps (see [Active Keymaps](active-keymaps)), and it is used when point is within the overlay, where the front- and rear-advance properties define whether the boundaries are considered as being *within* or not.
`local-map`
The `local-map` property is similar to `keymap` but replaces the buffer’s local map rather than augmenting existing keymaps. This also means it has lower precedence than minor mode keymaps.
The `keymap` and `local-map` properties do not affect a string displayed by the `before-string`, `after-string`, or `display` properties. This is only relevant for mouse clicks and other mouse events that fall on the string, since point is never on the string. To bind special mouse events for the string, assign it a `keymap` or `local-map` text property. See [Special Properties](special-properties).
elisp None ### Bool-vectors
A bool-vector is much like a vector, except that it stores only the values `t` and `nil`. If you try to store any non-`nil` value into an element of the bool-vector, the effect is to store `t` there. As with all arrays, bool-vector indices start from 0, and the length cannot be changed once the bool-vector is created. Bool-vectors are constants when evaluated.
Several functions work specifically with bool-vectors; aside from that, you manipulate them with same functions used for other kinds of arrays.
Function: **make-bool-vector** *length initial*
Return a new bool-vector of length elements, each one initialized to initial.
Function: **bool-vector** *&rest objects*
This function creates and returns a bool-vector whose elements are the arguments, objects.
Function: **bool-vector-p** *object*
This returns `t` if object is a bool-vector, and `nil` otherwise.
There are also some bool-vector set operation functions, described below:
Function: **bool-vector-exclusive-or** *a b &optional c*
Return *bitwise exclusive or* of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length.
Function: **bool-vector-union** *a b &optional c*
Return *bitwise or* of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length.
Function: **bool-vector-intersection** *a b &optional c*
Return *bitwise and* of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length.
Function: **bool-vector-set-difference** *a b &optional c*
Return *set difference* of bool vectors a and b. If optional argument c is given, the result of this operation is stored into c. All arguments should be bool vectors of the same length.
Function: **bool-vector-not** *a &optional b*
Return *set complement* of bool vector a. If optional argument b is given, the result of this operation is stored into b. All arguments should be bool vectors of the same length.
Function: **bool-vector-subsetp** *a b*
Return `t` if every `t` value in a is also `t` in b, `nil` otherwise. All arguments should be bool vectors of the same length.
Function: **bool-vector-count-consecutive** *a b i*
Return the number of consecutive elements in a equal b starting at i. `a` is a bool vector, b is `t` or `nil`, and i is an index into `a`.
Function: **bool-vector-count-population** *a*
Return the number of elements that are `t` in bool vector a.
The printed form represents up to 8 boolean values as a single character:
```
(bool-vector t nil t nil)
⇒ #&4"^E"
(bool-vector)
⇒ #&0""
```
You can use `vconcat` to print a bool-vector like other vectors:
```
(vconcat (bool-vector nil t nil t))
⇒ [nil t nil t]
```
Here is another example of creating, examining, and updating a bool-vector:
```
(setq bv (make-bool-vector 5 t))
⇒ #&5"^_"
(aref bv 1)
⇒ t
(aset bv 3 nil)
⇒ nil
bv
⇒ #&5"^W"
```
These results make sense because the binary codes for control-\_ and control-W are 11111 and 10111, respectively.
| programming_docs |
elisp None #### Classification of List Forms
A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The remaining list elements constitute the *arguments* for the function, macro, or special form.
The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is *not* evaluated, as it would be in some Lisp dialects such as Scheme.
elisp None ### Generic Functions
Functions defined using `defun` have a hard-coded set of assumptions about the types and expected values of their arguments. For example, a function that was designed to handle values of its argument that are either numbers or lists of numbers will fail or signal an error if called with a value of any other type, such as a vector or a string. This happens because the implementation of the function is not prepared to deal with types other than those assumed during the design.
By contrast, object-oriented programs use *polymorphic functions*: a set of specialized functions having the same name, each one of which was written for a certain specific set of argument types. Which of the functions is actually called is decided at run time based on the types of the actual arguments.
Emacs provides support for polymorphism. Like other Lisp environments, notably Common Lisp and its Common Lisp Object System (CLOS), this support is based on *generic functions*. The Emacs generic functions closely follow CLOS, including use of similar names, so if you have experience with CLOS, the rest of this section will sound very familiar.
A generic function specifies an abstract operation, by defining its name and list of arguments, but (usually) no implementation. The actual implementation for several specific classes of arguments is provided by *methods*, which should be defined separately. Each method that implements a generic function has the same name as the generic function, but the method’s definition indicates what kinds of arguments it can handle by *specializing* the arguments defined by the generic function. These *argument specializers* can be more or less specific; for example, a `string` type is more specific than a more general type, such as `sequence`.
Note that, unlike in message-based OO languages, such as C++ and Simula, methods that implement generic functions don’t belong to a class, they belong to the generic function they implement.
When a generic function is invoked, it selects the applicable methods by comparing the actual arguments passed by the caller with the argument specializers of each method. A method is applicable if the actual arguments of the call are compatible with the method’s specializers. If more than one method is applicable, they are combined using certain rules, described below, and the combination then handles the call.
Macro: **cl-defgeneric** *name arguments [documentation] [options-and-methods…] &rest body*
This macro defines a generic function with the specified name and arguments. If body is present, it provides the default implementation. If documentation is present (it should always be), it specifies the documentation string for the generic function, in the form `(:documentation docstring)`. The optional options-and-methods can be one of the following forms:
`(declare declarations)` A declare form, as described in [Declare Form](declare-form).
`(:argument-precedence-order &rest args)` This form affects the sorting order for combining applicable methods. Normally, when two methods are compared during combination, method arguments are examined left to right, and the first method whose argument specializer is more specific will come before the other one. The order defined by this form overrides that, and the arguments are examined according to their order in this form, and not left to right.
`(:method [qualifiers…] args &rest body)` This form defines a method like `cl-defmethod` does.
Macro: **cl-defmethod** *name [extra] [qualifier] arguments [&context (expr spec)…] &rest [docstring] body*
This macro defines a particular implementation for the generic function called name. The implementation code is given by body. If present, docstring is the documentation string for the method. The arguments list, which must be identical in all the methods that implement a generic function, and must match the argument list of that function, provides argument specializers of the form `(arg spec)`, where arg is the argument name as specified in the `cl-defgeneric` call, and spec is one of the following specializer forms:
`type` This specializer requires the argument to be of the given type, one of the types from the type hierarchy described below.
`(eql object)` This specializer requires the argument be `eql` to the given object.
`(head object)` The argument must be a cons cell whose `car` is `eql` to object.
`struct-type` The argument must be an instance of a class named struct-type defined with `cl-defstruct` (see [Structures](https://www.gnu.org/software/emacs/manual/html_node/cl/Structures.html#Structures) in Common Lisp Extensions for GNU Emacs Lisp), or of one of its child classes.
Method definitions can make use of a new argument-list keyword, `&context`, which introduces extra specializers that test the environment at the time the method is run. This keyword should appear after the list of required arguments, but before any `&rest` or `&optional` keywords. The `&context` specializers look much like regular argument specializers—(expr spec)—except that expr is an expression to be evaluated in the current context, and the spec is a value to compare against. For example, `&context (overwrite-mode (eql t))` will make the method applicable only when `overwrite-mode` is turned on. The `&context` keyword can be followed by any number of context specializers. Because the context specializers are not part of the generic function’s argument signature, they may be omitted in methods that don’t require them.
The type specializer, `(arg type)`, can specify one of the *system types* in the following list. When a parent type is specified, an argument whose type is any of its more specific child types, as well as grand-children, grand-grand-children, etc. will also be compatible.
`integer` Parent type: `number`.
`number` `null` Parent type: `symbol`
`symbol` `string` Parent type: `array`.
`array` Parent type: `sequence`.
`cons` Parent type: `list`.
`list` Parent type: `sequence`.
`marker` `overlay` `float` Parent type: `number`.
`window-configuration` `process` `window` `subr` `compiled-function` `buffer` `char-table` Parent type: `array`.
`bool-vector` Parent type: `array`.
`vector` Parent type: `array`.
`frame` `hash-table` `font-spec` `font-entity` `font-object` The optional extra element, expressed as ‘`:extra string`’, allows you to add more methods, distinguished by string, for the same specializers and qualifiers.
The optional qualifier allows combining several applicable methods. If it is not present, the defined method is a *primary* method, responsible for providing the primary implementation of the generic function for the specialized arguments. You can also define *auxiliary methods*, by using one of the following values as qualifier:
`:before` This auxiliary method will run before the primary method. More accurately, all the `:before` methods will run before the primary, in the most-specific-first order.
`:after` This auxiliary method will run after the primary method. More accurately, all such methods will run after the primary, in the most-specific-last order.
`:around` This auxiliary method will run *instead* of the primary method. The most specific of such methods will be run before any other method. Such methods normally use `cl-call-next-method`, described below, to invoke the other auxiliary or primary methods.
Functions defined using `cl-defmethod` cannot be made interactive, i.e. commands (see [Defining Commands](defining-commands)), by adding the `interactive` form to them. If you need a polymorphic command, we recommend defining a normal command that calls a polymorphic function defined via `cl-defgeneric` and `cl-defmethod`.
Each time a generic function is called, it builds the *effective method* which will handle this invocation by combining the applicable methods defined for the function. The process of finding the applicable methods and producing the effective method is called *dispatch*. The applicable methods are those all of whose specializers are compatible with the actual arguments of the call. Since all of the arguments must be compatible with the specializers, they all determine whether a method is applicable. Methods that explicitly specialize more than one argument are called *multiple-dispatch methods*.
The applicable methods are sorted into the order in which they will be combined. The method whose left-most argument specializer is the most specific one will come first in the order. (Specifying `:argument-precedence-order` as part of `cl-defmethod` overrides that, as described above.) If the method body calls `cl-call-next-method`, the next most-specific method will run. If there are applicable `:around` methods, the most-specific of them will run first; it should call `cl-call-next-method` to run any of the less specific `:around` methods. Next, the `:before` methods run in the order of their specificity, followed by the primary method, and lastly the `:after` methods in the reverse order of their specificity.
Function: **cl-call-next-method** *&rest args*
When invoked from within the lexical body of a primary or an `:around` auxiliary method, call the next applicable method for the same generic function. Normally, it is called with no arguments, which means to call the next applicable method with the same arguments that the calling method was invoked. Otherwise, the specified arguments are used instead.
Function: **cl-next-method-p**
This function, when called from within the lexical body of a primary or an `:around` auxiliary method, returns non-`nil` if there is a next method to call.
elisp None ### Window Dividers
Window dividers are bars drawn between a frame’s windows. A right divider is drawn between a window and any adjacent windows on the right. Its width (thickness) is specified by the frame parameter `right-divider-width`. A bottom divider is drawn between a window and adjacent windows on the bottom or the echo area. Its width is specified by the frame parameter `bottom-divider-width`. In either case, specifying a width of zero means to not draw such dividers. See [Layout Parameters](layout-parameters).
Technically, a right divider belongs to the window on its left, which means that its width contributes to the total width of that window. A bottom divider belongs to the window above it, which means that its width contributes to the total height of that window. See [Window Sizes](window-sizes). When a window has both, a right and a bottom divider, the bottom divider prevails. This means that a bottom divider is drawn over the full total width of its window while the right divider ends above the bottom divider.
Dividers can be dragged with the mouse and are therefore useful for adjusting the sizes of adjacent windows with the mouse. They also serve to visually set apart adjacent windows when no scroll bars or mode lines are present. The following three faces allow the customization of the appearance of dividers:
`window-divider`
When a divider is less than three pixels wide, it is drawn solidly with the foreground of this face. For larger dividers this face is used for the inner part only, excluding the first and last pixel.
`window-divider-first-pixel`
This is the face used for drawing the first pixel of a divider that is at least three pixels wide. To obtain a solid appearance, set this to the same value used for the `window-divider` face.
`window-divider-last-pixel` This is the face used for drawing the last pixel of a divider that is at least three pixels wide. To obtain a solid appearance, set this to the same value used for the `window-divider` face.
You can get the sizes of the dividers of a specific window with the following two functions.
Function: **window-right-divider-width** *&optional window*
Return the width (thickness) in pixels of window’s right divider. window must be a live window and defaults to the selected one. The return value is always zero for a rightmost window.
Function: **window-bottom-divider-width** *&optional window*
Return the width (thickness) in pixels of window’s bottom divider. window must be a live window and defaults to the selected one. The return value is zero for the minibuffer window or a bottommost window on a minibuffer-less frame.
elisp None ### Documentation Groups
Emacs can list functions based on various groupings. For instance, `string-trim` and `mapconcat` are “string” functions, so `M-x shortdoc-display-group RET string RET` will give an overview of functions that operate on strings.
The documentation groups are created with the `define-short-documentation-group` macro.
Macro: **define-short-documentation-group** *group &rest functions*
Define group as a group of functions, and provide short summaries of using those functions. The optional argument functions is a list whose elements are of the form:
```
(func [keyword val]…)
```
The following keywords are recognized:
`:eval`
The value should be a form that has no side effect when evaluated. The form will be used in the documentation by printing it with `prin1` (see [Output Functions](output-functions)). However, if the form is a string, it will be inserted as-is, and the string will then be `read` to yield the form. In any case, the form will then be evaluated, and the result used. For instance:
```
:eval (concat "foo" "bar" "zot")
:eval "(make-string 5 ?x)"
```
will result in:
```
(concat "foo" "bar" "zot")
⇒ "foobarzot"
(make-string 5 ?x)
⇒ "xxxxx"
```
(The reason for allowing both Lisp forms and strings here is so that printing could be controlled in the few cases where a certain presentation of the form is wished for. In the example, ‘`?x`’ would otherwise have been printed as ‘`120`’ if it hadn’t been included in a string.)
`:no-eval` This is like `:eval`, except that the form will not be evaluated. In these cases, a `:result` element of some kind (see below) should be included.
```
:no-eval (file-symlink-p "/tmp/foo")
:eg-result t
```
`:no-eval*`
Like `:no-eval`, but always inserts ‘`[it depends]`’ as the result. For instance:
```
:no-eval* (buffer-string)
```
will result in:
```
(buffer-string)
→ [it depends]
```
`:no-value`
Like `:no-eval`, but is used when the function in question has no well-defined return value, and is used for side effect only.
`:result`
Used to output the result from non-evaluating example forms.
```
:no-eval (setcar list 'c)
:result c
```
`:eg-result`
Used to output an example result from non-evaluating example forms. For instance:
```
:no-eval (looking-at "f[0-9]")
:eg-result t
```
will result in:
```
(looking-at "f[0-9]")
eg. → t
```
`:result-string` `:eg-result-string`
These two are the same as `:result` and `:eg-result`, respectively, but are inserted as is. This is useful when the result is unreadable or should be of a particular form:
```
:no-eval (find-file "/tmp/foo")
:eg-result-string "#<buffer foo>"
:no-eval (default-file-modes)
:eg-result-string "#o755"
```
`:no-manual`
Indicates that this function is not documented in the manual.
`:args`
By default, the function’s actual argument list is shown. If `:args` is present, they are used instead.
```
:args (regexp string)
```
Here’s a very short example:
```
(define-short-documentation-group string
"Creating Strings"
(substring
:eval (substring "foobar" 0 3)
:eval (substring "foobar" 3))
(concat
:eval (concat "foo" "bar" "zot")))
```
The first argument is the name of the group to be defined, and then follows any number of function descriptions.
A function can belong to any number of documentation groups.
In addition to function descriptions, the list can also have string elements, which are used to divide a documentation group into sections.
Function: **shortdoc-add-function** *shortdoc-add-function group section elem*
Lisp packages can add functions to groups with this command. Each elem should be a function description, as described above. group is the function group, and section is what section in the function group to insert the function into.
If group doesn’t exist, it will be created. If section doesn’t exist, it will be added to the end of the function group.
elisp None ### Blinking Parentheses
This section describes the mechanism by which Emacs shows a matching open parenthesis when the user inserts a close parenthesis.
Variable: **blink-paren-function**
The value of this variable should be a function (of no arguments) to be called whenever a character with close parenthesis syntax is inserted. The value of `blink-paren-function` may be `nil`, in which case nothing is done.
User Option: **blink-matching-paren**
If this variable is `nil`, then `blink-matching-open` does nothing.
User Option: **blink-matching-paren-distance**
This variable specifies the maximum distance to scan for a matching parenthesis before giving up.
User Option: **blink-matching-delay**
This variable specifies the number of seconds to keep indicating the matching parenthesis. A fraction of a second often gives good results, but the default is 1, which works on all systems.
Command: **blink-matching-open**
This function is the default value of `blink-paren-function`. It assumes that point follows a character with close parenthesis syntax and applies the appropriate effect momentarily to the matching opening character. If that character is not already on the screen, it displays the character’s context in the echo area. To avoid long delays, this function does not search farther than `blink-matching-paren-distance` characters.
Here is an example of calling this function explicitly.
```
(defun interactive-blink-matching-open ()
"Indicate momentarily the start of parenthesized sexp before point."
(interactive)
```
```
(let ((blink-matching-paren-distance
(buffer-size))
(blink-matching-paren t))
(blink-matching-open)))
```
elisp None ### Mutexes
A *mutex* is an exclusive lock. At any moment, zero or one threads may own a mutex. If a thread attempts to acquire a mutex, and the mutex is already owned by some other thread, then the acquiring thread will block until the mutex becomes available.
Emacs Lisp mutexes are of a type called *recursive*, which means that a thread can re-acquire a mutex it owns any number of times. A mutex keeps a count of how many times it has been acquired, and each acquisition of a mutex must be paired with a release. The last release by a thread of a mutex reverts it to the unowned state, potentially allowing another thread to acquire the mutex.
Function: **mutexp** *object*
This function returns `t` if object represents an Emacs mutex, `nil` otherwise.
Function: **make-mutex** *&optional name*
Create a new mutex and return it. If name is specified, it is a name given to the mutex. It must be a string. The name is for debugging purposes only; it has no meaning to Emacs.
Function: **mutex-name** *mutex*
Return the name of mutex, as specified to `make-mutex`.
Function: **mutex-lock** *mutex*
This will block until this thread acquires mutex, or until this thread is signaled using `thread-signal`. If mutex is already owned by this thread, this simply returns.
Function: **mutex-unlock** *mutex*
Release mutex. If mutex is not owned by this thread, this will signal an error.
Macro: **with-mutex** *mutex body…*
This macro is the simplest and safest way to evaluate forms while holding a mutex. It acquires mutex, invokes body, and then releases mutex. It returns the result of body.
| programming_docs |
elisp None #### Nonlocal Exits in Modules
Emacs Lisp supports nonlocal exits, whereby program control is transferred from one point in a program to another remote point. See [Nonlocal Exits](nonlocal-exits). Thus, Lisp functions called by your module might exit nonlocally by calling `signal` or `throw`, and your module functions must handle such nonlocal exits properly. Such handling is needed because C programs will not automatically release resources and perform other cleanups in these cases; your module code must itself do it. The module API provides facilities for that, described in this subsection. They are generally available since Emacs 25; those of them that became available in later releases explicitly call out the first Emacs version where they became part of the API.
When some Lisp code called by a module function signals an error or throws, the nonlocal exit is trapped, and the pending exit and its associated data are stored in the environment. Whenever a nonlocal exit is pending in the environment, any module API function called with a pointer to that environment will return immediately without any processing (the functions `non_local_exit_check`, `non_local_exit_get`, and `non_local_exit_clear` are exceptions from this rule). If your module function then does nothing and returns to Emacs, a pending nonlocal exit will cause Emacs to act on it: signal an error or throw to the corresponding `catch`.
So the simplest “handling” of nonlocal exits in module functions is to do nothing special and let the rest of your code to run as if nothing happened. However, this can cause two classes of problems:
* - Your module function might use uninitialized or undefined values, since API functions return immediately without producing the expected results.
* - Your module might leak resources, because it might not have the opportunity to release them.
Therefore, we recommend that your module functions check for nonlocal exit conditions and recover from them, using the functions described below.
Function: *enum emacs\_funcall\_exit* **non\_local\_exit\_check** *(emacs\_env \*env)*
This function returns the kind of nonlocal exit condition stored in env. The possible values are:
`emacs_funcall_exit_return` The last API function exited normally.
`emacs_funcall_exit_signal` The last API function signaled an error.
`emacs_funcall_exit_throw` The last API function exited via `throw`.
Function: *enum emacs\_funcall\_exit* **non\_local\_exit\_get** *(emacs\_env \*env, emacs\_value \*symbol, emacs\_value \*data)*
This function returns the kind of nonlocal exit condition stored in env, like `non_local_exit_check` does, but it also returns the full information about the nonlocal exit, if any. If the return value is `emacs_funcall_exit_signal`, the function stores the error symbol in `*symbol` and the error data in `*data` (see [Signaling Errors](signaling-errors)). If the return value is `emacs_funcall_exit_throw`, the function stores the `catch` tag symbol in `*symbol` and the `throw` value in `*data`. The function doesn’t store anything in memory pointed by these arguments when the return value is `emacs_funcall_exit_return`.
You should check nonlocal exit conditions where it matters: before you allocated some resource or after you allocated a resource that might need freeing, or where a failure means further processing is impossible or infeasible.
Once your module function detected that a nonlocal exit is pending, it can either return to Emacs (after performing the necessary local cleanup), or it can attempt to recover from the nonlocal exit. The following API functions will help with these tasks.
Function: *void* **non\_local\_exit\_clear** *(emacs\_env \*env)*
This function clears the pending nonlocal exit conditions and data from env. After calling it, the module API functions will work normally. Use this function if your module function can recover from nonlocal exits of the Lisp functions it calls and continue, and also before calling any of the following two functions (or any other API functions, if you want them to perform their intended processing when a nonlocal exit is pending).
Function: *void* **non\_local\_exit\_throw** *(emacs\_env \*env, emacs\_value tag, emacs\_value value)*
This function throws to the Lisp `catch` symbol represented by tag, passing it value as the value to return. Your module function should in general return soon after calling this function. One use of this function is when you want to re-throw a non-local exit from one of the called API or Lisp functions.
Function: *void* **non\_local\_exit\_signal** *(emacs\_env \*env, emacs\_value symbol, emacs\_value data)*
This function signals the error represented by the error symbol symbol with the specified error data data. The module function should return soon after calling this function. This function could be useful, e.g., for signaling errors from module functions to Emacs.
elisp None ### Mouse Position
The functions `mouse-position` and `set-mouse-position` give access to the current position of the mouse.
Function: **mouse-position**
This function returns a description of the position of the mouse. The value looks like `(frame x . y)`, where x and y are integers giving the (possibly rounded) position in multiples of the default character size of frame (see [Frame Font](frame-font)) relative to the native position of frame (see [Frame Geometry](frame-geometry)).
Variable: **mouse-position-function**
If non-`nil`, the value of this variable is a function for `mouse-position` to call. `mouse-position` calls this function just before returning, with its normal return value as the sole argument, and it returns whatever this function returns to it.
This abnormal hook exists for the benefit of packages like `xt-mouse.el` that need to do mouse handling at the Lisp level.
Variable: **tty-menu-calls-mouse-position-function**
If non-`nil`, TTY menus will call `mouse-position-function` as described above. This exists for cases where `mouse-position-function` is not safe to be called by the TTY menus, such as if it could trigger redisplay.
Function: **set-mouse-position** *frame x y*
This function *warps the mouse* to position x, y in frame frame. The arguments x and y are integers, giving the position in multiples of the default character size of frame (see [Frame Font](frame-font)) relative to the native position of frame (see [Frame Geometry](frame-geometry)).
The resulting mouse position is constrained to the native frame of frame. If frame is not visible, this function does nothing. The return value is not significant.
Function: **mouse-pixel-position**
This function is like `mouse-position` except that it returns coordinates in units of pixels rather than units of characters.
Function: **set-mouse-pixel-position** *frame x y*
This function warps the mouse like `set-mouse-position` except that x and y are in units of pixels rather than units of characters.
The resulting mouse position is not constrained to the native frame of frame. If frame is not visible, this function does nothing. The return value is not significant.
On a graphical terminal the following two functions allow the absolute position of the mouse cursor to be retrieved and set.
Function: **mouse-absolute-pixel-position**
This function returns a cons cell (x . y) of the coordinates of the mouse cursor position in pixels, relative to a position (0, 0) of the selected frame’s display.
Function: **set-mouse-absolute-pixel-position** *x y*
This function moves the mouse cursor to the position (x, y). The coordinates x and y are interpreted in pixels relative to a position (0, 0) of the selected frame’s display.
The following function can tell whether the mouse cursor is currently visible on a frame:
Function: **frame-pointer-visible-p** *&optional frame*
This predicate function returns non-`nil` if the mouse pointer displayed on frame is visible; otherwise it returns `nil`. frame omitted or `nil` means the selected frame. This is useful when `make-pointer-invisible` is set to `t`: it allows you to know if the pointer has been hidden. See [Mouse Avoidance](https://www.gnu.org/software/emacs/manual/html_node/emacs/Mouse-Avoidance.html#Mouse-Avoidance) in The Emacs Manual.
elisp None ### Process Information
Several functions return information about processes.
Command: **list-processes** *&optional query-only buffer*
This command displays a listing of all living processes. In addition, it finally deletes any process whose status was ‘`Exited`’ or ‘`Signaled`’. It returns `nil`.
The processes are shown in a buffer named `\*Process List\*` (unless you specify otherwise using the optional argument buffer), whose major mode is Process Menu mode.
If query-only is non-`nil`, it only lists processes whose query flag is non-`nil`. See [Query Before Exit](query-before-exit).
Function: **process-list**
This function returns a list of all processes that have not been deleted.
```
(process-list)
⇒ (#<process display-time> #<process shell>)
```
Function: **num-processors** *&optional query*
This function returns the number of processors, a positive integer. Each usable thread execution unit counts as a processor. By default, the count includes the number of available processors, which you can override by setting the [`OMP_NUM_THREADS` environment variable of OpenMP](https://www.openmp.org/spec-html/5.1/openmpse59.html). If the optional argument query is `current`, this function ignores `OMP_NUM_THREADS`; if query is `all`, this function also counts processors that are on the system but are not available to the current process.
Function: **get-process** *name*
This function returns the process named name (a string), or `nil` if there is none. The argument name can also be a process object, in which case it is returned.
```
(get-process "shell")
⇒ #<process shell>
```
Function: **process-command** *process*
This function returns the command that was executed to start process. This is a list of strings, the first string being the program executed and the rest of the strings being the arguments that were given to the program. For a network, serial, or pipe connection, this is either `nil`, which means the process is running or `t` (process is stopped).
```
(process-command (get-process "shell"))
⇒ ("bash" "-i")
```
Function: **process-contact** *process &optional key no-block*
This function returns information about how a network, a serial, or a pipe connection was set up. When key is `nil`, it returns `(hostname service)` for a network connection, `(port speed)` for a serial connection, and `t` for a pipe connection. For an ordinary child process, this function always returns `t` when called with a `nil` key.
If key is `t`, the value is the complete status information for the connection, server, serial port, or pipe; that is, the list of keywords and values specified in `make-network-process`, `make-serial-process`, or `make-pipe-process`, except that some of the values represent the current status instead of what you specified.
For a network process, the values include (see `make-network-process` for a complete list):
`:buffer` The associated value is the process buffer.
`:filter` The associated value is the process filter function. See [Filter Functions](filter-functions).
`:sentinel` The associated value is the process sentinel function. See [Sentinels](sentinels).
`:remote` In a connection, the address in internal format of the remote peer.
`:local` The local address, in internal format.
`:service` In a server, if you specified `t` for service, this value is the actual port number.
`:local` and `:remote` are included even if they were not specified explicitly in `make-network-process`.
For a serial connection, see `make-serial-process` and `serial-process-configure` for the list of keys. For a pipe connection, see `make-pipe-process` for the list of keys.
If key is a keyword, the function returns the value corresponding to that keyword.
If process is a non-blocking network stream that hasn’t been fully set up yet, then this function will block until that has happened. If given the optional no-block parameter, this function will return `nil` instead of blocking.
Function: **process-id** *process*
This function returns the PID of process. This is an integral number that distinguishes the process process from all other processes running on the same computer at the current time. The PID of a process is chosen by the operating system kernel when the process is started and remains constant as long as the process exists. For network, serial, and pipe connections, this function returns `nil`.
Function: **process-name** *process*
This function returns the name of process, as a string.
Function: **process-status** *process-name*
This function returns the status of process-name as a symbol. The argument process-name must be a process, a buffer, or a process name (a string).
The possible values for an actual subprocess are:
`run` for a process that is running.
`stop` for a process that is stopped but continuable.
`exit` for a process that has exited.
`signal` for a process that has received a fatal signal.
`open` for a network, serial, or pipe connection that is open.
`closed` for a network, serial, or pipe connection that is closed. Once a connection is closed, you cannot reopen it, though you might be able to open a new connection to the same place.
`connect` for a non-blocking connection that is waiting to complete.
`failed` for a non-blocking connection that has failed to complete.
`listen` for a network server that is listening.
`nil` if process-name is not the name of an existing process.
```
(process-status (get-buffer "*shell*"))
⇒ run
```
For a network, serial, or pipe connection, `process-status` returns one of the symbols `open`, `stop`, or `closed`. The latter means that the other side closed the connection, or Emacs did `delete-process`. The value `stop` means that `stop-process` was called on the connection.
Function: **process-live-p** *process*
This function returns non-`nil` if process is alive. A process is considered alive if its status is `run`, `open`, `listen`, `connect` or `stop`.
Function: **process-type** *process*
This function returns the symbol `network` for a network connection or server, `serial` for a serial port connection, `pipe` for a pipe connection, or `real` for a subprocess created for running a program.
Function: **process-exit-status** *process*
This function returns the exit status of process or the signal number that killed it. (Use the result of `process-status` to determine which of those it is.) If process has not yet terminated, the value is 0. For network, serial, and pipe connections that are already closed, the value is either 0 or 256, depending on whether the connection was closed normally or abnormally.
Function: **process-tty-name** *process*
This function returns the terminal name that process is using for its communication with Emacs—or `nil` if it is using pipes instead of a pty (see `process-connection-type` in [Asynchronous Processes](asynchronous-processes)). If process represents a program running on a remote host, the terminal name used by that program on the remote host is provided as process property `remote-tty`. If process represents a network, serial, or pipe connection, the value is `nil`.
Function: **process-coding-system** *process*
This function returns a cons cell `(decode . encode)`, describing the coding systems in use for decoding output from, and encoding input to, process (see [Coding Systems](coding-systems)).
Function: **set-process-coding-system** *process &optional decoding-system encoding-system*
This function specifies the coding systems to use for subsequent output from and input to process. It will use decoding-system to decode subprocess output, and encoding-system to encode subprocess input.
Every process also has a property list that you can use to store miscellaneous values associated with the process.
Function: **process-get** *process propname*
This function returns the value of the propname property of process.
Function: **process-put** *process propname value*
This function sets the value of the propname property of process to value.
Function: **process-plist** *process*
This function returns the process plist of process.
Function: **set-process-plist** *process plist*
This function sets the process plist of process to plist.
elisp None #### Select among Command Alternatives
The macro `define-alternatives` can be used to define *generic commands*. These are interactive functions whose implementation can be selected from several alternatives, as a matter of user preference.
Macro: **define-alternatives** *command &rest customizations*
Define the new command command, a symbol.
When a user runs `M-x command RET` for the first time, Emacs prompts for which real form of the command to use, and records the selection by way of a custom variable. Using a prefix argument repeats this process of choosing an alternative.
The variable `command-alternatives` should contain an alist with alternative implementations of command. Until this variable is set, `define-alternatives` has no effect.
If customizations is non-`nil`, it should consist of alternating `defcustom` keywords (typically `:group` and `:version`) and values to add to the declaration of `command-alternatives`.
elisp None Operating System Interface
--------------------------
This chapter is about starting and getting out of Emacs, access to values in the operating system environment, and terminal input, output.
See [Building Emacs](building-emacs), for related information. See [Display](display), for additional operating system status information pertaining to the terminal and the screen.
| | | |
| --- | --- | --- |
| • [Starting Up](starting-up) | | Customizing Emacs startup processing. |
| • [Getting Out](getting-out) | | How exiting works (permanent or temporary). |
| • [System Environment](system-environment) | | Distinguish the name and kind of system. |
| • [User Identification](user-identification) | | Finding the name and user id of the user. |
| • [Time of Day](time-of-day) | | Getting the current time. |
| • [Time Zone Rules](time-zone-rules) | | Rules for time zones and daylight saving time. |
| • [Time Conversion](time-conversion) | | Converting among timestamp forms. |
| • [Time Parsing](time-parsing) | | Converting timestamps to text and vice versa. |
| • [Processor Run Time](processor-run-time) | | Getting the run time used by Emacs. |
| • [Time Calculations](time-calculations) | | Adding, subtracting, comparing times, etc. |
| • [Timers](timers) | | Setting a timer to call a function at a certain time. |
| • [Idle Timers](idle-timers) | | Setting a timer to call a function when Emacs has been idle for a certain length of time. |
| • [Terminal Input](terminal-input) | | Accessing and recording terminal input. |
| • [Terminal Output](terminal-output) | | Controlling and recording terminal output. |
| • [Sound Output](sound-output) | | Playing sounds on the computer’s speaker. |
| • [X11 Keysyms](x11-keysyms) | | Operating on key symbols for X Windows. |
| • [Batch Mode](batch-mode) | | Running Emacs without terminal interaction. |
| • [Session Management](session-management) | | Saving and restoring state with X Session Management. |
| • [Desktop Notifications](desktop-notifications) | | Desktop notifications. |
| • [File Notifications](file-notifications) | | File notifications. |
| • [Dynamic Libraries](dynamic-libraries) | | On-demand loading of support libraries. |
| • [Security Considerations](security-considerations) | | Running Emacs in an unfriendly environment. |
| programming_docs |
elisp None ### Disassembled Byte-Code
People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into human-readable form.
The byte-code interpreter is implemented as a simple stack machine. It pushes values onto a stack of its own, then pops them off to use them in calculations whose results are themselves pushed back on the stack. When a byte-code function returns, it pops a value off the stack and returns it as the value of the function.
In addition to the stack, byte-code functions can use, bind, and set ordinary Lisp variables, by transferring values between variables and the stack.
Command: **disassemble** *object &optional buffer-or-name*
This command displays the disassembled code for object. In interactive use, or if buffer-or-name is `nil` or omitted, the output goes in a buffer named `\*Disassemble\*`. If buffer-or-name is non-`nil`, it must be a buffer or the name of an existing buffer. Then the output goes there, at point, and point is left before the output.
The argument object can be a function name, a lambda expression (see [Lambda Expressions](lambda-expressions)), or a byte-code object (see [Byte-Code Objects](byte_002dcode-objects)). If it is a lambda expression, `disassemble` compiles it and disassembles the resulting compiled code.
Here are two examples of using the `disassemble` function. We have added explanatory comments to help you relate the byte-code to the Lisp source; these do not appear in the output of `disassemble`.
```
(defun factorial (integer)
"Compute factorial of an integer."
(if (= 1 integer) 1
(* integer (factorial (1- integer)))))
⇒ factorial
```
```
(factorial 4)
⇒ 24
```
```
(disassemble 'factorial)
-| byte-code for factorial:
doc: Compute factorial of an integer.
args: (integer)
```
```
0 varref integer ; Get the value of `integer` and
; push it onto the stack.
1 constant 1 ; Push 1 onto stack.
```
```
2 eqlsign ; Pop top two values off stack, compare
; them, and push result onto stack.
```
```
3 goto-if-nil 1 ; Pop and test top of stack;
; if `nil`, go to 1, else continue.
6 constant 1 ; Push 1 onto top of stack.
7 return ; Return the top element of the stack.
```
```
8:1 varref integer ; Push value of `integer` onto stack.
9 constant factorial ; Push `factorial` onto stack.
10 varref integer ; Push value of `integer` onto stack.
11 sub1 ; Pop `integer`, decrement value,
; push new value onto stack.
12 call 1 ; Call function `factorial` using first
; (i.e., top) stack element as argument;
; push returned value onto stack.
```
```
13 mult ; Pop top two values off stack, multiply
; them, and push result onto stack.
14 return ; Return the top element of the stack.
```
The `silly-loop` function is somewhat more complex:
```
(defun silly-loop (n)
"Return time before and after N iterations of a loop."
(let ((t1 (current-time-string)))
(while (> (setq n (1- n))
0))
(list t1 (current-time-string))))
⇒ silly-loop
```
```
(disassemble 'silly-loop)
-| byte-code for silly-loop:
doc: Return time before and after N iterations of a loop.
args: (n)
```
```
0 constant current-time-string ; Push `current-time-string`
; onto top of stack.
```
```
1 call 0 ; Call `current-time-string` with no
; argument, push result onto stack.
```
```
2 varbind t1 ; Pop stack and bind `t1` to popped value.
```
```
3:1 varref n ; Get value of `n` from the environment
; and push the value on the stack.
4 sub1 ; Subtract 1 from top of stack.
```
```
5 dup ; Duplicate top of stack; i.e., copy the top
; of the stack and push copy onto stack.
6 varset n ; Pop the top of the stack,
; and bind `n` to the value.
;; (In effect, the sequence `dup varset` copies the top of the stack
;; into the value of `n` without popping it.)
```
```
7 constant 0 ; Push 0 onto stack.
8 gtr ; Pop top two values off stack,
; test if n is greater than 0
; and push result onto stack.
```
```
9 goto-if-not-nil 1 ; Goto 1 if `n` > 0
; (this continues the while loop)
; else continue.
```
```
12 varref t1 ; Push value of `t1` onto stack.
13 constant current-time-string ; Push `current-time-string`
; onto the top of the stack.
14 call 0 ; Call `current-time-string` again.
```
```
15 unbind 1 ; Unbind `t1` in local environment.
16 list2 ; Pop top two elements off stack, create a
; list of them, and push it onto stack.
17 return ; Return value of the top of stack.
```
elisp None ### Parsing HTML and XML
Emacs can be compiled with built-in libxml2 support.
Function: **libxml-available-p**
This function returns non-`nil` if built-in libxml2 support is available in this Emacs session.
When libxml2 support is available, the following functions can be used to parse HTML or XML text into Lisp object trees.
Function: **libxml-parse-html-region** *start end &optional base-url discard-comments*
This function parses the text between start and end as HTML, and returns a list representing the HTML *parse tree*. It attempts to handle real-world HTML by robustly coping with syntax mistakes.
The optional argument base-url, if non-`nil`, should be a string specifying the base URL for relative URLs occurring in links.
If the optional argument discard-comments is non-`nil`, any top-level comment is discarded. (This argument is obsolete and will be removed in future Emacs versions. To remove comments, use the `xml-remove-comments` utility function on the data before you call the parsing function.)
In the parse tree, each HTML node is represented by a list in which the first element is a symbol representing the node name, the second element is an alist of node attributes, and the remaining elements are the subnodes.
The following example demonstrates this. Given this (malformed) HTML document:
```
<html><head></head><body width=101><div class=thing>Foo<div>Yes
```
A call to `libxml-parse-html-region` returns this DOM (document object model):
```
(html nil
(head nil)
(body ((width . "101"))
(div ((class . "thing"))
"Foo"
(div nil
"Yes"))))
```
Function: **shr-insert-document** *dom*
This function renders the parsed HTML in dom into the current buffer. The argument dom should be a list as generated by `libxml-parse-html-region`. This function is, e.g., used by [EWW](https://www.gnu.org/software/emacs/manual/html_node/eww/index.html#Top) in The Emacs Web Wowser Manual.
Function: **libxml-parse-xml-region** *start end &optional base-url discard-comments*
This function is the same as `libxml-parse-html-region`, except that it parses the text as XML rather than HTML (so it is stricter about syntax).
| | | |
| --- | --- | --- |
| • [Document Object Model](document-object-model) | | Access, manipulate and search the DOM. |
elisp None #### Overview
The function `insert-file-contents`:
* initially, inserts bytes from the file into the buffer;
* decodes bytes to characters as appropriate;
* processes formats as defined by entries in `format-alist`; and
* calls functions in `after-insert-file-functions`.
The function `write-region`:
* initially, calls functions in `write-region-annotate-functions`;
* processes formats as defined by entries in `format-alist`;
* encodes characters to bytes as appropriate; and
* modifies the file with the bytes.
This shows the symmetry of the lowest-level operations; reading and writing handle things in opposite order. The rest of this section describes the two facilities surrounding the three variables named above, as well as some related functions. [Coding Systems](coding-systems), for details on character encoding and decoding.
elisp None #### Condition Variable Type
A *condition variable* is a device for a more complex thread synchronization than the one supported by a mutex. A thread can wait on a condition variable, to be woken up when some other thread notifies the condition.
Condition variable objects have no read syntax. They print in hash notation, giving the name of the condition variable (if it has been given a name) or its address in core:
```
(make-condition-variable (make-mutex))
⇒ #<condvar 01c45ae8>
```
elisp None ### Special Events
Certain *special events* are handled at a very low level—as soon as they are read. The `read-event` function processes these events itself, and never returns them. Instead, it keeps waiting for the first event that is not special and returns that one.
Special events do not echo, they are never grouped into key sequences, and they never appear in the value of `last-command-event` or `(this-command-keys)`. They do not discard a numeric argument, they cannot be unread with `unread-command-events`, they may not appear in a keyboard macro, and they are not recorded in a keyboard macro while you are defining one.
Special events do, however, appear in `last-input-event` immediately after they are read, and this is the way for the event’s definition to find the actual event.
The events types `iconify-frame`, `make-frame-visible`, `delete-frame`, `drag-n-drop`, `language-change`, and user signals like `sigusr1` are normally handled in this way. The keymap which defines how to handle special events—and which events are special—is in the variable `special-event-map` (see [Controlling Active Maps](controlling-active-maps)).
elisp None ### Input Events
The Emacs command loop reads a sequence of *input events* that represent keyboard or mouse activity, or system events sent to Emacs. The events for keyboard activity are characters or symbols; other events are always lists. This section describes the representation and meaning of input events in detail.
Function: **eventp** *object*
This function returns non-`nil` if object is an input event or event type.
Note that any non-`nil` symbol might be used as an event or an event type; `eventp` cannot distinguish whether a symbol is intended by Lisp code to be used as an event.
| | | |
| --- | --- | --- |
| • [Keyboard Events](keyboard-events) | | Ordinary characters – keys with symbols on them. |
| • [Function Keys](function-keys) | | Function keys – keys with names, not symbols. |
| • [Mouse Events](mouse-events) | | Overview of mouse events. |
| • [Click Events](click-events) | | Pushing and releasing a mouse button. |
| • [Drag Events](drag-events) | | Moving the mouse before releasing the button. |
| • [Button-Down Events](button_002ddown-events) | | A button was pushed and not yet released. |
| • [Repeat Events](repeat-events) | | Double and triple click (or drag, or down). |
| • [Motion Events](motion-events) | | Just moving the mouse, not pushing a button. |
| • [Focus Events](focus-events) | | Moving the mouse between frames. |
| • [Misc Events](misc-events) | | Other events the system can generate. |
| • [Event Examples](event-examples) | | Examples of the lists for mouse events. |
| • [Classifying Events](classifying-events) | | Finding the modifier keys in an event symbol. Event types. |
| • [Accessing Mouse](accessing-mouse) | | Functions to extract info from mouse events. |
| • [Accessing Scroll](accessing-scroll) | | Functions to get info from scroll bar events. |
| • [Strings of Events](strings-of-events) | | Special considerations for putting keyboard character events in a string. |
elisp None #### User-Chosen Coding Systems
Function: **select-safe-coding-system** *from to &optional default-coding-system accept-default-p file*
This function selects a coding system for encoding specified text, asking the user to choose if necessary. Normally the specified text is the text in the current buffer between from and to. If from is a string, the string specifies the text to encode, and to is ignored.
If the specified text includes raw bytes (see [Text Representations](text-representations)), `select-safe-coding-system` suggests `raw-text` for its encoding.
If default-coding-system is non-`nil`, that is the first coding system to try; if that can handle the text, `select-safe-coding-system` returns that coding system. It can also be a list of coding systems; then the function tries each of them one by one. After trying all of them, it next tries the current buffer’s value of `buffer-file-coding-system` (if it is not `undecided`), then the default value of `buffer-file-coding-system` and finally the user’s most preferred coding system, which the user can set using the command `prefer-coding-system` (see [Recognizing Coding Systems](https://www.gnu.org/software/emacs/manual/html_node/emacs/Recognize-Coding.html#Recognize-Coding) in The GNU Emacs Manual).
If one of those coding systems can safely encode all the specified text, `select-safe-coding-system` chooses it and returns it. Otherwise, it asks the user to choose from a list of coding systems which can encode all the text, and returns the user’s choice.
default-coding-system can also be a list whose first element is `t` and whose other elements are coding systems. Then, if no coding system in the list can handle the text, `select-safe-coding-system` queries the user immediately, without trying any of the three alternatives described above. This is handy for checking only the coding systems in the list.
The optional argument accept-default-p determines whether a coding system selected without user interaction is acceptable. If it’s omitted or `nil`, such a silent selection is always acceptable. If it is non-`nil`, it should be a function; `select-safe-coding-system` calls this function with one argument, the base coding system of the selected coding system. If the function returns `nil`, `select-safe-coding-system` rejects the silently selected coding system, and asks the user to select a coding system from a list of possible candidates.
If the variable `select-safe-coding-system-accept-default-p` is non-`nil`, it should be a function taking a single argument. It is used in place of accept-default-p, overriding any value supplied for this argument.
As a final step, before returning the chosen coding system, `select-safe-coding-system` checks whether that coding system is consistent with what would be selected if the contents of the region were read from a file. (If not, this could lead to data corruption in a file subsequently re-visited and edited.) Normally, `select-safe-coding-system` uses `buffer-file-name` as the file for this purpose, but if file is non-`nil`, it uses that file instead (this can be relevant for `write-region` and similar functions). If it detects an apparent inconsistency, `select-safe-coding-system` queries the user before selecting the coding system.
Variable: **select-safe-coding-system-function**
This variable names the function to be called to request the user to select a proper coding system for encoding text when the default coding system for an output operation cannot safely encode that text. The default value of this variable is `select-safe-coding-system`. Emacs primitives that write text to files, such as `write-region`, or send text to other processes, such as `process-send-region`, normally call the value of this variable, unless `coding-system-for-write` is bound to a non-`nil` value (see [Specifying Coding Systems](specifying-coding-systems)).
Here are two functions you can use to let the user specify a coding system, with completion. See [Completion](completion).
Function: **read-coding-system** *prompt &optional default*
This function reads a coding system using the minibuffer, prompting with string prompt, and returns the coding system name as a symbol. If the user enters null input, default specifies which coding system to return. It should be a symbol or a string.
Function: **read-non-nil-coding-system** *prompt*
This function reads a coding system using the minibuffer, prompting with string prompt, and returns the coding system name as a symbol. If the user tries to enter null input, it asks the user to try again. See [Coding Systems](coding-systems).
elisp None ### Examining Text Near Point
Many functions are provided to look at the characters around point. Several simple functions are described here. See also `looking-at` in [Regexp Search](regexp-search).
In the following four functions, “beginning” or “end” of buffer refers to the beginning or end of the accessible portion.
Function: **char-after** *&optional position*
This function returns the character in the current buffer at (i.e., immediately after) position position. If position is out of range for this purpose, either before the beginning of the buffer, or at or beyond the end, then the value is `nil`. The default for position is point.
In the following example, assume that the first character in the buffer is ‘`@`’:
```
(string (char-after 1))
⇒ "@"
```
Function: **char-before** *&optional position*
This function returns the character in the current buffer immediately before position position. If position is out of range for this purpose, either at or before the beginning of the buffer, or beyond the end, then the value is `nil`. The default for position is point.
Function: **following-char**
This function returns the character following point in the current buffer. This is similar to `(char-after (point))`. However, if point is at the end of the buffer, then `following-char` returns 0.
Remember that point is always between characters, and the cursor normally appears over the character following point. Therefore, the character returned by `following-char` is the character the cursor is over.
In this example, point is between the ‘`a`’ and the ‘`c`’.
```
---------- Buffer: foo ----------
Gentlemen may cry ``Pea∗ce! Peace!,''
but there is no peace.
---------- Buffer: foo ----------
```
```
(string (preceding-char))
⇒ "a"
(string (following-char))
⇒ "c"
```
Function: **preceding-char**
This function returns the character preceding point in the current buffer. See above, under `following-char`, for an example. If point is at the beginning of the buffer, `preceding-char` returns 0.
Function: **bobp**
This function returns `t` if point is at the beginning of the buffer. If narrowing is in effect, this means the beginning of the accessible portion of the text. See also `point-min` in [Point](point).
Function: **eobp**
This function returns `t` if point is at the end of the buffer. If narrowing is in effect, this means the end of accessible portion of the text. See also `point-max` in See [Point](point).
Function: **bolp**
This function returns `t` if point is at the beginning of a line. See [Text Lines](text-lines). The beginning of the buffer (or of its accessible portion) always counts as the beginning of a line.
Function: **eolp**
This function returns `t` if point is at the end of a line. The end of the buffer (or of its accessible portion) is always considered the end of a line.
| programming_docs |
elisp None ### Window Sizes
Emacs provides miscellaneous functions for finding the height and width of a window. The return value of many of these functions can be specified either in units of pixels or in units of lines and columns. On a graphical display, the latter actually correspond to the height and width of a default character specified by the frame’s default font as returned by `frame-char-height` and `frame-char-width` (see [Frame Font](frame-font)). Thus, if a window is displaying text with a different font or size, the reported line height and column width for that window may differ from the actual number of text lines or columns displayed within it.
The *total height* of a window is the number of lines comprising its body and its top and bottom decorations (see [Basic Windows](basic-windows)).
Function: **window-total-height** *&optional window round*
This function returns the total height, in lines, of the window window. If window is omitted or `nil`, it defaults to the selected window. If window is an internal window, the return value is the total height occupied by its descendant windows.
If a window’s pixel height is not an integral multiple of its frame’s default character height, the number of lines occupied by the window is rounded internally. This is done in a way such that, if the window is a parent window, the sum of the total heights of all its child windows internally equals the total height of their parent. This means that although two windows have the same pixel height, their internal total heights may differ by one line. This means also, that if window is vertically combined and has a next sibling, the topmost row of that sibling can be calculated as the sum of this window’s topmost row and total height (see [Coordinates and Windows](coordinates-and-windows))
If the optional argument round is `ceiling`, this function returns the smallest integer larger than window’s pixel height divided by the character height of its frame; if it is `floor`, it returns the largest integer smaller than said value; with any other round it returns the internal value of windows’s total height.
The *total width* of a window is the number of columns comprising its body and its left and right decorations (see [Basic Windows](basic-windows)).
Function: **window-total-width** *&optional window round*
This function returns the total width, in columns, of the window window. If window is omitted or `nil`, it defaults to the selected window. If window is internal, the return value is the total width occupied by its descendant windows.
If a window’s pixel width is not an integral multiple of its frame’s character width, the number of columns occupied by the window is rounded internally. This is done in a way such that, if the window is a parent window, the sum of the total widths of all its children internally equals the total width of their parent. This means that although two windows have the same pixel width, their internal total widths may differ by one column. This means also, that if this window is horizontally combined and has a next sibling, the leftmost column of that sibling can be calculated as the sum of this window’s leftmost column and total width (see [Coordinates and Windows](coordinates-and-windows)). The optional argument round behaves as it does for `window-total-height`.
Function: **window-total-size** *&optional window horizontal round*
This function returns either the total height in lines or the total width in columns of the window window. If horizontal is omitted or `nil`, this is equivalent to calling `window-total-height` for window; otherwise it is equivalent to calling `window-total-width` for window. The optional argument round behaves as it does for `window-total-height`.
The following two functions can be used to return the total size of a window in units of pixels.
Function: **window-pixel-height** *&optional window*
This function returns the total height of window window in pixels. window must be a valid window and defaults to the selected one.
The return value includes the heights of window’s top and bottom decorations. If window is an internal window, its pixel height is the pixel height of the screen areas spanned by its children.
Function: **window-pixel-width** *&optional window*
This function returns the width of window window in pixels. window must be a valid window and defaults to the selected one.
The return value includes the widths of window’s left and right decorations. If window is an internal window, its pixel width is the width of the screen areas spanned by its children.
The following functions can be used to determine whether a given window has any adjacent windows.
Function: **window-full-height-p** *&optional window*
This function returns non-`nil` if window has no other window above or below it in its frame. More precisely, this means that the total height of window equals the total height of the root window on that frame. The minibuffer window does not count in this regard. If window is omitted or `nil`, it defaults to the selected window.
Function: **window-full-width-p** *&optional window*
This function returns non-`nil` if window has no other window to the left or right in its frame, i.e., its total width equals that of the root window on that frame. If window is omitted or `nil`, it defaults to the selected window.
The *body height* of a window is the height of its body, which does not include any of its top or bottom decorations (see [Basic Windows](basic-windows)).
Function: **window-body-height** *&optional window pixelwise*
This function returns the height, in lines, of the body of window window. If window is omitted or `nil`, it defaults to the selected window; otherwise it must be a live window.
If the optional argument pixelwise is non-`nil`, this function returns the body height of window counted in pixels.
If pixelwise is `nil`, the return value is rounded down to the nearest integer, if necessary. This means that if a line at the bottom of the text area is only partially visible, that line is not counted. It also means that the height of a window’s body can never exceed its total height as returned by `window-total-height`.
The *body width* of a window is the width of its body and of the text area, which does not include any of its left or right decorations (see [Basic Windows](basic-windows)).
Note that when one or both fringes are removed (by setting their width to zero), the display engine reserves two character cells, one on each side of the window, for displaying the continuation and truncation glyphs, which leaves 2 columns less for text display. (The function `window-max-chars-per-line`, described below, takes this peculiarity into account.)
Function: **window-body-width** *&optional window pixelwise*
This function returns the width, in columns, of the body of window window. If window is omitted or `nil`, it defaults to the selected window; otherwise it must be a live window.
If the optional argument pixelwise is non-`nil`, this function returns the body width of window in units of pixels.
If pixelwise is `nil`, the return value is rounded down to the nearest integer, if necessary. This means that if a column on the right of the text area is only partially visible, that column is not counted. It also means that the width of a window’s body can never exceed its total width as returned by `window-total-width`.
Function: **window-body-size** *&optional window horizontal pixelwise*
This function returns the body height or body width of window. If horizontal is omitted or `nil`, it is equivalent to calling `window-body-height` for window; otherwise it is equivalent to calling `window-body-width`. In either case, the optional argument pixelwise is passed to the function called.
The pixel heights of a window’s mode, tab and header line can be retrieved with the functions given below. Their return value is usually accurate unless the window has not been displayed before: In that case, the return value is based on an estimate of the font used for the window’s frame.
Function: **window-mode-line-height** *&optional window*
This function returns the height in pixels of window’s mode line. window must be a live window and defaults to the selected one. If window has no mode line, the return value is zero.
Function: **window-tab-line-height** *&optional window*
This function returns the height in pixels of window’s tab line. window must be a live window and defaults to the selected one. If window has no tab line, the return value is zero.
Function: **window-header-line-height** *&optional window*
This function returns the height in pixels of window’s header line. window must be a live window and defaults to the selected one. If window has no header line, the return value is zero.
Functions for retrieving the height and/or width of window dividers (see [Window Dividers](window-dividers)), fringes (see [Fringes](fringes)), scroll bars (see [Scroll Bars](scroll-bars)), and display margins (see [Display Margins](display-margins)) are described in the corresponding sections.
If your Lisp program needs to make layout decisions, you will find the following function useful:
Function: **window-max-chars-per-line** *&optional window face*
This function returns the number of characters displayed in the specified face face in the specified window window (which must be a live window). If face was remapped (see [Face Remapping](face-remapping)), the information is returned for the remapped face. If omitted or `nil`, face defaults to the default face, and window defaults to the selected window.
Unlike `window-body-width`, this function accounts for the actual size of face’s font, instead of working in units of the canonical character width of window’s frame (see [Frame Font](frame-font)). It also accounts for space used by the continuation glyph, if window lacks one or both of its fringes.
Commands that change the size of windows (see [Resizing Windows](resizing-windows)), or split them (see [Splitting Windows](splitting-windows)), obey the variables `window-min-height` and `window-min-width`, which specify the smallest allowable window height and width. They also obey the variable `window-size-fixed`, with which a window can be *fixed* in size (see [Preserving Window Sizes](preserving-window-sizes)).
User Option: **window-min-height**
This option specifies the minimum total height, in lines, of any window. Its value has to accommodate at least one text line and any top or bottom decorations.
User Option: **window-min-width**
This option specifies the minimum total width, in columns, of any window. Its value has to accommodate at least two text columns and any left or right decorations.
The following function tells how small a specific window can get taking into account the sizes of its areas and the values of `window-min-height`, `window-min-width` and `window-size-fixed` (see [Preserving Window Sizes](preserving-window-sizes)).
Function: **window-min-size** *&optional window horizontal ignore pixelwise*
This function returns the minimum size of window. window must be a valid window and defaults to the selected one. The optional argument horizontal non-`nil` means to return the minimum number of columns of window; otherwise return the minimum number of window’s lines.
The return value makes sure that all components of window remain fully visible if window’s size were actually set to it. With horizontal `nil` it includes any top or bottom decorations. With horizontal non-`nil` it includes any left or right decorations of window.
The optional argument ignore, if non-`nil`, means ignore restrictions imposed by fixed size windows, `window-min-height` or `window-min-width` settings. If ignore equals `safe`, live windows may get as small as `window-safe-min-height` lines and `window-safe-min-width` columns. If ignore is a window, ignore restrictions for that window only. Any other non-`nil` value means ignore all of the above restrictions for all windows.
The optional argument pixelwise non-`nil` means to return the minimum size of window counted in pixels.
elisp None ### Windows and Frames
Each window belongs to exactly one frame (see [Frames](frames)). For all windows belonging to a specific frame, we sometimes also say that these windows are *owned* by that frame or simply that they are on that frame.
Function: **window-frame** *&optional window*
This function returns the specified window’s frame—the frame that window belongs to. If window is omitted or `nil`, it defaults to the selected window (see [Selecting Windows](selecting-windows)).
Function: **window-list** *&optional frame minibuffer window*
This function returns a list of all live windows owned by the specified frame. If frame is omitted or `nil`, it defaults to the selected frame (see [Input Focus](input-focus)).
The optional argument minibuffer specifies whether to include the minibuffer window (see [Minibuffer Windows](minibuffer-windows)) in that list. If minibuffer is `t`, the minibuffer window is included. If `nil` or omitted, the minibuffer window is included only if it is active. If minibuffer is neither `nil` nor `t`, the minibuffer window is never included.
The optional argument window, if non-`nil`, must be a live window on the specified frame; then window will be the first element in the returned list. If window is omitted or `nil`, the window selected within frame (see [Selecting Windows](selecting-windows)) is the first element.
Windows on the same frame are organized into a *window tree*, whose leaf nodes are the live windows. The internal nodes of a window tree are not live; they exist for the purpose of organizing the relationships between live windows. The root node of a window tree is called the *root window*. It is either a live window or an internal window. If it is a live window, then the frame has just one window besides the minibuffer window, or the frame is a minibuffer-only frame, see [Frame Layout](frame-layout).
A minibuffer window (see [Minibuffer Windows](minibuffer-windows)) that is not alone on its frame does not have a parent window, so it strictly speaking is not part of its frame’s window tree. Nonetheless, it is a sibling window of the frame’s root window, and thus can be reached from the root window via `window-next-sibling`, see below. Also, the function `window-tree` described at the end of this section lists the minibuffer window alongside the actual window tree.
Function: **frame-root-window** *&optional frame-or-window*
This function returns the root window for frame-or-window. The argument frame-or-window should be either a window or a frame; if omitted or `nil`, it defaults to the selected frame. If frame-or-window is a window, the return value is the root window of that window’s frame.
When a live window is split (see [Splitting Windows](splitting-windows)), there are two live windows where previously there was one. One of these is represented by the same Lisp window object as the original window, and the other is represented by a newly-created Lisp window object. Both of these live windows become leaf nodes of the window tree, as *child windows* of a single internal window. If necessary, Emacs automatically creates this internal window, which is also called the *parent window*, and assigns it to the appropriate position in the window tree. The set of windows that share the same parent are called *siblings*.
Function: **window-parent** *&optional window*
This function returns the parent window of window. If window is omitted or `nil`, it defaults to the selected window. The return value is `nil` if window has no parent (i.e., it is a minibuffer window or the root window of its frame).
A parent window always has at least two child windows. If this number were to fall to one as a result of window deletion (see [Deleting Windows](deleting-windows)), Emacs automatically deletes the parent window too, and its sole remaining child window takes its place in the window tree.
A child window can be either a live window, or an internal window (which in turn would have its own child windows). Therefore, each internal window can be thought of as occupying a certain rectangular *screen area*—the union of the areas occupied by the live windows that are ultimately descended from it.
For each internal window, the screen areas of the immediate children are arranged either vertically or horizontally (never both). If the child windows are arranged one above the other, they are said to form a *vertical combination*; if they are arranged side by side, they are said to form a *horizontal combination*. Consider the following example:
```
______________________________________
| ______ ____________________________ |
|| || __________________________ ||
|| ||| |||
|| ||| |||
|| ||| |||
|| |||____________W4____________|||
|| || __________________________ ||
|| ||| |||
|| ||| |||
|| |||____________W5____________|||
||__W2__||_____________W3_____________ |
|__________________W1__________________|
```
The root window of this frame is an internal window, W1. Its child windows form a horizontal combination, consisting of the live window W2 and the internal window W3. The child windows of W3 form a vertical combination, consisting of the live windows W4 and W5. Hence, the live windows in this window tree are W2, W4, and W5.
The following functions can be used to retrieve a child window of an internal window, and the siblings of a child window. Their window argument always defaults to the selected window (see [Selecting Windows](selecting-windows)).
Function: **window-top-child** *&optional window*
This function returns the topmost child window of window, if window is an internal window whose children form a vertical combination. For any other type of window, the return value is `nil`.
Function: **window-left-child** *&optional window*
This function returns the leftmost child window of window, if window is an internal window whose children form a horizontal combination. For any other type of window, the return value is `nil`.
Function: **window-child** *window*
This function returns the first child window of the internal window window—the topmost child window for a vertical combination, or the leftmost child window for a horizontal combination. If window is a live window, the return value is `nil`.
Function: **window-combined-p** *&optional window horizontal*
This function returns a non-`nil` value if and only if window is part of a vertical combination.
If the optional argument horizontal is non-`nil`, this means to return non-`nil` if and only if window is part of a horizontal combination.
Function: **window-next-sibling** *&optional window*
This function returns the next sibling of the specified window. The return value is `nil` if window is the last child of its parent.
Function: **window-prev-sibling** *&optional window*
This function returns the previous sibling of the specified window. The return value is `nil` if window is the first child of its parent.
The functions `window-next-sibling` and `window-prev-sibling` should not be confused with the functions `next-window` and `previous-window`, which return the next and previous window in the cyclic ordering of windows (see [Cyclic Window Ordering](cyclic-window-ordering)).
The following functions can be useful to locate a window within its frame.
Function: **frame-first-window** *&optional frame-or-window*
This function returns the live window at the upper left corner of the frame specified by frame-or-window. The argument frame-or-window must denote a window or a live frame and defaults to the selected frame. If frame-or-window specifies a window, this function returns the first window on that window’s frame. Under the assumption that the frame from our canonical example is selected `(frame-first-window)` returns W2.
Function: **window-at-side-p** *&optional window side*
This function returns `t` if window is located at side of its containing frame. The argument window must be a valid window and defaults to the selected one. The argument side can be any of the symbols `left`, `top`, `right` or `bottom`. The default value `nil` is handled like `bottom`.
Note that this function disregards the minibuffer window (see [Minibuffer Windows](minibuffer-windows)). Hence, with side equal to `bottom` it may return `t` also when the minibuffer window appears right below window.
Function: **window-in-direction** *direction &optional window ignore sign wrap minibuf*
This function returns the nearest live window in direction direction as seen from the position of `window-point` in window window. The argument direction must be one of `above`, `below`, `left` or `right`. The optional argument window must denote a live window and defaults to the selected one.
This function does not return a window whose `no-other-window` parameter is non-`nil` (see [Window Parameters](window-parameters)). If the nearest window’s `no-other-window` parameter is non-`nil`, this function tries to find another window in the indicated direction whose `no-other-window` parameter is `nil`. If the optional argument ignore is non-`nil`, a window may be returned even if its `no-other-window` parameter is non-`nil`.
If the optional argument sign is a negative number, it means to use the right or bottom edge of window as reference position instead of `window-point`. If sign is a positive number, it means to use the left or top edge of window as reference position.
If the optional argument wrap is non-`nil`, this means to wrap direction around frame borders. For example, if window is at the top of the frame and direction is `above`, then this function usually returns the frame’s minibuffer window if it’s active and a window at the bottom of the frame otherwise.
If the optional argument minibuf is `t`, this function may return the minibuffer window even when it’s not active. If the optional argument minibuf is `nil`, this means to return the minibuffer window if and only if it is currently active. If minibuf is neither `nil` nor `t`, this function never returns the minibuffer window. However, if wrap is non-`nil`, it always acts as if minibuf were `nil`.
If it doesn’t find a suitable window, this function returns `nil`.
Don’t use this function to check whether there is *no* window in direction. Calling `window-at-side-p` described above is a much more efficient way to do that.
The following function retrieves the entire window tree of a frame:
Function: **window-tree** *&optional frame*
This function returns a list representing the window tree for frame frame. If frame is omitted or `nil`, it defaults to the selected frame.
The return value is a list of the form `(root mini)`, where root represents the window tree of the frame’s root window, and mini is the frame’s minibuffer window.
If the root window is live, root is that window itself. Otherwise, root is a list `(dir edges w1
w2 ...)` where dir is `nil` for a horizontal combination and `t` for a vertical combination, edges gives the size and position of the combination, and the remaining elements are the child windows. Each child window may again be a window object (for a live window) or a list with the same format as above (for an internal window). The edges element is a list `(left
top right bottom)`, similar to the value returned by `window-edges` (see [Coordinates and Windows](coordinates-and-windows)).
| programming_docs |
elisp None ### The Thread List
The `list-threads` command lists all the currently alive threads. In the resulting buffer, each thread is identified either by the name passed to `make-thread` (see [Basic Thread Functions](basic-thread-functions)), or by its unique internal identifier if it was not created with a name. The status of each thread at the time of the creation or last update of the buffer is shown, in addition to the object the thread was blocked on at the time, if it was blocked.
Variable: **thread-list-refresh-seconds**
The `\*Threads\*` buffer will automatically update twice per second. You can make the refresh rate faster or slower by customizing this variable.
Here are the commands available in the thread list buffer:
`b`
Show a backtrace of the thread at point. This will show where in its code the thread had yielded or was blocked at the moment you pressed `b`. Be aware that the backtrace is a snapshot; the thread could have meanwhile resumed execution, and be in a different state, or could have exited.
You may use `g` in the thread’s backtrace buffer to get an updated backtrace, as backtrace buffers do not automatically update. See [Backtraces](backtraces), for a description of backtraces and the other commands which work on them.
`s`
Signal the thread at point. After `s`, type `q` to send a quit signal or `e` to send an error signal. Threads may implement handling of signals, but the default behavior is to exit on any signal. Therefore you should only use this command if you understand how to restart the target thread, because your Emacs session may behave incorrectly if necessary threads are killed.
`g` Update the list of threads and their statuses.
elisp None ### Performance of Byte-Compiled Code
A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example:
```
(defun silly-loop (n)
"Return the time, in seconds, to run N iterations of a loop."
(let ((t1 (float-time)))
(while (> (setq n (1- n)) 0))
(- (float-time) t1)))
⇒ silly-loop
```
```
(silly-loop 50000000)
⇒ 10.235304117202759
```
```
(byte-compile 'silly-loop)
⇒ [Compiled code not shown]
```
```
(silly-loop 50000000)
⇒ 3.705854892730713
```
In this example, the interpreted code required 10 seconds to run, whereas the byte-compiled code required less than 4 seconds. These results are representative, but actual results may vary.
elisp None ### Simple Packages
A simple package consists of a single Emacs Lisp source file. The file must conform to the Emacs Lisp library header conventions (see [Library Headers](https://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html)). The package’s attributes are taken from the various headers, as illustrated by the following example:
```
;;; superfrobnicator.el --- Frobnicate and bifurcate flanges
;; Copyright (C) 2011 Free Software Foundation, Inc.
```
```
;; Author: J. R. Hacker <[email protected]>
;; Version: 1.3
;; Package-Requires: ((flange "1.0"))
;; Keywords: multimedia, hypermedia
;; URL: https://example.com/jrhacker/superfrobnicate
…
;;; Commentary:
;; This package provides a minor mode to frobnicate and/or
;; bifurcate any flanges you desire. To activate it, just type
…
;;;###autoload
(define-minor-mode superfrobnicator-mode
…
```
The name of the package is the same as the base name of the file, as written on the first line. Here, it is ‘`superfrobnicator`’.
The brief description is also taken from the first line. Here, it is ‘`Frobnicate and bifurcate flanges`’.
The version number comes from the ‘`Package-Version`’ header, if it exists, or from the ‘`Version`’ header otherwise. One or the other *must* be present. Here, the version number is 1.3.
If the file has a ‘`;;; Commentary:`’ section, this section is used as the long description. (When displaying the description, Emacs omits the ‘`;;; Commentary:`’ line, as well as the leading comment characters in the commentary itself.)
If the file has a ‘`Package-Requires`’ header, that is used as the package dependencies. In the above example, the package depends on the ‘`flange`’ package, version 1.0 or higher. See [Library Headers](https://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html), for a description of the ‘`Package-Requires`’ header. If the header is omitted, the package has no dependencies.
The ‘`Keywords`’ and ‘`URL`’ headers are optional, but recommended. The command `describe-package` uses these to add links to its output. The ‘`Keywords`’ header should contain at least one standard keyword from the `finder-known-keywords` list.
The file ought to also contain one or more autoload magic comments, as explained in [Packaging Basics](packaging-basics). In the above example, a magic comment autoloads `superfrobnicator-mode`.
See [Package Archives](package-archives), for an explanation of how to add a single-file package to a package archive.
elisp None ### Loading Non-ASCII Characters
When Emacs Lisp programs contain string constants with non-ASCII characters, these can be represented within Emacs either as unibyte strings or as multibyte strings (see [Text Representations](text-representations)). Which representation is used depends on how the file is read into Emacs. If it is read with decoding into multibyte representation, the text of the Lisp program will be multibyte text, and its string constants will be multibyte strings. If a file containing Latin-1 characters (for example) is read without decoding, the text of the program will be unibyte text, and its string constants will be unibyte strings. See [Coding Systems](coding-systems).
In most Emacs Lisp programs, the fact that non-ASCII strings are multibyte strings should not be noticeable, since inserting them in unibyte buffers converts them to unibyte automatically. However, if this does make a difference, you can force a particular Lisp file to be interpreted as unibyte by writing ‘`coding: raw-text`’ in a local variables section. With that designator, the file will unconditionally be interpreted as unibyte. This can matter when making keybindings to non-ASCII characters written as `?vliteral`.
elisp None ### Text Terminal Colors
Text terminals usually support only a small number of colors, and the computer uses small integers to select colors on the terminal. This means that the computer cannot reliably tell what the selected color looks like; instead, you have to inform your application which small integers correspond to which colors. However, Emacs does know the standard set of colors and will try to use them automatically.
The functions described in this section control how terminal colors are used by Emacs.
Several of these functions use or return *rgb values*, described in [Color Names](color-names).
These functions accept a display (either a frame or the name of a terminal) as an optional argument. We hope in the future to make Emacs support different colors on different text terminals; then this argument will specify which terminal to operate on (the default being the selected frame’s terminal; see [Input Focus](input-focus)). At present, though, the frame argument has no effect.
Function: **tty-color-define** *name number &optional rgb frame*
This function associates the color name name with color number number on the terminal.
The optional argument rgb, if specified, is an rgb value, a list of three numbers that specify what the color actually looks like. If you do not specify rgb, then this color cannot be used by `tty-color-approximate` to approximate other colors, because Emacs will not know what it looks like.
Function: **tty-color-clear** *&optional frame*
This function clears the table of defined colors for a text terminal.
Function: **tty-color-alist** *&optional frame*
This function returns an alist recording the known colors supported by a text terminal.
Each element has the form `(name number . rgb)` or `(name number)`. Here, name is the color name, number is the number used to specify it to the terminal. If present, rgb is a list of three color values (for red, green, and blue) that says what the color actually looks like.
Function: **tty-color-approximate** *rgb &optional frame*
This function finds the closest color, among the known colors supported for display, to that described by the rgb value rgb (a list of color values). The return value is an element of `tty-color-alist`.
Function: **tty-color-translate** *color &optional frame*
This function finds the closest color to color among the known colors supported for display and returns its index (an integer). If the name color is not defined, the value is `nil`.
elisp None #### Evaluation Notation
A Lisp expression that you can evaluate is called a *form*. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with ‘`⇒`’:
```
(car '(1 2))
⇒ 1
```
You can read this as “`(car '(1 2))` evaluates to 1”.
When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with ‘`→`’. We may or may not show the result of the evaluation of the expanded form.
```
(third '(a b c))
→ (car (cdr (cdr '(a b c))))
⇒ c
```
To help describe one form, we sometimes show another form that produces identical results. The exact equivalence of two forms is indicated with ‘`≡`’.
```
(make-sparse-keymap) ≡ (list 'keymap)
```
elisp None ### Local Variables
Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to give a variable a *local value*—a value that takes effect only within a certain part of a Lisp program. When a variable has a local value, we say that it is *locally bound* to that value, and that it is a *local variable*.
For example, when a function is called, its argument variables receive local values, which are the actual arguments supplied to the function call; these local bindings take effect within the body of the function. To take another example, the `let` special form explicitly establishes local bindings for specific variables, which take effect only within the body of the `let` form.
We also speak of the *global binding*, which is where (conceptually) the global value is kept.
Establishing a local binding saves away the variable’s previous value (or lack of one). We say that the previous value is *shadowed*. Both global and local values may be shadowed. If a local binding is in effect, using `setq` on the local variable stores the specified value in the local binding. When that local binding is no longer in effect, the previously shadowed value (or lack of one) comes back.
A variable can have more than one local binding at a time (e.g., if there are nested `let` forms that bind the variable). The *current binding* is the local binding that is actually in effect. It determines the value returned by evaluating the variable symbol, and it is the binding acted on by `setq`.
For most purposes, you can think of the current binding as the innermost local binding, or the global binding if there is no local binding. To be more precise, a rule called the *scoping rule* determines where in a program a local binding takes effect. The default scoping rule in Emacs Lisp is called *dynamic scoping*, which simply states that the current binding at any given point in the execution of a program is the most recently-created binding for that variable that still exists. For details about dynamic scoping, and an alternative scoping rule called *lexical scoping*, see [Variable Scoping](variable-scoping).
The special forms `let` and `let*` exist to create local bindings:
Special Form: **let** *(bindings…) forms…*
This special form sets up local bindings for a certain set of variables, as specified by bindings, and then evaluates all of the forms in textual order. Its return value is the value of the last form in forms. The local bindings set up by `let` will be in effect only within the body of forms.
Each of the bindings is either (i) a symbol, in which case that symbol is locally bound to `nil`; or (ii) a list of the form `(symbol value-form)`, in which case symbol is locally bound to the result of evaluating value-form. If value-form is omitted, `nil` is used.
All of the value-forms in bindings are evaluated in the order they appear and *before* binding any of the symbols to them. Here is an example of this: `z` is bound to the old value of `y`, which is 2, not the new value of `y`, which is 1.
```
(setq y 2)
⇒ 2
```
```
(let ((y 1)
(z y))
(list y z))
⇒ (1 2)
```
On the other hand, the order of *bindings* is unspecified: in the following example, either 1 or 2 might be printed.
```
(let ((x 1)
(x 2))
(print x))
```
Therefore, avoid binding a variable more than once in a single `let` form.
Special Form: **let\*** *(bindings…) forms…*
This special form is like `let`, but it binds each variable right after computing its local value, before computing the local value for the next variable. Therefore, an expression in bindings can refer to the preceding symbols bound in this `let*` form. Compare the following example with the example above for `let`.
```
(setq y 2)
⇒ 2
```
```
(let* ((y 1)
(z y)) ; Use the just-established value of `y`.
(list y z))
⇒ (1 1)
```
Special Form: **letrec** *(bindings…) forms…*
This special form is like `let*`, but all the variables are bound before any of the local values are computed. The values are then assigned to the locally bound variables. This is only useful when lexical binding is in effect, and you want to create closures that refer to bindings that would otherwise not yet be in effect when using `let*`.
For instance, here’s a closure that removes itself from a hook after being run once:
```
(letrec ((hookfun (lambda ()
(message "Run once")
(remove-hook 'post-command-hook hookfun))))
(add-hook 'post-command-hook hookfun))
```
Special Form: **dlet** *(bindings…) forms…*
This special form is like `let`, but it binds all variables dynamically. This is rarely useful—you usually want to bind normal variables lexically, and special variables (i.e., variables that are defined with `defvar`) dynamically, and this is what `let` does.
`dlet` can be useful when interfacing with old code that assumes that certain variables are dynamically bound (see [Dynamic Binding](dynamic-binding)), but it’s impractical to `defvar` these variables. `dlet` will temporarily make the bound variables special, execute the forms, and then make the variables non-special again.
Special Form: **named-let** *name bindings &rest body*
This special form is a looping construct inspired from the Scheme language. It is similar to `let`: It binds the variables in bindings, and then evaluates body. However, `named-let` also binds name to a local function whose formal arguments are the variables in bindings and whose body is body. This allows body to call itself recursively by calling name, where the arguments passed to name are used as the new values of the bound variables in the recursive invocation.
Example of a loop summing a list of numbers:
```
(named-let sum ((numbers '(1 2 3 4))
(running-sum 0))
(if numbers
(sum (cdr numbers) (+ running-sum (car numbers)))
running-sum))
⇒ 10
```
Recursive calls to name that occur in *tail positions* in body are guaranteed to be optimized as *tail calls*, which means that they will not consume any additional stack space no matter how deeply the recursion runs. Such recursive calls will effectively jump to the top of the loop with new values for the variables.
A function call is in the tail position if it’s the very last thing done so that the value returned by the call is the value of body itself, as is the case in the recursive call to `sum` above.
Here is a complete list of the other facilities that create local bindings:
* Function calls (see [Functions](functions)).
* Macro calls (see [Macros](macros)).
* `condition-case` (see [Errors](errors)).
Variables can also have buffer-local bindings (see [Buffer-Local Variables](buffer_002dlocal-variables)); a few variables have terminal-local bindings (see [Multiple Terminals](multiple-terminals)). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on where you are in Emacs.
User Option: **max-specpdl-size**
This variable defines the limit on the total number of local variable bindings and `unwind-protect` cleanups (see [Cleaning Up from Nonlocal Exits](cleanups)) that are allowed before Emacs signals an error (with data `"Variable binding depth exceeds
max-specpdl-size"`).
This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function. `max-lisp-eval-depth` provides another limit on depth of nesting. See [Eval](eval#Definition-of-max_002dlisp_002deval_002ddepth).
The default value is 1600. Entry to the Lisp debugger increases the value, if there is little room left, to make sure the debugger itself has room to execute.
elisp None ### Nonlocal Exits
A *nonlocal exit* is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited.
| | | |
| --- | --- | --- |
| • [Catch and Throw](catch-and-throw) | | Nonlocal exits for the program’s own purposes. |
| • [Examples of Catch](examples-of-catch) | | Showing how such nonlocal exits can be written. |
| • [Errors](errors) | | How errors are signaled and handled. |
| • [Cleanups](cleanups) | | Arranging to run a cleanup form if an error happens. |
elisp None #### The Outside Context
Edebug tries to be transparent to the program you are debugging, but it does not succeed completely. Edebug also tries to be transparent when you evaluate expressions with `e` or with the evaluation list buffer, by temporarily restoring the outside context. This section explains precisely what context Edebug restores, and how Edebug fails to be completely transparent.
| | | |
| --- | --- | --- |
| • [Checking Whether to Stop](checking-whether-to-stop) | | When Edebug decides what to do. |
| • [Edebug Display Update](edebug-display-update) | | When Edebug updates the display. |
| • [Edebug Recursive Edit](edebug-recursive-edit) | | When Edebug stops execution. |
elisp None ### Transaction Queues
You can use a *transaction queue* to communicate with a subprocess using transactions. First use `tq-create` to create a transaction queue communicating with a specified process. Then you can call `tq-enqueue` to send a transaction.
Function: **tq-create** *process*
This function creates and returns a transaction queue communicating with process. The argument process should be a subprocess capable of sending and receiving streams of bytes. It may be a child process, or it may be a TCP connection to a server, possibly on another machine.
Function: **tq-enqueue** *queue question regexp closure fn &optional delay-question*
This function sends a transaction to queue queue. Specifying the queue has the effect of specifying the subprocess to talk to.
The argument question is the outgoing message that starts the transaction. The argument fn is the function to call when the corresponding answer comes back; it is called with two arguments: closure, and the answer received.
The argument regexp is a regular expression that should match text at the end of the entire answer, but nothing before; that’s how `tq-enqueue` determines where the answer ends.
If the argument delay-question is non-`nil`, delay sending this question until the process has finished replying to any previous questions. This produces more reliable results with some processes.
Function: **tq-close** *queue*
Shut down transaction queue queue, waiting for all pending transactions to complete, and then terminate the connection or child process.
Transaction queues are implemented by means of a filter function. See [Filter Functions](filter-functions).
| programming_docs |
elisp None #### XPM Images
To use XPM format, specify `xpm` as the image type. The additional image property `:color-symbols` is also meaningful with the `xpm` image type:
`:color-symbols symbols` The value, symbols, should be an alist whose elements have the form `(name . color)`. In each element, name is the name of a color as it appears in the image file, and color specifies the actual color to use for displaying that name.
elisp None Hash Tables
-----------
A hash table is a very fast kind of lookup table, somewhat like an alist (see [Association Lists](association-lists)) in that it maps keys to corresponding values. It differs from an alist in these ways:
* Lookup in a hash table is extremely fast for large tables—in fact, the time required is essentially *independent* of how many elements are stored in the table. For smaller tables (a few tens of elements) alists may still be faster because hash tables have a more-or-less constant overhead.
* The correspondences in a hash table are in no particular order.
* There is no way to share structure between two hash tables, the way two alists can share a common tail.
Emacs Lisp provides a general-purpose hash table data type, along with a series of functions for operating on them. Hash tables have a special printed representation, which consists of ‘`#s`’ followed by a list specifying the hash table properties and contents. See [Creating Hash](creating-hash). (Hash notation, the initial ‘`#`’ character used in the printed representations of objects with no read representation, has nothing to do with hash tables. See [Printed Representation](printed-representation).)
Obarrays are also a kind of hash table, but they are a different type of object and are used only for recording interned symbols (see [Creating Symbols](creating-symbols)).
| | | |
| --- | --- | --- |
| • [Creating Hash](creating-hash) | | Functions to create hash tables. |
| • [Hash Access](hash-access) | | Reading and writing the hash table contents. |
| • [Defining Hash](defining-hash) | | Defining new comparison methods. |
| • [Other Hash](other-hash) | | Miscellaneous. |
elisp None ### Desktop Save Mode
*Desktop Save Mode* is a feature to save the state of Emacs from one session to another. The user-level commands for using Desktop Save Mode are described in the GNU Emacs Manual (see [Saving Emacs Sessions](https://www.gnu.org/software/emacs/manual/html_node/emacs/Saving-Emacs-Sessions.html#Saving-Emacs-Sessions) in the GNU Emacs Manual). Modes whose buffers visit a file, don’t have to do anything to use this feature.
For buffers not visiting a file to have their state saved, the major mode must bind the buffer local variable `desktop-save-buffer` to a non-`nil` value.
Variable: **desktop-save-buffer**
If this buffer-local variable is non-`nil`, the buffer will have its state saved in the desktop file at desktop save. If the value is a function, it is called at desktop save with argument desktop-dirname, and its value is saved in the desktop file along with the state of the buffer for which it was called. When file names are returned as part of the auxiliary information, they should be formatted using the call
```
(desktop-file-name file-name desktop-dirname)
```
For buffers not visiting a file to be restored, the major mode must define a function to do the job, and that function must be listed in the alist `desktop-buffer-mode-handlers`.
Variable: **desktop-buffer-mode-handlers**
Alist with elements
```
(major-mode . restore-buffer-function)
```
The function restore-buffer-function will be called with argument list
```
(buffer-file-name buffer-name desktop-buffer-misc)
```
and it should return the restored buffer. Here desktop-buffer-misc is the value returned by the function optionally bound to `desktop-save-buffer`.
elisp None ### Size of Displayed Text
Since not all characters have the same width, these functions let you check the width of a character. See [Primitive Indent](primitive-indent), and [Screen Lines](screen-lines), for related functions.
Function: **char-width** *char*
This function returns the width in columns of the character char, if it were displayed in the current buffer (i.e., taking into account the buffer’s display table, if any; see [Display Tables](display-tables)). The width of a tab character is usually `tab-width` (see [Usual Display](usual-display)).
Function: **string-width** *string &optional from to*
This function returns the width in columns of the string string, if it were displayed in the current buffer and the selected window. Optional arguments from and to specify the substring of string to consider, and are interpreted as in `substring` (see [Creating Strings](creating-strings)).
The return value is an approximation: it only considers the values returned by `char-width` for the constituent characters, always takes a tab character as taking `tab-width` columns, ignores display properties and fonts, etc. For these reasons, we recommend using `window-text-pixel-size`, described below, instead.
Function: **truncate-string-to-width** *string width &optional start-column padding ellipsis ellipsis-text-property*
This function returns a new string that is a truncation of string which fits within width columns on display.
If string is narrower than width, the result is equal to string; otherwise excess characters are omitted from the result. If a multi-column character in string exceeds the goal width, that character is omitted from the result. Thus, the result can sometimes fall short of width, but cannot go beyond it.
The optional argument start-column specifies the starting column; it defaults to zero. If this is non-`nil`, then the first start-column columns of the string are omitted from the result. If one multi-column character in string extends across the column start-column, that character is omitted.
The optional argument padding, if non-`nil`, is a padding character added at the beginning and end of the result string, to extend it to exactly width columns. The padding character is appended at the end of the result if it falls short of width, as many times as needed to reach width. It is also prepended at the beginning of the result if a multi-column character in string extends across the column start-column.
If ellipsis is non-`nil`, it should be a string which will replace the end of string when it is truncated. In this case, more characters will be removed from string to free enough space for ellipsis to fit within width columns. However, if the display width of string is less than the display width of ellipsis, ellipsis will not be appended to the result. If ellipsis is non-`nil` and not a string, it stands for the value returned by the function `truncate-string-ellipsis`, described below.
The optional argument ellipsis-text-property, if non-`nil`, means hide the excess parts of string with a `display` text property (see [Display Property](display-property)) showing the ellipsis, instead of actually truncating the string.
```
(truncate-string-to-width "\tab\t" 12 4)
⇒ "ab"
(truncate-string-to-width "\tab\t" 12 4 ?\s)
⇒ " ab "
```
This function uses `string-width` and `char-width` to find the suitable truncation point when string is too wide, so it suffers from the same basic issues as `string-width` does. In particular, when character composition happens within string, the display width of a string could be smaller than the sum of widths of the constituent characters, and this function might return inaccurate results.
Function: **truncate-string-ellipsis**
This function returns the string to be used as an ellipses in `truncate-string-to-width` and other similar contexts. The value is that of the variable `truncate-string-ellipsis`, if it’s non-`nil`, the string with the single character U+2026 HORIZONTAL ELLIPSIS if that character can be displayed on the selected frame, and the string ‘`...`’ otherwise.
The following function returns the size in pixels of text as if it were displayed in a given window. This function is used by `fit-window-to-buffer` and `fit-frame-to-buffer` (see [Resizing Windows](resizing-windows)) to make a window exactly as large as the text it contains.
Function: **window-text-pixel-size** *&optional window from to x-limit y-limit mode-lines*
This function returns the size of the text of window’s buffer in pixels. window must be a live window and defaults to the selected one. The return value is a cons of the maximum pixel-width of any text line and the maximum pixel-height of all text lines. This function exists to allow Lisp programs to adjust the dimensions of window to the buffer text it needs to display.
The optional argument from, if non-`nil`, specifies the first text position to consider, and defaults to the minimum accessible position of the buffer. If from is `t`, it stands for the minimum accessible position that is not a newline character. The optional argument to, if non-`nil`, specifies the last text position to consider, and defaults to the maximum accessible position of the buffer. If to is `t`, it stands for the maximum accessible position that is not a newline character.
The optional argument x-limit, if non-`nil`, specifies the maximum X coordinate beyond which text should be ignored; it is therefore also the largest value of pixel-width that the function can return. If x-limit `nil` or omitted, it means to use the pixel-width of window’s body (see [Window Sizes](window-sizes)); this default means that text of truncated lines wider than the window will be ignored. This default is useful when the caller does not intend to change the width of window. Otherwise, the caller should specify here the maximum width window’s body may assume; in particular, if truncated lines are expected and their text needs to be accounted for, x-limit should be set to a large value. Since calculating the width of long lines can take some time, it’s always a good idea to make this argument as small as needed; in particular, if the buffer might contain long lines that will be truncated anyway.
The optional argument y-limit, if non-`nil`, specifies the maximum Y coordinate beyond which text is to be ignored; it is therefore also the maximum pixel-height that the function can return. If y-limit is nil or omitted, it means to considers all the lines of text till the buffer position specified by to. Since calculating the pixel-height of a large buffer can take some time, it makes sense to specify this argument; in particular, if the caller does not know the size of the buffer.
The optional argument mode-lines `nil` or omitted means to not include the height of the mode-, tab- or header-line of window in the return value. If it is either the symbol `mode-line`, `tab-line` or `header-line`, include only the height of that line, if present, in the return value. If it is `t`, include the height of all of these lines, if present, in the return value.
`window-text-pixel-size` treats the text displayed in a window as a whole and does not care about the size of individual lines. The following function does.
Function: **window-lines-pixel-dimensions** *&optional window first last body inverse left*
This function calculates the pixel dimensions of each line displayed in the specified window. It does so by walking window’s current glyph matrix—a matrix storing the glyph (see [Glyphs](glyphs)) of each buffer character currently displayed in window. If successful, it returns a list of cons pairs representing the x- and y-coordinates of the lower right corner of the last character of each line. Coordinates are measured in pixels from an origin (0, 0) at the top-left corner of window. window must be a live window and defaults to the selected one.
If the optional argument first is an integer, it denotes the index (starting with 0) of the first line of window’s glyph matrix to be returned. Note that if window has a header line, the line with index 0 is that header line. If first is `nil`, the first line to be considered is determined by the value of the optional argument body: If body is non-`nil`, this means to start with the first line of window’s body, skipping any header line, if present. Otherwise, this function will start with the first line of window’s glyph matrix, possibly the header line.
If the optional argument last is an integer, it denotes the index of the last line of window’s glyph matrix that shall be returned. If last is `nil`, the last line to be considered is determined by the value of body: If body is non-`nil`, this means to use the last line of window’s body, omitting window’s mode line, if present. Otherwise, this means to use the last line of window which may be the mode line.
The optional argument inverse, if `nil`, means that the y-pixel value returned for any line specifies the distance in pixels from the left edge (body edge if body is non-`nil`) of window to the right edge of the last glyph of that line. inverse non-`nil` means that the y-pixel value returned for any line specifies the distance in pixels from the right edge of the last glyph of that line to the right edge (body edge if body is non-`nil`) of window. This is useful for determining the amount of slack space at the end of each line.
The optional argument left, if non-`nil` means to return the x- and y-coordinates of the lower left corner of the leftmost character on each line. This is the value that should be used for windows that mostly display text from right to left.
If left is non-`nil` and inverse is `nil`, this means that the y-pixel value returned for any line specifies the distance in pixels from the left edge of the last (leftmost) glyph of that line to the right edge (body edge if body is non-`nil`) of window. If left and inverse are both non-`nil`, the y-pixel value returned for any line specifies the distance in pixels from the left edge (body edge if body is non-`nil`) of window to the left edge of the last (leftmost) glyph of that line.
This function returns `nil` if the current glyph matrix of window is not up-to-date which usually happens when Emacs is busy, for example, when processing a command. The value should be retrievable though when this function is run from an idle timer with a delay of zero seconds.
Function: **line-pixel-height**
This function returns the height in pixels of the line at point in the selected window. The value includes the line spacing of the line (see [Line Height](line-height)).
When a buffer is displayed with line numbers (see [Display Custom](https://www.gnu.org/software/emacs/manual/html_node/emacs/Display-Custom.html#Display-Custom) in The GNU Emacs Manual), it is sometimes useful to know the width taken for displaying the line numbers. The following function is for Lisp programs which need this information for layout calculations.
Function: **line-number-display-width** *&optional pixelwise*
This function returns the width used for displaying the line numbers in the selected window. If the optional argument pixelwise is the symbol `columns`, the return value is a float number of the frame’s canonical columns; if pixelwise is `t` or any other non-`nil` value, the value is an integer and is measured in pixels. If pixelwise is omitted or `nil`, the value is the integer number of columns of the font defined for the `line-number` face, and doesn’t include the 2 columns used to pad the numbers on display. If line numbers are not displayed in the selected window, the value is zero regardless of the value of pixelwise. Use `with-selected-window` (see [Selecting Windows](selecting-windows)) if you need this information about another window.
elisp None ### Deleting Processes
*Deleting a process* disconnects Emacs immediately from the subprocess. Processes are deleted automatically after they terminate, but not necessarily right away. You can delete a process explicitly at any time. If you explicitly delete a terminated process before it is deleted automatically, no harm results. Deleting a running process sends a signal to terminate it (and its child processes, if any), and calls the process sentinel. See [Sentinels](sentinels).
When a process is deleted, the process object itself continues to exist as long as other Lisp objects point to it. All the Lisp primitives that work on process objects accept deleted processes, but those that do I/O or send signals will report an error. The process mark continues to point to the same place as before, usually into a buffer where output from the process was being inserted.
User Option: **delete-exited-processes**
This variable controls automatic deletion of processes that have terminated (due to calling `exit` or to a signal). If it is `nil`, then they continue to exist until the user runs `list-processes`. Otherwise, they are deleted immediately after they exit.
Function: **delete-process** *process*
This function deletes a process, killing it with a `SIGKILL` signal if the process was running a program. The argument may be a process, the name of a process, a buffer, or the name of a buffer. (A buffer or buffer-name stands for the process that `get-buffer-process` returns.) Calling `delete-process` on a running process terminates it, updates the process status, and runs the sentinel immediately. If the process has already terminated, calling `delete-process` has no effect on its status, or on the running of its sentinel (which will happen sooner or later).
If the process object represents a network, serial, or pipe connection, its status changes to `closed`; otherwise, it changes to `signal`, unless the process already exited. See [process-status](process-information).
```
(delete-process "*shell*")
⇒ nil
```
elisp None #### Defining Clickable Text
*Clickable text* is text that can be clicked, with either the mouse or via a keyboard command, to produce some result. Many major modes use clickable text to implement textual hyper-links, or *links* for short.
The easiest way to insert and manipulate links is to use the `button` package. See [Buttons](buttons). In this section, we will explain how to manually set up clickable text in a buffer, using text properties. For simplicity, we will refer to the clickable text as a *link*.
Implementing a link involves three separate steps: (1) indicating clickability when the mouse moves over the link; (2) making RET or `mouse-2` on that link do something; and (3) setting up a `follow-link` condition so that the link obeys `mouse-1-click-follows-link`.
To indicate clickability, add the `mouse-face` text property to the text of the link; then Emacs will highlight the link when the mouse moves over it. In addition, you should define a tooltip or echo area message, using the `help-echo` text property. See [Special Properties](special-properties). For instance, here is how Dired indicates that file names are clickable:
```
(if (dired-move-to-filename)
(add-text-properties
(point)
(save-excursion
(dired-move-to-end-of-filename)
(point))
'(mouse-face highlight
help-echo "mouse-2: visit this file in other window")))
```
To make the link clickable, bind RET and `mouse-2` to commands that perform the desired action. Each command should check to see whether it was called on a link, and act accordingly. For instance, Dired’s major mode keymap binds `mouse-2` to the following command:
```
(defun dired-mouse-find-file-other-window (event)
"In Dired, visit the file or directory name you click on."
(interactive "e")
(let ((window (posn-window (event-end event)))
(pos (posn-point (event-end event)))
file)
(if (not (windowp window))
(error "No file chosen"))
(with-current-buffer (window-buffer window)
(goto-char pos)
(setq file (dired-get-file-for-visit)))
(if (file-directory-p file)
(or (and (cdr dired-subdir-alist)
(dired-goto-subdir file))
(progn
(select-window window)
(dired-other-window file)))
(select-window window)
(find-file-other-window (file-name-sans-versions file t)))))
```
This command uses the functions `posn-window` and `posn-point` to determine where the click occurred, and `dired-get-file-for-visit` to determine which file to visit.
Instead of binding the mouse command in a major mode keymap, you can bind it within the link text, using the `keymap` text property (see [Special Properties](special-properties)). For instance:
```
(let ((map (make-sparse-keymap)))
(define-key map [mouse-2] 'operate-this-button)
(put-text-property link-start link-end 'keymap map))
```
With this method, you can easily define different commands for different links. Furthermore, the global definition of RET and `mouse-2` remain available for the rest of the text in the buffer.
The basic Emacs command for clicking on links is `mouse-2`. However, for compatibility with other graphical applications, Emacs also recognizes `mouse-1` clicks on links, provided the user clicks on the link quickly without moving the mouse. This behavior is controlled by the user option `mouse-1-click-follows-link`. See [Mouse References](https://www.gnu.org/software/emacs/manual/html_node/emacs/Mouse-References.html#Mouse-References) in The GNU Emacs Manual.
To set up the link so that it obeys `mouse-1-click-follows-link`, you must either (1) apply a `follow-link` text or overlay property to the link text, or (2) bind the `follow-link` event to a keymap (which can be a major mode keymap or a local keymap specified via the `keymap` text property). The value of the `follow-link` property, or the binding for the `follow-link` event, acts as a condition for the link action. This condition tells Emacs two things: the circumstances under which a `mouse-1` click should be regarded as occurring inside the link, and how to compute an action code that says what to translate the `mouse-1` click into. The link action condition can be one of the following:
`mouse-face`
If the condition is the symbol `mouse-face`, a position is inside a link if there is a non-`nil` `mouse-face` property at that position. The action code is always `t`.
For example, here is how Info mode handles mouse-1:
```
(define-key Info-mode-map [follow-link] 'mouse-face)
```
a function
If the condition is a function, func, then a position pos is inside a link if `(func pos)` evaluates to non-`nil`. The value returned by func serves as the action code.
For example, here is how pcvs enables `mouse-1` to follow links on file names only:
```
(define-key map [follow-link]
(lambda (pos)
(eq (get-char-property pos 'face) 'cvs-filename-face)))
```
anything else If the condition value is anything else, then the position is inside a link and the condition itself is the action code. Clearly, you should specify this kind of condition only when applying the condition via a text or overlay property on the link text (so that it does not apply to the entire buffer).
The action code tells `mouse-1` how to follow the link:
a string or vector
If the action code is a string or vector, the `mouse-1` event is translated into the first element of the string or vector; i.e., the action of the `mouse-1` click is the local or global binding of that character or symbol. Thus, if the action code is `"foo"`, `mouse-1` translates into `f`. If it is `[foo]`, `mouse-1` translates into foo.
anything else For any other non-`nil` action code, the `mouse-1` event is translated into a `mouse-2` event at the same position.
To define `mouse-1` to activate a button defined with `define-button-type`, give the button a `follow-link` property. The property value should be a link action condition, as described above. See [Buttons](buttons). For example, here is how Help mode handles `mouse-1`:
```
(define-button-type 'help-xref
'follow-link t
'action #'help-button-action)
```
To define `mouse-1` on a widget defined with `define-widget`, give the widget a `:follow-link` property. The property value should be a link action condition, as described above. For example, here is how the `link` widget specifies that a mouse-1 click shall be translated to RET:
```
(define-widget 'link 'item
"An embedded link."
:button-prefix 'widget-link-prefix
:button-suffix 'widget-link-suffix
:follow-link "\C-m"
:help-echo "Follow the link."
:format "%[%t%]")
```
Function: **mouse-on-link-p** *pos*
This function returns non-`nil` if position pos in the current buffer is on a link. pos can also be a mouse event location, as returned by `event-start` (see [Accessing Mouse](accessing-mouse)).
| programming_docs |
elisp None ### Indirect Buffers
An *indirect buffer* shares the text of some other buffer, which is called the *base buffer* of the indirect buffer. In some ways it is the analogue, for buffers, of a symbolic link among files. The base buffer may not itself be an indirect buffer.
The text of the indirect buffer is always identical to the text of its base buffer; changes made by editing either one are visible immediately in the other. This includes the text properties as well as the characters themselves.
In all other respects, the indirect buffer and its base buffer are completely separate. They have different names, independent values of point, independent narrowing, independent markers and overlays (though inserting or deleting text in either buffer relocates the markers and overlays for both), independent major modes, and independent buffer-local variable bindings.
An indirect buffer cannot visit a file, but its base buffer can. If you try to save the indirect buffer, that actually saves the base buffer.
Killing an indirect buffer has no effect on its base buffer. Killing the base buffer effectively kills the indirect buffer in that it cannot ever again be the current buffer.
Command: **make-indirect-buffer** *base-buffer name &optional clone inhibit-buffer-hooks*
This creates and returns an indirect buffer named name whose base buffer is base-buffer. The argument base-buffer may be a live buffer or the name (a string) of an existing buffer. If name is the name of an existing buffer, an error is signaled.
If clone is non-`nil`, then the indirect buffer originally shares the state of base-buffer such as major mode, minor modes, buffer local variables and so on. If clone is omitted or `nil` the indirect buffer’s state is set to the default state for new buffers.
If base-buffer is an indirect buffer, its base buffer is used as the base for the new buffer. If, in addition, clone is non-`nil`, the initial state is copied from the actual base buffer, not from base-buffer.
See [Creating Buffers](creating-buffers), for the meaning of inhibit-buffer-hooks.
Command: **clone-indirect-buffer** *newname display-flag &optional norecord*
This function creates and returns a new indirect buffer that shares the current buffer’s base buffer and copies the rest of the current buffer’s attributes. (If the current buffer is not indirect, it is used as the base buffer.)
If display-flag is non-`nil`, as it always is in interactive calls, that means to display the new buffer by calling `pop-to-buffer`. If norecord is non-`nil`, that means not to put the new buffer to the front of the buffer list.
Function: **buffer-base-buffer** *&optional buffer*
This function returns the base buffer of buffer, which defaults to the current buffer. If buffer is not indirect, the value is `nil`. Otherwise, the value is another buffer, which is never an indirect buffer.
elisp None ### Datagrams
A *datagram* connection communicates with individual packets rather than streams of data. Each call to `process-send` sends one datagram packet (see [Input to Processes](input-to-processes)), and each datagram received results in one call to the filter function.
The datagram connection doesn’t have to talk with the same remote peer all the time. It has a *remote peer address* which specifies where to send datagrams to. Each time an incoming datagram is passed to the filter function, the peer address is set to the address that datagram came from; that way, if the filter function sends a datagram, it will go back to that place. You can specify the remote peer address when you create the datagram connection using the `:remote` keyword. You can change it later on by calling `set-process-datagram-address`.
Function: **process-datagram-address** *process*
If process is a datagram connection or server, this function returns its remote peer address.
Function: **set-process-datagram-address** *process address*
If process is a datagram connection or server, this function sets its remote peer address to address.
elisp None #### Syntax of Regular Expressions
Regular expressions have a syntax in which a few characters are special constructs and the rest are *ordinary*. An ordinary character is a simple regular expression that matches that character and nothing else. The special characters are ‘`.`’, ‘`\*`’, ‘`+`’, ‘`?`’, ‘`[`’, ‘`^`’, ‘`$`’, and ‘`\`’; no new special characters will be defined in the future. The character ‘`]`’ is special if it ends a character alternative (see later). The character ‘`-`’ is special inside a character alternative. A ‘`[:`’ and balancing ‘`:]`’ enclose a character class inside a character alternative. Any other character appearing in a regular expression is ordinary, unless a ‘`\`’ precedes it.
For example, ‘`f`’ is not a special character, so it is ordinary, and therefore ‘`f`’ is a regular expression that matches the string ‘`f`’ and no other string. (It does *not* match the string ‘`fg`’, but it does match a *part* of that string.) Likewise, ‘`o`’ is a regular expression that matches only ‘`o`’.
Any two regular expressions a and b can be concatenated. The result is a regular expression that matches a string if a matches some amount of the beginning of that string and b matches the rest of the string.
As a simple example, we can concatenate the regular expressions ‘`f`’ and ‘`o`’ to get the regular expression ‘`fo`’, which matches only the string ‘`fo`’. Still trivial. To do something more powerful, you need to use one of the special regular expression constructs.
| | | |
| --- | --- | --- |
| • [Regexp Special](regexp-special) | | Special characters in regular expressions. |
| • [Char Classes](char-classes) | | Character classes used in regular expressions. |
| • [Regexp Backslash](regexp-backslash) | | Backslash-sequences in regular expressions. |
elisp None ### Record Functions
Function: **recordp** *object*
This function returns `t` if object is a record.
```
(recordp #s(a))
⇒ t
```
Function: **record** *type &rest objects*
This function creates and returns a record whose type is type and remaining slots are the rest of the arguments, objects.
```
(record 'foo 23 [bar baz] "rats")
⇒ #s(foo 23 [bar baz] "rats")
```
Function: **make-record** *type length object*
This function returns a new record with type type and length more slots, each initialized to object.
```
(setq sleepy (make-record 'foo 9 'Z))
⇒ #s(foo Z Z Z Z Z Z Z Z Z)
```
elisp None #### Motion by Text Lines
Text lines are portions of the buffer delimited by newline characters, which are regarded as part of the previous line. The first text line begins at the beginning of the buffer, and the last text line ends at the end of the buffer whether or not the last character is a newline. The division of the buffer into text lines is not affected by the width of the window, by line continuation in display, or by how tabs and control characters are displayed.
Command: **beginning-of-line** *&optional count*
This function moves point to the beginning of the current line. With an argument count not `nil` or 1, it moves forward count-1 lines and then to the beginning of the line.
This function does not move point across a field boundary (see [Fields](fields)) unless doing so would move beyond there to a different line; therefore, if count is `nil` or 1, and point starts at a field boundary, point does not move. To ignore field boundaries, either bind `inhibit-field-text-motion` to `t`, or use the `forward-line` function instead. For instance, `(forward-line 0)` does the same thing as `(beginning-of-line)`, except that it ignores field boundaries.
If this function reaches the end of the buffer (or of the accessible portion, if narrowing is in effect), it positions point there. No error is signaled.
Function: **line-beginning-position** *&optional count*
Return the position that `(beginning-of-line count)` would move to.
Command: **end-of-line** *&optional count*
This function moves point to the end of the current line. With an argument count not `nil` or 1, it moves forward count-1 lines and then to the end of the line.
This function does not move point across a field boundary (see [Fields](fields)) unless doing so would move beyond there to a different line; therefore, if count is `nil` or 1, and point starts at a field boundary, point does not move. To ignore field boundaries, bind `inhibit-field-text-motion` to `t`.
If this function reaches the end of the buffer (or of the accessible portion, if narrowing is in effect), it positions point there. No error is signaled.
Function: **line-end-position** *&optional count*
Return the position that `(end-of-line count)` would move to.
Command: **forward-line** *&optional count*
This function moves point forward count lines, to the beginning of the line following that. If count is negative, it moves point -count lines backward, to the beginning of a line preceding that. If count is zero, it moves point to the beginning of the current line. If count is `nil`, that means 1.
If `forward-line` encounters the beginning or end of the buffer (or of the accessible portion) before finding that many lines, it sets point there. No error is signaled.
`forward-line` returns the difference between count and the number of lines actually moved. If you attempt to move down five lines from the beginning of a buffer that has only three lines, point stops at the end of the last line, and the value will be 2. As an explicit exception, if the last accessible line is non-empty, but has no newline (e.g., if the buffer ends without a newline), the function sets point to the end of that line, and the value returned by the function counts that line as one line successfully moved.
In an interactive call, count is the numeric prefix argument.
Function: **count-lines** *start end &optional ignore-invisible-lines*
This function returns the number of lines between the positions start and end in the current buffer. If start and end are equal, then it returns 0. Otherwise it returns at least 1, even if start and end are on the same line. This is because the text between them, considered in isolation, must contain at least one line unless it is empty.
If the optional ignore-invisible-lines is non-`nil`, invisible lines will not be included in the count.
Command: **count-words** *start end*
This function returns the number of words between the positions start and end in the current buffer.
This function can also be called interactively. In that case, it prints a message reporting the number of lines, words, and characters in the buffer, or in the region if the region is active.
Function: **line-number-at-pos** *&optional pos absolute*
This function returns the line number in the current buffer corresponding to the buffer position pos. If pos is `nil` or omitted, the current buffer position is used. If absolute is `nil`, the default, counting starts at `(point-min)`, so the value refers to the contents of the accessible portion of the (potentially narrowed) buffer. If absolute is non-`nil`, ignore any narrowing and return the absolute line number.
Also see the functions `bolp` and `eolp` in [Near Point](near-point). These functions do not move point, but test whether it is already at the beginning or end of a line.
elisp None ### Pattern-Matching Conditional
Aside from the four basic conditional forms, Emacs Lisp also has a pattern-matching conditional form, the `pcase` macro, a hybrid of `cond` and `cl-case` (see [Conditionals](https://www.gnu.org/software/emacs/manual/html_node/cl/Conditionals.html#Conditionals) in Common Lisp Extensions) that overcomes their limitations and introduces the *pattern matching programming style*. The limitations that `pcase` overcomes are:
* The `cond` form chooses among alternatives by evaluating the predicate condition of each of its clauses (see [Conditionals](conditionals)). The primary limitation is that variables let-bound in condition are not available to the clause’s body-forms. Another annoyance (more an inconvenience than a limitation) is that when a series of condition predicates implement equality tests, there is a lot of repeated code. (`cl-case` solves this inconvenience.)
* The `cl-case` macro chooses among alternatives by evaluating the equality of its first argument against a set of specific values. Its limitations are two-fold:
1. The equality tests use `eql`.
2. The values must be known and written in advance.These render `cl-case` unsuitable for strings or compound data structures (e.g., lists or vectors). (`cond` doesn’t have these limitations, but it has others, see above.)
Conceptually, the `pcase` macro borrows the first-arg focus of `cl-case` and the clause-processing flow of `cond`, replacing condition with a generalization of the equality test which is a variant of *pattern matching*, and adding facilities so that you can concisely express a clause’s predicate, and arrange to share let-bindings between a clause’s predicate and body-forms.
The concise expression of a predicate is known as a *pattern*. When the predicate, called on the value of the first arg, returns non-`nil`, we say that “the pattern matches the value” (or sometimes “the value matches the pattern”).
| | | |
| --- | --- | --- |
| • [The `pcase` macro](pcase-macro) | | Includes examples and caveats. |
| • [Extending `pcase`](extending-pcase) | | Define new kinds of patterns. |
| • [Backquote-Style Patterns](backquote-patterns) | | Structural patterns matching. |
| • [Destructuring with pcase Patterns](destructuring-with-pcase-patterns) | | Using pcase patterns to extract subfields. |
elisp None ### Buffer Modification
Emacs keeps a flag called the *modified flag* for each buffer, to record whether you have changed the text of the buffer. This flag is set to `t` whenever you alter the contents of the buffer, and cleared to `nil` when you save it. Thus, the flag shows whether there are unsaved changes. The flag value is normally shown in the mode line (see [Mode Line Variables](mode-line-variables)), and controls saving (see [Saving Buffers](saving-buffers)) and auto-saving (see [Auto-Saving](auto_002dsaving)).
Some Lisp programs set the flag explicitly. For example, the function `set-visited-file-name` sets the flag to `t`, because the text does not match the newly-visited file, even if it is unchanged from the file formerly visited.
The functions that modify the contents of buffers are described in [Text](text).
Function: **buffer-modified-p** *&optional buffer*
This function returns `t` if the buffer buffer has been modified since it was last read in from a file or saved, or `nil` otherwise. If buffer is not supplied, the current buffer is tested.
Function: **set-buffer-modified-p** *flag*
This function marks the current buffer as modified if flag is non-`nil`, or as unmodified if the flag is `nil`.
Another effect of calling this function is to cause unconditional redisplay of the mode line for the current buffer. In fact, the function `force-mode-line-update` works by doing this:
```
(set-buffer-modified-p (buffer-modified-p))
```
Function: **restore-buffer-modified-p** *flag*
Like `set-buffer-modified-p`, but does not force redisplay of mode lines.
Command: **not-modified** *&optional arg*
This command marks the current buffer as unmodified, and not needing to be saved. If arg is non-`nil`, it marks the buffer as modified, so that it will be saved at the next suitable occasion. Interactively, arg is the prefix argument.
Don’t use this function in programs, since it prints a message in the echo area; use `set-buffer-modified-p` (above) instead.
Function: **buffer-modified-tick** *&optional buffer*
This function returns buffer’s modification-count. This is a counter that increments every time the buffer is modified. If buffer is `nil` (or omitted), the current buffer is used.
Function: **buffer-chars-modified-tick** *&optional buffer*
This function returns buffer’s character-change modification-count. Changes to text properties leave this counter unchanged; however, each time text is inserted or removed from the buffer, the counter is reset to the value that would be returned by `buffer-modified-tick`. By comparing the values returned by two `buffer-chars-modified-tick` calls, you can tell whether a character change occurred in that buffer in between the calls. If buffer is `nil` (or omitted), the current buffer is used.
Sometimes there’s a need for modifying buffer in a way that doesn’t really change its text, like if only its text properties are changed. If your program needs to modify a buffer without triggering any hooks and features that react to buffer modifications, use the `with-silent-modifications` macro.
Macro: **with-silent-modifications** *body…*
Execute body pretending it does not modify the buffer. This includes checking whether the buffer’s file is locked (see [File Locks](file-locks)), running buffer modification hooks (see [Change Hooks](change-hooks)), etc. Note that if body actually modifies the buffer text (as opposed to its text properties), its undo data may become corrupted.
elisp None ### Querying Before Exit
When Emacs exits, it terminates all its subprocesses. For subprocesses that run a program, it sends them the `SIGHUP` signal; connections are simply closed. Because subprocesses may be doing valuable work, Emacs normally asks the user to confirm that it is ok to terminate them. Each process has a query flag, which, if non-`nil`, says that Emacs should ask for confirmation before exiting and thus killing that process. The default for the query flag is `t`, meaning *do* query.
Function: **process-query-on-exit-flag** *process*
This returns the query flag of process.
Function: **set-process-query-on-exit-flag** *process flag*
This function sets the query flag of process to flag. It returns flag.
Here is an example of using `set-process-query-on-exit-flag` on a shell process to avoid querying:
```
(set-process-query-on-exit-flag (get-process "shell") nil)
⇒ nil
```
User Option: **confirm-kill-processes**
If this user option is set to `t` (the default), then Emacs asks for confirmation before killing processes on exit. If it is `nil`, Emacs kills processes without confirmation, i.e., the query flag of all processes is ignored.
elisp None ### Keymap Basics
A keymap is a Lisp data structure that specifies *key bindings* for various key sequences.
A single keymap directly specifies definitions for individual events. When a key sequence consists of a single event, its binding in a keymap is the keymap’s definition for that event. The binding of a longer key sequence is found by an iterative process: first find the definition of the first event (which must itself be a keymap); then find the second event’s definition in that keymap, and so on until all the events in the key sequence have been processed.
If the binding of a key sequence is a keymap, we call the key sequence a *prefix key*. Otherwise, we call it a *complete key* (because no more events can be added to it). If the binding is `nil`, we call the key *undefined*. Examples of prefix keys are `C-c`, `C-x`, and `C-x 4`. Examples of defined complete keys are `X`, RET, and `C-x 4 C-f`. Examples of undefined complete keys are `C-x C-g`, and `C-c 3`. See [Prefix Keys](prefix-keys), for more details.
The rule for finding the binding of a key sequence assumes that the intermediate bindings (found for the events before the last) are all keymaps; if this is not so, the sequence of events does not form a unit—it is not really one key sequence. In other words, removing one or more events from the end of any valid key sequence must always yield a prefix key. For example, `C-f C-n` is not a key sequence; `C-f` is not a prefix key, so a longer sequence starting with `C-f` cannot be a key sequence.
The set of possible multi-event key sequences depends on the bindings for prefix keys; therefore, it can be different for different keymaps, and can change when bindings are changed. However, a one-event sequence is always a key sequence, because it does not depend on any prefix keys for its well-formedness.
At any time, several primary keymaps are *active*—that is, in use for finding key bindings. These are the *global map*, which is shared by all buffers; the *local keymap*, which is usually associated with a specific major mode; and zero or more *minor mode keymaps*, which belong to currently enabled minor modes. (Not all minor modes have keymaps.) The local keymap bindings shadow (i.e., take precedence over) the corresponding global bindings. The minor mode keymaps shadow both local and global keymaps. See [Active Keymaps](active-keymaps), for details.
| programming_docs |
elisp None #### Focus Events
Window systems provide general ways for the user to control which window gets keyboard input. This choice of window is called the *focus*. When the user does something to switch between Emacs frames, that generates a *focus event*. The normal definition of a focus event, in the global keymap, is to select a new frame within Emacs, as the user would expect. See [Input Focus](input-focus), which also describes hooks related to focus events.
Focus events are represented in Lisp as lists that look like this:
```
(switch-frame new-frame)
```
where new-frame is the frame switched to.
Some X window managers are set up so that just moving the mouse into a window is enough to set the focus there. Usually, there is no need for a Lisp program to know about the focus change until some other kind of input arrives. Emacs generates a focus event only when the user actually types a keyboard key or presses a mouse button in the new frame; just moving the mouse between frames does not generate a focus event.
A focus event in the middle of a key sequence would garble the sequence. So Emacs never generates a focus event in the middle of a key sequence. If the user changes focus in the middle of a key sequence—that is, after a prefix key—then Emacs reorders the events so that the focus event comes either before or after the multi-event key sequence, and not within it.
elisp None ### Calling Functions
Defining functions is only half the battle. Functions don’t do anything until you *call* them, i.e., tell them to run. Calling a function is also known as *invocation*.
The most common way of invoking a function is by evaluating a list. For example, evaluating the list `(concat "a" "b")` calls the function `concat` with arguments `"a"` and `"b"`. See [Evaluation](evaluation), for a description of evaluation.
When you write a list as an expression in your program, you specify which function to call, and how many arguments to give it, in the text of the program. Usually that’s just what you want. Occasionally you need to compute at run time which function to call. To do that, use the function `funcall`. When you also need to determine at run time how many arguments to pass, use `apply`.
Function: **funcall** *function &rest arguments*
`funcall` calls function with arguments, and returns whatever function returns.
Since `funcall` is a function, all of its arguments, including function, are evaluated before `funcall` is called. This means that you can use any expression to obtain the function to be called. It also means that `funcall` does not see the expressions you write for the arguments, only their values. These values are *not* evaluated a second time in the act of calling function; the operation of `funcall` is like the normal procedure for calling a function, once its arguments have already been evaluated.
The argument function must be either a Lisp function or a primitive function. Special forms and macros are not allowed, because they make sense only when given the unevaluated argument expressions. `funcall` cannot provide these because, as we saw above, it never knows them in the first place.
If you need to use `funcall` to call a command and make it behave as if invoked interactively, use `funcall-interactively` (see [Interactive Call](interactive-call)).
```
(setq f 'list)
⇒ list
```
```
(funcall f 'x 'y 'z)
⇒ (x y z)
```
```
(funcall f 'x 'y '(z))
⇒ (x y (z))
```
```
(funcall 'and t nil)
error→ Invalid function: #<subr and>
```
Compare these examples with the examples of `apply`.
Function: **apply** *function &rest arguments*
`apply` calls function with arguments, just like `funcall` but with one difference: the last of arguments is a list of objects, which are passed to function as separate arguments, rather than a single list. We say that `apply` *spreads* this list so that each individual element becomes an argument.
`apply` with a single argument is special: the first element of the argument, which must be a non-empty list, is called as a function with the remaining elements as individual arguments. Passing two or more arguments will be faster.
`apply` returns the result of calling function. As with `funcall`, function must either be a Lisp function or a primitive function; special forms and macros do not make sense in `apply`.
```
(setq f 'list)
⇒ list
```
```
(apply f 'x 'y 'z)
error→ Wrong type argument: listp, z
```
```
(apply '+ 1 2 '(3 4))
⇒ 10
```
```
(apply '+ '(1 2 3 4))
⇒ 10
```
```
(apply 'append '((a b c) nil (x y z) nil))
⇒ (a b c x y z)
```
```
(apply '(+ 3 4))
⇒ 7
```
For an interesting example of using `apply`, see [Definition of mapcar](mapping-functions#Definition-of-mapcar).
Sometimes it is useful to fix some of the function’s arguments at certain values, and leave the rest of arguments for when the function is actually called. The act of fixing some of the function’s arguments is called *partial application* of the function[12](#FOOT12). The result is a new function that accepts the rest of arguments and calls the original function with all the arguments combined.
Here’s how to do partial application in Emacs Lisp:
Function: **apply-partially** *func &rest args*
This function returns a new function which, when called, will call func with the list of arguments composed from args and additional arguments specified at the time of the call. If func accepts n arguments, then a call to `apply-partially` with `m <= n` arguments will produce a new function of `n - m` arguments[13](#FOOT13).
Here’s how we could define the built-in function `1+`, if it didn’t exist, using `apply-partially` and `+`, another built-in function[14](#FOOT14):
```
(defalias '1+ (apply-partially '+ 1)
"Increment argument by one.")
```
```
(1+ 10)
⇒ 11
```
It is common for Lisp functions to accept functions as arguments or find them in data structures (especially in hook variables and property lists) and call them using `funcall` or `apply`. Functions that accept function arguments are often called *functionals*.
Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function:
Function: **identity** *argument*
This function returns argument and has no side effects.
Function: **ignore** *&rest arguments*
This function ignores any arguments and returns `nil`.
Function: **always** *&rest arguments*
This function ignores any arguments and returns `t`.
Some functions are user-visible *commands*, which can be called interactively (usually by a key sequence). It is possible to invoke such a command exactly as though it was called interactively, by using the `call-interactively` function. See [Interactive Call](interactive-call).
elisp None ### Buffer-Local Variables
Global and local variable bindings are found in most programming languages in one form or another. Emacs, however, also supports additional, unusual kinds of variable binding, such as *buffer-local* bindings, which apply only in one buffer. Having different values for a variable in different buffers is an important customization method. (Variables can also have bindings that are local to each terminal. See [Multiple Terminals](multiple-terminals).)
| | | |
| --- | --- | --- |
| • [Intro to Buffer-Local](intro-to-buffer_002dlocal) | | Introduction and concepts. |
| • [Creating Buffer-Local](creating-buffer_002dlocal) | | Creating and destroying buffer-local bindings. |
| • [Default Value](default-value) | | The default value is seen in buffers that don’t have their own buffer-local values. |
elisp None ### Writing Emacs Primitives
Lisp primitives are Lisp functions implemented in C. The details of interfacing the C function so that Lisp can call it are handled by a few C macros. The only way to really understand how to write new C code is to read the source, but we can explain some things here.
An example of a special form is the definition of `or`, from `eval.c`. (An ordinary function would have the same general appearance.)
```
DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
doc: /* Eval args until one of them yields non-nil,
then return that value.
The remaining args are not evalled at all.
If all args return nil, return nil.
```
```
usage: (or CONDITIONS...) */)
(Lisp_Object args)
{
Lisp_Object val = Qnil;
```
```
while (CONSP (args))
{
val = eval_sub (XCAR (args));
if (!NILP (val))
break;
args = XCDR (args);
maybe_quit ();
}
```
```
return val;
}
```
Let’s start with a precise explanation of the arguments to the `DEFUN` macro. Here is a template for them:
```
DEFUN (lname, fname, sname, min, max, interactive, doc)
```
lname
This is the name of the Lisp symbol to define as the function name; in the example above, it is `or`.
fname
This is the C function name for this function. This is the name that is used in C code for calling the function. The name is, by convention, ‘`F`’ prepended to the Lisp name, with all dashes (‘`-`’) in the Lisp name changed to underscores. Thus, to call this function from C code, call `For`.
sname
This is a C variable name to use for a structure that holds the data for the subr object that represents the function in Lisp. This structure conveys the Lisp symbol name to the initialization routine that will create the symbol and store the subr object as its definition. By convention, this name is always fname with ‘`F`’ replaced with ‘`S`’.
min
This is the minimum number of arguments that the function requires. The function `or` allows a minimum of zero arguments.
max
This is the maximum number of arguments that the function accepts, if there is a fixed maximum. Alternatively, it can be `UNEVALLED`, indicating a special form that receives unevaluated arguments, or `MANY`, indicating an unlimited number of evaluated arguments (the equivalent of `&rest`). Both `UNEVALLED` and `MANY` are macros. If max is a number, it must be more than min but less than 8.
interactive
This is an interactive specification, a string such as might be used as the argument of `interactive` in a Lisp function (see [Using Interactive](using-interactive)). In the case of `or`, it is `0` (a null pointer), indicating that `or` cannot be called interactively. A value of `""` indicates a function that should receive no arguments when called interactively. If the value begins with a ‘`"(`’, the string is evaluated as a Lisp form. For example:
```
DEFUN ("foo", Ffoo, Sfoo, 0, 3,
"(list (read-char-by-name \"Insert character: \")\
(prefix-numeric-value current-prefix-arg)\
t)",
doc: /* … */)
```
doc
This is the documentation string. It uses C comment syntax rather than C string syntax because comment syntax requires nothing special to include multiple lines. The ‘`doc:`’ identifies the comment that follows as the documentation string. The ‘`/\*`’ and ‘`\*/`’ delimiters that begin and end the comment are not part of the documentation string.
If the last line of the documentation string begins with the keyword ‘`usage:`’, the rest of the line is treated as the argument list for documentation purposes. This way, you can use different argument names in the documentation string from the ones used in the C code. ‘`usage:`’ is required if the function has an unlimited number of arguments.
Some primitives have multiple definitions, one per platform (e.g., `x-create-frame`). In such cases, rather than writing the same documentation string in each definition, only one definition has the actual documentation. The others have placeholders beginning with ‘`SKIP`’, which are ignored by the function that parses the `DOC` file.
All the usual rules for documentation strings in Lisp code (see [Documentation Tips](https://www.gnu.org/software/emacs/manual/html_node/elisp/Documentation-Tips.html)) apply to C code documentation strings too.
The documentation string can be followed by a list of C function attributes for the C function that implements the primitive, like this:
```
DEFUN ("bar", Fbar, Sbar, 0, UNEVALLED, 0
doc: /* … */
attributes: attr1 attr2 …)
```
You can specify more than a single attribute, one after the other. Currently, only the following attributes are recognized:
`noreturn`
Declares the C function as one that never returns. This corresponds to the C11 keyword `_Noreturn` and to `\_\_attribute\_\_ ((\_\_noreturn\_\_))` attribute of GCC (see [Function Attributes](https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#Function-Attributes) in Using the GNU Compiler Collection).
`const`
Declares that the function does not examine any values except its arguments, and has no effects except the return value. This corresponds to `\_\_attribute\_\_ ((\_\_const\_\_))` attribute of GCC.
`noinline` This corresponds to `\_\_attribute\_\_ ((\_\_noinline\_\_))` attribute of GCC, which prevents the function from being considered for inlining. This might be needed, e.g., to countermand effects of link-time optimizations on stack-based variables.
After the call to the `DEFUN` macro, you must write the argument list for the C function, including the types for the arguments. If the primitive accepts a fixed maximum number of Lisp arguments, there must be one C argument for each Lisp argument, and each argument must be of type `Lisp_Object`. (Various macros and functions for creating values of type `Lisp_Object` are declared in the file `lisp.h`.) If the primitive is a special form, it must accept a Lisp list containing its unevaluated Lisp arguments as a single argument of type `Lisp_Object`. If the primitive has no upper limit on the number of evaluated Lisp arguments, it must have exactly two C arguments: the first is the number of Lisp arguments, and the second is the address of a block containing their values. These have types `ptrdiff_t` and `Lisp\_Object *`, respectively. Since `Lisp_Object` can hold any Lisp object of any data type, you can determine the actual data type only at run time; so if you want a primitive to accept only a certain type of argument, you must check the type explicitly using a suitable predicate (see [Type Predicates](type-predicates)).
Within the function `For` itself, the local variable `args` refers to objects controlled by Emacs’s stack-marking garbage collector. Although the garbage collector does not reclaim objects reachable from C `Lisp_Object` stack variables, it may move some of the components of an object, such as the contents of a string or the text of a buffer. Therefore, functions that access these components must take care to refetch their addresses after performing Lisp evaluation. This means that instead of keeping C pointers to string contents or buffer text, the code should keep the buffer or string position, and recompute the C pointer from the position after performing Lisp evaluation. Lisp evaluation can occur via calls to `eval_sub` or `Feval`, either directly or indirectly.
Note the call to `maybe_quit` inside the loop: this function checks whether the user pressed `C-g`, and if so, aborts the processing. You should do that in any loop that can potentially require a large number of iterations; in this case, the list of arguments could be very long. This increases Emacs responsiveness and improves user experience.
You must not use C initializers for static or global variables unless the variables are never written once Emacs is dumped. These variables with initializers are allocated in an area of memory that becomes read-only (on certain operating systems) as a result of dumping Emacs. See [Pure Storage](pure-storage).
Defining the C function is not enough to make a Lisp primitive available; you must also create the Lisp symbol for the primitive and store a suitable subr object in its function cell. The code looks like this:
```
defsubr (&sname);
```
Here sname is the name you used as the third argument to `DEFUN`.
If you add a new primitive to a file that already has Lisp primitives defined in it, find the function (near the end of the file) named `syms_of_something`, and add the call to `defsubr` there. If the file doesn’t have this function, or if you create a new file, add to it a `syms_of_filename` (e.g., `syms_of_myfile`). Then find the spot in `emacs.c` where all of these functions are called, and add a call to `syms_of_filename` there.
The function `syms_of_filename` is also the place to define any C variables that are to be visible as Lisp variables. `DEFVAR_LISP` makes a C variable of type `Lisp_Object` visible in Lisp. `DEFVAR_INT` makes a C variable of type `int` visible in Lisp with a value that is always an integer. `DEFVAR_BOOL` makes a C variable of type `int` visible in Lisp with a value that is either `t` or `nil`. Note that variables defined with `DEFVAR_BOOL` are automatically added to the list `byte-boolean-vars` used by the byte compiler.
These macros all expect three arguments:
`lname` The name of the variable to be used by Lisp programs.
`vname` The name of the variable in the C sources.
`doc` The documentation for the variable, as a C comment. See [Documentation Basics](documentation-basics), for more details.
By convention, when defining variables of a “native” type (`int` and `bool`), the name of the C variable is the name of the Lisp variable with `-` replaced by `_`. When the variable has type `Lisp_Object`, the convention is to also prefix the C variable name with `V`. i.e.
```
DEFVAR_INT ("my-int-variable", my_int_variable,
doc: /* An integer variable. */);
DEFVAR_LISP ("my-lisp-variable", Vmy_lisp_variable,
doc: /* A Lisp variable. */);
```
There are situations in Lisp where you need to refer to the symbol itself rather than the value of that symbol. One such case is when temporarily overriding the value of a variable, which in Lisp is done with `let`. In C sources, this is done by defining a corresponding, constant symbol, and using `specbind`. By convention, `Qmy_lisp_variable` corresponds to `Vmy_lisp_variable`; to define it, use the `DEFSYM` macro. i.e.
```
DEFSYM (Qmy_lisp_variable, "my-lisp-variable");
```
To perform the actual binding:
```
specbind (Qmy_lisp_variable, Qt);
```
In Lisp symbols sometimes need to be quoted, to achieve the same effect in C you again use the corresponding constant symbol `Qmy_lisp_variable`. For example, when creating a buffer-local variable (see [Buffer-Local Variables](buffer_002dlocal-variables)) in Lisp you would write:
```
(make-variable-buffer-local 'my-lisp-variable)
```
In C the corresponding code uses `Fmake_variable_buffer_local` in combination with `DEFSYM`, i.e.
```
DEFSYM (Qmy_lisp_variable, "my-lisp-variable");
Fmake_variable_buffer_local (Qmy_lisp_variable);
```
If you want to make a Lisp variable that is defined in C behave like one declared with `defcustom`, add an appropriate entry to `cus-start.el`. See [Variable Definitions](variable-definitions), for a description of the format to use.
If you directly define a file-scope C variable of type `Lisp_Object`, you must protect it from garbage-collection by calling `staticpro` in `syms_of_filename`, like this:
```
staticpro (&variable);
```
Here is another example function, with more complicated arguments. This comes from the code in `window.c`, and it demonstrates the use of macros and functions to manipulate Lisp objects.
```
DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
Scoordinates_in_window_p, 2, 2, 0,
doc: /* Return non-nil if COORDINATES are in WINDOW.
…
```
```
or `right-margin' is returned. */)
(register Lisp_Object coordinates, Lisp_Object window)
{
struct window *w;
struct frame *f;
int x, y;
Lisp_Object lx, ly;
```
```
w = decode_live_window (window);
f = XFRAME (w->frame);
CHECK_CONS (coordinates);
lx = Fcar (coordinates);
ly = Fcdr (coordinates);
CHECK_NUMBER (lx);
CHECK_NUMBER (ly);
x = FRAME_PIXEL_X_FROM_CANON_X (f, lx) + FRAME_INTERNAL_BORDER_WIDTH (f);
y = FRAME_PIXEL_Y_FROM_CANON_Y (f, ly) + FRAME_INTERNAL_BORDER_WIDTH (f);
```
```
switch (coordinates_in_window (w, x, y))
{
case ON_NOTHING: /* NOT in window at all. */
return Qnil;
```
```
…
```
```
case ON_MODE_LINE: /* In mode line of window. */
return Qmode_line;
```
```
…
```
```
case ON_SCROLL_BAR: /* On scroll-bar of window. */
/* Historically we are supposed to return nil in this case. */
return Qnil;
```
```
default:
emacs_abort ();
}
}
```
Note that C code cannot call functions by name unless they are defined in C. The way to call a function written in Lisp is to use `Ffuncall`, which embodies the Lisp function `funcall`. Since the Lisp function `funcall` accepts an unlimited number of arguments, in C it takes two: the number of Lisp-level arguments, and a one-dimensional array containing their values. The first Lisp-level argument is the Lisp function to call, and the rest are the arguments to pass to it.
The C functions `call0`, `call1`, `call2`, and so on, provide handy ways to call a Lisp function conveniently with a fixed number of arguments. They work by calling `Ffuncall`.
`eval.c` is a very good file to look through for examples; `lisp.h` contains the definitions for some important macros and functions.
If you define a function which is side-effect free or pure, give it a non-`nil` `side-effect-free` or `pure` property, respectively (see [Standard Properties](standard-properties)).
| programming_docs |
elisp None ### Object Internals
Emacs Lisp provides a rich set of the data types. Some of them, like cons cells, integers and strings, are common to nearly all Lisp dialects. Some others, like markers and buffers, are quite special and needed to provide the basic support to write editor commands in Lisp. To implement such a variety of object types and provide an efficient way to pass objects between the subsystems of an interpreter, there is a set of C data structures and a special type to represent the pointers to all of them, which is known as *tagged pointer*.
In C, the tagged pointer is an object of type `Lisp_Object`. Any initialized variable of such a type always holds the value of one of the following basic data types: integer, symbol, string, cons cell, float, or vectorlike object. Each of these data types has the corresponding tag value. All tags are enumerated by `enum Lisp_Type` and placed into a 3-bit bitfield of the `Lisp_Object`. The rest of the bits is the value itself. Integers are immediate, i.e., directly represented by those *value bits*, and all other objects are represented by the C pointers to a corresponding object allocated from the heap. Width of the `Lisp_Object` is platform- and configuration-dependent: usually it’s equal to the width of an underlying platform pointer (i.e., 32-bit on a 32-bit machine and 64-bit on a 64-bit one), but also there is a special configuration where `Lisp_Object` is 64-bit but all pointers are 32-bit. The latter trick was designed to overcome the limited range of values for Lisp integers on a 32-bit system by using 64-bit `long long` type for `Lisp_Object`.
The following C data structures are defined in `lisp.h` to represent the basic data types beyond integers:
`struct Lisp_Cons`
Cons cell, an object used to construct lists.
`struct Lisp_String`
String, the basic object to represent a sequence of characters.
`struct Lisp_Vector`
Array, a fixed-size set of Lisp objects which may be accessed by an index.
`struct Lisp_Symbol`
Symbol, the unique-named entity commonly used as an identifier.
`struct Lisp_Float` Floating-point value.
These types are the first-class citizens of an internal type system. Since the tag space is limited, all other types are the subtypes of `Lisp_Vectorlike`. Vector subtypes are enumerated by `enum pvec_type`, and nearly all complex objects like windows, buffers, frames, and processes fall into this category.
Below there is a description of a few subtypes of `Lisp_Vectorlike`. Buffer object represents the text to display and edit. Window is the part of display structure which shows the buffer or is used as a container to recursively place other windows on the same frame. (Do not confuse Emacs Lisp window object with the window as an entity managed by the user interface system like X; in Emacs terminology, the latter is called frame.) Finally, process object is used to manage the subprocesses.
| | | |
| --- | --- | --- |
| • [Buffer Internals](buffer-internals) | | Components of a buffer structure. |
| • [Window Internals](window-internals) | | Components of a window structure. |
| • [Process Internals](process-internals) | | Components of a process structure. |
elisp None ### Documentation Basics
A documentation string is written using the Lisp syntax for strings, with double-quote characters surrounding the text. It is, in fact, an actual Lisp string. When the string appears in the proper place in a function or variable definition, it serves as the function’s or variable’s documentation.
In a function definition (a `lambda` or `defun` form), the documentation string is specified after the argument list, and is normally stored directly in the function object. See [Function Documentation](function-documentation). You can also put function documentation in the `function-documentation` property of a function name (see [Accessing Documentation](accessing-documentation)).
In a variable definition (a `defvar` form), the documentation string is specified after the initial value. See [Defining Variables](defining-variables). The string is stored in the variable’s `variable-documentation` property.
Sometimes, Emacs does not keep documentation strings in memory. There are two such circumstances. Firstly, to save memory, the documentation for preloaded functions and variables (including primitives) is kept in a file named `DOC`, in the directory specified by `doc-directory` (see [Accessing Documentation](accessing-documentation)). Secondly, when a function or variable is loaded from a byte-compiled file, Emacs avoids loading its documentation string (see [Docs and Compilation](docs-and-compilation)). In both cases, Emacs looks up the documentation string from the file only when needed, such as when the user calls `C-h f` (`describe-function`) for a function.
Documentation strings can contain special *key substitution sequences*, referring to key bindings which are looked up only when the user views the documentation. This allows the help commands to display the correct keys even if a user rearranges the default key bindings. See [Keys in Documentation](keys-in-documentation).
In the documentation string of an autoloaded command (see [Autoload](autoload)), these key-substitution sequences have an additional special effect: they cause `C-h f` on the command to trigger autoloading. (This is needed for correctly setting up the hyperlinks in the `\*Help\*` buffer.)
elisp None ### Character Sets
An Emacs *character set*, or *charset*, is a set of characters in which each character is assigned a numeric code point. (The Unicode Standard calls this a *coded character set*.) Each Emacs charset has a name which is a symbol. A single character can belong to any number of different character sets, but it will generally have a different code point in each charset. Examples of character sets include `ascii`, `iso-8859-1`, `greek-iso8859-7`, and `windows-1255`. The code point assigned to a character in a charset is usually different from its code point used in Emacs buffers and strings.
Emacs defines several special character sets. The character set `unicode` includes all the characters whose Emacs code points are in the range `0..#x10FFFF`. The character set `emacs` includes all ASCII and non-ASCII characters. Finally, the `eight-bit` charset includes the 8-bit raw bytes; Emacs uses it to represent raw bytes encountered in text.
Function: **charsetp** *object*
Returns `t` if object is a symbol that names a character set, `nil` otherwise.
Variable: **charset-list**
The value is a list of all defined character set names.
Function: **charset-priority-list** *&optional highestp*
This function returns a list of all defined character sets ordered by their priority. If highestp is non-`nil`, the function returns a single character set of the highest priority.
Function: **set-charset-priority** *&rest charsets*
This function makes charsets the highest priority character sets.
Function: **char-charset** *character &optional restriction*
This function returns the name of the character set of highest priority that character belongs to. ASCII characters are an exception: for them, this function always returns `ascii`.
If restriction is non-`nil`, it should be a list of charsets to search. Alternatively, it can be a coding system, in which case the returned charset must be supported by that coding system (see [Coding Systems](coding-systems)).
Function: **charset-plist** *charset*
This function returns the property list of the character set charset. Although charset is a symbol, this is not the same as the property list of that symbol. Charset properties include important information about the charset, such as its documentation string, short name, etc.
Function: **put-charset-property** *charset propname value*
This function sets the propname property of charset to the given value.
Function: **get-charset-property** *charset propname*
This function returns the value of charsets property propname.
Command: **list-charset-chars** *charset*
This command displays a list of characters in the character set charset.
Emacs can convert between its internal representation of a character and the character’s codepoint in a specific charset. The following two functions support these conversions.
Function: **decode-char** *charset code-point*
This function decodes a character that is assigned a code-point in charset, to the corresponding Emacs character, and returns it. If charset doesn’t contain a character of that code point, the value is `nil`.
For backward compatibility, if code-point doesn’t fit in a Lisp fixnum (see [most-positive-fixnum](integer-basics)), it can be specified as a cons cell `(high . low)`, where low are the lower 16 bits of the value and high are the high 16 bits. This usage is obsolescent.
Function: **encode-char** *char charset*
This function returns the code point assigned to the character char in charset. If charset doesn’t have a codepoint for char, the value is `nil`.
The following function comes in handy for applying a certain function to all or part of the characters in a charset:
Function: **map-charset-chars** *function charset &optional arg from-code to-code*
Call function for characters in charset. function is called with two arguments. The first one is a cons cell `(from . to)`, where from and to indicate a range of characters contained in charset. The second argument passed to function is arg, or `nil` if arg is omitted.
By default, the range of codepoints passed to function includes all the characters in charset, but optional arguments from-code and to-code limit that to the range of characters between these two codepoints of charset. If either of them is `nil`, it defaults to the first or last codepoint of charset, respectively. Note that from-code and to-code are charset’s codepoints, not the Emacs codes of characters; by contrast, the values from and to in the cons cell passed to function *are* Emacs character codes. Those Emacs character codes are either Unicode code points, or Emacs internal code points that extend Unicode and are beyond the Unicode range of characters `0..#x10FFFF` (see [Text Representations](text-representations)). The latter happens rarely, with legacy CJK charsets for codepoints of charset which specify characters not yet unified with Unicode.
elisp None ### Inhibiting Interaction
It’s sometimes useful to be able to run Emacs as a headless server process that responds to commands given over a network connection. However, Emacs is primarily a platform for interactive usage, so many commands prompt the user for feedback in certain anomalous situations. This makes this use case more difficult, since the server process will just hang waiting for user input.
Binding the `inhibit-interaction` variable to something non-`nil` makes Emacs signal a `inhibited-interaction` error instead of prompting, which can then be used by the server process to handle these situations.
Here’s a typical use case:
```
(let ((inhibit-interaction t))
(respond-to-client
(condition-case err
(my-client-handling-function)
(inhibited-interaction err))))
```
If `my-client-handling-function` ends up calling something that asks the user for something (via `y-or-n-p` or `read-from-minibuffer` or the like), an `inhibited-interaction` error is signalled instead. The server code then catches that error and reports it to the client.
elisp None ### Creating, Copying and Deleting Directories
Most Emacs Lisp file-manipulation functions get errors when used on files that are directories. For example, you cannot delete a directory with `delete-file`. These special functions exist to create and delete directories.
Command: **make-directory** *dirname &optional parents*
This command creates a directory named dirname. If parents is non-`nil`, as is always the case in an interactive call, that means to create the parent directories first, if they don’t already exist. `mkdir` is an alias for this.
Command: **make-empty-file** *filename &optional parents*
This command creates an empty file named filename. As `make-directory`, this command creates parent directories if parents is non-`nil`. If filename already exists, this command signals an error.
Command: **copy-directory** *dirname newname &optional keep-time parents copy-contents*
This command copies the directory named dirname to newname. If newname is a directory name, dirname will be copied to a subdirectory there. See [Directory Names](directory-names).
It always sets the file modes of the copied files to match the corresponding original file.
The third argument keep-time non-`nil` means to preserve the modification time of the copied files. A prefix arg makes keep-time non-`nil`.
The fourth argument parents says whether to create parent directories if they don’t exist. Interactively, this happens by default.
The fifth argument copy-contents, if non-`nil`, means to copy the contents of dirname directly into newname if the latter is a directory name, instead of copying dirname into it as a subdirectory.
Command: **delete-directory** *dirname &optional recursive trash*
This command deletes the directory named dirname. The function `delete-file` does not work for files that are directories; you must use `delete-directory` for them. If recursive is `nil`, and the directory contains any files, `delete-directory` signals an error. If recursive is non-`nil`, there is no error merely because the directory or its files are deleted by some other process before `delete-directory` gets to them.
`delete-directory` only follows symbolic links at the level of parent directories.
If the optional argument trash is non-`nil` and the variable `delete-by-moving-to-trash` is non-`nil`, this command moves the file into the system Trash instead of deleting it. See [Miscellaneous File Operations](https://www.gnu.org/software/emacs/manual/html_node/emacs/Misc-File-Ops.html#Misc-File-Ops) in The GNU Emacs Manual. When called interactively, trash is `t` if no prefix argument is given, and `nil` otherwise.
elisp None ### Creating Frames
To create a new frame, call the function `make-frame`.
Command: **make-frame** *&optional parameters*
This function creates and returns a new frame, displaying the current buffer.
The parameters argument is an alist that specifies frame parameters for the new frame. See [Frame Parameters](frame-parameters). If you specify the `terminal` parameter in parameters, the new frame is created on that terminal. Otherwise, if you specify the `window-system` frame parameter in parameters, that determines whether the frame should be displayed on a text terminal or a graphical terminal. See [Window Systems](window-systems). If neither is specified, the new frame is created in the same terminal as the selected frame.
Any parameters not mentioned in parameters default to the values in the alist `default-frame-alist` (see [Initial Parameters](initial-parameters)); parameters not specified there default from the X resources or its equivalent on your operating system (see [X Resources](https://www.gnu.org/software/emacs/manual/html_node/emacs/X-Resources.html#X-Resources) in The GNU Emacs Manual). After the frame is created, this function applies any parameters specified in `frame-inherited-parameters` (see below) it has no assigned yet, taking the values from the frame that was selected when `make-frame` was called.
Note that on multi-monitor displays (see [Multiple Terminals](multiple-terminals)), the window manager might position the frame differently than specified by the positional parameters in parameters (see [Position Parameters](position-parameters)). For example, some window managers have a policy of displaying the frame on the monitor that contains the largest part of the window (a.k.a. the *dominating* monitor).
This function itself does not make the new frame the selected frame. See [Input Focus](input-focus). The previously selected frame remains selected. On graphical terminals, however, the window system may select the new frame for its own reasons.
Variable: **before-make-frame-hook**
A normal hook run by `make-frame` before it creates the frame.
Variable: **after-make-frame-functions**
An abnormal hook run by `make-frame` after it created the frame. Each function in `after-make-frame-functions` receives one argument, the frame just created.
Note that any functions added to these hooks by your initial file are usually not run for the initial frame, since Emacs reads the initial file only after creating that frame. However, if the initial frame is specified to use a separate minibuffer frame (see [Minibuffers and Frames](minibuffers-and-frames)), the functions will be run for both, the minibuffer-less and the minibuffer frame.
Variable: **frame-inherited-parameters**
This variable specifies the list of frame parameters that a newly created frame inherits from the currently selected frame. For each parameter (a symbol) that is an element in this list and has not been assigned earlier when processing `make-frame`, the function sets the value of that parameter in the created frame to its value in the selected frame.
User Option: **server-after-make-frame-hook**
A normal hook run when the Emacs server creates a client frame. When this hook is called, the created frame is the selected one. See [Emacs Server](https://www.gnu.org/software/emacs/manual/html_node/emacs/Emacs-Server.html#Emacs-Server) in The GNU Emacs Manual.
elisp None ### Time of Day
This section explains how to determine the current time and time zone.
Many functions like `current-time` and `file-attributes` return *Lisp timestamp* values that count seconds, and that can represent absolute time by counting seconds since the *epoch* of 1970-01-01 00:00:00 UTC.
Although traditionally Lisp timestamps were integer pairs, their form has evolved and programs ordinarily should not depend on the current default form. If your program needs a particular timestamp form, you can use the `time-convert` function to convert it to the needed form. See [Time Conversion](time-conversion).
There are currently three forms of Lisp timestamps, each of which represents a number of seconds:
* An integer. Although this is the simplest form, it cannot represent subsecond timestamps.
* A pair of integers `(ticks . hz)`, where hz is positive. This represents ticks/hz seconds, which is the same time as plain ticks if hz is 1. A common value for hz is 1000000000, for a nanosecond-resolution clock.[28](#FOOT28)
* A list of four integers `(high low micro
pico)`, where 0≤low<65536, 0≤micro<1000000, and 0≤pico<1000000. This represents the number of seconds using the formula: high \* 2\*\*16 + low + micro \* 10\*\*-6 + pico \* 10\*\*-12. In some cases, functions may default to returning two- or three-element lists, with omitted micro and pico components defaulting to zero. On all current machines pico is a multiple of 1000, but this may change as higher-resolution clocks become available.
Function arguments, e.g., the time argument to `current-time-string`, accept a more-general *time value* format, which can be a Lisp timestamp, `nil` for the current time, a single floating-point number for seconds, or a list `(high low micro)` or `(high
low)` that is a truncated list timestamp with missing elements taken to be zero.
Time values can be converted to and from calendrical and other forms. Some of these conversions rely on operating system functions that limit the range of possible time values, and signal an error such as ‘`"Specified time is not representable"`’ if the limits are exceeded. For instance, a system may not support years before 1970, or years before 1901, or years far in the future. You can convert a time value into a human-readable string using `format-time-string`, into a Lisp timestamp using `time-convert`, and into other forms using `decode-time` and `float-time`. These functions are described in the following sections.
Function: **current-time-string** *&optional time zone*
This function returns the current time and date as a human-readable string. The format does not vary for the initial part of the string, which contains the day of week, month, day of month, and time of day in that order: the number of characters used for these fields is always the same, although (unless you require English weekday or month abbreviations regardless of locale) it is typically more convenient to use `format-time-string` than to extract fields from the output of `current-time-string`, as the year might not have exactly four digits, and additional information may some day be added at the end.
The argument time, if given, specifies a time to format, instead of the current time. The optional argument zone defaults to the current time zone rule. See [Time Zone Rules](time-zone-rules). The operating system limits the range of time and zone values.
```
(current-time-string)
⇒ "Fri Nov 1 15:59:49 2019"
```
Function: **current-time**
This function returns the current time as a Lisp timestamp. Although the timestamp takes the form `(high low
micro pico)` in the current Emacs release, this is planned to change in a future Emacs version. You can use the `time-convert` function to convert a timestamp to some other form. See [Time Conversion](time-conversion).
Function: **float-time** *&optional time*
This function returns the current time as a floating-point number of seconds since the epoch. The optional argument time, if given, specifies a time to convert instead of the current time.
*Warning*: Since the result is floating point, it may not be exact. Do not use this function if precise time stamps are required. For example, on typical systems `(float-time '(1 . 10))` displays as ‘`0.1`’ but is slightly greater than 1/10.
`time-to-seconds` is an alias for this function.
| programming_docs |
elisp None #### Instrumenting for Edebug
In order to use Edebug to debug Lisp code, you must first *instrument* the code. Instrumenting code inserts additional code into it, to invoke Edebug at the proper places.
When you invoke command `C-M-x` (`eval-defun`) with a prefix argument on a function definition, it instruments the definition before evaluating it. (This does not modify the source code itself.) If the variable `edebug-all-defs` is non-`nil`, that inverts the meaning of the prefix argument: in this case, `C-M-x` instruments the definition *unless* it has a prefix argument. The default value of `edebug-all-defs` is `nil`. The command `M-x edebug-all-defs` toggles the value of the variable `edebug-all-defs`.
If `edebug-all-defs` is non-`nil`, then the commands `eval-region`, `eval-current-buffer`, and `eval-buffer` also instrument any definitions they evaluate. Similarly, `edebug-all-forms` controls whether `eval-region` should instrument *any* form, even non-defining forms. This doesn’t apply to loading or evaluations in the minibuffer. The command `M-x edebug-all-forms` toggles this option.
Another command, `M-x edebug-eval-top-level-form`, is available to instrument any top-level form regardless of the values of `edebug-all-defs` and `edebug-all-forms`. `edebug-defun` is an alias for `edebug-eval-top-level-form`.
While Edebug is active, the command `I` (`edebug-instrument-callee`) instruments the definition of the function or macro called by the list form after point, if it is not already instrumented. This is possible only if Edebug knows where to find the source for that function; for this reason, after loading Edebug, `eval-region` records the position of every definition it evaluates, even if not instrumenting it. See also the `i` command (see [Jumping](jumping)), which steps into the call after instrumenting the function.
Edebug knows how to instrument all the standard special forms, `interactive` forms with an expression argument, anonymous lambda expressions, and other defining forms. However, Edebug cannot determine on its own what a user-defined macro will do with the arguments of a macro call, so you must provide that information using Edebug specifications; for details, see [Edebug and Macros](edebug-and-macros).
When Edebug is about to instrument code for the first time in a session, it runs the hook `edebug-setup-hook`, then sets it to `nil`. You can use this to load Edebug specifications associated with a package you are using, but only when you use Edebug.
If Edebug detects a syntax error while instrumenting, it leaves point at the erroneous code and signals an `invalid-read-syntax` error. Example:
```
error→ Invalid read syntax: "Expected lambda expression"
```
One potential reason for such a failure to instrument is that some macro definitions are not yet known to Emacs. To work around this, load the file which defines the function you are about to instrument.
To remove instrumentation from a definition, simply re-evaluate its definition in a way that does not instrument. There are two ways of evaluating forms that never instrument them: from a file with `load`, and from the minibuffer with `eval-expression` (`M-:`).
A different way to remove the instrumentation from a definition is to use the `edebug-remove-instrumentation` command. It also allows removing the instrumentation from everything that has been instrumented.
See [Edebug Eval](edebug-eval), for other evaluation functions available inside of Edebug.
elisp None #### Action Functions for Buffer Display
An *action function* is a function `display-buffer` calls for choosing a window to display a buffer. Action functions take two arguments: buffer, the buffer to display, and alist, an action alist (see [Buffer Display Action Alists](buffer-display-action-alists)). They are supposed to return a window displaying buffer if they succeed and `nil` if they fail.
The following basic action functions are defined in Emacs.
Function: **display-buffer-same-window** *buffer alist*
This function tries to display buffer in the selected window. It fails if the selected window is a minibuffer window or is dedicated to another buffer (see [Dedicated Windows](dedicated-windows)). It also fails if alist has a non-`nil` `inhibit-same-window` entry.
Function: **display-buffer-reuse-window** *buffer alist*
This function tries to display buffer by finding a window that is already displaying it. Windows on the selected frame are preferred to windows on other frames.
If alist has a non-`nil` `inhibit-same-window` entry, the selected window is not eligible for reuse. The set of frames to search for a window already displaying buffer can be specified with the help of the `reusable-frames` action alist entry. If alist contains no `reusable-frames` entry, this function searches just the selected frame.
If this function chooses a window on another frame, it makes that frame visible and, unless alist contains an `inhibit-switch-frame` entry, raises that frame if necessary.
Function: **display-buffer-reuse-mode-window** *buffer alist*
This function tries to display buffer by finding a window that is displaying a buffer in a given mode.
If alist contains a `mode` entry, its value specifies a major mode (a symbol) or a list of major modes. If alist contains no `mode` entry, the current major mode of buffer is used instead. A window is a candidate if it displays a buffer whose mode derives from one of the modes specified thusly.
The behavior is also controlled by alist entries for `inhibit-same-window`, `reusable-frames` and `inhibit-switch-frame`, like `display-buffer-reuse-window` does.
Function: **display-buffer-pop-up-window** *buffer alist*
This function tries to display buffer by splitting the largest or least recently-used window (usually located on the selected frame). It actually performs the split by calling the function specified by `split-window-preferred-function` (see [Choosing Window Options](choosing-window-options)).
The size of the new window can be adjusted by supplying `window-height` and `window-width` entries in alist. If alist contains a `preserve-size` entry, Emacs will also try to preserve the size of the new window during future resize operations (see [Preserving Window Sizes](preserving-window-sizes)).
This function fails if no window can be split. More often than not, this happens because no window is large enough to allow splitting. Setting `split-height-threshold` or `split-width-threshold` to lower values may help in this regard. Splitting also fails when the selected frame has an `unsplittable` frame parameter; see [Buffer Parameters](buffer-parameters).
Function: **display-buffer-in-previous-window** *buffer alist*
This function tries to display buffer in a window where it was displayed previously.
If alist contains a non-`nil` `inhibit-same-window` entry, the selected window is not eligible for use. A dedicated window is usable only if it already shows buffer. If alist contains a `previous-window` entry, the window specified by that entry is usable even if it never showed buffer before.
If alist contains a `reusable-frames` entry (see [Buffer Display Action Alists](buffer-display-action-alists)), its value determines which frames to search for a suitable window. If alist contains no `reusable-frames` entry, this function searches just the selected frame if `display-buffer-reuse-frames` and `pop-up-frames` are both `nil`; it searches all frames on the current terminal if either of those variables is non-`nil`.
If more than one window qualifies as usable according to these rules, this function makes a choice in the following order of preference:
* The window specified by any `previous-window` alist entry, provided it is not the selected window.
* A window that showed buffer before, provided it is not the selected window.
* The selected window if it is either specified by a `previous-window` alist entry or showed buffer before.
Function: **display-buffer-use-some-window** *buffer alist*
This function tries to display buffer by choosing an existing window and displaying the buffer in that window. It can fail if all windows are dedicated to other buffers (see [Dedicated Windows](dedicated-windows)).
Function: **display-buffer-use-least-recent-window** *buffer alist*
This function is like `display-buffer-use-some-window`, but will not reuse the current window, and will use the least recently switched-to window.
Function: **display-buffer-in-direction** *buffer alist*
This function tries to display buffer at a location specified by alist. For this purpose, alist should contain a `direction` entry whose value is one of `left`, `above` (or `up`), `right` and `below` (or `down`). Other values are usually interpreted as `below`.
If alist also contains a `window` entry, its value specifies a reference window. That value can be a special symbol like `main` which stands for the selected frame’s main window (see [Side Window Options and Functions](side-window-options-and-functions)) or `root` standing for the selected frame’s root window (see [Windows and Frames](windows-and-frames)). It can also specify an arbitrary valid window. Any other value (or omitting the `window` entry entirely) means to use the selected window as reference window.
This function first tries to reuse a window in the specified direction that already shows buffer. If no such window exists, it tries to split the reference window in order to produce a new window in the specified direction. If this fails as well, it will try to display buffer in an existing window in the specified direction. In either case, the window chosen will appear on the side of the reference window specified by the `direction` entry, sharing at least one edge with the reference window.
If the reference window is live, the edge the chosen window will share with it is always the opposite of the one specified by the `direction` entry. For example, if the value of the `direction` entry is `left`, the chosen window’s right edge coordinate (see [Coordinates and Windows](coordinates-and-windows)) will equal the reference window’s left edge coordinate.
If the reference window is internal, a reused window must share with it the edge specified by the `direction` entry. Hence if, for example, the reference window is the frame’s root window and the value of the `direction` entry is `left`, a reused window must be on the left of the frame. This means that the left edge coordinate of the chosen window and that of the reference window are the same.
A new window, however, will be created by splitting the reference window such that the chosen window will share the opposite edge with the reference window. In our example, a new root window would be created with a new live window and the reference window as its children. The chosen window’s right edge coordinate would then equal the left edge coordinate of the reference window. Its left edge coordinate would equal the left edge coordinate of the frame’s new root window.
Four special values for `direction` entries allow to implicitly specify the selected frame’s main window as the reference window: `leftmost`, `top`, `rightmost` and `bottom`. This means that instead of, for example, `(direction . left) (window . main)` one can just specify `(direction . leftmost)`. An existing `window` alist entry is ignored in such cases.
Function: **display-buffer-below-selected** *buffer alist*
This function tries to display buffer in a window below the selected window. If there is a window below the selected one and that window already displays buffer, it reuses that window.
If there is no such window, this function tries to create a new window by splitting the selected one, and displays buffer there. It will also try to adjust that window’s size provided alist contains a suitable `window-height` or `window-width` entry, see above.
If splitting the selected window fails and there is a non-dedicated window below the selected one showing some other buffer, this function tries to use that window for showing buffer.
If alist contains a `window-min-height` entry, this function ensures that the window used is or can become at least as high as specified by that entry’s value. Note that this is only a guarantee. In order to actually resize the window used, alist must also provide an appropriate `window-height` entry.
Function: **display-buffer-at-bottom** *buffer alist*
This function tries to display buffer in a window at the bottom of the selected frame.
This either tries to split the window at the bottom of the frame or the frame’s root window, or to reuse an existing window at the bottom of the selected frame.
Function: **display-buffer-pop-up-frame** *buffer alist*
This function creates a new frame, and displays the buffer in that frame’s window. It actually performs the frame creation by calling the function specified in `pop-up-frame-function` (see [Choosing Window Options](choosing-window-options)). If alist contains a `pop-up-frame-parameters` entry, the associated value is added to the newly created frame’s parameters.
Function: **display-buffer-in-child-frame** *buffer alist*
This function tries to display buffer in a child frame (see [Child Frames](child-frames)) of the selected frame, either reusing an existing child frame or by making a new one. If alist has a non-`nil` `child-frame-parameters` entry, the corresponding value is an alist of frame parameters to give the new frame. A `parent-frame` parameter specifying the selected frame is provided by default. If the child frame should become the child of another frame, a corresponding entry must be added to alist.
The appearance of child frames is largely dependent on the parameters provided via alist. It is advisable to use at least ratios to specify the size (see [Size Parameters](size-parameters)) and the position (see [Position Parameters](position-parameters)) of the child frame, and to add a `keep-ratio` parameter (see [Frame Interaction Parameters](frame-interaction-parameters)), in order to make sure that the child frame remains visible. For other parameters that should be considered see [Child Frames](child-frames).
Function: **display-buffer-use-some-frame** *buffer alist*
This function tries to display buffer by finding a frame that meets a predicate (by default any frame other than the selected frame).
If this function chooses a window on another frame, it makes that frame visible and, unless alist contains an `inhibit-switch-frame` entry, raises that frame if necessary.
If alist has a non-`nil` `frame-predicate` entry, its value is a function taking one argument (a frame), returning non-`nil` if the frame is a candidate; this function replaces the default predicate.
If alist has a non-`nil` `inhibit-same-window` entry, the selected window is not used; thus if the selected frame has a single window, it is not used.
Function: **display-buffer-no-window** *buffer alist*
If alist has a non-`nil` `allow-no-window` entry, then this function does not display buffer and returns the symbol `fail`. This constitutes the only exception to the convention that an action function returns either `nil` or a window showing buffer. If alist has no such `allow-no-window` entry, this function returns `nil`.
If this function returns `fail`, `display-buffer` will skip the execution of any further display actions and return `nil` immediately. If this function returns `nil`, `display-buffer` will continue with the next display action, if any.
It is assumed that when a caller of `display-buffer` specifies a non-`nil` `allow-no-window` entry, it is also able to handle a `nil` return value.
Two other action functions are described in their proper sections—`display-buffer-in-side-window` (see [Displaying Buffers in Side Windows](displaying-buffers-in-side-windows)) and `display-buffer-in-atom-window` (see [Atomic Windows](atomic-windows)).
elisp None ### Converting Text Representations
Emacs can convert unibyte text to multibyte; it can also convert multibyte text to unibyte, provided that the multibyte text contains only ASCII and 8-bit raw bytes. In general, these conversions happen when inserting text into a buffer, or when putting text from several strings together in one string. You can also explicitly convert a string’s contents to either representation.
Emacs chooses the representation for a string based on the text from which it is constructed. The general rule is to convert unibyte text to multibyte text when combining it with other multibyte text, because the multibyte representation is more general and can hold whatever characters the unibyte text has.
When inserting text into a buffer, Emacs converts the text to the buffer’s representation, as specified by `enable-multibyte-characters` in that buffer. In particular, when you insert multibyte text into a unibyte buffer, Emacs converts the text to unibyte, even though this conversion cannot in general preserve all the characters that might be in the multibyte text. The other natural alternative, to convert the buffer contents to multibyte, is not acceptable because the buffer’s representation is a choice made by the user that cannot be overridden automatically.
Converting unibyte text to multibyte text leaves ASCII characters unchanged, and converts bytes with codes 128 through 255 to the multibyte representation of raw eight-bit bytes.
Converting multibyte text to unibyte converts all ASCII and eight-bit characters to their single-byte form, but loses information for non-ASCII characters by discarding all but the low 8 bits of each character’s codepoint. Converting unibyte text to multibyte and back to unibyte reproduces the original unibyte text.
The next two functions either return the argument string, or a newly created string with no text properties.
Function: **string-to-multibyte** *string*
This function returns a multibyte string containing the same sequence of characters as string. If string is a multibyte string, it is returned unchanged. The function assumes that string includes only ASCII characters and raw 8-bit bytes; the latter are converted to their multibyte representation corresponding to the codepoints `#x3FFF80` through `#x3FFFFF`, inclusive (see [codepoints](text-representations)).
Function: **string-to-unibyte** *string*
This function returns a unibyte string containing the same sequence of characters as string. If string is a unibyte string, it is returned unchanged. Otherwise, ASCII characters and characters in the `eight-bit` charset are converted to their corresponding byte values. Use this function for string arguments that contain only ASCII and eight-bit characters; the function signals an error if any other characters are encountered.
Function: **byte-to-string** *byte*
This function returns a unibyte string containing a single byte of character data, byte. It signals an error if byte is not an integer between 0 and 255.
Function: **multibyte-char-to-unibyte** *char*
This converts the multibyte character char to a unibyte character, and returns that character. If char is neither ASCII nor eight-bit, the function returns -1.
Function: **unibyte-char-to-multibyte** *char*
This converts the unibyte character char to a multibyte character, assuming char is either ASCII or raw 8-bit byte.
elisp None ### Key Sequences
A *key sequence*, or *key* for short, is a sequence of one or more input events that form a unit. Input events include characters, function keys, mouse actions, or system events external to Emacs, such as `iconify-frame` (see [Input Events](input-events)). The Emacs Lisp representation for a key sequence is a string or vector. Unless otherwise stated, any Emacs Lisp function that accepts a key sequence as an argument can handle both representations.
In the string representation, alphanumeric characters ordinarily stand for themselves; for example, `"a"` represents `a` and `"2"` represents `2`. Control character events are prefixed by the substring `"\C-"`, and meta characters by `"\M-"`; for example, `"\C-x"` represents the key `C-x`. In addition, the TAB, RET, ESC, and DEL events are represented by `"\t"`, `"\r"`, `"\e"`, and `"\d"` respectively. The string representation of a complete key sequence is the concatenation of the string representations of the constituent events; thus, `"\C-xl"` represents the key sequence `C-x l`.
Key sequences containing function keys, mouse button events, system events, or non-ASCII characters such as `C-=` or `H-a` cannot be represented as strings; they have to be represented as vectors.
In the vector representation, each element of the vector represents an input event, in its Lisp form. See [Input Events](input-events). For example, the vector `[?\C-x ?l]` represents the key sequence `C-x l`.
For examples of key sequences written in string and vector representations, [Init Rebinding](https://www.gnu.org/software/emacs/manual/html_node/emacs/Init-Rebinding.html#Init-Rebinding) in The GNU Emacs Manual.
Function: **kbd** *keyseq-text*
This function converts the text keyseq-text (a string constant) into a key sequence (a string or vector constant). The contents of keyseq-text should use the same syntax as in the buffer invoked by the `C-x C-k RET` (`kmacro-edit-macro`) command; in particular, you must surround function key names with ‘`<…>`’. See [Edit Keyboard Macro](https://www.gnu.org/software/emacs/manual/html_node/emacs/Edit-Keyboard-Macro.html#Edit-Keyboard-Macro) in The GNU Emacs Manual.
```
(kbd "C-x") ⇒ "\C-x"
(kbd "C-x C-f") ⇒ "\C-x\C-f"
(kbd "C-x 4 C-f") ⇒ "\C-x4\C-f"
(kbd "X") ⇒ "X"
(kbd "RET") ⇒ "\^M"
(kbd "C-c SPC") ⇒ "\C-c "
(kbd "<f1> SPC") ⇒ [f1 32]
(kbd "C-M-<down>") ⇒ [C-M-down]
```
| programming_docs |
elisp None ### Processor Run time
Emacs provides several functions and primitives that return time, both elapsed and processor time, used by the Emacs process.
Command: **emacs-uptime** *&optional format*
This function returns a string representing the Emacs *uptime*—the elapsed wall-clock time this instance of Emacs is running. The string is formatted by `format-seconds` according to the optional argument format. For the available format descriptors, see [format-seconds](time-parsing). If format is `nil` or omitted, it defaults to `"%Y, %D, %H, %M,
%z%S"`.
When called interactively, it prints the uptime in the echo area.
Function: **get-internal-run-time**
This function returns the processor run time used by Emacs, as a Lisp timestamp (see [Time of Day](time-of-day)).
Note that the time returned by this function excludes the time Emacs was not using the processor, and if the Emacs process has several threads, the returned value is the sum of the processor times used up by all Emacs threads.
If the system doesn’t provide a way to determine the processor run time, `get-internal-run-time` returns the same time as `current-time`.
Command: **emacs-init-time**
This function returns the duration of the Emacs initialization (see [Startup Summary](startup-summary)) in seconds, as a string. When called interactively, it prints the duration in the echo area.
elisp None #### Parser State
A *parser state* is a list of (currently) eleven elements describing the state of the syntactic parser, after it parses the text between a specified starting point and a specified end point in the buffer using `parse-partial-sexp` (see [Low-Level Parsing](low_002dlevel-parsing)). Parsing functions such as `syntax-ppss` (see [Position Parse](position-parse)) also return a parser state as the value. `parse-partial-sexp` can accept a parser state as an argument, for resuming parsing.
Here are the meanings of the elements of the parser state:
0. The depth in parentheses, counting from 0. **Warning:** this can be negative if there are more close parens than open parens between the parser’s starting point and end point.
1. The character position of the start of the innermost parenthetical grouping containing the stopping point; `nil` if none.
2. The character position of the start of the last complete subexpression terminated; `nil` if none.
3. Non-`nil` if inside a string. More precisely, this is the character that will terminate the string, or `t` if a generic string delimiter character should terminate it.
4. `t` if inside a non-nestable comment (of any comment style; see [Syntax Flags](syntax-flags)); or the comment nesting level if inside a comment that can be nested.
5. `t` if the end point is just after a quote character.
6. The minimum parenthesis depth encountered during this scan.
7. What kind of comment is active: `nil` if not in a comment or in a comment of style ‘`a`’; 1 for a comment of style ‘`b`’; 2 for a comment of style ‘`c`’; and `syntax-table` for a comment that should be ended by a generic comment delimiter character.
8. The string or comment start position. While inside a comment, this is the position where the comment began; while inside a string, this is the position where the string began. When outside of strings and comments, this element is `nil`.
9. The list of the positions of the currently open parentheses, starting with the outermost.
10. When the last buffer position scanned was the (potential) first character of a two character construct (comment delimiter or escaped/char-quoted character pair), the syntax-code (see [Syntax Table Internals](syntax-table-internals)) of that position. Otherwise `nil`.
Elements 1, 2, and 6 are ignored in a state which you pass as an argument to `parse-partial-sexp` to continue parsing. Elements 9 and 10 are mainly used internally by the parser code.
Some additional useful information is available from a parser state using these functions:
Function: **syntax-ppss-toplevel-pos** *state*
This function extracts, from parser state state, the last position scanned in the parse which was at top level in grammatical structure. “At top level” means outside of any parentheses, comments, or strings.
The value is `nil` if state represents a parse which has arrived at a top level position.
Function: **syntax-ppss-context** *state*
Return `string` if the end position of the scan returning state is in a string, and `comment` if it’s in a comment. Otherwise return `nil`.
elisp None #### Constructs in the Mode Line
Strings used as mode line constructs can use certain `%`-constructs to substitute various kinds of data. The following is a list of the defined `%`-constructs, and what they mean.
In any construct except ‘`%%`’, you can add a decimal integer after the ‘`%`’ to specify a minimum field width. If the width is less, the field is padded to that width. Purely numeric constructs (‘`c`’, ‘`i`’, ‘`I`’, and ‘`l`’) are padded by inserting spaces to the left, and others are padded by inserting spaces to the right.
`%b`
The current buffer name, obtained with the `buffer-name` function. See [Buffer Names](buffer-names).
`%c`
The current column number of point, counting from zero starting at the left margin of the window.
`%C`
The current column number of point, counting from one starting at the left margin of the window.
`%e`
When Emacs is nearly out of memory for Lisp objects, a brief message saying so. Otherwise, this is empty.
`%f`
The visited file name, obtained with the `buffer-file-name` function. See [Buffer File Name](buffer-file-name).
`%F`
The title (only on a window system) or the name of the selected frame. See [Basic Parameters](basic-parameters).
`%i`
The size of the accessible part of the current buffer; basically `(- (point-max) (point-min))`.
`%I`
Like ‘`%i`’, but the size is printed in a more readable way by using ‘`k`’ for 10^3, ‘`M`’ for 10^6, ‘`G`’ for 10^9, etc., to abbreviate.
`%l`
The current line number of point, counting within the accessible portion of the buffer.
`%n`
‘`Narrow`’ when narrowing is in effect; nothing otherwise (see `narrow-to-region` in [Narrowing](narrowing)).
`%o`
The degree of *travel* of the window through (the visible portion of) the buffer, i.e. the size of the text above the top of the window expressed as a percentage of all the text outside the window, or ‘`Top`’, ‘`Bottom`’ or ‘`All`’.
`%p`
The percentage of the buffer text above the **top** of window, or ‘`Top`’, ‘`Bottom`’ or ‘`All`’. Note that the default mode line construct truncates this to three characters.
`%P`
The percentage of the buffer text that is above the **bottom** of the window (which includes the text visible in the window, as well as the text above the top), plus ‘`Top`’ if the top of the buffer is visible on screen; or ‘`Bottom`’ or ‘`All`’.
`%q`
The percentages of text above both the **top** and the **bottom** of the window, separated by ‘`-`’, or ‘`All`’.
`%s`
The status of the subprocess belonging to the current buffer, obtained with `process-status`. See [Process Information](process-information).
`%z`
The mnemonics of keyboard, terminal, and buffer coding systems.
`%Z`
Like ‘`%z`’, but including the end-of-line format.
`%*`
‘`%`’ if the buffer is read only (see `buffer-read-only`); ‘`\*`’ if the buffer is modified (see `buffer-modified-p`); ‘`-`’ otherwise. See [Buffer Modification](buffer-modification).
`%+`
‘`\*`’ if the buffer is modified (see `buffer-modified-p`); ‘`%`’ if the buffer is read only (see `buffer-read-only`); ‘`-`’ otherwise. This differs from ‘`%\*`’ only for a modified read-only buffer. See [Buffer Modification](buffer-modification).
`%&`
‘`\*`’ if the buffer is modified, and ‘`-`’ otherwise.
`%@`
‘`@`’ if the buffer’s `default-directory` (see [File Name Expansion](file-name-expansion)) is on a remote machine, and ‘`-`’ otherwise.
`%[`
An indication of the depth of recursive editing levels (not counting minibuffer levels): one ‘`[`’ for each editing level. See [Recursive Editing](recursive-editing).
`%]`
One ‘`]`’ for each recursive editing level (not counting minibuffer levels).
`%-`
Dashes sufficient to fill the remainder of the mode line.
`%%` The character ‘`%`’—this is how to include a literal ‘`%`’ in a string in which `%`-constructs are allowed.
The following `%`-construct is still supported, but it is obsolete, since you can get the same result using the variable `mode-name`.
`%m` The value of `mode-name`.
elisp None Major and Minor Modes
---------------------
A *mode* is a set of definitions that customize Emacs behavior in useful ways. There are two varieties of modes: *minor modes*, which provide features that users can turn on and off while editing; and *major modes*, which are used for editing or interacting with a particular kind of text. Each buffer has exactly one *major mode* at a time.
This chapter describes how to write both major and minor modes, how to indicate them in the mode line, and how they run hooks supplied by the user. For related topics such as keymaps and syntax tables, see [Keymaps](keymaps), and [Syntax Tables](syntax-tables).
| | | |
| --- | --- | --- |
| • [Hooks](hooks) | | How to use hooks; how to write code that provides hooks. |
| • [Major Modes](major-modes) | | Defining major modes. |
| • [Minor Modes](minor-modes) | | Defining minor modes. |
| • [Mode Line Format](mode-line-format) | | Customizing the text that appears in the mode line. |
| • [Imenu](imenu) | | Providing a menu of definitions made in a buffer. |
| • [Font Lock Mode](font-lock-mode) | | How modes can highlight text according to syntax. |
| • [Auto-Indentation](auto_002dindentation) | | How to teach Emacs to indent for a major mode. |
| • [Desktop Save Mode](desktop-save-mode) | | How modes can have buffer state saved between Emacs sessions. |
elisp None Keymaps
-------
The command bindings of input events are recorded in data structures called *keymaps*. Each entry in a keymap associates (or *binds*) an individual event type, either to another keymap or to a command. When an event type is bound to a keymap, that keymap is used to look up the next input event; this continues until a command is found. The whole process is called *key lookup*.
| | | |
| --- | --- | --- |
| • [Key Sequences](key-sequences) | | Key sequences as Lisp objects. |
| • [Keymap Basics](keymap-basics) | | Basic concepts of keymaps. |
| • [Format of Keymaps](format-of-keymaps) | | What a keymap looks like as a Lisp object. |
| • [Creating Keymaps](creating-keymaps) | | Functions to create and copy keymaps. |
| • [Inheritance and Keymaps](inheritance-and-keymaps) | | How one keymap can inherit the bindings of another keymap. |
| • [Prefix Keys](prefix-keys) | | Defining a key with a keymap as its definition. |
| • [Active Keymaps](active-keymaps) | | How Emacs searches the active keymaps for a key binding. |
| • [Searching Keymaps](searching-keymaps) | | A pseudo-Lisp summary of searching active maps. |
| • [Controlling Active Maps](controlling-active-maps) | | Each buffer has a local keymap to override the standard (global) bindings. A minor mode can also override them. |
| • [Key Lookup](key-lookup) | | Finding a key’s binding in one keymap. |
| • [Functions for Key Lookup](functions-for-key-lookup) | | How to request key lookup. |
| • [Changing Key Bindings](changing-key-bindings) | | Redefining a key in a keymap. |
| • [Remapping Commands](remapping-commands) | | A keymap can translate one command to another. |
| • [Translation Keymaps](translation-keymaps) | | Keymaps for translating sequences of events. |
| • [Key Binding Commands](key-binding-commands) | | Interactive interfaces for redefining keys. |
| • [Scanning Keymaps](scanning-keymaps) | | Looking through all keymaps, for printing help. |
| • [Menu Keymaps](menu-keymaps) | | Defining a menu as a keymap. |
elisp None ### Sequences
This section describes functions that accept any kind of sequence.
Function: **sequencep** *object*
This function returns `t` if object is a list, vector, string, bool-vector, or char-table, `nil` otherwise. See also `seqp` below.
Function: **length** *sequence*
This function returns the number of elements in sequence. The function signals the `wrong-type-argument` error if the argument is not a sequence or is a dotted list; it signals the `circular-list` error if the argument is a circular list. For a char-table, the value returned is always one more than the maximum Emacs character code.
See [Definition of safe-length](list-elements#Definition-of-safe_002dlength), for the related function `safe-length`.
```
(length '(1 2 3))
⇒ 3
```
```
(length ())
⇒ 0
```
```
(length "foobar")
⇒ 6
```
```
(length [1 2 3])
⇒ 3
```
```
(length (make-bool-vector 5 nil))
⇒ 5
```
See also `string-bytes`, in [Text Representations](text-representations).
If you need to compute the width of a string on display, you should use `string-width` (see [Size of Displayed Text](size-of-displayed-text)), not `length`, since `length` only counts the number of characters, but does not account for the display width of each character.
Function: **length<** *sequence length*
Return non-`nil` if sequence is shorter than length. This may be more efficient than computing the length of sequence if sequence is a long list.
Function: **length>** *sequence length*
Return non-`nil` if sequence is longer than length.
Function: **length=** *sequence length*
Return non-`nil` if the length of sequence is equal to length.
Function: **elt** *sequence index*
This function returns the element of sequence indexed by index. Legitimate values of index are integers ranging from 0 up to one less than the length of sequence. If sequence is a list, out-of-range values behave as for `nth`. See [Definition of nth](list-elements#Definition-of-nth). Otherwise, out-of-range values trigger an `args-out-of-range` error.
```
(elt [1 2 3 4] 2)
⇒ 3
```
```
(elt '(1 2 3 4) 2)
⇒ 3
```
```
;; We use `string` to show clearly which character `elt` returns.
(string (elt "1234" 2))
⇒ "3"
```
```
(elt [1 2 3 4] 4)
error→ Args out of range: [1 2 3 4], 4
```
```
(elt [1 2 3 4] -1)
error→ Args out of range: [1 2 3 4], -1
```
This function generalizes `aref` (see [Array Functions](array-functions)) and `nth` (see [Definition of nth](list-elements#Definition-of-nth)).
Function: **copy-sequence** *seqr*
This function returns a copy of seqr, which should be either a sequence or a record. The copy is the same type of object as the original, and it has the same elements in the same order. However, if seqr is empty, like a string or a vector of zero length, the value returned by this function might not be a copy, but an empty object of the same type and identical to seqr.
Storing a new element into the copy does not affect the original seqr, and vice versa. However, the elements of the copy are not copies; they are identical (`eq`) to the elements of the original. Therefore, changes made within these elements, as found via the copy, are also visible in the original.
If the argument is a string with text properties, the property list in the copy is itself a copy, not shared with the original’s property list. However, the actual values of the properties are shared. See [Text Properties](text-properties).
This function does not work for dotted lists. Trying to copy a circular list may cause an infinite loop.
See also `append` in [Building Lists](building-lists), `concat` in [Creating Strings](creating-strings), and `vconcat` in [Vector Functions](vector-functions), for other ways to copy sequences.
```
(setq bar (list 1 2))
⇒ (1 2)
```
```
(setq x (vector 'foo bar))
⇒ [foo (1 2)]
```
```
(setq y (copy-sequence x))
⇒ [foo (1 2)]
```
```
(eq x y)
⇒ nil
```
```
(equal x y)
⇒ t
```
```
(eq (elt x 1) (elt y 1))
⇒ t
```
```
;; Replacing an element of one sequence.
(aset x 0 'quux)
x ⇒ [quux (1 2)]
y ⇒ [foo (1 2)]
```
```
;; Modifying the inside of a shared element.
(setcar (aref x 1) 69)
x ⇒ [quux (69 2)]
y ⇒ [foo (69 2)]
```
Function: **reverse** *sequence*
This function creates a new sequence whose elements are the elements of sequence, but in reverse order. The original argument sequence is *not* altered. Note that char-tables cannot be reversed.
```
(setq x '(1 2 3 4))
⇒ (1 2 3 4)
```
```
(reverse x)
⇒ (4 3 2 1)
x
⇒ (1 2 3 4)
```
```
(setq x [1 2 3 4])
⇒ [1 2 3 4]
```
```
(reverse x)
⇒ [4 3 2 1]
x
⇒ [1 2 3 4]
```
```
(setq x "xyzzy")
⇒ "xyzzy"
```
```
(reverse x)
⇒ "yzzyx"
x
⇒ "xyzzy"
```
Function: **nreverse** *sequence*
This function reverses the order of the elements of sequence. Unlike `reverse` the original sequence may be modified.
For example:
```
(setq x (list 'a 'b 'c))
⇒ (a b c)
```
```
x
⇒ (a b c)
(nreverse x)
⇒ (c b a)
```
```
;; The cons cell that was first is now last.
x
⇒ (a)
```
To avoid confusion, we usually store the result of `nreverse` back in the same variable which held the original list:
```
(setq x (nreverse x))
```
Here is the `nreverse` of our favorite example, `(a b c)`, presented graphically:
```
Original list head: Reversed list:
------------- ------------- ------------
| car | cdr | | car | cdr | | car | cdr |
| a | nil |<-- | b | o |<-- | c | o |
| | | | | | | | | | | | |
------------- | --------- | - | -------- | -
| | | |
------------- ------------
```
For the vector, it is even simpler because you don’t need setq:
```
(setq x (copy-sequence [1 2 3 4]))
⇒ [1 2 3 4]
(nreverse x)
⇒ [4 3 2 1]
x
⇒ [4 3 2 1]
```
Note that unlike `reverse`, this function doesn’t work with strings. Although you can alter string data by using `aset`, it is strongly encouraged to treat strings as immutable even when they are mutable. See [Mutability](mutability).
Function: **sort** *sequence predicate*
This function sorts sequence stably. Note that this function doesn’t work for all sequences; it may be used only for lists and vectors. If sequence is a list, it is modified destructively. This functions returns the sorted sequence and compares elements using predicate. A stable sort is one in which elements with equal sort keys maintain their relative order before and after the sort. Stability is important when successive sorts are used to order elements according to different criteria.
The argument predicate must be a function that accepts two arguments. It is called with two elements of sequence. To get an increasing order sort, the predicate should return non-`nil` if the first element is “less” than the second, or `nil` if not.
The comparison function predicate must give reliable results for any given pair of arguments, at least within a single call to `sort`. It must be *antisymmetric*; that is, if a is less than b, b must not be less than a. It must be *transitive*—that is, if a is less than b, and b is less than c, then a must be less than c. If you use a comparison function which does not meet these requirements, the result of `sort` is unpredictable.
The destructive aspect of `sort` for lists is that it rearranges the cons cells forming sequence by changing CDRs. A nondestructive sort function would create new cons cells to store the elements in their sorted order. If you wish to make a sorted copy without destroying the original, copy it first with `copy-sequence` and then sort.
Sorting does not change the CARs of the cons cells in sequence; the cons cell that originally contained the element `a` in sequence still has `a` in its CAR after sorting, but it now appears in a different position in the list due to the change of CDRs. For example:
```
(setq nums (list 1 3 2 6 5 4 0))
⇒ (1 3 2 6 5 4 0)
```
```
(sort nums #'<)
⇒ (0 1 2 3 4 5 6)
```
```
nums
⇒ (1 2 3 4 5 6)
```
**Warning**: Note that the list in `nums` no longer contains 0; this is the same cons cell that it was before, but it is no longer the first one in the list. Don’t assume a variable that formerly held the argument now holds the entire sorted list! Instead, save the result of `sort` and use that. Most often we store the result back into the variable that held the original list:
```
(setq nums (sort nums #'<))
```
For the better understanding of what stable sort is, consider the following vector example. After sorting, all items whose `car` is 8 are grouped at the beginning of `vector`, but their relative order is preserved. All items whose `car` is 9 are grouped at the end of `vector`, but their relative order is also preserved:
```
(setq
vector
(vector '(8 . "xxx") '(9 . "aaa") '(8 . "bbb") '(9 . "zzz")
'(9 . "ppp") '(8 . "ttt") '(8 . "eee") '(9 . "fff")))
⇒ [(8 . "xxx") (9 . "aaa") (8 . "bbb") (9 . "zzz")
(9 . "ppp") (8 . "ttt") (8 . "eee") (9 . "fff")]
```
```
(sort vector (lambda (x y) (< (car x) (car y))))
⇒ [(8 . "xxx") (8 . "bbb") (8 . "ttt") (8 . "eee")
(9 . "aaa") (9 . "zzz") (9 . "ppp") (9 . "fff")]
```
See [Sorting](sorting), for more functions that perform sorting. See `documentation` in [Accessing Documentation](accessing-documentation), for a useful example of `sort`.
The `seq.el` library provides the following additional sequence manipulation macros and functions, prefixed with `seq-`. To use them, you must first load the `seq` library.
All functions defined in this library are free of side-effects; i.e., they do not modify any sequence (list, vector, or string) that you pass as an argument. Unless otherwise stated, the result is a sequence of the same type as the input. For those functions that take a predicate, this should be a function of one argument.
The `seq.el` library can be extended to work with additional types of sequential data-structures. For that purpose, all functions are defined using `cl-defgeneric`. See [Generic Functions](generic-functions), for more details about using `cl-defgeneric` for adding extensions.
Function: **seq-elt** *sequence index*
This function returns the element of sequence at the specified index, which is an integer whose valid value range is zero to one less than the length of sequence. For out-of-range values on built-in sequence types, `seq-elt` behaves like `elt`. For the details, see [Definition of elt](#Definition-of-elt).
```
(seq-elt [1 2 3 4] 2)
⇒ 3
```
`seq-elt` returns places settable using `setf` (see [Setting Generalized Variables](setting-generalized-variables)).
```
(setq vec [1 2 3 4])
(setf (seq-elt vec 2) 5)
vec
⇒ [1 2 5 4]
```
Function: **seq-length** *sequence*
This function returns the number of elements in sequence. For built-in sequence types, `seq-length` behaves like `length`. See [Definition of length](#Definition-of-length).
Function: **seqp** *object*
This function returns non-`nil` if object is a sequence (a list or array), or any additional type of sequence defined via `seq.el` generic functions. This is an extensible variant of `sequencep`.
```
(seqp [1 2])
⇒ t
```
```
(seqp 2)
⇒ nil
```
Function: **seq-drop** *sequence n*
This function returns all but the first n (an integer) elements of sequence. If n is negative or zero, the result is sequence.
```
(seq-drop [1 2 3 4 5 6] 3)
⇒ [4 5 6]
```
```
(seq-drop "hello world" -4)
⇒ "hello world"
```
Function: **seq-take** *sequence n*
This function returns the first n (an integer) elements of sequence. If n is negative or zero, the result is `nil`.
```
(seq-take '(1 2 3 4) 3)
⇒ (1 2 3)
```
```
(seq-take [1 2 3 4] 0)
⇒ []
```
Function: **seq-take-while** *predicate sequence*
This function returns the members of sequence in order, stopping before the first one for which predicate returns `nil`.
```
(seq-take-while (lambda (elt) (> elt 0)) '(1 2 3 -1 -2))
⇒ (1 2 3)
```
```
(seq-take-while (lambda (elt) (> elt 0)) [-1 4 6])
⇒ []
```
Function: **seq-drop-while** *predicate sequence*
This function returns the members of sequence in order, starting from the first one for which predicate returns `nil`.
```
(seq-drop-while (lambda (elt) (> elt 0)) '(1 2 3 -1 -2))
⇒ (-1 -2)
```
```
(seq-drop-while (lambda (elt) (< elt 0)) [1 4 6])
⇒ [1 4 6]
```
Function: **seq-do** *function sequence*
This function applies function to each element of sequence in turn (presumably for side effects), and returns sequence.
Function: **seq-map** *function sequence*
This function returns the result of applying function to each element of sequence. The returned value is a list.
```
(seq-map #'1+ '(2 4 6))
⇒ (3 5 7)
```
```
(seq-map #'symbol-name [foo bar])
⇒ ("foo" "bar")
```
Function: **seq-map-indexed** *function sequence*
This function returns the result of applying function to each element of sequence and its index within seq. The returned value is a list.
```
(seq-map-indexed (lambda (elt idx)
(list idx elt))
'(a b c))
⇒ ((0 a) (1 b) (2 c))
```
Function: **seq-mapn** *function &rest sequences*
This function returns the result of applying function to each element of sequences. The arity (see [subr-arity](what-is-a-function)) of function must match the number of sequences. Mapping stops at the end of the shortest sequence, and the returned value is a list.
```
(seq-mapn #'+ '(2 4 6) '(20 40 60))
⇒ (22 44 66)
```
```
(seq-mapn #'concat '("moskito" "bite") ["bee" "sting"])
⇒ ("moskitobee" "bitesting")
```
Function: **seq-filter** *predicate sequence*
This function returns a list of all the elements in sequence for which predicate returns non-`nil`.
```
(seq-filter (lambda (elt) (> elt 0)) [1 -1 3 -3 5])
⇒ (1 3 5)
```
```
(seq-filter (lambda (elt) (> elt 0)) '(-1 -3 -5))
⇒ nil
```
Function: **seq-remove** *predicate sequence*
This function returns a list of all the elements in sequence for which predicate returns `nil`.
```
(seq-remove (lambda (elt) (> elt 0)) [1 -1 3 -3 5])
⇒ (-1 -3)
```
```
(seq-remove (lambda (elt) (< elt 0)) '(-1 -3 -5))
⇒ nil
```
Function: **seq-reduce** *function sequence initial-value*
This function returns the result of calling function with initial-value and the first element of sequence, then calling function with that result and the second element of sequence, then with that result and the third element of sequence, etc. function should be a function of two arguments.
function is called with two arguments. intial-value (and then the accumulated value) is used as the first argument, and the elements in sequence are used for the second argument.
If sequence is empty, this returns initial-value without calling function.
```
(seq-reduce #'+ [1 2 3 4] 0)
⇒ 10
```
```
(seq-reduce #'+ '(1 2 3 4) 5)
⇒ 15
```
```
(seq-reduce #'+ '() 3)
⇒ 3
```
Function: **seq-some** *predicate sequence*
This function returns the first non-`nil` value returned by applying predicate to each element of sequence in turn.
```
(seq-some #'numberp ["abc" 1 nil])
⇒ t
```
```
(seq-some #'numberp ["abc" "def"])
⇒ nil
```
```
(seq-some #'null ["abc" 1 nil])
⇒ t
```
```
(seq-some #'1+ [2 4 6])
⇒ 3
```
Function: **seq-find** *predicate sequence &optional default*
This function returns the first element in sequence for which predicate returns non-`nil`. If no element matches predicate, the function returns default.
Note that this function has an ambiguity if the found element is identical to default, as in that case it cannot be known whether an element was found or not.
```
(seq-find #'numberp ["abc" 1 nil])
⇒ 1
```
```
(seq-find #'numberp ["abc" "def"])
⇒ nil
```
Function: **seq-every-p** *predicate sequence*
This function returns non-`nil` if applying predicate to every element of sequence returns non-`nil`.
```
(seq-every-p #'numberp [2 4 6])
⇒ t
```
```
(seq-every-p #'numberp [2 4 "6"])
⇒ nil
```
Function: **seq-empty-p** *sequence*
This function returns non-`nil` if sequence is empty.
```
(seq-empty-p "not empty")
⇒ nil
```
```
(seq-empty-p "")
⇒ t
```
Function: **seq-count** *predicate sequence*
This function returns the number of elements in sequence for which predicate returns non-`nil`.
```
(seq-count (lambda (elt) (> elt 0)) [-1 2 0 3 -2])
⇒ 2
```
Function: **seq-sort** *function sequence*
This function returns a copy of sequence that is sorted according to function, a function of two arguments that returns non-`nil` if the first argument should sort before the second.
Function: **seq-sort-by** *function predicate sequence*
This function is similar to `seq-sort`, but the elements of sequence are transformed by applying function on them before being sorted. function is a function of one argument.
```
(seq-sort-by #'seq-length #'> ["a" "ab" "abc"])
⇒ ["abc" "ab" "a"]
```
Function: **seq-contains-p** *sequence elt &optional function*
This function returns non-`nil` if at least one element in sequence is equal to elt. If the optional argument function is non-`nil`, it is a function of two arguments to use instead of the default `equal`.
```
(seq-contains-p '(symbol1 symbol2) 'symbol1)
⇒ t
```
```
(seq-contains-p '(symbol1 symbol2) 'symbol3)
⇒ nil
```
Function: **seq-set-equal-p** *sequence1 sequence2 &optional testfn*
This function checks whether sequence1 and sequence2 contain the same elements, regardless of the order. If the optional argument testfn is non-`nil`, it is a function of two arguments to use instead of the default `equal`.
```
(seq-set-equal-p '(a b c) '(c b a))
⇒ t
```
```
(seq-set-equal-p '(a b c) '(c b))
⇒ nil
```
```
(seq-set-equal-p '("a" "b" "c") '("c" "b" "a"))
⇒ t
```
```
(seq-set-equal-p '("a" "b" "c") '("c" "b" "a") #'eq)
⇒ nil
```
Function: **seq-position** *sequence elt &optional function*
This function returns the index of the first element in sequence that is equal to elt. If the optional argument function is non-`nil`, it is a function of two arguments to use instead of the default `equal`.
```
(seq-position '(a b c) 'b)
⇒ 1
```
```
(seq-position '(a b c) 'd)
⇒ nil
```
Function: **seq-uniq** *sequence &optional function*
This function returns a list of the elements of sequence with duplicates removed. If the optional argument function is non-`nil`, it is a function of two arguments to use instead of the default `equal`.
```
(seq-uniq '(1 2 2 1 3))
⇒ (1 2 3)
```
```
(seq-uniq '(1 2 2.0 1.0) #'=)
⇒ (1 2)
```
Function: **seq-subseq** *sequence start &optional end*
This function returns a subset of sequence from start to end, both integers (end defaults to the last element). If start or end is negative, it counts from the end of sequence.
```
(seq-subseq '(1 2 3 4 5) 1)
⇒ (2 3 4 5)
```
```
(seq-subseq '[1 2 3 4 5] 1 3)
⇒ [2 3]
```
```
(seq-subseq '[1 2 3 4 5] -3 -1)
⇒ [3 4]
```
Function: **seq-concatenate** *type &rest sequences*
This function returns a sequence of type type made of the concatenation of sequences. type may be: `vector`, `list` or `string`.
```
(seq-concatenate 'list '(1 2) '(3 4) [5 6])
⇒ (1 2 3 4 5 6)
```
```
(seq-concatenate 'string "Hello " "world")
⇒ "Hello world"
```
Function: **seq-mapcat** *function sequence &optional type*
This function returns the result of applying `seq-concatenate` to the result of applying function to each element of sequence. The result is a sequence of type type, or a list if type is `nil`.
```
(seq-mapcat #'seq-reverse '((3 2 1) (6 5 4)))
⇒ (1 2 3 4 5 6)
```
Function: **seq-partition** *sequence n*
This function returns a list of the elements of sequence grouped into sub-sequences of length n. The last sequence may contain less elements than n. n must be an integer. If n is a negative integer or 0, the return value is `nil`.
```
(seq-partition '(0 1 2 3 4 5 6 7) 3)
⇒ ((0 1 2) (3 4 5) (6 7))
```
Function: **seq-union** *sequence1 sequence2 &optional function*
This function returns a list of the elements that appear either in sequence1 or sequence2. The elements of the returned list are all unique, in the sense that no two elements there will compare equal. If the optional argument function is non-`nil`, it should be a function of two arguments to use to compare elements, instead of the default `equal`.
```
(seq-union [1 2 3] [3 5])
⇒ (1 2 3 5)
```
Function: **seq-intersection** *sequence1 sequence2 &optional function*
This function returns a list of the elements that appear both in sequence1 and sequence2. If the optional argument function is non-`nil`, it is a function of two arguments to use to compare elements instead of the default `equal`.
```
(seq-intersection [2 3 4 5] [1 3 5 6 7])
⇒ (3 5)
```
Function: **seq-difference** *sequence1 sequence2 &optional function*
This function returns a list of the elements that appear in sequence1 but not in sequence2. If the optional argument function is non-`nil`, it is a function of two arguments to use to compare elements instead of the default `equal`.
```
(seq-difference '(2 3 4 5) [1 3 5 6 7])
⇒ (2 4)
```
Function: **seq-group-by** *function sequence*
This function separates the elements of sequence into an alist whose keys are the result of applying function to each element of sequence. Keys are compared using `equal`.
```
(seq-group-by #'integerp '(1 2.1 3 2 3.2))
⇒ ((t 1 3 2) (nil 2.1 3.2))
```
```
(seq-group-by #'car '((a 1) (b 2) (a 3) (c 4)))
⇒ ((b (b 2)) (a (a 1) (a 3)) (c (c 4)))
```
Function: **seq-into** *sequence type*
This function converts the sequence sequence into a sequence of type type. type can be one of the following symbols: `vector`, `string` or `list`.
```
(seq-into [1 2 3] 'list)
⇒ (1 2 3)
```
```
(seq-into nil 'vector)
⇒ []
```
```
(seq-into "hello" 'vector)
⇒ [104 101 108 108 111]
```
Function: **seq-min** *sequence*
This function returns the smallest element of sequence. The elements of sequence must be numbers or markers (see [Markers](markers)).
```
(seq-min [3 1 2])
⇒ 1
```
```
(seq-min "Hello")
⇒ 72
```
Function: **seq-max** *sequence*
This function returns the largest element of sequence. The elements of sequence must be numbers or markers.
```
(seq-max [1 3 2])
⇒ 3
```
```
(seq-max "Hello")
⇒ 111
```
Macro: **seq-doseq** *(var sequence) body…*
This macro is like `dolist` (see [dolist](iteration)), except that sequence can be a list, vector or string. This is primarily useful for side-effects.
Macro: **seq-let** *var-sequence val-sequence body…*
This macro binds the variables defined in var-sequence to the values that are the corresponding elements of val-sequence. This is known as *destructuring binding*. The elements of var-sequence can themselves include sequences, allowing for nested destructuring.
The var-sequence sequence can also include the `&rest` marker followed by a variable name to be bound to the rest of val-sequence.
```
(seq-let [first second] [1 2 3 4]
(list first second))
⇒ (1 2)
```
```
(seq-let (_ a _ b) '(1 2 3 4)
(list a b))
⇒ (2 4)
```
```
(seq-let [a [b [c]]] [1 [2 [3]]]
(list a b c))
⇒ (1 2 3)
```
```
(seq-let [a b &rest others] [1 2 3 4]
others)
```
```
⇒ [3 4]
```
The `pcase` patterns provide an alternative facility for destructuring binding, see [Destructuring with pcase Patterns](destructuring-with-pcase-patterns).
Macro: **seq-setq** *var-sequence val-sequence*
This macro works similarly to `seq-let`, except that values are assigned to variables as if by `setq` instead of as in a `let` binding.
```
(let ((a nil)
(b nil))
(seq-setq (_ a _ b) '(1 2 3 4))
(list a b))
⇒ (2 4)
```
Function: **seq-random-elt** *sequence*
This function returns an element of sequence taken at random.
```
(seq-random-elt [1 2 3 4])
⇒ 3
(seq-random-elt [1 2 3 4])
⇒ 2
(seq-random-elt [1 2 3 4])
⇒ 4
(seq-random-elt [1 2 3 4])
⇒ 2
(seq-random-elt [1 2 3 4])
⇒ 1
```
If sequence is empty, this function signals an error.
| programming_docs |
elisp None ### C Integer Types
Here are some guidelines for use of integer types in the Emacs C source code. These guidelines sometimes give competing advice; common sense is advised.
* Avoid arbitrary limits. For example, avoid `int len = strlen
(s);` unless the length of `s` is required for other reasons to fit in `int` range.
* Do not assume that signed integer arithmetic wraps around on overflow. This is no longer true of Emacs porting targets: signed integer overflow has undefined behavior in practice, and can dump core or even cause earlier or later code to behave illogically. Unsigned overflow does wrap around reliably, modulo a power of two.
* Prefer signed types to unsigned, as code gets confusing when signed and unsigned types are combined. Many other guidelines assume that types are signed; in the rarer cases where unsigned types are needed, similar advice may apply to the unsigned counterparts (e.g., `size_t` instead of `ptrdiff_t`, or `uintptr_t` instead of `intptr_t`).
* Prefer `int` for Emacs character codes, in the range 0 .. 0x3FFFFF. More generally, prefer `int` for integers known to be in `int` range, e.g., screen column counts.
* Prefer `ptrdiff_t` for sizes, i.e., for integers bounded by the maximum size of any individual C object or by the maximum number of elements in any C array. This is part of Emacs’s general preference for signed types. Using `ptrdiff_t` limits objects to `PTRDIFF_MAX` bytes, but larger objects would cause trouble anyway since they would break pointer subtraction, so this does not impose an arbitrary limit.
* Avoid `ssize_t` except when communicating to low-level APIs that have `ssize_t`-related limitations. Although it’s equivalent to `ptrdiff_t` on typical platforms, `ssize_t` is occasionally narrower, so using it for size-related calculations could overflow. Also, `ptrdiff_t` is more ubiquitous and better-standardized, has standard `printf` formats, and is the basis for Emacs’s internal size-overflow checking. When using `ssize_t`, please note that POSIX requires support only for values in the range -1 .. `SSIZE_MAX`.
* Normally, prefer `intptr_t` for internal representations of pointers, or for integers bounded only by the number of objects that can exist at any given time or by the total number of bytes that can be allocated. However, prefer `uintptr_t` to represent pointer arithmetic that could cross page boundaries. For example, on a machine with a 32-bit address space an array could cross the 0x7fffffff/0x80000000 boundary, which would cause an integer overflow when adding 1 to `(intptr_t) 0x7fffffff`.
* Prefer the Emacs-defined type `EMACS_INT` for representing values converted to or from Emacs Lisp fixnums, as fixnum arithmetic is based on `EMACS_INT`.
* When representing a system value (such as a file size or a count of seconds since the Epoch), prefer the corresponding system type (e.g., `off_t`, `time_t`). Do not assume that a system type is signed, unless this assumption is known to be safe. For example, although `off_t` is always signed, `time_t` need not be.
* Prefer `intmax_t` for representing values that might be any signed integer value. A `printf`-family function can print such a value via a format like `"%"PRIdMAX`.
* Prefer `bool`, `false` and `true` for booleans. Using `bool` can make programs easier to read and a bit faster than using `int`. Although it is also OK to use `int`, `0` and `1`, this older style is gradually being phased out. When using `bool`, respect the limitations of the replacement implementation of `bool`, as documented in the source file `lib/stdbool.in.h`. In particular, boolean bitfields should be of type `bool_bf`, not `bool`, so that they work correctly even when compiling Objective C with standard GCC.
* In bitfields, prefer `unsigned int` or `signed int` to `int`, as `int` is less portable: it might be signed, and might not be. Single-bit bit fields should be `unsigned int` or `bool_bf` so that their values are 0 or 1.
elisp None #### Displaying Messages in the Echo Area
This section describes the standard functions for displaying messages in the echo area.
Function: **message** *format-string &rest arguments*
This function displays a message in the echo area. format-string is a format string, and arguments are the objects for its format specifications, like in the `format-message` function (see [Formatting Strings](formatting-strings)). The resulting formatted string is displayed in the echo area; if it contains `face` text properties, it is displayed with the specified faces (see [Faces](faces)). The string is also added to the `\*Messages\*` buffer, but without text properties (see [Logging Messages](logging-messages)).
Typically grave accent and apostrophe in the format translate to matching curved quotes, e.g., "Missing `%s'" might result in "Missing ‘foo’". See [Text Quoting Style](text-quoting-style), for how to influence or inhibit this translation.
In batch mode, the message is printed to the standard error stream, followed by a newline.
When `inhibit-message` is non-`nil`, no message will be displayed in the echo area, it will only be logged to ‘`\*Messages\*`’.
If format-string is `nil` or the empty string, `message` clears the echo area; if the echo area has been expanded automatically, this brings it back to its normal size. If the minibuffer is active, this brings the minibuffer contents back onto the screen immediately.
```
(message "Reverting `%s'..." (buffer-name))
-| Reverting ‘subr.el’...
⇒ "Reverting ‘subr.el’..."
```
```
---------- Echo Area ----------
Reverting ‘subr.el’...
---------- Echo Area ----------
```
To automatically display a message in the echo area or in a pop-buffer, depending on its size, use `display-message-or-buffer` (see below).
**Warning:** If you want to use your own string as a message verbatim, don’t just write `(message string)`. If string contains ‘`%`’, ‘```’, or ‘`'`’ it may be reformatted, with undesirable results. Instead, use `(message
"%s" string)`.
Variable: **set-message-function**
If this variable is non-`nil`, it should be a function of one argument, the text of a message to display in the echo area. This function will be called by `message` and related functions. If the function returns `nil`, the message is displayed in the echo area as usual. If this function returns a string, that string is displayed in the echo area instead of the original one. If this function returns other non-`nil` values, that means the message was already handled, so `message` will not display anything in the echo area. See also `clear-message-function` that can be used to clear the message displayed by this function.
The default value is the function that displays the message at the end of the minibuffer when the minibuffer is active. However, if the text shown in the active minibuffer has the `minibuffer-message` text property (see [Special Properties](special-properties)) on some character, the message will be displayed before the first character having that property.
Variable: **clear-message-function**
If this variable is non-`nil`, `message` and related functions call it with no arguments when their argument message is `nil` or the empty string.
Usually this function is called when the next input event arrives after displaying an echo-area message. The function is expected to clear the message displayed by its counterpart function specified by `set-message-function`.
The default value is the function that clears the message displayed in an active minibuffer.
Variable: **inhibit-message**
When this variable is non-`nil`, `message` and related functions will not use the Echo Area to display messages.
Macro: **with-temp-message** *message &rest body*
This construct displays a message in the echo area temporarily, during the execution of body. It displays message, executes body, then returns the value of the last body form while restoring the previous echo area contents.
Function: **message-or-box** *format-string &rest arguments*
This function displays a message like `message`, but may display it in a dialog box instead of the echo area. If this function is called in a command that was invoked using the mouse—more precisely, if `last-nonmenu-event` (see [Command Loop Info](command-loop-info)) is either `nil` or a list—then it uses a dialog box or pop-up menu to display the message. Otherwise, it uses the echo area. (This is the same criterion that `y-or-n-p` uses to make a similar decision; see [Yes-or-No Queries](yes_002dor_002dno-queries).)
You can force use of the mouse or of the echo area by binding `last-nonmenu-event` to a suitable value around the call.
Function: **message-box** *format-string &rest arguments*
This function displays a message like `message`, but uses a dialog box (or a pop-up menu) whenever that is possible. If it is impossible to use a dialog box or pop-up menu, because the terminal does not support them, then `message-box` uses the echo area, like `message`.
Function: **display-message-or-buffer** *message &optional buffer-name action frame*
This function displays the message message, which may be either a string or a buffer. If it is shorter than the maximum height of the echo area, as defined by `max-mini-window-height`, it is displayed in the echo area, using `message`. Otherwise, `display-buffer` is used to show it in a pop-up buffer.
Returns either the string shown in the echo area, or when a pop-up buffer is used, the window used to display it.
If message is a string, then the optional argument buffer-name is the name of the buffer used to display it when a pop-up buffer is used, defaulting to `\*Message\*`. In the case where message is a string and displayed in the echo area, it is not specified whether the contents are inserted into the buffer anyway.
The optional arguments action and frame are as for `display-buffer`, and only used if a buffer is displayed.
Function: **current-message**
This function returns the message currently being displayed in the echo area, or `nil` if there is none.
elisp None ### Backquote
*Backquote constructs* allow you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form `quote` (described in the previous section; see [Quoting](quoting)). For example, these two forms yield identical results:
```
`(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
```
```
'(a list of (+ 2 3) elements)
⇒ (a list of (+ 2 3) elements)
```
The special marker ‘`,`’ inside of the argument to backquote indicates a value that isn’t constant. The Emacs Lisp evaluator evaluates the argument of ‘`,`’, and puts the value in the list structure:
```
`(a list of ,(+ 2 3) elements)
⇒ (a list of 5 elements)
```
Substitution with ‘`,`’ is allowed at deeper levels of the list structure also. For example:
```
`(1 2 (3 ,(+ 4 5)))
⇒ (1 2 (3 9))
```
You can also *splice* an evaluated value into the resulting list, using the special marker ‘`,@`’. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ‘```’ is often unreadable. Here are some examples:
```
(setq some-list '(2 3))
⇒ (2 3)
```
```
(cons 1 (append some-list '(4) some-list))
⇒ (1 2 3 4 2 3)
```
```
`(1 ,@some-list 4 ,@some-list)
⇒ (1 2 3 4 2 3)
```
```
(setq list '(hack foo bar))
⇒ (hack foo bar)
```
```
(cons 'use
(cons 'the
(cons 'words (append (cdr list) '(as elements)))))
⇒ (use the words foo bar as elements)
```
```
`(use the words ,@(cdr list) as elements)
⇒ (use the words foo bar as elements)
```
If a subexpression of a backquote construct has no substitutions or splices, it acts like `quote` in that it yields conses, vectors and strings that might be shared and should not be modified. See [Self-Evaluating Forms](self_002devaluating-forms).
elisp None #### Finding the Parse State for a Position
For syntactic analysis, such as in indentation, often the useful thing is to compute the syntactic state corresponding to a given buffer position. This function does that conveniently.
Function: **syntax-ppss** *&optional pos*
This function returns the parser state that the parser would reach at position pos starting from the beginning of the visible portion of the buffer. See [Parser State](parser-state), for a description of the parser state.
The return value is the same as if you call the low-level parsing function `parse-partial-sexp` to parse from the beginning of the visible portion of the buffer to pos (see [Low-Level Parsing](low_002dlevel-parsing)). However, `syntax-ppss` uses caches to speed up the computation. Due to this optimization, the second value (previous complete subexpression) and sixth value (minimum parenthesis depth) in the returned parser state are not meaningful.
This function has a side effect: it adds a buffer-local entry to `before-change-functions` (see [Change Hooks](change-hooks)) for `syntax-ppss-flush-cache` (see below). This entry keeps the cache consistent as the buffer is modified. However, the cache might not be updated if `syntax-ppss` is called while `before-change-functions` is temporarily let-bound, or if the buffer is modified without running the hook, such as when using `inhibit-modification-hooks`. In those cases, it is necessary to call `syntax-ppss-flush-cache` explicitly.
Function: **syntax-ppss-flush-cache** *beg &rest ignored-args*
This function flushes the cache used by `syntax-ppss`, starting at position beg. The remaining arguments, ignored-args, are ignored; this function accepts them so that it can be directly used on hooks such as `before-change-functions` (see [Change Hooks](change-hooks)).
elisp None #### Replacing the Text that Matched
This function replaces all or part of the text matched by the last search. It works by means of the match data.
Function: **replace-match** *replacement &optional fixedcase literal string subexp*
This function performs a replacement operation on a buffer or string.
If you did the last search in a buffer, you should omit the string argument or specify `nil` for it, and make sure that the current buffer is the one in which you performed the last search. Then this function edits the buffer, replacing the matched text with replacement. It leaves point at the end of the replacement text.
If you performed the last search on a string, pass the same string as string. Then this function returns a new string, in which the matched text is replaced by replacement.
If fixedcase is non-`nil`, then `replace-match` uses the replacement text without case conversion; otherwise, it converts the replacement text depending upon the capitalization of the text to be replaced. If the original text is all upper case, this converts the replacement text to upper case. If all words of the original text are capitalized, this capitalizes all the words of the replacement text. If all the words are one-letter and they are all upper case, they are treated as capitalized words rather than all-upper-case words.
If literal is non-`nil`, then replacement is inserted exactly as it is, the only alterations being case changes as needed. If it is `nil` (the default), then the character ‘`\`’ is treated specially. If a ‘`\`’ appears in replacement, then it must be part of one of the following sequences:
‘`\&`’
This stands for the entire text being replaced.
‘`\n`’, where n is a digit
This stands for the text that matched the nth subexpression in the original regexp. Subexpressions are those expressions grouped inside ‘`\(…\)`’. If the nth subexpression never matched, an empty string is substituted.
‘`\\`’
This stands for a single ‘`\`’ in the replacement text.
‘`\?`’ This stands for itself (for compatibility with `replace-regexp` and related commands; see [Regexp Replace](https://www.gnu.org/software/emacs/manual/html_node/emacs/Regexp-Replace.html#Regexp-Replace) in The GNU Emacs Manual).
Any other character following ‘`\`’ signals an error.
The substitutions performed by ‘`\&`’ and ‘`\n`’ occur after case conversion, if any. Therefore, the strings they substitute are never case-converted.
If subexp is non-`nil`, that says to replace just subexpression number subexp of the regexp that was matched, not the entire match. For example, after matching ‘`foo \(ba\*r\)`’, calling `replace-match` with 1 as subexp means to replace just the text that matched ‘`\(ba\*r\)`’.
Function: **match-substitute-replacement** *replacement &optional fixedcase literal string subexp*
This function returns the text that would be inserted into the buffer by `replace-match`, but without modifying the buffer. It is useful if you want to present the user with actual replacement result, with constructs like ‘`\n`’ or ‘`\&`’ substituted with matched groups. Arguments replacement and optional fixedcase, literal, string and subexp have the same meaning as for `replace-match`.
elisp None #### Window Management Parameters
The following frame parameters control various aspects of the frame’s interaction with the window manager or window system. They have no effect on text terminals.
`visibility`
The state of visibility of the frame. There are three possibilities: `nil` for invisible, `t` for visible, and `icon` for iconified. See [Visibility of Frames](visibility-of-frames).
`auto-raise`
If non-`nil`, Emacs automatically raises the frame when it is selected. Some window managers do not allow this.
`auto-lower`
If non-`nil`, Emacs automatically lowers the frame when it is deselected. Some window managers do not allow this.
`icon-type`
The type of icon to use for this frame. If the value is a string, that specifies a file containing a bitmap to use; `nil` specifies no icon (in which case the window manager decides what to show); any other non-`nil` value specifies the default Emacs icon.
`icon-name`
The name to use in the icon for this frame, when and if the icon appears. If this is `nil`, the frame’s title is used.
`window-id`
The ID number which the graphical display uses for this frame. Emacs assigns this parameter when the frame is created; changing the parameter has no effect on the actual ID number.
`outer-window-id`
The ID number of the outermost window-system window in which the frame exists. As with `window-id`, changing this parameter has no actual effect.
`wait-for-wm`
If non-`nil`, tell Xt to wait for the window manager to confirm geometry changes. Some window managers, including versions of Fvwm2 and KDE, fail to confirm, so Xt hangs. Set this to `nil` to prevent hanging with those window managers.
`sticky`
If non-`nil`, the frame is visible on all virtual desktops on systems with virtual desktops.
`inhibit-double-buffering`
If non-`nil`, the frame is drawn to the screen without double buffering. Emacs normally attempts to use double buffering, where available, to reduce flicker. Set this property if you experience display bugs or pine for that retro, flicker-y feeling.
`skip-taskbar`
If non-`nil`, this tells the window manager to remove the frame’s icon from the taskbar associated with the frame’s display and inhibit switching to the frame’s window via the combination `Alt-TAB`. On MS-Windows, iconifying such a frame will "roll in" its window-system window at the bottom of the desktop. Some window managers may not honor this parameter.
`no-focus-on-map`
If non-`nil`, this means that the frame does not want to receive input focus when it is mapped (see [Visibility of Frames](visibility-of-frames)). Some window managers may not honor this parameter.
`no-accept-focus`
If non-`nil`, this means that the frame does not want to receive input focus via explicit mouse clicks or when moving the mouse into it either via `focus-follows-mouse` (see [Input Focus](input-focus)) or `mouse-autoselect-window` (see [Mouse Window Auto-selection](mouse-window-auto_002dselection)). This may have the unwanted side-effect that a user cannot scroll a non-selected frame with the mouse. Some window managers may not honor this parameter.
`undecorated`
If non-`nil`, this frame’s window-system window is drawn without decorations, like the title, minimize/maximize boxes and external borders. This usually means that the window cannot be dragged, resized, iconified, maximized or deleted with the mouse. If `nil`, the frame’s window is usually drawn with all the elements listed above unless their display has been suspended via window manager settings.
Under X, Emacs uses the Motif window manager hints to turn off decorations. Some window managers may not honor these hints.
NS builds consider the tool bar to be a decoration, and therefore hide it on an undecorated frame.
`override-redirect`
If non-`nil`, this means that this is an *override redirect* frame—a frame not handled by window managers under X. Override redirect frames have no window manager decorations, can be positioned and resized only via Emacs’ positioning and resizing functions and are usually drawn on top of all other frames. Setting this parameter has no effect on MS-Windows.
`ns-appearance`
Only available on macOS, if set to `dark` draw this frame’s window-system window using the “vibrant dark” theme, and if set to `light` use the “aqua” theme, otherwise use the system default. The “vibrant dark” theme can be used to set the toolbar and scrollbars to a dark appearance when using an Emacs theme with a dark background.
`ns-transparent-titlebar` Only available on macOS, if non-`nil`, set the titlebar and toolbar to be transparent. This effectively sets the background color of both to match the Emacs background color.
| programming_docs |
elisp None #### The Init File
When you start Emacs, it normally attempts to load your *init file*. This is either a file named `.emacs` or `.emacs.el` in your home directory, or a file named `init.el` in a subdirectory named `.emacs.d` in your home directory.
The command-line switches ‘`-q`’, ‘`-Q`’, and ‘`-u`’ control whether and where to find the init file; ‘`-q`’ (and the stronger ‘`-Q`’) says not to load an init file, while ‘`-u user`’ says to load user’s init file instead of yours. See [Entering Emacs](https://www.gnu.org/software/emacs/manual/html_node/emacs/Entering-Emacs.html#Entering-Emacs) in The GNU Emacs Manual. If neither option is specified, Emacs uses the `LOGNAME` environment variable, or the `USER` (most systems) or `USERNAME` (MS systems) variable, to find your home directory and thus your init file; this way, even if you have su’d, Emacs still loads your own init file. If those environment variables are absent, though, Emacs uses your user-id to find your home directory.
Emacs also attempts to load a second init file, called the *early init file*, if it exists. This is a file named `early-init.el` in your `~/.emacs.d` directory. The difference between the early init file and the regular init file is that the early init file is loaded much earlier during the startup process, so you can use it to customize some things that are initialized before loading the regular init file. For example, you can customize the process of initializing the package system, by setting variables such as package-load-list or package-enable-at-startup. See [Package Installation](https://www.gnu.org/software/emacs/manual/html_node/emacs/Package-Installation.html#Package-Installation) in The GNU Emacs Manual.
An Emacs installation may have a *default init file*, which is a Lisp library named `default.el`. Emacs finds this file through the standard search path for libraries (see [How Programs Do Loading](how-programs-do-loading)). The Emacs distribution does not come with this file; it is intended for local customizations. If the default init file exists, it is loaded whenever you start Emacs. But your own personal init file, if any, is loaded first; if it sets `inhibit-default-init` to a non-`nil` value, then Emacs does not subsequently load the `default.el` file. In batch mode, or if you specify ‘`-q`’ (or ‘`-Q`’), Emacs loads neither your personal init file nor the default init file.
Another file for site-customization is `site-start.el`. Emacs loads this *before* the user’s init file. You can inhibit the loading of this file with the option ‘`--no-site-file`’.
User Option: **site-run-file**
This variable specifies the site-customization file to load before the user’s init file. Its normal value is `"site-start"`. The only way you can change it with real effect is to do so before dumping Emacs.
See [Init File Examples](https://www.gnu.org/software/emacs/manual/html_node/emacs/Init-Examples.html#Init-Examples) in The GNU Emacs Manual, for examples of how to make various commonly desired customizations in your `.emacs` file.
User Option: **inhibit-default-init**
If this variable is non-`nil`, it prevents Emacs from loading the default initialization library file. The default value is `nil`.
Variable: **before-init-hook**
This normal hook is run, once, just before loading all the init files (`site-start.el`, your init file, and `default.el`). (The only way to change it with real effect is before dumping Emacs.)
Variable: **after-init-hook**
This normal hook is run, once, just after loading all the init files (`site-start.el`, your init file, and `default.el`), before loading the terminal-specific library (if started on a text terminal) and processing the command-line action arguments.
Variable: **emacs-startup-hook**
This normal hook is run, once, just after handling the command line arguments. In batch mode, Emacs does not run this hook.
Variable: **window-setup-hook**
This normal hook is very similar to `emacs-startup-hook`. The only difference is that it runs slightly later, after setting of the frame parameters. See [window-setup-hook](startup-summary).
Variable: **user-init-file**
This variable holds the absolute file name of the user’s init file. If the actual init file loaded is a compiled file, such as `.emacs.elc`, the value refers to the corresponding source file.
Variable: **user-emacs-directory**
This variable holds the name of the Emacs default directory. It defaults to `${XDG\_CONFIG\_HOME-'~/.config'}/emacs/` if that directory exists and `~/.emacs.d/` and `~/.emacs` do not exist, otherwise to `~/.emacs.d/` on all platforms but MS-DOS. Here, `${XDG\_CONFIG\_HOME-'~/.config'}` stands for the value of the environment variable `XDG_CONFIG_HOME` if that variable is set, and for `~/.config` otherwise. See [How Emacs Finds Your Init File](https://www.gnu.org/software/emacs/manual/html_node/emacs/Find-Init.html#Find-Init) in The GNU Emacs Manual.
elisp None #### Errors
When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it *signals* an *error*.
When an error is signaled, Emacs’s default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type `C-f` at the end of the buffer.
In complicated programs, simple termination may not be what you want. For example, the program may have made temporary changes in data structures, or created temporary buffers that should be deleted before the program is finished. In such cases, you would use `unwind-protect` to establish *cleanup expressions* to be evaluated in case of error. (See [Cleanups](cleanups).) Occasionally, you may wish the program to continue execution despite an error in a subroutine. In these cases, you would use `condition-case` to establish *error handlers* to recover control in case of error.
Resist the temptation to use error handling to transfer control from one part of the program to another; use `catch` and `throw` instead. See [Catch and Throw](catch-and-throw).
| | | |
| --- | --- | --- |
| • [Signaling Errors](signaling-errors) | | How to report an error. |
| • [Processing of Errors](processing-of-errors) | | What Emacs does when you report an error. |
| • [Handling Errors](handling-errors) | | How you can trap errors and continue execution. |
| • [Error Symbols](error-symbols) | | How errors are classified for trapping them. |
elisp None #### XBM Images
To use XBM format, specify `xbm` as the image type. This image format doesn’t require an external library, so images of this type are always supported.
Additional image properties supported for the `xbm` image type are:
`:foreground foreground`
The value, foreground, should be a string specifying the image foreground color, or `nil` for the default color. This color is used for each pixel in the XBM that is 1. The default is the frame’s foreground color.
`:background background` The value, background, should be a string specifying the image background color, or `nil` for the default color. This color is used for each pixel in the XBM that is 0. The default is the frame’s background color.
If you specify an XBM image using data within Emacs instead of an external file, use the following three properties:
`:data data`
The value, data, specifies the contents of the image. There are three formats you can use for data:
* A vector of strings or bool-vectors, each specifying one line of the image. Do specify `:height` and `:width`.
* A string containing the same byte sequence as an XBM file would contain. You must not specify `:height` and `:width` in this case, because omitting them is what indicates the data has the format of an XBM file. The file contents specify the height and width of the image.
* A string or a bool-vector containing the bits of the image (plus perhaps some extra bits at the end that will not be used). It should contain at least `stride * height` bits, where stride is the smallest multiple of 8 greater than or equal to the width of the image. In this case, you should specify `:height`, `:width` and `:stride`, both to indicate that the string contains just the bits rather than a whole XBM file, and to specify the size of the image.
`:width width`
The value, width, specifies the width of the image, in pixels.
`:height height`
The value, height, specifies the height of the image, in pixels.
Note that `:width` and `:height` can only be used if passing in data that doesn’t specify the width and height (e.g., a string or a vector containing the bits of the image). XBM files usually specify this themselves, and it’s an error to use these two properties on these files. Also note that `:width` and `:height` are used by most other image formats to specify what the displayed image is supposed to be, which usually means performing some sort of scaling. This isn’t supported for XBM images.
`:stride stride` The number of bool vector entries stored for each row; the smallest multiple of 8 greater than or equal to width.
elisp None Loading
-------
Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file. Such a file is also called a *Lisp library*.
The load functions evaluate all the expressions in a file just as the `eval-buffer` function evaluates all the expressions in a buffer. The difference is that the load functions read and evaluate the text in the file as found on disk, not the text in an Emacs buffer.
The loaded file must contain Lisp expressions, either as source code or as byte-compiled code. Each form in the file is called a *top-level form*. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions.
Emacs can also load compiled dynamic modules: shared libraries that provide additional functionality for use in Emacs Lisp programs, just like a package written in Emacs Lisp would. When a dynamic module is loaded, Emacs calls a specially-named initialization function which the module needs to implement, and which exposes the additional functions and variables to Emacs Lisp programs.
For on-demand loading of external libraries which are known in advance to be required by certain Emacs primitives, see [Dynamic Libraries](dynamic-libraries).
| | | |
| --- | --- | --- |
| • [How Programs Do Loading](how-programs-do-loading) | | The `load` function and others. |
| • [Load Suffixes](load-suffixes) | | Details about the suffixes that `load` tries. |
| • [Library Search](library-search) | | Finding a library to load. |
| • [Loading Non-ASCII](loading-non_002dascii) | | Non-ASCII characters in Emacs Lisp files. |
| • [Autoload](autoload) | | Setting up a function to autoload. |
| • [Repeated Loading](repeated-loading) | | Precautions about loading a file twice. |
| • [Named Features](named-features) | | Loading a library if it isn’t already loaded. |
| • [Where Defined](where-defined) | | Finding which file defined a certain symbol. |
| • [Unloading](unloading) | | How to unload a library that was loaded. |
| • [Hooks for Loading](hooks-for-loading) | | Providing code to be run when particular libraries are loaded. |
| • [Dynamic Modules](dynamic-modules) | | Modules provide additional Lisp primitives. |
elisp None ### Locales
In POSIX, locales control which language to use in language-related features. These Emacs variables control how Emacs interacts with these features.
Variable: **locale-coding-system**
This variable specifies the coding system to use for decoding system error messages and—on X Window system only—keyboard input, for sending batch output to the standard output and error streams, for encoding the format argument to `format-time-string`, and for decoding the return value of `format-time-string`.
Variable: **system-messages-locale**
This variable specifies the locale to use for generating system error messages. Changing the locale can cause messages to come out in a different language or in a different orthography. If the variable is `nil`, the locale is specified by environment variables in the usual POSIX fashion.
Variable: **system-time-locale**
This variable specifies the locale to use for formatting time values. Changing the locale can cause messages to appear according to the conventions of a different language. If the variable is `nil`, the locale is specified by environment variables in the usual POSIX fashion.
Function: **locale-info** *item*
This function returns locale data item for the current POSIX locale, if available. item should be one of these symbols:
`codeset`
Return the character set as a string (locale item `CODESET`).
`days`
Return a 7-element vector of day names (locale items `DAY_1` through `DAY_7`);
`months`
Return a 12-element vector of month names (locale items `MON_1` through `MON_12`).
`paper` Return a list `(width height)` of 2 integers, for the default paper size measured in millimeters (locale items `_NL_PAPER_WIDTH` and `_NL_PAPER_HEIGHT`).
If the system can’t provide the requested information, or if item is not one of those symbols, the value is `nil`. All strings in the return value are decoded using `locale-coding-system`. See [Locales](https://www.gnu.org/software/libc/manual/html_node/Locales.html#Locales) in The GNU Libc Manual, for more information about locales and locale items.
elisp None #### Additional Options for Displaying Buffers
The behavior of buffer display actions (see [Choosing Window](choosing-window)) can be further modified by the following user options.
User Option: **pop-up-windows**
If the value of this variable is non-`nil`, `display-buffer` is allowed to split an existing window to make a new window for displaying in. This is the default.
This variable is provided for backward compatibility only. It is obeyed by `display-buffer` via a special mechanism in `display-buffer-fallback-action`, which calls the action function `display-buffer-pop-up-window` (see [Buffer Display Action Functions](buffer-display-action-functions)) when the value of this option is non-`nil`. It is not consulted by `display-buffer-pop-up-window` itself, which the user may specify directly in `display-buffer-alist` etc.
User Option: **split-window-preferred-function**
This variable specifies a function for splitting a window, in order to make a new window for displaying a buffer. It is used by the `display-buffer-pop-up-window` action function to actually split the window.
The value must be a function that takes one argument, a window, and returns either a new window (which will be used to display the desired buffer) or `nil` (which means the splitting failed). The default value is `split-window-sensibly`, which is documented next.
Function: **split-window-sensibly** *&optional window*
This function tries to split window and return the newly created window. If window cannot be split, it returns `nil`. If window is omitted or `nil`, it defaults to the selected window.
This function obeys the usual rules that determine when a window may be split (see [Splitting Windows](splitting-windows)). It first tries to split by placing the new window below, subject to the restriction imposed by `split-height-threshold` (see below), in addition to any other restrictions. If that fails, it tries to split by placing the new window to the right, subject to `split-width-threshold` (see below). If that also fails, and the window is the only window on its frame, this function again tries to split and place the new window below, disregarding `split-height-threshold`. If this fails as well, this function gives up and returns `nil`.
User Option: **split-height-threshold**
This variable specifies whether `split-window-sensibly` is allowed to split the window placing the new window below. If it is an integer, that means to split only if the original window has at least that many lines. If it is `nil`, that means not to split this way.
User Option: **split-width-threshold**
This variable specifies whether `split-window-sensibly` is allowed to split the window placing the new window to the right. If the value is an integer, that means to split only if the original window has at least that many columns. If the value is `nil`, that means not to split this way.
User Option: **even-window-sizes**
This variable, if non-`nil`, causes `display-buffer` to even window sizes whenever it reuses an existing window, and that window is adjacent to the selected one.
If its value is `width-only`, sizes are evened only if the reused window is on the left or right of the selected one and the selected window is wider than the reused one. If its value is `height-only` sizes are evened only if the reused window is above or beneath the selected window and the selected window is higher than the reused one. Any other non-`nil` value means to even sizes in any of these cases provided the selected window is larger than the reused one in the sense of their combination.
User Option: **pop-up-frames**
If the value of this variable is non-`nil`, that means `display-buffer` may display buffers by making new frames. The default is `nil`.
A non-`nil` value also means that when `display-buffer` is looking for a window already displaying buffer-or-name, it can search any visible or iconified frame, not just the selected frame.
This variable is provided mainly for backward compatibility. It is obeyed by `display-buffer` via a special mechanism in `display-buffer-fallback-action`, which calls the action function `display-buffer-pop-up-frame` (see [Buffer Display Action Functions](buffer-display-action-functions)) if the value is non-`nil`. (This is done before attempting to split a window.) This variable is not consulted by `display-buffer-pop-up-frame` itself, which the user may specify directly in `display-buffer-alist` etc.
User Option: **pop-up-frame-function**
This variable specifies a function for creating a new frame, in order to make a new window for displaying a buffer. It is used by the `display-buffer-pop-up-frame` action function.
The value should be a function that takes no arguments and returns a frame, or `nil` if no frame could be created. The default value is a function that creates a frame using the parameters specified by `pop-up-frame-alist` (see below).
User Option: **pop-up-frame-alist**
This variable holds an alist of frame parameters (see [Frame Parameters](frame-parameters)), which is used by the function specified by `pop-up-frame-function` to make a new frame. The default is `nil`.
This option is provided for backward compatibility only. Note, that when `display-buffer-pop-up-frame` calls the function specified by `pop-up-frame-function`, it prepends the value of all `pop-up-frame-parameters` action alist entries to `pop-up-frame-alist` so that the values specified by the action alist entry effectively override any corresponding values of `pop-up-frame-alist`.
Hence, users should set up a `pop-up-frame-parameters` action alist entry in `display-buffer-alist` instead of customizing `pop-up-frame-alist`. Only this will guarantee that the value of a parameter specified by the user overrides the value of that parameter specified by the caller of `display-buffer`.
Many efforts in the design of `display-buffer` have been given to maintain compatibility with code that uses older options like `pop-up-windows`, `pop-up-frames`, `pop-up-frame-alist`, `same-window-buffer-names` and `same-window-regexps`. Lisp Programs and users should refrain from using these options. Above we already warned against customizing `pop-up-frame-alist`. Here we describe how to convert the remaining options to use display actions instead.
`pop-up-windows`
This variable is `t` by default. Instead of customizing it to `nil` and thus telling `display-buffer` what not to do, it’s much better to list in `display-buffer-base-action` the action functions it should try instead as, for example:
```
(customize-set-variable
'display-buffer-base-action
'((display-buffer-reuse-window display-buffer-same-window
display-buffer-in-previous-window
display-buffer-use-some-window)))
```
`pop-up-frames`
Instead of customizing this variable to `t`, customize `display-buffer-base-action`, for example, as follows:
```
(customize-set-variable
'display-buffer-base-action
'((display-buffer-reuse-window display-buffer-pop-up-frame)
(reusable-frames . 0)))
```
`same-window-buffer-names` `same-window-regexps`
Instead of adding a buffer name or a regular expression to one of these options use a `display-buffer-alist` entry for that buffer specifying the action function `display-buffer-same-window`.
```
(customize-set-variable
'display-buffer-alist
(cons '("\\*foo\\*" (display-buffer-same-window))
display-buffer-alist))
```
| programming_docs |
elisp None #### Process Internals
The fields of a process (for a complete list, see the definition of `struct Lisp_Process` in `process.h`) include:
`name`
A Lisp string, the name of the process.
`command`
A list containing the command arguments that were used to start this process. For a network or serial process, it is `nil` if the process is running or `t` if the process is stopped.
`filter`
A Lisp function used to accept output from the process.
`sentinel`
A Lisp function called whenever the state of the process changes.
`buffer`
The associated buffer of the process.
`pid`
An integer, the operating system’s process ID. Pseudo-processes such as network or serial connections use a value of 0.
`childp`
A flag, `t` if this is really a child process. For a network or serial connection, it is a plist based on the arguments to `make-network-process` or `make-serial-process`.
`mark`
A marker indicating the position of the end of the last output from this process inserted into the buffer. This is often but not always the end of the buffer.
`kill_without_query`
If this is non-zero, killing Emacs while this process is still running does not ask for confirmation about killing the process.
`raw_status`
The raw process status, as returned by the `wait` system call.
`status`
The process status, as `process-status` should return it. This is a Lisp symbol, a cons cell, or a list.
`tick` `update_tick`
If these two fields are not equal, a change in the status of the process needs to be reported, either by running the sentinel or by inserting a message in the process buffer.
`pty_flag`
Non-zero if communication with the subprocess uses a pty; zero if it uses a pipe.
`infd`
The file descriptor for input from the process.
`outfd`
The file descriptor for output to the process.
`tty_name`
The name of the terminal that the subprocess is using, or `nil` if it is using pipes.
`decode_coding_system`
Coding-system for decoding the input from this process.
`decoding_buf`
A working buffer for decoding.
`decoding_carryover`
Size of carryover in decoding.
`encode_coding_system`
Coding-system for encoding the output to this process.
`encoding_buf`
A working buffer for encoding.
`inherit_coding_system_flag`
Flag to set `coding-system` of the process buffer from the coding system used to decode process output.
`type`
Symbol indicating the type of process: `real`, `network`, `serial`.
elisp None ### Creating Strings
The following functions create strings, either from scratch, or by putting strings together, or by taking them apart. (For functions that create strings based on the modified contents of other strings, like `string-replace` and `replace-regexp-in-string`, see [Search and Replace](search-and-replace).)
Function: **make-string** *count character &optional multibyte*
This function returns a string made up of count repetitions of character. If count is negative, an error is signaled.
```
(make-string 5 ?x)
⇒ "xxxxx"
(make-string 0 ?x)
⇒ ""
```
Normally, if character is an ASCII character, the result is a unibyte string. But if the optional argument multibyte is non-`nil`, the function will produce a multibyte string instead. This is useful when you later need to concatenate the result with non-ASCII strings or replace some of its characters with non-ASCII characters.
Other functions to compare with this one include `make-vector` (see [Vectors](vectors)) and `make-list` (see [Building Lists](building-lists)).
Function: **string** *&rest characters*
This returns a string containing the characters characters.
```
(string ?a ?b ?c)
⇒ "abc"
```
Function: **substring** *string &optional start end*
This function returns a new string which consists of those characters from string in the range from (and including) the character at the index start up to (but excluding) the character at the index end. The first character is at index zero. With one argument, this function just copies string.
```
(substring "abcdefg" 0 3)
⇒ "abc"
```
In the above example, the index for ‘`a`’ is 0, the index for ‘`b`’ is 1, and the index for ‘`c`’ is 2. The index 3—which is the fourth character in the string—marks the character position up to which the substring is copied. Thus, ‘`abc`’ is copied from the string `"abcdefg"`.
A negative number counts from the end of the string, so that -1 signifies the index of the last character of the string. For example:
```
(substring "abcdefg" -3 -1)
⇒ "ef"
```
In this example, the index for ‘`e`’ is -3, the index for ‘`f`’ is -2, and the index for ‘`g`’ is -1. Therefore, ‘`e`’ and ‘`f`’ are included, and ‘`g`’ is excluded.
When `nil` is used for end, it stands for the length of the string. Thus,
```
(substring "abcdefg" -3 nil)
⇒ "efg"
```
Omitting the argument end is equivalent to specifying `nil`. It follows that `(substring string 0)` returns a copy of all of string.
```
(substring "abcdefg" 0)
⇒ "abcdefg"
```
But we recommend `copy-sequence` for this purpose (see [Sequence Functions](sequence-functions)).
If the characters copied from string have text properties, the properties are copied into the new string also. See [Text Properties](text-properties).
`substring` also accepts a vector for the first argument. For example:
```
(substring [a b (c) "d"] 1 3)
⇒ [b (c)]
```
A `wrong-type-argument` error is signaled if start is not an integer or if end is neither an integer nor `nil`. An `args-out-of-range` error is signaled if start indicates a character following end, or if either integer is out of range for string.
Contrast this function with `buffer-substring` (see [Buffer Contents](buffer-contents)), which returns a string containing a portion of the text in the current buffer. The beginning of a string is at index 0, but the beginning of a buffer is at index 1.
Function: **substring-no-properties** *string &optional start end*
This works like `substring` but discards all text properties from the value. Also, start may be omitted or `nil`, which is equivalent to 0. Thus, `(substring-no-properties string)` returns a copy of string, with all text properties removed.
Function: **concat** *&rest sequences*
This function returns a string consisting of the characters in the arguments passed to it (along with their text properties, if any). The arguments may be strings, lists of numbers, or vectors of numbers; they are not themselves changed. If `concat` receives no arguments, it returns an empty string.
```
(concat "abc" "-def")
⇒ "abc-def"
(concat "abc" (list 120 121) [122])
⇒ "abcxyz"
;; `nil` is an empty sequence.
(concat "abc" nil "-def")
⇒ "abc-def"
(concat "The " "quick brown " "fox.")
⇒ "The quick brown fox."
(concat)
⇒ ""
```
This function does not always allocate a new string. Callers are advised not rely on the result being a new string nor on it being `eq` to an existing string.
In particular, mutating the returned value may inadvertently change another string, alter a constant string in the program, or even raise an error. To obtain a string that you can safely mutate, use `copy-sequence` on the result.
For information about other concatenation functions, see the description of `mapconcat` in [Mapping Functions](mapping-functions), `vconcat` in [Vector Functions](vector-functions), and `append` in [Building Lists](building-lists). For concatenating individual command-line arguments into a string to be used as a shell command, see [combine-and-quote-strings](shell-arguments).
Function: **split-string** *string &optional separators omit-nulls trim*
This function splits string into substrings based on the regular expression separators (see [Regular Expressions](regular-expressions)). Each match for separators defines a splitting point; the substrings between splitting points are made into a list, which is returned.
If separators is `nil` (or omitted), the default is the value of `split-string-default-separators` and the function behaves as if omit-nulls were `t`.
If omit-nulls is `nil` (or omitted), the result contains null strings whenever there are two consecutive matches for separators, or a match is adjacent to the beginning or end of string. If omit-nulls is `t`, these null strings are omitted from the result.
If the optional argument trim is non-`nil`, it should be a regular expression to match text to trim from the beginning and end of each substring. If trimming makes the substring empty, it is treated as null.
If you need to split a string into a list of individual command-line arguments suitable for `call-process` or `start-process`, see [split-string-and-unquote](shell-arguments).
Examples:
```
(split-string " two words ")
⇒ ("two" "words")
```
The result is not `("" "two" "words" "")`, which would rarely be useful. If you need such a result, use an explicit value for separators:
```
(split-string " two words "
split-string-default-separators)
⇒ ("" "two" "words" "")
```
```
(split-string "Soup is good food" "o")
⇒ ("S" "up is g" "" "d f" "" "d")
(split-string "Soup is good food" "o" t)
⇒ ("S" "up is g" "d f" "d")
(split-string "Soup is good food" "o+")
⇒ ("S" "up is g" "d f" "d")
```
Empty matches do count, except that `split-string` will not look for a final empty match when it already reached the end of the string using a non-empty match or when string is empty:
```
(split-string "aooob" "o*")
⇒ ("" "a" "" "b" "")
(split-string "ooaboo" "o*")
⇒ ("" "" "a" "b" "")
(split-string "" "")
⇒ ("")
```
However, when separators can match the empty string, omit-nulls is usually `t`, so that the subtleties in the three previous examples are rarely relevant:
```
(split-string "Soup is good food" "o*" t)
⇒ ("S" "u" "p" " " "i" "s" " " "g" "d" " " "f" "d")
(split-string "Nice doggy!" "" t)
⇒ ("N" "i" "c" "e" " " "d" "o" "g" "g" "y" "!")
(split-string "" "" t)
⇒ nil
```
Somewhat odd, but predictable, behavior can occur for certain “non-greedy” values of separators that can prefer empty matches over non-empty matches. Again, such values rarely occur in practice:
```
(split-string "ooo" "o*" t)
⇒ nil
(split-string "ooo" "\\|o+" t)
⇒ ("o" "o" "o")
```
Variable: **split-string-default-separators**
The default value of separators for `split-string`. Its usual value is `"[ \f\t\n\r\v]+"`.
Function: **string-clean-whitespace** *string*
Clean up the whitespace in string by collapsing stretches of whitespace to a single space character, as well as removing all whitespace from the start and the end of string.
Function: **string-trim-left** *string &optional regexp*
Remove the leading text that matches regexp from string. regexp defaults to ‘`[ \t\n\r]+`’.
Function: **string-trim-right** *string &optional regexp*
Remove the trailing text that matches regexp from string. regexp defaults to ‘`[ \t\n\r]+`’.
Function: **string-trim** *string &optional trim-left trim-right*
Remove the leading text that matches trim-left and trailing text that matches trim-right from string. Both regexps default to ‘`[ \t\n\r]+`’.
Function: **string-fill** *string length*
Attempt to Word-wrap string so that no lines are longer than length. Filling is done on whitespace boundaries only. If there are individual words that are longer than length, these will not be shortened.
Function: **string-limit** *string length &optional end coding-system*
If string is shorter than length characters, string is returned as is. Otherwise, return a substring of string consisting of the first length characters. If the optional end parameter is given, return a string of the length last characters instead.
If coding-system is non-`nil`, string will be encoded before limiting, and the result will be a unibyte string that’s shorter than `length` bytes. If string contains characters that are encoded into several bytes (for instance, when using `utf-8`), the resulting unibyte string is never truncated in the middle of a character representation.
This function measures the string length in characters or bytes, and thus is generally inappropriate if you need to shorten strings for display purposes; use `truncate-string-to-width` or `window-text-pixel-size` instead (see [Size of Displayed Text](size-of-displayed-text)).
Function: **string-lines** *string &optional omit-nulls*
Split string into a list of strings on newline boundaries. If omit-nulls, remove empty lines from the results.
Function: **string-pad** *string length &optional padding start*
Pad string to be of the given length using padding as the padding character. padding defaults to the space character. If string is longer than length, no padding is done. If start is `nil` or omitted, the padding is appended to the characters of string, and if it’s non-`nil`, the padding is prepended to string’s characters.
Function: **string-chop-newline** *string*
Remove the final newline, if any, from string.
elisp None ### Shorthands
The symbol *shorthands*, sometimes known as “renamed symbols”, are symbolic forms found in Lisp source. They’re just like regular symbolic forms, except that when the Lisp reader encounters them, it produces symbols which have a different and usually longer *print name* (see [Symbol Components](symbol-components)).
It is useful to think of shorthands as *abbreviating* the full names of intended symbols. Despite this, do not confuse shorthands with the Abbrev system see [Abbrevs](abbrevs).
Shorthands make Emacs Lisp’s *namespacing etiquette* easier to work with. Since all symbols are stored in a single obarray (see [Creating Symbols](creating-symbols)), programmers commonly prefix each symbol name with the name of the library where it originates. For example, the functions `text-property-search-forward` and `text-property-search-backward` both belong to the `text-property-search.el` library (see [Loading](loading)). By properly prefixing symbol names, one effectively prevents clashes between similarly named symbols which belong to different libraries and thus do different things. However, this practice commonly originates very long symbols names, which are inconvenient to type and read after a while. Shorthands solve these issues in a clean way.
Variable: **read-symbol-shorthands**
This variable’s value is an alist whose elements have the form `(shorthand-prefix . longhand-prefix)`. Each element instructs the Lisp reader to read every symbol form which starts with shorthand-prefix as if it started with longhand-prefix instead.
This variable may only be set in file-local variables (see [Local Variables in Files](https://www.gnu.org/software/emacs/manual/html_node/emacs/File-Variables.html#File-Variables) in The GNU Emacs Manual).
Here’s an example of shorthands usage in a hypothetical string manipulating library `some-nice-string-utils.el`.
```
(defun some-nice-string-utils-split (separator s &optional omit-nulls)
"A match-data saving variant of `split-string'."
(save-match-data (split-string s separator omit-nulls)))
(defun some-nice-string-utils-lines (s)
"Split string S at newline characters into a list of strings."
(some-nice-string-utils-split "\\(\r\n\\|[\n\r]\\)" s))
```
As can be seen, it’s quite tedious to read or develop this code since the symbol names to type are so long. We can use shorthands to alleviate that.
```
(defun snu-split (separator s &optional omit-nulls)
"A match-data saving variation on `split-string'."
(save-match-data (split-string s separator omit-nulls)))
(defun snu-lines (s)
"Split string S into a list of strings on newline characters."
(snu-split "\\(\r\n\\|[\n\r]\\)" s))
;; Local Variables:
;; read-symbol-shorthands: (("snu-" . "some-nice-string-utils-"))
;; End:
```
Even though the two excerpts look different, they are quite identical after the Lisp reader processes them. Both will lead to the very same symbols being interned (see [Creating Symbols](creating-symbols)). Thus loading or byte-compiling any of the two files has equivalent results. The shorthands `snu-split` and `snu-lines` used in the second version are *not* interned in the obarray. This is easily seen by moving point to the location where the shorthands are used and waiting for ElDoc (see [Local Variables in Files](https://www.gnu.org/software/emacs/manual/html_node/emacs/Lisp-Doc.html#Lisp-Doc) in The GNU Emacs Manual) to hint at the true full name of the symbol under point in the echo area.
Since `read-symbol-shorthands` is a file-local variable, it is possible that multiple libraries depending on `some-nice-string-utils-lines.el` refer to the same symbols under *different* shorthands, or not using shorthands at all. In the next example, the `my-tricks.el` library refers to the symbol `some-nice-string-utils-lines` using the `sns-` prefix instead of `snu-`.
```
(defun t-reverse-lines (s) (string-join (reverse (sns-lines s)) "\n")
;; Local Variables:
;; read-symbol-shorthands: (("t-" . "my-tricks-")
;; ("sns-" . "some-nice-string-utils-"))
;; End:
```
#### Exceptions
There are two exceptions to rules governing Shorthand transformations:
* Symbol forms comprised entirely of characters in the Emacs Lisp symbol constituent class (see [Syntax Class Table](syntax-class-table)) are not transformed. For example, it’s possible to use `-` or `/=` as shorthand prefixes, but that won’t shadow the arithmetic *functions* of those names.
* Symbol forms whose names start with ‘`#\_`’ are not transformed.
elisp None #### Primitives to manipulate advices
Macro: **add-function** *where place function &optional props*
This macro is the handy way to add the advice function to the function stored in place (see [Generalized Variables](generalized-variables)).
where determines how function is composed with the existing function, e.g., whether function should be called before, or after the original function. See [Advice Combinators](advice-combinators), for the list of available ways to compose the two functions.
When modifying a variable (whose name will usually end with `-function`), you can choose whether function is used globally or only in the current buffer: if place is just a symbol, then function is added to the global value of place. Whereas if place is of the form `(local symbol)`, where symbol is an expression which returns the variable name, then function will only be added in the current buffer. Finally, if you want to modify a lexical variable, you will have to use `(var variable)`.
Every function added with `add-function` can be accompanied by an association list of properties props. Currently only two of those properties have a special meaning:
`name`
This gives a name to the advice, which `remove-function` can use to identify which function to remove. Typically used when function is an anonymous function.
`depth`
This specifies how to order the advice, should several pieces of advice be present. By default, the depth is 0. A depth of 100 indicates that this piece of advice should be kept as deep as possible, whereas a depth of -100 indicates that it should stay as the outermost piece. When two pieces of advice specify the same depth, the most recently added one will be outermost.
For `:before` advice, being outermost means that this advice will be run first, before any other advice, whereas being innermost means that it will run right before the original function, with no other advice run between itself and the original function. Similarly, for `:after` advice innermost means that it will run right after the original function, with no other advice run in between, whereas outermost means that it will be run right at the end after all other advice. An innermost `:override` piece of advice will only override the original function and other pieces of advice will apply to it, whereas an outermost `:override` piece of advice will override not only the original function but all other advice applied to it as well.
If function is not interactive, then the combined function will inherit the interactive spec, if any, of the original function. Else, the combined function will be interactive and will use the interactive spec of function. One exception: if the interactive spec of function is a function (i.e., a `lambda` expression or an `fbound` symbol rather than an expression or a string), then the interactive spec of the combined function will be a call to that function with the interactive spec of the original function as sole argument. To interpret the spec received as argument, use `advice-eval-interactive-spec`.
Note: The interactive spec of function will apply to the combined function and should hence obey the calling convention of the combined function rather than that of function. In many cases, it makes no difference since they are identical, but it does matter for `:around`, `:filter-args`, and `:filter-return`, where function receives different arguments than the original function stored in place.
Macro: **remove-function** *place function*
This macro removes function from the function stored in place. This only works if function was added to place using `add-function`.
function is compared with functions added to place using `equal`, to try and make it work also with lambda expressions. It is additionally compared also with the `name` property of the functions added to place, which can be more reliable than comparing lambda expressions using `equal`.
Function: **advice-function-member-p** *advice function-def*
Return non-`nil` if advice is already in function-def. Like for `remove-function` above, instead of advice being the actual function, it can also be the `name` of the piece of advice.
Function: **advice-function-mapc** *f function-def*
Call the function f for every piece of advice that was added to function-def. f is called with two arguments: the advice function and its properties.
Function: **advice-eval-interactive-spec** *spec*
Evaluate the interactive spec just like an interactive call to a function with such a spec would, and then return the corresponding list of arguments that was built. E.g., `(advice-eval-interactive-spec "r\nP")` will return a list of three elements, containing the boundaries of the region and the current prefix argument.
For instance, if you want to make the `C-x m` (`compose-mail`) command prompt for a ‘`From:`’ header, you could say something like this:
```
(defun my-compose-mail-advice (orig &rest args)
"Read From: address interactively."
(interactive
(lambda (spec)
(let* ((user-mail-address
(completing-read "From: "
'("[email protected]"
"[email protected]")))
(from (message-make-from user-full-name
user-mail-address))
(spec (advice-eval-interactive-spec spec)))
;; Put the From header into the OTHER-HEADERS argument.
(push (cons 'From from) (nth 2 spec))
spec)))
(apply orig args))
(advice-add 'compose-mail :around #'my-compose-mail-advice)
```
| programming_docs |
elisp None #### Frame Configuration Type
A *frame configuration* stores information about the positions, sizes, and contents of the windows in all frames. It is not a primitive type—it is actually a list whose CAR is `frame-configuration` and whose CDR is an alist. Each alist element describes one frame, which appears as the CAR of that element.
See [Frame Configurations](frame-configurations), for a description of several functions related to frame configurations.
elisp None #### Looking Up Fonts
Function: **x-list-fonts** *name &optional reference-face frame maximum width*
This function returns a list of available font names that match name. name should be a string containing a font name in either the Fontconfig, GTK+, or XLFD format (see [Fonts](https://www.gnu.org/software/emacs/manual/html_node/emacs/Fonts.html#Fonts) in The GNU Emacs Manual). Within an XLFD string, wildcard characters may be used: the ‘`\*`’ character matches any substring, and the ‘`?`’ character matches any single character. Case is ignored when matching font names.
If the optional arguments reference-face and frame are specified, the returned list includes only fonts that are the same size as reference-face (a face name) currently is on the frame frame.
The optional argument maximum sets a limit on how many fonts to return. If it is non-`nil`, then the return value is truncated after the first maximum matching fonts. Specifying a small value for maximum can make this function much faster, in cases where many fonts match the pattern.
The optional argument width specifies a desired font width. If it is non-`nil`, the function only returns those fonts whose characters are (on average) width times as wide as reference-face.
Function: **x-family-fonts** *&optional family frame*
This function returns a list describing the available fonts for family family on frame. If family is omitted or `nil`, this list applies to all families, and therefore, it contains all available fonts. Otherwise, family must be a string; it may contain the wildcards ‘`?`’ and ‘`\*`’.
The list describes the display that frame is on; if frame is omitted or `nil`, it applies to the selected frame’s display (see [Input Focus](input-focus)).
Each element in the list is a vector of the following form:
```
[family width point-size weight slant
fixed-p full registry-and-encoding]
```
The first five elements correspond to face attributes; if you specify these attributes for a face, it will use this font.
The last three elements give additional information about the font. fixed-p is non-`nil` if the font is fixed-pitch. full is the full name of the font, and registry-and-encoding is a string giving the registry and encoding of the font.
elisp None ### Mapping Functions
A *mapping function* applies a given function (*not* a special form or macro) to each element of a list or other collection. Emacs Lisp has several such functions; this section describes `mapcar`, `mapc`, `mapconcat`, and `mapcan`, which map over a list. See [Definition of mapatoms](creating-symbols#Definition-of-mapatoms), for the function `mapatoms` which maps over the symbols in an obarray. See [Definition of maphash](hash-access#Definition-of-maphash), for the function `maphash` which maps over key/value associations in a hash table.
These mapping functions do not allow char-tables because a char-table is a sparse array whose nominal range of indices is very large. To map over a char-table in a way that deals properly with its sparse nature, use the function `map-char-table` (see [Char-Tables](char_002dtables)).
Function: **mapcar** *function sequence*
`mapcar` applies function to each element of sequence in turn, and returns a list of the results.
The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string. The result is always a list. The length of the result is the same as the length of sequence. For example:
```
(mapcar #'car '((a b) (c d) (e f)))
⇒ (a c e)
(mapcar #'1+ [1 2 3])
⇒ (2 3 4)
(mapcar #'string "abc")
⇒ ("a" "b" "c")
```
```
;; Call each function in `my-hooks`.
(mapcar 'funcall my-hooks)
```
```
(defun mapcar* (function &rest args)
"Apply FUNCTION to successive cars of all ARGS.
Return the list of results."
;; If no list is exhausted,
(if (not (memq nil args))
;; apply function to CARs.
(cons (apply function (mapcar #'car args))
(apply #'mapcar* function
;; Recurse for rest of elements.
(mapcar #'cdr args)))))
```
```
(mapcar* #'cons '(a b c) '(1 2 3 4))
⇒ ((a . 1) (b . 2) (c . 3))
```
Function: **mapcan** *function sequence*
This function applies function to each element of sequence, like `mapcar`, but instead of collecting the results into a list, it returns a single list with all the elements of the results (which must be lists), by altering the results (using `nconc`; see [Rearrangement](rearrangement)). Like with `mapcar`, sequence can be of any type except a char-table.
```
;; Contrast this:
(mapcar #'list '(a b c d))
⇒ ((a) (b) (c) (d))
;; with this:
(mapcan #'list '(a b c d))
⇒ (a b c d)
```
Function: **mapc** *function sequence*
`mapc` is like `mapcar` except that function is used for side-effects only—the values it returns are ignored, not collected into a list. `mapc` always returns sequence.
Function: **mapconcat** *function sequence separator*
`mapconcat` applies function to each element of sequence; the results, which must be sequences of characters (strings, vectors, or lists), are concatenated into a single string return value. Between each pair of result sequences, `mapconcat` inserts the characters from separator, which also must be a string, or a vector or list of characters. See [Sequences Arrays Vectors](sequences-arrays-vectors).
The argument function must be a function that can take one argument and returns a sequence of characters: a string, a vector, or a list. The argument sequence can be any kind of sequence except a char-table; that is, a list, a vector, a bool-vector, or a string.
```
(mapconcat #'symbol-name
'(The cat in the hat)
" ")
⇒ "The cat in the hat"
```
```
(mapconcat (lambda (x) (format "%c" (1+ x)))
"HAL-8000"
"")
⇒ "IBM.9111"
```
elisp None ### Security Considerations
Like any application, Emacs can be run in a secure environment, where the operating system enforces rules about access and the like. With some care, Emacs-based applications can also be part of a security perimeter that checks such rules. Although the default settings for Emacs work well for a typical software development environment, they may require adjustment in environments containing untrusted users that may include attackers. Here is a compendium of security issues that may be helpful if you are developing such applications. It is by no means complete; it is intended to give you an idea of the security issues involved, rather than to be a security checklist.
File local variables
A file that Emacs visits can contain variable settings that affect the buffer visiting that file; See [File Local Variables](file-local-variables). Similarly, a directory can specify local variable values common to all files in that directory; see [Directory Local Variables](directory-local-variables). Although Emacs takes some effort to protect against misuse of these variables, a security hole can be created merely by a package setting `safe-local-variable` too optimistically, a problem that is all too common. To disable this feature for both files and directories, set `enable-local-variables` to `nil`.
Access control
Although Emacs normally respects access permissions of the underlying operating system, in some cases it handles accesses specially. For example, file names can have handlers that treat the files specially, with their own access checking. See [Magic File Names](magic-file-names). Also, a buffer can be read-only even if the corresponding file is writable, and vice versa, which can result in messages such as ‘`File passwd is write-protected; try to save anyway? (yes or no)`’. See [Read Only Buffers](read-only-buffers).
Authentication
Emacs has several functions that deal with passwords, e.g., `read-passwd`. See [Reading a Password](reading-a-password). Although these functions do not attempt to broadcast passwords to the world, their implementations are not proof against determined attackers with access to Emacs internals. For example, even if Elisp code uses `clear-string` to scrub a password from its memory after using it, remnants of the password may still reside in the garbage-collected free list. See [Modifying Strings](modifying-strings).
Code injection
Emacs can send commands to many other applications, and applications should take care that strings sent as operands of these commands are not misinterpreted as directives. For example, when using a shell command to rename a file a to b, do not simply use the string `mv a b`, because either file name might start with ‘`-`’, or might contain shell metacharacters like ‘`;`’. Although functions like `shell-quote-argument` can help avoid this sort of problem, they are not panaceas; for example, on a POSIX platform `shell-quote-argument` quotes shell metacharacters but not leading ‘`-`’. On MS-Windows, quoting for ‘`%`’ assumes none of the environment variables have ‘`^`’ in their name. See [Shell Arguments](shell-arguments). Typically it is safer to use `call-process` than a subshell. See [Synchronous Processes](synchronous-processes). And it is safer yet to use builtin Emacs functions; for example, use `(rename-file "a" "b" t)` instead of invoking `mv`. See [Changing Files](changing-files).
Coding systems
Emacs attempts to infer the coding systems of the files and network connections it accesses. See [Coding Systems](coding-systems). If Emacs infers incorrectly, or if the other parties to the network connection disagree with Emacs’s inferences, the resulting system could be unreliable. Also, even when it infers correctly, Emacs often can use bytes that other programs cannot. For example, although to Emacs the null byte is just a character like any other, many other applications treat it as a string terminator and mishandle strings or files containing null bytes.
Environment and configuration variables
POSIX specifies several environment variables that can affect how Emacs behaves. Any environment variable whose name consists entirely of uppercase ASCII letters, digits, and the underscore may affect the internal behavior of Emacs. Emacs uses several such variables, e.g., `EMACSLOADPATH`. See [Library Search](library-search). On some platforms some environment variables (e.g., `PATH`, `POSIXLY_CORRECT`, `SHELL`, `TMPDIR`) need to have properly-configured values in order to get standard behavior for any utility Emacs might invoke. Even seemingly-benign variables like `TZ` may have security implications. See [System Environment](system-environment).
Emacs has customization and other variables with similar considerations. For example, if the variable `shell-file-name` specifies a shell with nonstandard behavior, an Emacs-based application may misbehave.
Installation
When Emacs is installed, if the installation directory hierarchy can be modified by untrusted users, the application cannot be trusted. This applies also to the directory hierarchies of the programs that Emacs uses, and of the files that Emacs reads and writes.
Network access
Emacs often accesses the network, and you may want to configure it to avoid network accesses that it would normally do. For example, unless you set `tramp-mode` to `nil`, file names using a certain syntax are interpreted as being network files, and are retrieved across the network. See [The Tramp Manual](https://www.gnu.org/software/emacs/manual/html_node/tramp/index.html#Top) in The Tramp Manual.
Race conditions
Emacs applications have the same sort of race-condition issues that other applications do. For example, even when `(file-readable-p "foo.txt")` returns `t`, it could be that `foo.txt` is unreadable because some other program changed the file’s permissions between the call to `file-readable-p` and now. See [Testing Accessibility](testing-accessibility).
Resource limits When Emacs exhausts memory or other operating system resources, its behavior can be less reliable, in that computations that ordinarily run to completion may abort back to the top level. This may cause Emacs to neglect operations that it normally would have done.
elisp None ### Scroll Bars
Normally the frame parameter `vertical-scroll-bars` controls whether the windows in the frame have vertical scroll bars, and whether they are on the left or right. The frame parameter `scroll-bar-width` specifies how wide they are (`nil` meaning the default).
The frame parameter `horizontal-scroll-bars` controls whether the windows in the frame have horizontal scroll bars. The frame parameter `scroll-bar-height` specifies how high they are (`nil` meaning the default). See [Layout Parameters](layout-parameters).
Horizontal scroll bars are not available on all platforms. The function `horizontal-scroll-bars-available-p` which takes no argument returns non-`nil` if they are available on your system.
The following three functions take as argument a live frame which defaults to the selected one.
Function: **frame-current-scroll-bars** *&optional frame*
This function reports the scroll bar types for frame frame. The value is a cons cell `(vertical-type .
horizontal-type)`, where vertical-type is either `left`, `right`, or `nil` (which means no vertical scroll bar.) horizontal-type is either `bottom` or `nil` (which means no horizontal scroll bar).
Function: **frame-scroll-bar-width** *&optional frame*
This function returns the width of vertical scroll bars of frame in pixels.
Function: **frame-scroll-bar-height** *&optional frame*
This function returns the height of horizontal scroll bars of frame in pixels.
You can override the frame specific settings for individual windows by using the following function:
Function: **set-window-scroll-bars** *window &optional width vertical-type height horizontal-type persistent*
This function sets the width and/or height and the types of scroll bars for window window. If window is `nil`, the selected window is used.
width specifies the width of the vertical scroll bar in pixels (`nil` means use the width specified for the frame). vertical-type specifies whether to have a vertical scroll bar and, if so, where. The possible values are `left`, `right`, `t`, which means to use the frame’s default, and `nil` for no vertical scroll bar.
height specifies the height of the horizontal scroll bar in pixels (`nil` means use the height specified for the frame). horizontal-type specifies whether to have a horizontal scroll bar. The possible values are `bottom`, `t`, which means to use the frame’s default, and `nil` for no horizontal scroll bar. Note that for a mini window the value `t` has the same meaning as `nil`, namely to not show a horizontal scroll bar. You have to explicitly specify `bottom` in order to show a horizontal scroll bar in a mini window.
If window is not large enough to accommodate a scroll bar of the desired dimension, this leaves the corresponding scroll bar unchanged.
The values specified here may be later overridden by invoking `set-window-buffer` (see [Buffers and Windows](buffers-and-windows)) on window with its keep-margins argument `nil` or omitted. However, if the optional fifth argument persistent is non-`nil` and the other arguments are processed successfully, the values specified here unconditionally survive subsequent invocations of `set-window-buffer`.
Using the persistent argument of `set-window-scroll-bars` and `set-window-fringes` (see [Fringe Size/Pos](fringe-size_002fpos)) you can reliably and permanently turn off scroll bars and/or fringes in any minibuffer window by adding the following snippet to your early init file (see [Init File](init-file)).
```
(add-hook 'after-make-frame-functions
(lambda (frame)
(set-window-scroll-bars
(minibuffer-window frame) 0 nil 0 nil t)
(set-window-fringes
(minibuffer-window frame) 0 0 nil t)))
```
The following four functions take as argument a live window which defaults to the selected one.
Function: **window-scroll-bars** *&optional window*
This function returns a list of the form `(width
columns vertical-type height lines
horizontal-type persistent)`.
The value width is the value that was specified for the width of the vertical scroll bar (which may be `nil`); columns is the (possibly rounded) number of columns that the vertical scroll bar actually occupies.
The value height is the value that was specified for the height of the horizontal scroll bar (which may be `nil`); lines is the (possibly rounded) number of lines that the horizontally scroll bar actually occupies.
The value of persistent is the value specified for window with the last successful invocation of `set-window-scroll-bars`, `nil` if there never was one.
Function: **window-current-scroll-bars** *&optional window*
This function reports the scroll bar type for window window. The value is a cons cell `(vertical-type .
horizontal-type)`. Unlike `window-scroll-bars`, this reports the scroll bar type actually used, once frame defaults and `scroll-bar-mode` are taken into account.
Function: **window-scroll-bar-width** *&optional window*
This function returns the width in pixels of window’s vertical scrollbar.
Function: **window-scroll-bar-height** *&optional window*
This function returns the height in pixels of window’s horizontal scrollbar.
If you do not specify a window’s scroll bar settings via `set-window-scroll-bars`, the buffer-local variables `vertical-scroll-bar`, `horizontal-scroll-bar`, `scroll-bar-width` and `scroll-bar-height` in the buffer being displayed control the window’s scroll bars. The function `set-window-buffer` examines these variables. If you change them in a buffer that is already visible in a window, you can make the window take note of the new values by calling `set-window-buffer` specifying the same buffer that is already displayed.
You can control the appearance of scroll bars for a particular buffer by setting the following variables which automatically become buffer-local when set.
Variable: **vertical-scroll-bar**
This variable specifies the location of the vertical scroll bar. The possible values are `left`, `right`, `t`, which means to use the frame’s default, and `nil` for no scroll bar.
Variable: **horizontal-scroll-bar**
This variable specifies the location of the horizontal scroll bar. The possible values are `bottom`, `t`, which means to use the frame’s default, and `nil` for no scroll bar.
Variable: **scroll-bar-width**
This variable specifies the width of the buffer’s vertical scroll bars, measured in pixels. A value of `nil` means to use the value specified by the frame.
Variable: **scroll-bar-height**
This variable specifies the height of the buffer’s horizontal scroll bar, measured in pixels. A value of `nil` means to use the value specified by the frame.
Finally you can toggle the display of scroll bars on all frames by customizing the variables `scroll-bar-mode` and `horizontal-scroll-bar-mode`.
User Option: **scroll-bar-mode**
This variable controls whether and where to put vertical scroll bars in all frames. The possible values are `nil` for no scroll bars, `left` to put scroll bars on the left and `right` to put scroll bars on the right.
User Option: **horizontal-scroll-bar-mode**
This variable controls whether to display horizontal scroll bars on all frames.
| programming_docs |
elisp None ### Comments
A *comment* is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, an unescaped semicolon (‘`;`’) starts a comment if it is not within a string or character constant. The comment continues to the end of line. The Lisp reader discards comments; they do not become part of the Lisp objects which represent the program within the Lisp system.
The ‘`#@count`’ construct, which skips the next count characters, is useful for program-generated comments containing binary data. The Emacs Lisp byte compiler uses this in its output files (see [Byte Compilation](byte-compilation)). It isn’t meant for source files, however.
See [Comment Tips](https://www.gnu.org/software/emacs/manual/html_node/elisp/Comment-Tips.html), for conventions for formatting comments.
elisp None #### Basic Parameters
These frame parameters give the most basic information about the frame. `title` and `name` are meaningful on all terminals.
`display`
The display on which to open this frame. It should be a string of the form ‘`host:dpy.screen`’, just like the `DISPLAY` environment variable. See [Multiple Terminals](multiple-terminals), for more details about display names.
`display-type`
This parameter describes the range of possible colors that can be used in this frame. Its value is `color`, `grayscale` or `mono`.
`title`
If a frame has a non-`nil` title, it appears in the window system’s title bar at the top of the frame, and also in the mode line of windows in that frame if `mode-line-frame-identification` uses ‘`%F`’ (see [%-Constructs](_0025_002dconstructs)). This is normally the case when Emacs is not using a window system, and can only display one frame at a time. See [Frame Titles](frame-titles).
`name`
The name of the frame. The frame name serves as a default for the frame title, if the `title` parameter is unspecified or `nil`. If you don’t specify a name, Emacs sets the frame name automatically (see [Frame Titles](frame-titles)).
If you specify the frame name explicitly when you create the frame, the name is also used (instead of the name of the Emacs executable) when looking up X resources for the frame.
`explicit-name` If the frame name was specified explicitly when the frame was created, this parameter will be that name. If the frame wasn’t explicitly named, this parameter will be `nil`.
elisp None ### Character Codes
The unibyte and multibyte text representations use different character codes. The valid character codes for unibyte representation range from 0 to `#xFF` (255)—the values that can fit in one byte. The valid character codes for multibyte representation range from 0 to `#x3FFFFF`. In this code space, values 0 through `#x7F` (127) are for ASCII characters, and values `#x80` (128) through `#x3FFF7F` (4194175) are for non-ASCII characters.
Emacs character codes are a superset of the Unicode standard. Values 0 through `#x10FFFF` (1114111) correspond to Unicode characters of the same codepoint; values `#x110000` (1114112) through `#x3FFF7F` (4194175) represent characters that are not unified with Unicode; and values `#x3FFF80` (4194176) through `#x3FFFFF` (4194303) represent eight-bit raw bytes.
Function: **characterp** *charcode*
This returns `t` if charcode is a valid character, and `nil` otherwise.
```
(characterp 65)
⇒ t
```
```
(characterp 4194303)
⇒ t
```
```
(characterp 4194304)
⇒ nil
```
Function: **max-char**
This function returns the largest value that a valid character codepoint can have.
```
(characterp (max-char))
⇒ t
```
```
(characterp (1+ (max-char)))
⇒ nil
```
Function: **char-from-name** *string &optional ignore-case*
This function returns the character whose Unicode name is string. If ignore-case is non-`nil`, case is ignored in string. This function returns `nil` if string does not name a character.
```
;; U+03A3
(= (char-from-name "GREEK CAPITAL LETTER SIGMA") #x03A3)
⇒ t
```
Function: **get-byte** *&optional pos string*
This function returns the byte at character position pos in the current buffer. If the current buffer is unibyte, this is literally the byte at that position. If the buffer is multibyte, byte values of ASCII characters are the same as character codepoints, whereas eight-bit raw bytes are converted to their 8-bit codes. The function signals an error if the character at pos is non-ASCII.
The optional argument string means to get a byte value from that string instead of the current buffer.
elisp None ### Operating System Environment
Emacs provides access to variables in the operating system environment through various functions. These variables include the name of the system, the user’s UID, and so on.
Variable: **system-configuration**
This variable holds the standard GNU configuration name for the hardware/software configuration of your system, as a string. For example, a typical value for a 64-bit GNU/Linux system is ‘`"x86\_64-unknown-linux-gnu"`’.
Variable: **system-type**
The value of this variable is a symbol indicating the type of operating system Emacs is running on. The possible values are:
`aix`
IBM’s AIX.
`berkeley-unix`
Berkeley BSD and its variants.
`cygwin`
Cygwin, a POSIX layer on top of MS-Windows.
`darwin`
Darwin (macOS).
`gnu`
The GNU system (using the GNU kernel, which consists of the HURD and Mach).
`gnu/linux`
A GNU/Linux system—that is, a variant GNU system, using the Linux kernel. (These systems are the ones people often call “Linux”, but actually Linux is just the kernel, not the whole system.)
`gnu/kfreebsd`
A GNU (glibc-based) system with a FreeBSD kernel.
`hpux`
Hewlett-Packard HPUX operating system.
`nacl`
Google Native Client (NaCl) sandboxing system.
`ms-dos`
Microsoft’s DOS. Emacs compiled with DJGPP for MS-DOS binds `system-type` to `ms-dos` even when you run it on MS-Windows.
`usg-unix-v`
AT&T Unix System V.
`windows-nt`
Microsoft Windows NT, 9X and later. The value of `system-type` is always `windows-nt`, e.g., even on Windows 10.
We do not wish to add new symbols to make finer distinctions unless it is absolutely necessary! In fact, we hope to eliminate some of these alternatives in the future. If you need to make a finer distinction than `system-type` allows for, you can test `system-configuration`, e.g., against a regexp.
Function: **system-name**
This function returns the name of the machine you are running on, as a string.
User Option: **mail-host-address**
If this variable is non-`nil`, it is used instead of `system-name` for purposes of generating email addresses. For example, it is used when constructing the default value of `user-mail-address`. See [User Identification](user-identification).
Command: **getenv** *var &optional frame*
This function returns the value of the environment variable var, as a string. var should be a string. If var is undefined in the environment, `getenv` returns `nil`. It returns ‘`""`’ if var is set but null. Within Emacs, a list of environment variables and their values is kept in the variable `process-environment`.
```
(getenv "USER")
⇒ "lewis"
```
The shell command `printenv` prints all or part of the environment:
```
bash$ printenv
PATH=/usr/local/bin:/usr/bin:/bin
USER=lewis
```
```
TERM=xterm
SHELL=/bin/bash
HOME=/home/lewis
```
```
…
```
Command: **setenv** *variable &optional value substitute*
This command sets the value of the environment variable named variable to value. variable should be a string. Internally, Emacs Lisp can handle any string. However, normally variable should be a valid shell identifier, that is, a sequence of letters, digits and underscores, starting with a letter or underscore. Otherwise, errors may occur if subprocesses of Emacs try to access the value of variable. If value is omitted or `nil` (or, interactively, with a prefix argument), `setenv` removes variable from the environment. Otherwise, value should be a string.
If the optional argument substitute is non-`nil`, Emacs calls the function `substitute-env-vars` to expand any environment variables in value.
`setenv` works by modifying `process-environment`; binding that variable with `let` is also reasonable practice.
`setenv` returns the new value of variable, or `nil` if it removed variable from the environment.
Macro: **with-environment-variables** *variables body…*
This macro sets the environment variables according to variables temporarily when executing body. The previous values are restored when the form finishes. The argument variables should be a list of pairs of strings of the form `(var value)`, where var is the name of the environment variable and value is that variable’s value.
```
(with-environment-variables (("LANG" "C")
("LANGUAGE" "en_US:en"))
(call-process "ls" nil t))
```
Variable: **process-environment**
This variable is a list of strings, each describing one environment variable. The functions `getenv` and `setenv` work by means of this variable.
```
process-environment
⇒ ("PATH=/usr/local/bin:/usr/bin:/bin"
"USER=lewis"
```
```
"TERM=xterm"
"SHELL=/bin/bash"
"HOME=/home/lewis"
…)
```
If `process-environment` contains multiple elements that specify the same environment variable, the first of these elements specifies the variable, and the others are ignored.
Variable: **initial-environment**
This variable holds the list of environment variables Emacs inherited from its parent process when Emacs started.
Variable: **path-separator**
This variable holds a string that says which character separates directories in a search path (as found in an environment variable). Its value is `":"` for Unix and GNU systems, and `";"` for MS systems.
Function: **path-separator**
This function returns the connection-local value of variable `path-separator`. That is `";"` for MS systems and a local `default-directory`, and `":"` for Unix and GNU systems, or a remote `default-directory`.
Function: **parse-colon-path** *path*
This function takes a search path string such as the value of the `PATH` environment variable, and splits it at the separators, returning a list of directories. `nil` in this list means the current directory. Although the function’s name says “colon”, it actually uses the value of variable `path-separator`.
```
(parse-colon-path ":/foo:/bar")
⇒ (nil "/foo/" "/bar/")
```
Variable: **invocation-name**
This variable holds the program name under which Emacs was invoked. The value is a string, and does not include a directory name.
Variable: **invocation-directory**
This variable holds the directory in which the Emacs executable was located when it was run, or `nil` if that directory cannot be determined.
Variable: **installation-directory**
If non-`nil`, this is a directory within which to look for the `lib-src` and `etc` subdirectories. In an installed Emacs, it is normally `nil`. It is non-`nil` when Emacs can’t find those directories in their standard installed locations, but can find them in a directory related somehow to the one containing the Emacs executable (i.e., `invocation-directory`).
Function: **load-average** *&optional use-float*
This function returns the current 1-minute, 5-minute, and 15-minute system load averages, in a list. The load average indicates the number of processes trying to run on the system.
By default, the values are integers that are 100 times the system load averages, but if use-float is non-`nil`, then they are returned as floating-point numbers without multiplying by 100.
If it is impossible to obtain the load average, this function signals an error. On some platforms, access to load averages requires installing Emacs as setuid or setgid so that it can read kernel information, and that usually isn’t advisable.
If the 1-minute load average is available, but the 5- or 15-minute averages are not, this function returns a shortened list containing the available averages.
```
(load-average)
⇒ (169 48 36)
```
```
(load-average t)
⇒ (1.69 0.48 0.36)
```
The shell command `uptime` returns similar information.
Function: **emacs-pid**
This function returns the process ID of the Emacs process, as an integer.
Variable: **tty-erase-char**
This variable holds the erase character that was selected in the system’s terminal driver, before Emacs was started.
Variable: **null-device**
This variable holds the system null device. Its value is `"/dev/null"` for Unix and GNU systems, and `"NUL"` for MS systems.
Function: **null-device**
This function returns the connection-local value of variable `null-device`. That is `"NUL"` for MS systems and a local `default-directory`, and `"/dev/null"` for Unix and GNU systems, or a remote `default-directory`.
elisp None ### Changing Key Bindings
The way to rebind a key is to change its entry in a keymap. If you change a binding in the global keymap, the change is effective in all buffers (though it has no direct effect in buffers that shadow the global binding with a local one). If you change the current buffer’s local map, that usually affects all buffers using the same major mode. The `global-set-key` and `local-set-key` functions are convenient interfaces for these operations (see [Key Binding Commands](key-binding-commands)). You can also use `define-key`, a more general function; then you must explicitly specify the map to change.
When choosing the key sequences for Lisp programs to rebind, please follow the Emacs conventions for use of various keys (see [Key Binding Conventions](https://www.gnu.org/software/emacs/manual/html_node/elisp/Key-Binding-Conventions.html)).
In writing the key sequence to rebind, it is good to use the special escape sequences for control and meta characters (see [String Type](string-type)). The syntax ‘`\C-`’ means that the following character is a control character and ‘`\M-`’ means that the following character is a meta character. Thus, the string `"\M-x"` is read as containing a single `M-x`, `"\C-f"` is read as containing a single `C-f`, and `"\M-\C-x"` and `"\C-\M-x"` are both read as containing a single `C-M-x`. You can also use this escape syntax in vectors, as well as others that aren’t allowed in strings; one example is ‘`[?\C-\H-x home]`’. See [Character Type](character-type).
The key definition and lookup functions accept an alternate syntax for event types in a key sequence that is a vector: you can use a list containing modifier names plus one base event (a character or function key name). For example, `(control ?a)` is equivalent to `?\C-a` and `(hyper control left)` is equivalent to `C-H-left`. One advantage of such lists is that the precise numeric codes for the modifier bits don’t appear in compiled files.
The functions below signal an error if keymap is not a keymap, or if key is not a string or vector representing a key sequence. You can use event types (symbols) as shorthand for events that are lists. The `kbd` function (see [Key Sequences](key-sequences)) is a convenient way to specify the key sequence.
Function: **define-key** *keymap key binding*
This function sets the binding for key in keymap. (If key is more than one event long, the change is actually made in another keymap reached from keymap.) The argument binding can be any Lisp object, but only certain types are meaningful. (For a list of meaningful types, see [Key Lookup](key-lookup).) The value returned by `define-key` is binding.
If key is `[t]`, this sets the default binding in keymap. When an event has no binding of its own, the Emacs command loop uses the keymap’s default binding, if there is one.
Every prefix of key must be a prefix key (i.e., bound to a keymap) or undefined; otherwise an error is signaled. If some prefix of key is undefined, then `define-key` defines it as a prefix key so that the rest of key can be defined as specified.
If there was previously no binding for key in keymap, the new binding is added at the beginning of keymap. The order of bindings in a keymap makes no difference for keyboard input, but it does matter for menu keymaps (see [Menu Keymaps](menu-keymaps)).
This example creates a sparse keymap and makes a number of bindings in it:
```
(setq map (make-sparse-keymap))
⇒ (keymap)
```
```
(define-key map "\C-f" 'forward-char)
⇒ forward-char
```
```
map
⇒ (keymap (6 . forward-char))
```
```
;; Build sparse submap for `C-x` and bind `f` in that.
(define-key map (kbd "C-x f") 'forward-word)
⇒ forward-word
```
```
map
⇒ (keymap
(24 keymap ; C-x
(102 . forward-word)) ; f
(6 . forward-char)) ; C-f
```
```
;; Bind `C-p` to the `ctl-x-map`.
(define-key map (kbd "C-p") ctl-x-map)
;; ctl-x-map
⇒ [nil … find-file … backward-kill-sentence]
```
```
;; Bind `C-f` to `foo` in the `ctl-x-map`.
(define-key map (kbd "C-p C-f") 'foo)
⇒ 'foo
```
```
map
⇒ (keymap ; Note `foo` in `ctl-x-map`.
(16 keymap [nil … foo … backward-kill-sentence])
(24 keymap
(102 . forward-word))
(6 . forward-char))
```
Note that storing a new binding for `C-p C-f` actually works by changing an entry in `ctl-x-map`, and this has the effect of changing the bindings of both `C-p C-f` and `C-x C-f` in the default global map.
The function `substitute-key-definition` scans a keymap for keys that have a certain binding and rebinds them with a different binding. Another feature which is cleaner and can often produce the same results is to remap one command into another (see [Remapping Commands](remapping-commands)).
Function: **substitute-key-definition** *olddef newdef keymap &optional oldmap*
This function replaces olddef with newdef for any keys in keymap that were bound to olddef. In other words, olddef is replaced with newdef wherever it appears. The function returns `nil`.
For example, this redefines `C-x C-f`, if you do it in an Emacs with standard bindings:
```
(substitute-key-definition
'find-file 'find-file-read-only (current-global-map))
```
If oldmap is non-`nil`, that changes the behavior of `substitute-key-definition`: the bindings in oldmap determine which keys to rebind. The rebindings still happen in keymap, not in oldmap. Thus, you can change one map under the control of the bindings in another. For example,
```
(substitute-key-definition
'delete-backward-char 'my-funny-delete
my-map global-map)
```
puts the special deletion command in `my-map` for whichever keys are globally bound to the standard deletion command.
Here is an example showing a keymap before and after substitution:
```
(setq map (list 'keymap
(cons ?1 olddef-1)
(cons ?2 olddef-2)
(cons ?3 olddef-1)))
⇒ (keymap (49 . olddef-1) (50 . olddef-2) (51 . olddef-1))
```
```
(substitute-key-definition 'olddef-1 'newdef map)
⇒ nil
```
```
map
⇒ (keymap (49 . newdef) (50 . olddef-2) (51 . newdef))
```
Function: **suppress-keymap** *keymap &optional nodigits*
This function changes the contents of the full keymap keymap by remapping `self-insert-command` to the command `undefined` (see [Remapping Commands](remapping-commands)). This has the effect of undefining all printing characters, thus making ordinary insertion of text impossible. `suppress-keymap` returns `nil`.
If nodigits is `nil`, then `suppress-keymap` defines digits to run `digit-argument`, and `-` to run `negative-argument`. Otherwise it makes them undefined like the rest of the printing characters.
The `suppress-keymap` function does not make it impossible to modify a buffer, as it does not suppress commands such as `yank` and `quoted-insert`. To prevent any modification of a buffer, make it read-only (see [Read Only Buffers](read-only-buffers)).
Since this function modifies keymap, you would normally use it on a newly created keymap. Operating on an existing keymap that is used for some other purpose is likely to cause trouble; for example, suppressing `global-map` would make it impossible to use most of Emacs.
This function can be used to initialize the local keymap of a major mode for which insertion of text is not desirable. But usually such a mode should be derived from `special-mode` (see [Basic Major Modes](basic-major-modes)); then its keymap will automatically inherit from `special-mode-map`, which is already suppressed. Here is how `special-mode-map` is defined:
```
(defvar special-mode-map
(let ((map (make-sparse-keymap)))
(suppress-keymap map)
(define-key map "q" 'quit-window)
…
map))
```
| programming_docs |
elisp None ### Generators
A *generator* is a function that produces a potentially-infinite stream of values. Each time the function produces a value, it suspends itself and waits for a caller to request the next value.
Macro: **iter-defun** *name args [doc] [declare] [interactive] body…*
`iter-defun` defines a generator function. A generator function has the same signature as a normal function, but works differently. Instead of executing body when called, a generator function returns an iterator object. That iterator runs body to generate values, emitting a value and pausing where `iter-yield` or `iter-yield-from` appears. When body returns normally, `iter-next` signals `iter-end-of-sequence` with body’s result as its condition data.
Any kind of Lisp code is valid inside body, but `iter-yield` and `iter-yield-from` cannot appear inside `unwind-protect` forms.
Macro: **iter-lambda** *args [doc] [interactive] body…*
`iter-lambda` produces an unnamed generator function that works just like a generator function produced with `iter-defun`.
Macro: **iter-yield** *value*
When it appears inside a generator function, `iter-yield` indicates that the current iterator should pause and return value from `iter-next`. `iter-yield` evaluates to the `value` parameter of next call to `iter-next`.
Macro: **iter-yield-from** *iterator*
`iter-yield-from` yields all the values that iterator produces and evaluates to the value that iterator’s generator function returns normally. While it has control, iterator receives values sent to the iterator using `iter-next`.
To use a generator function, first call it normally, producing a *iterator* object. An iterator is a specific instance of a generator. Then use `iter-next` to retrieve values from this iterator. When there are no more values to pull from an iterator, `iter-next` raises an `iter-end-of-sequence` condition with the iterator’s final value.
It’s important to note that generator function bodies only execute inside calls to `iter-next`. A call to a function defined with `iter-defun` produces an iterator; you must drive this iterator with `iter-next` for anything interesting to happen. Each call to a generator function produces a *different* iterator, each with its own state.
Function: **iter-next** *iterator value*
Retrieve the next value from iterator. If there are no more values to be generated (because iterator’s generator function returned), `iter-next` signals the `iter-end-of-sequence` condition; the data value associated with this condition is the value with which iterator’s generator function returned.
value is sent into the iterator and becomes the value to which `iter-yield` evaluates. value is ignored for the first `iter-next` call to a given iterator, since at the start of iterator’s generator function, the generator function is not evaluating any `iter-yield` form.
Function: **iter-close** *iterator*
If iterator is suspended inside an `unwind-protect`’s `bodyform` and becomes unreachable, Emacs will eventually run unwind handlers after a garbage collection pass. (Note that `iter-yield` is illegal inside an `unwind-protect`’s `unwindforms`.) To ensure that these handlers are run before then, use `iter-close`.
Some convenience functions are provided to make working with iterators easier:
Macro: **iter-do** *(var iterator) body …*
Run body with var bound to each value that iterator produces.
The Common Lisp loop facility also contains features for working with iterators. See [Loop Facility](https://www.gnu.org/software/emacs/manual/html_node/cl/Loop-Facility.html#Loop-Facility) in Common Lisp Extensions.
The following piece of code demonstrates some important principles of working with iterators.
```
(require 'generator)
(iter-defun my-iter (x)
(iter-yield (1+ (iter-yield (1+ x))))
;; Return normally
-1)
(let* ((iter (my-iter 5))
(iter2 (my-iter 0)))
;; Prints 6
(print (iter-next iter))
;; Prints 9
(print (iter-next iter 8))
;; Prints 1; iter and iter2 have distinct states
(print (iter-next iter2 nil))
;; We expect the iter sequence to end now
(condition-case x
(iter-next iter)
(iter-end-of-sequence
;; Prints -1, which my-iter returned normally
(print (cdr x)))))
```
elisp None #### Evaluation List Buffer
You can use the *evaluation list buffer*, called `\*edebug\*`, to evaluate expressions interactively. You can also set up the *evaluation list* of expressions to be evaluated automatically each time Edebug updates the display.
`E` Switch to the evaluation list buffer `\*edebug\*` (`edebug-visit-eval-list`).
In the `\*edebug\*` buffer you can use the commands of Lisp Interaction mode (see [Lisp Interaction](https://www.gnu.org/software/emacs/manual/html_node/emacs/Lisp-Interaction.html#Lisp-Interaction) in The GNU Emacs Manual) as well as these special commands:
`C-j`
Evaluate the expression before point, in the outside context, and insert the value in the buffer (`edebug-eval-print-last-sexp`). With prefix argument of zero (`C-u 0 C-j`), don’t shorten long items (like strings and lists).
`C-x C-e`
Evaluate the expression before point, in the context outside of Edebug (`edebug-eval-last-sexp`).
`C-c C-u`
Build a new evaluation list from the contents of the buffer (`edebug-update-eval-list`).
`C-c C-d`
Delete the evaluation list group that point is in (`edebug-delete-eval-item`).
`C-c C-w` Switch back to the source code buffer at the current stop point (`edebug-where`).
You can evaluate expressions in the evaluation list window with `C-j` or `C-x C-e`, just as you would in `\*scratch\*`; but they are evaluated in the context outside of Edebug.
The expressions you enter interactively (and their results) are lost when you continue execution; but you can set up an *evaluation list* consisting of expressions to be evaluated each time execution stops.
To do this, write one or more *evaluation list groups* in the evaluation list buffer. An evaluation list group consists of one or more Lisp expressions. Groups are separated by comment lines.
The command `C-c C-u` (`edebug-update-eval-list`) rebuilds the evaluation list, scanning the buffer and using the first expression of each group. (The idea is that the second expression of the group is the value previously computed and displayed.)
Each entry to Edebug redisplays the evaluation list by inserting each expression in the buffer, followed by its current value. It also inserts comment lines so that each expression becomes its own group. Thus, if you type `C-c C-u` again without changing the buffer text, the evaluation list is effectively unchanged.
If an error occurs during an evaluation from the evaluation list, the error message is displayed in a string as if it were the result. Therefore, expressions using variables that are not currently valid do not interrupt your debugging.
Here is an example of what the evaluation list window looks like after several expressions have been added to it:
```
(current-buffer)
#<buffer *scratch*>
;---------------------------------------------------------------
(selected-window)
#<window 16 on *scratch*>
;---------------------------------------------------------------
(point)
196
;---------------------------------------------------------------
bad-var
"Symbol's value as variable is void: bad-var"
;---------------------------------------------------------------
(recursion-depth)
0
;---------------------------------------------------------------
this-command
eval-last-sexp
;---------------------------------------------------------------
```
To delete a group, move point into it and type `C-c C-d`, or simply delete the text for the group and update the evaluation list with `C-c C-u`. To add a new expression to the evaluation list, insert the expression at a suitable place, insert a new comment line, then type `C-c C-u`. You need not insert dashes in the comment line—its contents don’t matter.
After selecting `\*edebug\*`, you can return to the source code buffer with `C-c C-w`. The `\*edebug\*` buffer is killed when you continue execution, and recreated next time it is needed.
elisp None ### Temporary Displays
Temporary displays are used by Lisp programs to put output into a buffer and then present it to the user for perusal rather than for editing. Many help commands use this feature.
Macro: **with-output-to-temp-buffer** *buffer-name body…*
This function executes the forms in body while arranging to insert any output they print into the buffer named buffer-name, which is first created if necessary, and put into Help mode. (See the similar form `with-temp-buffer-window` below.) Finally, the buffer is displayed in some window, but that window is not selected.
If the forms in body do not change the major mode in the output buffer, so that it is still Help mode at the end of their execution, then `with-output-to-temp-buffer` makes this buffer read-only at the end, and also scans it for function and variable names to make them into clickable cross-references. See [Tips for Documentation Strings](https://www.gnu.org/software/emacs/manual/html_node/elisp/Documentation-Tips.html#Docstring-hyperlinks), in particular the item on hyperlinks in documentation strings, for more details.
The string buffer-name specifies the temporary buffer, which need not already exist. The argument must be a string, not a buffer. The buffer is erased initially (with no questions asked), and it is marked as unmodified after `with-output-to-temp-buffer` exits.
`with-output-to-temp-buffer` binds `standard-output` to the temporary buffer, then it evaluates the forms in body. Output using the Lisp output functions within body goes by default to that buffer (but screen display and messages in the echo area, although they are “output” in the general sense of the word, are not affected). See [Output Functions](output-functions).
Several hooks are available for customizing the behavior of this construct; they are listed below.
The value of the last form in body is returned.
```
---------- Buffer: foo ----------
This is the contents of foo.
---------- Buffer: foo ----------
```
```
(with-output-to-temp-buffer "foo"
(print 20)
(print standard-output))
⇒ #<buffer foo>
---------- Buffer: foo ----------
20
#<buffer foo>
---------- Buffer: foo ----------
```
User Option: **temp-buffer-show-function**
If this variable is non-`nil`, `with-output-to-temp-buffer` calls it as a function to do the job of displaying a help buffer. The function gets one argument, which is the buffer it should display.
It is a good idea for this function to run `temp-buffer-show-hook` just as `with-output-to-temp-buffer` normally would, inside of `save-selected-window` and with the chosen window and buffer selected.
Variable: **temp-buffer-setup-hook**
This normal hook is run by `with-output-to-temp-buffer` before evaluating body. When the hook runs, the temporary buffer is current. This hook is normally set up with a function to put the buffer in Help mode.
Variable: **temp-buffer-show-hook**
This normal hook is run by `with-output-to-temp-buffer` after displaying the temporary buffer. When the hook runs, the temporary buffer is current, and the window it was displayed in is selected.
Macro: **with-temp-buffer-window** *buffer-or-name action quit-function body…*
This macro is similar to `with-output-to-temp-buffer`. Like that construct, it executes body while arranging to insert any output it prints into the buffer named buffer-or-name and displays that buffer in some window. Unlike `with-output-to-temp-buffer`, however, it does not automatically switch that buffer to Help mode.
The argument buffer-or-name specifies the temporary buffer. It can be either a buffer, which must already exist, or a string, in which case a buffer of that name is created, if necessary. The buffer is marked as unmodified and read-only when `with-temp-buffer-window` exits.
This macro does not call `temp-buffer-show-function`. Rather, it passes the action argument to `display-buffer` (see [Choosing Window](choosing-window)) in order to display the buffer.
The value of the last form in body is returned, unless the argument quit-function is specified. In that case, it is called with two arguments: the window showing the buffer and the result of body. The final return value is then whatever quit-function returns.
This macro uses the normal hooks `temp-buffer-window-setup-hook` and `temp-buffer-window-show-hook` in place of the analogous hooks run by `with-output-to-temp-buffer`.
The two constructs described next are mostly identical to `with-temp-buffer-window` but differ from it as specified:
Macro: **with-current-buffer-window** *buffer-or-name action quit-function &rest body*
This macro is like `with-temp-buffer-window` but unlike that makes the buffer specified by buffer-or-name current for running body.
A window showing a temporary buffer can be fitted to the size of that buffer using the following mode:
User Option: **temp-buffer-resize-mode**
When this minor mode is enabled, windows showing a temporary buffer are automatically resized to fit their buffer’s contents.
A window is resized if and only if it has been specially created for the buffer. In particular, windows that have shown another buffer before are not resized. By default, this mode uses `fit-window-to-buffer` (see [Resizing Windows](resizing-windows)) for resizing. You can specify a different function by customizing the options `temp-buffer-max-height` and `temp-buffer-max-width` below.
User Option: **temp-buffer-max-height**
This option specifies the maximum height (in lines) of a window displaying a temporary buffer when `temp-buffer-resize-mode` is enabled. It can also be a function to be called to choose the height for such a buffer. It gets one argument, the buffer, and should return a positive integer. At the time the function is called, the window to be resized is selected.
User Option: **temp-buffer-max-width**
This option specifies the maximum width of a window (in columns) displaying a temporary buffer when `temp-buffer-resize-mode` is enabled. It can also be a function to be called to choose the width for such a buffer. It gets one argument, the buffer, and should return a positive integer. At the time the function is called, the window to be resized is selected.
The following function uses the current buffer for temporary display:
Function: **momentary-string-display** *string position &optional char message*
This function momentarily displays string in the current buffer at position. It has no effect on the undo list or on the buffer’s modification status.
The momentary display remains until the next input event. If the next input event is char, `momentary-string-display` ignores it and returns. Otherwise, that event remains buffered for subsequent use as input. Thus, typing char will simply remove the string from the display, while typing (say) `C-f` will remove the string from the display and later (presumably) move point forward. The argument char is a space by default.
The return value of `momentary-string-display` is not meaningful.
If the string string does not contain control characters, you can do the same job in a more general way by creating (and then subsequently deleting) an overlay with a `before-string` property. See [Overlay Properties](overlay-properties).
If message is non-`nil`, it is displayed in the echo area while string is displayed in the buffer. If it is `nil`, a default message says to type char to continue.
In this example, point is initially located at the beginning of the second line:
```
---------- Buffer: foo ----------
This is the contents of foo.
∗Second line.
---------- Buffer: foo ----------
```
```
(momentary-string-display
"**** Important Message! ****"
(point) ?\r
"Type RET when done reading")
⇒ t
```
```
---------- Buffer: foo ----------
This is the contents of foo.
**** Important Message! ****Second line.
---------- Buffer: foo ----------
---------- Echo Area ----------
Type RET when done reading
---------- Echo Area ----------
```
elisp None ### Byte-Code Function Objects
Byte-compiled functions have a special data type: they are *byte-code function objects*. Whenever such an object appears as a function to be called, Emacs uses the byte-code interpreter to execute the byte-code.
Internally, a byte-code function object is much like a vector; its elements can be accessed using `aref`. Its printed representation is like that for a vector, with an additional ‘`#`’ before the opening ‘`[`’. It must have at least four elements; there is no maximum number, but only the first six elements have any normal use. They are:
argdesc
The descriptor of the arguments. This can either be a list of arguments, as described in [Argument List](argument-list), or an integer encoding the required number of arguments. In the latter case, the value of the descriptor specifies the minimum number of arguments in the bits zero to 6, and the maximum number of arguments in bits 8 to 14. If the argument list uses `&rest`, then bit 7 is set; otherwise it’s cleared.
If argdesc is a list, the arguments will be dynamically bound before executing the byte code. If argdesc is an integer, the arguments will be instead pushed onto the stack of the byte-code interpreter, before executing the code.
byte-code
The string containing the byte-code instructions.
constants
The vector of Lisp objects referenced by the byte code. These include symbols used as function names and variable names.
stacksize
The maximum stack size this function needs.
docstring
The documentation string (if any); otherwise, `nil`. The value may be a number or a list, in case the documentation string is stored in a file. Use the function `documentation` to get the real documentation string (see [Accessing Documentation](accessing-documentation)).
interactive The interactive spec (if any). This can be a string or a Lisp expression. It is `nil` for a function that isn’t interactive.
Here’s an example of a byte-code function object, in printed representation. It is the definition of the command `backward-sexp`.
```
#[256
"\211\204^G^@\300\262^A\301^A[!\207"
[1 forward-sexp]
3
1793299
"^p"]
```
The primitive way to create a byte-code object is with `make-byte-code`:
Function: **make-byte-code** *&rest elements*
This function constructs and returns a byte-code function object with elements as its elements.
You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte compiler to create these objects; it makes the elements consistent (we hope).
elisp None #### Round-Trip Specification
The most general of the two facilities is controlled by the variable `format-alist`, a list of *file format* specifications, which describe textual representations used in files for the data in an Emacs buffer. The descriptions for reading and writing are paired, which is why we call this “round-trip” specification (see [Format Conversion Piecemeal](format-conversion-piecemeal), for non-paired specification).
Variable: **format-alist**
This list contains one format definition for each defined file format. Each format definition is a list of this form:
```
(name doc-string regexp from-fn to-fn modify mode-fn preserve)
```
Here is what the elements in a format definition mean:
name
The name of this format.
doc-string
A documentation string for the format.
regexp
A regular expression which is used to recognize files represented in this format. If `nil`, the format is never applied automatically.
from-fn
A shell command or function to decode data in this format (to convert file data into the usual Emacs data representation).
A shell command is represented as a string; Emacs runs the command as a filter to perform the conversion.
If from-fn is a function, it is called with two arguments, begin and end, which specify the part of the buffer it should convert. It should convert the text by editing it in place. Since this can change the length of the text, from-fn should return the modified end position.
One responsibility of from-fn is to make sure that the beginning of the file no longer matches regexp. Otherwise it is likely to get called again. Also, from-fn must not involve buffers or files other than the one being decoded, otherwise the internal buffer used for formatting might be overwritten.
to-fn
A shell command or function to encode data in this format—that is, to convert the usual Emacs data representation into this format.
If to-fn is a string, it is a shell command; Emacs runs the command as a filter to perform the conversion.
If to-fn is a function, it is called with three arguments: begin and end, which specify the part of the buffer it should convert, and buffer, which specifies which buffer. There are two ways it can do the conversion:
* By editing the buffer in place. In this case, to-fn should return the end-position of the range of text, as modified.
* By returning a list of annotations. This is a list of elements of the form `(position . string)`, where position is an integer specifying the relative position in the text to be written, and string is the annotation to add there. The list must be sorted in order of position when to-fn returns it. When `write-region` actually writes the text from the buffer to the file, it intermixes the specified annotations at the corresponding positions. All this takes place without modifying the buffer.
to-fn must not involve buffers or files other than the one being encoded, otherwise the internal buffer used for formatting might be overwritten.
modify
A flag, `t` if the encoding function modifies the buffer, and `nil` if it works by returning a list of annotations.
mode-fn
A minor-mode function to call after visiting a file converted from this format. The function is called with one argument, the integer 1; that tells a minor-mode function to enable the mode.
preserve A flag, `t` if `format-write-file` should not remove this format from `buffer-file-format`.
The function `insert-file-contents` automatically recognizes file formats when it reads the specified file. It checks the text of the beginning of the file against the regular expressions of the format definitions, and if it finds a match, it calls the decoding function for that format. Then it checks all the known formats over again. It keeps checking them until none of them is applicable.
Visiting a file, with `find-file-noselect` or the commands that use it, performs conversion likewise (because it calls `insert-file-contents`); it also calls the mode function for each format that it decodes. It stores a list of the format names in the buffer-local variable `buffer-file-format`.
Variable: **buffer-file-format**
This variable states the format of the visited file. More precisely, this is a list of the file format names that were decoded in the course of visiting the current buffer’s file. It is always buffer-local in all buffers.
When `write-region` writes data into a file, it first calls the encoding functions for the formats listed in `buffer-file-format`, in the order of appearance in the list.
Command: **format-write-file** *file format &optional confirm*
This command writes the current buffer contents into the file file in a format based on format, which is a list of format names. It constructs the actual format starting from format, then appending any elements from the value of `buffer-file-format` with a non-`nil` preserve flag (see above), if they are not already present in format. It then updates `buffer-file-format` with this format, making it the default for future saves. Except for the format argument, this command is similar to `write-file`. In particular, confirm has the same meaning and interactive treatment as the corresponding argument to `write-file`. See [Definition of write-file](saving-buffers#Definition-of-write_002dfile).
Command: **format-find-file** *file format*
This command finds the file file, converting it according to format format. It also makes format the default if the buffer is saved later.
The argument format is a list of format names. If format is `nil`, no conversion takes place. Interactively, typing just RET for format specifies `nil`.
Command: **format-insert-file** *file format &optional beg end*
This command inserts the contents of file file, converting it according to format format. If beg and end are non-`nil`, they specify which part of the file to read, as in `insert-file-contents` (see [Reading from Files](reading-from-files)).
The return value is like what `insert-file-contents` returns: a list of the absolute file name and the length of the data inserted (after conversion).
The argument format is a list of format names. If format is `nil`, no conversion takes place. Interactively, typing just RET for format specifies `nil`.
Variable: **buffer-auto-save-file-format**
This variable specifies the format to use for auto-saving. Its value is a list of format names, just like the value of `buffer-file-format`; however, it is used instead of `buffer-file-format` for writing auto-save files. If the value is `t`, the default, auto-saving uses the same format as a regular save in the same buffer. This variable is always buffer-local in all buffers.
| programming_docs |
elisp None #### Invoking the Debugger
Here we describe in full detail the function `debug` that is used to invoke the debugger.
Command: **debug** *&rest debugger-args*
This function enters the debugger. It switches buffers to a buffer named `\*Backtrace\*` (or `\*Backtrace\*<2>` if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, showing the backtrace buffer in Debugger mode.
The Debugger mode `c`, `d`, `j`, and `r` commands exit the recursive edit; then `debug` switches back to the previous buffer and returns to whatever called `debug`. This is the only way the function `debug` can return to its caller.
The use of the debugger-args is that `debug` displays the rest of its arguments at the top of the `\*Backtrace\*` buffer, so that the user can see them. Except as described below, this is the *only* way these arguments are used.
However, certain values for first argument to `debug` have a special significance. (Normally, these values are used only by the internals of Emacs, and not by programmers calling `debug`.) Here is a table of these special values:
`lambda`
A first argument of `lambda` means `debug` was called because of entry to a function when `debug-on-next-call` was non-`nil`. The debugger displays ‘`Debugger entered--entering a function:`’ as a line of text at the top of the buffer.
`debug`
`debug` as first argument means `debug` was called because of entry to a function that was set to debug on entry. The debugger displays the string ‘`Debugger entered--entering a function:`’, just as in the `lambda` case. It also marks the stack frame for that function so that it will invoke the debugger when exited.
`t`
When the first argument is `t`, this indicates a call to `debug` due to evaluation of a function call form when `debug-on-next-call` is non-`nil`. The debugger displays ‘`Debugger entered--beginning evaluation of function call form:`’ as the top line in the buffer.
`exit`
When the first argument is `exit`, it indicates the exit of a stack frame previously marked to invoke the debugger on exit. The second argument given to `debug` in this case is the value being returned from the frame. The debugger displays ‘`Debugger entered--returning value:`’ in the top line of the buffer, followed by the value being returned.
`error`
When the first argument is `error`, the debugger indicates that it is being entered because an error or `quit` was signaled and not handled, by displaying ‘`Debugger entered--Lisp error:`’ followed by the error signaled and any arguments to `signal`. For example,
```
(let ((debug-on-error t))
(/ 1 0))
```
```
------ Buffer: *Backtrace* ------
Debugger entered--Lisp error: (arith-error)
/(1 0)
...
------ Buffer: *Backtrace* ------
```
If an error was signaled, presumably the variable `debug-on-error` is non-`nil`. If `quit` was signaled, then presumably the variable `debug-on-quit` is non-`nil`.
`nil` Use `nil` as the first of the debugger-args when you want to enter the debugger explicitly. The rest of the debugger-args are printed on the top line of the buffer. You can use this feature to display messages—for example, to remind yourself of the conditions under which `debug` is called.
elisp None ### Frame Parameters
A frame has many parameters that control its appearance and behavior. Just what parameters a frame has depends on what display mechanism it uses.
Frame parameters exist mostly for the sake of graphical displays. Most frame parameters have no effect when applied to a frame on a text terminal; only the `height`, `width`, `name`, `title`, `menu-bar-lines`, `buffer-list` and `buffer-predicate` parameters do something special. If the terminal supports colors, the parameters `foreground-color`, `background-color`, `background-mode` and `display-type` are also meaningful. If the terminal supports frame transparency, the parameter `alpha` is also meaningful.
By default, frame parameters are saved and restored by the desktop library functions (see [Desktop Save Mode](desktop-save-mode)) when the variable `desktop-restore-frames` is non-`nil`. It’s the responsibility of applications that their parameters are included in `frameset-persistent-filter-alist` to avoid that they get meaningless or even harmful values in restored sessions.
| | | |
| --- | --- | --- |
| • [Parameter Access](parameter-access) | | How to change a frame’s parameters. |
| • [Initial Parameters](initial-parameters) | | Specifying frame parameters when you make a frame. |
| • [Window Frame Parameters](window-frame-parameters) | | List of frame parameters for window systems. |
| • [Geometry](geometry) | | Parsing geometry specifications. |
elisp None ### Comparison of Characters and Strings
Function: **char-equal** *character1 character2*
This function returns `t` if the arguments represent the same character, `nil` otherwise. This function ignores differences in case if `case-fold-search` is non-`nil`.
```
(char-equal ?x ?x)
⇒ t
(let ((case-fold-search nil))
(char-equal ?x ?X))
⇒ nil
```
Function: **string=** *string1 string2*
This function returns `t` if the characters of the two strings match exactly. Symbols are also allowed as arguments, in which case the symbol names are used. Case is always significant, regardless of `case-fold-search`.
This function is equivalent to `equal` for comparing two strings (see [Equality Predicates](equality-predicates)). In particular, the text properties of the two strings are ignored; use `equal-including-properties` if you need to distinguish between strings that differ only in their text properties. However, unlike `equal`, if either argument is not a string or symbol, `string=` signals an error.
```
(string= "abc" "abc")
⇒ t
(string= "abc" "ABC")
⇒ nil
(string= "ab" "ABC")
⇒ nil
```
For technical reasons, a unibyte and a multibyte string are `equal` if and only if they contain the same sequence of character codes and all these codes are either in the range 0 through 127 (ASCII) or 160 through 255 (`eight-bit-graphic`). However, when a unibyte string is converted to a multibyte string, all characters with codes in the range 160 through 255 are converted to characters with higher codes, whereas ASCII characters remain unchanged. Thus, a unibyte string and its conversion to multibyte are only `equal` if the string is all ASCII. Character codes 160 through 255 are not entirely proper in multibyte text, even though they can occur. As a consequence, the situation where a unibyte and a multibyte string are `equal` without both being all ASCII is a technical oddity that very few Emacs Lisp programmers ever get confronted with. See [Text Representations](text-representations).
Function: **string-equal** *string1 string2*
`string-equal` is another name for `string=`.
Function: **string-collate-equalp** *string1 string2 &optional locale ignore-case*
This function returns `t` if string1 and string2 are equal with respect to collation rules. A collation rule is not only determined by the lexicographic order of the characters contained in string1 and string2, but also further rules about relations between these characters. Usually, it is defined by the locale environment Emacs is running with and by the Standard C library against which Emacs was linked[3](#FOOT3).
For example, characters with different code points but the same meaning, like different grave accent Unicode characters, might, in some locales, be considered as equal:
```
(string-collate-equalp (string ?\uFF40) (string ?\u1FEF))
⇒ t
```
The optional argument locale, a string, overrides the setting of your current locale identifier for collation. The value is system dependent; a locale `"en_US.UTF-8"` is applicable on POSIX systems, while it would be, e.g., `"enu_USA.1252"` on MS-Windows systems.
If ignore-case is non-`nil`, characters are converted to lower-case before comparing them.
To emulate Unicode-compliant collation on MS-Windows systems, bind `w32-collate-ignore-punctuation` to a non-`nil` value, since the codeset part of the locale cannot be `"UTF-8"` on MS-Windows.
If your system does not support a locale environment, this function behaves like `string-equal`.
Do *not* use this function to compare file names for equality, as filesystems generally don’t honor linguistic equivalence of strings that collation implements.
Function: **string<** *string1 string2*
This function compares two strings a character at a time. It scans both the strings at the same time to find the first pair of corresponding characters that do not match. If the lesser character of these two is the character from string1, then string1 is less, and this function returns `t`. If the lesser character is the one from string2, then string1 is greater, and this function returns `nil`. If the two strings match entirely, the value is `nil`.
Pairs of characters are compared according to their character codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; digits and many punctuation characters have a lower numeric value than upper case letters. An ASCII character is less than any non-ASCII character; a unibyte non-ASCII character is always less than any multibyte non-ASCII character (see [Text Representations](text-representations)).
```
(string< "abc" "abd")
⇒ t
(string< "abd" "abc")
⇒ nil
(string< "123" "abc")
⇒ t
```
When the strings have different lengths, and they match up to the length of string1, then the result is `t`. If they match up to the length of string2, the result is `nil`. A string of no characters is less than any other string.
```
(string< "" "abc")
⇒ t
(string< "ab" "abc")
⇒ t
(string< "abc" "")
⇒ nil
(string< "abc" "ab")
⇒ nil
(string< "" "")
⇒ nil
```
Symbols are also allowed as arguments, in which case their print names are compared.
Function: **string-lessp** *string1 string2*
`string-lessp` is another name for `string<`.
Function: **string-greaterp** *string1 string2*
This function returns the result of comparing string1 and string2 in the opposite order, i.e., it is equivalent to calling `(string-lessp string2 string1)`.
Function: **string-collate-lessp** *string1 string2 &optional locale ignore-case*
This function returns `t` if string1 is less than string2 in collation order. A collation order is not only determined by the lexicographic order of the characters contained in string1 and string2, but also further rules about relations between these characters. Usually, it is defined by the locale environment Emacs is running with.
For example, punctuation and whitespace characters might be ignored for sorting (see [Sequence Functions](sequence-functions)):
```
(sort (list "11" "12" "1 1" "1 2" "1.1" "1.2") 'string-collate-lessp)
⇒ ("11" "1 1" "1.1" "12" "1 2" "1.2")
```
This behavior is system-dependent; e.g., punctuation and whitespace are never ignored on Cygwin, regardless of locale.
The optional argument locale, a string, overrides the setting of your current locale identifier for collation. The value is system dependent; a locale `"en_US.UTF-8"` is applicable on POSIX systems, while it would be, e.g., `"enu_USA.1252"` on MS-Windows systems. The locale value of `"POSIX"` or `"C"` lets `string-collate-lessp` behave like `string-lessp`:
```
(sort (list "11" "12" "1 1" "1 2" "1.1" "1.2")
(lambda (s1 s2) (string-collate-lessp s1 s2 "POSIX")))
⇒ ("1 1" "1 2" "1.1" "1.2" "11" "12")
```
If ignore-case is non-`nil`, characters are converted to lower-case before comparing them.
To emulate Unicode-compliant collation on MS-Windows systems, bind `w32-collate-ignore-punctuation` to a non-`nil` value, since the codeset part of the locale cannot be `"UTF-8"` on MS-Windows.
If your system does not support a locale environment, this function behaves like `string-lessp`.
Function: **string-version-lessp** *string1 string2*
This function compares strings lexicographically, except it treats sequences of numerical characters as if they comprised a base-ten number, and then compares the numbers. So ‘`foo2.png`’ is “smaller” than ‘`foo12.png`’ according to this predicate, even if ‘`12`’ is lexicographically “smaller” than ‘`2`’.
Function: **string-prefix-p** *string1 string2 &optional ignore-case*
This function returns non-`nil` if string1 is a prefix of string2; i.e., if string2 starts with string1. If the optional argument ignore-case is non-`nil`, the comparison ignores case differences.
Function: **string-suffix-p** *suffix string &optional ignore-case*
This function returns non-`nil` if suffix is a suffix of string; i.e., if string ends with suffix. If the optional argument ignore-case is non-`nil`, the comparison ignores case differences.
Function: **string-search** *needle haystack &optional start-pos*
Return the position of the first instance of needle in haystack, both of which are strings. If start-pos is non-`nil`, start searching from that position in needle. Return `nil` if no match was found. This function only considers the characters in the strings when doing the comparison; text properties are ignored. Matching is always case-sensitive.
Function: **compare-strings** *string1 start1 end1 string2 start2 end2 &optional ignore-case*
This function compares a specified part of string1 with a specified part of string2. The specified part of string1 runs from index start1 (inclusive) up to index end1 (exclusive); `nil` for start1 means the start of the string, while `nil` for end1 means the length of the string. Likewise, the specified part of string2 runs from index start2 up to index end2.
The strings are compared by the numeric values of their characters. For instance, str1 is considered less than str2 if its first differing character has a smaller numeric value. If ignore-case is non-`nil`, characters are converted to upper-case, using the current buffer’s case-table (see [Case Tables](case-tables)), before comparing them. Unibyte strings are converted to multibyte for comparison (see [Text Representations](text-representations)), so that a unibyte string and its conversion to multibyte are always regarded as equal.
If the specified portions of the two strings match, the value is `t`. Otherwise, the value is an integer which indicates how many leading characters agree, and which string is less. Its absolute value is one plus the number of characters that agree at the beginning of the two strings. The sign is negative if string1 (or its specified portion) is less.
Function: **string-distance** *string1 string2 &optional bytecompare*
This function returns the *Levenshtein distance* between the source string string1 and the target string string2. The Levenshtein distance is the number of single-character changes—deletions, insertions, or replacements—required to transform the source string into the target string; it is one possible definition of the *edit distance* between strings.
Letter-case of the strings is significant for the computed distance, but their text properties are ignored. If the optional argument bytecompare is non-`nil`, the function calculates the distance in terms of bytes instead of characters. The byte-wise comparison uses the internal Emacs representation of characters, so it will produce inaccurate results for multibyte strings that include raw bytes (see [Text Representations](text-representations)); make the strings unibyte by encoding them (see [Explicit Encoding](explicit-encoding)) if you need accurate results with raw bytes.
Function: **assoc-string** *key alist &optional case-fold*
This function works like `assoc`, except that key must be a string or symbol, and comparison is done using `compare-strings`. Symbols are converted to strings before testing. If case-fold is non-`nil`, key and the elements of alist are converted to upper-case before comparison. Unlike `assoc`, this function can also match elements of the alist that are strings or symbols rather than conses. In particular, alist can be a list of strings or symbols rather than an actual alist. See [Association Lists](association-lists).
See also the function `compare-buffer-substrings` in [Comparing Text](comparing-text), for a way to compare text in buffers. The function `string-match`, which matches a regular expression against a string, can be used for a kind of string comparison; see [Regexp Search](regexp-search).
elisp None ### Predicates for Strings
For more information about general sequence and array predicates, see [Sequences Arrays Vectors](sequences-arrays-vectors), and [Arrays](arrays).
Function: **stringp** *object*
This function returns `t` if object is a string, `nil` otherwise.
Function: **string-or-null-p** *object*
This function returns `t` if object is a string or `nil`. It returns `nil` otherwise.
Function: **char-or-string-p** *object*
This function returns `t` if object is a string or a character (i.e., an integer), `nil` otherwise.
elisp None #### Layout Parameters
These frame parameters enable or disable various parts of the frame, or control their sizes.
`border-width`
The width in pixels of the frame’s outer border (see [Frame Geometry](frame-geometry)).
`internal-border-width`
The width in pixels of the frame’s internal border (see [Frame Geometry](frame-geometry)).
`child-frame-border-width`
The width in pixels of the frame’s internal border (see [Frame Geometry](frame-geometry)) if the given frame is a child frame (see [Child Frames](child-frames)). If this is `nil`, the value specified by the `internal-border-width` parameter is used instead.
`vertical-scroll-bars`
Whether the frame has scroll bars (see [Scroll Bars](scroll-bars)) for vertical scrolling, and which side of the frame they should be on. The possible values are `left`, `right`, and `nil` for no scroll bars.
`horizontal-scroll-bars`
Whether the frame has scroll bars for horizontal scrolling (`t` and `bottom` mean yes, `nil` means no).
`scroll-bar-width`
The width of vertical scroll bars, in pixels, or `nil` meaning to use the default width.
`scroll-bar-height`
The height of horizontal scroll bars, in pixels, or `nil` meaning to use the default height.
`left-fringe` `right-fringe`
The default width of the left and right fringes of windows in this frame (see [Fringes](fringes)). If either of these is zero, that effectively removes the corresponding fringe.
When you use `frame-parameter` to query the value of either of these two frame parameters, the return value is always an integer. When using `set-frame-parameter`, passing a `nil` value imposes an actual default value of 8 pixels.
`right-divider-width`
The width (thickness) reserved for the right divider (see [Window Dividers](window-dividers)) of any window on the frame, in pixels. A value of zero means to not draw right dividers.
`bottom-divider-width`
The width (thickness) reserved for the bottom divider (see [Window Dividers](window-dividers)) of any window on the frame, in pixels. A value of zero means to not draw bottom dividers.
`menu-bar-lines`
The number of lines to allocate at the top of the frame for a menu bar (see [Menu Bar](menu-bar)). The default is one if Menu Bar mode is enabled and zero otherwise. See [Menu Bars](https://www.gnu.org/software/emacs/manual/html_node/emacs/Menu-Bars.html#Menu-Bars) in The GNU Emacs Manual. For an external menu bar (see [Frame Layout](frame-layout)), this value remains unchanged even when the menu bar wraps to two or more lines. In that case, the `menu-bar-size` value returned by `frame-geometry` (see [Frame Geometry](frame-geometry)) allows to derive whether the menu bar actually occupies one or more lines.
`tool-bar-lines`
The number of lines to use for the tool bar (see [Tool Bar](tool-bar)). The default is one if Tool Bar mode is enabled and zero otherwise. See [Tool Bars](https://www.gnu.org/software/emacs/manual/html_node/emacs/Tool-Bars.html#Tool-Bars) in The GNU Emacs Manual. This value may change whenever the tool bar wraps (see [Frame Layout](frame-layout)).
`tool-bar-position`
The position of the tool bar when Emacs was built with GTK+. Its value can be one of `top`, `bottom` `left`, `right`. The default is `top`.
`tab-bar-lines`
The number of lines to use for the tab bar (see [Tab Bars](https://www.gnu.org/software/emacs/manual/html_node/emacs/Tab-Bars.html#Tab-Bars) in The GNU Emacs Manual). The default is one if Tab Bar mode is enabled and zero otherwise. This value may change whenever the tab bar wraps (see [Frame Layout](frame-layout)).
`line-spacing`
Additional space to leave below each text line, in pixels (a positive integer). See [Line Height](line-height), for more information.
`no-special-glyphs` If this is non-`nil`, it suppresses the display of any truncation and continuation glyphs (see [Truncation](truncation)) for all buffers displayed by this frame. This is useful to eliminate such glyphs when fitting a frame to its buffer via `fit-frame-to-buffer` (see [Resizing Windows](resizing-windows)).
| programming_docs |
elisp None #### Regular Expression Functions
These functions operate on regular expressions.
Function: **regexp-quote** *string*
This function returns a regular expression whose only exact match is string. Using this regular expression in `looking-at` will succeed only if the next characters in the buffer are string; using it in a search function will succeed if the text being searched contains string. See [Regexp Search](regexp-search).
This allows you to request an exact string match or search when calling a function that wants a regular expression.
```
(regexp-quote "^The cat$")
⇒ "\\^The cat\\$"
```
One use of `regexp-quote` is to combine an exact string match with context described as a regular expression. For example, this searches for the string that is the value of string, surrounded by whitespace:
```
(re-search-forward
(concat "\\s-" (regexp-quote string) "\\s-"))
```
The returned string may be string itself if it does not contain any special characters.
Function: **regexp-opt** *strings &optional paren*
This function returns an efficient regular expression that will match any of the strings in the list strings. This is useful when you need to make matching or searching as fast as possible—for example, for Font Lock mode[23](#FOOT23).
If strings is the empty list, the return value is a regexp that never matches anything.
The optional argument paren can be any of the following:
a string
The resulting regexp is preceded by paren and followed by ‘`\)`’, e.g. use ‘`"\\(?1:"`’ to produce an explicitly numbered group.
`words`
The resulting regexp is surrounded by ‘`\<\(`’ and ‘`\)\>`’.
`symbols`
The resulting regexp is surrounded by ‘`\\_<\(`’ and ‘`\)\\_>`’ (this is often appropriate when matching programming-language keywords and the like).
non-`nil`
The resulting regexp is surrounded by ‘`\(`’ and ‘`\)`’.
`nil` The resulting regexp is surrounded by ‘`\(?:`’ and ‘`\)`’, if it is necessary to ensure that a postfix operator appended to it will apply to the whole expression.
The returned regexp is ordered in such a way that it will always match the longest string possible.
Up to reordering, the resulting regexp of `regexp-opt` is equivalent to but usually more efficient than that of a simplified version:
```
(defun simplified-regexp-opt (strings &optional paren)
(let ((parens
(cond
((stringp paren) (cons paren "\\)"))
((eq paren 'words) '("\\<\\(" . "\\)\\>"))
((eq paren 'symbols) '("\\_<\\(" . "\\)\\_>"))
((null paren) '("\\(?:" . "\\)"))
(t '("\\(" . "\\)")))))
(concat (car parens)
(mapconcat 'regexp-quote strings "\\|")
(cdr parens))))
```
Function: **regexp-opt-depth** *regexp*
This function returns the total number of grouping constructs (parenthesized expressions) in regexp. This does not include shy groups (see [Regexp Backslash](regexp-backslash)).
Function: **regexp-opt-charset** *chars*
This function returns a regular expression matching a character in the list of characters chars.
```
(regexp-opt-charset '(?a ?b ?c ?d ?e))
⇒ "[a-e]"
```
Variable: **regexp-unmatchable**
This variable contains a regexp that is guaranteed not to match any string at all. It is particularly useful as default value for variables that may be set to a pattern that actually matches something.
elisp None ### Deleting Frames
A *live frame* is one that has not been deleted. When a frame is deleted, it is removed from its terminal display, although it may continue to exist as a Lisp object until there are no more references to it.
Command: **delete-frame** *&optional frame force*
This function deletes the frame frame. The argument frame must specify a live frame (see below) and defaults to the selected frame.
It first deletes any child frame of frame (see [Child Frames](child-frames)) and any frame whose `delete-before` frame parameter (see [Frame Interaction Parameters](frame-interaction-parameters)) specifies frame. All such deletions are performed recursively; so this step makes sure that no other frames with frame as their ancestor will exist. Then, unless frame specifies a tooltip, this function runs the hook `delete-frame-functions` (each function getting one argument, frame) before actually killing the frame. After actually killing the frame and removing the frame from the frame list, `delete-frame` runs `after-delete-frame-functions`.
Note that a frame cannot be deleted as long as its minibuffer serves as surrogate minibuffer for another frame (see [Minibuffers and Frames](minibuffers-and-frames)). Normally, you cannot delete a frame if all other frames are invisible, but if force is non-`nil`, then you are allowed to do so.
Function: **frame-live-p** *frame*
This function returns non-`nil` if the frame frame has not been deleted. The possible non-`nil` return values are like those of `framep`. See [Frames](frames).
Some window managers provide a command to delete a window. These work by sending a special message to the program that operates the window. When Emacs gets one of these commands, it generates a `delete-frame` event, whose normal definition is a command that calls the function `delete-frame`. See [Misc Events](misc-events).
Command: **delete-other-frames** *&optional frame iconify*
This command deletes all frames on frame’s terminal, except frame. If frame uses another frame’s minibuffer, that minibuffer frame is left untouched. The argument frame must specify a live frame and defaults to the selected frame. Internally, this command works by calling `delete-frame` with force `nil` for all frames that shall be deleted.
This function does not delete any of frame’s child frames (see [Child Frames](child-frames)). If frame is a child frame, it deletes frame’s siblings only.
With the prefix argument iconify, the frames are iconified rather than deleted.
elisp None #### Specification Examples
It may be easier to understand Edebug specifications by studying the examples provided here.
Consider a hypothetical macro `my-test-generator` that runs tests on supplied lists of data. Although it is Edebug’s default behavior to not instrument arguments as code, as controlled by `edebug-eval-macro-args` (see [Instrumenting Macro Calls](instrumenting-macro-calls)), it can be useful to explicitly document that the arguments are data:
```
(def-edebug-spec my-test-generator (&rest sexp))
```
A `let` special form has a sequence of bindings and a body. Each of the bindings is either a symbol or a sublist with a symbol and optional expression. In the specification below, notice the `gate` inside of the sublist to prevent backtracking once a sublist is found.
```
(def-edebug-spec let
((&rest
&or symbolp (gate symbolp &optional form))
body))
```
Edebug uses the following specifications for `defun` and the associated argument list and `interactive` specifications. It is necessary to handle interactive forms specially since an expression argument is actually evaluated outside of the function body. (The specification for `defmacro` is very similar to that for `defun`, but allows for the `declare` statement.)
```
(def-edebug-spec defun
(&define name lambda-list
[&optional stringp] ; Match the doc string, if present.
[&optional ("interactive" interactive)]
def-body))
(def-edebug-elem-spec 'lambda-list
'(([&rest arg]
[&optional ["&optional" arg &rest arg]]
&optional ["&rest" arg]
)))
(def-edebug-elem-spec 'interactive
'(&optional &or stringp def-form)) ; Notice: `def-form`
```
The specification for backquote below illustrates how to match dotted lists and use `nil` to terminate recursion. It also illustrates how components of a vector may be matched. (The actual specification defined by Edebug is a little different, and does not support dotted lists because doing so causes very deep recursion that could fail.)
```
(def-edebug-spec \` (backquote-form)) ; Alias just for clarity.
(def-edebug-elem-spec 'backquote-form
'(&or ([&or "," ",@"] &or ("quote" backquote-form) form)
(backquote-form . [&or nil backquote-form])
(vector &rest backquote-form)
sexp))
```
elisp None #### Accessing Symbol Properties
The following functions can be used to access symbol properties.
Function: **get** *symbol property*
This function returns the value of the property named property in symbol’s property list. If there is no such property, it returns `nil`. Thus, there is no distinction between a value of `nil` and the absence of the property.
The name property is compared with the existing property names using `eq`, so any object is a legitimate property.
See `put` for an example.
Function: **put** *symbol property value*
This function puts value onto symbol’s property list under the property name property, replacing any previous property value. The `put` function returns value.
```
(put 'fly 'verb 'transitive)
⇒'transitive
(put 'fly 'noun '(a buzzing little bug))
⇒ (a buzzing little bug)
(get 'fly 'verb)
⇒ transitive
(symbol-plist 'fly)
⇒ (verb transitive noun (a buzzing little bug))
```
Function: **symbol-plist** *symbol*
This function returns the property list of symbol.
Function: **setplist** *symbol plist*
This function sets symbol’s property list to plist. Normally, plist should be a well-formed property list, but this is not enforced. The return value is plist.
```
(setplist 'foo '(a 1 b (2 3) c nil))
⇒ (a 1 b (2 3) c nil)
(symbol-plist 'foo)
⇒ (a 1 b (2 3) c nil)
```
For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (see [Abbrevs](abbrevs)).
You could define `put` in terms of `setplist` and `plist-put`, as follows:
```
(defun put (symbol prop value)
(setplist symbol
(plist-put (symbol-plist symbol) prop value)))
```
Function: **function-get** *symbol property &optional autoload*
This function is identical to `get`, except that if symbol is the name of a function alias, it looks in the property list of the symbol naming the actual function. See [Defining Functions](defining-functions). If the optional argument autoload is non-`nil`, and symbol is auto-loaded, this function will try to autoload it, since autoloading might set property of symbol. If autoload is the symbol `macro`, only try autoloading if symbol is an auto-loaded macro.
Function: **function-put** *function property value*
This function sets property of function to value. function should be a symbol. This function is preferred to calling `put` for setting properties of a function, because it will allow us some day to implement remapping of old properties to new ones.
elisp None ### POSIX Regular Expression Searching
The usual regular expression functions do backtracking when necessary to handle the ‘`\|`’ and repetition constructs, but they continue this only until they find *some* match. Then they succeed and report the first match found.
This section describes alternative search functions which perform the full backtracking specified by the POSIX standard for regular expression matching. They continue backtracking until they have tried all possibilities and found all matches, so they can report the longest match, as required by POSIX. This is much slower, so use these functions only when you really need the longest match.
The POSIX search and match functions do not properly support the non-greedy repetition operators (see [non-greedy](regexp-special)). This is because POSIX backtracking conflicts with the semantics of non-greedy repetition.
Command: **posix-search-forward** *regexp &optional limit noerror count*
This is like `re-search-forward` except that it performs the full backtracking specified by the POSIX standard for regular expression matching.
Command: **posix-search-backward** *regexp &optional limit noerror count*
This is like `re-search-backward` except that it performs the full backtracking specified by the POSIX standard for regular expression matching.
Function: **posix-looking-at** *regexp*
This is like `looking-at` except that it performs the full backtracking specified by the POSIX standard for regular expression matching.
Function: **posix-string-match** *regexp string &optional start*
This is like `string-match` except that it performs the full backtracking specified by the POSIX standard for regular expression matching.
elisp None #### Simple Match Data Access
This section explains how to use the match data to find out what was matched by the last search or match operation, if it succeeded.
You can ask about the entire matching text, or about a particular parenthetical subexpression of a regular expression. The count argument in the functions below specifies which. If count is zero, you are asking about the entire match. If count is positive, it specifies which subexpression you want.
Recall that the subexpressions of a regular expression are those expressions grouped with escaped parentheses, ‘`\(…\)`’. The countth subexpression is found by counting occurrences of ‘`\(`’ from the beginning of the whole regular expression. The first subexpression is numbered 1, the second 2, and so on. Only regular expressions can have subexpressions—after a simple string search, the only information available is about the entire match.
Every successful search sets the match data. Therefore, you should query the match data immediately after searching, before calling any other function that might perform another search. Alternatively, you may save and restore the match data (see [Saving Match Data](saving-match-data)) around the call to functions that could perform another search. Or use the functions that explicitly do not modify the match data; e.g., `string-match-p`.
A search which fails may or may not alter the match data. In the current implementation, it does not, but we may change it in the future. Don’t try to rely on the value of the match data after a failing search.
Function: **match-string** *count &optional in-string*
This function returns, as a string, the text matched in the last search or match operation. It returns the entire text if count is zero, or just the portion corresponding to the countth parenthetical subexpression, if count is positive.
If the last such operation was done against a string with `string-match`, then you should pass the same string as the argument in-string. After a buffer search or match, you should omit in-string or pass `nil` for it; but you should make sure that the current buffer when you call `match-string` is the one in which you did the searching or matching. Failure to follow this advice will lead to incorrect results.
The value is `nil` if count is out of range, or for a subexpression inside a ‘`\|`’ alternative that wasn’t used or a repetition that repeated zero times.
Function: **match-string-no-properties** *count &optional in-string*
This function is like `match-string` except that the result has no text properties.
Function: **match-beginning** *count*
If the last regular expression search found a match, this function returns the position of the start of the matching text or of a subexpression of it.
If count is zero, then the value is the position of the start of the entire match. Otherwise, count specifies a subexpression in the regular expression, and the value of the function is the starting position of the match for that subexpression.
The value is `nil` for a subexpression inside a ‘`\|`’ alternative that wasn’t used or a repetition that repeated zero times.
Function: **match-end** *count*
This function is like `match-beginning` except that it returns the position of the end of the match, rather than the position of the beginning.
Here is an example of using the match data, with a comment showing the positions within the text:
```
(string-match "\\(qu\\)\\(ick\\)"
"The quick fox jumped quickly.")
;0123456789
⇒ 4
```
```
(match-string 0 "The quick fox jumped quickly.")
⇒ "quick"
(match-string 1 "The quick fox jumped quickly.")
⇒ "qu"
(match-string 2 "The quick fox jumped quickly.")
⇒ "ick"
```
```
(match-beginning 1) ; The beginning of the match
⇒ 4 ; with ‘`qu`’ is at index 4.
```
```
(match-beginning 2) ; The beginning of the match
⇒ 6 ; with ‘`ick`’ is at index 6.
```
```
(match-end 1) ; The end of the match
⇒ 6 ; with ‘`qu`’ is at index 6.
(match-end 2) ; The end of the match
⇒ 9 ; with ‘`ick`’ is at index 9.
```
Here is another example. Point is initially located at the beginning of the line. Searching moves point to between the space and the word ‘`in`’. The beginning of the entire match is at the 9th character of the buffer (‘`T`’), and the beginning of the match for the first subexpression is at the 13th character (‘`c`’).
```
(list
(re-search-forward "The \\(cat \\)")
(match-beginning 0)
(match-beginning 1))
⇒ (17 9 13)
```
```
---------- Buffer: foo ----------
I read "The cat ∗in the hat comes back" twice.
^ ^
9 13
---------- Buffer: foo ----------
```
(In this case, the index returned is a buffer position; the first character of the buffer counts as 1.)
elisp None #### Pixel Specification for Spaces
The value of the `:width`, `:align-to`, `:height`, and `:ascent` properties can be a special kind of expression that is evaluated during redisplay. The result of the evaluation is used as an absolute number of pixels.
The following expressions are supported:
```
expr ::= num | (num) | unit | elem | pos | image | xwidget | form
num ::= integer | float | symbol
unit ::= in | mm | cm | width | height
```
```
elem ::= left-fringe | right-fringe | left-margin | right-margin
| scroll-bar | text
pos ::= left | center | right
form ::= (num . expr) | (op expr ...)
op ::= + | -
```
The form num specifies a fraction of the default frame font height or width. The form `(num)` specifies an absolute number of pixels. If num is a symbol, symbol, its buffer-local variable binding is used; that binding can be either a number or a cons cell of the forms shown above (including yet another cons cell whose `car` is a symbol that has a buffer-local binding).
The `in`, `mm`, and `cm` units specify the number of pixels per inch, millimeter, and centimeter, respectively. The `width` and `height` units correspond to the default width and height of the current face. An image specification of the form `(image . props)` (see [Image Descriptors](image-descriptors)) corresponds to the width or height of the specified image. Similarly, an xwidget specification of the form `(xwidget . props)` stands for the width or height of the specified xwidget. See [Xwidgets](xwidgets).
The elements `left-fringe`, `right-fringe`, `left-margin`, `right-margin`, `scroll-bar`, and `text` specify the width of the corresponding area of the window. When the window displays line numbers (see [Size of Displayed Text](size-of-displayed-text)), the width of the `text` area is decreased by the screen space taken by the line-number display.
The `left`, `center`, and `right` positions can be used with `:align-to` to specify a position relative to the left edge, center, or right edge of the text area. When the window displays line numbers, the `left` and the `center` positions are offset to account for the screen space taken by the line-number display.
Any of the above window elements (except `text`) can also be used with `:align-to` to specify that the position is relative to the left edge of the given area. Once the base offset for a relative position has been set (by the first occurrence of one of these symbols), further occurrences of these symbols are interpreted as the width of the specified area. For example, to align to the center of the left-margin, use
```
:align-to (+ left-margin (0.5 . left-margin))
```
If no specific base offset is set for alignment, it is always relative to the left edge of the text area. For example, ‘`:align-to 0`’ in a header-line aligns with the first text column in the text area. When the window displays line numbers, the text is considered to start where the space used for line-number display ends.
A value of the form `(num . expr)` stands for the product of the values of num and expr. For example, `(2 . in)` specifies a width of 2 inches, while `(0.5 .
image)` specifies half the width (or height) of the specified image (which should be given by its image spec).
The form `(+ expr ...)` adds up the value of the expressions. The form `(- expr ...)` negates or subtracts the value of the expressions.
| programming_docs |
elisp None #### Coverage Testing
Edebug provides rudimentary coverage testing and display of execution frequency.
Coverage testing works by comparing the result of each expression with the previous result; each form in the program is considered covered if it has returned two different values since you began testing coverage in the current Emacs session. Thus, to do coverage testing on your program, execute it under various conditions and note whether it behaves correctly; Edebug will tell you when you have tried enough different conditions that each form has returned two different values.
Coverage testing makes execution slower, so it is only done if `edebug-test-coverage` is non-`nil`. Frequency counting is performed for all executions of an instrumented function, even if the execution mode is Go-nonstop, and regardless of whether coverage testing is enabled.
Use `C-x X =` (`edebug-display-freq-count`) to display both the coverage information and the frequency counts for a definition. Just `=` (`edebug-temp-display-freq-count`) displays the same information temporarily, only until you type another key.
Command: **edebug-display-freq-count**
This command displays the frequency count data for each line of the current definition.
It inserts frequency counts as comment lines after each line of code. You can undo all insertions with one `undo` command. The counts appear under the ‘`(`’ before an expression or the ‘`)`’ after an expression, or on the last character of a variable. To simplify the display, a count is not shown if it is equal to the count of an earlier expression on the same line.
The character ‘`=`’ following the count for an expression says that the expression has returned the same value each time it was evaluated. In other words, it is not yet covered for coverage testing purposes.
To clear the frequency count and coverage data for a definition, simply reinstrument it with `eval-defun`.
For example, after evaluating `(fac 5)` with a source breakpoint, and setting `edebug-test-coverage` to `t`, when the breakpoint is reached, the frequency data looks like this:
```
(defun fac (n)
(if (= n 0) (edebug))
;#6 1 = =5
(if (< 0 n)
;#5 =
(* n (fac (1- n)))
;# 5 0
1))
;# 0
```
The comment lines show that `fac` was called 6 times. The first `if` statement returned 5 times with the same result each time; the same is true of the condition on the second `if`. The recursive call of `fac` did not return at all.
elisp None ### Standard Regular Expressions Used in Editing
This section describes some variables that hold regular expressions used for certain purposes in editing:
User Option: **page-delimiter**
This is the regular expression describing line-beginnings that separate pages. The default value is `"^\014"` (i.e., `"^^L"` or `"^\C-l"`); this matches a line that starts with a formfeed character.
The following two regular expressions should *not* assume the match always starts at the beginning of a line; they should not use ‘`^`’ to anchor the match. Most often, the paragraph commands do check for a match only at the beginning of a line, which means that ‘`^`’ would be superfluous. When there is a nonzero left margin, they accept matches that start after the left margin. In that case, a ‘`^`’ would be incorrect. However, a ‘`^`’ is harmless in modes where a left margin is never used.
User Option: **paragraph-separate**
This is the regular expression for recognizing the beginning of a line that separates paragraphs. (If you change this, you may have to change `paragraph-start` also.) The default value is `"[ \t\f]*$"`, which matches a line that consists entirely of spaces, tabs, and form feeds (after its left margin).
User Option: **paragraph-start**
This is the regular expression for recognizing the beginning of a line that starts *or* separates paragraphs. The default value is `"\f\\|[ \t]*$"`, which matches a line containing only whitespace or starting with a form feed (after its left margin).
User Option: **sentence-end**
If non-`nil`, the value should be a regular expression describing the end of a sentence, including the whitespace following the sentence. (All paragraph boundaries also end sentences, regardless.)
If the value is `nil`, as it is by default, then the function `sentence-end` constructs the regexp. That is why you should always call the function `sentence-end` to obtain the regexp to be used to recognize the end of a sentence.
Function: **sentence-end**
This function returns the value of the variable `sentence-end`, if non-`nil`. Otherwise it returns a default value based on the values of the variables `sentence-end-double-space` (see [Definition of sentence-end-double-space](filling#Definition-of-sentence_002dend_002ddouble_002dspace)), `sentence-end-without-period`, and `sentence-end-without-space`.
elisp None #### Menu Example
Here is a complete example of defining a menu keymap. It is the definition of the ‘`Replace`’ submenu in the ‘`Edit`’ menu in the menu bar, and it uses the extended menu item format (see [Extended Menu Items](extended-menu-items)). First we create the keymap, and give it a name:
```
(defvar menu-bar-replace-menu (make-sparse-keymap "Replace"))
```
Next we define the menu items:
```
(define-key menu-bar-replace-menu [tags-repl-continue]
'(menu-item "Continue Replace" multifile-continue
:help "Continue last tags replace operation"))
(define-key menu-bar-replace-menu [tags-repl]
'(menu-item "Replace in tagged files" tags-query-replace
:help "Interactively replace a regexp in all tagged files"))
(define-key menu-bar-replace-menu [separator-replace-tags]
'(menu-item "--"))
;; …
```
Note the symbols which the bindings are made for; these appear inside square brackets, in the key sequence being defined. In some cases, this symbol is the same as the command name; sometimes it is different. These symbols are treated as function keys, but they are not real function keys on the keyboard. They do not affect the functioning of the menu itself, but they are echoed in the echo area when the user selects from the menu, and they appear in the output of `where-is` and `apropos`.
The menu in this example is intended for use with the mouse. If a menu is intended for use with the keyboard, that is, if it is bound to a key sequence ending with a keyboard event, then the menu items should be bound to characters or real function keys, that can be typed with the keyboard.
The binding whose definition is `("--")` is a separator line. Like a real menu item, the separator has a key symbol, in this case `separator-replace-tags`. If one menu has two separators, they must have two different key symbols.
Here is how we make this menu appear as an item in the parent menu:
```
(define-key menu-bar-edit-menu [replace]
(list 'menu-item "Replace" menu-bar-replace-menu))
```
Note that this incorporates the submenu keymap, which is the value of the variable `menu-bar-replace-menu`, rather than the symbol `menu-bar-replace-menu` itself. Using that symbol in the parent menu item would be meaningless because `menu-bar-replace-menu` is not a command.
If you wanted to attach the same replace menu to a mouse click, you can do it this way:
```
(define-key global-map [C-S-down-mouse-1]
menu-bar-replace-menu)
```
elisp None ### Notifications on File Changes
Several operating systems support watching of filesystems for changes to files or their attributes. If configured properly, Emacs links a respective library like `inotify`, `kqueue`, `gfilenotify`, or `w32notify` statically. These libraries enable watching of filesystems on the local machine.
It is also possible to watch filesystems on remote machines, see [Remote Files](https://www.gnu.org/software/emacs/manual/html_node/emacs/Remote-Files.html#Remote-Files) in The GNU Emacs Manual. This does not depend on one of the libraries linked to Emacs.
Since all these libraries emit different events upon notified file changes, Emacs provides a special library `filenotify` which presents a unified interface to applications. Lisp programs that want to receive file notifications should always use this library in preference to the native ones. This section documents the `filenotify` library functions and variables.
Function: **file-notify-add-watch** *file flags callback*
Add a watch for filesystem events pertaining to file. This arranges for filesystem events pertaining to file to be reported to Emacs.
The returned value is a descriptor for the added watch. Its type depends on the underlying library, and in general cannot be assumed to be an integer as in the example below. It should be used for comparison by `equal` only.
If the file cannot be watched for some reason, this function signals a `file-notify-error` error.
Sometimes, mounted filesystems cannot be watched for file changes. This is not detected by this function, and so a non-`nil` return value does not guarantee that changes on file will be actually notified.
flags is a list of conditions to set what will be watched for. It can include the following symbols:
`change` watch for changes in file’s contents
`attribute-change` watch for changes in file attributes, like permissions or modification time
If file is a directory, `change` watches for file creation and deletion in that directory. Some of the native file notification libraries also report file changes in that case. This does not work recursively.
When any event happens, Emacs will call the callback function passing it a single argument event, which is of the form
```
(descriptor action file [file1])
```
descriptor is the same object as the one returned by this function. action is the description of the event. It could be any one of the following symbols:
`created` file was created
`deleted` file was deleted
`changed` file’s contents has changed; with `w32notify` library, reports attribute changes as well
`renamed` file has been renamed to file1
`attribute-changed` a file attribute was changed
`stopped` watching file has stopped
Note that the `w32notify` library does not report `attribute-changed` events. When some file’s attribute, like permissions or modification time, has changed, this library reports a `changed` event. Likewise, the `kqueue` library does not reliably report file attribute changes when watching a directory.
The `stopped` event means that watching the file has been discontinued. This could be because `file-notify-rm-watch` was called (see below), or because the file being watched was deleted, or due to another error reported from the underlying library which makes further watching impossible.
file and file1 are the name of the file(s) whose event is being reported. For example:
```
(require 'filenotify)
⇒ filenotify
```
```
(defun my-notify-callback (event)
(message "Event %S" event))
⇒ my-notify-callback
```
```
(file-notify-add-watch
"/tmp" '(change attribute-change) 'my-notify-callback)
⇒ 35025468
```
```
(write-region "foo" nil "/tmp/foo")
⇒ Event (35025468 created "/tmp/.#foo")
Event (35025468 created "/tmp/foo")
Event (35025468 changed "/tmp/foo")
Event (35025468 deleted "/tmp/.#foo")
```
```
(write-region "bla" nil "/tmp/foo")
⇒ Event (35025468 created "/tmp/.#foo")
Event (35025468 changed "/tmp/foo")
Event (35025468 deleted "/tmp/.#foo")
```
```
(set-file-modes "/tmp/foo" (default-file-modes) 'nofollow)
⇒ Event (35025468 attribute-changed "/tmp/foo")
```
Whether the action `renamed` is returned depends on the used watch library. Otherwise, the actions `deleted` and `created` could be returned in a random order.
```
(rename-file "/tmp/foo" "/tmp/bla")
⇒ Event (35025468 renamed "/tmp/foo" "/tmp/bla")
```
```
(delete-file "/tmp/bla")
⇒ Event (35025468 deleted "/tmp/bla")
```
Function: **file-notify-rm-watch** *descriptor*
Removes an existing file watch specified by its descriptor. descriptor should be an object returned by `file-notify-add-watch`.
Function: **file-notify-valid-p** *descriptor*
Checks a watch specified by its descriptor for validity. descriptor should be an object returned by `file-notify-add-watch`.
A watch can become invalid if the file or directory it watches is deleted, or if the watcher thread exits abnormally for any other reason. Removing the watch by calling `file-notify-rm-watch` also makes it invalid.
```
(make-directory "/tmp/foo")
⇒ Event (35025468 created "/tmp/foo")
```
```
(setq desc
(file-notify-add-watch
"/tmp/foo" '(change) 'my-notify-callback))
⇒ 11359632
```
```
(file-notify-valid-p desc)
⇒ t
```
```
(write-region "bla" nil "/tmp/foo/bla")
⇒ Event (11359632 created "/tmp/foo/.#bla")
Event (11359632 created "/tmp/foo/bla")
Event (11359632 changed "/tmp/foo/bla")
Event (11359632 deleted "/tmp/foo/.#bla")
```
```
;; Deleting a file in the directory doesn't invalidate the watch.
(delete-file "/tmp/foo/bla")
⇒ Event (11359632 deleted "/tmp/foo/bla")
```
```
(write-region "bla" nil "/tmp/foo/bla")
⇒ Event (11359632 created "/tmp/foo/.#bla")
Event (11359632 created "/tmp/foo/bla")
Event (11359632 changed "/tmp/foo/bla")
Event (11359632 deleted "/tmp/foo/.#bla")
```
```
;; Deleting the directory invalidates the watch.
;; Events arrive for different watch descriptors.
(delete-directory "/tmp/foo" 'recursive)
⇒ Event (35025468 deleted "/tmp/foo")
Event (11359632 deleted "/tmp/foo/bla")
Event (11359632 deleted "/tmp/foo")
Event (11359632 stopped "/tmp/foo")
```
```
(file-notify-valid-p desc)
⇒ nil
```
elisp None #### Record Type
A *record* is much like a `vector`. However, the first element is used to hold its type as returned by `type-of`. The purpose of records is to allow programmers to create objects with new types that are not built into Emacs.
See [Records](records), for functions that work with records.
elisp None #### Printing in Edebug
If an expression in your program produces a value containing circular list structure, you may get an error when Edebug attempts to print it.
One way to cope with circular structure is to set `print-length` or `print-level` to truncate the printing. Edebug does this for you; it binds `print-length` and `print-level` to the values of the variables `edebug-print-length` and `edebug-print-level` (so long as they have non-`nil` values). See [Output Variables](output-variables).
User Option: **edebug-print-length**
If non-`nil`, Edebug binds `print-length` to this value while printing results. The default value is `50`.
User Option: **edebug-print-level**
If non-`nil`, Edebug binds `print-level` to this value while printing results. The default value is `50`.
You can also print circular structures and structures that share elements more informatively by binding `print-circle` to a non-`nil` value.
Here is an example of code that creates a circular structure:
```
(setq a (list 'x 'y))
(setcar a a)
```
If `print-circle` is non-`nil`, printing functions (e.g., `prin1`) will print `a` as ‘`#1=(#1# y)`’. The ‘`#1=`’ notation labels the structure that follows it with the label ‘`1`’, and the ‘`#1#`’ notation references the previously labeled structure. This notation is used for any shared elements of lists or vectors.
User Option: **edebug-print-circle**
If non-`nil`, Edebug binds `print-circle` to this value while printing results. The default value is `t`.
For further details about how printing can be customized, see see [Output Functions](output-functions).
elisp None #### Putting Keyboard Events in Strings
In most of the places where strings are used, we conceptualize the string as containing text characters—the same kind of characters found in buffers or files. Occasionally Lisp programs use strings that conceptually contain keyboard characters; for example, they may be key sequences or keyboard macro definitions. However, storing keyboard characters in a string is a complex matter, for reasons of historical compatibility, and it is not always possible.
We recommend that new programs avoid dealing with these complexities by not storing keyboard events in strings. Here is how to do that:
* Use vectors instead of strings for key sequences, when you plan to use them for anything other than as arguments to `lookup-key` and `define-key`. For example, you can use `read-key-sequence-vector` instead of `read-key-sequence`, and `this-command-keys-vector` instead of `this-command-keys`.
* Use vectors to write key sequence constants containing meta characters, even when passing them directly to `define-key`.
* When you have to look at the contents of a key sequence that might be a string, use `listify-key-sequence` (see [Event Input Misc](event-input-misc)) first, to convert it to a list.
The complexities stem from the modifier bits that keyboard input characters can include. Aside from the Meta modifier, none of these modifier bits can be included in a string, and the Meta modifier is allowed only in special cases.
The earliest GNU Emacs versions represented meta characters as codes in the range of 128 to 255. At that time, the basic character codes ranged from 0 to 127, so all keyboard character codes did fit in a string. Many Lisp programs used ‘`\M-`’ in string constants to stand for meta characters, especially in arguments to `define-key` and similar functions, and key sequences and sequences of events were always represented as strings.
When we added support for larger basic character codes beyond 127, and additional modifier bits, we had to change the representation of meta characters. Now the flag that represents the Meta modifier in a character is 2\*\*27 and such numbers cannot be included in a string.
To support programs with ‘`\M-`’ in string constants, there are special rules for including certain meta characters in a string. Here are the rules for interpreting a string as a sequence of input characters:
* If the keyboard character value is in the range of 0 to 127, it can go in the string unchanged.
* The meta variants of those characters, with codes in the range of 2\*\*27 to 2\*\*27+127, can also go in the string, but you must change their numeric values. You must set the 2\*\*7 bit instead of the 2\*\*27 bit, resulting in a value between 128 and 255. Only a unibyte string can include these codes.
* Non-ASCII characters above 256 can be included in a multibyte string.
* Other keyboard character events cannot fit in a string. This includes keyboard events in the range of 128 to 255.
Functions such as `read-key-sequence` that construct strings of keyboard input characters follow these rules: they construct vectors instead of strings, when the events won’t fit in a string.
When you use the read syntax ‘`\M-`’ in a string, it produces a code in the range of 128 to 255—the same code that you get if you modify the corresponding keyboard event to put it in the string. Thus, meta events in strings work consistently regardless of how they get into the strings.
However, most programs would do well to avoid these issues by following the recommendations at the beginning of this section.
elisp None ### Yes-or-No Queries
This section describes functions used to ask the user a yes-or-no question. The function `y-or-n-p` can be answered with a single character; it is useful for questions where an inadvertent wrong answer will not have serious consequences. `yes-or-no-p` is suitable for more momentous questions, since it requires three or four characters to answer.
If either of these functions is called in a command that was invoked using the mouse—more precisely, if `last-nonmenu-event` (see [Command Loop Info](command-loop-info)) is either `nil` or a list—then it uses a dialog box or pop-up menu to ask the question. Otherwise, it uses keyboard input. You can force use either of the mouse or of keyboard input by binding `last-nonmenu-event` to a suitable value around the call.
Both `yes-or-no-p` and `y-or-n-p` use the minibuffer.
Function: **y-or-n-p** *prompt*
This function asks the user a question, expecting input in the minibuffer. It returns `t` if the user types `y`, `nil` if the user types `n`. This function also accepts SPC to mean yes and DEL to mean no. It accepts `C-]` and `C-g` to quit, because the question uses the minibuffer and for that reason the user might try to use `C-]` to get out. The answer is a single character, with no RET needed to terminate it. Upper and lower case are equivalent.
“Asking the question” means printing prompt in the minibuffer, followed by the string ‘`(y or n)` ’. If the input is not one of the expected answers (`y`, `n`, `SPC`, `DEL`, or something that quits), the function responds ‘`Please answer y or n.`’, and repeats the request.
This function actually uses the minibuffer, but does not allow editing of the answer. The cursor moves to the minibuffer while the question is being asked.
The answers and their meanings, even ‘`y`’ and ‘`n`’, are not hardwired, and are specified by the keymap `query-replace-map` (see [Search and Replace](search-and-replace)). In particular, if the user enters the special responses `recenter`, `scroll-up`, `scroll-down`, `scroll-other-window`, or `scroll-other-window-down` (respectively bound to `C-l`, `C-v`, `M-v`, `C-M-v` and `C-M-S-v` in `query-replace-map`), this function performs the specified window recentering or scrolling operation, and poses the question again.
If you bind `help-form` (see [Help Functions](help-functions)) to a non-`nil` value while calling `y-or-n-p`, then pressing `help-char` causes it to evaluate `help-form` and display the result. `help-char` is automatically added to prompt.
Function: **y-or-n-p-with-timeout** *prompt seconds default*
Like `y-or-n-p`, except that if the user fails to answer within seconds seconds, this function stops waiting and returns default. It works by setting up a timer; see [Timers](timers). The argument seconds should be a number.
Function: **yes-or-no-p** *prompt*
This function asks the user a question, expecting input in the minibuffer. It returns `t` if the user enters ‘`yes`’, `nil` if the user types ‘`no`’. The user must type RET to finalize the response. Upper and lower case are equivalent.
`yes-or-no-p` starts by displaying prompt in the minibuffer, followed by ‘`(yes or no)` ’. The user must type one of the expected responses; otherwise, the function responds ‘`Please answer yes or no.`’, waits about two seconds and repeats the request.
`yes-or-no-p` requires more work from the user than `y-or-n-p` and is appropriate for more crucial decisions.
Here is an example:
```
(yes-or-no-p "Do you really want to remove everything?")
;; After evaluation of the preceding expression,
;; the following prompt appears,
;; with an empty minibuffer:
```
```
---------- Buffer: minibuffer ----------
Do you really want to remove everything? (yes or no)
---------- Buffer: minibuffer ----------
```
If the user first types `y RET`, which is invalid because this function demands the entire word ‘`yes`’, it responds by displaying these prompts, with a brief pause between them:
```
---------- Buffer: minibuffer ----------
Please answer yes or no.
Do you really want to remove everything? (yes or no)
---------- Buffer: minibuffer ----------
```
| programming_docs |
elisp None #### Jumping
The commands described in this section execute until they reach a specified location. All except `i` make a temporary breakpoint to establish the place to stop, then switch to go mode. Any other breakpoint reached before the intended stop point will also stop execution. See [Breakpoints](breakpoints), for the details on breakpoints.
These commands may fail to work as expected in case of nonlocal exit, as that can bypass the temporary breakpoint where you expected the program to stop.
`h`
Proceed to the stop point near where point is (`edebug-goto-here`).
`f`
Run the program for one expression (`edebug-forward-sexp`).
`o`
Run the program until the end of the containing sexp (`edebug-step-out`).
`i` Step into the function or macro called by the form after point (`edebug-step-in`).
The `h` command proceeds to the stop point at or after the current location of point, using a temporary breakpoint.
The `f` command runs the program forward over one expression. More precisely, it sets a temporary breakpoint at the position that `forward-sexp` would reach, then executes in go mode so that the program will stop at breakpoints.
With a prefix argument n, the temporary breakpoint is placed n sexps beyond point. If the containing list ends before n more elements, then the place to stop is after the containing expression.
You must check that the position `forward-sexp` finds is a place that the program will really get to. In `cond`, for example, this may not be true.
For flexibility, the `f` command does `forward-sexp` starting at point, rather than at the stop point. If you want to execute one expression *from the current stop point*, first type `w` (`edebug-where`) to move point there, and then type `f`.
The `o` command continues out of an expression. It places a temporary breakpoint at the end of the sexp containing point. If the containing sexp is a function definition itself, `o` continues until just before the last sexp in the definition. If that is where you are now, it returns from the function and then stops. In other words, this command does not exit the currently executing function unless you are positioned after the last sexp.
Normally, the `h`, `f`, and `o` commands display “Break” and pause for `edebug-sit-for-seconds` before showing the result of the form just evaluated. You can avoid this pause by setting `edebug-sit-on-break` to `nil`. See [Edebug Options](edebug-options).
The `i` command steps into the function or macro called by the list form after point, and stops at its first stop point. Note that the form need not be the one about to be evaluated. But if the form is a function call about to be evaluated, remember to use this command before any of the arguments are evaluated, since otherwise it will be too late.
The `i` command instruments the function or macro it’s supposed to step into, if it isn’t instrumented already. This is convenient, but keep in mind that the function or macro remains instrumented unless you explicitly arrange to deinstrument it.
elisp None #### Array Type
An *array* is composed of an arbitrary number of slots for holding or referring to other Lisp objects, arranged in a contiguous block of memory. Accessing any element of an array takes approximately the same amount of time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.)
Emacs defines four types of array: strings, vectors, bool-vectors, and char-tables.
A string is an array of characters and a vector is an array of arbitrary objects. A bool-vector can hold only `t` or `nil`. These kinds of array may have any length up to the largest fixnum, subject to system architecture limits and available memory. Char-tables are sparse arrays indexed by any valid character code; they can hold arbitrary objects.
The first element of an array has index zero, the second element has index 1, and so on. This is called *zero-origin* indexing. For example, an array of four elements has indices 0, 1, 2, and 3. The largest possible index value is one less than the length of the array. Once an array is created, its length is fixed.
All Emacs Lisp arrays are one-dimensional. (Most other programming languages support multidimensional arrays, but they are not essential; you can get the same effect with nested one-dimensional arrays.) Each type of array has its own read syntax; see the following sections for details.
The array type is a subset of the sequence type, and contains the string type, the vector type, the bool-vector type, and the char-table type.
elisp None ### Interactive Call
After the command loop has translated a key sequence into a command, it invokes that command using the function `command-execute`. If the command is a function, `command-execute` calls `call-interactively`, which reads the arguments and calls the command. You can also call these functions yourself.
Note that the term “command”, in this context, refers to an interactively callable function (or function-like object), or a keyboard macro. It does not refer to the key sequence used to invoke a command (see [Keymaps](keymaps)).
Function: **commandp** *object &optional for-call-interactively*
This function returns `t` if object is a command. Otherwise, it returns `nil`.
Commands include strings and vectors (which are treated as keyboard macros), lambda expressions that contain a top-level `interactive` form (see [Using Interactive](using-interactive)), byte-code function objects made from such lambda expressions, autoload objects that are declared as interactive (non-`nil` fourth argument to `autoload`), and some primitive functions. Also, a symbol is considered a command if it has a non-`nil` `interactive-form` property, or if its function definition satisfies `commandp`.
If for-call-interactively is non-`nil`, then `commandp` returns `t` only for objects that `call-interactively` could call—thus, not for keyboard macros.
See `documentation` in [Accessing Documentation](accessing-documentation), for a realistic example of using `commandp`.
Function: **call-interactively** *command &optional record-flag keys*
This function calls the interactively callable function command, providing arguments according to its interactive calling specifications. It returns whatever command returns.
If, for instance, you have a function with the following signature:
```
(defun foo (begin end)
(interactive "r")
...)
```
then saying
```
(call-interactively 'foo)
```
will call `foo` with the region (`point` and `mark`) as the arguments.
An error is signaled if command is not a function or if it cannot be called interactively (i.e., is not a command). Note that keyboard macros (strings and vectors) are not accepted, even though they are considered commands, because they are not functions. If command is a symbol, then `call-interactively` uses its function definition.
If record-flag is non-`nil`, then this command and its arguments are unconditionally added to the list `command-history`. Otherwise, the command is added only if it uses the minibuffer to read an argument. See [Command History](command-history).
The argument keys, if given, should be a vector which specifies the sequence of events to supply if the command inquires which events were used to invoke it. If keys is omitted or `nil`, the default is the return value of `this-command-keys-vector`. See [Definition of this-command-keys-vector](command-loop-info#Definition-of-this_002dcommand_002dkeys_002dvector).
Function: **funcall-interactively** *function &rest arguments*
This function works like `funcall` (see [Calling Functions](calling-functions)), but it makes the call look like an interactive invocation: a call to `called-interactively-p` inside function will return `t`. If function is not a command, it is called without signaling an error.
Function: **command-execute** *command &optional record-flag keys special*
This function executes command. The argument command must satisfy the `commandp` predicate; i.e., it must be an interactively callable function or a keyboard macro.
A string or vector as command is executed with `execute-kbd-macro`. A function is passed to `call-interactively` (see above), along with the record-flag and keys arguments.
If command is a symbol, its function definition is used in its place. A symbol with an `autoload` definition counts as a command if it was declared to stand for an interactively callable function. Such a definition is handled by loading the specified library and then rechecking the definition of the symbol.
The argument special, if given, means to ignore the prefix argument and not clear it. This is used for executing special events (see [Special Events](special-events)).
Command: **execute-extended-command** *prefix-argument*
This function reads a command name from the minibuffer using `completing-read` (see [Completion](completion)). Then it uses `command-execute` to call the specified command. Whatever that command returns becomes the value of `execute-extended-command`.
If the command asks for a prefix argument, it receives the value prefix-argument. If `execute-extended-command` is called interactively, the current raw prefix argument is used for prefix-argument, and thus passed on to whatever command is run.
`execute-extended-command` is the normal definition of `M-x`, so it uses the string ‘`M-x` ’ as a prompt. (It would be better to take the prompt from the events used to invoke `execute-extended-command`, but that is painful to implement.) A description of the value of the prefix argument, if any, also becomes part of the prompt.
```
(execute-extended-command 3)
---------- Buffer: Minibuffer ----------
3 M-x forward-word RET
---------- Buffer: Minibuffer ----------
⇒ t
```
This command heeds the `read-extended-command-predicate` variable, which can filter out commands that are not applicable to the current major mode (or enabled minor modes). By default, the value of this variable is `nil`, and no commands are filtered out. However, customizing it to invoke the function `command-completion-default-include-p` will perform mode-dependent filtering. `read-extended-command-predicate` can be any predicate function; it will be called with two parameters: the command’s symbol and the current buffer. If should return non-`nil` if the command is to be included when completing in that buffer.
Command: **execute-extended-command-for-buffer** *prefix-argument*
This is like `execute-extended-command`, but limits the commands offered for completion to those commands that are of particular relevance to the current major mode (and enabled minor modes). This includes commands that are tagged with the modes (see [Using Interactive](using-interactive)), and also commands that are bound to locally active keymaps. This command is the normal definition of `M-S-x` (that’s “meta shift x”).
elisp None #### Major Mode Conventions
The code for every major mode should follow various coding conventions, including conventions for local keymap and syntax table initialization, function and variable names, and hooks.
If you use the `define-derived-mode` macro, it will take care of many of these conventions automatically. See [Derived Modes](derived-modes). Note also that Fundamental mode is an exception to many of these conventions, because it represents the default state of Emacs.
The following list of conventions is only partial. Each major mode should aim for consistency in general with other Emacs major modes, as this makes Emacs as a whole more coherent. It is impossible to list here all the possible points where this issue might come up; if the Emacs developers point out an area where your major mode deviates from the usual conventions, please make it compatible.
* Define a major mode command whose name ends in ‘`-mode`’. When called with no arguments, this command should switch to the new mode in the current buffer by setting up the keymap, syntax table, and buffer-local variables in an existing buffer. It should not change the buffer’s contents.
* Write a documentation string for this command that describes the special commands available in this mode. See [Mode Help](mode-help). The documentation string may include the special documentation substrings, ‘`\[command]`’, ‘`\{keymap}`’, and ‘`\<keymap>`’, which allow the help display to adapt automatically to the user’s own key bindings. See [Keys in Documentation](keys-in-documentation).
* The major mode command should start by calling `kill-all-local-variables`. This runs the normal hook `change-major-mode-hook`, then gets rid of the buffer-local variables of the major mode previously in effect. See [Creating Buffer-Local](creating-buffer_002dlocal).
* The major mode command should set the variable `major-mode` to the major mode command symbol. This is how `describe-mode` discovers which documentation to print.
* The major mode command should set the variable `mode-name` to the “pretty” name of the mode, usually a string (but see [Mode Line Data](mode-line-data), for other possible forms). The name of the mode appears in the mode line.
* Calling the major mode command twice in direct succession should not fail and should do the same thing as calling the command only once. In other words, the major mode command should be idempotent.
* Since all global names are in the same name space, all the global variables, constants, and functions that are part of the mode should have names that start with the major mode name (or with an abbreviation of it if the name is long). See [Coding Conventions](https://www.gnu.org/software/emacs/manual/html_node/elisp/Coding-Conventions.html).
* In a major mode for editing some kind of structured text, such as a programming language, indentation of text according to structure is probably useful. So the mode should set `indent-line-function` to a suitable function, and probably customize other variables for indentation. See [Auto-Indentation](auto_002dindentation).
* The major mode should usually have its own keymap, which is used as the local keymap in all buffers in that mode. The major mode command should call `use-local-map` to install this local map. See [Active Keymaps](active-keymaps), for more information. This keymap should be stored permanently in a global variable named `modename-mode-map`. Normally the library that defines the mode sets this variable.
See [Tips for Defining](tips-for-defining), for advice about how to write the code to set up the mode’s keymap variable.
* The key sequences bound in a major mode keymap should usually start with `C-c`, followed by a control character, a digit, or `{`, `}`, `<`, `>`, `:` or `;`. The other punctuation characters are reserved for minor modes, and ordinary letters are reserved for users. A major mode can also rebind the keys `M-n`, `M-p` and `M-s`. The bindings for `M-n` and `M-p` should normally be some kind of moving forward and backward, but this does not necessarily mean cursor motion.
It is legitimate for a major mode to rebind a standard key sequence if it provides a command that does the same job in a way better suited to the text this mode is used for. For example, a major mode for editing a programming language might redefine `C-M-a` to move to the beginning of a function in a way that works better for that language. The recommended way of tailoring `C-M-a` to the needs of a major mode is to set `beginning-of-defun-function` (see [List Motion](list-motion)) to invoke the function specific to the mode.
It is also legitimate for a major mode to rebind a standard key sequence whose standard meaning is rarely useful in that mode. For instance, minibuffer modes rebind `M-r`, whose standard meaning is rarely of any use in the minibuffer. Major modes such as Dired or Rmail that do not allow self-insertion of text can reasonably redefine letters and other printing characters as special commands.
* Major modes for editing text should not define RET to do anything other than insert a newline. However, it is ok for specialized modes for text that users don’t directly edit, such as Dired and Info modes, to redefine RET to do something entirely different.
* Major modes should not alter options that are primarily a matter of user preference, such as whether Auto-Fill mode is enabled. Leave this to each user to decide. However, a major mode should customize other variables so that Auto-Fill mode will work usefully *if* the user decides to use it.
* The mode may have its own syntax table or may share one with other related modes. If it has its own syntax table, it should store this in a variable named `modename-mode-syntax-table`. See [Syntax Tables](syntax-tables).
* If the mode handles a language that has a syntax for comments, it should set the variables that define the comment syntax. See [Options Controlling Comments](https://www.gnu.org/software/emacs/manual/html_node/emacs/Options-for-Comments.html#Options-for-Comments) in The GNU Emacs Manual.
* The mode may have its own abbrev table or may share one with other related modes. If it has its own abbrev table, it should store this in a variable named `modename-mode-abbrev-table`. If the major mode command defines any abbrevs itself, it should pass `t` for the system-flag argument to `define-abbrev`. See [Defining Abbrevs](defining-abbrevs).
* The mode should specify how to do highlighting for Font Lock mode, by setting up a buffer-local value for the variable `font-lock-defaults` (see [Font Lock Mode](font-lock-mode)).
* Each face that the mode defines should, if possible, inherit from an existing Emacs face. See [Basic Faces](basic-faces), and [Faces for Font Lock](faces-for-font-lock).
* Consider adding a mode-specific menu to the menu bar. This should preferably include the most important menu-specific settings and commands that will allow users discovering the main features quickly and efficiently.
* Consider adding mode-specific context menus for the mode, to be used if and when users activate the `context-menu-mode` (see [Menu Mouse Clicks](https://www.gnu.org/software/emacs/manual/html_node/emacs/Menu-Mouse-Clicks.html#Menu-Mouse-Clicks) in The Emacs Manual). To this end, define a mode-specific function which builds one or more menus depending on the location of the `mouse-3` click in the buffer, and then add that function to the buffer-local value of `context-menu-functions`.
* The mode should specify how Imenu should find the definitions or sections of a buffer, by setting up a buffer-local value for the variable `imenu-generic-expression`, for the two variables `imenu-prev-index-position-function` and `imenu-extract-index-name-function`, or for the variable `imenu-create-index-function` (see [Imenu](imenu)).
* The mode can tell ElDoc mode how to retrieve different types of documentation for whatever is at point, by adding one or more buffer-local entries to the special hook `eldoc-documentation-functions`.
* The mode can specify how to complete various keywords by adding one or more buffer-local entries to the special hook `completion-at-point-functions`. See [Completion in Buffers](completion-in-buffers).
* To make a buffer-local binding for an Emacs customization variable, use `make-local-variable` in the major mode command, not `make-variable-buffer-local`. The latter function would make the variable local to every buffer in which it is subsequently set, which would affect buffers that do not use this mode. It is undesirable for a mode to have such global effects. See [Buffer-Local Variables](buffer_002dlocal-variables). With rare exceptions, the only reasonable way to use `make-variable-buffer-local` in a Lisp package is for a variable which is used only within that package. Using it on a variable used by other packages would interfere with them.
* Each major mode should have a normal *mode hook* named `modename-mode-hook`. The very last thing the major mode command should do is to call `run-mode-hooks`. This runs the normal hook `change-major-mode-after-body-hook`, the mode hook, the function `hack-local-variables` (when the buffer is visiting a file), and then the normal hook `after-change-major-mode-hook`. See [Mode Hooks](mode-hooks).
* The major mode command may start by calling some other major mode command (called the *parent mode*) and then alter some of its settings. A mode that does this is called a *derived mode*. The recommended way to define one is to use the `define-derived-mode` macro, but this is not required. Such a mode should call the parent mode command inside a `delay-mode-hooks` form. (Using `define-derived-mode` does this automatically.) See [Derived Modes](derived-modes), and [Mode Hooks](mode-hooks).
* If something special should be done if the user switches a buffer from this mode to any other major mode, this mode can set up a buffer-local value for `change-major-mode-hook` (see [Creating Buffer-Local](creating-buffer_002dlocal)).
* If this mode is appropriate only for specially-prepared text produced by the mode itself (rather than by the user typing at the keyboard or by an external file), then the major mode command symbol should have a property named `mode-class` with value `special`, put on as follows:
```
(put 'funny-mode 'mode-class 'special)
```
This tells Emacs that new buffers created while the current buffer is in Funny mode should not be put in Funny mode, even though the default value of `major-mode` is `nil`. By default, the value of `nil` for `major-mode` means to use the current buffer’s major mode when creating new buffers (see [Auto Major Mode](auto-major-mode)), but with such `special` modes, Fundamental mode is used instead. Modes such as Dired, Rmail, and Buffer List use this feature.
The function `view-buffer` does not enable View mode in buffers whose mode-class is special, because such modes usually provide their own View-like bindings.
The `define-derived-mode` macro automatically marks the derived mode as special if the parent mode is special. Special mode is a convenient parent for such modes to inherit from; See [Basic Major Modes](basic-major-modes).
* If you want to make the new mode the default for files with certain recognizable names, add an element to `auto-mode-alist` to select the mode for those file names (see [Auto Major Mode](auto-major-mode)). If you define the mode command to autoload, you should add this element in the same file that calls `autoload`. If you use an autoload cookie for the mode command, you can also use an autoload cookie for the form that adds the element (see [autoload cookie](autoload#autoload-cookie)). If you do not autoload the mode command, it is sufficient to add the element in the file that contains the mode definition.
* The top-level forms in the file defining the mode should be written so that they may be evaluated more than once without adverse consequences. For instance, use `defvar` or `defcustom` to set mode-related variables, so that they are not reinitialized if they already have a value (see [Defining Variables](defining-variables)).
| programming_docs |
elisp None ### Introduction to Reading and Printing
*Reading* a Lisp object means parsing a Lisp expression in textual form and producing a corresponding Lisp object. This is how Lisp programs get into Lisp from files of Lisp code. We call the text the *read syntax* of the object. For example, the text ‘`(a . 5)`’ is the read syntax for a cons cell whose CAR is `a` and whose CDR is the number 5.
*Printing* a Lisp object means producing text that represents that object—converting the object to its *printed representation* (see [Printed Representation](printed-representation)). Printing the cons cell described above produces the text ‘`(a . 5)`’.
Reading and printing are more or less inverse operations: printing the object that results from reading a given piece of text often produces the same text, and reading the text that results from printing an object usually produces a similar-looking object. For example, printing the symbol `foo` produces the text ‘`foo`’, and reading that text returns the symbol `foo`. Printing a list whose elements are `a` and `b` produces the text ‘`(a b)`’, and reading that text produces a list (but not the same list) with elements `a` and `b`.
However, these two operations are not precisely inverse to each other. There are three kinds of exceptions:
* Printing can produce text that cannot be read. For example, buffers, windows, frames, subprocesses and markers print as text that starts with ‘`#`’; if you try to read this text, you get an error. There is no way to read those data types.
* One object can have multiple textual representations. For example, ‘`1`’ and ‘`01`’ represent the same integer, and ‘`(a b)`’ and ‘`(a . (b))`’ represent the same list. Reading will accept any of the alternatives, but printing must choose one of them.
* Comments can appear at certain points in the middle of an object’s read sequence without affecting the result of reading it.
elisp None ### Information from the Command Loop
The editor command loop sets several Lisp variables to keep status records for itself and for commands that are run. With the exception of `this-command` and `last-command` it’s generally a bad idea to change any of these variables in a Lisp program.
Variable: **last-command**
This variable records the name of the previous command executed by the command loop (the one before the current command). Normally the value is a symbol with a function definition, but this is not guaranteed.
The value is copied from `this-command` when a command returns to the command loop, except when the command has specified a prefix argument for the following command.
This variable is always local to the current terminal and cannot be buffer-local. See [Multiple Terminals](multiple-terminals).
Variable: **real-last-command**
This variable is set up by Emacs just like `last-command`, but never altered by Lisp programs.
Variable: **last-repeatable-command**
This variable stores the most recently executed command that was not part of an input event. This is the command `repeat` will try to repeat, See [Repeating](https://www.gnu.org/software/emacs/manual/html_node/emacs/Repeating.html#Repeating) in The GNU Emacs Manual.
Variable: **this-command**
This variable records the name of the command now being executed by the editor command loop. Like `last-command`, it is normally a symbol with a function definition.
The command loop sets this variable just before running a command, and copies its value into `last-command` when the command finishes (unless the command specified a prefix argument for the following command).
Some commands set this variable during their execution, as a flag for whatever command runs next. In particular, the functions for killing text set `this-command` to `kill-region` so that any kill commands immediately following will know to append the killed text to the previous kill.
If you do not want a particular command to be recognized as the previous command in the case where it got an error, you must code that command to prevent this. One way is to set `this-command` to `t` at the beginning of the command, and set `this-command` back to its proper value at the end, like this:
```
(defun foo (args…)
(interactive …)
(let ((old-this-command this-command))
(setq this-command t)
…do the work…
(setq this-command old-this-command)))
```
We do not bind `this-command` with `let` because that would restore the old value in case of error—a feature of `let` which in this case does precisely what we want to avoid.
Variable: **this-original-command**
This has the same value as `this-command` except when command remapping occurs (see [Remapping Commands](remapping-commands)). In that case, `this-command` gives the command actually run (the result of remapping), and `this-original-command` gives the command that was specified to run but remapped into another command.
Variable: **current-minibuffer-command**
This has the same value as `this-command`, but is bound recursively when entering a minibuffer. This variable can be used from minibuffer hooks and the like to determine what command opened the current minibuffer session.
Function: **this-command-keys**
This function returns a string or vector containing the key sequence that invoked the present command. Any events read by the command using `read-event` without a timeout get tacked on to the end.
However, if the command has called `read-key-sequence`, it returns the last read key sequence. See [Key Sequence Input](key-sequence-input). The value is a string if all events in the sequence were characters that fit in a string. See [Input Events](input-events).
```
(this-command-keys)
;; Now use `C-u C-x C-e` to evaluate that.
⇒ "^X^E"
```
Function: **this-command-keys-vector**
Like `this-command-keys`, except that it always returns the events in a vector, so you don’t need to deal with the complexities of storing input events in a string (see [Strings of Events](strings-of-events)).
Function: **clear-this-command-keys** *&optional keep-record*
This function empties out the table of events for `this-command-keys` to return. Unless keep-record is non-`nil`, it also empties the records that the function `recent-keys` (see [Recording Input](recording-input)) will subsequently return. This is useful after reading a password, to prevent the password from echoing inadvertently as part of the next command in certain cases.
Variable: **last-nonmenu-event**
This variable holds the last input event read as part of a key sequence, not counting events resulting from mouse menus.
One use of this variable is for telling `x-popup-menu` where to pop up a menu. It is also used internally by `y-or-n-p` (see [Yes-or-No Queries](yes_002dor_002dno-queries)).
Variable: **last-command-event**
This variable is set to the last input event that was read by the command loop as part of a command. The principal use of this variable is in `self-insert-command`, which uses it to decide which character to insert.
```
last-command-event
;; Now use `C-u C-x C-e` to evaluate that.
⇒ 5
```
The value is 5 because that is the ASCII code for `C-e`.
Variable: **last-event-frame**
This variable records which frame the last input event was directed to. Usually this is the frame that was selected when the event was generated, but if that frame has redirected input focus to another frame, the value is the frame to which the event was redirected. See [Input Focus](input-focus).
If the last event came from a keyboard macro, the value is `macro`.
elisp None ### Asking Multiple-Choice Questions
This section describes facilities for asking the user more complex questions or several similar questions.
When you have a series of similar questions to ask, such as “Do you want to save this buffer?” for each buffer in turn, you should use `map-y-or-n-p` to ask the collection of questions, rather than asking each question individually. This gives the user certain convenient facilities such as the ability to answer the whole series at once.
Function: **map-y-or-n-p** *prompter actor list &optional help action-alist no-cursor-in-echo-area*
This function asks the user a series of questions, reading a single-character answer in the echo area for each one.
The value of list specifies the objects to ask questions about. It should be either a list of objects or a generator function. If it is a function, it will be called with no arguments, and should return either the next object to ask about, or `nil`, meaning to stop asking questions.
The argument prompter specifies how to ask each question. If prompter is a string, the question text is computed like this:
```
(format prompter object)
```
where object is the next object to ask about (as obtained from list). See [Formatting Strings](formatting-strings), for more information about `format`.
If prompter is not a string, it should be a function of one argument (the object to ask about) and should return the question text for that object. If the value prompter returns is a string, that is the question to ask the user. The function can also return `t`, meaning to act on this object without asking the user, or `nil`, which means to silently ignore this object.
The argument actor says how to act on the objects for which the user answers yes. It should be a function of one argument, and will be called with each object from list for which the user answers yes.
If the argument help is given, it should be a list of this form:
```
(singular plural action)
```
where singular is a string containing a singular noun that describes a single object to be acted on, plural is the corresponding plural noun, and action is a transitive verb describing what actor does with the objects.
If you don’t specify help, it defaults to the list `("object" "objects" "act on")`.
Each time a question is asked, the user can answer as follows:
`y`, `Y`, or `SPC`
act on the object
`n`, `N`, or `DEL`
skip the object
`!` act on all the following objects
`ESC` or `q`
exit (skip all following objects)
`.` (period) act on the object and then exit
`C-h` get help
These are the same answers that `query-replace` accepts. The keymap `query-replace-map` defines their meaning for `map-y-or-n-p` as well as for `query-replace`; see [Search and Replace](search-and-replace).
You can use action-alist to specify additional possible answers and what they mean. If provided, action-alist should be an alist whose elements are of the form `(char function help)`. Each of the alist elements defines one additional answer. In each element, char is a character (the answer); function is a function of one argument (an object from list); and help is a string. When the user responds with char, `map-y-or-n-p` calls function. If it returns non-`nil`, the object is considered to have been acted upon, and `map-y-or-n-p` advances to the next object in list. If it returns `nil`, the prompt is repeated for the same object. If the user requests help, the text in help is used to describe these additional answers.
Normally, `map-y-or-n-p` binds `cursor-in-echo-area` while prompting. But if no-cursor-in-echo-area is non-`nil`, it does not do that.
If `map-y-or-n-p` is called in a command that was invoked using the mouse—more precisely, if `last-nonmenu-event` (see [Command Loop Info](command-loop-info)) is either `nil` or a list—then it uses a dialog box or pop-up menu to ask the question. In this case, it does not use keyboard input or the echo area. You can force use either of the mouse or of keyboard input by binding `last-nonmenu-event` to a suitable value around the call.
The return value of `map-y-or-n-p` is the number of objects acted on.
If you need to ask the user a question that might have more than just 2 answers, use `read-answer`.
Function: **read-answer** *question answers*
This function prompts the user with text in question, which should end in the ‘`SPC`’ character. The function includes in the prompt the possible responses in answers by appending them to the end of question. The possible responses are provided in answers as an alist whose elements are of the following form:
```
(long-answer short-answer help-message)
```
where long-answer is the complete text of the user response, a string; short-answer is a short form of the same response, a single character or a function key; and help-message is the text that describes the meaning of the answer. If the variable `read-answer-short` is non-`nil`, the prompt will show the short variants of the possible answers and the user is expected to type the single characters/keys shown in the prompt; otherwise the prompt will show the long variants of the answers, and the user is expected to type the full text of one of the answers and end by pressing RET. If `use-dialog-box` is non-`nil`, and this function was invoked by mouse events, the question and the answers will be displayed in a GUI dialog box.
The function returns the text of the long-answer selected by the user, regardless of whether long or short answers were shown in the prompt and typed by the user.
Here is an example of using this function:
```
(let ((read-answer-short t))
(read-answer "Foo "
'(("yes" ?y "perform the action")
("no" ?n "skip to the next")
("all" ?! "perform for the rest without more questions")
("help" ?h "show help")
("quit" ?q "exit"))))
```
Function: **read-char-from-minibuffer** *prompt &optional chars history*
This function uses the minibuffer to read and return a single character. Optionally, it ignores any input that is not a member of chars, a list of accepted characters. The history argument specifies the history list symbol to use; if it is omitted or `nil`, this function doesn’t use the history.
If you bind `help-form` (see [Help Functions](help-functions)) to a non-`nil` value while calling `read-char-from-minibuffer`, then pressing `help-char` causes it to evaluate `help-form` and display the result.
elisp None #### A Sample Variable Description
A *variable* is a name that can be *bound* (or *set*) to an object. The object to which a variable is bound is called a *value*; we say also that variable holds that value. Although nearly all variables can be set by the user, certain variables exist specifically so that users can change them; these are called *user options*. Ordinary variables and user options are described using a format like that for functions, except that there are no arguments.
Here is a description of the imaginary `electric-future-map` variable.
Variable: **electric-future-map**
The value of this variable is a full keymap used by Electric Command Future mode. The functions in this map allow you to edit commands you have not yet thought about executing.
User option descriptions have the same format, but ‘`Variable`’ is replaced by ‘`User Option`’.
elisp None ### Examining Buffer Contents
This section describes functions that allow a Lisp program to convert any portion of the text in the buffer into a string.
Function: **buffer-substring** *start end*
This function returns a string containing a copy of the text of the region defined by positions start and end in the current buffer. If the arguments are not positions in the accessible portion of the buffer, `buffer-substring` signals an `args-out-of-range` error.
Here’s an example which assumes Font-Lock mode is not enabled:
```
---------- Buffer: foo ----------
This is the contents of buffer foo
---------- Buffer: foo ----------
```
```
(buffer-substring 1 10)
⇒ "This is t"
```
```
(buffer-substring (point-max) 10)
⇒ "he contents of buffer foo\n"
```
If the text being copied has any text properties, these are copied into the string along with the characters they belong to. See [Text Properties](text-properties). However, overlays (see [Overlays](overlays)) in the buffer and their properties are ignored, not copied.
For example, if Font-Lock mode is enabled, you might get results like these:
```
(buffer-substring 1 10)
⇒ #("This is t" 0 1 (fontified t) 1 9 (fontified t))
```
Function: **buffer-substring-no-properties** *start end*
This is like `buffer-substring`, except that it does not copy text properties, just the characters themselves. See [Text Properties](text-properties).
Function: **buffer-string**
This function returns the contents of the entire accessible portion of the current buffer, as a string. If the text being copied has any text properties, these are copied into the string along with the characters they belong to.
If you need to make sure the resulting string, when copied to a different location, will not change its visual appearance due to reordering of bidirectional text, use the `buffer-substring-with-bidi-context` function (see [buffer-substring-with-bidi-context](bidirectional-display)).
Function: **filter-buffer-substring** *start end &optional delete*
This function filters the buffer text between start and end using a function specified by the variable `filter-buffer-substring-function`, and returns the result.
The default filter function consults the obsolete wrapper hook `filter-buffer-substring-functions` (see the documentation string of the macro `with-wrapper-hook` for the details about this obsolete facility), and the obsolete variable `buffer-substring-filters`. If both of these are `nil`, it returns the unaltered text from the buffer, i.e., what `buffer-substring` would return.
If delete is non-`nil`, the function deletes the text between start and end after copying it, like `delete-and-extract-region`.
Lisp code should use this function instead of `buffer-substring`, `buffer-substring-no-properties`, or `delete-and-extract-region` when copying into user-accessible data structures such as the kill-ring, X clipboard, and registers. Major and minor modes can modify `filter-buffer-substring-function` to alter such text as it is copied out of the buffer.
Variable: **filter-buffer-substring-function**
The value of this variable is a function that `filter-buffer-substring` will call to do the actual work. The function receives three arguments, the same as those of `filter-buffer-substring`, which it should treat as per the documentation of that function. It should return the filtered text (and optionally delete the source text).
The following two variables are obsoleted by `filter-buffer-substring-function`, but are still supported for backward compatibility.
Variable: **filter-buffer-substring-functions**
This obsolete variable is a wrapper hook, whose members should be functions that accept four arguments: fun, start, end, and delete. fun is a function that takes three arguments (start, end, and delete), and returns a string. In both cases, the start, end, and delete arguments are the same as those of `filter-buffer-substring`.
The first hook function is passed a fun that is equivalent to the default operation of `filter-buffer-substring`, i.e., it returns the buffer-substring between start and end (processed by any `buffer-substring-filters`) and optionally deletes the original text from the buffer. In most cases, the hook function will call fun once, and then do its own processing of the result. The next hook function receives a fun equivalent to this, and so on. The actual return value is the result of all the hook functions acting in sequence.
Variable: **buffer-substring-filters**
The value of this obsolete variable should be a list of functions that accept a single string argument and return another string. The default `filter-buffer-substring` function passes the buffer substring to the first function in this list, and the return value of each function is passed to the next function. The return value of the last function is passed to `filter-buffer-substring-functions`.
Function: **current-word** *&optional strict really-word*
This function returns the symbol (or word) at or near point, as a string. The return value includes no text properties.
If the optional argument really-word is non-`nil`, it finds a word; otherwise, it finds a symbol (which includes both word characters and symbol constituent characters).
If the optional argument strict is non-`nil`, then point must be in or next to the symbol or word—if no symbol or word is there, the function returns `nil`. Otherwise, a nearby symbol or word on the same line is acceptable.
Function: **thing-at-point** *thing &optional no-properties*
Return the thing around or next to point, as a string.
The argument thing is a symbol which specifies a kind of syntactic entity. Possibilities include `symbol`, `list`, `sexp`, `defun`, `filename`, `existing-filename`, `url`, `word`, `sentence`, `whitespace`, `line`, `page`, `string`, and others.
When the optional argument no-properties is non-`nil`, this function strips text properties from the return value.
```
---------- Buffer: foo ----------
Gentlemen may cry ``Pea∗ce! Peace!,''
but there is no peace.
---------- Buffer: foo ----------
(thing-at-point 'word)
⇒ "Peace"
(thing-at-point 'line)
⇒ "Gentlemen may cry ``Peace! Peace!,''\n"
(thing-at-point 'whitespace)
⇒ nil
```
Variable: **thing-at-point-provider-alist**
This variable allows users and modes to tweak how `thing-at-point` works. It’s an association list of things and functions (called with zero parameters) to return that thing. Entries for thing will be evaluated in turn until a non-`nil` result is returned.
For instance, a major mode could say:
```
(setq-local thing-at-point-provider-alist
(append thing-at-point-provider-alist
'((url . my-mode--url-at-point))))
```
If no providers have a non-`nil` return, the thing will be computed the standard way.
| programming_docs |
elisp None #### Writing Module Functions
The main reason for writing an Emacs module is to make additional functions available to Lisp programs that load the module. This subsection describes how to write such *module functions*.
A module function has the following general form and signature:
Function: *emacs\_value* **emacs\_function** *(emacs\_env \*env, ptrdiff\_t nargs, emacs\_value \*args, void \*data)*
The env argument provides a pointer to the API environment, needed to access Emacs objects and functions. The nargs argument is the required number of arguments, which can be zero (see `make_function` below for more flexible specification of the argument number), and args is a pointer to the array of the function arguments. The argument data points to additional data required by the function, which was arranged when `make_function` (see below) was called to create an Emacs function from `emacs_function`.
Module functions use the type `emacs_value` to communicate Lisp objects between Emacs and the module (see [Module Values](module-values)). The API, described below and in the following subsections, provides facilities for conversion between basic C data types and the corresponding `emacs_value` objects.
A module function always returns a value. If the function returns normally, the Lisp code which called it will see the Lisp object corresponding to the `emacs_value` value the function returned. However, if the user typed `C-g`, or if the module function or its callees signaled an error or exited nonlocally (see [Module Nonlocal](module-nonlocal)), Emacs will ignore the returned value and quit or throw as it does when Lisp code encounters the same situations.
The header `emacs-module.h` provides the type `emacs_function` as an alias type for a function pointer to a module function.
After writing your C code for a module function, you should make a Lisp function object from it using the `make_function` function, whose pointer is provided in the environment (recall that the pointer to the environment is returned by `get_environment`). This is normally done in the module initialization function (see [module initialization function](module-initialization#module-initialization-function)), after verifying the API compatibility.
Function: *emacs\_value* **make\_function** *(emacs\_env \*env, ptrdiff\_t min\_arity, ptrdiff\_t max\_arity, emacs\_function func, const char \*docstring, void \*data)*
This returns an Emacs function created from the C function func, whose signature is as described for `emacs_function` above. The arguments min\_arity and max\_arity specify the minimum and maximum number of arguments that func can accept. The max\_arity argument can have the special value `emacs_variadic_function`, which makes the function accept an unlimited number of arguments, like the `&rest` keyword in Lisp (see [Argument List](argument-list)).
The argument data is a way to arrange for arbitrary additional data to be passed to func when it is called. Whatever pointer is passed to `make_function` will be passed unaltered to func.
The argument docstring specifies the documentation string for the function. It should be either an ASCII string, or a UTF-8 encoded non-ASCII string, or a `NULL` pointer; in the latter case the function will have no documentation. The documentation string can end with a line that specifies the advertised calling convention, see [Function Documentation](function-documentation).
Since every module function must accept the pointer to the environment as its first argument, the call to `make_function` could be made from any module function, but you will normally want to do that from the module initialization function, so that all the module functions are known to Emacs once the module is loaded.
Finally, you should bind the Lisp function to a symbol, so that Lisp code could call your function by name. For that, use the module API function `intern` (see [intern](module-misc#intern)) whose pointer is also provided in the environment that module functions can access.
Combining the above steps, code that arranges for a C function `module_func` to be callable as `module-func` from Lisp will look like this, as part of the module initialization function:
```
emacs_env *env = runtime->get_environment (runtime);
emacs_value func = env->make_function (env, min_arity, max_arity,
module_func, docstring, data);
emacs_value symbol = env->intern (env, "module-func");
emacs_value args[] = {symbol, func};
env->funcall (env, env->intern (env, "defalias"), 2, args);
```
This makes the symbol `module-func` known to Emacs by calling `env->intern`, then invokes `defalias` from Emacs to bind the function to that symbol. Note that it is possible to use `fset` instead of `defalias`; the differences are described in [defalias](defining-functions).
Module functions including the `emacs_module_init` function (see [module initialization function](module-initialization#module-initialization-function)) may only interact with Emacs by calling environment functions from some live `emacs_env` pointer while being called directly or indirectly from Emacs. In other words, if a module function wants to call Lisp functions or Emacs primitives, convert `emacs_value` objects to and from C datatypes (see [Module Values](module-values)), or interact with Emacs in any other way, some call from Emacs to `emacs_module_init` or to a module function must be in the call stack. Module functions may not interact with Emacs while garbage collection is running; see [Garbage Collection](garbage-collection). They may only interact with Emacs from Lisp interpreter threads (including the main thread) created by Emacs; see [Threads](threads). The `--module-assertions` command-line option can detect some violations of the above requirements. See [Initial Options](https://www.gnu.org/software/emacs/manual/html_node/emacs/Initial-Options.html#Initial-Options) in The GNU Emacs Manual.
Using the module API, it is possible to define more complex function and data types: inline functions, macros, etc. However, the resulting C code will be cumbersome and hard to read. Therefore, we recommend that you limit the module code which creates functions and data structures to the absolute minimum, and leave the rest for a Lisp package that will accompany your module, because doing these additional tasks in Lisp is much easier, and will produce a much more readable code. For example, given a module function `module-func` defined as above, one way of making a macro `module-macro` based on it is with the following simple Lisp wrapper:
```
(defmacro module-macro (&rest args)
"Documentation string for the macro."
(module-func args))
```
The Lisp package which goes with your module could then load the module using the `load` primitive (see [Dynamic Modules](dynamic-modules)) when the package is loaded into Emacs.
By default, module functions created by `make_function` are not interactive. To make them interactive, you can use the following function.
Function: *void* **make\_interactive** *(emacs\_env \*env, emacs\_value function, emacs\_value spec)*
This function, which is available since Emacs 28, makes the function function interactive using the interactive specification spec. Emacs interprets spec like the argument to the `interactive` form. [Using Interactive](using-interactive), and see [Interactive Codes](interactive-codes). function must be an Emacs module function returned by `make_function`.
Note that there is no native module support for retrieving the interactive specification of a module function. Use the function `interactive-form` for that. [Using Interactive](using-interactive). It is not possible to make a module function non-interactive once you have made it interactive using `make_interactive`.
If you want to run some code when a module function object (i.e., an object returned by `make_function`) is garbage-collected, you can install a *function finalizer*. Function finalizers are available since Emacs 28. For example, if you have passed some heap-allocated structure to the data argument of `make_function`, you can use the finalizer to deallocate the structure. See [(libc)Basic Allocation](https://www.gnu.org/software/libc/manual/html_node/Basic-Allocation.html#Basic-Allocation), and see [(libc)Freeing after Malloc](https://www.gnu.org/software/libc/manual/html_node/Freeing-after-Malloc.html#Freeing-after-Malloc). The finalizer function has the following signature:
```
void finalizer (void *data)
```
Here, data receives the value passed to data when calling `make_function`. Note that the finalizer can’t interact with Emacs in any way.
Directly after calling `make_function`, the newly-created function doesn’t have a finalizer. Use `set_function_finalizer` to add one, if desired.
Function: *void* **emacs\_finalizer** *(void \*ptr)*
The header `emacs-module.h` provides the type `emacs_finalizer` as a type alias for an Emacs finalizer function.
Function: *emacs\_finalizer* **get\_function\_finalizer** *(emacs\_env \*env, emacs\_value arg)*
This function, which is available since Emacs 28, returns the function finalizer associated with the module function represented by arg. arg must refer to a module function, that is, an object returned by `make_function`. If no finalizer is associated with the function, `NULL` is returned.
Function: *void* **set\_function\_finalizer** *(emacs\_env \*env, emacs\_value arg, emacs\_finalizer fin)*
This function, which is available since Emacs 28, sets the function finalizer associated with the module function represented by arg to fin. arg must refer to a module function, that is, an object returned by `make_function`. fin can either be `NULL` to clear arg’s function finalizer, or a pointer to a function to be called when the object represented by arg is garbage-collected. At most one function finalizer can be set per function; if arg already has a finalizer, it is replaced by fin.
elisp None ### Input Streams
Most of the Lisp functions for reading text take an *input stream* as an argument. The input stream specifies where or how to get the characters of the text to be read. Here are the possible types of input stream:
buffer
The input characters are read from buffer, starting with the character directly after point. Point advances as characters are read.
marker
The input characters are read from the buffer that marker is in, starting with the character directly after the marker. The marker position advances as characters are read. The value of point in the buffer has no effect when the stream is a marker.
string
The input characters are taken from string, starting at the first character in the string and using as many characters as required.
function
The input characters are generated by function, which must support two kinds of calls:
* When it is called with no arguments, it should return the next character.
* When it is called with one argument (always a character), function should save the argument and arrange to return it on the next call. This is called *unreading* the character; it happens when the Lisp reader reads one character too many and wants to put it back where it came from. In this case, it makes no difference what value function returns.
`t`
`t` used as a stream means that the input is read from the minibuffer. In fact, the minibuffer is invoked once and the text given by the user is made into a string that is then used as the input stream. If Emacs is running in batch mode (see [Batch Mode](batch-mode)), standard input is used instead of the minibuffer. For example,
```
(message "%s" (read t))
```
will in batch mode read a Lisp expression from standard input and print the result to standard output.
`nil`
`nil` supplied as an input stream means to use the value of `standard-input` instead; that value is the *default input stream*, and must be a non-`nil` input stream.
symbol A symbol as input stream is equivalent to the symbol’s function definition (if any).
Here is an example of reading from a stream that is a buffer, showing where point is located before and after:
```
---------- Buffer: foo ----------
This∗ is the contents of foo.
---------- Buffer: foo ----------
```
```
(read (get-buffer "foo"))
⇒ is
```
```
(read (get-buffer "foo"))
⇒ the
```
```
---------- Buffer: foo ----------
This is the∗ contents of foo.
---------- Buffer: foo ----------
```
Note that the first read skips a space. Reading skips any amount of whitespace preceding the significant text.
Here is an example of reading from a stream that is a marker, initially positioned at the beginning of the buffer shown. The value read is the symbol `This`.
```
---------- Buffer: foo ----------
This is the contents of foo.
---------- Buffer: foo ----------
```
```
(setq m (set-marker (make-marker) 1 (get-buffer "foo")))
⇒ #<marker at 1 in foo>
```
```
(read m)
⇒ This
```
```
m
⇒ #<marker at 5 in foo> ;; Before the first space.
```
Here we read from the contents of a string:
```
(read "(When in) the course")
⇒ (When in)
```
The following example reads from the minibuffer. The prompt is: ‘`Lisp expression:` ’. (That is always the prompt used when you read from the stream `t`.) The user’s input is shown following the prompt.
```
(read t)
⇒ 23
---------- Buffer: Minibuffer ----------
Lisp expression: 23 RET
---------- Buffer: Minibuffer ----------
```
Finally, here is an example of a stream that is a function, named `useless-stream`. Before we use the stream, we initialize the variable `useless-list` to a list of characters. Then each call to the function `useless-stream` obtains the next character in the list or unreads a character by adding it to the front of the list.
```
(setq useless-list (append "XY()" nil))
⇒ (88 89 40 41)
```
```
(defun useless-stream (&optional unread)
(if unread
(setq useless-list (cons unread useless-list))
(prog1 (car useless-list)
(setq useless-list (cdr useless-list)))))
⇒ useless-stream
```
Now we read using the stream thus constructed:
```
(read 'useless-stream)
⇒ XY
```
```
useless-list
⇒ (40 41)
```
Note that the open and close parentheses remain in the list. The Lisp reader encountered the open parenthesis, decided that it ended the input, and unread it. Another attempt to read from the stream at this point would read ‘`()`’ and return `nil`.
elisp None #### Formatted Text Properties
These text properties affect the behavior of the fill commands. They are used for representing formatted text. See [Filling](filling), and [Margins](margins).
`hard`
If a newline character has this property, it is a “hard” newline. The fill commands do not alter hard newlines and do not move words across them. However, this property takes effect only if the `use-hard-newlines` minor mode is enabled. See [Hard and Soft Newlines](https://www.gnu.org/software/emacs/manual/html_node/emacs/Hard-and-Soft-Newlines.html#Hard-and-Soft-Newlines) in The GNU Emacs Manual.
`right-margin`
This property specifies an extra right margin for filling this part of the text.
`left-margin`
This property specifies an extra left margin for filling this part of the text.
`justification` This property specifies the style of justification for filling this part of the text.
elisp None #### Altering List Elements with setcar
Changing the CAR of a cons cell is done with `setcar`. When used on a list, `setcar` replaces one element of a list with a different element.
Function: **setcar** *cons object*
This function stores object as the new CAR of cons, replacing its previous CAR. In other words, it changes the CAR slot of cons to refer to object. It returns the value object. For example:
```
(setq x (list 1 2))
⇒ (1 2)
```
```
(setcar x 4)
⇒ 4
```
```
x
⇒ (4 2)
```
When a cons cell is part of the shared structure of several lists, storing a new CAR into the cons changes one element of each of these lists. Here is an example:
```
;; Create two lists that are partly shared.
(setq x1 (list 'a 'b 'c))
⇒ (a b c)
(setq x2 (cons 'z (cdr x1)))
⇒ (z b c)
```
```
;; Replace the CAR of a shared link.
(setcar (cdr x1) 'foo)
⇒ foo
x1 ; Both lists are changed.
⇒ (a foo c)
x2
⇒ (z foo c)
```
```
;; Replace the CAR of a link that is not shared.
(setcar x1 'baz)
⇒ baz
x1 ; Only one list is changed.
⇒ (baz foo c)
x2
⇒ (z foo c)
```
Here is a graphical depiction of the shared structure of the two lists in the variables `x1` and `x2`, showing why replacing `b` changes them both:
```
--- --- --- --- --- ---
x1---> | | |----> | | |--> | | |--> nil
--- --- --- --- --- ---
| --> | |
| | | |
--> a | --> b --> c
|
--- --- |
x2--> | | |--
--- ---
|
|
--> z
```
Here is an alternative form of box diagram, showing the same relationship:
```
x1:
-------------- -------------- --------------
| car | cdr | | car | cdr | | car | cdr |
| a | o------->| b | o------->| c | nil |
| | | -->| | | | | |
-------------- | -------------- --------------
|
x2: |
-------------- |
| car | cdr | |
| z | o----
| | |
--------------
```
elisp None ### Cyclic Ordering of Windows
When you use the command `C-x o` (`other-window`) to select some other window, it moves through live windows in a specific order. For any given configuration of windows, this order never varies. It is called the *cyclic ordering of windows*.
The ordering is determined by a depth-first traversal of each frame’s window tree, retrieving the live windows which are the leaf nodes of the tree (see [Windows and Frames](windows-and-frames)). If the minibuffer is active, the minibuffer window is included too. The ordering is cyclic, so the last window in the sequence is followed by the first one.
Function: **next-window** *&optional window minibuf all-frames*
This function returns a live window, the one following window in the cyclic ordering of windows. window should be a live window; if omitted or `nil`, it defaults to the selected window.
The optional argument minibuf specifies whether minibuffer windows should be included in the cyclic ordering. Normally, when minibuf is `nil`, a minibuffer window is included only if it is currently active; this matches the behavior of `C-x o`. (Note that a minibuffer window is active as long as its minibuffer is in use; see [Minibuffers](minibuffers)).
If minibuf is `t`, the cyclic ordering includes all minibuffer windows. If minibuf is neither `t` nor `nil`, minibuffer windows are not included even if they are active.
The optional argument all-frames specifies which frames to consider:
* `nil` means to consider windows on window’s frame. If the minibuffer window is considered (as specified by the minibuf argument), then frames that share the minibuffer window are considered too.
* `t` means to consider windows on all existing frames.
* `visible` means to consider windows on all visible frames.
* 0 means to consider windows on all visible or iconified frames.
* A frame means to consider windows on that specific frame.
* Anything else means to consider windows on window’s frame, and no others.
If more than one frame is considered, the cyclic ordering is obtained by appending the orderings for those frames, in the same order as the list of all live frames (see [Finding All Frames](finding-all-frames)).
Function: **previous-window** *&optional window minibuf all-frames*
This function returns a live window, the one preceding window in the cyclic ordering of windows. The other arguments are handled like in `next-window`.
Command: **other-window** *count &optional all-frames*
This function selects a live window, one count places from the selected window in the cyclic ordering of windows. If count is a positive number, it skips count windows forwards; if count is negative, it skips -count windows backwards; if count is zero, that simply re-selects the selected window. When called interactively, count is the numeric prefix argument.
The optional argument all-frames has the same meaning as in `next-window`, like a `nil` minibuf argument to `next-window`.
This function does not select a window that has a non-`nil` `no-other-window` window parameter (see [Window Parameters](window-parameters)), provided that `ignore-window-parameters` is `nil`.
If the `other-window` parameter of the selected window is a function, and `ignore-window-parameters` is `nil`, that function will be called with the arguments count and all-frames instead of the normal operation of this function.
Function: **walk-windows** *fun &optional minibuf all-frames*
This function calls the function fun once for each live window, with the window as the argument.
It follows the cyclic ordering of windows. The optional arguments minibuf and all-frames specify the set of windows included; these have the same arguments as in `next-window`. If all-frames specifies a frame, the first window walked is the first window on that frame (the one returned by `frame-first-window`), not necessarily the selected window.
If fun changes the window configuration by splitting or deleting windows, that does not alter the set of windows walked, which is determined prior to calling fun for the first time.
Function: **one-window-p** *&optional no-mini all-frames*
This function returns `t` if the selected window is the only live window, and `nil` otherwise.
If the minibuffer window is active, it is normally considered (so that this function returns `nil`). However, if the optional argument no-mini is non-`nil`, the minibuffer window is ignored even if active. The optional argument all-frames has the same meaning as for `next-window`.
The following functions return a window which satisfies some criterion, without selecting it:
Function: **get-lru-window** *&optional all-frames dedicated not-selected no-other*
This function returns a live window which is heuristically the least recently used one. The *least recently used window* is the least recently selected one—the window whose use time is less than the use time of all other live windows (see [Selecting Windows](selecting-windows)). The optional argument all-frames has the same meaning as in `next-window`.
If any full-width windows are present, only those windows are considered. A minibuffer window is never a candidate. A dedicated window (see [Dedicated Windows](dedicated-windows)) is never a candidate unless the optional argument dedicated is non-`nil`. The selected window is never returned, unless it is the only candidate. However, if the optional argument not-selected is non-`nil`, this function returns `nil` in that case. The optional argument no-other, if non-`nil`, means to never return a window whose `no-other-window` parameter is non-`nil`.
Function: **get-mru-window** *&optional all-frames dedicated not-selected no-other*
This function is like `get-lru-window`, but it returns the most recently used window instead. The *most recently used window* is the most recently selected one—the window whose use time exceeds the use time of all other live windows (see [Selecting Windows](selecting-windows)). The meaning of the arguments is the same as for `get-lru-window`.
Since in practice the most recently used window is always the selected one, it usually makes sense to call this function with a non-`nil` not-selected argument only.
Function: **get-largest-window** *&optional all-frames dedicated not-selected no-other*
This function returns the window with the largest area (height times width). If there are two candidate windows of the same size, it prefers the one that comes first in the cyclic ordering of windows, starting from the selected window. The meaning of the arguments is the same as for `get-lru-window`.
Function: **get-window-with-predicate** *predicate &optional minibuf all-frames default*
This function calls the function predicate for each of the windows in the cyclic order of windows in turn, passing it the window as an argument. If the predicate returns non-`nil` for any window, this function stops and returns that window. If no such window is found, the return value is default (which defaults to `nil`).
The optional arguments minibuf and all-frames specify the windows to search, and have the same meanings as in `next-window`.
| programming_docs |
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