question
stringlengths 25
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stringlengths 4
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|---|---|
I read it on several sites that precompiled headers and address space layout randomization is incompatible. What makes them incompatible? What would it take to fix it?
My question is OS independent I heard this is a problem on linux with grsecurity and on newer windows versions. (And I don't know what else has ASLR.)
|
Precompiled headers are not incompatible with ASLR.
The particular technique used to implement precompiled headers that was used by older versions of the Visual C++ compiler was broken by ASLR. There isan article on the Visual C++ Team Blogwith details on the problem and an explanation of how it was fixed. There is a hotpatch available for Visual C++ 2008 SP1 and the problem is not present in Visual C++ 2010.
There may have been a similar problem on Linux, I don't really know. I'm no expert on Linux, Linux security, or GCC.
|
I am going through a windows device driver and I saw struct code like this:
```
struct driver1
{
UINT64 Readable : 1;
UINT64 Writable : 1;
UINT64 Executable : 1;
UINT64 Control : 3;
UINT64 Status : 1;
UINT64 Reserved : 51;
UINT64 Available1 : 5;
UINT64 IsMapped : 1;
};
```
Does eachUINT64represent a single bit ? Does a colon represent bits?
|
This means thatReadable,WritableandExecutableeach take up a bit,Controltakes 3,Reserved51 and so on.
Refer tohttp://publications.gbdirect.co.uk/c_book/chapter6/bitfields.htmlfor more info.
The UINT64 simply means that the entire bitfield will be inside a 64 bit unsigned integer.
|
```
unsigned char x = 93;
unsigned char a = x << 4;
printf("a = %d\n", a);
```
I do understand how bit operators work but I don't understand the binary representation of x.
How is a = 208?
|
```
93 = 01011101
```
Shift that left 4 bits and it leaves the following (only 8 bits in your result):
```
11010000 = 208
```
|
I need to create a shared vector, with the same size as the block.
```
__global__ func()
{
const int size = blockDim.x;
__shared__ float* Vec[size];
..
}
```
I get this error
```
error : expression must have a constant value
```
I cannot understand where the problem is, sinceblockDim.xis "constant" for each block of threads?
|
If you look at the section about__shared__of theCUDA C++ Programming Guide, there's some text on how to specify a size for anexterndeclared shared array. Although it's a bit more complicated, this is the syntax on how to specify execution-time sized shared arrays. The way you're doing it won't work.
|
I'm compiling several C/C++ packages from source on Ubuntu. I compare the results from my build to running with the debian install, and find that my executables are slower.
I usedreadelf -ato compare the two executables. Things like the relocation section for '.rela.dyn' are substantially different, but I'm not sure what documentation I should be looking for to make sense of differences like this.
Does anyone have any advise for trying to tweak gcc to produce executables like the ones I find in the deb package?
|
Well, as it is open source, you should get the compile flags they use to generate the executables :)
my2c
|
I want to learn OpenGL ES, but so far I don't know anything about it and I was wondering if there is a tutorial which stars from 0 knowledge. Remember that I don't know anything about graphics, drawing, etc.
|
This is a great start since it's specific to iPhone:
OpenGL ES from the Ground Up: Table of Contents
|
I'm writing a program in C on Debian Linux x64. In part of the code, I have the following if statement:
```
if (*all_drops >= (npes * 128)) {
break;
}
```
all_dropsis anint*which has been assigned anint's worth of memory, whilenpesis anintwith the value 2. Therefore, the if statement should be true when the value inall_dropsis at least 256. However, I've had the variable get above 1000 without the if statement evaluating true.
I'm not sure if there's some nuance in comparing dereferenced pointers to other numbers, but I've searched Google and haven't been able to find any similar issues (which leads me to believe I am missing something). Any help you could give would be greatly appreciated.
|
This works as intented:http://ideone.com/DjHGI
I guess the problem can be in the way you're initializing*all_drops
|
Is there a c atoi() equivalent for wide chars on Linux? I can find something for MS (wtoi) but I can find anything in a standard Linux lib.
|
You can usewcstolto convert from wide strings to integer values.
|
So, my basic problem is that I'm trying to write a program for a small project I'm working on for fun.
Basically, my issue is this: I need to take user input, as an int, say 15, then manipulate the number so that it returns the result of 1 + 5, being 6. Or for another example say the number is 29, will give you 2 + 9 = 11, which would then need to be reduced down again to 1 + 1 = 2. That could probably be handled easily, but I'm stuck on how to actually split the int apart without having to take the numbers in one by one. I guess it's possible to with RegEx, but I was looking for a more efficient method.
|
This is not a particularly good job for a regex. The usual way would be to get individual digits as the remainder after dividing by 10.
|
Hi does anyone know a way to send signal SIGUSR1/2 without loss to another pid ?
the problem is when i put kill(pid, SIGUSR1) in a while loop, my other program recieve only few of them, its seems there is a loss and i have to wait between two calls to kill().
Im actually forced to used usleep() in my loop in order to receive all the SIGUSR signals ive sent.
I'd like to recieve all of them as fast as it is possible.
Thx.
|
You need to usesigaction()rather thansignal()becausesigaction()can control which signals are blocked while the current signal is being handled. Also, your receiver has to process each signal received; that is relatively costly compared to sending a signal, so your killer (signalling) process can outrun the 'killed' (receiving) process with ease. You should think about why you are needing to send so many signals. They are a mechanism to use in special circumstances.
|
For unpacking complex binary strings with mixed doubles and integers using Ruby'sString.unpackI need to determine offsets within the binary string. Commonly, doubles are 8 bytes and integers are 4 bytes, but in order to make my code machine-independent, I would like to query these sizes from within my Ruby code.
What is the easiest way to determine the size of integers and doubles from within Ruby, i.e., request the response of a request to C'ssizeof( type )method?
|
I have found a workable solution, by usingArray#pack, the inverse method ofString#unpack. By packing a string of 1 integer or 1 double it is possible to determine the size of integers and doubles from within ruby itself which gives the same results as usingsizeof(int)orsizeof(double)(because of the implementation ofArray#packin C):
```
[1.to_i].pack("i").size # 4
[1.to_f].pack("d").size # 8
```
|
This question already has answers here:Why are these constructs using pre and post-increment undefined behavior?(15 answers)Closed5 years ago.
I came across a very interesting fact while executing the following a simple c program:
```
#include<stdio.h>
int main( )
{
int k=0;
printf("%d%d%d", k++, k, ++k);
return 0;
}
```
in windows it was showing output as: 1 1 1
but in linux(ubuntu) it was showing as: 1 2 2
why is it so?
|
It's undefined behaviour. When there are no / ambiguous sequence points. See this wikipedia article:
http://en.wikipedia.org/wiki/Sequence_point
|
Often stacks in C are dependent upon datatype used to declare them. For example,
```
int arr[5]; //creates an integer array of size 5 for stack use
char arr[5]; //creates a character array of size 5 for stack use
```
are both limited to holding integer and character datatypes respectively and presumes that the programmer knows what data is generated during the runtime. What if I want a stack which can hold any datatype?
I initially thought of implementing it as a union, but the approach is not only difficult but also flawed. Any other suggestions?
|
I would use a structure like this:
```
struct THolder
{
int dataType; // this is a value representing the type
void *val; // this is the value
};
```
Then use an array ofTHolderto store your values.
|
EDIT: Removed code/explanation because this project has been given again and students can easily find the solution via this post.
To clarify what happened, I simply passed the wrong length/size in my recvfrom() call.
|
In this line:
```
if(recvfrom( temp->sockfd, sendHostIP, BUFFER_LEN, 0, (struct sockaddr *)&recvAddr, &recvLen) < 0)
errorMsg("recvfrom");
```
You passBUFFER_LENas the length (256), butsendHostIPis only of lengthMAXHOSTNAMELEN(64).
This causesrecvfrom()to overflow that buffer. The same problem occurs when you read in tolocalHostIP.
|
This method in Cocos2d:
```
/** Returns opposite of point.
@return CGPoint
@since v0.7.2
*/
static inline CGPoint
ccpNeg(const CGPoint v)
{
return ccp(-v.x, -v.y);
}
```
Why does it say 'CGPoint' after inline? Why not
```
static inline ccpNeg(...
```
?
|
Because the function wants to return a CGPoint.
|
I need to create a shared vector, with the same size as the block.
```
__global__ func()
{
const int size = blockDim.x;
__shared__ float* Vec[size];
..
}
```
I get this error
```
error : expression must have a constant value
```
I cannot understand where the problem is, sinceblockDim.xis "constant" for each block of threads?
|
If you look at the section about__shared__of theCUDA C++ Programming Guide, there's some text on how to specify a size for anexterndeclared shared array. Although it's a bit more complicated, this is the syntax on how to specify execution-time sized shared arrays. The way you're doing it won't work.
|
I'm compiling several C/C++ packages from source on Ubuntu. I compare the results from my build to running with the debian install, and find that my executables are slower.
I usedreadelf -ato compare the two executables. Things like the relocation section for '.rela.dyn' are substantially different, but I'm not sure what documentation I should be looking for to make sense of differences like this.
Does anyone have any advise for trying to tweak gcc to produce executables like the ones I find in the deb package?
|
Well, as it is open source, you should get the compile flags they use to generate the executables :)
my2c
|
I want to learn OpenGL ES, but so far I don't know anything about it and I was wondering if there is a tutorial which stars from 0 knowledge. Remember that I don't know anything about graphics, drawing, etc.
|
This is a great start since it's specific to iPhone:
OpenGL ES from the Ground Up: Table of Contents
|
I'm writing a program in C on Debian Linux x64. In part of the code, I have the following if statement:
```
if (*all_drops >= (npes * 128)) {
break;
}
```
all_dropsis anint*which has been assigned anint's worth of memory, whilenpesis anintwith the value 2. Therefore, the if statement should be true when the value inall_dropsis at least 256. However, I've had the variable get above 1000 without the if statement evaluating true.
I'm not sure if there's some nuance in comparing dereferenced pointers to other numbers, but I've searched Google and haven't been able to find any similar issues (which leads me to believe I am missing something). Any help you could give would be greatly appreciated.
|
This works as intented:http://ideone.com/DjHGI
I guess the problem can be in the way you're initializing*all_drops
|
Is there a c atoi() equivalent for wide chars on Linux? I can find something for MS (wtoi) but I can find anything in a standard Linux lib.
|
You can usewcstolto convert from wide strings to integer values.
|
So, my basic problem is that I'm trying to write a program for a small project I'm working on for fun.
Basically, my issue is this: I need to take user input, as an int, say 15, then manipulate the number so that it returns the result of 1 + 5, being 6. Or for another example say the number is 29, will give you 2 + 9 = 11, which would then need to be reduced down again to 1 + 1 = 2. That could probably be handled easily, but I'm stuck on how to actually split the int apart without having to take the numbers in one by one. I guess it's possible to with RegEx, but I was looking for a more efficient method.
|
This is not a particularly good job for a regex. The usual way would be to get individual digits as the remainder after dividing by 10.
|
Hi does anyone know a way to send signal SIGUSR1/2 without loss to another pid ?
the problem is when i put kill(pid, SIGUSR1) in a while loop, my other program recieve only few of them, its seems there is a loss and i have to wait between two calls to kill().
Im actually forced to used usleep() in my loop in order to receive all the SIGUSR signals ive sent.
I'd like to recieve all of them as fast as it is possible.
Thx.
|
You need to usesigaction()rather thansignal()becausesigaction()can control which signals are blocked while the current signal is being handled. Also, your receiver has to process each signal received; that is relatively costly compared to sending a signal, so your killer (signalling) process can outrun the 'killed' (receiving) process with ease. You should think about why you are needing to send so many signals. They are a mechanism to use in special circumstances.
|
For unpacking complex binary strings with mixed doubles and integers using Ruby'sString.unpackI need to determine offsets within the binary string. Commonly, doubles are 8 bytes and integers are 4 bytes, but in order to make my code machine-independent, I would like to query these sizes from within my Ruby code.
What is the easiest way to determine the size of integers and doubles from within Ruby, i.e., request the response of a request to C'ssizeof( type )method?
|
I have found a workable solution, by usingArray#pack, the inverse method ofString#unpack. By packing a string of 1 integer or 1 double it is possible to determine the size of integers and doubles from within ruby itself which gives the same results as usingsizeof(int)orsizeof(double)(because of the implementation ofArray#packin C):
```
[1.to_i].pack("i").size # 4
[1.to_f].pack("d").size # 8
```
|
This question already has answers here:Why are these constructs using pre and post-increment undefined behavior?(15 answers)Closed5 years ago.
I came across a very interesting fact while executing the following a simple c program:
```
#include<stdio.h>
int main( )
{
int k=0;
printf("%d%d%d", k++, k, ++k);
return 0;
}
```
in windows it was showing output as: 1 1 1
but in linux(ubuntu) it was showing as: 1 2 2
why is it so?
|
It's undefined behaviour. When there are no / ambiguous sequence points. See this wikipedia article:
http://en.wikipedia.org/wiki/Sequence_point
|
Often stacks in C are dependent upon datatype used to declare them. For example,
```
int arr[5]; //creates an integer array of size 5 for stack use
char arr[5]; //creates a character array of size 5 for stack use
```
are both limited to holding integer and character datatypes respectively and presumes that the programmer knows what data is generated during the runtime. What if I want a stack which can hold any datatype?
I initially thought of implementing it as a union, but the approach is not only difficult but also flawed. Any other suggestions?
|
I would use a structure like this:
```
struct THolder
{
int dataType; // this is a value representing the type
void *val; // this is the value
};
```
Then use an array ofTHolderto store your values.
|
EDIT: Removed code/explanation because this project has been given again and students can easily find the solution via this post.
To clarify what happened, I simply passed the wrong length/size in my recvfrom() call.
|
In this line:
```
if(recvfrom( temp->sockfd, sendHostIP, BUFFER_LEN, 0, (struct sockaddr *)&recvAddr, &recvLen) < 0)
errorMsg("recvfrom");
```
You passBUFFER_LENas the length (256), butsendHostIPis only of lengthMAXHOSTNAMELEN(64).
This causesrecvfrom()to overflow that buffer. The same problem occurs when you read in tolocalHostIP.
|
In reference to the thread:memmove implementation in C, I did not understand why would there be a memory overlap for 2 different variables? i.e. is this a normal scenario that the compiler allocates same 'common space' to 2 different variables and why is this policy used by compilers in such cases?
|
Nothing to do with the compiler. Consider the following:
```
int x[100];
memmove(&x[1], &x[0], 99*sizeof(int));
```
|
```
int x = 5;
int *xPtr = &x;
void **xPtrPtr = &xPtr;
printf("%d\n", *(int*)*xPtrPtr);
```
I have a void pointer pointing to an int pointer. What is the syntax for properly casting the void pointer when I want to dereference? The above code quits with:
```
error: invalid conversion from ‘int**’ to ‘void**’
```
Thank you!
|
For void pointers, you don't really need to know how many indirections there are.
```
#include <iostream>
int main(void){
int x = 5;
int* pX = &x;
void* pV = &pX;
std::cout << "x = " << **(int**)pV << std::endl;
// better use C++-Style casts from the beginning
// or you'll be stuck with the lazyness of writing the C-versions:
std::cout << "x = " << **reinterpret_cast<int**>(pV) << std::endl;
std::cin.get();
}
```
Output:
```
x = 5
x = 5
```
See this here.
|
Closed.This question isoff-topic. It is not currently accepting answers.Want to improve this question?Update the questionso it'son-topicfor Stack Overflow.Closed12 years ago.Improve this question
If you hit any non-existent page on SO, likehttps://stackoverflow.com/not-found, you'll see a small code snippet on the right side of the page (as an illustration)
What exactly does this piece of code do? (I'm asking simply out of curiosity)
|
This might get closed as a duplicate on meta too, since it's been answeredhere. The suggestion for it ishere.
|
Hello every one I have following function in c and want to map in JNA what could be equivlent of this in JNA.
2.3. SI_Open
Description: Opens a device (using device number as returned by SI_GetNumDevices) and returns
a handle which will be used for subsequent accesses.
```
SI_STATUS SI_Open (DWORD DeviceNum, HANDLE *Handle)
```
Parameters:
DeviceNum—Device index. 0 for first device, 1 for 2nd, etc.Handle—Pointer to a variable where the handle to the device will be stored.
Return Value:
SI_SUCCESS
SI_DEVICE_NOT_FOUND
SI_INVALID_PARAMETER
|
SI_STATUSis not a common type you should look at your header file for this answer I will take for granted thatSI_STATUStype isDWORD(or 32 bit integer).
This would give the following mapping in jna
```
int SI_Open(int DeviceNum, PointerByReference handle)
```
|
Hi All
I wrote a static library (libA) that uses another library (libB).
How can I link libB to my libA in eclipse (linux)? i cannot find linkage options in project properties, they are not on there usual place.
Thx
|
Static libraries don't link. They are an archive of object files (.ofiles). These archives are then taken as arguments during a linking phase, have their object files extracted and linked into the application at that time. As such, static libraries cannot link against anything.
|
```
x=4+2%-8;
```
This gives output=6
Please anyone explain how?
|
x=4+2%-8;is equivalent tox = 4 + (2 % -8);which givesx = 4 + 2which is 6.
More:C Operator Precedence Table
|
I have a simple question
How to simply convert integer (getting values 0-8) to char, e.g. char[2] in C?
Thanks
|
```
main()
{
int i = 247593;
char str[10];
sprintf(str, "%d", i);
// Now str contains the integer as characters
}
```
Hope it will be helpful to you.
|
I am working on a iPhone game. Everything is working fine, but what is the correnct way to reset a array of ints in Objective-C?
The array holds the coordinates for walls in each level. The array is set, every time a new level is loaded - But how can i "clear" the array?
LevelData.h
```
int wallH[50][2];
int wallV[50][2];
```
LevelData.m
```
-(void)setLevelOne {
// (50, 70)
wallH[0][0] = 50;
wallH[0][1] = 70;
// (50, 90)
wallH[1][0] = 50;
wallH[1][1] = 90;
// And so on ...
// The same for wallV (Vertical walls in the level)
}
```
How can I "clear" the array, so it's ready for the next level?
[wallH release] doesn't work - We are talking about a c array :/
|
You can zero array usingmemsetfunction (in your case you know the number of elements in array so you can use "hardcoded" 100 determining size):
```
memset(wallH, 0, 100*sizeof(wallH[0][0]);
```
|
What is the benefit of usingNSNumberfrom Foundation Framework instead of basic C types (int, float, double)?
Using NSNumber:
```
NSNumber *intNumber;
NSInteger myInt;
intNumber = [NSNumber numberWithInteger: 100];
myInt = [intNumber integerValue];
```
Using pure C:
```
int intNumber;
intNumber = 100;
```
Seems a lot easier and economic to use C.
I know NSNumber is an object (or class?) type, but why would I use them instead simple C variables? When should I use them?
|
The purpose ofNSNumberis simply to box primitive types in objects (pointer types), so you can use them in situations that require pointer-type values to work.
One common example: you have to useNSNumberif you want to persist numeric values in Core Data entities.
You can and should use primitives for calculations (unless with decimals, in which case you useNSDecimalorNSDecimalNumber).
|
I have written my own assertion debug.
```
#define ASSERT_EQUALS(a,b) \
do { \
if ((a)!=(b)) \
{ \
printf(". ASSERT_EQUALS (%s:%d) %d!=%d\n",__FUNCTION__,__LINE__,a,b); \
} \
} while (0)
```
However its only compatible with integer types. Is there a way I can change this so I can support float/double types as well?
Thanks.
|
Maybe you should just print them as floats.
```
#define ASSERT_EQUALS(a, b) \
do { \
if ((a)!=(b)) { \
printf(". ASSERT_EQUALS (%s:%d) %f!=%f\n",__FUNCTION__,__LINE__,(float)(a),(float)(b)); \
} \
} while (0)
```
It looks bad with integers, for example1will show up as1.00000, but it will work for both types.
|
I've a question about passing in parameters via the command line.
Mymain()looks like
```
int main(int argc, char **argv){
int b, d, n, flag;
char *init_d, tst_dir[100];
argv++;
init_d=*(argv++);
//printf(); <--------What do I have to do to init_d so that I can print it later?
```
Ifargvis a pointer to an array of pointers I'm assigninginit_dto point to the value being pointed to by the pointerargvpoints to? (If that even makes sense)
I assume I have to get that value into a character array in order to print it out but if I don't know the size of the "string" I am passing in, I am not sure how to achieve this.
For instance if i run my code './myprogram hello' compared to './myprogram alongerinput'
|
You can print the arguments without transferring them into character arrays. They are null-terminated C strings andprintfeats them for breakfast:
```
for (i=0; i<argc; i++)
printf("%s\n", argv[i]);
```
|
Is there any mechanism to send asynchronous events/signals through JNI from C to Java or viceversa? I need to catch a signal/event from C to Java.
|
It sounds a bit tricky, as JNI is supposed to be one-way only: Java code can invoke a native method, which of course may call back into Java code, but can't itself initiate the process.
But it isn't impossible, I think the most straightforward solution is to have a native call that blocks until the signal is received.
Another option would be to write a JVMTI agent, which can interfere with a running VM actively.
But if we're talking about UNIX signals, you can handle then with the not public API (therefore not guaranteed in the future and Sun VM specific) but existingsun.misc.Signalclass.
|
It is little bit weird. I just play with the unsigned char type and negative values. I have the following code.
```
#include <stdio.h>
int main(int argc, char* agrv[]){
unsigned char c = -3;
printf("%d, %u, %d, %u\n", c, c, ~c, ~c);
}
```
The output is,
```
253, 253, -254, 4294967042
```
I can not figure out the last three values. What does %d and %u really do?
|
The%dformat prints out anint, and%uprints out anunsigned int. All arithmetic onunsigned charvalues is done by first casting them tointand doing the operations onintvalues, and so~c(which is equal to-1 - (int)c) will return a negativeintvalue. An explicit cast would be needed to get theunsigned charresult before printing it out (and the call toprintfwould cast it back tointanyway).
|
In reference to the thread:memmove implementation in C, I did not understand why would there be a memory overlap for 2 different variables? i.e. is this a normal scenario that the compiler allocates same 'common space' to 2 different variables and why is this policy used by compilers in such cases?
|
Nothing to do with the compiler. Consider the following:
```
int x[100];
memmove(&x[1], &x[0], 99*sizeof(int));
```
|
```
int x = 5;
int *xPtr = &x;
void **xPtrPtr = &xPtr;
printf("%d\n", *(int*)*xPtrPtr);
```
I have a void pointer pointing to an int pointer. What is the syntax for properly casting the void pointer when I want to dereference? The above code quits with:
```
error: invalid conversion from ‘int**’ to ‘void**’
```
Thank you!
|
For void pointers, you don't really need to know how many indirections there are.
```
#include <iostream>
int main(void){
int x = 5;
int* pX = &x;
void* pV = &pX;
std::cout << "x = " << **(int**)pV << std::endl;
// better use C++-Style casts from the beginning
// or you'll be stuck with the lazyness of writing the C-versions:
std::cout << "x = " << **reinterpret_cast<int**>(pV) << std::endl;
std::cin.get();
}
```
Output:
```
x = 5
x = 5
```
See this here.
|
Closed.This question isoff-topic. It is not currently accepting answers.Want to improve this question?Update the questionso it'son-topicfor Stack Overflow.Closed12 years ago.Improve this question
If you hit any non-existent page on SO, likehttps://stackoverflow.com/not-found, you'll see a small code snippet on the right side of the page (as an illustration)
What exactly does this piece of code do? (I'm asking simply out of curiosity)
|
This might get closed as a duplicate on meta too, since it's been answeredhere. The suggestion for it ishere.
|
Hello every one I have following function in c and want to map in JNA what could be equivlent of this in JNA.
2.3. SI_Open
Description: Opens a device (using device number as returned by SI_GetNumDevices) and returns
a handle which will be used for subsequent accesses.
```
SI_STATUS SI_Open (DWORD DeviceNum, HANDLE *Handle)
```
Parameters:
DeviceNum—Device index. 0 for first device, 1 for 2nd, etc.Handle—Pointer to a variable where the handle to the device will be stored.
Return Value:
SI_SUCCESS
SI_DEVICE_NOT_FOUND
SI_INVALID_PARAMETER
|
SI_STATUSis not a common type you should look at your header file for this answer I will take for granted thatSI_STATUStype isDWORD(or 32 bit integer).
This would give the following mapping in jna
```
int SI_Open(int DeviceNum, PointerByReference handle)
```
|
Hi All
I wrote a static library (libA) that uses another library (libB).
How can I link libB to my libA in eclipse (linux)? i cannot find linkage options in project properties, they are not on there usual place.
Thx
|
Static libraries don't link. They are an archive of object files (.ofiles). These archives are then taken as arguments during a linking phase, have their object files extracted and linked into the application at that time. As such, static libraries cannot link against anything.
|
```
x=4+2%-8;
```
This gives output=6
Please anyone explain how?
|
x=4+2%-8;is equivalent tox = 4 + (2 % -8);which givesx = 4 + 2which is 6.
More:C Operator Precedence Table
|
I have a simple question
How to simply convert integer (getting values 0-8) to char, e.g. char[2] in C?
Thanks
|
```
main()
{
int i = 247593;
char str[10];
sprintf(str, "%d", i);
// Now str contains the integer as characters
}
```
Hope it will be helpful to you.
|
I am working on a iPhone game. Everything is working fine, but what is the correnct way to reset a array of ints in Objective-C?
The array holds the coordinates for walls in each level. The array is set, every time a new level is loaded - But how can i "clear" the array?
LevelData.h
```
int wallH[50][2];
int wallV[50][2];
```
LevelData.m
```
-(void)setLevelOne {
// (50, 70)
wallH[0][0] = 50;
wallH[0][1] = 70;
// (50, 90)
wallH[1][0] = 50;
wallH[1][1] = 90;
// And so on ...
// The same for wallV (Vertical walls in the level)
}
```
How can I "clear" the array, so it's ready for the next level?
[wallH release] doesn't work - We are talking about a c array :/
|
You can zero array usingmemsetfunction (in your case you know the number of elements in array so you can use "hardcoded" 100 determining size):
```
memset(wallH, 0, 100*sizeof(wallH[0][0]);
```
|
What is the benefit of usingNSNumberfrom Foundation Framework instead of basic C types (int, float, double)?
Using NSNumber:
```
NSNumber *intNumber;
NSInteger myInt;
intNumber = [NSNumber numberWithInteger: 100];
myInt = [intNumber integerValue];
```
Using pure C:
```
int intNumber;
intNumber = 100;
```
Seems a lot easier and economic to use C.
I know NSNumber is an object (or class?) type, but why would I use them instead simple C variables? When should I use them?
|
The purpose ofNSNumberis simply to box primitive types in objects (pointer types), so you can use them in situations that require pointer-type values to work.
One common example: you have to useNSNumberif you want to persist numeric values in Core Data entities.
You can and should use primitives for calculations (unless with decimals, in which case you useNSDecimalorNSDecimalNumber).
|
I have written my own assertion debug.
```
#define ASSERT_EQUALS(a,b) \
do { \
if ((a)!=(b)) \
{ \
printf(". ASSERT_EQUALS (%s:%d) %d!=%d\n",__FUNCTION__,__LINE__,a,b); \
} \
} while (0)
```
However its only compatible with integer types. Is there a way I can change this so I can support float/double types as well?
Thanks.
|
Maybe you should just print them as floats.
```
#define ASSERT_EQUALS(a, b) \
do { \
if ((a)!=(b)) { \
printf(". ASSERT_EQUALS (%s:%d) %f!=%f\n",__FUNCTION__,__LINE__,(float)(a),(float)(b)); \
} \
} while (0)
```
It looks bad with integers, for example1will show up as1.00000, but it will work for both types.
|
I've a question about passing in parameters via the command line.
Mymain()looks like
```
int main(int argc, char **argv){
int b, d, n, flag;
char *init_d, tst_dir[100];
argv++;
init_d=*(argv++);
//printf(); <--------What do I have to do to init_d so that I can print it later?
```
Ifargvis a pointer to an array of pointers I'm assigninginit_dto point to the value being pointed to by the pointerargvpoints to? (If that even makes sense)
I assume I have to get that value into a character array in order to print it out but if I don't know the size of the "string" I am passing in, I am not sure how to achieve this.
For instance if i run my code './myprogram hello' compared to './myprogram alongerinput'
|
You can print the arguments without transferring them into character arrays. They are null-terminated C strings andprintfeats them for breakfast:
```
for (i=0; i<argc; i++)
printf("%s\n", argv[i]);
```
|
Is there any mechanism to send asynchronous events/signals through JNI from C to Java or viceversa? I need to catch a signal/event from C to Java.
|
It sounds a bit tricky, as JNI is supposed to be one-way only: Java code can invoke a native method, which of course may call back into Java code, but can't itself initiate the process.
But it isn't impossible, I think the most straightforward solution is to have a native call that blocks until the signal is received.
Another option would be to write a JVMTI agent, which can interfere with a running VM actively.
But if we're talking about UNIX signals, you can handle then with the not public API (therefore not guaranteed in the future and Sun VM specific) but existingsun.misc.Signalclass.
|
Hello every one I have following function in c and want to map in JNA what could be equivlent of this in JNA.
2.3. SI_Open
Description: Opens a device (using device number as returned by SI_GetNumDevices) and returns
a handle which will be used for subsequent accesses.
```
SI_STATUS SI_Open (DWORD DeviceNum, HANDLE *Handle)
```
Parameters:
DeviceNum—Device index. 0 for first device, 1 for 2nd, etc.Handle—Pointer to a variable where the handle to the device will be stored.
Return Value:
SI_SUCCESS
SI_DEVICE_NOT_FOUND
SI_INVALID_PARAMETER
|
SI_STATUSis not a common type you should look at your header file for this answer I will take for granted thatSI_STATUStype isDWORD(or 32 bit integer).
This would give the following mapping in jna
```
int SI_Open(int DeviceNum, PointerByReference handle)
```
|
Hi All
I wrote a static library (libA) that uses another library (libB).
How can I link libB to my libA in eclipse (linux)? i cannot find linkage options in project properties, they are not on there usual place.
Thx
|
Static libraries don't link. They are an archive of object files (.ofiles). These archives are then taken as arguments during a linking phase, have their object files extracted and linked into the application at that time. As such, static libraries cannot link against anything.
|
```
x=4+2%-8;
```
This gives output=6
Please anyone explain how?
|
x=4+2%-8;is equivalent tox = 4 + (2 % -8);which givesx = 4 + 2which is 6.
More:C Operator Precedence Table
|
I have a simple question
How to simply convert integer (getting values 0-8) to char, e.g. char[2] in C?
Thanks
|
```
main()
{
int i = 247593;
char str[10];
sprintf(str, "%d", i);
// Now str contains the integer as characters
}
```
Hope it will be helpful to you.
|
I am working on a iPhone game. Everything is working fine, but what is the correnct way to reset a array of ints in Objective-C?
The array holds the coordinates for walls in each level. The array is set, every time a new level is loaded - But how can i "clear" the array?
LevelData.h
```
int wallH[50][2];
int wallV[50][2];
```
LevelData.m
```
-(void)setLevelOne {
// (50, 70)
wallH[0][0] = 50;
wallH[0][1] = 70;
// (50, 90)
wallH[1][0] = 50;
wallH[1][1] = 90;
// And so on ...
// The same for wallV (Vertical walls in the level)
}
```
How can I "clear" the array, so it's ready for the next level?
[wallH release] doesn't work - We are talking about a c array :/
|
You can zero array usingmemsetfunction (in your case you know the number of elements in array so you can use "hardcoded" 100 determining size):
```
memset(wallH, 0, 100*sizeof(wallH[0][0]);
```
|
What is the benefit of usingNSNumberfrom Foundation Framework instead of basic C types (int, float, double)?
Using NSNumber:
```
NSNumber *intNumber;
NSInteger myInt;
intNumber = [NSNumber numberWithInteger: 100];
myInt = [intNumber integerValue];
```
Using pure C:
```
int intNumber;
intNumber = 100;
```
Seems a lot easier and economic to use C.
I know NSNumber is an object (or class?) type, but why would I use them instead simple C variables? When should I use them?
|
The purpose ofNSNumberis simply to box primitive types in objects (pointer types), so you can use them in situations that require pointer-type values to work.
One common example: you have to useNSNumberif you want to persist numeric values in Core Data entities.
You can and should use primitives for calculations (unless with decimals, in which case you useNSDecimalorNSDecimalNumber).
|
I have written my own assertion debug.
```
#define ASSERT_EQUALS(a,b) \
do { \
if ((a)!=(b)) \
{ \
printf(". ASSERT_EQUALS (%s:%d) %d!=%d\n",__FUNCTION__,__LINE__,a,b); \
} \
} while (0)
```
However its only compatible with integer types. Is there a way I can change this so I can support float/double types as well?
Thanks.
|
Maybe you should just print them as floats.
```
#define ASSERT_EQUALS(a, b) \
do { \
if ((a)!=(b)) { \
printf(". ASSERT_EQUALS (%s:%d) %f!=%f\n",__FUNCTION__,__LINE__,(float)(a),(float)(b)); \
} \
} while (0)
```
It looks bad with integers, for example1will show up as1.00000, but it will work for both types.
|
I've a question about passing in parameters via the command line.
Mymain()looks like
```
int main(int argc, char **argv){
int b, d, n, flag;
char *init_d, tst_dir[100];
argv++;
init_d=*(argv++);
//printf(); <--------What do I have to do to init_d so that I can print it later?
```
Ifargvis a pointer to an array of pointers I'm assigninginit_dto point to the value being pointed to by the pointerargvpoints to? (If that even makes sense)
I assume I have to get that value into a character array in order to print it out but if I don't know the size of the "string" I am passing in, I am not sure how to achieve this.
For instance if i run my code './myprogram hello' compared to './myprogram alongerinput'
|
You can print the arguments without transferring them into character arrays. They are null-terminated C strings andprintfeats them for breakfast:
```
for (i=0; i<argc; i++)
printf("%s\n", argv[i]);
```
|
Is there any mechanism to send asynchronous events/signals through JNI from C to Java or viceversa? I need to catch a signal/event from C to Java.
|
It sounds a bit tricky, as JNI is supposed to be one-way only: Java code can invoke a native method, which of course may call back into Java code, but can't itself initiate the process.
But it isn't impossible, I think the most straightforward solution is to have a native call that blocks until the signal is received.
Another option would be to write a JVMTI agent, which can interfere with a running VM actively.
But if we're talking about UNIX signals, you can handle then with the not public API (therefore not guaranteed in the future and Sun VM specific) but existingsun.misc.Signalclass.
|
It is little bit weird. I just play with the unsigned char type and negative values. I have the following code.
```
#include <stdio.h>
int main(int argc, char* agrv[]){
unsigned char c = -3;
printf("%d, %u, %d, %u\n", c, c, ~c, ~c);
}
```
The output is,
```
253, 253, -254, 4294967042
```
I can not figure out the last three values. What does %d and %u really do?
|
The%dformat prints out anint, and%uprints out anunsigned int. All arithmetic onunsigned charvalues is done by first casting them tointand doing the operations onintvalues, and so~c(which is equal to-1 - (int)c) will return a negativeintvalue. An explicit cast would be needed to get theunsigned charresult before printing it out (and the call toprintfwould cast it back tointanyway).
|
What is a good way to detect bugs where I overwrite an array bound?
```
int a[100];
for (int i = 0; i<1000; i++) a[i] = i;
```
It would be helpful to collect a list of different strategies that people have used in their experience to uncover bugs of this type.
For example, doing a backtrace on from the point of the memory fault (for me often this doesn't work because the stack has been corrupted).
|
Valgrind will spot this sort of thing pretty reliably!
|
In C Programming language and I use 32 bit system,
I have a struct and this struct size is multiple of four.
But I look at Linker Map file and size is multiple of eight
Example
```
typedef struct _str
{
U64 var1,
U32 var2
} STR;
```
Size of this struct is 16.
But
```
typedef struct _str
{
U32 var1,
U32 var2,
U32 var3
} STR2;
```
Size of STR2 is 12.
I am working on 32 bit ARM microcontroller.
I dont know why
|
The first structure is padded in order to get it aligned on a U64 boundary: it is equivalent to
```
struct STR
{
U64 var1;
U32 var2;
U8 pad[4]; /* sizeof(U64) - sizeof(U32) */
};
```
So when used in an array of struct STR [], each U64 is well aligned regarding to ABI requirements.
Have a look atProcedure Call Standard for ARM Architecture, 4.1 Fundamental Data Types .
|
Is there alternative forstrrev()in C? I am interested in a function that does the same but it must be included in the standard library.
I am aware that operation can be easily implemented by user defined function but I am looking for a library alternative.Precisely there are few functions take for examplerindex(identical tostrrchr()) which is more common in perl but still works in gcc-4.3.4,I am inquisitive to know if there is anything like that forstrrev()since the common reversing function in perl i.ereverse<>is not working in C.
Compiler specification: [C (gcc-4.3.4)]
Thanks,
PS:I am aware of C++'s STL reverse but my question is strictly about C.
|
As far as I know, there is no such a function in C standard libraries. If you don't mind using C++, there isreversetemplate function inalgorithms.
|
I am learning about OpenMP concurrency, and tried my hand at some existing code I have. In this code, I tried to make all the for loops parallel. However, this seems to make the program MUCH slower, at least 10x slower, or even more than the single threaded version.
Here is the code:http://pastebin.com/zyLzuWU2
I also used pthreads, which turns out to be faster than the single threaded version.
Now the question is, what am I doing wrong in my OpenMP implementation that is causing this slowdown?
Thanks!
edit: the single threaded version is just the one without all the #pragmas
|
One problem I see with your code is that you are using OpenMP across loops that are very small (8 or 64 iterations, for example). This will not be efficient due to overheads. If you want to use OpenMP for the n-queens problem, look at OpenMP 3.0 tasks and thread parallelism for branch-and-bound problems.
|
What are the possible effect of returning a static type data. And when should we actually use it?
```
static ssize_t
my_read(int fd, char *ptr)
{
//code from Stevens Unix Network programming.
if (something)
return (-1)
if (something else)
return (0)
return (1)
}
```
why static here?
Thanks.
|
The function is static, not the return type. This means that its name is only visible from within the current compilation unit, which is used as an encapsulation mechanism.
The function can still be called from elsewhere through a function pointer, however.
See alsothis discussion on the generalstatickeywordfor more context.
|
I have the following old c code.
```
const char *c[3];
c[0] = "ABC";
c[1] = "EFG";
c[2] = 0;
c_function(c);
```
Now, I need to use Python to call old c function. I have the following code.
```
c_names = (c_char_p * 3)()
c_names[0] = "ABC";
c_names[1] = "EFG";
// c[2] = 0??
libc = CDLL("c_library.dll")
libc.c_function(c_names)
```
May I know what is the Python equivalent forc[2] = 0;?
|
Noneand0both work:
```
>>> import ctypes
>>> x=ctypes.c_char_p(0)
>>> x
c_char_p(None)
>>> x=ctypes.c_char_p(None)
>>> x
c_char_p(None)
```
|
we gotif(expression) {...}for example. We all know if expression is true, it will execute lines in braces. But what is "True" in C?
Is that != 0 means true as I think?
Thank you
|
Here is what the standard has to say.
§6.8.4 Selection statementsSyntaxselection-statement:if ( expression ) statementif ( expression ) statement else statementswitch ( expression ) statement§6.8.4.1 TheifstatementConstraintsThe controlling expression of an if statement shall have scalar type.SemanticsIn both forms, the first substatement is executed if the expression compares unequal to 0. In the else form, the second substatement is executed if the expression compares equal to 0. If the first substatement is reached via a label, the second substatement is not executed.Anelseis associated with the lexically nearest preceding if that is allowed by the
syntax.
|
```
/*
180 * Create a regular file
181 */
182 static int
183 ufs_create(ap)
184 struct vop_create_args /* {
185 struct vnode *a_dvp;
186 struct vnode **a_vpp;
187 struct componentname *a_cnp;
188 struct vattr *a_vap;
189 } */ *ap;
190 {
191 int error;
192
193 error =
194 ufs_makeinode(MAKEIMODE(ap->a_vap->va_type, ap->a_vap->va_mode),
195 ap->a_dvp, ap->a_vpp, ap->a_cnp);
196 if (error)
197 return (error);
198 return (0);
199 }
```
Please help me to get information from line 182-189...this is strange for me.. What is this function title means? (I mean, what is return value, what is input parameter?)
Thank you all.
|
The return type isintand it takes one argument, astruct vop_create_args*namedap. This is K&R notation.
|
Is it possible to implementstrlen()in theCpreprocessor?
Given:
```
#define MYSTRING "bob"
```
Is there some preprocessor macro,X, which would let me say:
```
#define MYSTRING_LEN X(MYSTRING)
```
|
It doesn't use the preprocessor, but sizeof is resolved at compile time. If your string is in an array, you can use that to determine its length at compile time:
```
static const char string[] = "bob";
#define STRLEN(s) (sizeof(s)/sizeof(s[0]))
```
Keep in mind the fact thatSTRLENabove will include the null terminator, unlikestrlen().
|
```
#include <stdio.h>
#include <stdlib.h>
int main()
{
char a[]="shevchenko ac milan";
printf("%s",&a);
}
```
This is prints "shevchenko ac milan"
```
but
#include <stdio.h>
#include <stdlib.h>
int main()
{
char a[]="shevchenko ac milan";
printf("%s",&a+1);
}
```
Why does this print junk characters?
|
&ais of typepointer to achar[20].
When you do&a+1you'll go to the nextchar[20]item in memory, thus you'll go aftera.
You should instead have achar*: by summing 1 to it you'll go to the nextchar.
To obtain achar*you can just usea(it decays in achar*by doing this), and thusa+1to go the next char.
|
```
#include <stdio.h>
#include <stdlib.h>
int main()
{
char a="9jhjhi";
printf("%s",a);
}
```
Why does this throw a segmentation fault? What happens behind the screens?
|
You need to usechar *a = "...".
printfwhen passed%swill run through a string looking for a0/NULLbyte. In your case you are not assigning a string literal toa, and in fact your compiler should have thrown a warning that you were trying to initialize acharfrom a pointer.
```
#include <stdio.h>
#include <stdlib.h>
int main()
{
char *a="9jhjhi";
printf("%s",a);
}
```
|
The following line of code was calling thecurrentPlaybackTimemethod on my singletonMAMusicPlayerclass twice:
```
float trackPosition = MAX(floor([[MAMusicPlayer sharedPlayer] currentPlaybackTime]),0.0f);
[scrubber setValue:trackPosition];
```
Through trial and error I managed to resolve the issue by adjusting the code to:
```
float trackPosition = floor([[MAMusicPlayer sharedPlayer] currentTrackPosition]);
[scrubber setValue:MAX(trackPosition,0.0f)];
```
However I still do not understand why the first example should call the method twice. Since this is probably fairly fundamental, I thought it best to ask for clarification.
|
MAX(a, b)is probably a macro defined as((a > b) ? a : b), so when terms are substituted foraandb, each term is evaluated twice.
|
Is there any reason why I never see main's prototype declared in C programs, ie:
```
int main(int argc, char* argv[]);
int main(int argc, char* argv[])
{
return 0;
}
```
Always seemed inconsistent..
|
C language standard, draftn1256:
5.1.2.2.1 Program startup1 The function called at program startup is namedmain.The implementation declares no
prototype for this function. It shall be defined with a return type ofintand with no
parameters:int main(void) { /* ... */ }or with two parameters (referred to here asargcandargv, though any names may be
used, as they are local to the function in which they are declared):int main(int argc, char *argv[]) { /* ... */ }or equivalent;9)or in some other implementation-defined manner.
Emphasis mine.
|
This question already has answers here:Closed12 years ago.
Possible Duplicate:Is there a performance difference between i++ and ++i in C++?
I understand that:
```
i++;
```
Is supposed to take more instructions than
```
++i;
```
Because it's generating an intermediate result that you don't need to use, so people put "++i" on lines of code that have no use for the intermediate result.
However, compilers are really smart, and they can tell if a result is being used or not. They can tell all kinds of things. But they aren't magic
So I am curious - on modern compilers does picking one way over the other actually matter or will it simply compile to the same machine code regardless?
|
It does not matter when using it on anint. However, when using iterators or other objects that overloadoperator++, it might still make a difference.
|
is there a macro or a global variable which through which I can learn about the name of the object file the current translation unit will be used for in C? I am only aware of the source file macro.
Thanks.
|
Not in standard C.
The preprocessor may be implemented as a separate program that has no knowledge of object files, so it can't be a built-in macro.
A compiler could in theory produce a symbol with the object file name, but that would be invalidated as soon as the object file is renamed by the user.
You can get the build system to#definethe object name:
```
# Compile ${module}.c to ${module}.o
cc ${CPPFLAGS} ${CFLAGS} -DOBJNAME=${module}.o -c -o ${module.o} ${module}.c
```
but the point about renaming still stands.
(This is just a snippet of shell code, but it could be turned into aMakefilerule, I suppose.)
|
In What lines will this code fail (meaning: don't do what they are supposed to do), and why?
```
int main(void) {
char student[64] = "some guy";
char* teacher;
/* line1 */ strcpy(teacher, student);
/* line2 */ teacher=student;
/* line3 */ strcpy(student, "Alber Einstein");
/* line4 */ student = teacher;
}
```
|
Line 1 causes undefined behaviour. Line 4 won't even compile. Since this seems like it could just as easily be a homework question, and I don't like to give the whole thing away, a quick read of thecomp.lang.c FAQor theC language specificationwill explain why.
|
What will be printed out? 6 6 or 6 7? And why?
```
void foo()
{
static int x = 5;
x++;
printf("%d", x);
}
int main()
{
foo();
foo();
return 0;
}
```
|
There are two issues here, lifetime and scope.
The scope of variable is where the variable name can be seen. Here,xis visible only inside functionfoo().
The lifetime of a variable is the period over which it exists. Ifxwere defined without the keywordstatic, the lifetime would be from the entry intofoo()to the return fromfoo(); so it would be re-initialized to 5 on every call.
The keywordstaticacts to extend the lifetime of a variable to the lifetime of the programme; e.g. initialization occurs once and once only and then the variable retains its value - whatever it has come to be - over all future calls tofoo().
|
i am beginner in c programming, i am currently using gedit of ubuntu 10.04 to write c prog, i want to plot a graph, but i am able to do it, can any one tell me hw it can be done or else is there any way to extract the data from the output to spreadsheet where i can plot the req, graph??
I appreciate your help..n thanx!!!
|
Medsphere has some pretty greatGTK# widgetsfor graphing (among other things), but you'll need to be a little more clear about your input/output requirements to get more specific help.
|
running the configure script for mozart oz gives me this:
checking for stdint.h... (cached) yeschecking for gethostbyaddr in -lnsl... nochecking for gethostbyaddr in -lc... nochecking for gethostbyaddr... noconfigure: error: Function gethostbyaddr is not available.The system cannot be built.
configure: error: /home/stewart/dev/mozart/mozart/platform/emulator/configure failed for emulatorconfigure: error: /home/stewart/dev/mozart/mozart/platform/configure failed for platform
how do I get gethostbyaddr on my latest ubuntu 64 machine?
|
do a
sudo apt-get install tk-dev tcl-dev build-essential libc6-dev g++-multilib ia32-libs lib32z1-dev lib32gmp3-dev
this fixes it.
|
From msdn
For functions declared with the naked
attribute, the compiler generates code
without prolog and epilog code. You
can use this feature to write your own
prolog/epilog code sequences using
inline assembler code. Naked functions
are particularly useful in writing
virtual device drivers.
```
__declspec(naked) declarator
```
What is "prolog and epilog code". I seen a libraries written in C code running on device or firmware using only libc. It calls functions without a problem, what does the naked keyword do and why is it needed?
note: I'm unsure what calling convention the functions use in those libs.
|
Prolog: Code that runs before the function body, typically code that handles function entry and argument handling.
Epilog: Code that runs after the function body, typically code that handles return of function and return value.
With "naked" you have to/get the opportunity to write this stuff yourself.
|
I'm giving a try at OpenCL, and in order to put this in production I'd like to be able to bind dynamically to OpenCL.DLL (when under Windows), in order to handle 'gracefully' the case where no OpenCL is installed on the host computer.
Is there any available library (or code snippet) that takes care of this dynamic binding in C or C++, much like GLEW does for OpenGL ? I'd like to avoid the hassle to do it myself.
Thanks,
|
Here you go:
http://clcc.sourceforge.net/clew_8h.html
|
Yesterday on an interview the interviewer asked me a question:
Why doesn't the following code give the desired answer?
```
int a = 100000, b = 100000;
long int c = a * b ;
```
The language is C.
I've told the interviewer that we count first the 100,000 * 100,000 as an int(overflow) and just then cast it to long.
|
I'm guessing the clue would be aninteger overflowto occur, but with such low values, I don't see that happening.
Maximum (positive) value for int (usually 32bit) is:2,147,483,647
The result of your calculation is:100,000,000
UPDATE:
With your updated question:100000 * 100000instead of10000 * 10000results in10,000,000,000, whichwillcause an overflow to occur. This value is then cast to a long afterwards.
To prevent such an overflow the correct approach would be to cast one of the two values in the multiplication to a long (usually 64bit). E.g.(long)100000 * 100000
|
I'm trying to convert a string to a double, but since I'm working on a Windows native application (as in linking only to ntdll.dll), I don't have most of the standard library available. I can use basic FP support in math.h, but that's basically it.
How do I convert a string to the double closest to the rational number expressed in that string?
|
If you really want to get the nearest, the problem is quite hard and you need arbitrary precision arithmetic to achieve that result. Seeftp://ftp.ccs.neu.edu/pub/people/will/howtoread.psfor instance.
|
I have this particular problem where i have to write a c program that reads numerical data from a text file. The data is tab delimited. Here is a sample from the text file.
```
1 23099 345565 345569
2 908 66766 66768
```
This is data for clients and each client has a row.Each column represents customer no.,previous balance,previous reading, current reading.Then i have to generate a doc. document
that summarizes all this information and calculates the balance I can write a function that does this but how do i create an xls document
and a word document where all the results are summarized using the program? The text document has only numerical data. Any ideas
|
The easiest way is to create a csv file and not a xls file.
Office can open those csv files with good results.
And it is way easier to create a ascii text file with commaseparated values,
than to create something into a closed format like the ms office formats.
|
Here is my code.
```
if(fseek(file,position,SEEK_SET)!=0)
{
throw std::runtime_error("can't seek to specified position");
}
```
I used to assume that even ifpositionis greater than num of characters in file this code would work correct (i.e. throw error), but it wouldn't. So I want to know how can I handle seeking failure when trying to seek out of file range?
|
Well, you can always check for file length before doing thefseek.
```
void safe_seek(FILE* f, off_t offset) {
fseek(f, 0, SEEK_END);
off_t file_length = ftell(f);
if (file_length < offset) {
// throw!
}
fseek(f, offset, SEEK_SET);
}
```
Be aware though that this is not thread safe.
|
I know that using isDialogMessage() function we can able to switch the focus from one control to another.
My doubt is
is it possible to override that the control should focus from first button to third button.If possible how to customize WS_TABSTOP property
I also like to know that on what basis the focus is shifted from control to control.
How to set the tabindex property for each control?
|
The order for tab stop depends on the order in which the controls have been added on the dialog.
You can open the dialog in the resource editor in Visual Studio and pressCTRL+Dto see and modify the tab order.
To modify just press on the number that appears in the top left of the control. The control with the number 1 will be the first control to have focus on your dialog, the order is given by consecutive numbers.
Using this method you can set any tab order.
Hope this helps.
|
Is it possible to somehow refer to the value I am returning from a function? An example explains better:
```
CFTypeRef foo()
{
CFTypeRef valueRef = NULL;
bar(&valueRef); // fills valueRef with some data
return valueRef;
}
```
I thought it would be nice to rewrite this as:
```
CFTypeRef foo()
{
bar(&__retvalue);
}
```
Where of course__retvaluewould be some magical token. Does this make sense? Is it possible to do that? If not, why?
|
This is not possible. At a low level, the return value is usually returned in a processor register, making it impossible to pass as a reference.
|
I am wondering what the difference is between these two variables in C:
```
float price = 3.00;
```
and
```
float price = 3.00f;
```
What is the use of suffixfin this case?
|
3.00is interpreted as adouble, as opposed to3.00fwhich is seen by the compiler as afloat.
Thefsuffix simply tells the compiler which is afloatand which is adouble.
SeeMSDN (C++)
|
By using functions like prctl, or pthread_set_name_np it's possible to change the name of a thread. The limit both functions imposes, at least in Linux 2.6.38, is that the name cannot be longer than 15 characters (NULL termination being the 16th byte).
Where is this 15 character limit imposed, and is there any (even unorthodox) way around it?
Update:
As mentioned in the comments, this is imposed by the kernel.
The definition can be found here:http://lxr.linux.no/linux+v2.6.37/include/linux/sched.h#L245
|
15-char limit is enforced by the kernel:
```
struct task_struct::comm[TASK_COMM_LEN]
```
which is 16-byte wide.
You have to recompile the kernel if you want to increase that.
|
Can we generate Control flow graph of the c program by Turbo C compiler?
I want to know that there is something given by the compiler to generate a CFG of a C program.
|
Don't think Turbo C has this feature.
Though, modern IDEs have capabilities to draw Class Diagrams, but I personally haven't seen any IDEs drawing Flow diagrams.
Edit:
Hereis a list of tools that does what you want to achieve.
|
friends, I have a piece of code which can perform simple add, subtraction, multiplication, division, and formula with brackets.
Is there some kind of conversion tool for translating code from C to Smalltalk?
Or any other relatively easy way to achieve this?
Thanks in advance.
|
This should be trivial to write withPetitParser
|
What are the possible effect of returning a static type data. And when should we actually use it?
```
static ssize_t
my_read(int fd, char *ptr)
{
//code from Stevens Unix Network programming.
if (something)
return (-1)
if (something else)
return (0)
return (1)
}
```
why static here?
Thanks.
|
The function is static, not the return type. This means that its name is only visible from within the current compilation unit, which is used as an encapsulation mechanism.
The function can still be called from elsewhere through a function pointer, however.
See alsothis discussion on the generalstatickeywordfor more context.
|
I have the following old c code.
```
const char *c[3];
c[0] = "ABC";
c[1] = "EFG";
c[2] = 0;
c_function(c);
```
Now, I need to use Python to call old c function. I have the following code.
```
c_names = (c_char_p * 3)()
c_names[0] = "ABC";
c_names[1] = "EFG";
// c[2] = 0??
libc = CDLL("c_library.dll")
libc.c_function(c_names)
```
May I know what is the Python equivalent forc[2] = 0;?
|
Noneand0both work:
```
>>> import ctypes
>>> x=ctypes.c_char_p(0)
>>> x
c_char_p(None)
>>> x=ctypes.c_char_p(None)
>>> x
c_char_p(None)
```
|
we gotif(expression) {...}for example. We all know if expression is true, it will execute lines in braces. But what is "True" in C?
Is that != 0 means true as I think?
Thank you
|
Here is what the standard has to say.
§6.8.4 Selection statementsSyntaxselection-statement:if ( expression ) statementif ( expression ) statement else statementswitch ( expression ) statement§6.8.4.1 TheifstatementConstraintsThe controlling expression of an if statement shall have scalar type.SemanticsIn both forms, the first substatement is executed if the expression compares unequal to 0. In the else form, the second substatement is executed if the expression compares equal to 0. If the first substatement is reached via a label, the second substatement is not executed.Anelseis associated with the lexically nearest preceding if that is allowed by the
syntax.
|
```
/*
180 * Create a regular file
181 */
182 static int
183 ufs_create(ap)
184 struct vop_create_args /* {
185 struct vnode *a_dvp;
186 struct vnode **a_vpp;
187 struct componentname *a_cnp;
188 struct vattr *a_vap;
189 } */ *ap;
190 {
191 int error;
192
193 error =
194 ufs_makeinode(MAKEIMODE(ap->a_vap->va_type, ap->a_vap->va_mode),
195 ap->a_dvp, ap->a_vpp, ap->a_cnp);
196 if (error)
197 return (error);
198 return (0);
199 }
```
Please help me to get information from line 182-189...this is strange for me.. What is this function title means? (I mean, what is return value, what is input parameter?)
Thank you all.
|
The return type isintand it takes one argument, astruct vop_create_args*namedap. This is K&R notation.
|
Does -fomit-frame-pointeralwaysomit the frame pointer? Is there ever a situation where both the pc and fp need to be setup? Does dynamic stack growth force the fp to be setup? Asking specifically for MIPS32. Thanks!
|
The frame pointer is not really needed for correct execution, except sometimes for exception unwind. Dynamic stack growth usually requires some kind of a frame pointer, but it is not tied to a particular register, but rather allocated through normal data flow analysis.
Basically,-fomit-frame-pointerdemotes the FP from a fixed register assignment to a pseudo register and makes initialisation subject to dead store elimination. So the answer to the first question is no, it doesn't omit it always.
|
Is there a "good" way to write "pointer to something" in C/C++ ?I use to writevoid foo( char *str );But sometimes I find it quite illogical because the type ofstris "pointer to char", then it should more logical to attach the*to the type name.Is there a rule to write pointers ?
```
char*str;
char* str;
char *str;
char * str;
```
|
There is no strict rule, but bear in mind that the*attaches to the variable, so:
```
char *str1, *str2; // str1 and str2 are pointers
char* str1, str2; // str1 is a pointer, str2 is a char
```
Some people like to dochar * str1as well, but it's up to you or your company's coding standard.
|
I want to check a header in my source file and if its already defined, I want to undefine it.
Is it possible?
|
#undefis the opposite of#defineand actually cancels it.
It's very useful when you need a macro for some task but does not wish to "pollute" the symbol tables
```
// referencing FOO is an error (not yet defined)
#define FOO(arg_) ....
FOO(A)
FOO(B)
FOO(C)
#undef FOO
// referencing FOO now is an error (not defined)
```
There is no way to cancel a#includedirective.
|
I have a sorted array ( in ascending order) and I want to find the subscript
of a number in the array which is the the next highest number to a given
number. For example,if I have 67 as a given number and if I have an array
x=(23,36,45,62,79,103,109), then how do I get the subscript 5 from x (to
get 79 which is the next highest to 67) without using a for loop?
|
Is this homework?
You can do it pretty easily with a recursive call, splitting the array in half each time.
|
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