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.\" copyright (c) 2008 michael kerrisk <[email protected]>
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.\"
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.\" %%%license_start(gplv2+_sw_3_para)
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.\" this program is free software; you can redistribute it and/or modify
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.\" it under the terms of the gnu general public license as published by
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.\" the free software foundation; either version 2 of the license, or
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.\" (at your option) any later version.
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.\"
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.\" this program is distributed in the hope that it will be useful,
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.\" but without any warranty; without even the implied warranty of
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.\" merchantability or fitness for a particular purpose. see the
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.\" gnu general public license for more details.
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.\"
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.\" you should have received a copy of the gnu general public
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.\" license along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%license_end
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.\"
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.th timerfd_create 2 2021-03-22 linux "linux programmer's manual"
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.sh name
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timerfd_create, timerfd_settime, timerfd_gettime \-
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timers that notify via file descriptors
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.sh synopsis
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.nf
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.b #include <sys/timerfd.h>
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.pp
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.bi "int timerfd_create(int " clockid ", int " flags );
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.pp
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.bi "int timerfd_settime(int " fd ", int " flags ,
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.bi " const struct itimerspec *" new_value ,
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.bi " struct itimerspec *" old_value );
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.bi "int timerfd_gettime(int " fd ", struct itimerspec *" curr_value );
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.fi
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.sh description
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these system calls create and operate on a timer
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that delivers timer expiration notifications via a file descriptor.
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they provide an alternative to the use of
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.br setitimer (2)
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or
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.br timer_create (2),
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with the advantage that the file descriptor may be monitored by
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.br select (2),
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.br poll (2),
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and
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.br epoll (7).
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.pp
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the use of these three system calls is analogous to the use of
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.br timer_create (2),
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.br timer_settime (2),
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and
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.br timer_gettime (2).
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(there is no analog of
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.br timer_getoverrun (2),
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since that functionality is provided by
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.br read (2),
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as described below.)
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.\"
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.ss timerfd_create()
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.br timerfd_create ()
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creates a new timer object,
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and returns a file descriptor that refers to that timer.
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the
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.i clockid
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argument specifies the clock that is used to mark the progress
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of the timer, and must be one of the following:
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.tp
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.b clock_realtime
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a settable system-wide real-time clock.
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.tp
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.b clock_monotonic
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a nonsettable monotonically increasing clock that measures time
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from some unspecified point in the past that does not change
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after system startup.
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.tp
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.br clock_boottime " (since linux 3.15)"
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.\" commit 4a2378a943f09907fb1ae35c15de917f60289c14
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like
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.br clock_monotonic ,
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this is a monotonically increasing clock.
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however, whereas the
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.br clock_monotonic
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clock does not measure the time while a system is suspended, the
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.br clock_boottime
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clock does include the time during which the system is suspended.
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this is useful for applications that need to be suspend-aware.
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.br clock_realtime
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is not suitable for such applications, since that clock is affected
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by discontinuous changes to the system clock.
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.tp
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.br clock_realtime_alarm " (since linux 3.11)"
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.\" commit 11ffa9d6065f344a9bd769a2452f26f2f671e5f8
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this clock is like
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.br clock_realtime ,
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but will wake the system if it is suspended.
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the caller must have the
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.b cap_wake_alarm
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capability in order to set a timer against this clock.
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.tp
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.br clock_boottime_alarm " (since linux 3.11)"
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.\" commit 11ffa9d6065f344a9bd769a2452f26f2f671e5f8
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