timer_getoverrun man page on Archlinux

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TIMER_GETOVERRUN(2)	   Linux Programmer's Manual	   TIMER_GETOVERRUN(2)

       timer_getoverrun - get overrun count for a POSIX per-process timer

       #include <time.h>

       int timer_getoverrun(timer_t timerid);

       Link with -lrt.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       timer_getoverrun(): _POSIX_C_SOURCE >= 199309L

       timer_getoverrun()  returns  the "overrun count" for the timer referred
       to by timerid.  An application can use the overrun count to  accurately
       calculate the number of timer expirations that would have occurred over
       a given time interval.  Timer overruns can occur	 both  when  receiving
       expiration  notifications  via  signals (SIGEV_SIGNAL), and via threads

       When expiration notifications are delivered via a signal, overruns  can
       occur  as  follows.  Regardless of whether or not a real-time signal is
       used for timer notifications, the system queues at most one signal  per
       timer.	(This is the behavior specified by POSIX.1-2001.  The alterna‐
       tive, queuing one signal for each timer expiration, could easily result
       in  overflowing	the  allowed limits for queued signals on the system.)
       Because of system scheduling delays, or because the signal may be  tem‐
       porarily	 blocked, there can be a delay between the time when the noti‐
       fication signal is generated and the time when it is  delivered	(e.g.,
       caught  by  a signal handler) or accepted (e.g., using sigwaitinfo(2)).
       In this interval, further timer expirations may occur.  The timer over‐
       run  count  is the number of additional timer expirations that occurred
       between the time when the signal was generated and when it  was	deliv‐
       ered or accepted.

       Timer  overruns can also occur when expiration notifications are deliv‐
       ered via invocation of a thread, since there may be an arbitrary	 delay
       between	an expiration of the timer and the invocation of the notifica‐
       tion thread, and in that delay interval, additional  timer  expirations
       may occur.

       On  success, timer_getoverrun() returns the overrun count of the speci‐
       fied timer; this count may be 0 if no overruns have occurred.  On fail‐
       ure, -1 is returned, and errno is set to indicate the error.

       EINVAL timerid is not a valid timer ID.

       This system call is available since Linux 2.6.


       When  timer  notifications are delivered via signals (SIGEV_SIGNAL), on
       Linux it is also possible to obtain the overrun count via the  si_over‐
       run  field  of the siginfo_t structure (see sigaction(2)).  This allows
       an application to avoid the overhead of making a system call to	obtain
       the overrun count, but is a nonportable extension to POSIX.1-2001.

       POSIX.1-2001  discusses	timer  overruns	 only  in the context of timer
       notifications using signals.

       POSIX.1-2001 specifies that if the timer overrun count is equal	to  or
       greater	than  an  implementation-defined maximum, DELAYTIMER_MAX, then
       timer_getoverrun() should return DELAYTIMER_MAX.	 However,  Linux  does
       not implement this feature: instead, if the timer overrun value exceeds
       the maximum representable integer, the counter  cycles,	starting  once
       more from low values.

       See timer_create(2).

       clock_gettime(2), sigaction(2), signalfd(2), sigwaitinfo(2), timer_cre‐
       ate(2), timer_delete(2), timer_settime(2), signal(7), time(7)

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2009-02-20		   TIMER_GETOVERRUN(2)

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