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TIMER_GETOVERRUN(2)        Linux Programmer's Manual       TIMER_GETOVERRUN(2)
NAME
       timer_getoverrun - get overrun count for a POSIX per-process timer
SYNOPSIS
       #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
DESCRIPTION
       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
       (SIGEV_THREAD).
       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.  The alternative,
       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.
RETURN VALUE
       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.
ERRORS
       EINVAL timerid is not a valid timer ID.
VERSIONS
       This system call is available since Linux 2.6.
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.
NOTES
       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.
       POSIX.1 discusses timer overruns only in the context of timer notifica-
       tions using signals.
BUGS
       POSIX.1 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.
EXAMPLE
       See timer_create(2).
SEE ALSO
       clock_gettime(2), sigaction(2), signalfd(2), sigwaitinfo(2), timer_cre-
       ate(2), timer_delete(2), timer_settime(2), signal(7), time(7)
COLOPHON
       This page is part of release 4.15 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.
Linux                             2017-09-15               TIMER_GETOVERRUN(2)