TIMER_GETOVERRUN(2)       (2020-12-21)        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 expira-
          tions 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 sys-
          tem 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 temporarily blocked, there can be a delay
          between the time when the notification 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
          overrun count is the number of additional timer expirations
          that occurred between the time when the signal was generated
          and when it was delivered or accepted.

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

     RETURN VALUE
          On success, timer_getoverrun() returns the overrun count of

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     TIMER_GETOVERRUN(2)       (2020-12-21)        TIMER_GETOVERRUN(2)

          the specified timer; this count may be 0 if no overruns have
          occurred.  On failure, -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_overrun 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 notifications 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, before Linux 4.19, if the timer
          overrun value exceeds the maximum representable integer, the
          counter cycles, starting once more from low values.  Since
          Linux 4.19, timer_getoverrun() returns DELAYTIMER_MAX
          (defined as INT_MAX in <limits.h>) in this case (and the
          overrun value is reset to 0).

     EXAMPLES
          See timer_create(2).

     SEE ALSO
          clock_gettime(2), sigaction(2), signalfd(2), sigwaitinfo(2),
          timer_create(2), timer_delete(2), timer_settime(2),
          signal(7), time(7)

     COLOPHON
          This page is part of release 5.10 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/.

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