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.  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.

       On success, timer_getoverrun() returns the overrun count of 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.

       EINVAL timerid is not a valid timer ID.

       This system call is available since Linux 2.6.

       POSIX.1-2001, POSIX.1-2008.

       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.

       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.

       See timer_create(2).

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

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Linux                             2017-09-15               TIMER_GETOVERRUN(2)