timer_getoverrun(2) — Linux manual page

timer_getoverrun(2)        System Calls Manual       timer_getoverrun(2)

NAME

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

LIBRARY

       Real-time library (librt, -lrt)

SYNOPSIS

       #include <time.h>

       int timer_getoverrun(timer_t timerid);

   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
       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 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

       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.

STANDARDS

       POSIX.1-2008.

HISTORY

       Linux 2.6.  POSIX.1-2001.

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

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Linux man-pages 6.9.1          2024-05-02            timer_getoverrun(2)

Pages that refer to this page: sigaction(2), syscalls(2), timer_create(2), timer_delete(2), timerfd_create(2), timer_settime(2), timer_t(3type), ualarm(3), usleep(3), signal-safety(7)

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