timer_getoverrun(2) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #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         top

       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         top

       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         top

       EINVAL timerid is not a valid timer ID.

VERSIONS         top

       This system call is available since Linux 2.6.

CONFORMING TO         top

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

NOTES         top

       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 discusses timer overruns only in the context of timer
       notifications using signals.

BUGS         top

       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.

EXAMPLES         top

       See timer_create(2).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09              TIMER_GETOVERRUN(2)

Pages that refer to this page: rt_sigaction(2)sigaction(2)syscalls(2)timer_create(2)timer_delete(2)timerfd_create(2)timerfd_gettime(2)timerfd_settime(2)timer_gettime(2)timer_settime(2)ualarm(3)usleep(3)signal-safety(7)