timer_getoverrun

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
       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
       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/.



Linux                              2020-12-21                TIMER_GETOVERRUN(2)