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

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