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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 >= 199309

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 num-
       ber  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 notifi-
       cations,  the  system  queues  at most one signal per timer.  (This is the behavior
       specified by POSIX.1-2001.  The alternative, queuing  one  signal  for  each  timer
       expiration,  could  easily result in overflowing the allowed limits for queued sig-
       nals 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 notifi-
       cation 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, fur-
       ther timer expirations may occur.  The timer overrun count is the number  of  addi-
       tional  timer expirations that occurred between the time when the signal was gener-
       ated and when it was delivered or accepted.

       Timer overruns can also occur when expiration notifications are delivered via invo-
       cation  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  inter-
       val, 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

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  non-portable  extension
       POSIX.1-2001.

       POSIX.1-2001  only  discusses  timer overruns in the context of timer notifications
       using signals.

BUGS
       POSIX.1-2001 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.

EXAMPLE
       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 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-02-20               TIMER_GETOVERRUN(2)

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