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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 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_SIG-
       NAL), 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.1-2001.  The alternative, queuing one signal for  each  timer  expira-
       tion, could easily result in overflowing the allowed limits for queued signals on the system.)  Because of sys-
       tem 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

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  applica-
       tion 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 imple-
       ment 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_set-
       time(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  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-02-20               TIMER_GETOVERRUN(2)