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.1-2001.  The alternative, queuing one signal for each timer expiration, could easily result  in  over-
       flowing 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, fur-
       ther 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 application to avoid the
       overhead of making a system call to obtain the overrun count, but is a nonportable extension to 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), sig-
       nal(7), time(7)

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-02-20                                        TIMER_GETOVERRUN(2)