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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 >= 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  tem-
     porarily  blocked, there can be a delay between the time when the noti-
     fication 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 over-
     run  count  is the number of additional timer expirations that occurred
     between the time when the signal was generated and when it  was  deliv-
     ered or accepted.

     Timer  overruns can also occur when expiration notifications are deliv-
     ered via invocation of a thread, since there may be an arbitrary  delay
     between  an expiration of the timer and the invocation of the notifica-
     tion thread, and in that delay interval, additional  timer  expirations
     may occur.

RETURN VALUE

     On  success, timer_getoverrun() returns the overrun count of the speci-
     fied timer; this count may be 0 if no overruns have occurred.  On fail-
     ure, -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_over-
     run  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 notifica-
     tions 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,  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).