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  expira‐
       tions  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  sys‐
       tem 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  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 deliv‐
       ered (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 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  discusses  timer  overruns  only 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 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2009-02-20                        TIMER_GETOVERRUN(2)