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 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, 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_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.

       POSIX.1 discusses timer overruns only in the context of timer notifications 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).

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 4.15 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 https://www.kernel.org/doc/man-pages/.