Ubuntu Manpage: timer_getoverrun - get overrun count for a POSIX per-process timer

NAME

       timer_getoverrun - get overrun count for a POSIX per-process timer

LIBRARY

       Real-time library (librt, -lrt)

SYNOPSIS

       #include <time.h>

       int timer_getoverrun(timer_t timerid);

   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

       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.

STANDARDS

       POSIX.1-2008.

HISTORY

       Linux 2.6.  POSIX.1-2001.

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, before Linux
       4.19, if the timer overrun value exceeds the maximum representable integer, the counter cycles,  starting
       once  more  from  low  values.   Since  Linux 4.19, timer_getoverrun() returns DELAYTIMER_MAX (defined as
       INT_MAX in <limits.h>) in this case (and the overrun value is reset to 0).

EXAMPLES

       See timer_create(2).