timer_getoverrun(2) - Linux manual page (original) (raw)

timergetoverrun(2) System Calls Manual timergetoverrun(2)

NAME top

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

LIBRARY top

   Real-time library (_librt_, _-lrt_)

SYNOPSIS top

   **#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 top

   **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)](../man2/sigwaitinfo.2.html)).  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 top

   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](../man3/errno.3.html)_ is set to indicate the
   error.

ERRORS top

   **EINVAL** _timerid_ is not a valid timer ID.

VERSIONS top

   When timer notifications are delivered via signals (**SIGEV_SIGNAL**),
   on Linux it is also possible to obtain the overrun count via the
   _sioverrun_ field of the _siginfot_ structure (see [sigaction(2)](../man2/sigaction.2.html)).
   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 top

   POSIX.1-2008.

HISTORY top

   Linux 2.6.  POSIX.1-2001.

BUGS top

   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 top

   See [timer_create(2)](../man2/timer%5Fcreate.2.html).

SEE ALSO top

   [clock_gettime(2)](../man2/clock%5Fgettime.2.html), [sigaction(2)](../man2/sigaction.2.html), [signalfd(2)](../man2/signalfd.2.html), [sigwaitinfo(2)](../man2/sigwaitinfo.2.html),
   [timer_create(2)](../man2/timer%5Fcreate.2.html), [timer_delete(2)](../man2/timer%5Fdelete.2.html), [timer_settime(2)](../man2/timer%5Fsettime.2.html), [signal(7)](../man7/signal.7.html),
   [time(7)](../man7/time.7.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 timergetoverrun(2)

Pages that refer to this page:sigaction(2), syscalls(2), timer_create(2), timer_delete(2), timerfd_create(2), timer_settime(2), timer_t(3type), ualarm(3), usleep(3), signal-safety(7)