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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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timer_getoverrun(2) System Calls Manual timer_getoverrun(2)
timer_getoverrun - get overrun count for a POSIX per-process timer
Real-time library (librt, -lrt)
#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
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.
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.
EINVAL timerid is not a valid timer ID.
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.
POSIX.1-2008.
Linux 2.6. POSIX.1-2001.
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).
See timer_create(2).
clock_gettime(2), sigaction(2), signalfd(2), sigwaitinfo(2),
timer_create(2), timer_delete(2), timer_settime(2), signal(7),
time(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 timer_getoverrun(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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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