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TIMER_GETOVERRUN(2) Linux Programmer's Manual TIMER_GETOVERRUN(2)
timer_getoverrun - get overrun count for a POSIX per-process timer
#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
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.
This system call is available since Linux 2.6.
POSIX.1-2001, POSIX.1-2008.
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 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.
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 release 4.12 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/.
Linux 2015-08-08 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), ualarm(3), usleep(3), signal-safety(7)