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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.1-2001. 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.
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.1-2001.
POSIX.1-2001 only discusses timer overruns in the context of timer
notifications using signals.
POSIX.1-2001 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 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2009-02-20 TIMER_GETOVERRUN(2)
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