NAME
timer_getoverrun - get overrun count for a POSIX per-process timer
SYNOPSIS
#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 >= 199309
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.1-2001. 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
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
This system call is available since Linux 2.6.
CONFORMING TO
POSIX.1-2001
NOTES
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.
BUGS
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.
EXAMPLE
See timer_create(2).
SEE ALSO
clock_gettime(2), sigaction(2), signalfd(2), sigwaitinfo(2),
timer_create(2), timer_delete(2), timer_settime(2), signal(7), time(7)
COLOPHON
This page is part of release 3.24 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/.