TIMER_GETOVERRUN(2) Linux Programmer's Manual TIMER_GETOVERRUN(2)
timer_getoverrun - get overrun count for a POSIX per-process timer
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 expira-
tions 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
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 sys-
tem 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 deliv-
ered (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.
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 discusses timer overruns only in the context of timer notifications using
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.
clock_gettime(2), sigaction(2), signalfd(2), sigwaitinfo(2), timer_create(2),
timer_delete(2), timer_settime(2), signal(7), time(7)
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Linux 2009-02-20 TIMER_GETOVERRUN(2)