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
(): _POSIX_C_SOURCE >= 199309L
() 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
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
(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.
- 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
Linux it is also possible to obtain the overrun count via the
field of the siginfo_t
(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
POSIX.1 specifies that if the timer overrun count is equal to or greater than an
implementation-defined maximum, DELAYTIMER_MAX
() 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
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