timer_getoverrun - get overrun count for a POSIX per-process timer
int timer_getoverrun(timer_t timerid);
Link with -lrt.
_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
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
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.
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.
- timerid is not a valid timer ID.
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, before
Linux 4.19, if the timer overrun value exceeds the maximum representable
integer, the counter cycles, starting once more from low values. Since Linux
4.19, timer_getoverrun() returns DELAYTIMER_MAX (defined as
INT_MAX in <limits.h>) in this case (and the overrun value
is reset to 0).
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