returns the "overrun count" for the timer referred to by
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
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
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
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
returns the overrun count of the specified timer;
this count may be 0 if no overruns have occurred.
On failure, -1 is returned, and
is set to indicate the error.
is not a valid timer ID.
This system call is available since Linux 2.6.
When timer notifications are delivered via signals
on Linux it is also possible to obtain the overrun count via the
field of the
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,
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.