TIMER_GETOVERRUN
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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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 >= 199309L
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.
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, POSIX.1-2008.
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.
POSIX.1 discusses timer overruns only in the context of
timer notifications using signals.
BUGS
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, 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 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- EXAMPLE
-
- SEE ALSO
-
- COLOPHON
-
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