1TIMER_GETOVERRUN(2)        Linux Programmer's Manual       TIMER_GETOVERRUN(2)
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NAME

6       timer_getoverrun - get overrun count for a POSIX per-process timer
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SYNOPSIS

9       #include <time.h>
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11       int timer_getoverrun(timer_t timerid);
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13       Link with -lrt.
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15   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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17       timer_getoverrun(): _POSIX_C_SOURCE >= 199309L
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DESCRIPTION

20       timer_getoverrun()  returns  the "overrun count" for the timer referred
21       to by timerid.  An application can use the overrun count to  accurately
22       calculate the number of timer expirations that would have occurred over
23       a given time interval.  Timer overruns can occur  both  when  receiving
24       expiration  notifications  via  signals (SIGEV_SIGNAL), and via threads
25       (SIGEV_THREAD).
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27       When expiration notifications are delivered via a signal, overruns  can
28       occur  as  follows.  Regardless of whether or not a real-time signal is
29       used for timer notifications, the system queues at most one signal  per
30       timer.   (This  is the behavior specified by POSIX.1.  The alternative,
31       queuing one signal for each timer expiration, could  easily  result  in
32       overflowing  the  allowed  limits  for  queued  signals on the system.)
33       Because of system scheduling delays, or because the signal may be  tem‐
34       porarily  blocked, there can be a delay between the time when the noti‐
35       fication signal is generated and the time when it is  delivered  (e.g.,
36       caught  by  a signal handler) or accepted (e.g., using sigwaitinfo(2)).
37       In this interval, further timer expirations may occur.  The timer over‐
38       run  count  is the number of additional timer expirations that occurred
39       between the time when the signal was generated and when it  was  deliv‐
40       ered or accepted.
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42       Timer  overruns can also occur when expiration notifications are deliv‐
43       ered via invocation of a thread, since there may be an arbitrary  delay
44       between  an expiration of the timer and the invocation of the notifica‐
45       tion thread, and in that delay interval, additional  timer  expirations
46       may occur.
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RETURN VALUE

49       On  success, timer_getoverrun() returns the overrun count of the speci‐
50       fied timer; this count may be 0 if no overruns have occurred.  On fail‐
51       ure, -1 is returned, and errno is set to indicate the error.
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ERRORS

54       EINVAL timerid is not a valid timer ID.
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VERSIONS

57       This system call is available since Linux 2.6.
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CONFORMING TO

60       POSIX.1-2001, POSIX.1-2008.
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NOTES

63       When  timer  notifications are delivered via signals (SIGEV_SIGNAL), on
64       Linux it is also possible to obtain the overrun count via the  si_over‐
65       run  field  of the siginfo_t structure (see sigaction(2)).  This allows
66       an application to avoid the overhead of making a system call to  obtain
67       the overrun count, but is a nonportable extension to POSIX.1.
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69       POSIX.1 discusses timer overruns only in the context of timer notifica‐
70       tions using signals.
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BUGS

73       POSIX.1 specifies that if the  timer  overrun  count  is  equal  to  or
74       greater  than  an  implementation-defined maximum, DELAYTIMER_MAX, then
75       timer_getoverrun() should return DELAYTIMER_MAX.  However,  Linux  does
76       not implement this feature: instead, if the timer overrun value exceeds
77       the maximum representable integer, the counter  cycles,  starting  once
78       more from low values.
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EXAMPLE

81       See timer_create(2).
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SEE ALSO

84       clock_gettime(2), sigaction(2), signalfd(2), sigwaitinfo(2), timer_cre‐
85       ate(2), timer_delete(2), timer_settime(2), signal(7), time(7)
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COLOPHON

88       This page is part of release 4.15 of the Linux  man-pages  project.   A
89       description  of  the project, information about reporting bugs, and the
90       latest    version    of    this    page,    can     be     found     at
91       https://www.kernel.org/doc/man-pages/.
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95Linux                             2017-09-15               TIMER_GETOVERRUN(2)
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