man2/timer_getoverrun.2

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  23 .TH TIMER_GETOVERRUN 2 2009-02-20 Linux "Linux Programmer's Manual"
  24 .SH NAME
  25 timer_getoverrun \- get overrun count for a POSIX per-process timer
  26 .SH SYNOPSIS
  27 .nf
  28 .B  #include <time.h>
  29
  30 .BI "int timer_getoverrun(timer_t " timerid );
  31 .fi
  32
  33 Link with \fI\-lrt\fP.
  34 .sp
  35 .in -4n
  36 Feature Test Macro Requirements for glibc (see
  37 .BR feature_test_macros (7)):
  38 .in
  39 .sp
  40 .BR timer_getoverrun ():
  41 _POSIX_C_SOURCE >= 199309
  42 .SH DESCRIPTION
  43 .BR timer_getoverrun ()
  44 returns the "overrun count" for the timer referred to by
  45 .IR timerid .
  46 An application can use the overrun count to accurately calculate the number
  47 of timer expirations that would have occurred over a given time interval.
  48 Timer overruns can occur both when receiving expiration notifications
  49 via signals
  50 .RB ( SIGEV_SIGNAL ),
  51 and via threads
  52 .RB ( SIGEV_THREAD ).
  53
  54 When expiration notifications are delivered via a signal,
  55 overruns can occur as follows.
  56 Regardless of whether or not a real-time signal is used for
  57 timer notifications,
  58 the system queues at most one signal per timer.
  59 (This is the behavior specified by POSIX.1-2001.
  60 The alternative, queuing one signal for each timer expiration,
  61 could easily result in overflowing the allowed limits for
  62 queued signals on the system.)
  63 Because of system scheduling delays,
  64 or because the signal may be temporarily blocked,
  65 there can be a delay between the time when the notification
  66 signal is generated and the time when it
  67 is delivered (e.g., caught by a signal handler) or accepted (e.g., using
  68 .BR sigwaitinfo (2)).
  69 In this interval, further timer expirations may occur.
  70 The timer overrun count is the number of additional
  71 timer expirations that occurred between the time when the signal
  72 was generated and when it was delivered or accepted.
  73
  74 Timer overruns can also occur when expiration notifications
  75 are delivered via invocation of a thread,
  76 since there may be an arbitrary delay between an expiration of the timer
  77 and the invocation of the notification thread,
  78 and in that delay interval, additional timer expirations may occur
  79 .SH RETURN VALUE
  80 On success,
  81 .BR timer_getoverrun ()
  82 returns the overrun count of the specified timer;
  83 this count may be 0 if no overruns have occurred.
  84 On failure, \-1 is returned, and
  85 .I errno
  86 is set to indicate the error.
  87 .SH ERRORS
  88 .TP
  89 .B EINVAL
  90 .I timerid
  91 is not a valid timer ID.
  92 .SH VERSIONS
  93 This system call is available since Linux 2.6.
  94 .SH CONFORMING TO
  95 POSIX.1-2001
  96 .SH NOTES
  97 When timer notifications are delivered via signals
  98 .RB ( SIGEV_SIGNAL ),
  99 on Linux it is also possible to obtain the overrun count via the
 100 .I si_overrun
 101 field of the
 102 .I siginfo_t
 103 structure (see
 104 .BR sigaction (2)).
 105 This allows an application to avoid the overhead of making
 106 a system call to obtain the overrun count,
 107 but is a nonportable extension to POSIX.1-2001.
 108
 109 POSIX.1-2001 only discusses timer overruns in the context of
 110 timer notifications using signals.
 111 .\" FIXME . Austin bug filed, 11 Feb 09
 112 .SH BUGS
 113 POSIX.1-2001 specifies that if the timer overrun count
 114 is equal to or greater than an implementation-defined maximum,
 115 .BR DELAYTIMER_MAX ,
 116 then
 117 .BR timer_getoverrun ()
 118 should return
 119 .BR DELAYTIMER_MAX .
 120 However, Linux does not implement this feature: instead,
 121 if the timer overrun value exceeds the maximum representable integer,
 122 the counter cycles, starting once more from low values.
 123 .\" Bug filed: http://bugzilla.kernel.org/show_bug.cgi?id=12665
 124 .\" http://thread.gmane.org/gmane.linux.kernel/113276/
 125 .SH EXAMPLE
 126 See
 127 .BR timer_create (2).
 128 .SH SEE ALSO
 129 .BR clock_gettime (2),
 130 .BR sigaction (2),
 131 .BR signalfd (2),
 132 .BR sigwaitinfo (2),
 133 .BR timer_create (2),
 134 .BR timer_delete (2),
 135 .BR timer_settime (2),
 136 .BR signal (7),
 137 .BR time (7)