man2/timer_getoverrun.2

   1 .\" Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk
   2 .\"     <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .TH TIMER_GETOVERRUN 2 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
   7 .SH NAME
   8 timer_getoverrun \- get overrun count for a POSIX per-process timer
   9 .SH LIBRARY
  10 Real-time library
  11 .RI ( librt ", " \-lrt )
  12 .SH SYNOPSIS
  13 .nf
  14 .B  #include <time.h>
  15 .PP
  16 .BI "int timer_getoverrun(timer_t " timerid );
  17 .fi
  18 .PP
  19 .RS -4
  20 Feature Test Macro Requirements for glibc (see
  21 .BR feature_test_macros (7)):
  22 .RE
  23 .PP
  24 .BR timer_getoverrun ():
  25 .nf
  26     _POSIX_C_SOURCE >= 199309L
  27 .fi
  28 .SH DESCRIPTION
  29 .BR timer_getoverrun ()
  30 returns the "overrun count" for the timer referred to by
  31 .IR timerid .
  32 An application can use the overrun count to accurately calculate the number
  33 of timer expirations that would have occurred over a given time interval.
  34 Timer overruns can occur both when receiving expiration notifications
  35 via signals
  36 .RB ( SIGEV_SIGNAL ),
  37 and via threads
  38 .RB ( SIGEV_THREAD ).
  39 .PP
  40 When expiration notifications are delivered via a signal,
  41 overruns can occur as follows.
  42 Regardless of whether or not a real-time signal is used for
  43 timer notifications,
  44 the system queues at most one signal per timer.
  45 (This is the behavior specified by POSIX.1.
  46 The alternative, queuing one signal for each timer expiration,
  47 could easily result in overflowing the allowed limits for
  48 queued signals on the system.)
  49 Because of system scheduling delays,
  50 or because the signal may be temporarily blocked,
  51 there can be a delay between the time when the notification
  52 signal is generated and the time when it
  53 is delivered (e.g., caught by a signal handler) or accepted (e.g., using
  54 .BR sigwaitinfo (2)).
  55 In this interval, further timer expirations may occur.
  56 The timer overrun count is the number of additional
  57 timer expirations that occurred between the time when the signal
  58 was generated and when it was delivered or accepted.
  59 .PP
  60 Timer overruns can also occur when expiration notifications
  61 are delivered via invocation of a thread,
  62 since there may be an arbitrary delay between an expiration of the timer
  63 and the invocation of the notification thread,
  64 and in that delay interval, additional timer expirations may occur.
  65 .SH RETURN VALUE
  66 On success,
  67 .BR timer_getoverrun ()
  68 returns the overrun count of the specified timer;
  69 this count may be 0 if no overruns have occurred.
  70 On failure, \-1 is returned, and
  71 .I errno
  72 is set to indicate the error.
  73 .SH ERRORS
  74 .TP
  75 .B EINVAL
  76 .I timerid
  77 is not a valid timer ID.
  78 .SH VERSIONS
  79 This system call is available since Linux 2.6.
  80 .SH STANDARDS
  81 POSIX.1-2001, POSIX.1-2008.
  82 .SH NOTES
  83 When timer notifications are delivered via signals
  84 .RB ( SIGEV_SIGNAL ),
  85 on Linux it is also possible to obtain the overrun count via the
  86 .I si_overrun
  87 field of the
  88 .I siginfo_t
  89 structure (see
  90 .BR sigaction (2)).
  91 This allows an application to avoid the overhead of making
  92 a system call to obtain the overrun count,
  93 but is a nonportable extension to POSIX.1.
  94 .PP
  95 POSIX.1 discusses timer overruns only in the context of
  96 timer notifications using signals.
  97 .\" FIXME . Austin bug filed, 11 Feb 09
  98 .\" https://www.austingroupbugs.net/view.php?id=95
  99 .SH BUGS
 100 POSIX.1 specifies that if the timer overrun count
 101 is equal to or greater than an implementation-defined maximum,
 102 .BR DELAYTIMER_MAX ,
 103 then
 104 .BR timer_getoverrun ()
 105 should return
 106 .BR DELAYTIMER_MAX .
 107 However, before Linux 4.19,
 108 .\" http://bugzilla.kernel.org/show_bug.cgi?id=12665
 109 if the timer overrun value exceeds the maximum representable integer,
 110 the counter cycles, starting once more from low values.
 111 Since Linux 4.19,
 112 .\" commit 78c9c4dfbf8c04883941445a195276bb4bb92c76
 113 .BR timer_getoverrun ()
 114 returns
 115 .B DELAYTIMER_MAX
 116 (defined as
 117 .B INT_MAX
 118 in
 119 .IR <limits.h> )
 120 in this case (and the overrun value is reset to 0).
 121 .SH EXAMPLES
 122 See
 123 .BR timer_create (2).
 124 .SH SEE ALSO
 125 .BR clock_gettime (2),
 126 .BR sigaction (2),
 127 .BR signalfd (2),
 128 .BR sigwaitinfo (2),
 129 .BR timer_create (2),
 130 .BR timer_delete (2),
 131 .BR timer_settime (2),
 132 .BR signal (7),
 133 .BR time (7)