1TIMER_GETOVERRUN(3P)       POSIX Programmer's Manual      TIMER_GETOVERRUN(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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NAME

12       timer_getoverrun, timer_gettime,  timer_settime  -  per-process  timers
13       (REALTIME)
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SYNOPSIS

16       #include <time.h>
17
18       int timer_getoverrun(timer_t timerid);
19       int timer_gettime(timer_t timerid, struct itimerspec *value);
20       int timer_settime(timer_t timerid, int flags,
21              const struct itimerspec *restrict value,
22              struct itimerspec *restrict ovalue);
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24

DESCRIPTION

26       The  timer_gettime()  function shall store the amount of time until the
27       specified timer, timerid, expires and the reload  value  of  the  timer
28       into the space pointed to by the value argument. The it_value member of
29       this structure shall contain  the  amount  of  time  before  the  timer
30       expires,  or  zero  if the timer is disarmed. This value is returned as
31       the interval until timer expiration, even if the timer was  armed  with
32       absolute time. The it_interval member of value shall contain the reload
33       value last set by timer_settime().
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35       The timer_settime() function shall set the time until the next  expira‐
36       tion  of the timer specified by timerid from the it_value member of the
37       value argument and arm the timer if the it_value  member  of  value  is
38       non-zero. If the specified timer was already armed when timer_settime()
39       is called, this call shall reset the time until next expiration to  the
40       value  specified.  If  the  it_value member of value is zero, the timer
41       shall be disarmed. The effect of disarming or resetting  a  timer  with
42       pending expiration notifications is unspecified.
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44       If  the flag TIMER_ABSTIME is not set in the argument flags, timer_set‐
45       time() shall behave as if the time until next expiration is set  to  be
46       equal  to  the interval specified by the it_value member of value. That
47       is, the timer shall expire in it_value nanoseconds from when  the  call
48       is  made.  If  the  flag  TIMER_ABSTIME  is  set in the argument flags,
49       timer_settime() shall behave as if the time until  next  expiration  is
50       set  to  be equal to the difference between the absolute time specified
51       by the it_value member of value and the  current  value  of  the  clock
52       associated  with  timerid.   That  is,  the timer shall expire when the
53       clock reaches the value specified by the it_value member of  value.  If
54       the  specified  time has already passed, the function shall succeed and
55       the expiration notification shall be made.
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57       The reload value of the timer shall be set to the  value  specified  by
58       the  it_interval member of value. When a timer is armed with a non-zero
59       it_interval, a periodic (or repetitive) timer is specified.
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61       Time values that are between two consecutive non-negative integer  mul‐
62       tiples  of the resolution of the specified timer shall be rounded up to
63       the larger multiple of the resolution.  Quantization  error  shall  not
64       cause the timer to expire earlier than the rounded time value.
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66       If  the argument ovalue is not NULL, the timer_settime() function shall
67       store, in the location referenced by ovalue, a value  representing  the
68       previous amount of time before the timer would have expired, or zero if
69       the timer was disarmed, together with the previous timer reload  value.
70       Timers shall not expire before their scheduled time.
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72       Only  a  single signal shall be queued to the process for a given timer
73       at any point in time. When a timer for which a signal is still  pending
74       expires,  no  signal  shall be queued, and a timer overrun shall occur.
75       When a timer expiration  signal  is  delivered  to  or  accepted  by  a
76       process, if the implementation supports the Realtime Signals Extension,
77       the timer_getoverrun() function shall return the timer expiration over‐
78       run  count for the specified timer. The overrun count returned contains
79       the number of extra timer expirations that occurred  between  the  time
80       the  signal  was  generated  (queued)  and  when  it  was  delivered or
81       accepted, up to but not including an implementation-defined maximum  of
82       {DELAYTIMER_MAX}.  If  the  number of such extra expirations is greater
83       than or equal to {DELAYTIMER_MAX}, then the overrun count shall be  set
84       to  {DELAYTIMER_MAX}.  The  value  returned by timer_getoverrun() shall
85       apply to the most recent expiration signal delivery or  acceptance  for
86       the  timer.   If no expiration signal has been delivered for the timer,
87       or if the Realtime Signals Extension is not supported, the return value
88       of timer_getoverrun() is unspecified.
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RETURN VALUE

91       If  the timer_getoverrun() function succeeds, it shall return the timer
92       expiration overrun count as explained above.
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94       If the timer_gettime() or timer_settime() functions succeed, a value of
95       0 shall be returned.
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97       If  an  error  occurs for any of these functions, the value -1 shall be
98       returned, and errno set to indicate the error.
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ERRORS

101       The timer_getoverrun(), timer_gettime(), and timer_settime()  functions
102       shall fail if:
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104       EINVAL The  timerid  argument  does not correspond to an ID returned by
105              timer_create() but not yet deleted by timer_delete().
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108       The timer_settime() function shall fail if:
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110       EINVAL A value structure specified a nanosecond value less than zero or
111              greater  than  or equal to 1000 million, and the it_value member
112              of that structure did not specify zero seconds and nanoseconds.
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115       The following sections are informative.
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EXAMPLES

118       None.
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APPLICATION USAGE

121       None.
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RATIONALE

124       Practical clocks tick at a finite rate, with rates  of  100  hertz  and
125       1000  hertz  being  common.  The inverse of this tick rate is the clock
126       resolution, also called the clock granularity, which in either case  is
127       expressed  as  a time duration, being 10 milliseconds and 1 millisecond
128       respectively for these common  rates.   The  granularity  of  practical
129       clocks  implies  that if one reads a given clock twice in rapid succes‐
130       sion, one may get the same time value twice; and that timers must  wait
131       for  the  next  clock  tick  after  the theoretical expiration time, to
132       ensure that a timer never returns too soon. Note also that  the  granu‐
133       larity of the clock may be significantly coarser than the resolution of
134       the data format used to set and get time and interval values. Also note
135       that  some  implementations  may  choose to adjust time and/or interval
136       values to exactly match the ticks of the underlying clock.
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138       This volume of IEEE Std 1003.1-2001 defines  functions  that  allow  an
139       application  to  determine  the implementation-supported resolution for
140       the clocks and requires an implementation to  document  the  resolution
141       supported  for timers and nanosleep() if they differ from the supported
142       clock resolution.  This is more of a procurement issue than  a  runtime
143       application issue.
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FUTURE DIRECTIONS

146       None.
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SEE ALSO

149       clock_getres(),   timer_create(),   the   Base  Definitions  volume  of
150       IEEE Std 1003.1-2001, <time.h>
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153       Portions of this text are reprinted and reproduced in  electronic  form
154       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
155       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
156       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
157       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
158       event of any discrepancy between this version and the original IEEE and
159       The Open Group Standard, the original IEEE and The Open Group  Standard
160       is  the  referee document. The original Standard can be obtained online
161       at http://www.opengroup.org/unix/online.html .
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165IEEE/The Open Group                  2003                 TIMER_GETOVERRUN(3P)
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