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NAME
       timer_getoverrun, timer_gettime, timer_settime - per-process timers (REALTIME)


SYNOPSIS
       #include <time.h>

       int timer_getoverrun(timer_t timerid);
       int timer_gettime(timer_t timerid, struct itimerspec *value);
       int timer_settime(timer_t timerid, int flags,
              const struct itimerspec *restrict value,
              struct itimerspec *restrict ovalue);


DESCRIPTION
       The  timer_gettime()  function shall store the amount of time until the specified timer, timerid, expires
       and the reload value of the timer into the space pointed to by the value argument. The it_value member of
       this  structure  shall  contain  the  amount  of  time  before the timer expires, or zero if the timer is
       disarmed. This value is returned as the interval until timer expiration, even if the timer was armed with
       absolute   time.   The  it_interval  member  of  value  shall  contain  the  reload  value  last  set  by
       timer_settime().

       The timer_settime() function shall set the time until the next  expiration  of  the  timer  specified  by
       timerid  from the it_value member of the value argument and arm the timer if the it_value member of value
       is non-zero. If the specified timer was already armed when timer_settime() is  called,  this  call  shall
       reset the time until next expiration to the value specified. If the it_value member of value is zero, the
       timer shall be  disarmed.  The  effect  of  disarming  or  resetting  a  timer  with  pending  expiration
       notifications is unspecified.

       If  the  flag TIMER_ABSTIME is not set in the argument flags, timer_settime() shall behave as if the time
       until next expiration is set to be equal to the interval specified by the it_value member of value.  That
       is,  the timer shall expire in it_value nanoseconds from when the call is made. If the flag TIMER_ABSTIME
       is set in the argument flags, timer_settime() shall behave as if the time until next expiration is set to
       be  equal  to  the difference between the absolute time specified by the it_value member of value and the
       current value of the clock associated with timerid.  That is, the  timer  shall  expire  when  the  clock
       reaches  the  value  specified by the it_value member of value. If the specified time has already passed,
       the function shall succeed and the expiration notification shall be made.

       The reload value of the timer shall be set to the value specified by the  it_interval  member  of  value.
       When a timer is armed with a non-zero it_interval, a periodic (or repetitive) timer is specified.

       Time  values  that  are  between  two consecutive non-negative integer multiples of the resolution of the
       specified timer shall be rounded up to the larger multiple of the resolution.  Quantization  error  shall
       not cause the timer to expire earlier than the rounded time value.

       If  the argument ovalue is not NULL, the timer_settime() function shall store, in the location referenced
       by ovalue, a value representing the previous amount of time before the timer would have expired, or  zero
       if  the timer was disarmed, together with the previous timer reload value. Timers shall not expire before
       their scheduled time.

       Only a single signal shall be queued to the process for a given timer at any point in time. When a  timer
       for  which a signal is still pending expires, no signal shall be queued, and a timer overrun shall occur.
        When a timer expiration signal is delivered to or accepted by a process, if the implementation  supports
       the Realtime Signals Extension, the timer_getoverrun() function shall return the timer expiration overrun
       count for the specified timer. The overrun count returned contains the number of extra timer  expirations
       that  occurred  between the time the signal was generated (queued) and when it was delivered or accepted,
       up to but not including an implementation-defined maximum of {DELAYTIMER_MAX}.  If  the  number  of  such
       extra  expirations  is  greater than or equal to {DELAYTIMER_MAX}, then the overrun count shall be set to
       {DELAYTIMER_MAX}. The value returned by timer_getoverrun() shall apply  to  the  most  recent  expiration
       signal  delivery  or acceptance for the timer.  If no expiration signal has been delivered for the timer,
       or if the Realtime Signals Extension  is  not  supported,  the  return  value  of  timer_getoverrun()  is
       unspecified.


RETURN VALUE
       If  the  timer_getoverrun()  function  succeeds,  it  shall  return the timer expiration overrun count as
       explained above.

       If the timer_gettime() or timer_settime() functions succeed, a value of 0 shall be returned.

       If an error occurs for any of these functions, the value -1 shall be returned, and errno set to  indicate
       the error.


ERRORS
       The timer_getoverrun(), timer_gettime(), and timer_settime() functions shall fail if:

       EINVAL The  timerid  argument does not correspond to an ID returned by timer_create() but not yet deleted
              by timer_delete().

       The timer_settime() function shall fail if:

       EINVAL A value structure specified a nanosecond value less than zero or greater than  or  equal  to  1000
              million, and the it_value member of that structure did not specify zero seconds and nanoseconds.

       The following sections are informative.


EXAMPLES
       None.


APPLICATION USAGE
       None.


RATIONALE
       Practical  clocks tick at a finite rate, with rates of 100 hertz and 1000 hertz being common. The inverse
       of this tick rate is the clock resolution, also called the clock granularity, which  in  either  case  is
       expressed  as  a  time  duration,  being  10 milliseconds and 1 millisecond respectively for these common
       rates.  The granularity of practical clocks implies that if one  reads  a  given  clock  twice  in  rapid
       succession,  one  may  get  the  same time value twice; and that timers must wait for the next clock tick
       after the theoretical expiration time, to ensure that a timer never returns too soon. Note also that  the
       granularity  of the clock may be significantly coarser than the resolution of the data format used to set
       and get time and interval values. Also note that some implementations may choose to  adjust  time  and/or
       interval values to exactly match the ticks of the underlying clock.

       This  volume  of  IEEE Std 1003.1-2001  defines  functions  that  allow  an  application to determine the
       implementation-supported resolution for the  clocks  and  requires  an  implementation  to  document  the
       resolution supported for timers and nanosleep() if they differ from the supported clock resolution.  This
       is more of a procurement issue than a runtime application issue.


FUTURE DIRECTIONS
       None.


SEE ALSO
       clock_getres() , timer_create() , the Base Definitions volume of IEEE Std 1003.1-2001, <time.h>


       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,
       Inc  and  The  Open Group. In the event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .