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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>

COPYRIGHT

       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 .