NAME

       clock_nanosleep - high resolution sleep with specifiable clock (ADVANCED REALTIME)

SYNOPSIS

       #include <time.h>

       int clock_nanosleep(clockid_t clock_id, int flags,
              const struct timespec *rqtp, struct timespec *rmtp);

DESCRIPTION

       If  the  flag  TIMER_ABSTIME is not set in the flags argument, the clock_nanosleep() function shall cause
       the current thread to be suspended from execution until either the time interval specified  by  the  rqtp
       argument  has  elapsed,  or  a  signal  is  delivered to the calling thread and its action is to invoke a
       signal-catching function, or the process is terminated. The clock used to measure the time shall  be  the
       clock specified by clock_id.

       If  the  flag  TIMER_ABSTIME is set in the flags argument, the clock_nanosleep() function shall cause the
       current thread to be suspended from execution until either the time  value  of  the  clock  specified  by
       clock_id  reaches  the  absolute  time  specified  by  the rqtp argument, or a signal is delivered to the
       calling thread and its action is to invoke a signal-catching function, or the process is terminated.  If,
       at  the time of the call, the time value specified by rqtp is less than or equal to the time value of the
       specified clock, then clock_nanosleep() shall return immediately and the calling  process  shall  not  be
       suspended.

       The  suspension  time  caused by this function may be longer than requested because the argument value is
       rounded up to an integer multiple of the sleep resolution, or because of the scheduling of other activity
       by the system. But, except for the case of being interrupted by a signal, the  suspension  time  for  the
       relative clock_nanosleep() function (that is, with the TIMER_ABSTIME flag not set) shall not be less than
       the  time  interval  specified  by  rqtp,  as measured by the corresponding clock. The suspension for the
       absolute clock_nanosleep() function (that is, with the TIMER_ABSTIME flag set)  shall  be  in  effect  at
       least  until the value of the corresponding clock reaches the absolute time specified by rqtp, except for
       the case of being interrupted by a signal.

       The use of the clock_nanosleep() function shall have no effect on the action or blockage of any signal.

       The clock_nanosleep() function shall fail if the clock_id argument refers to the CPU-time  clock  of  the
       calling thread. It is unspecified whether clock_id values of other CPU-time clocks are allowed.

RETURN VALUE

       If  the clock_nanosleep() function returns because the requested time has elapsed, its return value shall
       be zero.

       If the clock_nanosleep() function returns because it has been interrupted by a signal,  it  shall  return
       the  corresponding error value. For the relative clock_nanosleep() function, if the rmtp argument is non-
       NULL, the timespec structure referenced by it shall be updated to contain the amount of time remaining in
       the interval (the requested time minus the time actually slept).  If  the  rmtp  argument  is  NULL,  the
       remaining  time  is  not returned. The absolute clock_nanosleep() function has no effect on the structure
       referenced by rmtp.

       If clock_nanosleep() fails, it shall return the corresponding error value.

ERRORS

       The clock_nanosleep() function shall fail if:

       EINTR  The clock_nanosleep() function was interrupted by a signal.

       EINVAL The rqtp argument specified a nanosecond value less than zero or greater than  or  equal  to  1000
              million;  or  the  TIMER_ABSTIME  flag was specified in flags and the rqtp argument is outside the
              range for the clock specified by clock_id; or the clock_id  argument  does  not  specify  a  known
              clock, or specifies the CPU-time clock of the calling thread.

       ENOTSUP
              The  clock_id  argument  specifies a clock for which clock_nanosleep() is not supported, such as a
              CPU-time clock.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       Calling clock_nanosleep() with the value TIMER_ABSTIME not set in the flags argument and with a  clock_id
       of CLOCK_REALTIME is equivalent to calling nanosleep() with the same rqtp and rmtp arguments.

RATIONALE

       The  nanosleep()  function  specifies  that  the  system-wide clock CLOCK_REALTIME is used to measure the
       elapsed time for this time service. However, with the introduction of the monotonic clock CLOCK_MONOTONIC
       a new relative sleep function is needed to  allow  an  application  to  take  advantage  of  the  special
       characteristics of this clock.

       There are many applications in which a process needs to be suspended and then activated multiple times in
       a  periodic  way;  for  example,  to poll the status of a non-interrupting device or to refresh a display
       device. For these cases, it is known that precise periodic activation cannot be achieved with a  relative
       sleep()  or nanosleep() function call. Suppose, for example, a periodic process that is activated at time
       T0, executes for a while, and then wants to suspend itself until time T0+ T, the period being T.  If this
       process wants to use the nanosleep() function, it must first call  clock_gettime()  to  get  the  current
       time,  then  calculate  the  difference between the current time and T0+ T and, finally, call nanosleep()
       using the computed interval. However, the process could be preempted by a different process  between  the
       two  function  calls,  and  in  this case the interval computed would be wrong; the process would wake up
       later than desired. This problem would not occur with the absolute clock_nanosleep() function, since only
       one function call would be necessary to suspend the process until the  desired  time.   In  other  cases,
       however, a relative sleep is needed, and that is why both functionalities are required.

       Although  it  is possible to implement periodic processes using the timers interface, this implementation
       would require the use of signals, and the reservation of some signal numbers. In this regard, the reasons
       for including an absolute version of the clock_nanosleep() function in IEEE Std 1003.1-2001 are the  same
       as for the inclusion of the relative nanosleep().

       It  is  also possible to implement precise periodic processes using pthread_cond_timedwait(), in which an
       absolute timeout is specified that takes effect if the condition variable  involved  is  never  signaled.
       However,  the use of this interface is unnatural, and involves performing other operations on mutexes and
       condition variables that imply an unnecessary  overhead.  Furthermore,  pthread_cond_timedwait()  is  not
       available in implementations that do not support threads.

       Although  the interface of the relative and absolute versions of the new high resolution sleep service is
       the same clock_nanosleep() function, the rmtp argument is only used in the relative sleep. This  argument
       is needed in the relative clock_nanosleep() function to reissue the function call if it is interrupted by
       a  signal,  but  it  is  not  needed  in  the  absolute  clock_nanosleep()  function call; if the call is
       interrupted by a signal, the absolute clock_nanosleep() function can be invoked again with the same  rqtp
       argument used in the interrupted call.

FUTURE DIRECTIONS

       None.

SEE ALSO

       clock_getres()  ,  nanosleep()  ,  pthread_cond_timedwait()  ,  sleep()  , 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 .

IEEE/The Open Group                                   2003                                    CLOCK_NANOSLEEP(P)