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NANOSLEEP(2)		   Linux Programmers Manual		 NANOSLEEP(2)

       nanosleep - high-resolution sleep


       int nanosleep(const struct timespec *req, struct timespec *rem);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       nanosleep(): _POSIX_C_SOURCE >= 199309L

       nanosleep()  suspends  the execution of the calling thread until either
       at least the time specified in *req has elapsed, or the delivery  of  a
       signal  that triggers the invocation of a handler in the calling thread
       or that terminates the process.

       If the call is interrupted by a signal handler, nanosleep() returns -1,
       sets  errno  to EINTR, and writes the remaining time into the structure
       pointed to by rem unless rem is NULL.  The value of *rem  can  then  be
       used  to  call  nanosleep() again and complete the specified pause (but
       see NOTES).

       The structure timespec is  used	to  specify  intervals	of  time  with
       nanosecond precision.  It is defined as follows:

	   struct timespec {
	       time_t tv_sec;	     /* seconds */
	       long   tv_nsec;	     /* nanoseconds */

       The value of the nanoseconds field must be in the range 0 to 999999999.

       Compared to sleep(3)  and  usleep(3),  nanosleep()  has	the  following
       advantages:  it	provides  a higher resolution for specifying the sleep
       interval; POSIX.1 explicitly specifies that it does not	interact  with
       signals; and it makes the task of resuming a sleep that has been inter
       rupted by a signal handler easier.

       On  successfully  sleeping  for	the  requested	interval,  nanosleep()
       returns	0.   If the call is interrupted by a signal handler or encoun
       ters an error, then it returns -1,  with  errno	set  to  indicate  the

       EFAULT Problem with copying information from user space.

       EINTR  The  pause has been interrupted by a non-blocked signal that was
	      delivered to the thread.	The  remaining	sleep  time  has  been
	      written into *rem so that the thread can easily call nanosleep()
	      again and continue with the pause.

       EINVAL The value in the tv_nsec	field  was  not  in  the  range  0  to
	      999999999 or tv_sec was negative.


   Old behavior
       In  order  to  support  applications requiring much more precise pauses
       (e.g., in order to control some	time-critical  hardware),  nanosleep()
       would handle pauses of up to 2 ms by busy waiting with microsecond pre
       cision when called from a thread scheduled  under  a  real-time	policy
       like  SCHED_FIFO  or  SCHED_RR.	 This special extension was removed in
       kernel 2.5.39, hence is still present in current 2.4 kernels,  but  not
       in 2.6 kernels.

       In  Linux  2.4,	if nanosleep() is stopped by a signal (e.g., SIGTSTP),
       then the call fails with the error EINTR after the thread is resumed by
       a  SIGCONT  signal.  If the system call is subsequently restarted, then
       the time that the thread spent in the  stopped  state  is  not  counted
       against the sleep interval.

       If the interval specified in req is not an exact multiple of the granu
       larity underlying clock	(see  time(7)),  then  the  interval  will  be
       rounded	up  to	the  next multiple.  Furthermore, after the sleep com
       pletes, there may still be a delay before the CPU becomes free to  once
       again execute the calling thread.

       The  fact  that nanosleep() sleeps for a relative interval can be prob
       lematic if the call is repeatedly restarted after being interrupted  by
       signals,  since	the time between the interruptions and restarts of the
       call will lead to drift in the time when the sleep  finally  completes.
       This  problem  can be avoided by using clock_nanosleep(2) with an abso
       lute time value.

       POSIX.1 specifies that nanosleep()  should  measure  time  against  the
       CLOCK_REALTIME  clock.	However,  Linux  measures  the	time using the
       CLOCK_MONOTONIC clock.	This  probably	does  not  matter,  since  the
       POSIX.1	specification  for  clock_settime()  says  that  discontinuous
       changes in CLOCK_REALTIME should not affect nanosleep():

	      Setting the value of the	CLOCK_REALTIME	clock  via  clock_set
	      time()  shall have no effect on threads that are blocked waiting
	      for a relative time service based upon this clock, including the
	      nanosleep()  function;  ...   Consequently,  these time services
	      shall expire when the requested relative interval elapses, inde
	      pendently of the new or old value of the clock.

       clock_nanosleep(2),  sched_setscheduler(2),  sleep(3), timer_create(3),
       usleep(3), time(7)

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-06-24			  NANOSLEEP(2)

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