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NANOSLEEP(2)		   Linux Programmer's Manual		  NANOSLEEP(2)

       nanosleep - high-resolution sleep

       #include <time.h>

       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 signal that was delivered to
	      the  thread  (see signal(7)).  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.

       POSIX.1-2001, POSIX.1-2008.

       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(2)  says  that	 discontinuous
       changes in CLOCK_REALTIME should not affect nanosleep():

	      Setting  the  value  of  the CLOCK_REALTIME clock via clock_set-
	      time(2) 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.

   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	 milliseconds  by  busy	 waiting  with
       microsecond precision 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, and is thus not available in Linux 2.6.0 and
       later kernels.

       If a program that catches signals and uses nanosleep() receives signals
       at  a very high rate, then scheduling delays and rounding errors in the
       kernel's calculation of the sleep  interval  and	 the  returned	remain
       value  mean  that  the remain value may steadily increase on successive
       restarts	 of  the  nanosleep()  call.   To  avoid  such	problems,  use
       clock_nanosleep(2)  with the TIMER_ABSTIME flag to sleep to an absolute

       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.  This problem is fixed in Linux  2.6.0  and
       later kernels.

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

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Linux				  2017-03-13			  NANOSLEEP(2)