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Linux 2.6 - man page for nanosleep (linux section 2)

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

NAME
       nanosleep - high-resolution sleep

SYNOPSIS
       #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

DESCRIPTION
       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 spec-
       ified 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
       interrupted by a signal handler easier.

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

ERRORS
       EFAULT Problem with copying information from user space.

       EINTR  The pause has been interrupted by a 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.

CONFORMING TO
       POSIX.1-2001.

NOTES
       If  the	interval  specified in req is not an exact multiple of the granularity underlying
       clock (see time(7)), then the interval will be rounded up to the next multiple.	 Further-
       more, after the sleep completes, 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 problematic if the call is
       repeatedly restarted after being interrupted by signals, since the time between the inter-
       ruptions 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 absolute 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_settime(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 ser-
	      vices shall expire when the requested relative interval elapses,	independently  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 ms  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, hence is still present in current 2.4 kernels, but not in 2.6 kernels.

BUGS
       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.

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

COLOPHON
       This  page  is  part of release 3.55 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					    2013-07-30				     NANOSLEEP(2)
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