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  struc-
       ture  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
       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.  Furthermore, 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 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 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() says that discontinuous changes
       in CLOCK_REALTIME should not affect nanosleep():

	      Setting the value of the CLOCK_REALTIME clock via clock_settime() 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, 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), sched_setscheduler(2), sleep(3), timer_create(2), usleep(3), time(7)

COLOPHON

       This  page is part of release 3.25 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								    2009-01-19							      NANOSLEEP(2)