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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 com-
       plete the specified pause (but see NOTES).

       The structure timespec is used to specify intervals of time with nanosecond  preci-
       sion.  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 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.

CONFORMING TO
       POSIX.1-2001.

NOTES
       If the interval specified in req is not an exact multiple of the granularity under-
       lying clock (see time(7)), then the interval will be rounded up to the next  multi-
       ple.  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; ...  Conse-
              quently, these time services shall expire when the requested relative inter-
              val 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 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-01-19                      NANOSLEEP(2)

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