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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 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.

RETURN VALUE
       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
       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  con-
              tinue 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 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.

   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.

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 3.27 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)

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