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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
       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

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       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.