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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(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  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),  timer_create(2), sleep(3),
       usleep(3), time(7)

COLOPHON

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