NAME

       nanosleep - high-resolution sleep

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <time.h>

       int nanosleep(const struct timespec *duration,
                     struct timespec *_Nullable 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 *duration 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 timespec(3) structure is used to specify intervals of time with nanosecond precision.

       The value of the nanoseconds field must be in the range [0, 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 duration, 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 (see
              signal(7)).  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, 999999999] or tv_sec was
              negative.

VERSIONS

       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 duration elapses, independently of the new
              or old value of the clock.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

       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
       milliseconds  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 Linux 2.5.39, and is thus not available in Linux 2.6.0 and later kernels.

NOTES

       If  the  duration  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.

BUGS

       If a program that catches signals and uses nanosleep() receives signals  at  a  very  high
       rate,  then scheduling delays and rounding errors in the kernel's calculation of the sleep
       interval and the returned remain value mean that the remain value may steadily increase on
       successive   restarts   of   the   nanosleep()   call.    To   avoid  such  problems,  use
       clock_nanosleep(2) with the TIMER_ABSTIME flag to sleep to an absolute deadline.

       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.   This  problem  is fixed in Linux 2.6.0 and later
       kernels.

SEE ALSO

       clock_nanosleep(2), restart_syscall(2), sched_setscheduler(2), timer_create(2),  sleep(3),
       timespec(3), usleep(3), time(7)