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       nanosleep - high-resolution sleep


       #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


       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.


       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.


       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 to 999999999  or  tv_sec  was


       POSIX.1-2001, POSIX.1-2008.


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


       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


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


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