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       nanosleep - high-resolution sleep


       Standard C library (libc, -lc)


       #include <time.h>

       int nanosleep(const struct timespec *req,
                     struct timespec *_Nullable rem);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           _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 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.


       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, 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 Linux 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),
       timespec(3), usleep(3), time(7)