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NAME

       clock_nanosleep - high-resolution sleep with specifiable clock

LIBRARY

       Standard C library (libc, -lc), since glibc 2.17

       Before glibc 2.17, Real-time library (librt, -lrt)

SYNOPSIS

       #include <time.h>

       int clock_nanosleep(clockid_t clockid, int flags,
                           const struct timespec *t,
                           struct timespec *_Nullable remain);

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

       clock_nanosleep():
           _POSIX_C_SOURCE >= 200112L

DESCRIPTION

       Like  nanosleep(2),  clock_nanosleep()  allows the calling thread to sleep for an interval
       specified with nanosecond precision.  It differs in allowing  the  caller  to  select  the
       clock  against  which  the  sleep  interval  is  to be measured, and in allowing the sleep
       interval to be specified as either an absolute or a relative value.

       The time values passed to and returned  by  this  call  are  specified  using  timespec(3)
       structures.

       The  clockid  argument  specifies  the  clock  against  which  the sleep interval is to be
       measured.  This argument can have one of the following values:

       CLOCK_REALTIME
              A settable system-wide real-time clock.

       CLOCK_TAI (since Linux 3.10)
              A system-wide clock derived from wall-clock time but counting leap seconds.

       CLOCK_MONOTONIC
              A nonsettable,  monotonically  increasing  clock  that  measures  time  since  some
              unspecified point in the past that does not change after system startup.

       CLOCK_BOOTTIME (since Linux 2.6.39)
              Identical to CLOCK_MONOTONIC, except that it also includes any time that the system
              is suspended.

       CLOCK_PROCESS_CPUTIME_ID
              A settable per-process clock that measures CPU time consumed by all threads in  the
              process.

       See  clock_getres(2)  for further details on these clocks.  In addition, the CPU clock IDs
       returned by clock_getcpuclockid(3) and pthread_getcpuclockid(3)  can  also  be  passed  in
       clockid.

       If flags is 0, then the value specified in t is interpreted as an interval relative to the
       current value of the clock specified by clockid.

       If flags is TIMER_ABSTIME, then t is interpreted as an absolute time as  measured  by  the
       clock,  clockid.   If  t  is  less  than  or equal to the current value of the clock, then
       clock_nanosleep() returns immediately without suspending the calling thread.

       clock_nanosleep() suspends the execution of the calling thread until either at  least  the
       time  specified by t has elapsed, or a signal is delivered that causes a signal handler to
       be called or that terminates the process.

       If the call is interrupted by a signal handler, clock_nanosleep()  fails  with  the  error
       EINTR.   In  addition,  if remain is not NULL, and flags was not TIMER_ABSTIME, it returns
       the  remaining  unslept  time  in  remain.   This  value  can  then  be   used   to   call
       clock_nanosleep() again and complete a (relative) sleep.

RETURN VALUE

       On  successfully sleeping for the requested interval, clock_nanosleep() returns 0.  If the
       call is interrupted by a signal handler or encounters an error, then it returns one of the
       positive error number listed in ERRORS.

ERRORS

       EFAULT t or remain specified an invalid address.

       EINTR  The sleep was interrupted by a signal handler; see signal(7).

       EINVAL The  value  in  the tv_nsec field was not in the range [0, 999999999] or tv_sec was
              negative.

       EINVAL clockid was  invalid.   (CLOCK_THREAD_CPUTIME_ID  is  not  a  permitted  value  for
              clockid.)

       ENOTSUP
              The kernel does not support sleeping against this clockid.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.  Linux 2.6, glibc 2.1.

NOTES

       If  the  interval  specified  in  t 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.

       Using an absolute timer is  useful  for  preventing  timer  drift  problems  of  the  type
       described in nanosleep(2).  (Such problems are exacerbated in programs that try to restart
       a relative sleep that is repeatedly interrupted by signals.)  To perform a relative  sleep
       that  avoids  these problems, call clock_gettime(2) for the desired clock, add the desired
       interval  to  the  returned  time  value,  and  then  call  clock_nanosleep()   with   the
       TIMER_ABSTIME flag.

       clock_nanosleep()  is  never  restarted  after  being  interrupted  by  a  signal handler,
       regardless of the use of the sigaction(2) SA_RESTART flag.

       The remain argument is unused, and unnecessary, when flags is TIMER_ABSTIME.  (An absolute
       sleep can be restarted using the same t argument.)

       POSIX.1  specifies  that  clock_nanosleep()  has  no effect on signals dispositions or the
       signal mask.

       POSIX.1  specifies  that  after  changing  the  value  of  the  CLOCK_REALTIME  clock  via
       clock_settime(2),  the  new  clock  value  shall  be used to determine the time at which a
       thread blocked on an absolute clock_nanosleep() will wake up; if the new clock value falls
       past  the  end  of  the  sleep  interval,  then  the  clock_nanosleep()  call  will return
       immediately.

       POSIX.1 specifies that changing the value of the CLOCK_REALTIME clock via clock_settime(2)
       shall have no effect on a thread that is blocked on a relative clock_nanosleep().

SEE ALSO

       clock_getres(2), nanosleep(2), restart_syscall(2), timer_create(2), sleep(3), timespec(3),
       usleep(3), time(7)