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NANOSLEEP(2)               Linux Programmer's Manual              NANOSLEEP(2)



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 fol-
       lows:

           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 non-blocked 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 specifica-
       tion for clock_settime() says that discontinuous changes in CLOCK_REALTIME should not affect nanosleep():

              Setting the value of the CLOCK_REALTIME clock via clock_settime() shall have no effect on  threads  that
              are  blocked  waiting for a relative time service based upon this clock, including the nanosleep() func-
              tion; ...  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-criti-
       cal 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), sleep(3), timer_create(2), usleep(3), time(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-01-19                      NANOSLEEP(2)