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 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  speci-
       fying  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  han-
       dler 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.  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 specification 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() 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  hard-
       ware), 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 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-01-19                                               NANOSLEEP(2)