nanosleep(2) — Linux manual page


nanosleep(2)               System Calls Manual              nanosleep(2)

NAME         top

       nanosleep - high-resolution sleep

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <time.h>

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

   Feature Test Macro Requirements for glibc (see

           _POSIX_C_SOURCE >= 199309L

DESCRIPTION         top

       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,

       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         top

       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         top

       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

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

VERSIONS         top

       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

              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.

STANDARDS         top


HISTORY         top


       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.

NOTES         top

       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.

BUGS         top

       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 kernels.

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                     nanosleep(2)

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