nanosleep(2) — Linux manual page


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

NAME         top

       nanosleep - high-resolution sleep

SYNOPSIS         top

       #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         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 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

       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 pause.

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

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

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.

       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

   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 kernel 2.5.39, and is thus not available in Linux
       2.6.0 and later kernels.

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), usleep(3), time(7)

COLOPHON         top

       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2017-09-15                     NANOSLEEP(2)

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