nanosleep

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

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(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 clock.

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

COLOPHON
       This page is part of release 5.10 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2017-09-15                       NANOSLEEP(2)