nanosleep (2)
Leading comments
Copyright (C) Markus Kuhn, 1996 and Copyright (C) Linux Foundation, 2008, written by Michael Kerrisk <mtk.manpages@gmail.com> %%%LICENSE_START(GPLv2+_DOC_FULL) This is free documentation; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU General Public License's references to "object code" and "executables" are to be inter...
NAME
nanosleep - high-resolution sleepSYNOPSIS
#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. 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.