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 ad-
vantages: it provides a higher resolution for specifying the sleep in-
terval; POSIX.1 explicitly specifies that it does not interact with
signals; and it makes the task of resuming a sleep that has been inter-
rupted by a signal handler easier.
RETURN VALUE
On successfully sleeping for the requested interval, nanosleep() re-
turns 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 granu-
larity underlying clock (see time(7)), then the interval will be
rounded up to the next multiple. Furthermore, after the sleep com-
pletes, 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 prob-
lematic 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 abso-
lute 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_set-
time(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, inde-
pendently 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 mi-
crosecond 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 5.07 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)