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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 inter-
rupted 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 encoun-
ters 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 con-
tinue 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.
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 ms by busy waiting with microsecond pre-
cision 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, hence is still present in current 2.4 kernels, but not
in 2.6 kernels.
BUGS
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.
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 3.69 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
http://www.kernel.org/doc/man-pages/.
Linux 2013-07-30 NANOSLEEP(2)
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