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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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nanosleep(2) System Calls Manual nanosleep(2)
nanosleep - high-resolution sleep
Standard C library (libc, -lc)
#include <time.h>
int nanosleep(const struct timespec *duration,
struct timespec *_Nullable rem);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
nanosleep():
_POSIX_C_SOURCE >= 199309L
nanosleep() suspends the execution of the calling thread until
either at least the time specified in *duration 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 timespec(3) structure is used to specify intervals of time
with nanosecond precision.
The value of the nanoseconds field must be in the range [0,
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.
On successfully sleeping for the requested duration, 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.
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,
999999999] or tv_sec was negative.
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 duration elapses, independently of the new or old
value of the clock.
POSIX.1-2008.
POSIX.1-2001.
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 Linux 2.5.39, and is thus
not available in Linux 2.6.0 and later kernels.
If the duration 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.
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.
clock_nanosleep(2), restart_syscall(2), sched_setscheduler(2),
timer_create(2), sleep(3), timespec(3), usleep(3), time(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 nanosleep(2)
Pages that refer to this page: pmsleep(1), clock_nanosleep(2), PR_SET_TIMERSLACK(2const), restart_syscall(2), syscalls(2), aio_suspend(3), getaddrinfo_a(3), pthread_cond_init(3), sleep(3), timespec(3type), ualarm(3), usleep(3), signal(7), time(7), time_namespaces(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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