.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH clock_nanosleep 2 2025-10-29 "Linux man-pages (unreleased)"
.SH NAME
clock_nanosleep \- high-resolution sleep with specifiable clock
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc ),
since glibc 2.17
.P
Before glibc 2.17,
Real-time library
.RI ( librt ,\~ \-lrt )
.SH SYNOPSIS
.B #include <time.h>
.nf
.P
.BI "int clock_nanosleep(clockid_t " clockid ", int " flags ,
.BI "                    const struct timespec *" t ,
.BI "                    struct timespec *_Nullable " remain );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR clock_nanosleep ():
.nf
    _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
Like
.BR nanosleep (2),
.BR clock_nanosleep ()
allows the calling thread to sleep for an interval specified
with nanosecond precision.
It differs in allowing the caller to select the clock against
which the sleep interval is to be measured,
and in allowing the sleep interval to be specified as
either an absolute or a relative value.
.P
The time values passed to and returned by this call are specified using
.BR timespec (3)
structures.
.P
The
.I clockid
argument specifies the clock against which the sleep interval
is to be measured.
This argument can have one of the following values:
.\" Look in time/posix-timers.c (kernel 5.6 sources) for the
.\" 'struct k_clock' structures that have an 'nsleep' method
.TP
.B CLOCK_REALTIME
A settable system-wide real-time clock.
.TP
.BR CLOCK_TAI " (since Linux 3.10)"
A system-wide clock derived from wall-clock time but counting leap seconds.
.TP
.B CLOCK_MONOTONIC
A nonsettable, monotonically increasing clock that measures time
since some unspecified point in the past that does not change after
system startup.
.\" On Linux this clock measures time since boot.
.TP
.BR CLOCK_BOOTTIME " (since Linux 2.6.39)"
Identical to
.BR CLOCK_MONOTONIC ,
except that it also includes any time that the system is suspended.
.TP
.B CLOCK_PROCESS_CPUTIME_ID
A settable per-process clock that measures CPU time consumed
by all threads in the process.
.\" There is some trickery between glibc and the kernel
.\" to deal with the CLOCK_PROCESS_CPUTIME_ID case.
.P
See
.BR clock_getres (2)
for further details on these clocks.
In addition, the CPU clock IDs returned by
.BR clock_getcpuclockid (3)
and
.BR pthread_getcpuclockid (3)
can also be passed in
.IR clockid .
.\" Sleeping against CLOCK_REALTIME_ALARM and CLOCK_BOOTTIME_ALARM
.\" is also possible (tested), with CAP_WAKE_ALARM, but I'm not
.\" sure if this is useful or needs to be documented.
.P
If
.I flags
is 0, then the value specified in
.I t
is interpreted as an interval relative to the current
value of the clock specified by
.IR clockid .
.P
If
.I flags
is
.BR TIMER_ABSTIME ,
then
.I t
is interpreted as an absolute time as measured by the clock,
.IR clockid .
If
.I t
is less than or equal to the current value of the clock,
then
.BR clock_nanosleep ()
returns immediately without suspending the calling thread.
.P
.BR clock_nanosleep ()
suspends the execution of the calling thread
until either at least the time specified by
.I t
has elapsed,
or a signal is delivered that causes a signal handler to be called or
that terminates the process.
.P
If the call is interrupted by a signal handler,
.BR clock_nanosleep ()
fails with the error
.BR EINTR .
In addition, if
.I remain
is not NULL, and
.I flags
was not
.BR TIMER_ABSTIME ,
it returns the remaining unslept time in
.IR remain .
This value can then be used to call
.BR clock_nanosleep ()
again and complete a (relative) sleep.
.SH RETURN VALUE
On successfully sleeping for the requested interval,
.BR clock_nanosleep ()
returns 0.
If the call is interrupted by a signal handler or encounters an error,
then it returns one of the positive error number listed in ERRORS.
.SH ERRORS
.TP
.B EFAULT
.I t
or
.I remain
specified an invalid address.
.TP
.B EINTR
The sleep was interrupted by a signal handler;
see
.BR signal (7).
.TP
.B EINVAL
The value in the
.I tv_nsec
field was not in the range [0, 999999999] or
.I tv_sec
was negative.
.TP
.B EINVAL
.I clockid
was invalid.
.RB ( CLOCK_THREAD_CPUTIME_ID
is not a permitted value for
.IR clockid .)
.TP
.B ENOTSUP
The kernel does not support sleeping against this
.IR clockid .
.SH STANDARDS
POSIX.1-2024.
.SH HISTORY
POSIX.1-2001.
Linux 2.6,
glibc 2.1.
.SH NOTES
If the interval specified in
.I t
is not an exact multiple of the granularity underlying clock (see
.BR 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.
.P
Using an absolute timer is useful for preventing
timer drift problems of the type described in
.BR nanosleep (2).
(Such problems are exacerbated in programs that try to restart
a relative sleep that is repeatedly interrupted by signals.)
To perform a relative sleep that avoids these problems, call
.BR clock_gettime (2)
for the desired clock,
add the desired interval to the returned time value,
and then call
.BR clock_nanosleep ()
with the
.B TIMER_ABSTIME
flag.
.P
.BR clock_nanosleep ()
is never restarted after being interrupted by a signal handler,
regardless of the use of the
.BR sigaction (2)
.B SA_RESTART
flag.
.P
The
.I remain
argument is unused, and unnecessary, when
.I flags
is
.BR TIMER_ABSTIME .
(An absolute sleep can be restarted using the same
.I t
argument.)
.P
POSIX.1 specifies that
.BR clock_nanosleep ()
has no effect on signals dispositions or the signal mask.
.P
POSIX.1 specifies that after changing the value of the
.B CLOCK_REALTIME
clock via
.BR clock_settime (2),
the new clock value shall be used to determine the time
at which a thread blocked on an absolute
.BR clock_nanosleep ()
will wake up;
if the new clock value falls past the end of the sleep interval, then the
.BR clock_nanosleep ()
call will return immediately.
.P
POSIX.1 specifies that
changing the value of the
.B CLOCK_REALTIME
clock via
.BR clock_settime (2)
shall have no effect on a thread that is blocked on a relative
.BR clock_nanosleep ().
.SH SEE ALSO
.BR clock_getres (2),
.BR nanosleep (2),
.BR restart_syscall (2),
.BR timer_create (2),
.BR sleep (3),
.BR timespec (3),
.BR usleep (3),
.BR time (7)