NANOSLEEP
Section: Linux Programmer's Manual (2)
Updated: 2009-01-19
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
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.
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()
says that discontinuous changes in
CLOCK_REALTIME
should not affect
nanosleep():
-
Setting the value of the
CLOCK_REALTIME
clock via
clock_settime()
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.
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
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,
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),
sched_setscheduler(2),
sleep(3),
timer_create(2),
usleep(3),
time(7)
COLOPHON
This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- Old behavior
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
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Time: 07:34:54 GMT, March 26, 2013