man-pages/man2/nanosleep.2

.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" Copyright (C) Markus Kuhn, 1996
.\"
.\" This is free documentation; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License as
.\" published by the Free Software Foundation; either version 2 of
.\" the License, or (at your option) any later version.
.\"
.\" The GNU General Public License's references to "object code"
.\" and "executables" are to be interpreted as the output of any
.\" document formatting or typesetting system, including
.\" intermediate and printed output.
.\"
.\" This manual is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
.\" GNU General Public License for more details.
.\"
.\" You should have received a copy of the GNU General Public
.\" License along with this manual; if not, write to the Free
.\" Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111,
.\" USA.
.\"
.\" 1996-04-10  Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"             First version written
.\" Modified, 2004-10-24, aeb
.TH NANOSLEEP 2 2004-10-24 "Linux 2.6.9" "Linux Programmer's Manual"
.SH NAME
nanosleep \- pause execution for a specified time
.SH SYNOPSIS
.B #define _POSIX_C_SOURCE 199309
.B #include <time.h>
.sp
\fBint nanosleep(const struct timespec *\fIreq\fB, struct timespec *\fIrem\fB);
.fi
.SH DESCRIPTION
.BR nanosleep ()
delays the execution of the program for at least the time specified in
.IR *req .
The function can return earlier if a signal has been delivered to the
process. In this case, it returns \-1, sets \fIerrno\fR to
.BR EINTR ,
and writes the
remaining time into the structure pointed to by
.IR rem
unless 
.I rem
is NULL.
The value of
.I *rem
can then be used to call 
.BR nanosleep ()
again and complete the specified pause.

The structure
.I timespec
is used to specify intervals of time with nanosecond precision. It is
specified in
.I <time.h>
and has the form
.sp
.RS
.nf
struct timespec {
    time_t tv_sec;        /* seconds */
    long   tv_nsec;       /* nanoseconds */
};
.fi
.RE
.PP
The value of the nanoseconds field must be in the range 0 to 999999999.

Compared to
.BR sleep  (3)
and
.BR usleep (3),
.BR nanosleep ()
has the advantage of not affecting any signals, it is standardized by
POSIX, it provides higher timing resolution, and it allows to continue
a sleep that has been interrupted by a signal more easily.
.SH "RETURN VALUE"
On successfully sleeping for the requested interval,
.BR nanosleep ()
returns 0.
If the call is interrupted by a signal handler or encounters an error,
then it returns \-1, with 
.I errno
set to indicate the error.
.SH ERRORS
.B EFAULT
Problem with copying information from user space.
.TP
.B EINTR
The pause has been interrupted by a non-blocked signal that was
delivered to the process. The remaining sleep time has been written
into *\fIrem\fR so that the process can easily call
.BR nanosleep ()
again and continue with the pause.
.TP
.B EINVAL
The value in the
.I tv_nsec
field was not in the range 0 to 999999999 or
.I tv_sec
was negative.
.SH BUGS
The current implementation of
.BR nanosleep ()
is based on the normal kernel timer mechanism, which has a resolution
of 1/\fIHZ\fR\ s (see
.BR time (7)).
Therefore,
.BR nanosleep ()
pauses always for at least the specified time, however it can take up
to 10 ms longer than specified until the process becomes runnable
again. For the same reason, the value returned in case of a delivered
signal in *\fIrem\fR is usually rounded to the next larger multiple of
1/\fIHZ\fR\ s.
.SS "Old behaviour"
In order to support applications requiring much more precise pauses
(e.g., in order to control some time-critical hardware),
.BR nanosleep ()
would handle pauses of up to 2\ ms by busy waiting with microsecond
precision when called from a process scheduled under a real-time policy
like
.I SCHED_FIFO
or
.IR 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.
.PP
In Linux 2.4, if
.BR nanosleep ()
is stopped by a signal (e.g., SIGTSTP), 
then the call fails with the error
.BR EINTR 
after the process is resumed by a SIGCONT signal.
If the system call is subsequently restarted, 
then the time that the process spent in the stopped state is 
\fInot\fP counted against the sleep interval.
.SH "CONFORMING TO"
POSIX.1b (formerly POSIX.4).
.SH "SEE ALSO"
.BR sched_setscheduler (2),
.BR timer_create (2),
.BR sleep (3),
.BR usleep (3)