man-pages/man2/nanosleep.2

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.\" Copyright (C) Markus Kuhn, 1996
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.\" 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
.ne 12
.ta 8n 16n 32n
struct timespec
{
	time_t	tv_sec;			/* seconds */
	long	tv_nsec;		/* nanoseconds */
};
.ta
.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 ERRORS
In case of an error or exception, the
.BR nanosleep ()
system call returns \-1 instead of 0 and sets
.I errno
to one of the following values:
.TP
.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 (i.e, 10\ ms on Linux/i386 and 1\ ms on Linux/Alpha).
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.
.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)