man-pages/man3p/sleep.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SLEEP" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" sleep 
.SH NAME
sleep \- suspend execution for an interval of time
.SH SYNOPSIS
.LP
\fB#include <unistd.h>
.br
.sp
unsigned sleep(unsigned\fP \fIseconds\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIsleep\fP() function shall cause the calling thread to be suspended
from execution until either the number of realtime
seconds specified by the argument \fIseconds\fP has elapsed or a signal
is delivered to the calling thread and its action is to
invoke a signal-catching function or to terminate the process. The
suspension time may be longer than requested due to the
scheduling of other activity by the system.
.LP
If a SIGALRM signal is generated for the calling process during execution
of \fIsleep\fP() and if the SIGALRM signal is being
ignored or blocked from delivery, it is unspecified whether \fIsleep\fP()
returns when the SIGALRM signal is scheduled. If the
signal is being blocked, it is also unspecified whether it remains
pending after \fIsleep\fP() returns or it is discarded.
.LP
If a SIGALRM signal is generated for the calling process during execution
of \fIsleep\fP(), except as a result of a prior call
to \fIalarm\fP(), and if the SIGALRM signal is not being ignored or
blocked from delivery,
it is unspecified whether that signal has any effect other than causing
\fIsleep\fP() to return.
.LP
If a signal-catching function interrupts \fIsleep\fP() and examines
or changes either the time a SIGALRM is scheduled to be
generated, the action associated with the SIGALRM signal, or whether
the SIGALRM signal is blocked from delivery, the results are
unspecified.
.LP
If a signal-catching function interrupts \fIsleep\fP() and calls \fIsiglongjmp\fP()
or \fIlongjmp\fP() to restore an environment saved prior to the \fIsleep\fP()
call, the
action associated with the SIGALRM signal and the time at which a
SIGALRM signal is scheduled to be generated are unspecified. It
is also unspecified whether the SIGALRM signal is blocked, unless
the process' signal mask is restored as part of the
environment.
.LP
Interactions between \fIsleep\fP() and any of \fIsetitimer\fP(), \fIualarm\fP(),
or \fIusleep\fP() are unspecified. 
.SH RETURN VALUE
.LP
If \fIsleep\fP() returns because the requested time has elapsed, the
value returned shall be 0. If \fIsleep\fP() returns due
to delivery of a signal, the return value shall be the "unslept" amount
(the requested time minus the time actually slept) in
seconds.
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
There are two general approaches to the implementation of the \fIsleep\fP()
function. One is to use the \fIalarm\fP() function to schedule a SIGALRM
signal and then suspend the process waiting for that
signal. The other is to implement an independent facility. This volume
of IEEE\ Std\ 1003.1-2001 permits either
approach.
.LP
In order to comply with the requirement that no primitive shall change
a process attribute unless explicitly described by this
volume of IEEE\ Std\ 1003.1-2001, an implementation using SIGALRM
must carefully take into account any SIGALRM signal
scheduled by previous \fIalarm\fP() calls, the action previously established
for SIGALRM,
and whether SIGALRM was blocked. If a SIGALRM has been scheduled before
the \fIsleep\fP() would ordinarily complete, the
\fIsleep\fP() must be shortened to that time and a SIGALRM generated
(possibly simulated by direct invocation of the
signal-catching function) before \fIsleep\fP() returns. If a SIGALRM
has been scheduled after the \fIsleep\fP() would ordinarily
complete, it must be rescheduled for the same time before \fIsleep\fP()
returns. The action and blocking for SIGALRM must be saved
and restored.
.LP
Historical implementations often implement the SIGALRM-based version
using \fIalarm\fP()
and \fIpause\fP(). One such implementation is prone to infinite hangups,
as described in \fIpause\fP() . Another such implementation uses the
C-language \fIsetjmp\fP() and \fIlongjmp\fP() functions to avoid that
window. That implementation introduces a different problem: when the
SIGALRM signal interrupts a signal-catching function installed
by the user to catch a different signal, the \fIlongjmp\fP() aborts
that signal-catching
function. An implementation based on \fIsigprocmask\fP(), \fIalarm\fP(),
and \fIsigsuspend\fP() can avoid these
problems.
.LP
Despite all reasonable care, there are several very subtle, but detectable
and unavoidable, differences between the two types of
implementations. These are the cases mentioned in this volume of IEEE\ Std\ 1003.1-2001
where some other activity relating
to SIGALRM takes place, and the results are stated to be unspecified.
All of these cases are sufficiently unusual as not to be of
concern to most applications.
.LP
See also the discussion of the term \fIrealtime\fP in \fIalarm\fP()
\&.
.LP
Since \fIsleep\fP() can be implemented using \fIalarm\fP(), the discussion
about alarms
occurring early under \fIalarm\fP() applies to \fIsleep\fP() as well.
.LP
Application writers should note that the type of the argument \fIseconds\fP
and the return value of \fIsleep\fP() is
\fBunsigned\fP. That means that a Strictly Conforming POSIX System
Interfaces Application cannot pass a value greater than the
minimum guaranteed value for {UINT_MAX}, which the ISO\ C standard
sets as 65535, and any application passing a larger value is
restricting its portability. A different type was considered, but
historical implementations, including those with a 16-bit
\fBint\fP type, consistently use either \fBunsigned\fP or \fBint\fP.
.LP
Scheduling delays may cause the process to return from the \fIsleep\fP()
function significantly after the requested time. In
such cases, the return value should be set to zero, since the formula
(requested time minus the time actually spent) yields a
negative number and \fIsleep\fP() returns an \fBunsigned\fP.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIalarm\fP() , \fIgetitimer\fP() , \fInanosleep\fP() , \fIpause\fP()
, \fIsigaction\fP()
, \fIsigsetjmp\fP() , \fIualarm\fP() , \fIusleep\fP() , the Base Definitions
volume of IEEE\ Std\ 1003.1-2001, \fI<unistd.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .