SLEEP(3P) POSIX Programmer's Manual SLEEP(3P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEsleep — suspend execution for an interval of time
SYNOPSIS
#include <unistd.h>
unsigned sleep(unsigned seconds);
DESCRIPTION
The sleep() function shall cause the calling thread to be suspended
from execution until either the number of realtime seconds specified by
the argument seconds has elapsed or a signal is delivered to the call‐
ing 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.
If a SIGALRM signal is generated for the calling process during execu‐
tion of sleep() and if the SIGALRM signal is being ignored or blocked
from delivery, it is unspecified whether sleep() returns when the
SIGALRM signal is scheduled. If the signal is being blocked, it is also
unspecified whether it remains pending after sleep() returns or it is
discarded.
If a SIGALRM signal is generated for the calling process during execu‐
tion of sleep(), except as a result of a prior call to alarm(), 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
sleep() to return.
If a signal-catching function interrupts sleep() 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 sig‐
nal is blocked from delivery, the results are unspecified.
If a signal-catching function interrupts sleep() and calls siglongjmp()
or longjmp() to restore an environment saved prior to the sleep() 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 sig‐
nal mask of the process is restored as part of the environment.
Interactions between sleep() and setitimer() are unspecified.
RETURN VALUE
If sleep() returns because the requested time has elapsed, the value
returned shall be 0. If sleep() 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.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
There are two general approaches to the implementation of the sleep()
function. One is to use the alarm() function to schedule a SIGALRM sig‐
nal and then suspend the calling thread waiting for that signal. The
other is to implement an independent facility. This volume of
POSIX.1‐2008 permits either approach.
In order to comply with the requirement that no primitive shall change
a process attribute unless explicitly described by this volume of
POSIX.1‐2008, an implementation using SIGALRM must carefully take into
account any SIGALRM signal scheduled by previous alarm() calls, the
action previously established for SIGALRM, and whether SIGALRM was
blocked. If a SIGALRM has been scheduled before the sleep() would ordi‐
narily complete, the sleep() must be shortened to that time and a
SIGALRM generated (possibly simulated by direct invocation of the sig‐
nal-catching function) before sleep() returns. If a SIGALRM has been
scheduled after the sleep() would ordinarily complete, it must be
rescheduled for the same time before sleep() returns. The action and
blocking for SIGALRM must be saved and restored.
Historical implementations often implement the SIGALRM-based version
using alarm() and pause(). One such implementation is prone to infi‐
nite hangups, as described in pause(). Another such implementation
uses the C-language setjmp() and longjmp() functions to avoid that win‐
dow. 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 longjmp() aborts that signal-
catching function. An implementation based on sigprocmask(), alarm(),
and sigsuspend() can avoid these problems.
Despite all reasonable care, there are several very subtle, but
detectable and unavoidable, differences between the two types of imple‐
mentations. These are the cases mentioned in this volume of
POSIX.1‐2008 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.
See also the discussion of the term realtime in alarm().
Since sleep() can be implemented using alarm(), the discussion about
alarms occurring early under alarm() applies to sleep() as well.
Application developers should note that the type of the argument sec‐
onds and the return value of sleep() is unsigned. 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 int type,
consistently use either unsigned or int.
Scheduling delays may cause the process to return from the sleep()
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 sleep()
returns an unsigned.
FUTURE DIRECTIONS
None.
SEE ALSOalarm(), getitimer(), nanosleep(), pause(), sigaction(), sigsetjmp()
The Base Definitions volume of POSIX.1‐2008, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SLEEP(3P)
