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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.
NAME
sleep — 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
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.
In single-threaded programs, sleep() may make use of SIGALRM. In multi-threaded programs, sleep() shall
not make use of SIGALRM and the remainder of this DESCRIPTION does not apply.
If a SIGALRM signal is generated for the calling process during execution 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 execution 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 signal
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 signal 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 signal and then suspend the calling thread waiting for that signal. The
other is to implement an independent facility. This volume of POSIX.1‐2017 permits either approach in
single-threaded programs, but the simple alarm/suspend implementation is not appropriate for multi-
threaded programs.
In order to comply with the requirement that no primitive shall change a process attribute unless
explicitly described by this volume of POSIX.1‐2017, 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
ordinarily complete, the sleep() must be shortened to that time and a SIGALRM generated (possibly
simulated by direct invocation of the signal-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 infinite hangups, as described in pause(). Another such implementation uses
the C-language setjmp() and longjmp() 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 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 implementations. These are the cases mentioned in this volume of POSIX.1‐2017
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 seconds 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
A future version of this standard may require that sleep() does not make use of SIGALRM in all programs,
not just multi-threaded programs.
SEE ALSO
alarm(), getitimer(), nanosleep(), pause(), sigaction(), sigsetjmp()
The Base Definitions volume of POSIX.1‐2017, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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