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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SLEEP(3P) POSIX Programmer's Manual SLEEP(3P)
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.
sleep — suspend execution for an interval of time
#include <unistd.h>
unsigned sleep(unsigned seconds);
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.
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.
No errors are defined.
The following sections are informative.
None.
None.
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(3p). 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(3p).
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.
A future version of this standard may require that sleep() does
not make use of SIGALRM in all programs, not just multi-threaded
programs.
alarm(3p), getitimer(3p), nanosleep(3p), pause(3p), sigaction(3p),
sigsetjmp(3p)
The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)
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 .
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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 SLEEP(3P)
Pages that refer to this page: unistd.h(0p), sleep(1p), alarm(3p), clock_nanosleep(3p), getitimer(3p), nanosleep(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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