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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.
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.1‐2008 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
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.1‐2008 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(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.
None.
alarm(3p), getitimer(3p), nanosleep(3p), pause(3p), sigaction(3p),
sigsetjmp(3p)
The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)
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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 SLEEP(3P)
Pages that refer to this page: unistd.h(0p), sleep(1p), alarm(3p), clock_nanosleep(3p), getitimer(3p), nanosleep(3p)