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

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 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.



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 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 in 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 and the return value of sleep() is 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 type, consistently use either or 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


The Base Definitions volume of POSIX.1‐2008,

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 .

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