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.
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 process’
signal mask is restored as part of the environment.
Interactions between sleep() and any of setitimer(), ualarm(), or
usleep() 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 process waiting for that signal. The other
is to implement an independent facility. This volume of
IEEE Std 1003.1-2001 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
IEEE Std 1003.1-2001, 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
IEEE Std 1003.1-2001 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 writers 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
None.
SEE ALSO
alarm() , getitimer() , nanosleep() , pause() , sigaction() ,
sigsetjmp() , ualarm() , usleep() , the Base Definitions volume of
IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .