The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


sleep - suspend execution for an interval of time


#include <unistd.h>

unsigned sleep(unsigned


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.

[XSI] [Option Start] Interactions between sleep() and any of setitimer(), ualarm(), or usleep() are unspecified. [Option End]


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.






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 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.




alarm(), getitimer(), nanosleep(), pause(), sigaction() , sigsetjmp(), ualarm(), usleep(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Threads Extension.

Issue 6

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/132 is applied, making a correction in the RATIONALE section.

End of informative text.

UNIX ® is a registered Trademark of The Open Group.
POSIX ® is a registered Trademark of The IEEE.
[ Main Index | XBD | XCU | XSH | XRAT ]