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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.

DESCRIPTION top

Thesleep() function shall cause the calling thread to be suspended from execution until either the number of realtime seconds specified by the argumentseconds 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 ofsleep() and if the SIGALRM signal is being ignored or blocked from delivery, it is unspecified whethersleep() returns when the SIGALRM signal is scheduled. If the signal is being blocked, it is also unspecified whether it remains pending aftersleep() returns or it is discarded. If a SIGALRM signal is generated for the calling process during execution ofsleep(), except as a result of a prior call toalarm(), and if the SIGALRM signal is not being ignored or blocked from delivery, it is unspecified whether that signal has any effect other than causingsleep() to return. If a signal-catching function interruptssleep() 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 interruptssleep() and callssiglongjmp() orlongjmp() to restore an environment saved prior to thesleep() 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 betweensleep() andsetitimer() are unspecified.

Ifsleep() returns because the requested time has elapsed, the value returned shall be 0. Ifsleep() 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.

RATIONALE top

There are two general approaches to the implementation of thesleep() function. One is to use thealarm() 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 previousalarm() calls, the action previously established for SIGALRM, and whether SIGALRM was blocked. If a SIGALRM has been scheduled before thesleep() would ordinarily complete, thesleep() must be shortened to that time and a SIGALRM generated (possibly simulated by direct invocation of the signal-catching function) beforesleep() returns. If a SIGALRM has been scheduled after thesleep() would ordinarily complete, it must be rescheduled for the same time beforesleep() returns. The action and blocking for SIGALRM must be saved and restored. Historical implementations often implement the SIGALRM-based version usingalarm() andpause(). One such implementation is prone to infinite hangups, as described inpause(3p). Another such implementation uses the C-languagesetjmp() andlongjmp() 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, thelongjmp() aborts that signal-catching function. An implementation based onsigprocmask(),alarm(), andsigsuspend() 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 termrealtime inalarm(3p). Sincesleep() can be implemented usingalarm(), the discussion about alarms occurring early underalarm() applies tosleep() as well. Application developers should note that the type of the argumentseconds and the return value ofsleep() isunsigned. 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-bitinttype, consistently use eitherunsignedorint. Scheduling delays may cause the process to return from thesleep() 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 andsleep() returns anunsigned.

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 athttp://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, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017SLEEP(3P)

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