sleep(3p) — Linux manual page


SLEEP(3P)               POSIX Programmer's Manual              SLEEP(3P)

PROLOG         top

       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.

NAME         top

       sleep — suspend execution for an interval of time

SYNOPSIS         top

       #include <unistd.h>

       unsigned sleep(unsigned seconds);

DESCRIPTION         top

       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.

       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 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

       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.

RETURN VALUE         top

       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         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top




RATIONALE         top

       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.1‐2017 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
       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.1‐2017 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

       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

       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.


       A future version of this standard may require that sleep() does
       not make use of SIGALRM in all programs, not just multi-threaded

SEE ALSO         top

       alarm(3p), getitimer(3p), nanosleep(3p), pause(3p),
       sigaction(3p), sigsetjmp(3p)

       The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)

COPYRIGHT         top

       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 at .

       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 .

IEEE/The Open Group               2017                         SLEEP(3P)

Pages that refer to this page: unistd.h(0p)sleep(1p)alarm(3p)clock_nanosleep(3p)getitimer(3p)nanosleep(3p)