PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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.

       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.

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

       None.

APPLICATION USAGE         top

       None.

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.1‐2008 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 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.1‐2008 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 realtime in alarm(3p).

       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
       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         top

       None.

SEE ALSO         top

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

       The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)

COPYRIGHT         top

       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 http://www.unix.org/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, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                           SLEEP(3P)