sleep

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



PROLOG
       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
       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
       signal mask of the process is restored as part of the environment.

       Interactions between sleep() and setitimer() 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 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().  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().

       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
       None.

SEE ALSO
       alarm(), getitimer(), nanosleep(), pause(), sigaction(), sigsetjmp()

       The Base Definitions volume of POSIX.1‐2008, <unistd.h>

COPYRIGHT
       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(3POSIX)