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 .