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

       Interactions between sleep() and any of setitimer(), ualarm(), or usleep() 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 process waiting for that signal. The other  is
       to implement an independent facility. This volume of IEEE Std 1003.1-2001 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  IEEE Std 1003.1-2001,  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
       IEEE Std 1003.1-2001  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  writers 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() , ualarm() , usleep() , the
       Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications Issue 6, Copyright (C) 2001-2003 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 http://www.opengroup.org/unix/online.html .

IEEE/The Open Group                                   2003                                              SLEEP(P)