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

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME

       sigwait - wait for queued signals

SYNOPSIS

       #include <signal.h>

       int sigwait(const sigset_t *restrict set, int *restrict sig);

DESCRIPTION

       The sigwait() function shall select a pending signal from set, atomically clear it from the system's set of  pending  signals,  and  return
       that  signal  number  in the location referenced by sig. If prior to the call to sigwait() there are multiple pending instances of a single
       signal number, it is implementation-defined whether upon successful return there are any remaining pending signals for that signal  number.
       If  the	implementation supports queued signals and there are multiple signals queued for the signal number selected, the first such queued
       signal shall cause a return from sigwait() and the remainder shall remain queued.  If no signal in set is pending at the time of the  call,
       the thread shall be suspended until one or more becomes pending. The signals defined by set shall have been blocked at the time of the call
       to sigwait(); otherwise, the behavior is undefined. The effect of sigwait() on the signal actions for the signals in set is unspecified.

       If more than one thread is using sigwait() to wait for the same signal, no more than one of these threads shall return from sigwait()  with
       the signal number. Which thread returns from sigwait() if more than a single thread is waiting is unspecified.

       Should  any  of the multiple pending signals in the range SIGRTMIN to SIGRTMAX be selected, it shall be the lowest numbered one. The selec-
       tion order between realtime and non-realtime signals, or between multiple pending non-realtime signals, is unspecified.

RETURN VALUE

       Upon successful completion, sigwait() shall store the signal number of the received signal at the location referenced  by  sig  and  return
       zero. Otherwise, an error number shall be returned to indicate the error.

ERRORS

       The sigwait() function may fail if:

       EINVAL The set argument contains an invalid or unsupported signal number.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       To  provide  a  convenient  way for a thread to wait for a signal, this volume of IEEE Std 1003.1-2001 provides the sigwait() function. For
       most cases where a thread has to wait for a signal, the sigwait() function should be quite convenient, efficient, and adequate.

       However, requests were made for a lower-level primitive than sigwait() and for semaphores that could be used by threads.  After	some  con-
       sideration, threads were allowed to use semaphores and sem_post() was defined to be async-signal and async-cancel-safe.

       In  summary, when it is necessary for code run in response to an asynchronous signal to notify a thread, sigwait() should be used to handle
       the signal. Alternatively, if the implementation provides semaphores, they also can be used, either following sigwait() or  from  within  a
       signal handling routine previously registered with sigaction().

FUTURE DIRECTIONS

       None.

SEE ALSO

       Signal Concepts, Realtime Signals, pause(), pthread_sigmask(), sigaction(), sigpending(), sigsuspend(), sigwaitinfo(), the Base Definitions
       volume of IEEE Std 1003.1-2001, <signal.h>, <time.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- 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							       SIGWAIT(3P)