.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SIGWAIT" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" sigwait 
.SH 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.
.SH NAME
sigwait \- wait for queued signals
.SH SYNOPSIS
.LP
\fB#include <signal.h>
.br
.sp
int sigwait(const sigset_t *restrict\fP \fIset\fP\fB, int *restrict\fP
\fIsig\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIsigwait\fP() function shall select a pending signal from \fIset\fP,
atomically clear it from the system's set of
pending signals, and return that signal number in the location referenced
by \fIsig\fP. If prior to the call to \fIsigwait\fP()
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 \fIsigwait\fP() and
the remainder shall remain queued.  If no signal in \fIset\fP is
pending at the time of the call, the thread shall be suspended until
one or more becomes pending. The signals defined by \fIset\fP
shall have been blocked at the time of the call to \fIsigwait\fP();
otherwise, the behavior is undefined. The effect of
\fIsigwait\fP() on the signal actions for the signals in \fIset\fP
is unspecified.
.LP
If more than one thread is using \fIsigwait\fP() to wait for the same
signal, no more than one of these threads shall return
from \fIsigwait\fP() with the signal number. Which thread returns
from \fIsigwait\fP() if more than a single thread is waiting is
unspecified.
.LP
Should any of the multiple pending signals in the range SIGRTMIN to
SIGRTMAX be selected, it shall be the lowest numbered one. The
selection order between realtime and non-realtime signals, or between
multiple pending non-realtime signals, is unspecified. 
.SH RETURN VALUE
.LP
Upon successful completion, \fIsigwait\fP() shall store the signal
number of the received signal at the location referenced by
\fIsig\fP and return zero. Otherwise, an error number shall be returned
to indicate the error.
.SH ERRORS
.LP
The \fIsigwait\fP() function may fail if:
.TP 7
.B EINVAL
The \fIset\fP argument contains an invalid or unsupported signal number.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
To provide a convenient way for a thread to wait for a signal, this
volume of IEEE\ Std\ 1003.1-2001 provides the
\fIsigwait\fP() function. For most cases where a thread has to wait
for a signal, the \fIsigwait\fP() function should be quite
convenient, efficient, and adequate.
.LP
However, requests were made for a lower-level primitive than \fIsigwait\fP()
and for semaphores that could be used by threads.
After some consideration, threads were allowed to use semaphores and
\fIsem_post\fP() was
defined to be async-signal and async-cancel-safe.
.LP
In summary, when it is necessary for code run in response to an asynchronous
signal to notify a thread, \fIsigwait\fP() should
be used to handle the signal. Alternatively, if the implementation
provides semaphores, they also can be used, either following
\fIsigwait\fP() or from within a signal handling routine previously
registered with \fIsigaction\fP().
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fISignal Concepts\fP, \fIRealtime
Signals\fP, \fIpause\fP(), \fIpthread_sigmask\fP(), \fIsigaction\fP(),
\fIsigpending\fP(), \fIsigsuspend\fP(), \fIsigwaitinfo\fP(),
the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<signal.h>\fP, \fI<time.h>\fP
.SH 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 .