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<basefont size="3"> <a name="sigwait"></a> <a name="tag_03_700"></a><!-- sigwait -->
<!--header start-->
<center><font size="2">The Open Group Base Specifications Issue 6<br>
IEEE Std 1003.1-2001<br>
Copyright &copy; 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
<!--header end-->
<hr size="2" noshade>
<h4><a name="tag_03_700_01"></a>NAME</h4>
<blockquote>sigwait - wait for queued signals</blockquote>
<h4><a name="tag_03_700_02"></a>SYNOPSIS</h4>
<blockquote class="synopsis">
<div class="box"><code><tt><sup>[<a href="javascript:open_code('CX')">CX</a>]</sup> <img src="../images/opt-start.gif" alt=
"[Option Start]" border="0"> #include &lt;<a href="../basedefs/signal.h.html">signal.h</a>&gt;<br>
<br>
int sigwait(const sigset_t *restrict</tt> <i>set</i><tt>, int *restrict</tt> <i>sig</i><tt>); <img src="../images/opt-end.gif"
alt="[Option End]" border="0"></tt></code></div>
<tt><br>
</tt></blockquote>
<h4><a name="tag_03_700_03"></a>DESCRIPTION</h4>
<blockquote>
<p>The <i>sigwait</i>() function shall select a pending signal from <i>set</i>, atomically clear it from the system's set of
pending signals, and return that signal number in the location referenced by <i>sig</i>. If prior to the call to <i>sigwait</i>()
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. <sup>[<a href="javascript:open_code('RTS')">RTS</a>]</sup> <img src=
"../images/opt-start.gif" alt="[Option Start]" border="0"> &nbsp;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 <i>sigwait</i>() and
the remainder shall remain queued. <img src="../images/opt-end.gif" alt="[Option End]" border="0"> If no signal in <i>set</i> is
pending at the time of the call, the thread shall be suspended until one or more becomes pending. The signals defined by <i>set</i>
shall have been blocked at the time of the call to <i>sigwait</i>(); otherwise, the behavior is undefined. The effect of
<i>sigwait</i>() on the signal actions for the signals in <i>set</i> is unspecified.</p>
<p>If more than one thread is using <i>sigwait</i>() to wait for the same signal, no more than one of these threads shall return
from <i>sigwait</i>() with the signal number. Which thread returns from <i>sigwait</i>() if more than a single thread is waiting is
unspecified.</p>
<p><sup>[<a href="javascript:open_code('RTS')">RTS</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
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. <img
src="../images/opt-end.gif" alt="[Option End]" border="0"></p>
</blockquote>
<h4><a name="tag_03_700_04"></a>RETURN VALUE</h4>
<blockquote>
<p>Upon successful completion, <i>sigwait</i>() shall store the signal number of the received signal at the location referenced by
<i>sig</i> and return zero. Otherwise, an error number shall be returned to indicate the error.</p>
</blockquote>
<h4><a name="tag_03_700_05"></a>ERRORS</h4>
<blockquote>
<p>The <i>sigwait</i>() function may fail if:</p>
<dl compact>
<dt>[EINVAL]</dt>
<dd>The <i>set</i> argument contains an invalid or unsupported signal number.</dd>
</dl>
</blockquote>
<hr>
<div class="box"><em>The following sections are informative.</em></div>
<h4><a name="tag_03_700_06"></a>EXAMPLES</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_700_07"></a>APPLICATION USAGE</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_700_08"></a>RATIONALE</h4>
<blockquote>
<p>To provide a convenient way for a thread to wait for a signal, this volume of IEEE&nbsp;Std&nbsp;1003.1-2001 provides the
<i>sigwait</i>() function. For most cases where a thread has to wait for a signal, the <i>sigwait</i>() function should be quite
convenient, efficient, and adequate.</p>
<p>However, requests were made for a lower-level primitive than <i>sigwait</i>() and for semaphores that could be used by threads.
After some consideration, threads were allowed to use semaphores and <a href="../functions/sem_post.html"><i>sem_post</i>()</a> was
defined to be async-signal and async-cancel-safe.</p>
<p>In summary, when it is necessary for code run in response to an asynchronous signal to notify a thread, <i>sigwait</i>() should
be used to handle the signal. Alternatively, if the implementation provides semaphores, they also can be used, either following
<i>sigwait</i>() or from within a signal handling routine previously registered with <a href=
"../functions/sigaction.html"><i>sigaction</i>()</a>.</p>
</blockquote>
<h4><a name="tag_03_700_09"></a>FUTURE DIRECTIONS</h4>
<blockquote>
<p>None.</p>
</blockquote>
<h4><a name="tag_03_700_10"></a>SEE ALSO</h4>
<blockquote>
<p><a href="xsh_chap02_04.html#tag_02_04"><i>Signal Concepts</i></a> , <a href="xsh_chap02_08.html#tag_02_08_01"><i>Realtime
Signals</i></a> , <a href="pause.html"><i>pause</i>()</a> , <a href="pthread_sigmask.html"><i>pthread_sigmask</i>()</a> , <a href=
"sigaction.html"><i>sigaction</i>()</a> , <a href="sigpending.html"><i>sigpending</i>()</a> , <a href=
"sigsuspend.html"><i>sigsuspend</i>()</a> , <a href="sigwaitinfo.html"><i>sigwaitinfo</i>()</a> , the Base Definitions volume of
IEEE&nbsp;Std&nbsp;1003.1-2001, <a href="../basedefs/signal.h.html"><i>&lt;signal.h&gt;</i></a>, <a href=
"../basedefs/time.h.html"><i>&lt;time.h&gt;</i></a></p>
</blockquote>
<h4><a name="tag_03_700_11"></a>CHANGE HISTORY</h4>
<blockquote>
<p>First released in Issue 5. Included for alignment with the POSIX Realtime Extension and the POSIX Threads Extension.</p>
</blockquote>
<h4><a name="tag_03_700_12"></a>Issue 6</h4>
<blockquote>
<p>The <b>restrict</b> keyword is added to the <i>sigwait</i>() prototype for alignment with the ISO/IEC&nbsp;9899:1999
standard.</p>
</blockquote>
<div class="box"><em>End of informative text.</em></div>
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