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<tr class="t-nv-h1"><td colspan="5"> Condition variables</td></tr>
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<td> <div>Defined in header <code><threads.h></code>
</div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-c11">
<td> <div><span class="mw-geshi c source-c"><span class="kw4">void</span> call_once<span class="br0">(</span> <a href="../thread.html"><span class="kw1022">once_flag</span></a><span class="sy2">*</span> flag, <span class="kw4">void</span> <span class="br0">(</span><span class="sy2">*</span>func<span class="br0">)</span><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td> (1) </td>
<td> <span class="t-mark-rev t-since-c11">(since C11)</span> </td>
</tr>
<tr class="t-dcl t-since-c11">
<td> <div><span class="mw-geshi c source-c"><span class="kw1">typedef</span> <a href="../thread.html"><span class="kw1022">once_flag</span></a> <span class="coMULTI">/* unspecified */</span></span></div></td>
<td> (2) </td>
<td> <span class="t-mark-rev t-since-c11">(since C11)</span> </td>
</tr>
<tr class="t-dcl t-since-c11">
<td> <div><span class="mw-geshi c source-c"><span class="co2">#define ONCE_FLAG_INIT /* unspecified */</span></span></div></td>
<td> (3) </td>
<td> <span class="t-mark-rev t-since-c11">(since C11)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<div class="t-li1"><span class="t-li">1)</span> Calls function <code>func</code> exactly once, even if invoked from several threads. The completion of the function <code>func</code> synchronizes with all previous or subsequent calls to <code>call_once</code> with the same <code>flag</code> variable.</div>
<div class="t-li1"><span class="t-li">2)</span> Complete object type capable of holding a flag used by <code>call_once</code></div>
<div class="t-li1"><span class="t-li">3)</span> Expands to a value that can be used to initialize an object of type <code>once_flag</code>.</div>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="ONCE_FLAG_INIT.html#Parameters"><span class="tocnumber">1</span> <span class="toctext">Parameters</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="ONCE_FLAG_INIT.html#Return_value"><span class="tocnumber">2</span> <span class="toctext">Return value</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="ONCE_FLAG_INIT.html#Notes"><span class="tocnumber">3</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="ONCE_FLAG_INIT.html#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="ONCE_FLAG_INIT.html#See_also"><span class="tocnumber">5</span> <span class="toctext">See also</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="ONCE_FLAG_INIT.html#References"><span class="tocnumber">6</span> <span class="toctext">References</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/thread/call_once&action=edit&section=1" title="Edit section: Parameters">edit</a>]</span> <span class="mw-headline" id="Parameters">Parameters</span></h3>
<table class="t-par-begin">
<tr class="t-par">
<td> flag
</td>
<td> -
</td>
<td> pointer to an object of type <code>call_once</code> that is used to ensure <code>func</code> is called only once
</td></tr>
<tr class="t-par">
<td> func
</td>
<td> -
</td>
<td> the function to execute only once
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/thread/call_once&action=edit&section=2" title="Edit section: Return value">edit</a>]</span> <span class="mw-headline" id="Return_value">Return value</span></h3>
<p>(none)
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/thread/call_once&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>The POSIX equivalent of this function is <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_once.html">pthread_once</a>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/thread/call_once&action=edit&section=4" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include <stdio.h></span>
<span class="co2">#include <threads.h></span>
 
<span class="kw4">void</span> do_once<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span>
<a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"called once"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">static</span> <a href="../thread.html"><span class="kw1022">once_flag</span></a> flag <span class="sy1">=</span> <a href="ONCE_FLAG_INIT.html"><span class="kw1023">ONCE_FLAG_INIT</span></a><span class="sy4">;</span>
<span class="kw4">int</span> func<span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span> data<span class="br0">)</span>
<span class="br0">{</span>
call_once<span class="br0">(</span><span class="sy3">&</span>flag, do_once<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../thread.html"><span class="kw996">thrd_t</span></a> t1, t2, t3, t4<span class="sy4">;</span>
<a href="thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&</span>t1, func, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&</span>t2, func, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&</span>t3, func, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&</span>t4, func, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
 
<a href="thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>t1, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>t2, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>t3, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>t4, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">called once</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/thread/call_once&action=edit&section=5" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../cpp/thread/call_once.html" title="cpp/thread/call once">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>call_once</span></span></div>
</td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/thread/call_once&action=edit&section=6" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011):
</li></ul>
<dl><dd><ul><li> 7.26.2.1 The call_once function (p: 378)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 7.26.1/3 ONCE_FLAG_INIT (p: 376)
</li></ul>
</dd></dl>
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