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<title>Issue 1494: Term "are serialized" not defined</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++11">C++11</a> status.</em></p>
<h3 id="1494"><a href="lwg-defects.html#1494">1494</a>. Term "are serialized" not defined</h3>
<p><b>Section:</b> 32.6.7.2 <a href="https://wg21.link/thread.once.callonce">[thread.once.callonce]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> INCITS <b>Opened:</b> 2010-08-25 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#thread.once.callonce">issues</a> in [thread.once.callonce].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses US-190</b></p>
<p>
The term "are serialized" is never defined (32.6.7.2 <a href="https://wg21.link/thread.once.callonce">[thread.once.callonce]</a> p. 2).
</p>
<p><i>[
Resolution proposed by ballot comment:
]</i></p>

<p>
Remove the sentence with "are serialized" from
paragraph 2. Add "Calls to <code>call_once</code> on the same
<code>once_flag</code> object shall not introduce data races
(17.6.4.8)." to paragraph 3.
</p>

<p><i>[
2010-11-01 Daniel translates NB comment into wording
]</i></p>



<p><i>[
2011-02-17: Hans proposes an alternative resolution
]</i></p>


<p><i>[
2011-02-25: Hans, Clark, and Lawrence update the suggested wording
]</i></p>


<p><i>[2011-02-26 Reflector discussion]</i></p>

<p>
Moved to Tentatively Ready after 5 votes.
</p> 


<p id="res-1494"><b>Proposed resolution:</b></p>
<p>Change 32.6.7.2 <a href="https://wg21.link/thread.once.callonce">[thread.once.callonce]</a> p.2+3 as indicated:</p>

<blockquote><pre>
template&lt;class Callable, class ...Args&gt;
void call_once(once_flag&amp; flag, Callable&amp;&amp; func, Args&amp;&amp;... args);
</pre><blockquote><p>
[..]
<p/>
2 <em>Effects</em>: <del>Calls to <code>call_once</code> on the same <code>once_flag</code> object are serialized.
If there has been a prior effective call to <code>call_once</code> on the same <code>once_flag object</code>, the call to <code>call_once</code> 
returns without invoking <code>func</code>. If there has been no prior effective call to <code>call_once</code> 
on the same <code>once_flag</code> object, <code>INVOKE(decay_copy( std::forward&lt;Callable&gt;(func)), 
decay_copy(std::forward&lt;Args&gt;(args))...)</code> is executed. The call to <code>call_once</code> is effective 
if and only if <code>INVOKE(decay_copy( std::forward&lt;Callable&gt;(func)), decay_copy(std::forward&lt;Args&gt;(args))...)</code> 
returns without throwing an exception. If an exception is thrown it is propagated to the caller.</del>
<ins>An execution of <code>call_once</code> that does not call its <code>func</code> is a passive execution.
An execution of <code>call_once</code> that calls its <code>func</code> is an active execution.
An active execution shall call <code><var>INVOKE</var>(decay_copy(std::forward&lt;Callable&gt;(func)),
decay_copy(std::forward&lt;Args&gt;(args))...)</code>. If such a call to <code>func</code> throws an exception,
the execution is exceptional, otherwise it is returning.
An exceptional execution shall propagate the exception to the caller of <code>call_once</code>.
Among all executions of <code>call_once</code> for any given <code>once_flag</code>: at most one shall be a 
returning execution; if there is a returning execution, it shall be the last active execution; and
there are passive executions only if there is a returning execution.
[<i>Note:</i> Passive executions allow other threads to reliably observe the results produced by the 
earlier returning execution. &mdash; <i>end note</i>]</ins>
<p/>
3 <em>Synchronization</em>: <del>The completion of an effective call to <code>call_once</code> on a <code>once_flag</code> 
object <em>synchronizes with</em> (6.10.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a>) all subsequent calls to <code>call_once</code> 
on the same <code>once_flag</code> object.</del><ins>For any given <code>once_flag</code>: all active executions occur in a total 
order; completion of an active execution synchronizes with (6.10.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a>) the start of 
the next one in this total order; and the returning execution synchronizes with the return from all passive 
executions.</ins>
</p></blockquote></blockquote>






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