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<title>Issue 3030: Who shall meet the requirements of try_lock?</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++20">C++20</a> status.</em></p>
<h3 id="3030"><a href="lwg-defects.html#3030">3030</a>. Who shall meet the requirements of <code>try_lock</code>?</h3>
<p><b>Section:</b> 32.6.6 <a href="https://wg21.link/thread.lock.algorithm">[thread.lock.algorithm]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2017-11-07 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#thread.lock.algorithm">issues</a> in [thread.lock.algorithm].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
32.6.6 <a href="https://wg21.link/thread.lock.algorithm">[thread.lock.algorithm]</a> says:
</p>
<blockquote><p>
"If a call to <code>try_lock()</code> fails, <code>unlock()</code> shall be called for all prior
arguments and there shall be no further calls to <code>try_lock()</code>."
</p></blockquote>
<p>
We try to use "shall" for requirements on the user (e.g. as in the
previous paragraph) which is absolutely not what is meant here.
</p>

<p><i>[2017-11 Albuquerque Wednesday night issues processing]</i></p>

<p>Moved to Ready</p>
<p><i>[2018-3-17 Adopted in Jacksonville]</i></p>



<p id="res-3030"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4700">N4700</a>.</p>

<ol>
<li><p>Change 32.6.6 <a href="https://wg21.link/thread.lock.algorithm">[thread.lock.algorithm]</a> as indicated:</p>

<blockquote>
<pre>
template &lt;class L1, class L2, class... L3&gt; int try_lock(L1&amp;, L2&amp;, L3&amp;...);
</pre>
<blockquote>
<p>
-1- <i>Requires:</i> [&hellip;]
<p/>
-2- <i>Effects:</i> Calls <code>try_lock()</code> for each argument in order beginning with the first until all arguments have
been processed or a call to <code>try_lock()</code> fails, either by returning <code>false</code> or by throwing an exception.
If a call to <code>try_lock()</code> fails, <code>unlock()</code> <del>shall be</del><ins>is</ins> called for all prior arguments 
<del>and there shall be</del><ins>with</ins> no further calls to <code>try_lock()</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class L1, class L2, class... L3&gt; void lock(L1&amp;, L2&amp;, L3&amp;...);
</pre>
<blockquote>
<p>
-4- <i>Requires:</i> [&hellip;]
<p/>
-5- <i>Effects:</i> All arguments are locked via a sequence of calls to <code>lock()</code>, <code>try_lock()</code>, 
or <code>unlock()</code> on each argument. The sequence of calls <del>shall</del><ins>does</ins> not result in deadlock, 
but is otherwise unspecified. [<i>Note:</i> A deadlock avoidance algorithm such as try-and-back-off must be used, but the 
specific algorithm is not specified to avoid over-constraining implementations. &mdash; <i>end note</i>] If a 
call to <code>lock()</code> or <code>try_lock()</code> throws an exception, <code>unlock()</code> <del>shall be</del><ins>is</ins> 
called for any argument that had been locked by a call to <code>lock()</code> or <code>try_lock()</code>.
</p>
</blockquote>
</blockquote>
</li>
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