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<title>Issue 905: Mutex specification questions</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#Dup">Dup</a> status.</em></p>
<h3 id="905"><a href="lwg-closed.html#905">905</a>. Mutex specification questions</h3>
<p><b>Section:</b> 32.6.4.2.2 <a href="https://wg21.link/thread.mutex.class">[thread.mutex.class]</a> <b>Status:</b> <a href="lwg-active.html#Dup">Dup</a>
 <b>Submitter:</b> Herb Sutter <b>Opened:</b> 2008-09-18 <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.mutex.class">issues</a> in [thread.mutex.class].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Dup">Dup</a> status.</p>
<p><b>Duplicate of:</b> <a href="lwg-defects.html#893" title="std::mutex issue (Status: C++11)">893</a></p>
<p><b>Discussion:</b></p>
<p>
A few questions on the current WP,
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2723.pdf">N2723</a>:
</p>
<p>
32.6.4 <a href="https://wg21.link/thread.mutex.requirements">[thread.mutex.requirements]</a>/24 says an expression
<code>mut.unlock()</code> "Throws: Nothing." I'm assuming that, per 99 [res.on.required], errors that violate the precondition "The
calling thread shall own the mutex" opens the door for throwing an
exception anyway, such as to report unbalanced unlock operations and
unlocking from a thread that does not have ownership. Right?
</p>
<p>
32.6.4.2.2 <a href="https://wg21.link/thread.mutex.class">[thread.mutex.class]</a>/3 (actually numbered paragraph "27"
in the WP; this is just a typo I think) says
</p>
<blockquote>
<p>
The behavior of a program is undefined if:
</p>
<ul>
<li>it destroys a <code>mutex</code> object owned by any thread,</li>
<li>a thread that owns a <code>mutex</code> object calls <code>lock()</code> or <code>try_lock()</code> on that object, or</li>
<li>a thread terminates while owning a <code>mutex</code> object.</li>
</ul>
</blockquote>

<p>
As already discussed, I think the second bullet should be removed, and
such a <code>lock()</code> or <code>try_lock()</code> should fail with an
exception or returning <code>false</code>, respectively.
</p>
<p>
A potential addition to the list would be
</p>
<ul>
<li>a thread unlocks a <code>mutex</code> it does not have ownership of.</li>
</ul>
<p>
but without that the status quo text endorses the technique of the
program logically transferring ownership of a mutex to another thread
with correctness enforced by programming discipline. Was that intended?
</p>

<p><i>[
Summit:
]</i></p>


<blockquote>
<p>
Two resolutions: "not a defect" and "duplicate", as follows:
</p>
<ul>
<li>
32.6.4 <a href="https://wg21.link/thread.mutex.requirements">[thread.mutex.requirements]</a>/24: NAD. If the precondition
fails the program has undefined behaviour and therefore an
implementation may throw an exception already.
</li>
<li>
32.6.4.2.2 <a href="https://wg21.link/thread.mutex.class">[thread.mutex.class]</a>/3 bullet 2: Already addressed by issue <a href="lwg-defects.html#893" title="std::mutex issue (Status: C++11)">893</a><sup><a href="https://cplusplus.github.io/LWG/issue893" title="Latest snapshot">(i)</a></sup>.
</li>
<li>
32.6.4.2.2 <a href="https://wg21.link/thread.mutex.class">[thread.mutex.class]</a>/3 proposed addition: NAD. This is
already covered by the mutex requirements, which have ownership as a
Precondition.
</li>
</ul>
</blockquote>


<p id="res-905"><b>Proposed resolution:</b></p>






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