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<title>Issue 980: mutex lock() missing error conditions</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#NAD">NAD</a> status.</em></p>
<h3 id="980"><a href="lwg-closed.html#980">980</a>. <code>mutex lock()</code> missing error conditions</h3>
<p><b>Section:</b> 32.6.4 <a href="https://wg21.link/thread.mutex.requirements">[thread.mutex.requirements]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Ion Gazta&ntilde;aga <b>Opened:</b> 2009-02-07 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#thread.mutex.requirements">active issues</a> in [thread.mutex.requirements].</p>
<p><b>View all other</b> <a href="lwg-index.html#thread.mutex.requirements">issues</a> in [thread.mutex.requirements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
POSIX 2008 adds two return values for <code>pthread_mutex_xxxlock()</code>:
<code>EOWNERDEAD</code> (<code>owner_dead</code>) and <code>ENOTRECOVERABLE</code>
(<code>state_not_recoverable</code>). In the first case the mutex is locked,
in the second case the mutex is not locked.
</p>

<p>
Throwing an exception in the first case can be incompatible with the use
of Locks, since the <code>Lock::owns_lock()</code> will be <code>false</code> when the lock is
being destroyed.
</p>

<p>
Consider:
</p>

<blockquote><pre>
//Suppose mutex.lock() throws "owner_dead"
unique_lock ul(&amp;mutex);
//mutex left locked if "owner_dead" is thrown
</pre></blockquote>

<p>
Throwing an exception with <code>owner_dead</code> might be also undesirable if
robust-mutex support is added to C++ and the user has the equivalent of
<code>pthread_mutex_consistent()</code> to notify the user has fixed the corrupted
data and the mutex state should be marked consistent.
</p>

<ol>
<li>
For <code>state_not_recoverable</code> add it to the list of Error conditions:
</li>
<li>
For <code>owner_dead</code>, no proposed resolution.
</li>
</ol>

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


<blockquote><p>
Not a defect. Handling these error conditions is an implementation
detail and must be handled below the C++ interface.
</p></blockquote>



<p id="res-980"><b>Proposed resolution:</b></p>

<p>
Add to 32.6.4 <a href="https://wg21.link/thread.mutex.requirements">[thread.mutex.requirements]</a>, p12:
</p>

<blockquote>
<p>
-12- <i>Error conditions:</i>
</p>

<ul>
<li>
<code>operation_not_permitted</code> -- if the thread does not have the necessary permission to change 
the state of the mutex.
</li>
<li>
<code>resource_deadlock_would_occur</code> -- if the current thread already owns the mutex and is able 
to detect it.
</li>
<li>
<code>device_or_resource_busy</code> --  if the mutex is already locked and blocking is not possible.
</li>
<li>
<ins><code>state_not_recoverable</code> -- if the state protected by the mutex is not recoverable.</ins>
</li>
</ul>
</blockquote>





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