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<title>Issue 1108: thread.req.exception overly constrains implementations</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="1108"><a href="lwg-defects.html#1108">1108</a>. thread.req.exception overly constrains implementations</h3>
<p><b>Section:</b> 32.2.2 <a href="https://wg21.link/thread.req.exception">[thread.req.exception]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Christopher Kohlhoff <b>Opened:</b> 2009-04-25 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</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>
The current formulation of 32.2.2 <a href="https://wg21.link/thread.req.exception">[thread.req.exception]</a>/2 reads:
</p>
<blockquote><p>
The <code>error_category</code> of the <code>error_code</code> reported by such an
exception's <code>code()</code> member function is as specified in the error
condition Clause.
</p></blockquote>
<p>
This constraint on the code's associated <code>error_categor</code> means an
implementation must perform a mapping from the system-generated
error to a <code>generic_category()</code> error code. The problems with this
include:
</p>

<ul>
<li>
The mapping is always performed, even if the resultant value is
 never used.
</li>
<li>
<p>
The original error produced by the operating system is lost.
</p>
</li>
</ul>
<p>
The latter was one of Peter Dimov's main objections (in a private
email discussion) to the original <code>error_code</code>-only design, and led to
the creation of <code>error_condition</code> in the first place. Specifically,
<code>error_code</code> and <code>error_condition</code> are intended to perform 
the following roles:
</p>
<ul>
<li>
<code>error_code</code> holds the original error produced by the operating
 system.
</li>
<li>
<code>error_condition</code> and the generic category provide a set of well
 known error constants that error codes may be tested against.
</li>
</ul>
<p>
Any mapping determining correspondence of the returned error code to
the conditions listed in the error condition clause falls under the
"latitude" granted to implementors in 19.5.3.5 <a href="https://wg21.link/syserr.errcat.objects">[syserr.errcat.objects]</a>.
(Although obviously their latitude is restricted a little by the
need to match the right error condition when returning an error code
from a library function.)
</p>
<p>
It is important that this <code>error_code&#47;error_condition</code> usage is done
correctly for the thread library since it is likely to set the pattern for future 
TR libraries that interact with the operating system.
</p>

<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote><p>
Move to Open, and recommend the issue be deferred until after the next
Committee Draft is issued.
</p></blockquote>

<p><i>[
2009-10 post-Santa Cruz:
]</i></p>


<blockquote><p>
Move to Tentatively Ready.
</p></blockquote>



<p id="res-1108"><b>Proposed resolution:</b></p>
<p>
Change 32.2.2 <a href="https://wg21.link/thread.req.exception">[thread.req.exception]</a> p.2:
</p>

<blockquote>
<p>
-2- <del>The <code>error_category</code> (19.5.1.1) of the <code>error_code</code> reported by
such an exception's <code>code()</code> member function 
is as specified in the error condition Clause.</del>
<ins>
The <code>error_code</code> reported by such an exception's <code>code()</code> member
function shall compare equal to one of the conditions specified in
the function's error condition Clause. [<i>Example:</i> When the thread
constructor fails:
</ins>
</p>
<blockquote><pre><ins>
ec.category() == implementation-defined // probably system_category
ec == errc::resource_unavailable_try_again // holds true
</ins></pre></blockquote>

<p><ins>
&mdash; <i>end example</i>]
</ins></p>

</blockquote>






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