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<title>Issue 3598: system_category().default_error_condition(0) is underspecified</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++23">C++23</a> status.</em></p>
<h3 id="3598"><a href="lwg-defects.html#3598">3598</a>. <code>system_category().default_error_condition(0)</code> is underspecified</h3>
<p><b>Section:</b> 19.5.3.5 <a href="https://wg21.link/syserr.errcat.objects">[syserr.errcat.objects]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2021-09-23 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#syserr.errcat.objects">issues</a> in [syserr.errcat.objects].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
19.5.3.5 <a href="https://wg21.link/syserr.errcat.objects">[syserr.errcat.objects]</a> says:
</p>
<blockquote><p>
If the argument <code>ev</code> corresponds to a POSIX <code>errno</code> value <code>posv</code>, 
the function shall return <code>error_condition(posv, generic_category())</code>. Otherwise, 
the function shall return <code>error_condition(ev, system_category())</code>. What constitutes 
correspondence for any given operating system is unspecified.
</p></blockquote>
<p>
What is "a POSIX <code>errno</code> value"? Does it mean a value equal to one of the <code>&lt;cerrno&gt;</code> 
<code>Exxx</code> macros? Because in that case, the value <code>0</code> is not "a POSIX <code>errno</code> value". 
So arguably <code>system_category().default_error_condition(0)</code> is required to return an 
<code>error_condition</code> using the "system" category, which means that <code>error_code{} == error_condition{}</code> 
is required to be <code>false</code>. This seems wrong.
<p/>
For POSIX-based systems the value <code>0</code> should definitely correspond to the generic category. 
Arguably that needs to be <code>true</code> for all systems, because the <code>std::error_code</code> API 
strongly encourages a model where zero means "no error".
<p/>
The proposed resolution has been implemented in libstdc++. Libc++ has always treated system error code 
<code>0</code> as corresponding to generic error code <code>0</code>.
</p>

<p><i>[2021-10-14; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after six votes in favour during reflector poll.
</p>

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3598"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 19.5.3.5 <a href="https://wg21.link/syserr.errcat.objects">[syserr.errcat.objects]</a> as indicated:</p>

<blockquote>
<pre>
const error_category&amp; system_category() noexcept;
</pre>
<blockquote>
<p>
-3- <i>Returns:</i> [&hellip;]
<p/>
-4- <i>Remarks:</i> The object's <code>equivalent</code> virtual functions shall behave as specified 
for class <code>error_category</code>. The object's <code>name</code> virtual function shall return a pointer 
to the string <code>"system"</code>. The object's <code>default_error_condition</code> virtual function 
shall behave as follows:
<p/>
If the argument <code>ev</code> <ins>is equal to <code>0</code>, the function returns 
<code>error_condition(0, generic_category())</code>. Otherwise, if <code>ev</code></ins> corresponds to a POSIX 
<code>errno</code> value <code>posv</code>, the function <del>shall return</del><ins>returns</ins> 
<code>error_condition(posv, generic_category())</code>. Otherwise, the function <del>shall return</del><ins>returns</ins> 
<code>error_condition(ev, system_category())</code>. What constitutes correspondence for any given operating 
system is unspecified.
<p/>
[<i>Note 1</i>: The number of potential system error codes is large and unbounded, and some might not 
correspond to any POSIX <code>errno</code> value. Thus implementations are given latitude in determining 
correspondence. &mdash; <i>end note</i>]
</p>
</blockquote>
</blockquote>
</li>

</ol>





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