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<title>Issue 2647: [filesys.ts] [PDTS] last_write_time() uses ill-formed cast</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#TS">TS</a> status.</em></p>
<h3 id="2647"><a href="lwg-defects.html#2647">2647</a>. [filesys.ts] [PDTS] <code>last_write_time()</code> uses ill-formed cast</h3>
<p><b>Section:</b> 15.25 [filesys.ts::fs.op.last_write_time] <b>Status:</b> <a href="lwg-active.html#TS">TS</a>
 <b>Submitter:</b> P.J. Plauger <b>Opened:</b> 2014-01-30 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#filesys.ts::fs.op.last_write_time">issues</a> in [filesys.ts::fs.op.last_write_time].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TS">TS</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses: filesys.ts</b></p>
<p>
In <code>last_write_time</code>, <code>static_cast&lt;file_time_type&gt;(-1)</code> is ill formed,
(possibly should be <code>chrono&lt;<i>unspecified-trivial-clock</i>&gt;::from_time_t(-1))</code>.
</p>

<p><i>[2014-02-08 Daniel comments and provides wording]</i></p>


<p>
I agree that the current wording needs to be fixed, but it is unclear to me whether invoking
<code>from_time_t</code> (which is only guaranteed to exist for <code>std::chrono::system_clock</code>)
with a value of <code>time_t(-1)</code> is a well-defined operation. Reason is that the C Standard
makes a difference between a <i>calendar time</i> and the value <code>time_t(-1)</code> as an error 
indication. But the specification of <code>system_clock::from_time_t</code> only says "A <code>time_point</code> 
object that represents the same point in time as <code>t</code> [&hellip;]" and it is not clear whether
<code>time_t(-1)</code> can be considered as a "point in time". Instead of relying on such a potentially
subtle semantics of the conversion result of <code>time_t(-1)</code> to <code>std::chrono::system_clock::time_point</code>
with a further conversion to <code>file_time_type</code> (which in theory could led to overflow or underflow and
thus another potential source of undefined behaviour), I suggest to change the current error value of
<code>last_write_time</code> to one of the well-define limiting values of <code>file_time_type</code>, e.g.
<code>file_time_type::min()</code>.
</p>

<p><i>[2014-02-13 LWG/SG-3 Issaquah: Proposed wording accepted.]</i></p>




<p id="res-2647"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="http://wiki.edg.com/twiki/pub/Wg21issaquah/SG3/working-draft.html">SG3 working draft</a>.</p>

<ol>
<li><p>Change the last write time prototype specification, 15.25 [fs.op.last_write_time], as indicated:</p>
<blockquote><pre>
file_time_type last_write_time(const path&amp; p);
file_time_type last_write_time(const path&amp; p, error_code&amp; ec) noexcept;
</pre>
<blockquote>
<p>
<i>Returns</i>: The time of last data modification of <code>p</code>, determined as if by the value of the POSIX <code>stat</code> 
structure member <code>st_mtime</code> obtained as if by POSIX <code>stat()</code>. The signature with argument <code>ec</code> returns 
<code><del>static_cast&lt;file_time_type&gt;(-1)</del><ins>file_time_type::min()</ins></code> if an error occurs.
</p>
</blockquote></blockquote>
</li>
</ol>





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