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<title>Issue 3464: istream::gcount() can overflow</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="3464"><a href="lwg-defects.html#3464">3464</a>. <code>istream::gcount()</code> can overflow</h3>
<p><b>Section:</b> 31.7.5.4 <a href="https://wg21.link/istream.unformatted">[istream.unformatted]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2020-07-10 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#istream.unformatted">issues</a> in [istream.unformatted].</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>
The standard doesn't say what <code>gcount()</code> should return if the last unformatted input 
operation extracted more than <code>numeric_limits&lt;streamsize&gt;::max()</code> characters. 
This is possible when using <code>istream::ignore(numeric_limits&lt;streamsize&gt;::max(), delim)</code>, 
which will keep extracting characters until the delimiter is found. On a 32-bit platform files 
larger than 2GB can overflow the counter, so can a streambuf reading from a network socket, 
or producing random characters.
<p/>
Libstdc++ saturates the counter in <code>istream::ignore</code>, so that <code>gcount()</code> returns 
<code>numeric_limits&lt;streamsize&gt;::max()</code>. Libc++ results in an integer overflow.
<p/>
As far as I'm aware, only <code>istream::ignore</code> can extract more than 
<code>numeric_limits&lt;streamsize&gt;::max()</code> characters at once. We could either fix it 
in the specification of <code>ignore</code>, or in <code>gcount</code>.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">

<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>. 
</p>

<b>Option A:</b>
<ol>
<li><p>Modify 31.7.5.4 <a href="https://wg21.link/istream.unformatted">[istream.unformatted]</a> as indicated:</p>

<blockquote>
<pre>
streamsize gcount() const;
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> None. This member function does not behave as an unformatted input function (as described
above).
<p/>
-3- <i>Returns:</i> The number of characters extracted by the last unformatted input member function called for
the object. <ins>If the number cannot be represented, returns <code>numeric_limits&lt;streamsize&gt;::max()</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>

<b>Option B:</b>
<ol>
<li><p>Modify 31.7.5.4 <a href="https://wg21.link/istream.unformatted">[istream.unformatted]</a> as indicated:</p>

<blockquote>
<pre>
basic_istream&lt;charT, traits&gt;&amp; ignore(streamsize n = 1, int_type delim = traits::eof());
</pre>
<blockquote>
<p>
-25- <i>Effects:</i> Behaves as an unformatted input function (as described above). After constructing a sentry
object, extracts characters and discards them. Characters are extracted until any of the following
occurs:
</p>
<ol style="list-style-type: none">
<li><p>(25.1) &mdash; <code>n != numeric_limits&lt;streamsize&gt;::max()</code> (17.3.5 <a href="https://wg21.link/numeric.limits">[numeric.limits]</a>) 
and <code>n</code> characters have been extracted so far</p></li>
<li><p>(25.2) &mdash; end-of-file occurs on the input sequence (in which case the function calls <code>setstate(eofbit)</code>,
which may throw <code>ios_base::failure</code> (31.5.4.4 <a href="https://wg21.link/iostate.flags">[iostate.flags]</a>));</p></li>
<li><p>(25.3) &mdash; <code>traits::eq_int_type(traits::to_int_type(c), delim)</code> for the next available input 
character <code>c</code> (in which case <code>c</code> is extracted).</p></li>
</ol>
<p>
<ins>-?- If the number of characters extracted is greater than <code>numeric_limits&lt;streamsize&gt;::max()</code> 
then for the purposes of <code>gcount()</code> the number is treated as <code>numeric_limits&lt;streamsize&gt;::max()</code>.</ins>
<p/>
-26- <i>Remarks:</i> The last condition will never occur if <code>traits::eq_int_type(delim, traits::eof())</code>.
<p/>
-27- <i>Returns:</i> <code>*this</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>

</blockquote>

<p><i>[2020-07-17; Moved to Ready in telecon]</i></p>

<p>
On the reflector Davis pointed out that there are other
members which can cause <code>gcount()</code> to overflow.
There was unanimous agreement on the reflector and the telecon
that Option A is better.
</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3464"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>. 
</p>

<ol>
<li><p>Modify 31.7.5.4 <a href="https://wg21.link/istream.unformatted">[istream.unformatted]</a> as indicated:</p>

<blockquote>
<pre>
streamsize gcount() const;
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> None. This member function does not behave as an unformatted input function (as described
above).
<p/>
-3- <i>Returns:</i> The number of characters extracted by the last unformatted input member function called for
the object. <ins>If the number cannot be represented, returns <code>numeric_limits&lt;streamsize&gt;::max()</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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