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<title>Issue 49: Underspecification of ios_base::sync_with_stdio</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#CD1">CD1</a> status.</em></p>
<h3 id="49"><a href="lwg-defects.html#49">49</a>. Underspecification of ios_base::sync_with_stdio</h3>
<p><b>Section:</b> 31.5.2.5 <a href="https://wg21.link/ios.members.static">[ios.members.static]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Matt Austern <b>Opened:</b> 1998-06-21 <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#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>Two problems</p>

<p>(1) 27.4.2.4 doesn't say what ios_base::sync_with_stdio(f)
returns. Does it return f, or does it return the previous
synchronization state? My guess is the latter, but the standard
doesn't say so.</p>

<p>(2) 27.4.2.4 doesn't say what it means for streams to be
synchronized with stdio.  Again, of course, I can make some
guesses. (And I'm unhappy about the performance implications of those
guesses, but that's another matter.)</p>


<p id="res-49"><b>Proposed resolution:</b></p>
<p>Change the following sentence in 31.5.2.5 <a href="https://wg21.link/ios.members.static">[ios.members.static]</a>
returns clause from:</p>

<blockquote>
  <p><code>true</code> if the standard iostream objects (27.3) are
  synchronized and otherwise returns <code>false</code>.</p>
</blockquote>

<p>to:</p>

<blockquote>
  <p><code>true</code> if the previous state of the standard iostream
  objects (27.3) was synchronized and otherwise returns
  <code>false</code>.</p>
</blockquote>

<p>Add the following immediately after 31.5.2.5 <a href="https://wg21.link/ios.members.static">[ios.members.static]</a>,
paragraph 2:</p>

<blockquote>
<p>When a standard iostream object str is <i>synchronized</i> with a
standard stdio stream f, the effect of inserting a character c by</p>
<pre>
  fputc(f, c);
</pre>

<p>is the same as the effect of</p>
<pre>
  str.rdbuf()-&gt;sputc(c);
</pre>

<p>for any sequence of characters; the effect of extracting a
character c by</p>
<pre>
  c = fgetc(f);
</pre>

<p>is the same as the effect of:</p>
<pre>
  c = str.rdbuf()-&gt;sbumpc(c);
</pre>

<p>for any sequences of characters; and the effect of pushing
back a character c by</p>
<pre>
  ungetc(c, f);
</pre>

<p>is the same as the effect of</p>
<pre>
  str.rdbuf()-&gt;sputbackc(c);
</pre>

<p>for any sequence of characters.  [<i>Footnote</i>: This implies
that operations on a standard iostream object can be mixed arbitrarily
with operations on the corresponding stdio stream.  In practical
terms, synchronization usually means that a standard iostream object
and a standard stdio object share a buffer. <i>--End Footnote</i>]</p>
</blockquote>

<p><i>[pre-Copenhagen: PJP and Matt contributed the definition
of "synchronization"]</i></p>


<p><i>[post-Copenhagen: proposed resolution was revised slightly:
text was added in the non-normative footnote to say that operations
on the two streams can be mixed arbitrarily.]</i></p>






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