main/libstdc++-v3/doc/html/manual/io_and_c.html

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   3   Input and Output
   4
   5 </th><td width="20%" align="right"> <a accesskey="n" href="atomics.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.io.c"></a>Interacting with C</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.io.c.FILE"></a>Using FILE* and file descriptors</h3></div></div></div><p>
   6       See the <a class="link" href="ext_io.html" title="Chapter 28. Input and Output">extensions</a> for using
   7       <span class="type">FILE</span> and <span class="type">file descriptors</span> with
   8       <code class="classname">ofstream</code> and
   9       <code class="classname">ifstream</code>.
  10     </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="std.io.c.sync"></a>Performance</h3></div></div></div><p>
  11       Pathetic Performance? Ditch C.
  12     </p><p>It sounds like a flame on C, but it isn't.  Really.  Calm down.
  13       I'm just saying it to get your attention.
  14    </p><p>Because the C++ library includes the C library, both C-style and
  15       C++-style I/O have to work at the same time.  For example:
  16    </p><pre class="programlisting">
  17      #include &lt;iostream&gt;
  18      #include &lt;cstdio&gt;
  19
  20      std::cout &lt;&lt; "Hel";
  21      std::printf ("lo, worl");
  22      std::cout &lt;&lt; "d!\n";
  23    </pre><p>This must do what you think it does.
  24    </p><p>Alert members of the audience will immediately notice that buffering
  25       is going to make a hash of the output unless special steps are taken.
  26    </p><p>The special steps taken by libstdc++, at least for version 3.0,
  27       involve doing very little buffering for the standard streams, leaving
  28       most of the buffering to the underlying C library.  (This kind of
  29       thing is tricky to get right.)
  30       The upside is that correctness is ensured.  The downside is that
  31       writing through <code class="code">cout</code> can quite easily lead to awful
  32       performance when the C++ I/O library is layered on top of the C I/O
  33       library (as it is for 3.0 by default).  Some patches have been applied
  34       which improve the situation for 3.1.
  35    </p><p>However, the C and C++ standard streams only need to be kept in sync
  36       when both libraries' facilities are in use.  If your program only uses
  37       C++ I/O, then there's no need to sync with the C streams.  The right
  38       thing to do in this case is to call
  39    </p><pre class="programlisting">
  40      #include <span class="emphasis"><em>any of the I/O headers such as ios, iostream, etc</em></span>
  41
  42      std::ios::sync_with_stdio(false);
  43    </pre><p>You must do this before performing any I/O via the C++ stream objects.
  44       Once you call this, the C++ streams will operate independently of the
  45       (unused) C streams.  For GCC 3.x, this means that <code class="code">cout</code> and
  46       company will become fully buffered on their own.
  47    </p><p>Note, by the way, that the synchronization requirement only applies to
  48       the standard streams (<code class="code">cin</code>, <code class="code">cout</code>,
  49       <code class="code">cerr</code>,
  50       <code class="code">clog</code>, and their wide-character counterparts).  File stream
  51       objects that you declare yourself have no such requirement and are fully
  52       buffered.
  53    </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="fstreams.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="io.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="atomics.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">File Based Streams </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 14.
  54   Atomics
  55
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