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

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  11     "><link rel="home" href="../index.html" title="The GNU C++ Library"><link rel="up" href="io.html" title="Chapter 13.  Input and Output"><link rel="prev" href="fstreams.html" title="File Based Streams"><link rel="next" href="atomics.html" title="Chapter 14.  Atomics"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Interacting with C</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="fstreams.html">Prev</a> </td><th width="60%" align="center">Chapter 13.
  12   Input and Output
  13
  14 </th><td width="20%" align="right"> <a accesskey="n" href="atomics.html">Next</a></td></tr></table><hr></div><div class="section" title="Interacting with C"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="std.io.c"></a>Interacting with C</h2></div></div></div><div class="section" title="Using FILE* and file descriptors"><div class="titlepage"><div><div><h3 class="title"><a name="std.io.c.FILE"></a>Using FILE* and file descriptors</h3></div></div></div><p>
  15       See the <a class="link" href="ext_io.html" title="Chapter 28. Input and Output">extensions</a> for using
  16       <span class="type">FILE</span> and <span class="type">file descriptors</span> with
  17       <code class="classname">ofstream</code> and
  18       <code class="classname">ifstream</code>.
  19     </p></div><div class="section" title="Performance"><div class="titlepage"><div><div><h3 class="title"><a name="std.io.c.sync"></a>Performance</h3></div></div></div><p>
  20       Pathetic Performance? Ditch C.
  21     </p><p>It sounds like a flame on C, but it isn't.  Really.  Calm down.
  22       I'm just saying it to get your attention.
  23    </p><p>Because the C++ library includes the C library, both C-style and
  24       C++-style I/O have to work at the same time.  For example:
  25    </p><pre class="programlisting">
  26      #include &lt;iostream&gt;
  27      #include &lt;cstdio&gt;
  28
  29      std::cout &lt;&lt; "Hel";
  30      std::printf ("lo, worl");
  31      std::cout &lt;&lt; "d!\n";
  32    </pre><p>This must do what you think it does.
  33    </p><p>Alert members of the audience will immediately notice that buffering
  34       is going to make a hash of the output unless special steps are taken.
  35    </p><p>The special steps taken by libstdc++, at least for version 3.0,
  36       involve doing very little buffering for the standard streams, leaving
  37       most of the buffering to the underlying C library.  (This kind of
  38       thing is tricky to get right.)
  39       The upside is that correctness is ensured.  The downside is that
  40       writing through <code class="code">cout</code> can quite easily lead to awful
  41       performance when the C++ I/O library is layered on top of the C I/O
  42       library (as it is for 3.0 by default).  Some patches have been applied
  43       which improve the situation for 3.1.
  44    </p><p>However, the C and C++ standard streams only need to be kept in sync
  45       when both libraries' facilities are in use.  If your program only uses
  46       C++ I/O, then there's no need to sync with the C streams.  The right
  47       thing to do in this case is to call
  48    </p><pre class="programlisting">
  49      #include <span class="emphasis"><em>any of the I/O headers such as ios, iostream, etc</em></span>
  50
  51      std::ios::sync_with_stdio(false);
  52    </pre><p>You must do this before performing any I/O via the C++ stream objects.
  53       Once you call this, the C++ streams will operate independently of the
  54       (unused) C streams.  For GCC 3.x, this means that <code class="code">cout</code> and
  55       company will become fully buffered on their own.
  56    </p><p>Note, by the way, that the synchronization requirement only applies to
  57       the standard streams (<code class="code">cin</code>, <code class="code">cout</code>,
  58       <code class="code">cerr</code>,
  59       <code class="code">clog</code>, and their wide-character counterchapters).  File stream
  60       objects that you declare yourself have no such requirement and are fully
  61       buffered.
  62    </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.
  63   Atomics
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