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jeremybenn |
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Performance</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="io_and_c.html" title="Chapter 28. Interacting with C" /><link rel="prev" href="io_and_c.html" title="Chapter 28. Interacting with C" /><link rel="next" href="extensions.html" title="Part XII. Extensions" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Performance</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="io_and_c.html">Prev</a> </td><th width="60%" align="center">Chapter 28. Interacting with C</th><td width="20%" align="right"> <a accesskey="n" href="extensions.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Performance"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.io.c.sync"></a>Performance</h2></div></div></div><p>
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Pathetic Performance? Ditch C.
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</p><p>It sounds like a flame on C, but it isn't. Really. Calm down.
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I'm just saying it to get your attention.
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</p><p>Because the C++ library includes the C library, both C-style and
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C++-style I/O have to work at the same time. For example:
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</p><pre class="programlisting">
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#include <iostream>
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#include <cstdio>
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std::cout << "Hel";
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std::printf ("lo, worl");
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std::cout << "d!\n";
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</pre><p>This must do what you think it does.
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</p><p>Alert members of the audience will immediately notice that buffering
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is going to make a hash of the output unless special steps are taken.
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</p><p>The special steps taken by libstdc++, at least for version 3.0,
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involve doing very little buffering for the standard streams, leaving
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most of the buffering to the underlying C library. (This kind of
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thing is tricky to get right.)
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The upside is that correctness is ensured. The downside is that
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writing through <code class="code">cout</code> can quite easily lead to awful
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performance when the C++ I/O library is layered on top of the C I/O
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library (as it is for 3.0 by default). Some patches have been applied
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which improve the situation for 3.1.
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</p><p>However, the C and C++ standard streams only need to be kept in sync
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when both libraries' facilities are in use. If your program only uses
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C++ I/O, then there's no need to sync with the C streams. The right
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thing to do in this case is to call
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</p><pre class="programlisting">
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#include <span class="emphasis"><em>any of the I/O headers such as ios, iostream, etc</em></span>
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std::ios::sync_with_stdio(false);
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</pre><p>You must do this before performing any I/O via the C++ stream objects.
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Once you call this, the C++ streams will operate independently of the
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(unused) C streams. For GCC 3.x, this means that <code class="code">cout</code> and
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company will become fully buffered on their own.
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</p><p>Note, by the way, that the synchronization requirement only applies to
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the standard streams (<code class="code">cin</code>, <code class="code">cout</code>,
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<code class="code">cerr</code>,
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<code class="code">clog</code>, and their wide-character counterparts). File stream
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objects that you declare yourself have no such requirement and are fully
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buffered.
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</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="io_and_c.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="io_and_c.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="extensions.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 28. Interacting with C </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Part XII.
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Extensions
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</td></tr></table></div></body></html>
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