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3 <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 V1.74.0" /><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" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.io.c.sync"></a>Performance</h2></div></div></div><p>
4 Pathetic Performance? Ditch C.
5 </p><p>It sounds like a flame on C, but it isn't. Really. Calm down.
6 I'm just saying it to get your attention.
7 </p><p>Because the C++ library includes the C library, both C-style and
8 C++-style I/O have to work at the same time. For example:
9 </p><pre class="programlisting">
10 #include <iostream>
11 #include <cstdio>
13 std::cout << "Hel";
14 std::printf ("lo, worl");
15 std::cout << "d!\n";
16 </pre><p>This must do what you think it does.
17 </p><p>Alert members of the audience will immediately notice that buffering
18 is going to make a hash of the output unless special steps are taken.
19 </p><p>The special steps taken by libstdc++, at least for version 3.0,
20 involve doing very little buffering for the standard streams, leaving
21 most of the buffering to the underlying C library. (This kind of
22 thing is tricky to get right.)
23 The upside is that correctness is ensured. The downside is that
24 writing through <code class="code">cout</code> can quite easily lead to awful
25 performance when the C++ I/O library is layered on top of the C I/O
26 library (as it is for 3.0 by default). Some patches have been applied
27 which improve the situation for 3.1.
28 </p><p>However, the C and C++ standard streams only need to be kept in sync
29 when both libraries' facilities are in use. If your program only uses
30 C++ I/O, then there's no need to sync with the C streams. The right
31 thing to do in this case is to call
32 </p><pre class="programlisting">
33 #include <span class="emphasis"><em>any of the I/O headers such as ios, iostream, etc</em></span>
35 std::ios::sync_with_stdio(false);
36 </pre><p>You must do this before performing any I/O via the C++ stream objects.
37 Once you call this, the C++ streams will operate independently of the
38 (unused) C streams. For GCC 3.x, this means that <code class="code">cout</code> and
39 company will become fully buffered on their own.
40 </p><p>Note, by the way, that the synchronization requirement only applies to
41 the standard streams (<code class="code">cin</code>, <code class="code">cout</code>,
42 <code class="code">cerr</code>,
43 <code class="code">clog</code>, and their wide-character counterparts). File stream
44 objects that you declare yourself have no such requirement and are fully
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