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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 V1.73.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt11ch28.html" title="Chapter 28. Interacting with C" /><link rel="prev" href="bk01pt11ch28.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="bk01pt11ch28.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> |
| Pathetic Performance? Ditch C. |
| </p><p>It sounds like a flame on C, but it isn't. Really. Calm down. |
| I'm just saying it to get your attention. |
| </p><p>Because the C++ library includes the C library, both C-style and |
| C++-style I/O have to work at the same time. For example: |
| </p><pre class="programlisting"> |
| #include <iostream> |
| #include <cstdio> |
| |
| std::cout << "Hel"; |
| std::printf ("lo, worl"); |
| std::cout << "d!\n"; |
| </pre><p>This must do what you think it does. |
| </p><p>Alert members of the audience will immediately notice that buffering |
| is going to make a hash of the output unless special steps are taken. |
| </p><p>The special steps taken by libstdc++, at least for version 3.0, |
| involve doing very little buffering for the standard streams, leaving |
| most of the buffering to the underlying C library. (This kind of |
| thing is tricky to get right.) |
| The upside is that correctness is ensured. The downside is that |
| writing through <code class="code">cout</code> can quite easily lead to awful |
| performance when the C++ I/O library is layered on top of the C I/O |
| library (as it is for 3.0 by default). Some patches have been applied |
| which improve the situation for 3.1. |
| </p><p>However, the C and C++ standard streams only need to be kept in sync |
| when both libraries' facilities are in use. If your program only uses |
| C++ I/O, then there's no need to sync with the C streams. The right |
| thing to do in this case is to call |
| </p><pre class="programlisting"> |
| #include <span class="emphasis"><em>any of the I/O headers such as ios, iostream, etc</em></span> |
| |
| std::ios::sync_with_stdio(false); |
| </pre><p>You must do this before performing any I/O via the C++ stream objects. |
| Once you call this, the C++ streams will operate independently of the |
| (unused) C streams. For GCC 3.x, this means that <code class="code">cout</code> and |
| company will become fully buffered on their own. |
| </p><p>Note, by the way, that the synchronization requirement only applies to |
| the standard streams (<code class="code">cin</code>, <code class="code">cout</code>, |
| <code class="code">cerr</code>, |
| <code class="code">clog</code>, and their wide-character counterparts). File stream |
| objects that you declare yourself have no such requirement and are fully |
| buffered. |
| </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt11ch28.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt11ch28.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. Extensions</td></tr></table></div></body></html> |
