<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><title>Interacting with C</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , library "/><link rel="home" href="../spine.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><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Interacting with C</th></tr><tr><td align="left"><a accesskey="p" href="fstreams.html">Prev</a> </td><th width="60%" align="center">Chapter 13. Input and Output </th><td 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"><a id="std.io.c"/>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 id="std.io.c.FILE"/>Using FILE* and file descriptors</h3></div></div></div><p> See the <a class="link" href="ext_io.html" title="Chapter 26. Input and Output">extensions</a> for using <span class="type">FILE</span> and <span class="type">file descriptors</span> with <code class="classname">ofstream</code> and <code class="classname">ifstream</code>. </p></div><div class="section" title="Performance"><div class="titlepage"><div><div><h3 class="title"><a id="std.io.c.sync"/>Performance</h3></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 counterchapters). File stream objects that you declare yourself have no such requirement and are fully buffered. </p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="fstreams.html">Prev</a> </td><td align="center"><a accesskey="u" href="io.html">Up</a></td><td align="right"> <a accesskey="n" href="atomics.html">Next</a></td></tr><tr><td align="left" valign="top">File Based Streams </td><td align="center"><a accesskey="h" href="../spine.html">Home</a></td><td align="right" valign="top"> Chapter 14. Atomics </td></tr></table></div></body></html>