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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:46 2005 by faqproc version 2.7 --><!-- from source file stdio.sgml dated Wed Dec 21 13:07:57 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/stdio/extconform.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:58 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 12.42</title><link href="structio.html" rev=precedes><link href="runtimesc2.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="structio.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="runtimesc2.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 12.42</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I write code to conform to these old, binary data file formats?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>It's hard,because of word size and byte order differences,floating-point formats,andstructure padding.Toget the control you need overtheseparticulars,you may have to read and writethingsa byte at a time,shuffling and rearranging as you go.(This isn't always as bad as it sounds,and gives you both portabilityof your codeand complete control.)</p><p>For example, to reada data structureconsisting of a character, a 32-bit integer, and a 16-bit integer,from the stream <TT>fp</TT>,into the C structure<pre>struct mystruct { char c; long int i32; int i16;} s;</pre>you might usecode like this:<pre> s.c = getc(fp); s.i32 = (long)getc(fp) << 24; s.i32 |= (long)getc(fp) << 16; s.i32 |= (unsigned)(getc(fp) << 8); s.i32 |= getc(fp); s.i16 = getc(fp) << 8; s.i16 |= getc(fp);</pre>This code assumes that <TT>getc</TT> reads8-bit characters,and that the data is storedmost significant byte first(``big endian'').The casts to <TT>(long)</TT> ensure that the 16- and 24-bit shiftsoperate on <TT>long</TT> values(see question <a href="../expr/intoverflow1.html">3.14</a>),and the cast to <TT>(unsigned)</TT> guards against sign extension.(In general,it's safer to use all <TT>unsigned</TT>types when writing code like this,but see question <a href="../expr/preservingrules.html">3.19</a>.)</p><p>The corresponding code to write the structure might look like:<pre> putc(s.c, fp); putc((unsigned)((s.i32 >> 24) & 0xff), fp); putc((unsigned)((s.i32 >> 16) & 0xff), fp); putc((unsigned)((s.i32 >> 8) & 0xff), fp); putc((unsigned)(s.i32 & 0xff), fp); putc((s.i16 >> 8) & 0xff, fp); putc(s.i16 & 0xff, fp);</pre></p><p>See also questions<a href="../struct/padding.html">2.12</a>,<a href="fopenbinary.html">12.38</a>,<a href="../strangeprob/ptralign.html">16.7</a>,and<a href="../misc/binaryfiles.html">20.5</a>.</p><!-- aend --><p><hr><a href="structio.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="runtimesc2.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/stdio/extconform.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:58 GMT --></html>⌨️ 快捷键说明
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