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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!-- but may not be published or retransmitted without explicit permission.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 cpp.sgml dated Wed Dec 21 13:52:14 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/cpp/ifendian.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:54 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 10.16</title><link href="iftypedef.html" rev=precedes><link href="ifdefsyntax.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="iftypedef.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="ifdefsyntax.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 10.16</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I use a preprocessor <TT>#if</TT> expression totellwhether a machine's byte orderis big-endian or little-endian?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>You probably can't.Theusual techniques for detecting endiannessinvolve pointers or arrays of <TT>char</TT>,or maybe unions,butpreprocessorarithmetic uses only long integers,andthere is no concept ofaddressing.Another tempting possibility is something like<pre> #if 'ABCD' == 0x41424344</pre>but this isn't reliable, either.At any rate,the integer formats used in preprocessor <TT>#if</TT> expressions are not necessarilythe same asthose that will be used at run time.</p><p>Are yousure you need to know the machine's endiannessexplicitly?Usually it's better to write code which doesn't care(see for example the code fragment in question <a href="../stdio/extconform.html">12.42</a>).See also question <a href="../misc/endiantest.html">20.9</a>.</p><p>References:ISO Sec. 6.8.1<br>H&S Sec. 7.11.1 p. 225<br></p><!-- aend --><p><hr><a href="iftypedef.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="ifdefsyntax.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/cpp/ifendian.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:54 GMT --></html>⌨️ 快捷键说明
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