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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!-- but may not be published or retransmitted without explicit permission.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 21:47:45 2005 by faqproc version 2.7 --><!-- from source file null.sgml dated Wed Dec 21 13:03:25 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/null/machnon0.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:48 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 5.5</title><link href="macro.html" rev=precedes><link href="safermacs.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="macro.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="safermacs.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 5.5</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How should <TT>NULL</TT> be defined on a machine which uses a nonzero bitpattern as the internal representation of a null pointer?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The same asonany other machine: as <TT>0</TT>(or some version of <TT>0</TT>; see question <a href="macro.html">5.4</a>).</p><p>Whenevera programmer requests a null pointer,eitherby writing ``<TT>0</TT>'' or ``<TT>NULL</TT>'',it is the compiler'sresponsibilityto generatewhatever bit pattern the machine uses for that null pointer.(Again, the compiler can tell that an unadorned<TT>0</TT> requests a nullpointer when the <TT>0</TT> is in a pointer context;see question <a href="null2.html">5.2</a>.)Therefore, #defining <TT>NULL</TT> as <TT>0</TT> on a machine for which internalnull pointers are nonzero is as valid as on any other:the compiler mustalwaysbe able togenerate the machine's correctnull pointers in response tounadorned <TT>0</TT>'s seen in pointer contexts.A constant <TT>0</TT> is a null pointer constant;<TT>NULL</TT> is just a convenient name for it(see also question <a href="varieties.html">5.13</a>).</p><p>(Section 4.1.5 of the C Standard states that <TT>NULL</TT>``expands to an implementation-defined null pointer constant,''which means that the implementation gets to choosewhich form of <TT>0</TT> to useand whether touse a <TT>void *</TT> cast;see questions <a href="safermacs.html">5.6</a> and <a href="long0.html">5.7</a>.``Implementation-defined'' here does <em>not</em> meanthat <TT>NULL</TT> might be <TT>#define</TT>d to match someimplementation-specific nonzero internal null pointer value.)</p><p>See also questions<a href="null2.html">5.2</a>,<a href="macsochange.html">5.10</a>and <a href="machexamp.html">5.17</a>.</p><p>References:ISO Sec. 7.1.6<br>Rationale Sec. 4.1.5<br></p><!-- aend --><p><hr><a href="macro.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="safermacs.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/null/machnon0.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:48 GMT --></html>⌨️ 快捷键说明
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