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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!-- but may not be published or retransmitted without explicit permission.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/varieties.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 5.13</title><link href="nullptrmacro.html" rev=precedes><link href="confusion.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="nullptrmacro.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="confusion.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 5.13</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>This is strange.<TT>NULL</TT> is guaranteed to be <TT>0</TT>, but the null pointer is not?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>When the term ``null'' or ``<TT>NULL</TT>'' is casually used,one of several things may be meant:<OL><li>The conceptual null pointer,the abstract languageconceptdefined in question<a href="null1.html">5.1</a>.It is implemented with...<li>The internal (or run-time) representation of a null pointer,which may or may not be all-bits-0andwhichmay bedifferent for different pointer types.The actual values should be of concern only to compiler writers.Authors of C programs never see them,since theyuse...<li>The null pointer constant,which is a constant integer 0<a href="fn22.html" rel=subdocument>[footnote]</a>(see question<a href="null2.html">5.2</a>).It is often hidden behind...<li>The <TT>NULL</TT> macro,which is <TT>#define</TT>d to be <TT>0</TT>(see question <a href="macro.html">5.4</a>).Finally,as red herrings,we have...<li>The ASCII null character (NUL),which does have all bits zero,but has nonecessaryrelation to the null pointer except in name;and...<li>The ``null string,''which is another name for the empty string (<TT>""</TT>).Usingtheterm ``null string'' can be confusing in C,becausean empty stringinvolves a null (<TT>'\0'</TT>) character,but <em>not</em> a null pointer,which brings us full circle...</OL></p><p>In other words, to paraphrasethe White Knight's description of his song in<I>Through the Looking-Glass</I>,the name of the null pointer is ``<TT>0</TT>'',but the name of thenull pointer is called ``<TT>NULL</TT>''(and we're not sure what the null pointer<em>is</em>).</p><p>Thisdocumentuses the phrase ``null pointer''(in lower case)for sense 1,the token ``<TT>0</TT>''or the phrase ``null pointer constant''for sense 3,and the capitalized word ``<TT>NULL</TT>''for sense 4.<a href="precise.html" rel=subdocument>[footnote]</a></p><p>Additional links:<a href="bull.html" rel=subdocument>mnemonic device</a></p><p>References:H&S Sec. 1.3 p. 325<br><I>Through the Looking-Glass</I>,chapter VIII.<br></p><!-- aend --><p><hr><a href="nullptrmacro.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="confusion.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/varieties.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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