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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!-- but may not be published or retransmitted without explicit permission.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/nullor0.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 5.9</title><link href="fcnptr.html" rev=precedes><link href="macsochange.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="fcnptr.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="macsochange.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 5.9</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>If <TT>NULL</TT> and <TT>0</TT> are equivalentas null pointer constants,which should I use?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Many programmers believe that <TT>NULL</TT>should be used in all pointer contexts,as a reminder that the value is to be thought of asa pointer.Others feel that the confusion surrounding <TT>NULL</TT>and <TT>0</TT> is only compoundedby hiding <TT>0</TT> behind a macro,and prefer to use unadorned <TT>0</TT> instead.There is no one right answer.(See also questions<a href="../bool/macros.html">9.4</a>and<a href="../style/stylewars.html">17.10</a>.)C programmers must understand that<TT>NULL</TT> and <TT>0</TT> are interchangeablein pointer contexts,and that an uncast <TT>0</TT> isperfectly acceptable.Any usage of <TT>NULL</TT>(as opposed to <TT>0</TT>) shouldbe considereda gentle reminderthat a pointer is involved;programmers should not depend on it(either for their own understanding or the compiler's)for distinguishing pointer <TT>0</TT>'s from integer <TT>0</TT>'s.</p><p>It is only in pointer contexts that <TT>NULL</TT> and 0 are equivalent.<TT>NULL</TT> should<em>not</em>be used when another kind of <TT>0</TT> is required,even though it might work,because doing so sends the wrong stylistic message.(Furthermore,ANSI allows the definition of <TT>NULL</TT> to be <TT>((void *)0)</TT>,which will not workat allin non-pointer contexts.)In particular, do not use <TT>NULL</TT> when the ASCIInull character(NUL)is desired.Provide your own definition<pre> #define NUL '\0'</pre>if you must.</p><p>References:K&R1 Sec. 5.4 pp. 97-8<br>K&R2 Sec. 5.4 p. 102<br></p><!-- aend --><p><hr><a href="fcnptr.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="macsochange.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/nullor0.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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