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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!-- but may not be published or retransmitted without explicit permission.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/macro.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 5.4</title><link href="ptrtest.html" rev=precedes><link href="machnon0.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="ptrtest.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="machnon0.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 5.4</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What is <TT>NULL</TT> and how is itdefined?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>As a matter of style,many programmers prefer not to haveunadorned <TT>0</TT>'s scattered through their programs,some representing numbersand some representing pointers.Therefore, the preprocessor macro <TT>NULL</TT> isdefined(byseveral headers, including<TT><stdio.h></TT>and<TT><stddef.h></TT>)as a null pointer constant,typically<TT>0</TT>or <TT>((void *)0)</TT>(see also question <a href="safermacs.html">5.6</a>).A programmer who wishes to make explicit the distinction between<TT>0</TT> the integer and <TT>0</TT> the null pointer constantcan then use <TT>NULL</TT> whenever anull pointer is required.</p><p>Using <TT>NULL</TT>is a stylistic convention only; the preprocessor turns <TT>NULL</TT>back into<TT>0</TT> which is then recognized by the compiler,in pointer contexts,as before.In particular, a cast may still be necessary before <TT>NULL</TT> (asbefore <TT>0</TT>) in a function call argument.The table under question<a href="null2.html">5.2</a> above applies for<TT>NULL</TT> as well as <TT>0</TT>(an unadorned <TT>NULL</TT>is equivalent to an unadorned 0).</p><p><TT>NULL</TT>should be used<em>only</em>as a pointer constant;see question<a href="nullor0.html">5.9</a>.</p><p>References:K&R1 Sec. 5.4 pp. 97-8<br>K&R2 Sec. 5.4 p. 102<br>ISO Sec. 7.1.6, Sec. 6.2.2.3<br>Rationale Sec. 4.1.5<br>H&S Sec. 5.3.2 p. 122, Sec. 11.1 p. 292<br></p><!-- aend --><p><hr><a href="ptrtest.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="machnon0.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/macro.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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