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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!-- but may not be published or retransmitted without explicit permission.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/null1.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 5.1</title><link href="../ptrs/nearfar2.html" rev=precedes><link href="null2.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="../ptrs/nearfar2.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="null2.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 5.1</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What is this infamous null pointer, anyway?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The language definition states that foreach pointer type,thereis a special value--the ``null pointer''--which isdistinguishable from allother pointer values andwhich is``guaranteed to compare unequal to a pointer toany object or function.''That is,a null pointer points definitively nowhere;it is not the address ofany object or function.Theaddress-of operator <TT>&</TT> will never yield a null pointer,nor will a successful call to<TT>malloc</TT>.<a href="nullsuccess.html" rel=subdocument>[footnote]</a>(<TT>malloc</TT>does returna null pointer when it fails,and this is a typical use of null pointers:as a ``special'' pointer value with some other meaning,usually ``not allocated'' or ``not pointing anywhere yet.'')</p><p>A null pointer is conceptually different from an uninitialized pointer.A null pointer is known not to point to any objector function;an uninitialized pointer might point anywhere.See also questions<a href="../decl/initval.html">1.30</a>,<a href="../malloc/malloc1.html">7.1</a>,and<a href="../malloc/calloc.html">7.31</a>.</p><p>As mentioned above,there isa null pointer for each pointer type,and the internal values of null pointers fordifferent typesmay be different.Although programmers need not know the internal values, thecompiler must always be informed which type of null pointer is required,so that it can make the distinction if necessary(seequestions<a href="null2.html">5.2</a>,<a href="machnon0.html">5.5</a>,and<a href="safermacs.html">5.6</a>).</p><p>References:K&R1 Sec. 5.4 pp. 97-8<br>K&R2 Sec. 5.4 p. 102<br>ISO Sec. 6.2.2.3<br>Rationale Sec. 3.2.2.3<br>H&S Sec. 5.3.2 pp. 121-3<br></p><!-- aend --><p><hr><a href="../ptrs/nearfar2.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="null2.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/null1.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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