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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!-- but may not be published or retransmitted without explicit permission.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/safermacs.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 5.6</title><link href="machnon0.html" rev=precedes><link href="long0.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="machnon0.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="long0.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 5.6</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>If <TT>NULL</TT> were defined asfollows:<pre> #define NULL ((char *)0)</pre>wouldn't that make function calls which pass an uncast <TT>NULL</TT> work?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Not inthe most general case.Thecomplicationis that there are machines whichuse differentinternalrepresentations forpointers to different types of data.The suggested definition would make uncast <TT>NULL</TT> arguments tofunctions expecting pointers to characters work correctly,but pointer arguments of other types could still(in the absence of prototypes)require explicit casts.Furthermore,legalconstructions such as<pre> FILE *fp = NULL;</pre>could fail.</p><p>Nevertheless,ANSI C allows the alternatedefinition<pre> #define NULL ((void *)0)</pre>for <TT>NULL</TT>.<a href="voidpassign.html" rel=subdocument>[footnote]</a>Besidespotentiallyhelpingincorrect programs towork(but only on machines with homogeneous pointers,thusquestionably valid assistance),this definition may catch programs which use <TT>NULL</TT> incorrectly(e.g. when the ASCII NUL character was really intended;see question<a href="nullor0.html">5.9</a>).See also question <a href="long0.html">5.7</a>.</p><p>At any rate, ANSI function prototypes ensure that most(though not quite all; see question <a href="null2.html">5.2</a>)pointer arguments are converted correctlywhen passed as function arguments,so the question is largely moot.</p><p>Programmers who are accustomed tomodern, ``flat'' memory architectures mayfind the idea of ``different kinds of pointers''very difficult to accept.See question<a href="machexamp.html">5.17</a>for someexamples.</p><p>References:Rationale Sec. 4.1.5<br></p><!-- aend --><p><hr><a href="machnon0.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="long0.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/safermacs.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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