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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!-- but may not be published or retransmitted without explicit permission.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/nullreq.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 5.11</title><link href="macsochange.html" rev=precedes><link href="nullptrmacro.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="macsochange.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="nullptrmacro.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 5.11</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I once used a compiler that wouldn't work unless <TT>NULL</TT> wasused.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Unless the code being compiled was nonportable,thatcompilerwas probably broken.</p><p>Perhaps the code used something like this nonportable version of an example from question <a href="null2.html">5.2</a>:<pre> execl("/bin/sh", "sh", "-c", "date", NULL); /* WRONG */</pre>Under a compiler which defines <TT>NULL</TT> to <TT>((void *)0)</TT>(see question <a href="safermacs.html">5.6</a>),this code will happen to work.<a href="voidpcharp.html" rel=subdocument>[footnote]</a>However,if pointers and integers have different sizes or representations,the (equally incorrect) code<pre> execl("/bin/sh", "sh", "-c", "date", 0); /* WRONG */</pre>may not work.</p><p>Correct, portable code uses an explicit cast:<pre> execl("/bin/sh", "sh", "-c", "date", (char *)NULL);</pre>With the cast,the code works correctlyno matter what the machine's integer and pointer representations are,and no matter which form of null pointer constantthe compiler has chosen as the definition of <TT>NULL</TT>.(The code fragment in question <a href="null2.html">5.2</a>,which used 0 instead of <TT>NULL</TT>,is equally correct;see also question <a href="nullor0.html">5.9</a>.)(In general, making decisions about a language based on thebehavior of one particular compiler is likely to be counterproductive.)</p><!-- aend --><p><hr><a href="macsochange.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="nullptrmacro.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/nullreq.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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