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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!-- but may not be published or retransmitted without explicit permission.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/null2.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 5.2</title><link href="null1.html" rev=precedes><link href="ptrtest.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="null1.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="ptrtest.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 5.2</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How do I get a null pointer in my programs?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>With a <a href="../sx1/index.html#null pointer constant"><dfn>null pointer constant</dfn></a>.</p><p>According to the language definition,an``integral constant expression with the value 0''in a pointer contextis converted into a null pointer at compiletime.That is, in aninitialization,assignment, or comparisonwhen one side is avariable or expressionof pointer type, the compiler can tell that a constant <TT>0</TT>on the other side requests a null pointer,and generate the correctly-typed null pointer value.Therefore, the following fragments are perfectly legal:<pre> char *p = 0; if(p != 0)</pre>(See also question <a href="ptrtest.html">5.3</a>.)</p><p>However, an argument being passed to a function is notnecessarilyrecognizable asa pointer context,and the compilermay not be able to tell that an unadorned <TT>0</TT> ``means''a nullpointer.To generate a null pointer in a function call context, anexplicit castmay berequired,to force the <TT>0</TT> to berecognized as a pointer.For example, theUnixsystem call<TT>execl</TT>takes avariable-length, null-pointer-terminated list of character pointer arguments,and is correctly called like this:<pre> execl("/bin/sh", "sh", "-c", "date", (char *)0);</pre>If the <TT>(char *)</TT> caston the last argumentwere omitted,the compiler would not know to pass a null pointer,and would pass an integer <TT>0</TT> instead.(Note that manyUnixmanualsget this example wrong;see also question <a href="nullreq.html">5.11</a>.)</p><p>When function prototypes are in scope, argument passing becomesan ``assignment context,''and most casts may safely be omitted,since the prototype tells thecompiler that a pointer is required,and of which type,enabling itto correctlyconvert an unadorned <TT>0</TT>.Function prototypes cannot provide the types for variablearguments in variable-length argument listshowever,so explicit casts are still required for those arguments.(See alsoquestion <a href="../varargs/proto2.html">15.3</a>.)Itisprobablysafest toproperlycastall null pointerconstants in function calls,toguard against varargs functions or those without prototypes.</p><p>Here is a summary of the rulesfor whennull pointer constants may be used by themselves,and when they require explicit casts:<blockquote>[TABLE GOES HERE]</blockquote></p><p>References:K&R1 Sec. A7.7 p. 190, Sec. A7.14 p. 192<br>K&R2 Sec. A7.10 p. 207, Sec. A7.17 p. 209<br>ISO Sec. 6.2.2.3<br>H&S Sec. 4.6.3 p. 95, Sec. 6.2.7 p. 171<br></p><!-- aend --><p><hr><a href="null1.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="ptrtest.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/null2.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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