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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!-- but may not be published or retransmitted without explicit permission.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 decl.sgml dated Wed Dec 21 12:56:18 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/decl/decldef.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:41 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 1.7</title><link href="mimic.html" rev=precedes><link href="opaque.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="mimic.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="opaque.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 1.7</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What's the best way to declare and defineglobal variablesand functions?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>First, though there can be many<a href="../sx1/index.html#declarations"><dfn>declarations</dfn></a>(and in many translation units)of a singleglobalvariableor function,there must be exactly one<a href="../sx1/index.html#definition"><dfn>definition</dfn></a>.<a href="unused.html" rel=subdocument>[footnote]</a>For global variables,thedefinition is the declaration that actually allocates space,and provides an initialization value, if any.For functions,the definition isthe ``declaration''that provides the function body.For example,these are declarations:<pre> extern int i; extern int f();</pre>and these are definitions:<pre> int i = 0; int f() { return 1; }</pre>(Actually, the keyword <TT>extern</TT> is optional in function declarations;see question <a href="extern.html">1.11</a>.)</p><p>When you need to share variables or functionsacross severalsource files,you will of course want to ensurethat all definitions and declarations are consistent.The best arrangementis toplace eachdefinition in some relevant.c file.Then,putan external declaration in aheader(``<TT>.h</TT>'')file,and <TT>#include</TT> itwherever the declaration is needed.The <TT>.c</TT> file containing the definition should also <TT>#include</TT> thesame header file,so the compiler can checkthat the definition matches the declarations.</p><p>This rule promotes a high degree of portability:itis consistent with the requirements of the ANSI C Standard,and is also consistent with most pre-ANSI compilers and linkers.(Unixcompilers and linkers typically use a ``common model''which allows multiple definitions,as long as at most one is initialized;thisbehavior is mentioned as a ``common extension'' bythe ANSI Standard,no<a href="common.html">pun</a>intended.A fewvery oldsystemsmight once have requiredan explicit initializer todistinguish a definition from an external declaration.)</p><p>It is possible to use preprocessor tricksto arrange thata line like<pre> DEFINE(int, i);</pre>need only be entered once in one header file,and turned into a definition or a declarationdepending on the setting of some macro,but it's not clear ifthis isworth the trouble,especially since it's usually abetterideato keep global variables to a minimum.</p><p>It'snot just a good ideato put global declarations in header files:if you want the compilertobe able tocatch inconsistent declarations for you,you <em>must</em> place them in header files.In particular,never place a prototype for an external functionin a .c file--ifthe definition of the function ever changes,it would be too easy to forget to change the prototype,and an incompatibleprototype is worse than useless.</p><p>See alsoquestions <a href="extarraysize.html">1.24</a>, <a href="../cpp/hfiles.html">10.6</a>, <a href="../style/srcfiles.html">17.2</a>, and <a href="../resources/lintvsansi.html">18.8</a>.</p><p>References:K&R1 Sec. 4.5 pp. 76-7<br>K&R2 Sec. 4.4 pp. 80-1<br>ISO Sec. 6.1.2.2, Sec. 6.7, Sec. 6.7.2, Sec. G.5.11<br>Rationale Sec. 3.1.2.2<br>H&S Sec. 4.8 pp. 101-104, Sec. 9.2.3 p. 267<br>CT&P Sec. 4.2 pp. 54-56<br></p><!-- aend --><p><hr><a href="mimic.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="opaque.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/decl/decldef.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:43 GMT --></html>⌨️ 快捷键说明
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