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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!-- but may not be published or retransmitted without explicit permission.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 21:47:46 2005 by faqproc version 2.7 --><!-- from source file cpp.sgml dated Wed Dec 21 13:52:14 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/cpp/hfiles.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:54 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 10.6</title><link href="constvsdefine.html" rev=precedes><link href="nestincl.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="constvsdefine.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="nestincl.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 10.6</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I'm splitting up a programinto multiple source filesfor the first time,and I'm wondering what to put in <TT>.c</TT> filesand what to put in <TT>.h</TT> files.(What does ``<TT>.h</TT>''mean,anyway?)</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>As a general rule, you should put these things in header(<TT>.h</TT>) files:<blockquote><pre>macro definitions (preprocessor <TT>#define</TT>s)structure, union, and enumeration declarationstypedef declarationsexternal function declarations (see also question <a href="../decl/extern.html">1.11</a>)global variable declarations</pre></blockquote></p><p>It's especially important to put a declaration or definition in a header file when it will be shared between several other files.Don't repeat a declaration or macro definition at the top of two or more source files;do put it in a header file and <TT>#include</TT> it wherever needed.The reason is not just to save typing:you want to set things up so that whenever a declaration or definition changes,you only have to change it in one place,with the update propagated toall source filesconsistently.(In particular, never put external functionprototypes in <TT>.c</TT> files.See also question <a href="../decl/decldef.html">1.7</a>.)</p><p>On the other hand,when a definition or declaration should remain private to one<TT>.c</TT>file, it'sfinetoleave it there.(Private file-scope functions andvariablesshouldalsobe declared<TT>static</TT>.See also question <a href="../struct/opaquetypes.html">2.4</a>.)</p><p>Finally,you should not put actual code(i.e. functionbodies)or global variable definitions(that is, definingor initializinginstances)in header files.Also,when you are building a projectout of multiple source files,you should compile each of them separately(using a compiler optionto request compilation only),and use the linkerto link the resultant object files together.(In an integrated development environment,all of this may be taken care of for you.)Don't try to ``link'' all of your source files togetherwith <TT>#include</TT>;the <TT>#include</TT> directive should be usedto pull inheader files,not other <TT>.c</TT> files.</p><p>See alsoquestions <a href="../decl/decldef.html">1.7</a>, <a href="nestincl.html">10.7</a>, and <a href="../style/srcfiles.html">17.2</a>.</p><p>Additional links:<a href="sd7.html" rel=subdocument>more explanation</a></p><p>References:K&R2 Sec. 4.5 pp. 81-2<br>H&S Sec. 9.2.3 p. 267<br>CT&P Sec. 4.6 pp. 66-7<br></p><!-- aend --><p><hr><a href="constvsdefine.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="nestincl.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 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