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| 22 | <title>Header Files (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Preprocessing.html" rel="up" title="Preprocessing">
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| 33 | <link href="include-Syntax.html" rel="next" title="include Syntax">
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| 57 | <body lang="en">
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| 58 | <span id="Header-Files"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Macros.html" accesskey="n" rel="next">Macros</a>, Previous: <a href="Preprocessing-Tokens.html" accesskey="p" rel="prev">Preprocessing Tokens</a>, Up: <a href="Preprocessing.html" accesskey="u" rel="up">Preprocessing</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Header-Files-1"></span><h3 class="section">26.4 Header Files</h3>
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| 64 |
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| 65 | <span id="index-header-file"></span>
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| 66 | <p>A header file is a file of C code, typically containing C declarations
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| 67 | and macro definitions (see <a href="Macros.html">Macros</a>), to be shared between several
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| 68 | source files. You request the use of a header file in your program by
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| 69 | <em>including</em> it, with the C preprocessing directive
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| 70 | <code>#include</code>.
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| 71 | </p>
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| 72 | <p>Header files serve two purposes.
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| 73 | </p>
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| 74 | <ul>
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| 75 | <li> <span id="index-system-header-files"></span>
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| 76 | System header files declare the interfaces to parts of the operating
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| 77 | system. You include them in your program to supply the definitions and
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| 78 | declarations that you need to invoke system calls and libraries.
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| 79 |
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| 80 | </li><li> Program-specific header files contain declarations for interfaces between the
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| 81 | source files of a particular program. It is a good idea to create a header
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| 82 | file for related declarations and macro definitions if all or most of them
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| 83 | are needed in several different source files.
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| 84 | </li></ul>
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| 85 |
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| 86 | <p>Including a header file produces the same results as copying the header
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| 87 | file into each source file that needs it. Such copying would be
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| 88 | time-consuming and error-prone. With a header file, the related
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| 89 | declarations appear in only one place. If they need to be changed, you
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| 90 | can change them in one place, and programs that include the header file
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| 91 | will then automatically use the new version when next recompiled. The header
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| 92 | file eliminates the labor of finding and changing all the copies as well
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| 93 | as the risk that a failure to change one copy will result in
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| 94 | inconsistencies within a program.
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| 95 | </p>
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| 96 | <p>In C, the usual convention is to give header files names that end with
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| 97 | <samp>.h</samp>. It is most portable to use only letters, digits, dashes, and
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| 98 | underscores in header file names, and at most one dot.
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| 99 | </p>
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| 100 | <table class="menu" border="0" cellspacing="0">
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| 101 | <tr><td align="left" valign="top">• <a href="include-Syntax.html" accesskey="1">include Syntax</a></td><td> </td><td align="left" valign="top">
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| 102 | </td></tr>
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| 103 | <tr><td align="left" valign="top">• <a href="include-Operation.html" accesskey="2">include Operation</a></td><td> </td><td align="left" valign="top">
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| 104 | </td></tr>
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| 105 | <tr><td align="left" valign="top">• <a href="Search-Path.html" accesskey="3">Search Path</a></td><td> </td><td align="left" valign="top">
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| 106 | </td></tr>
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| 107 | <tr><td align="left" valign="top">• <a href="Once_002dOnly-Headers.html" accesskey="4">Once-Only Headers</a></td><td> </td><td align="left" valign="top">
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| 108 | </td></tr>
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| 109 | <tr><td align="left" valign="top">• <a href="Computed-Includes.html" accesskey="5">Computed Includes</a></td><td> </td><td align="left" valign="top">
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| 110 | </td></tr>
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| 114 | <div class="header">
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| 115 | <p>
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| 116 | Next: <a href="Macros.html" accesskey="n" rel="next">Macros</a>, Previous: <a href="Preprocessing-Tokens.html" accesskey="p" rel="prev">Preprocessing Tokens</a>, Up: <a href="Preprocessing.html" accesskey="u" rel="up">Preprocessing</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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