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| 22 | <title>Preproc Overview (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="Directives.html" rel="next" title="Directives">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Preproc-Overview"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Directives.html" accesskey="n" rel="next">Directives</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="Preprocessing-Overview"></span><h3 class="section">26.1 Preprocessing Overview</h3>
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| 64 |
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| 65 | <p>GNU C performs preprocessing on each line of a C program as the first
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| 66 | stage of compilation. Preprocessing operates on a line only when it
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| 67 | contains a <em>preprocessing directive</em> or uses a <em>macro</em>—all
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| 68 | other lines pass through preprocessing unchanged.
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| 69 | </p>
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| 70 | <p>Here are some jobs that preprocessing does. The rest of
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| 71 | this chapter gives the details.
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| 72 | </p>
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| 73 | <ul>
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| 74 | <li> Inclusion of header files. These are files (usually containing
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| 75 | declarations and macro definitions) that can be substituted into your
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| 76 | program.
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| 77 |
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| 78 | </li><li> Macro expansion. You can define <em>macros</em>, which are abbreviations
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| 79 | for arbitrary fragments of C code. Preprocessing replaces the macros
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| 80 | with their definitions. Some macros are automatically predefined.
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| 81 |
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| 82 | </li><li> Conditional compilation. You can include or exclude parts of the
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| 83 | program according to various conditions.
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| 84 |
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| 85 | </li><li> Line control. If you use a program to combine or rearrange source files
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| 86 | into an intermediate file that is then compiled, you can use line
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| 87 | control to inform the compiler where each source line originally came
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| 88 | from.
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| 89 |
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| 90 | </li><li> Compilation control. <code>#pragma</code> and <code>_Pragma</code> invoke
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| 91 | some special compiler features in how to handle certain constructs.
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| 92 |
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| 93 | </li><li> Diagnostics. You can detect problems at compile time and issue errors
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| 94 | or warnings.
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| 95 | </li></ul>
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| 96 |
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| 97 | <p>Except for expansion of predefined macros, all these operations happen
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| 98 | only if you use preprocessing directives to request them.
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| 99 | </p>
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| 100 |
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| 101 |
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| 102 |
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| 103 | </body>
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| 104 | </html>
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