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| 22 | <title>Compilation (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Compilation (GNU C Language Manual)">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="index.html" rel="up" title="Top">
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| 33 | <link href="Directing-Compilation.html" rel="next" title="Directing Compilation">
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Compilation"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Directing-Compilation.html" accesskey="n" rel="next">Directing Compilation</a>, Previous: <a href="Floating-Point-in-Depth.html" accesskey="p" rel="prev">Floating Point in Depth</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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="Compilation-1"></span><h2 class="chapter">29 Compilation</h2>
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| 64 | <span id="index-object-file"></span>
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| 65 | <span id="index-compilation-module"></span>
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| 66 | <span id="index-make-rules"></span>
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| 67 |
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| 68 | <p>Early in the manual we explained how to compile a simple C program
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| 69 | that consists of a single source file (see <a href="Compile-Example.html">Compile Example</a>).
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| 70 | However, we handle only short programs that way. A typical C program
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| 71 | consists of many source files, each of which is a separate
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| 72 | <em>compilation module</em>—meaning that it has to be compiled
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| 73 | separately.
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| 74 | </p>
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| 75 | <p>The full details of how to compile with GCC are documented in xxxx.
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| 76 | Here we give only a simple introduction.
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| 77 | </p>
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| 78 | <p>These are the commands to compile two compilation modules,
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| 79 | <samp>foo.c</samp> and <samp>bar.c</samp>, with a command for each module:
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| 80 | </p>
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| 81 | <div class="example">
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| 82 | <pre class="example">gcc -c -O -g foo.c
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| 83 | gcc -c -O -g bar.c
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| 84 | </pre></div>
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| 85 |
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| 86 | <p>In these commands, <samp>-g</samp> says to generate debugging information,
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| 87 | <samp>-O</samp> says to do some optimization, and <samp>-c</samp> says to put
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| 88 | the compiled code for that module into a corresponding <em>object
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| 89 | file</em> and go no further. The object file for <samp>foo.c</samp> is called
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| 90 | <samp>foo.o</samp>, and so on.
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| 91 | </p>
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| 92 | <p>If you wish, you can specify the additional options <samp>-Wformat
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| 93 | -Wparenthesis -Wstrict-prototypes</samp>, which request additional warnings.
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| 94 | </p>
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| 95 | <p>One reason to divide a large program into multiple compilation modules
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| 96 | is to control how each module can access the internals of the others.
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| 97 | When a module declares a function or variable <code>extern</code>, other
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| 98 | modules can access it. The other functions and variables in
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| 99 | a module can’t be accessed from outside that module.
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| 100 | </p>
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| 101 | <p>The other reason for using multiple modules is so that changing
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| 102 | one source file does not require recompiling all of them in order
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| 103 | to try the modified program. Dividing a large program into many
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| 104 | substantial modules in this way typically makes recompilation much faster.
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| 105 | </p>
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| 106 | <span id="index-linking-object-files"></span>
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| 107 | <p>After you compile all the program’s modules, in order to run the
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| 108 | program you must <em>link</em> the object files into a combined
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| 109 | executable, like this:
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| 110 | </p>
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| 111 | <div class="example">
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| 112 | <pre class="example">gcc -o foo foo.o bar.o
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| 113 | </pre></div>
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| 114 |
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| 115 | <p>In this command, <samp>-o foo</samp> species the file name for the
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| 116 | executable file, and the other arguments are the object files to link.
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| 117 | Always specify the executable file name in a command that generates
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| 118 | one.
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| 119 | </p>
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| 120 | <p>Normally we don’t run any of these commands directly. Instead we
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| 121 | write a set of <em>make rules</em> for the program, then use the
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| 122 | <code>make</code> program to recompile only the source files that need to
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| 123 | be recompiled.
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| 124 | </p>
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| 125 |
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| 126 | <hr>
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| 127 | <div class="header">
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| 128 | <p>
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| 129 | Next: <a href="Directing-Compilation.html" accesskey="n" rel="next">Directing Compilation</a>, Previous: <a href="Floating-Point-in-Depth.html" accesskey="p" rel="prev">Floating Point in Depth</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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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