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| 22 | <title>Computed Includes (GNU C Language Manual)</title>
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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="Computed-Includes"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Once_002dOnly-Headers.html" accesskey="p" rel="prev">Once-Only Headers</a>, Up: <a href="Header-Files.html" accesskey="u" rel="up">Header Files</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="Computed-Includes-1"></span><h4 class="subsection">26.4.5 Computed Includes</h4>
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| 64 | <span id="index-computed-includes"></span>
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| 65 | <span id="index-macros-in-include"></span>
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| 66 |
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| 67 | <p>Sometimes it is necessary to select one of several different header
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| 68 | files to be included into your program. They might specify
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| 69 | configuration parameters to be used on different sorts of operating
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| 70 | systems, for instance. You could do this with a series of conditionals,
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">#if SYSTEM_1
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| 74 | # include "system_1.h"
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| 75 | #elif SYSTEM_2
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| 76 | # include "system_2.h"
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| 77 | #elif SYSTEM_3
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| 78 | /* <span class="roman">…</span> */
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| 79 | #endif
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| 80 | </pre></div>
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| 81 |
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| 82 | <p>That rapidly becomes tedious. Instead, GNU C offers the ability to use
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| 83 | a macro for the header name. This is called a <em>computed include</em>.
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| 84 | Instead of writing a header name as the direct argument of
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| 85 | <code>#include</code>, you simply put a macro name there instead:
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">#define SYSTEM_H "system_1.h"
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| 89 | /* <span class="roman">…</span> */
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| 90 | #include SYSTEM_H
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| 91 | </pre></div>
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| 92 |
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| 93 | <p><code>SYSTEM_H</code> is expanded, then <samp>system_1.h</samp> is included as if
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| 94 | the <code>#include</code> had been written with that name. <code>SYSTEM_H</code>
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| 95 | could be defined by your Makefile with a <samp>-D</samp> option.
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| 96 | </p>
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| 97 | <p>You must be careful when you define such a macro. <code>#define</code>
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| 98 | saves tokens, not text. GCC has no way of knowing that the macro will
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| 99 | be used as the argument of <code>#include</code>, so it generates ordinary
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| 100 | tokens, not a header name. This is unlikely to cause problems if you
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| 101 | use double-quote includes, which are syntactically similar to string
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| 102 | constants. If you use angle brackets, however, you may have trouble.
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| 103 | </p>
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| 104 | <p>The syntax of a computed include is actually a bit more general than the
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| 105 | above. If the first non-whitespace character after <code>#include</code> is
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| 106 | not ‘<samp>"</samp>’ or ‘<samp><</samp>’, then the entire line is macro-expanded
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| 107 | like running text would be.
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| 108 | </p>
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| 109 | <p>If the line expands to a single string constant, the contents of that
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| 110 | string constant are the file to be included. Preprocessing does not
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| 111 | re-examine the string for embedded quotes, but neither does it process
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| 112 | backslash escapes in the string. Therefore
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| 113 | </p>
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| 114 | <div class="example">
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| 115 | <pre class="example">#define HEADER "a\"b"
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| 116 | #include HEADER
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| 117 | </pre></div>
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| 118 |
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| 119 | <p>looks for a file named <samp>a\"b</samp>. Preprocessing searches for the
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| 120 | file according to the rules for double-quoted includes.
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| 121 | </p>
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| 122 | <p>If the line expands to a token stream beginning with a ‘<samp><</samp>’ token
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| 123 | and including a ‘<samp>></samp>’ token, then the tokens between the ‘<samp><</samp>’ and
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| 124 | the first ‘<samp>></samp>’ are combined to form the filename to be included.
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| 125 | Any whitespace between tokens is reduced to a single space; then any
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| 126 | space after the initial ‘<samp><</samp>’ is retained, but a trailing space
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| 127 | before the closing ‘<samp>></samp>’ is ignored. Preprocessing searches for the file
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| 128 | according to the rules for angle-bracket includes.
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| 129 | </p>
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| 130 | <p>In either case, if there are any tokens on the line after the file name,
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| 131 | an error occurs and the directive is not processed. It is also an error
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| 132 | if the result of expansion does not match either of the two expected
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| 133 | forms.
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| 134 | </p>
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| 135 | <p>These rules are implementation-defined behavior according to the C
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| 136 | standard. To minimize the risk of different compilers interpreting your
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| 137 | computed includes differently, we recommend you use only a single
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| 138 | object-like macro that expands to a string constant. That also
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| 139 | makes it clear to people reading your program.
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| 140 | </p>
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| 141 | <hr>
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| 142 | <div class="header">
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| 143 | <p>
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| 144 | Previous: <a href="Once_002dOnly-Headers.html" accesskey="p" rel="prev">Once-Only Headers</a>, Up: <a href="Header-Files.html" accesskey="u" rel="up">Header Files</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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