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| 22 | <title>Directives (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Directives (GNU C Language Manual)">
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| 29 | <link href="index.html" rel="start" title="Top">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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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="Preprocessing-Tokens.html" rel="next" title="Preprocessing Tokens">
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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="Directives"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Preprocessing-Tokens.html" accesskey="n" rel="next">Preprocessing Tokens</a>, Previous: <a href="Preproc-Overview.html" accesskey="p" rel="prev">Preproc Overview</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="Directives-1"></span><h3 class="section">26.2 Directives</h3>
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| 64 | <span id="index-directives"></span>
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| 65 | <span id="index-preprocessing-directives"></span>
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| 66 | <span id="index-directive-line"></span>
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| 67 | <span id="index-directive-name"></span>
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| 68 |
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| 69 | <p><em>Preprocessing directives</em> are lines in the program that start
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| 70 | with ‘<samp>#</samp>’. Whitespace is allowed before and after the ‘<samp>#</samp>’.
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| 71 | The ‘<samp>#</samp>’ is followed by an identifier, the <em>directive name</em>.
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| 72 | It specifies the operation to perform. Here are a couple of examples:
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| 73 | </p>
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| 74 | <div class="example">
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| 75 | <pre class="example">#define LIMIT 51
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| 76 | # undef LIMIT
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| 77 | # error You screwed up!
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| 78 | </pre></div>
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| 79 |
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| 80 | <p>We usually refer to a directive as <code>#<var>name</var></code> where <var>name</var>
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| 81 | is the directive name. For example, <code>#define</code> means the
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| 82 | directive that defines a macro.
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| 83 | </p>
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| 84 | <p>The ‘<samp>#</samp>’ that begins a directive cannot come from a macro
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| 85 | expansion. Also, the directive name is not macro expanded. Thus, if
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| 86 | <code>foo</code> is defined as a macro expanding to <code>define</code>, that does
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| 87 | not make <code>#foo</code> a valid preprocessing directive.
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| 88 | </p>
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| 89 | <p>The set of valid directive names is fixed. Programs cannot define new
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| 90 | preprocessing directives.
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| 91 | </p>
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| 92 | <p>Some directives require arguments; these make up the rest of the
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| 93 | directive line and must be separated from the directive name by
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| 94 | whitespace. For example, <code>#define</code> must be followed by a macro
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| 95 | name and the intended expansion of the macro.
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| 96 | </p>
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| 97 | <p>A preprocessing directive cannot cover more than one line. The line
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| 98 | can, however, be continued with backslash-newline, or by a
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| 99 | ‘<samp>/*<span class="roman">…</span>*/</samp>’-style comment that extends past the end of the
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| 100 | line. These will be replaced (by nothing, or by whitespace) before
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| 101 | the directive is processed.
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| 102 | </p>
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| 103 |
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| 104 |
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| 105 |
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| 106 | </body>
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| 107 | </html>
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