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| 22 | <title>Preprocessing Tokens (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Preprocessing Tokens (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="Preprocessing.html" rel="up" title="Preprocessing">
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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="Preprocessing-Tokens"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Header-Files.html" accesskey="n" rel="next">Header Files</a>, Previous: <a href="Directives.html" accesskey="p" rel="prev">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-Tokens-1"></span><h3 class="section">26.3 Preprocessing Tokens</h3>
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| 64 |
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| 65 | <span id="index-preprocessing-tokens"></span>
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| 66 | <p>Preprocessing divides C code (minus its comments) into
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| 67 | <em>tokens</em> that are similar to C tokens, but not exactly the same.
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| 68 | Here are the quirks of preprocessing tokens.
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| 69 | </p>
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| 70 | <p>The main classes of preprocessing tokens are identifiers,
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| 71 | preprocessing numbers, string constants, character constants, and
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| 72 | punctuators; there are a few others too.
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| 73 | </p>
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| 74 | <dl compact="compact">
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| 75 | <dt>identifier</dt>
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| 76 | <dd><span id="index-identifiers-1"></span>
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| 77 | <p>An <em>identifier</em> preprocessing token is syntactically like an
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| 78 | identifier in C: any sequence of letters, digits, or underscores, as
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| 79 | well as non-ASCII characters represented using ‘<samp>\U</samp>’ or ‘<samp>\u</samp>’,
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| 80 | that doesn’t begin with a digit.
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| 81 | </p>
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| 82 | <p>During preprocessing, the keywords of C have no special significance;
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| 83 | at that stage, they are simply identifiers. Thus, you can define a
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| 84 | macro whose name is a keyword. The only identifier that is special
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| 85 | during preprocessing is <code>defined</code> (see <a href="defined.html">defined</a>).
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| 86 | </p>
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| 87 | </dd>
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| 88 | <dt>preprocessing number</dt>
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| 89 | <dd><span id="index-numbers_002c-preprocessing"></span>
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| 90 | <span id="index-preprocessing-numbers"></span>
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| 91 | <p>A <em>preprocessing number</em> is something that preprocessing treats
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| 92 | textually as a number, including C numeric constants, and other
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| 93 | sequences of characters which resemble numeric constants.
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| 94 | Preprocessing does not try to verify that a preprocessing number is a
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| 95 | valid number in C, and indeed it need not be one.
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| 96 | </p>
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| 97 | <p>More precisely, preprocessing numbers begin with an optional period, a
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| 98 | required decimal digit, and then continue with any sequence of
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| 99 | letters, digits, underscores, periods, and exponents. Exponents are
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| 100 | the two-character sequences ‘<samp>e+</samp>’, ‘<samp>e-</samp>’, ‘<samp>E+</samp>’,
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| 101 | ‘<samp>E-</samp>’, ‘<samp>p+</samp>’, ‘<samp>p-</samp>’, ‘<samp>P+</samp>’, and ‘<samp>P-</samp>’. (The
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| 102 | exponents that begin with ‘<samp>p</samp>’ or ‘<samp>P</samp>’ are new to C99. They
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| 103 | are used for hexadecimal floating-point constants.)
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| 104 | </p>
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| 105 | <p>The reason behind this unusual syntactic class is that the full
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| 106 | complexity of numeric constants is irrelevant during preprocessing.
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| 107 | The distinction between lexically valid and invalid floating-point
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| 108 | numbers, for example, doesn’t matter at this stage. The use of
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| 109 | preprocessing numbers makes it possible to split an identifier at any
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| 110 | position and get exactly two tokens, and reliably paste them together
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| 111 | using the <code>##</code> operator (see <a href="Concatenation.html">Concatenation</a>).
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| 112 | </p>
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| 113 | </dd>
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| 114 | <dt>punctuator</dt>
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| 115 | <dd><p>A <em>punctuator</em> is syntactically like an operator.
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| 116 | These are the valid punctuators:
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| 117 | </p>
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| 118 | <div class="example">
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| 119 | <pre class="example">[ ] ( ) { } . ->
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| 120 | ++ -- & * + - ~ !
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| 121 | / % << >> < > <= >= == != ^ | && ||
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| 122 | ? : ; ...
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| 123 | = *= /= %= += -= <<= >>= &= ^= |=
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| 124 | , # ##
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| 125 | <: :> <% %> %: %:%:
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| 126 | </pre></div>
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| 127 |
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| 128 | </dd>
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| 129 | <dt>string constant</dt>
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| 130 | <dd><p>A string constant in the source code is recognized by preprocessing as
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| 131 | a single preprocessing token.
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| 132 | </p>
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| 133 | </dd>
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| 134 | <dt>character constant</dt>
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| 135 | <dd><p>A character constant in the source code is recognized by preprocessing
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| 136 | as a single preprocessing token.
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| 137 | </p>
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| 138 | </dd>
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| 139 | <dt>header name</dt>
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| 140 | <dd><p>Within the <code>#include</code> directive, preprocessing recognizes a
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| 141 | <em>header name</em> token. It consists of ‘<samp>"<var>name</var>"</samp>’, where
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| 142 | <var>name</var> is a sequence of source characters other than newline and
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| 143 | ‘<samp>"</samp>’, or ‘<samp><<var>name</var>></samp>’, where <var>name</var> is a sequence of
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| 144 | source characters other than newline and ‘<samp>></samp>’.
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| 145 | </p>
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| 146 | <p>In practice, it is more convenient to think that the <code>#include</code> line
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| 147 | is exempt from tokenization.
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| 148 | </p>
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| 149 | </dd>
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| 150 | <dt>other</dt>
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| 151 | <dd><p>Any other character that’s valid in a C source program
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| 152 | is treated as a separate preprocessing token.
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| 153 | </p></dd>
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| 154 | </dl>
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| 155 |
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| 156 | <p>Once the program is broken into preprocessing tokens, they remain
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| 157 | separate until the end of preprocessing. Macros that generate two
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| 158 | consecutive tokens insert whitespace to keep them separate, if
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| 159 | necessary. For example,
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| 160 | </p>
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| 161 | <div class="example">
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| 162 | <pre class="example">#define foo() bar
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| 163 | foo()baz
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| 164 | → bar baz
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| 165 | <em>not</em>
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| 166 | → barbaz
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| 167 | </pre></div>
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| 168 |
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| 169 | <p>The only exception is with the <code>##</code> preprocessing operator, which
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| 170 | pastes tokens together (see <a href="Concatenation.html">Concatenation</a>).
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| 171 | </p>
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| 172 | <p>Preprocessing treats the null character (code 0) as whitespace, but
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| 173 | generates a warning for it because it may be invisible to the user
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| 174 | (many terminals do not display it at all) and its presence in the file
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| 175 | is probably a mistake.
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| 176 | </p>
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| 177 | <hr>
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| 178 | <div class="header">
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| 179 | <p>
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| 180 | Next: <a href="Header-Files.html" accesskey="n" rel="next">Header Files</a>, Previous: <a href="Directives.html" accesskey="p" rel="prev">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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| 181 | </div>
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| 182 |
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| 183 |
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| 184 |
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| 185 | </body>
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| 186 | </html>
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