156 lines
6.7 KiB
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156 lines
6.7 KiB
HTML
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<title>The C Preprocessor: Traditional lexical analysis</title>
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Traditional-Mode.html#Traditional-Mode" rel="up" title="Traditional Mode">
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<link href="Traditional-macros.html#Traditional-macros" rel="next" title="Traditional macros">
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<a name="Traditional-lexical-analysis"></a>
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<div class="header">
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<p>
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Next: <a href="Traditional-macros.html#Traditional-macros" accesskey="n" rel="next">Traditional macros</a>, Up: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="u" rel="up">Traditional Mode</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Traditional-lexical-analysis-1"></a>
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<h3 class="section">10.1 Traditional lexical analysis</h3>
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<p>The traditional preprocessor does not decompose its input into tokens
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the same way a standards-conforming preprocessor does. The input is
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simply treated as a stream of text with minimal internal form.
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</p>
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<p>This implementation does not treat trigraphs (see <a href="Initial-processing.html#trigraphs">trigraphs</a>)
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specially since they were an invention of the standards committee. It
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handles arbitrarily-positioned escaped newlines properly and splices
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the lines as you would expect; many traditional preprocessors did not
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do this.
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</p>
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<p>The form of horizontal whitespace in the input file is preserved in
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the output. In particular, hard tabs remain hard tabs. This can be
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useful if, for example, you are preprocessing a Makefile.
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</p>
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<p>Traditional CPP only recognizes C-style block comments, and treats the
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‘<samp>/*</samp>’ sequence as introducing a comment only if it lies outside
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quoted text. Quoted text is introduced by the usual single and double
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quotes, and also by an initial ‘<samp><</samp>’ in a <code>#include</code>
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directive.
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</p>
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<p>Traditionally, comments are completely removed and are not replaced
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with a space. Since a traditional compiler does its own tokenization
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of the output of the preprocessor, this means that comments can
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effectively be used as token paste operators. However, comments
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behave like separators for text handled by the preprocessor itself,
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since it doesn’t re-lex its input. For example, in
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if foo/**/bar
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</pre></div>
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<p>‘<samp>foo</samp>’ and ‘<samp>bar</samp>’ are distinct identifiers and expanded
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separately if they happen to be macros. In other words, this
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directive is equivalent to
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if foo bar
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</pre></div>
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<p>rather than
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if foobar
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</pre></div>
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<p>Generally speaking, in traditional mode an opening quote need not have
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a matching closing quote. In particular, a macro may be defined with
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replacement text that contains an unmatched quote. Of course, if you
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attempt to compile preprocessed output containing an unmatched quote
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you will get a syntax error.
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</p>
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<p>However, all preprocessing directives other than <code>#define</code>
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require matching quotes. For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define m This macro's fine and has an unmatched quote
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"/* This is not a comment. */
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/* <span class="roman">This is a comment. The following #include directive
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is ill-formed.</span> */
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#include <stdio.h
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</pre></div>
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<p>Just as for the ISO preprocessor, what would be a closing quote can be
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escaped with a backslash to prevent the quoted text from closing.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Traditional-macros.html#Traditional-macros" accesskey="n" rel="next">Traditional macros</a>, Up: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="u" rel="up">Traditional Mode</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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