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116 lines
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<a name="Traditional-lexical-analysis"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Traditional-macros.html#Traditional-macros">Traditional macros</a>,
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Up: <a rel="up" accesskey="u" href="Traditional-Mode.html#Traditional-Mode">Traditional Mode</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>This implementation does not treat trigraphs (see <a href="trigraphs.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>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>Traditional CPP only recognizes C-style block comments, and treats the
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‘<samp><span class="samp">/*</span></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><span class="samp"><</span></samp>’ in a <code>#include</code>
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directive.
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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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<pre class="smallexample"> #if foo/**/bar
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</pre>
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<p class="noindent">‘<samp><span class="samp">foo</span></samp>’ and ‘<samp><span class="samp">bar</span></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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<pre class="smallexample"> #if foo bar
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</pre>
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<p class="noindent">rather than
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<pre class="smallexample"> #if foobar
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</pre>
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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>However, all preprocessing directives other than <code>#define</code>
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require matching quotes. For example:
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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>
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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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