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