<html lang="en"> <head> <title>Traditional lexical analysis - The C Preprocessor</title> <meta http-equiv="Content-Type" content="text/html"> <meta name="description" content="The C Preprocessor"> <meta name="generator" content="makeinfo 4.13"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="Traditional-Mode.html#Traditional-Mode" title="Traditional Mode"> <link rel="next" href="Traditional-macros.html#Traditional-macros" title="Traditional macros"> <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> <!-- Copyright (C) 1987-2015 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation. A copy of the license is included in the section entitled ``GNU Free Documentation License''. This manual contains no Invariant Sections. The Front-Cover Texts are (a) (see below), and the Back-Cover Texts are (b) (see below). (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development. --> <meta http-equiv="Content-Style-Type" content="text/css"> <style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller } span.sc { font-variant:small-caps } span.roman { font-family:serif; font-weight:normal; } span.sansserif { font-family:sans-serif; font-weight:normal; } --></style> </head> <body> <div class="node"> <a name="Traditional-lexical-analysis"></a> <p> Next: <a rel="next" accesskey="n" href="Traditional-macros.html#Traditional-macros">Traditional macros</a>, Up: <a rel="up" accesskey="u" href="Traditional-Mode.html#Traditional-Mode">Traditional Mode</a> <hr> </div> <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. </body></html>