1<html lang="en"> 2<head> 3<title>Traditional lexical analysis - The C Preprocessor</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="The C Preprocessor"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Traditional-Mode.html#Traditional-Mode" title="Traditional Mode"> 9<link rel="next" href="Traditional-macros.html#Traditional-macros" title="Traditional macros"> 10<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 11<!-- 12Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996, 131997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 142008, 2009, 2010, 2011 15Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation. 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The input is 63simply treated as a stream of text with minimal internal form. 64 65 <p>This implementation does not treat trigraphs (see <a href="trigraphs.html#trigraphs">trigraphs</a>) 66specially since they were an invention of the standards committee. It 67handles arbitrarily-positioned escaped newlines properly and splices 68the lines as you would expect; many traditional preprocessors did not 69do this. 70 71 <p>The form of horizontal whitespace in the input file is preserved in 72the output. In particular, hard tabs remain hard tabs. This can be 73useful if, for example, you are preprocessing a Makefile. 74 75 <p>Traditional CPP only recognizes C-style block comments, and treats the 76‘<samp><span class="samp">/*</span></samp>’ sequence as introducing a comment only if it lies outside 77quoted text. Quoted text is introduced by the usual single and double 78quotes, and also by an initial ‘<samp><span class="samp"><</span></samp>’ in a <code>#include</code> 79directive. 80 81 <p>Traditionally, comments are completely removed and are not replaced 82with a space. Since a traditional compiler does its own tokenization 83of the output of the preprocessor, this means that comments can 84effectively be used as token paste operators. However, comments 85behave like separators for text handled by the preprocessor itself, 86since it doesn't re-lex its input. For example, in 87 88<pre class="smallexample"> #if foo/**/bar 89</pre> 90 <p class="noindent">‘<samp><span class="samp">foo</span></samp>’ and ‘<samp><span class="samp">bar</span></samp>’ are distinct identifiers and expanded 91separately if they happen to be macros. In other words, this 92directive is equivalent to 93 94<pre class="smallexample"> #if foo bar 95</pre> 96 <p class="noindent">rather than 97 98<pre class="smallexample"> #if foobar 99</pre> 100 <p>Generally speaking, in traditional mode an opening quote need not have 101a matching closing quote. In particular, a macro may be defined with 102replacement text that contains an unmatched quote. Of course, if you 103attempt to compile preprocessed output containing an unmatched quote 104you will get a syntax error. 105 106 <p>However, all preprocessing directives other than <code>#define</code> 107require matching quotes. For example: 108 109<pre class="smallexample"> #define m This macro's fine and has an unmatched quote 110 "/* This is not a comment. */ 111 /* <span class="roman">This is a comment. The following #include directive 112 is ill-formed.</span> */ 113 #include <stdio.h 114</pre> 115 <p>Just as for the ISO preprocessor, what would be a closing quote can be 116escaped with a backslash to prevent the quoted text from closing. 117 118 </body></html> 119 120