1<html lang="en"> 2<head> 3<title>Implementation-defined behavior - 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="Implementation-Details.html#Implementation-Details" title="Implementation Details"> 9<link rel="next" href="Implementation-limits.html#Implementation-limits" title="Implementation limits"> 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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This term means that the 65implementation is free to do what it likes, but must document its choice 66and stick to it. 67<!-- FIXME: Check the C++ standard for more implementation-defined stuff. --> 68 69 <ul> 70<li>The mapping of physical source file multi-byte characters to the 71execution character set. 72 73 <p>The input character set can be specified using the 74<samp><span class="option">-finput-charset</span></samp> option, while the execution character set may 75be controlled using the <samp><span class="option">-fexec-charset</span></samp> and 76<samp><span class="option">-fwide-exec-charset</span></samp> options. 77 78 <li>Identifier characters. 79<a name="Identifier-characters"></a>The C and C++ standards allow identifiers to be composed of ‘<samp><span class="samp">_</span></samp>’ 80and the alphanumeric characters. C++ and C99 also allow universal 81character names, and C99 further permits implementation-defined 82characters. GCC currently only permits universal character names if 83<samp><span class="option">-fextended-identifiers</span></samp> is used, because the implementation of 84universal character names in identifiers is experimental. 85 86 <p>GCC allows the ‘<samp><span class="samp">$</span></samp>’ character in identifiers as an extension for 87most targets. This is true regardless of the <samp><span class="option">std=</span></samp> switch, 88since this extension cannot conflict with standards-conforming 89programs. When preprocessing assembler, however, dollars are not 90identifier characters by default. 91 92 <p>Currently the targets that by default do not permit ‘<samp><span class="samp">$</span></samp>’ are AVR, 93IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX 94operating system. 95 96 <p>You can override the default with <samp><span class="option">-fdollars-in-identifiers</span></samp> or 97<samp><span class="option">fno-dollars-in-identifiers</span></samp>. See <a href="fdollars_002din_002didentifiers.html#fdollars_002din_002didentifiers">fdollars-in-identifiers</a>. 98 99 <li>Non-empty sequences of whitespace characters. 100 101 <p>In textual output, each whitespace sequence is collapsed to a single 102space. For aesthetic reasons, the first token on each non-directive 103line of output is preceded with sufficient spaces that it appears in the 104same column as it did in the original source file. 105 106 <li>The numeric value of character constants in preprocessor expressions. 107 108 <p>The preprocessor and compiler interpret character constants in the 109same way; i.e. escape sequences such as ‘<samp><span class="samp">\a</span></samp>’ are given the 110values they would have on the target machine. 111 112 <p>The compiler evaluates a multi-character character constant a character 113at a time, shifting the previous value left by the number of bits per 114target character, and then or-ing in the bit-pattern of the new 115character truncated to the width of a target character. The final 116bit-pattern is given type <code>int</code>, and is therefore signed, 117regardless of whether single characters are signed or not (a slight 118change from versions 3.1 and earlier of GCC). If there are more 119characters in the constant than would fit in the target <code>int</code> the 120compiler issues a warning, and the excess leading characters are 121ignored. 122 123 <p>For example, <code>'ab'</code> for a target with an 8-bit <code>char</code> would be 124interpreted as ‘<samp><span class="samp">(int) ((unsigned char) 'a' * 256 + (unsigned char) 'b')</span></samp>’<!-- /@w -->, and <code>'\234a'</code> as ‘<samp><span class="samp">(int) ((unsigned char) '\234' * 256 + (unsigned char) 'a')</span></samp>’<!-- /@w -->. 125 126 <li>Source file inclusion. 127 128 <p>For a discussion on how the preprocessor locates header files, 129<a href="Include-Operation.html#Include-Operation">Include Operation</a>. 130 131 <li>Interpretation of the filename resulting from a macro-expanded 132‘<samp><span class="samp">#include</span></samp>’ directive. 133 134 <p>See <a href="Computed-Includes.html#Computed-Includes">Computed Includes</a>. 135 136 <li>Treatment of a ‘<samp><span class="samp">#pragma</span></samp>’ directive that after macro-expansion 137results in a standard pragma. 138 139 <p>No macro expansion occurs on any ‘<samp><span class="samp">#pragma</span></samp>’ directive line, so the 140question does not arise. 141 142 <p>Note that GCC does not yet implement any of the standard 143pragmas. 144 145 </ul> 146 147 </body></html> 148 149