1<html lang="en"> 2<head> 3<title>Local Labels - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Statement-Exprs.html#Statement-Exprs" title="Statement Exprs"> 10<link rel="next" href="Labels-as-Values.html#Labels-as-Values" title="Labels as Values"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free 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; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). 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A local label is just like an ordinary label, but you can 63only reference it (with a <code>goto</code> statement, or by taking its 64address) within the block in which it was declared. 65 66 <p>A local label declaration looks like this: 67 68<pre class="smallexample"> __label__ <var>label</var>; 69</pre> 70 <p class="noindent">or 71 72<pre class="smallexample"> __label__ <var>label1</var>, <var>label2</var>, /* <span class="roman">...</span> */; 73</pre> 74 <p>Local label declarations must come at the beginning of the block, 75before any ordinary declarations or statements. 76 77 <p>The label declaration defines the label <em>name</em>, but does not define 78the label itself. You must do this in the usual way, with 79<var>label</var><code>:</code>, within the statements of the statement expression. 80 81 <p>The local label feature is useful for complex macros. If a macro 82contains nested loops, a <code>goto</code> can be useful for breaking out of 83them. However, an ordinary label whose scope is the whole function 84cannot be used: if the macro can be expanded several times in one 85function, the label will be multiply defined in that function. A 86local label avoids this problem. For example: 87 88<pre class="smallexample"> #define SEARCH(value, array, target) \ 89 do { \ 90 __label__ found; \ 91 typeof (target) _SEARCH_target = (target); \ 92 typeof (*(array)) *_SEARCH_array = (array); \ 93 int i, j; \ 94 int value; \ 95 for (i = 0; i < max; i++) \ 96 for (j = 0; j < max; j++) \ 97 if (_SEARCH_array[i][j] == _SEARCH_target) \ 98 { (value) = i; goto found; } \ 99 (value) = -1; \ 100 found:; \ 101 } while (0) 102</pre> 103 <p>This could also be written using a statement-expression: 104 105<pre class="smallexample"> #define SEARCH(array, target) \ 106 ({ \ 107 __label__ found; \ 108 typeof (target) _SEARCH_target = (target); \ 109 typeof (*(array)) *_SEARCH_array = (array); \ 110 int i, j; \ 111 int value; \ 112 for (i = 0; i < max; i++) \ 113 for (j = 0; j < max; j++) \ 114 if (_SEARCH_array[i][j] == _SEARCH_target) \ 115 { value = i; goto found; } \ 116 value = -1; \ 117 found: \ 118 value; \ 119 }) 120</pre> 121 <p>Local label declarations also make the labels they declare visible to 122nested functions, if there are any. See <a href="Nested-Functions.html#Nested-Functions">Nested Functions</a>, for details. 123 124 </body></html> 125 126