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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below). A copy of the license isincluded in the section entitled "GNU Free Documentation License". <p>(a) The FSF's Front-Cover Text is: <p>A GNU Manual <p>(b) The FSF's Back-Cover Text is: <p>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 }--></style></head><body><div class="node"><p>Node: <a name="Local%20Labels">Local Labels</a>,Next: <a rel="next" accesskey="n" href="Labels-as-Values.html#Labels%20as%20Values">Labels as Values</a>,Previous: <a rel="previous" accesskey="p" href="Statement-Exprs.html#Statement%20Exprs">Statement Exprs</a>,Up: <a rel="up" accesskey="u" href="C-Extensions.html#C%20Extensions">C Extensions</a><hr><br></div><h3 class="section">Locally Declared Labels</h3><p>GCC allows you to declare <dfn>local labels</dfn> in any nested blockscope. A local label is just like an ordinary label, but you canonly reference it (with a <code>goto</code> statement, or by taking itsaddress) within the block in which it was declared. <p>A local label declaration looks like this:<pre class="smallexample"> __label__ <var>label</var>; </pre><p>or<pre class="smallexample"> __label__ <var>label1</var>, <var>label2</var>, /* <small class="dots">...</small> */; </pre> <p>Local label declarations must come at the beginning of the block,before any ordinary declarations or statements. <p>The label declaration defines the label <em>name</em>, but does not definethe label itself. You must do this in the usual way, with<code></code><var>label</var><code>:</code>, within the statements of the statement expression. <p>The local label feature is useful for complex macros. If a macrocontains nested loops, a <code>goto</code> can be useful for breaking out ofthem. However, an ordinary label whose scope is the whole functioncannot be used: if the macro can be expanded several times in onefunction, the label will be multiply defined in that function. Alocal label avoids this problem. For example:<pre class="smallexample"> #define SEARCH(value, array, target) \ do { \ __label__ found; \ typeof (target) _SEARCH_target = (target); \ typeof (*(array)) *_SEARCH_array = (array); \ int i, j; \ int value; \ for (i = 0; i < max; i++) \ for (j = 0; j < max; j++) \ if (_SEARCH_array[i][j] == _SEARCH_target) \ { (value) = i; goto found; } \ (value) = -1; \ found:; \ } while (0) </pre> <p>This could also be written using a statement-expression:<pre class="smallexample"> #define SEARCH(array, target) \ ({ \ __label__ found; \ typeof (target) _SEARCH_target = (target); \ typeof (*(array)) *_SEARCH_array = (array); \ int i, j; \ int value; \ for (i = 0; i < max; i++) \ for (j = 0; j < max; j++) \ if (_SEARCH_array[i][j] == _SEARCH_target) \ { value = i; goto found; } \ value = -1; \ found: \ value; \ }) </pre> <p>Local label declarations also make the labels they declare visible tonested functions, if there are any. See <a href="Nested-Functions.html#Nested%20Functions">Nested Functions</a>, for details. </body></html>⌨️ 快捷键说明
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