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162 lines
7.1 KiB
HTML
162 lines
7.1 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Local Labels (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Local Labels (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Local Labels (Using the GNU Compiler Collection (GCC))">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Labels-as-Values.html#Labels-as-Values" rel="next" title="Labels as Values">
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<link href="Statement-Exprs.html#Statement-Exprs" rel="prev" title="Statement Exprs">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Local-Labels"></a>
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<div class="header">
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<p>
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Next: <a href="Labels-as-Values.html#Labels-as-Values" accesskey="n" rel="next">Labels as Values</a>, Previous: <a href="Statement-Exprs.html#Statement-Exprs" accesskey="p" rel="prev">Statement Exprs</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Locally-Declared-Labels"></a>
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<h3 class="section">6.2 Locally Declared Labels</h3>
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<a name="index-local-labels"></a>
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<a name="index-macros_002c-local-labels"></a>
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<p>GCC allows you to declare <em>local labels</em> in any nested block
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scope. A local label is just like an ordinary label, but you can
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only reference it (with a <code>goto</code> statement, or by taking its
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address) within the block in which it is declared.
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</p>
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<p>A local label declaration looks like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">__label__ <var>label</var>;
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</pre></div>
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<p>or
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</p>
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<div class="smallexample">
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<pre class="smallexample">__label__ <var>label1</var>, <var>label2</var>, /* <span class="roman">…</span> */;
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</pre></div>
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<p>Local label declarations must come at the beginning of the block,
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before any ordinary declarations or statements.
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</p>
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<p>The label declaration defines the label <em>name</em>, but does not define
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the label itself. You must do this in the usual way, with
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<code><var>label</var>:</code>, within the statements of the statement expression.
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</p>
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<p>The local label feature is useful for complex macros. If a macro
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contains nested loops, a <code>goto</code> can be useful for breaking out of
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them. However, an ordinary label whose scope is the whole function
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cannot be used: if the macro can be expanded several times in one
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function, the label is multiply defined in that function. A
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local label avoids this problem. For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define SEARCH(value, array, target) \
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do { \
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__label__ found; \
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typeof (target) _SEARCH_target = (target); \
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typeof (*(array)) *_SEARCH_array = (array); \
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int i, j; \
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int value; \
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for (i = 0; i < max; i++) \
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for (j = 0; j < max; j++) \
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if (_SEARCH_array[i][j] == _SEARCH_target) \
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{ (value) = i; goto found; } \
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(value) = -1; \
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found:; \
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} while (0)
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</pre></div>
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<p>This could also be written using a statement expression:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define SEARCH(array, target) \
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({ \
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__label__ found; \
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typeof (target) _SEARCH_target = (target); \
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typeof (*(array)) *_SEARCH_array = (array); \
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int i, j; \
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int value; \
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for (i = 0; i < max; i++) \
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for (j = 0; j < max; j++) \
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if (_SEARCH_array[i][j] == _SEARCH_target) \
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{ value = i; goto found; } \
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value = -1; \
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found: \
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value; \
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})
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</pre></div>
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<p>Local label declarations also make the labels they declare visible to
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nested functions, if there are any. See <a href="Nested-Functions.html#Nested-Functions">Nested Functions</a>, for details.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Labels-as-Values.html#Labels-as-Values" accesskey="n" rel="next">Labels as Values</a>, Previous: <a href="Statement-Exprs.html#Statement-Exprs" accesskey="p" rel="prev">Statement Exprs</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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