<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
<html>
|
<!-- Copyright (C) 1988-2016 Free Software Foundation, Inc.
|
|
Permission is granted to copy, distribute and/or modify this document
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
any later version published by the Free Software Foundation; with the
|
Invariant Sections being "Funding Free Software", the Front-Cover
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
(see below). A copy of the license is included in the section entitled
|
"GNU Free Documentation License".
|
|
(a) The FSF's Front-Cover Text is:
|
|
A GNU Manual
|
|
(b) The FSF's Back-Cover Text is:
|
|
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. -->
|
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
|
<head>
|
<title>Using the GNU Compiler Collection (GCC): Local Labels</title>
|
|
<meta name="description" content="Using the GNU Compiler Collection (GCC): Local Labels">
|
<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Local Labels">
|
<meta name="resource-type" content="document">
|
<meta name="distribution" content="global">
|
<meta name="Generator" content="makeinfo">
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
<link href="index.html#Top" rel="start" title="Top">
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
|
<link href="Labels-as-Values.html#Labels-as-Values" rel="next" title="Labels as Values">
|
<link href="Statement-Exprs.html#Statement-Exprs" rel="prev" title="Statement Exprs">
|
<style type="text/css">
|
<!--
|
a.summary-letter {text-decoration: none}
|
blockquote.smallquotation {font-size: smaller}
|
div.display {margin-left: 3.2em}
|
div.example {margin-left: 3.2em}
|
div.indentedblock {margin-left: 3.2em}
|
div.lisp {margin-left: 3.2em}
|
div.smalldisplay {margin-left: 3.2em}
|
div.smallexample {margin-left: 3.2em}
|
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
|
div.smalllisp {margin-left: 3.2em}
|
kbd {font-style:oblique}
|
pre.display {font-family: inherit}
|
pre.format {font-family: inherit}
|
pre.menu-comment {font-family: serif}
|
pre.menu-preformatted {font-family: serif}
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
pre.smallexample {font-size: smaller}
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
pre.smalllisp {font-size: smaller}
|
span.nocodebreak {white-space:nowrap}
|
span.nolinebreak {white-space:nowrap}
|
span.roman {font-family:serif; font-weight:normal}
|
span.sansserif {font-family:sans-serif; font-weight:normal}
|
ul.no-bullet {list-style: none}
|
-->
|
</style>
|
|
|
</head>
|
|
<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
|
<a name="Local-Labels"></a>
|
<div class="header">
|
<p>
|
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>
|
</div>
|
<hr>
|
<a name="Locally-Declared-Labels"></a>
|
<h3 class="section">6.2 Locally Declared Labels</h3>
|
<a name="index-local-labels"></a>
|
<a name="index-macros_002c-local-labels"></a>
|
|
<p>GCC allows you to declare <em>local labels</em> in any nested block
|
scope. A local label is just like an ordinary label, but you can
|
only reference it (with a <code>goto</code> statement, or by taking its
|
address) within the block in which it is declared.
|
</p>
|
<p>A local label declaration looks like this:
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">__label__ <var>label</var>;
|
</pre></div>
|
|
<p>or
|
</p>
|
<div class="smallexample">
|
<pre class="smallexample">__label__ <var>label1</var>, <var>label2</var>, /* <span class="roman">…</span> */;
|
</pre></div>
|
|
<p>Local label declarations must come at the beginning of the block,
|
before any ordinary declarations or statements.
|
</p>
|
<p>The label declaration defines the label <em>name</em>, but does not define
|
the label itself. You must do this in the usual way, with
|
<code><var>label</var>:</code>, within the statements of the statement expression.
|
</p>
|
<p>The local label feature is useful for complex macros. If a macro
|
contains nested loops, a <code>goto</code> can be useful for breaking out of
|
them. However, an ordinary label whose scope is the whole function
|
cannot be used: if the macro can be expanded several times in one
|
function, the label is multiply defined in that function. A
|
local label avoids this problem. For example:
|
</p>
|
<div class="smallexample">
|
<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></div>
|
|
<p>This could also be written using a statement expression:
|
</p>
|
<div class="smallexample">
|
<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></div>
|
|
<p>Local label declarations also make the labels they declare visible to
|
nested functions, if there are any. See <a href="Nested-Functions.html#Nested-Functions">Nested Functions</a>, for details.
|
</p>
|
<hr>
|
<div class="header">
|
<p>
|
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>
|
</div>
|
|
|
|
</body>
|
</html>
|
