<html lang="en">
<head>
<title>Local Labels - 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 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
<link rel="prev" href="Statement-Exprs.html#Statement-Exprs" title="Statement Exprs">
<link rel="next" href="Labels-as-Values.html#Labels-as-Values" title="Labels as Values">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
2008 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 1.2 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.-->
<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 }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; } 
  span.sansserif { font-family:sans-serif; font-weight:normal; } 
--></style>
<link rel="stylesheet" type="text/css" href="../cs.css">
</head>
<body>
<div class="node">
<a name="Local-Labels"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>,
Previous:&nbsp;<a rel="previous" accesskey="p" href="Statement-Exprs.html#Statement-Exprs">Statement Exprs</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
<hr>
</div>

<h3 class="section">6.2 Locally Declared Labels</h3>

<p><a name="index-local-labels-2185"></a><a name="index-macros_002c-local-labels-2186"></a>
GCC allows you to declare <dfn>local labels</dfn> 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 was declared.

 <p>A local label declaration looks like this:

<pre class="smallexample">     __label__ <var>label</var>;
</pre>
 <p class="noindent">or

<pre class="smallexample">     __label__ <var>label1</var>, <var>label2</var>, /* <span class="roman">...</span> */;
</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 define
the label itself.  You must do this in the usual way, with
<var>label</var><code>:</code>, within the statements of the statement expression.

 <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 will be multiply defined in that function.  A
local 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 &lt; max; i++)                       \
         for (j = 0; j &lt; 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 &lt; max; i++)                       \
         for (j = 0; j &lt; 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 to
nested functions, if there are any.  See <a href="Nested-Functions.html#Nested-Functions">Nested Functions</a>, for details.

 </body></html>