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123 lines
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123 lines
5.6 KiB
HTML
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<title>Local Labels - Using the GNU Compiler Collection (GCC)</title>
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Statement-Exprs.html#Statement-Exprs" title="Statement Exprs">
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<div class="node">
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<a name="Local-Labels"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>,
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Previous: <a rel="previous" accesskey="p" href="Statement-Exprs.html#Statement-Exprs">Statement Exprs</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.2 Locally Declared Labels</h3>
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<p><a name="index-local-labels-2873"></a><a name="index-macros_002c-local-labels-2874"></a>
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GCC allows you to declare <dfn>local labels</dfn> 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>A local label declaration looks like this:
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<pre class="smallexample"> __label__ <var>label</var>;
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</pre>
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<p class="noindent">or
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<pre class="smallexample"> __label__ <var>label1</var>, <var>label2</var>, /* <span class="roman">...</span> */;
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</pre>
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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>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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<var>label</var><code>:</code>, within the statements of the statement expression.
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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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<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>
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<p>This could also be written using a statement expression:
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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>
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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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</body></html>
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