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| <title>Using the GNU Compiler Collection (GCC): Local Labels</title> |
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| <a name="Local-Labels"></a> |
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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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| <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> |
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