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| <title>Using the GNU Compiler Collection (GCC): Labels as Values</title> |
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| <a name="Labels-as-Values"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Nested-Functions.html#Nested-Functions" accesskey="n" rel="next">Nested Functions</a>, Previous: <a href="Local-Labels.html#Local-Labels" accesskey="p" rel="prev">Local Labels</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="Labels-as-Values-1"></a> |
| <h3 class="section">6.3 Labels as Values</h3> |
| <a name="index-labels-as-values"></a> |
| <a name="index-computed-gotos"></a> |
| <a name="index-goto-with-computed-label"></a> |
| <a name="index-address-of-a-label"></a> |
| |
| <p>You can get the address of a label defined in the current function |
| (or a containing function) with the unary operator ‘<samp>&&</samp>’. The |
| value has type <code>void *</code>. This value is a constant and can be used |
| wherever a constant of that type is valid. For example: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">void *ptr; |
| /* <span class="roman">…</span> */ |
| ptr = &&foo; |
| </pre></div> |
| |
| <p>To use these values, you need to be able to jump to one. This is done |
| with the computed goto statement<a name="DOCF3" href="#FOOT3"><sup>3</sup></a>, <code>goto *<var>exp</var>;</code>. For example, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">goto *ptr; |
| </pre></div> |
| |
| <p>Any expression of type <code>void *</code> is allowed. |
| </p> |
| <p>One way of using these constants is in initializing a static array that |
| serves as a jump table: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">static void *array[] = { &&foo, &&bar, &&hack }; |
| </pre></div> |
| |
| <p>Then you can select a label with indexing, like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">goto *array[i]; |
| </pre></div> |
| |
| <p>Note that this does not check whether the subscript is in bounds—array |
| indexing in C never does that. |
| </p> |
| <p>Such an array of label values serves a purpose much like that of the |
| <code>switch</code> statement. The <code>switch</code> statement is cleaner, so |
| use that rather than an array unless the problem does not fit a |
| <code>switch</code> statement very well. |
| </p> |
| <p>Another use of label values is in an interpreter for threaded code. |
| The labels within the interpreter function can be stored in the |
| threaded code for super-fast dispatching. |
| </p> |
| <p>You may not use this mechanism to jump to code in a different function. |
| If you do that, totally unpredictable things happen. The best way to |
| avoid this is to store the label address only in automatic variables and |
| never pass it as an argument. |
| </p> |
| <p>An alternate way to write the above example is |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">static const int array[] = { &&foo - &&foo, &&bar - &&foo, |
| &&hack - &&foo }; |
| goto *(&&foo + array[i]); |
| </pre></div> |
| |
| <p>This is more friendly to code living in shared libraries, as it reduces |
| the number of dynamic relocations that are needed, and by consequence, |
| allows the data to be read-only. |
| This alternative with label differences is not supported for the AVR target, |
| please use the first approach for AVR programs. |
| </p> |
| <p>The <code>&&foo</code> expressions for the same label might have different |
| values if the containing function is inlined or cloned. If a program |
| relies on them being always the same, |
| <code>__attribute__((__noinline__,__noclone__))</code> should be used to |
| prevent inlining and cloning. If <code>&&foo</code> is used in a static |
| variable initializer, inlining and cloning is forbidden. |
| </p> |
| <div class="footnote"> |
| <hr> |
| <h4 class="footnotes-heading">Footnotes</h4> |
| |
| <h3><a name="FOOT3" href="#DOCF3">(3)</a></h3> |
| <p>The analogous feature in |
| Fortran is called an assigned goto, but that name seems inappropriate in |
| C, where one can do more than simply store label addresses in label |
| variables.</p> |
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| Next: <a href="Nested-Functions.html#Nested-Functions" accesskey="n" rel="next">Nested Functions</a>, Previous: <a href="Local-Labels.html#Local-Labels" accesskey="p" rel="prev">Local Labels</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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