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| <h3 class="section">6.3 Labels as Values</h3> |
| |
| <p><a name="index-labels-as-values-2187"></a><a name="index-computed-gotos-2188"></a><a name="index-goto-with-computed-label-2189"></a><a name="index-address-of-a-label-2190"></a> |
| You can get the address of a label defined in the current function |
| (or a containing function) with the unary operator ‘<samp><span class="samp">&&</span></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: |
| |
| <pre class="smallexample"> void *ptr; |
| /* <span class="roman">...</span> */ |
| ptr = &&foo; |
| </pre> |
| <p>To use these values, you need to be able to jump to one. This is done |
| with the computed goto statement<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>, <code>goto *</code><var>exp</var><code>;</code>. For example, |
| |
| <pre class="smallexample"> goto *ptr; |
| </pre> |
| <p class="noindent">Any expression of type <code>void *</code> is allowed. |
| |
| <p>One way of using these constants is in initializing a static array that |
| will serve as a jump table: |
| |
| <pre class="smallexample"> static void *array[] = { &&foo, &&bar, &&hack }; |
| </pre> |
| <p>Then you can select a label with indexing, like this: |
| |
| <pre class="smallexample"> goto *array[i]; |
| </pre> |
| <p class="noindent">Note that this does not check whether the subscript is in bounds—array |
| indexing in C never does that. |
| |
| <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>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>You may not use this mechanism to jump to code in a different function. |
| If you do that, totally unpredictable things will 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>An alternate way to write the above example is |
| |
| <pre class="smallexample"> static const int array[] = { &&foo - &&foo, &&bar - &&foo, |
| &&hack - &&foo }; |
| goto *(&&foo + array[i]); |
| </pre> |
| <p class="noindent">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. |
| |
| <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. |
| |
| <div class="footnote"> |
| <hr> |
| <h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> 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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