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130 lines
5.8 KiB
HTML
130 lines
5.8 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Labels as Values - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Local-Labels.html#Local-Labels" title="Local Labels">
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<link rel="next" href="Nested-Functions.html#Nested-Functions" title="Nested Functions">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Labels-as-Values"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Nested-Functions.html#Nested-Functions">Nested Functions</a>,
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Previous: <a rel="previous" accesskey="p" href="Local-Labels.html#Local-Labels">Local Labels</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">6.3 Labels as Values</h3>
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<p><a name="index-labels-as-values-2875"></a><a name="index-computed-gotos-2876"></a><a name="index-goto-with-computed-label-2877"></a><a name="index-address-of-a-label-2878"></a>
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You can get the address of a label defined in the current function
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(or a containing function) with the unary operator ‘<samp><span class="samp">&&</span></samp>’. The
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value has type <code>void *</code>. This value is a constant and can be used
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wherever a constant of that type is valid. For example:
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<pre class="smallexample"> void *ptr;
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/* <span class="roman">...</span> */
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ptr = &&foo;
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</pre>
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<p>To use these values, you need to be able to jump to one. This is done
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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,
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<pre class="smallexample"> goto *ptr;
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</pre>
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<p class="noindent">Any expression of type <code>void *</code> is allowed.
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<p>One way of using these constants is in initializing a static array that
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serves as a jump table:
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<pre class="smallexample"> static void *array[] = { &&foo, &&bar, &&hack };
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</pre>
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<p class="noindent">Then you can select a label with indexing, like this:
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<pre class="smallexample"> goto *array[i];
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</pre>
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<p class="noindent">Note that this does not check whether the subscript is in bounds—array
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indexing in C never does that.
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<p>Such an array of label values serves a purpose much like that of the
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<code>switch</code> statement. The <code>switch</code> statement is cleaner, so
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use that rather than an array unless the problem does not fit a
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<code>switch</code> statement very well.
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<p>Another use of label values is in an interpreter for threaded code.
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The labels within the interpreter function can be stored in the
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threaded code for super-fast dispatching.
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<p>You may not use this mechanism to jump to code in a different function.
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If you do that, totally unpredictable things happen. The best way to
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avoid this is to store the label address only in automatic variables and
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never pass it as an argument.
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<p>An alternate way to write the above example is
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<pre class="smallexample"> static const int array[] = { &&foo - &&foo, &&bar - &&foo,
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&&hack - &&foo };
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goto *(&&foo + array[i]);
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</pre>
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<p class="noindent">This is more friendly to code living in shared libraries, as it reduces
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the number of dynamic relocations that are needed, and by consequence,
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allows the data to be read-only.
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This alternative with label differences is not supported for the AVR target,
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please use the first approach for AVR programs.
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<p>The <code>&&foo</code> expressions for the same label might have different
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values if the containing function is inlined or cloned. If a program
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relies on them being always the same,
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<code>__attribute__((__noinline__,__noclone__))</code> should be used to
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prevent inlining and cloning. If <code>&&foo</code> is used in a static
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variable initializer, inlining and cloning is forbidden.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> The analogous feature in
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Fortran is called an assigned goto, but that name seems inappropriate in
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C, where one can do more than simply store label addresses in label
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variables.</p>
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<hr></div>
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</body></html>
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