| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|---|
| 2 | <html>
|
|---|
| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
|
|---|
| 4 |
|
|---|
| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
|
|---|
| 6 | licensed to the FSF.)
|
|---|
| 7 |
|
|---|
| 8 | Permission is granted to copy, distribute and/or modify this document
|
|---|
| 9 | under the terms of the GNU Free Documentation License, Version 1.3 or
|
|---|
| 10 | any later version published by the Free Software Foundation; with the
|
|---|
| 11 | Invariant Sections being "GNU General Public License," with the
|
|---|
| 12 | Front-Cover Texts being "A GNU Manual," and with the Back-Cover
|
|---|
| 13 | Texts as in (a) below. A copy of the license is included in the
|
|---|
| 14 | section entitled "GNU Free Documentation License."
|
|---|
| 15 |
|
|---|
| 16 | (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
|
|---|
| 17 | modify this GNU manual. Buying copies from the FSF supports it in
|
|---|
| 18 | developing GNU and promoting software freedom." -->
|
|---|
| 19 | <!-- Created by GNU Texinfo 6.7, http://www.gnu.org/software/texinfo/ -->
|
|---|
| 20 | <head>
|
|---|
| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|---|
| 22 | <title>Label Value Uses (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Label Value Uses (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Label Value Uses (GNU C Language Manual)">
|
|---|
| 26 | <meta name="resource-type" content="document">
|
|---|
| 27 | <meta name="distribution" content="global">
|
|---|
| 28 | <meta name="Generator" content="makeinfo">
|
|---|
| 29 | <link href="index.html" rel="start" title="Top">
|
|---|
| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
|
|---|
| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|---|
| 32 | <link href="Labels-as-Values.html" rel="up" title="Labels as Values">
|
|---|
| 33 | <link href="Label-Value-Caveats.html" rel="next" title="Label Value Caveats">
|
|---|
| 34 | <link href="Labels-as-Values.html" rel="prev" title="Labels as Values">
|
|---|
| 35 | <style type="text/css">
|
|---|
| 36 | <!--
|
|---|
| 37 | a.summary-letter {text-decoration: none}
|
|---|
| 38 | blockquote.indentedblock {margin-right: 0em}
|
|---|
| 39 | div.display {margin-left: 3.2em}
|
|---|
| 40 | div.example {margin-left: 3.2em}
|
|---|
| 41 | div.lisp {margin-left: 3.2em}
|
|---|
| 42 | kbd {font-style: oblique}
|
|---|
| 43 | pre.display {font-family: inherit}
|
|---|
| 44 | pre.format {font-family: inherit}
|
|---|
| 45 | pre.menu-comment {font-family: serif}
|
|---|
| 46 | pre.menu-preformatted {font-family: serif}
|
|---|
| 47 | span.nolinebreak {white-space: nowrap}
|
|---|
| 48 | span.roman {font-family: initial; font-weight: normal}
|
|---|
| 49 | span.sansserif {font-family: sans-serif; font-weight: normal}
|
|---|
| 50 | ul.no-bullet {list-style: none}
|
|---|
| 51 | -->
|
|---|
| 52 | </style>
|
|---|
| 53 |
|
|---|
| 54 |
|
|---|
| 55 | </head>
|
|---|
| 56 |
|
|---|
| 57 | <body lang="en">
|
|---|
| 58 | <span id="Label-Value-Uses"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Label-Value-Caveats.html" accesskey="n" rel="next">Label Value Caveats</a>, Up: <a href="Labels-as-Values.html" accesskey="u" rel="up">Labels as Values</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
|
|---|
| 61 | </div>
|
|---|
| 62 | <hr>
|
|---|
| 63 | <span id="Label-Value-Uses-1"></span><h4 class="subsection">19.14.1 Label Value Uses</h4>
|
|---|
| 64 |
|
|---|
| 65 | <p>One use for label-valued constants is to initialize a static array to
|
|---|
| 66 | serve as a jump table:
|
|---|
| 67 | </p>
|
|---|
| 68 | <div class="example">
|
|---|
| 69 | <pre class="example">static void *array[] = { &&foo, &&bar, &&hack };
|
|---|
| 70 | </pre></div>
|
|---|
| 71 |
|
|---|
| 72 | <p>Then you can select a label with indexing, like this:
|
|---|
| 73 | </p>
|
|---|
| 74 | <div class="example">
|
|---|
| 75 | <pre class="example">goto *array[i];
|
|---|
| 76 | </pre></div>
|
|---|
| 77 |
|
|---|
| 78 | <p>Note that this does not check whether the subscript is in bounds—array
|
|---|
| 79 | indexing in C never checks that.
|
|---|
| 80 | </p>
|
|---|
| 81 | <p>You can make the table entries offsets instead of addresses
|
|---|
| 82 | by subtracting one label from the others. Here is an example:
|
|---|
| 83 | </p>
|
|---|
| 84 | <div class="example">
|
|---|
| 85 | <pre class="example">static const int array[] = { &&foo - &&foo, &&bar - &&foo,
|
|---|
| 86 | &&hack - &&foo };
|
|---|
| 87 | goto *(&&foo + array[i]);
|
|---|
| 88 | </pre></div>
|
|---|
| 89 |
|
|---|
| 90 | <p>Using offsets is preferable in shared libraries, as it avoids the need
|
|---|
| 91 | for dynamic relocation of the array elements; therefore, the array can
|
|---|
| 92 | be read-only.
|
|---|
| 93 | </p>
|
|---|
| 94 | <p>An array of label values or offsets serves a purpose much like that of
|
|---|
| 95 | the <code>switch</code> statement. The <code>switch</code> statement is cleaner,
|
|---|
| 96 | so use <code>switch</code> by preference when feasible.
|
|---|
| 97 | </p>
|
|---|
| 98 | <p>Another use of label values is in an interpreter for threaded code.
|
|---|
| 99 | The labels within the interpreter function can be stored in the
|
|---|
| 100 | threaded code for super-fast dispatching.
|
|---|
| 101 | </p>
|
|---|
| 102 |
|
|---|
| 103 |
|
|---|
| 104 |
|
|---|
| 105 | </body>
|
|---|
| 106 | </html>
|
|---|
