| 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>Unions (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Unions (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Unions (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="Structures.html" rel="up" title="Structures">
|
|---|
| 33 | <link href="Packing-With-Unions.html" rel="next" title="Packing With Unions">
|
|---|
| 34 | <link href="Structure-Assignment.html" rel="prev" title="Structure Assignment">
|
|---|
| 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="Unions"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Packing-With-Unions.html" accesskey="n" rel="next">Packing With Unions</a>, Previous: <a href="Structure-Assignment.html" accesskey="p" rel="prev">Structure Assignment</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</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="Unions-1"></span><h3 class="section">15.13 Unions</h3>
|
|---|
| 64 | <span id="index-unions"></span>
|
|---|
| 65 | <span id="index-union"></span>
|
|---|
| 66 |
|
|---|
| 67 | <p>A <em>union type</em> defines alternative ways of looking at the same
|
|---|
| 68 | piece of memory. Each alternative view is defined with a data type,
|
|---|
| 69 | and identified by a name. A union definition looks like this:
|
|---|
| 70 | </p>
|
|---|
| 71 | <div class="example">
|
|---|
| 72 | <pre class="example">union <var>name</var>
|
|---|
| 73 | {
|
|---|
| 74 | <var>alternative declarations</var><span class="roman">…</span>
|
|---|
| 75 | };
|
|---|
| 76 | </pre></div>
|
|---|
| 77 |
|
|---|
| 78 | <p>Each alternative declaration looks like a structure field declaration,
|
|---|
| 79 | except that it can’t be a bit field. For instance,
|
|---|
| 80 | </p>
|
|---|
| 81 | <div class="example">
|
|---|
| 82 | <pre class="example">union number
|
|---|
| 83 | {
|
|---|
| 84 | long int integer;
|
|---|
| 85 | double float;
|
|---|
| 86 | }
|
|---|
| 87 | </pre></div>
|
|---|
| 88 |
|
|---|
| 89 | <p>lets you store either an integer (type <code>long int</code>) or a floating
|
|---|
| 90 | point number (type <code>double</code>) in the same place in memory. The
|
|---|
| 91 | length and alignment of the union type are the maximum of all the
|
|---|
| 92 | alternatives—they do not have to be the same. In this union
|
|---|
| 93 | example, <code>double</code> probably takes more space than <code>long int</code>,
|
|---|
| 94 | but that doesn’t cause a problem in programs that use the union in the
|
|---|
| 95 | normal way.
|
|---|
| 96 | </p>
|
|---|
| 97 | <p>The members don’t have to be different in data type. Sometimes
|
|---|
| 98 | each member pertains to a way the data will be used. For instance,
|
|---|
| 99 | </p>
|
|---|
| 100 | <div class="example">
|
|---|
| 101 | <pre class="example">union datum
|
|---|
| 102 | {
|
|---|
| 103 | double latitude;
|
|---|
| 104 | double longitude;
|
|---|
| 105 | double height;
|
|---|
| 106 | double weight;
|
|---|
| 107 | int continent;
|
|---|
| 108 | }
|
|---|
| 109 | </pre></div>
|
|---|
| 110 |
|
|---|
| 111 | <p>This union holds one of several kinds of data; most kinds are floating
|
|---|
| 112 | points, but the value can also be a code for a continent which is an
|
|---|
| 113 | integer. You <em>could</em> use one member of type <code>double</code> to
|
|---|
| 114 | access all the values which have that type, but the different member
|
|---|
| 115 | names will make the program clearer.
|
|---|
| 116 | </p>
|
|---|
| 117 | <p>The alignment of a union type is the maximum of the alignments of the
|
|---|
| 118 | alternatives. The size of the union type is the maximum of the sizes
|
|---|
| 119 | of the alternatives, rounded up to a multiple of the alignment
|
|---|
| 120 | (because every type’s size must be a multiple of its alignment).
|
|---|
| 121 | </p>
|
|---|
| 122 | <p>All the union alternatives start at the address of the union itself.
|
|---|
| 123 | If an alternative is shorter than the union as a whole, it occupies
|
|---|
| 124 | the first part of the union’s storage, leaving the last part unused
|
|---|
| 125 | <em>for that alternative</em>.
|
|---|
| 126 | </p>
|
|---|
| 127 | <p><strong>Warning:</strong> if the code stores data using one union alternative
|
|---|
| 128 | and accesses it with another, the results depend on the kind of
|
|---|
| 129 | computer in use. Only wizards should try to do this. However, when
|
|---|
| 130 | you need to do this, a union is a clean way to do it.
|
|---|
| 131 | </p>
|
|---|
| 132 | <p>Assignment works on any union type by copying the entire value.
|
|---|
| 133 | </p>
|
|---|
| 134 | <hr>
|
|---|
| 135 | <div class="header">
|
|---|
| 136 | <p>
|
|---|
| 137 | Next: <a href="Packing-With-Unions.html" accesskey="n" rel="next">Packing With Unions</a>, Previous: <a href="Structure-Assignment.html" accesskey="p" rel="prev">Structure Assignment</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</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>
|
|---|
| 138 | </div>
|
|---|
| 139 |
|
|---|
| 140 |
|
|---|
| 141 |
|
|---|
| 142 | </body>
|
|---|
| 143 | </html>
|
|---|
