| 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>Structure Layout (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Structure Layout (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Structure Layout (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="Packed-Structures.html" rel="next" title="Packed Structures">
|
|---|
| 34 | <link href="Field-Offset.html" rel="prev" title="Field Offset">
|
|---|
| 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="Structure-Layout"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Packed-Structures.html" accesskey="n" rel="next">Packed Structures</a>, Previous: <a href="Field-Offset.html" accesskey="p" rel="prev">Field Offset</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="Structure-Layout-1"></span><h3 class="section">15.4 Structure Layout</h3>
|
|---|
| 64 | <span id="index-structure-layout"></span>
|
|---|
| 65 | <span id="index-layout-of-structures"></span>
|
|---|
| 66 |
|
|---|
| 67 | <p>The rest of this chapter covers advanced topics about structures. If
|
|---|
| 68 | you are just learning C, you can skip it.
|
|---|
| 69 | </p>
|
|---|
| 70 | <p>The precise layout of a <code>struct</code> type is crucial when using it to
|
|---|
| 71 | overlay hardware registers, to access data structures in shared
|
|---|
| 72 | memory, or to assemble and disassemble packets for network
|
|---|
| 73 | communication. It is also important for avoiding memory waste when
|
|---|
| 74 | the program makes many objects of that type. However, the layout
|
|---|
| 75 | depends on the target platform. Each platform has conventions for
|
|---|
| 76 | structure layout, which compilers need to follow.
|
|---|
| 77 | </p>
|
|---|
| 78 | <p>Here are the conventions used on most platforms.
|
|---|
| 79 | </p>
|
|---|
| 80 | <p>The structure’s fields appear in the structure layout in the order
|
|---|
| 81 | they are declared. When possible, consecutive fields occupy
|
|---|
| 82 | consecutive bytes within the structure. However, if a field’s type
|
|---|
| 83 | demands more alignment than it would get that way, C gives it the
|
|---|
| 84 | alignment it requires by leaving a gap after the previous field.
|
|---|
| 85 | </p>
|
|---|
| 86 | <p>Once all the fields have been laid out, it is possible to determine
|
|---|
| 87 | the structure’s alignment and size. The structure’s alignment is the
|
|---|
| 88 | maximum alignment of any of the fields in it. Then the structure’s
|
|---|
| 89 | size is rounded up to a multiple of its alignment. That may require
|
|---|
| 90 | leaving a gap at the end of the structure.
|
|---|
| 91 | </p>
|
|---|
| 92 | <p>Here are some examples, where we assume that <code>char</code> has size and
|
|---|
| 93 | alignment 1 (always true), and <code>int</code> has size and alignment 4
|
|---|
| 94 | (true on most kinds of computers):
|
|---|
| 95 | </p>
|
|---|
| 96 | <div class="example">
|
|---|
| 97 | <pre class="example">struct foo
|
|---|
| 98 | {
|
|---|
| 99 | char a, b;
|
|---|
| 100 | int c;
|
|---|
| 101 | };
|
|---|
| 102 | </pre></div>
|
|---|
| 103 |
|
|---|
| 104 | <p>This structure occupies 8 bytes, with an alignment of 4. <code>a</code> is
|
|---|
| 105 | at offset 0, <code>b</code> is at offset 1, and <code>c</code> is at offset 4.
|
|---|
| 106 | There is a gap of 2 bytes before <code>c</code>.
|
|---|
| 107 | </p>
|
|---|
| 108 | <p>Contrast that with this structure:
|
|---|
| 109 | </p>
|
|---|
| 110 | <div class="example">
|
|---|
| 111 | <pre class="example">struct foo
|
|---|
| 112 | {
|
|---|
| 113 | char a;
|
|---|
| 114 | int c;
|
|---|
| 115 | char b;
|
|---|
| 116 | };
|
|---|
| 117 | </pre></div>
|
|---|
| 118 |
|
|---|
| 119 | <p>This structure has size 12 and alignment 4. <code>a</code> is at offset 0,
|
|---|
| 120 | <code>c</code> is at offset 4, and <code>b</code> is at offset 8. There are two
|
|---|
| 121 | gaps: three bytes before <code>c</code>, and three bytes at the end.
|
|---|
| 122 | </p>
|
|---|
| 123 | <p>These two structures have the same contents at the C level, but one
|
|---|
| 124 | takes 8 bytes and the other takes 12 bytes due to the ordering of the
|
|---|
| 125 | fields. A reliable way to avoid this sort of wastage is to order the
|
|---|
| 126 | fields by size, biggest fields first.
|
|---|
| 127 | </p>
|
|---|
| 128 | <hr>
|
|---|
| 129 | <div class="header">
|
|---|
| 130 | <p>
|
|---|
| 131 | Next: <a href="Packed-Structures.html" accesskey="n" rel="next">Packed Structures</a>, Previous: <a href="Field-Offset.html" accesskey="p" rel="prev">Field Offset</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>
|
|---|
| 132 | </div>
|
|---|
| 133 |
|
|---|
| 134 |
|
|---|
| 135 |
|
|---|
| 136 | </body>
|
|---|
| 137 | </html>
|
|---|
