You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
117 lines
5.0 KiB
HTML
117 lines
5.0 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation; with the
|
|
Invariant Sections being "Funding Free Software", the Front-Cover
|
|
Texts being (a) (see below), and with the Back-Cover Texts being (b)
|
|
(see below). A copy of the license is included in the section entitled
|
|
"GNU Free Documentation License".
|
|
|
|
(a) The FSF's Front-Cover Text is:
|
|
|
|
A GNU Manual
|
|
|
|
(b) The FSF's Back-Cover Text is:
|
|
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
|
software. Copies published by the Free Software Foundation raise
|
|
funds for GNU development. -->
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Alignment (Using the GNU Compiler Collection (GCC))</title>
|
|
|
|
<meta name="description" content="Alignment (Using the GNU Compiler Collection (GCC))">
|
|
<meta name="keywords" content="Alignment (Using the GNU Compiler Collection (GCC))">
|
|
<meta name="resource-type" content="document">
|
|
<meta name="distribution" content="global">
|
|
<meta name="Generator" content="makeinfo">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<link href="index.html#Top" rel="start" title="Top">
|
|
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
|
|
<link href="Inline.html#Inline" rel="next" title="Inline">
|
|
<link href="Character-Escapes.html#Character-Escapes" rel="prev" title="Character Escapes">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
div.smalllisp {margin-left: 3.2em}
|
|
kbd {font-style: oblique}
|
|
pre.display {font-family: inherit}
|
|
pre.format {font-family: inherit}
|
|
pre.menu-comment {font-family: serif}
|
|
pre.menu-preformatted {font-family: serif}
|
|
pre.smalldisplay {font-family: inherit; font-size: smaller}
|
|
pre.smallexample {font-size: smaller}
|
|
pre.smallformat {font-family: inherit; font-size: smaller}
|
|
pre.smalllisp {font-size: smaller}
|
|
span.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="Alignment"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Inline.html#Inline" accesskey="n" rel="next">Inline</a>, Previous: <a href="Character-Escapes.html#Character-Escapes" accesskey="p" rel="prev">Character Escapes</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
|
|
</div>
|
|
<hr>
|
|
<a name="Inquiring-on-Alignment-of-Types-or-Variables"></a>
|
|
<h3 class="section">6.42 Inquiring on Alignment of Types or Variables</h3>
|
|
<a name="index-alignment"></a>
|
|
<a name="index-type-alignment"></a>
|
|
<a name="index-variable-alignment"></a>
|
|
|
|
<p>The keyword <code>__alignof__</code> allows you to inquire about how an object
|
|
is aligned, or the minimum alignment usually required by a type. Its
|
|
syntax is just like <code>sizeof</code>.
|
|
</p>
|
|
<p>For example, if the target machine requires a <code>double</code> value to be
|
|
aligned on an 8-byte boundary, then <code>__alignof__ (double)</code> is 8.
|
|
This is true on many RISC machines. On more traditional machine
|
|
designs, <code>__alignof__ (double)</code> is 4 or even 2.
|
|
</p>
|
|
<p>Some machines never actually require alignment; they allow reference to any
|
|
data type even at an odd address. For these machines, <code>__alignof__</code>
|
|
reports the smallest alignment that GCC gives the data type, usually as
|
|
mandated by the target ABI.
|
|
</p>
|
|
<p>If the operand of <code>__alignof__</code> is an lvalue rather than a type,
|
|
its value is the required alignment for its type, taking into account
|
|
any minimum alignment specified with GCC’s <code>__attribute__</code>
|
|
extension (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>). For example, after this
|
|
declaration:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">struct foo { int x; char y; } foo1;
|
|
</pre></div>
|
|
|
|
<p>the value of <code>__alignof__ (foo1.y)</code> is 1, even though its actual
|
|
alignment is probably 2 or 4, the same as <code>__alignof__ (int)</code>.
|
|
</p>
|
|
<p>It is an error to ask for the alignment of an incomplete type.
|
|
</p>
|
|
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|