118 lines
5.1 KiB
HTML
118 lines
5.1 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2016 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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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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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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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using the GNU Compiler Collection (GCC): Alignment</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Alignment">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Alignment">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Inline.html#Inline" rel="next" title="Inline">
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<link href="Character-Escapes.html#Character-Escapes" rel="prev" title="Character Escapes">
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<style type="text/css">
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Alignment"></a>
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<div class="header">
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<p>
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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>
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</div>
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<hr>
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<a name="Inquiring-on-Alignment-of-Types-or-Variables"></a>
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<h3 class="section">6.41 Inquiring on Alignment of Types or Variables</h3>
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<a name="index-alignment"></a>
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<a name="index-type-alignment"></a>
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<a name="index-variable-alignment"></a>
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<p>The keyword <code>__alignof__</code> allows you to inquire about how an object
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is aligned, or the minimum alignment usually required by a type. Its
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syntax is just like <code>sizeof</code>.
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</p>
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<p>For example, if the target machine requires a <code>double</code> value to be
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aligned on an 8-byte boundary, then <code>__alignof__ (double)</code> is 8.
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This is true on many RISC machines. On more traditional machine
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designs, <code>__alignof__ (double)</code> is 4 or even 2.
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</p>
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<p>Some machines never actually require alignment; they allow reference to any
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data type even at an odd address. For these machines, <code>__alignof__</code>
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reports the smallest alignment that GCC gives the data type, usually as
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mandated by the target ABI.
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</p>
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<p>If the operand of <code>__alignof__</code> is an lvalue rather than a type,
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its value is the required alignment for its type, taking into account
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any minimum alignment specified with GCC’s <code>__attribute__</code>
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extension (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>). For example, after this
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declaration:
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct foo { int x; char y; } foo1;
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</pre></div>
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<p>the value of <code>__alignof__ (foo1.y)</code> is 1, even though its actual
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alignment is probably 2 or 4, the same as <code>__alignof__ (int)</code>.
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</p>
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<p>It is an error to ask for the alignment of an incomplete type.
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</p>
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</body>
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</html>
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