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108 lines
4.5 KiB
HTML
108 lines
4.5 KiB
HTML
4 years ago
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<!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) 1988-2018 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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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Offsetof (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Offsetof (Using the GNU Compiler Collection (GCC))">
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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="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" rel="next" title="__sync Builtins">
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<link href="Vector-Extensions.html#Vector-Extensions" rel="prev" title="Vector Extensions">
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</head>
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<body lang="en">
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<a name="Offsetof"></a>
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<div class="header">
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<p>
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Next: <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" accesskey="n" rel="next">__sync Builtins</a>, Previous: <a href="Vector-Extensions.html#Vector-Extensions" accesskey="p" rel="prev">Vector Extensions</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="Support-for-offsetof"></a>
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<h3 class="section">6.51 Support for <code>offsetof</code></h3>
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<a name="index-_005f_005fbuiltin_005foffsetof"></a>
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<p>GCC implements for both C and C++ a syntactic extension to implement
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the <code>offsetof</code> macro.
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</p>
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<div class="smallexample">
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<pre class="smallexample">primary:
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"__builtin_offsetof" "(" <code>typename</code> "," offsetof_member_designator ")"
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offsetof_member_designator:
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<code>identifier</code>
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| offsetof_member_designator "." <code>identifier</code>
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| offsetof_member_designator "[" <code>expr</code> "]"
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</pre></div>
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<p>This extension is sufficient such that
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define offsetof(<var>type</var>, <var>member</var>) __builtin_offsetof (<var>type</var>, <var>member</var>)
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</pre></div>
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<p>is a suitable definition of the <code>offsetof</code> macro. In C++, <var>type</var>
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may be dependent. In either case, <var>member</var> may consist of a single
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identifier, or a sequence of member accesses and array references.
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</p>
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</body>
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</html>
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