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134 lines
6.5 KiB
HTML
134 lines
6.5 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>Structure-Layout Pragmas (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="Structure-Layout Pragmas (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="Structure-Layout Pragmas (Using the GNU Compiler Collection (GCC))">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="Pragmas.html#Pragmas" rel="up" title="Pragmas">
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<link href="Weak-Pragmas.html#Weak-Pragmas" rel="next" title="Weak Pragmas">
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<link href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas" rel="prev" title="Symbol-Renaming Pragmas">
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</style>
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</head>
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<body lang="en">
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<a name="Structure_002dLayout-Pragmas"></a>
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<div class="header">
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<p>
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Next: <a href="Weak-Pragmas.html#Weak-Pragmas" accesskey="n" rel="next">Weak Pragmas</a>, Previous: <a href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas" accesskey="p" rel="prev">Symbol-Renaming Pragmas</a>, Up: <a href="Pragmas.html#Pragmas" accesskey="u" rel="up">Pragmas</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="Structure_002dLayout-Pragmas-1"></a>
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<h4 class="subsection">6.61.10 Structure-Layout Pragmas</h4>
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<p>For compatibility with Microsoft Windows compilers, GCC supports a
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set of <code>#pragma</code> directives that change the maximum alignment of
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members of structures (other than zero-width bit-fields), unions, and
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classes subsequently defined. The <var>n</var> value below always is required
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to be a small power of two and specifies the new alignment in bytes.
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</p>
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<ol>
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<li> <code>#pragma pack(<var>n</var>)</code> simply sets the new alignment.
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</li><li> <code>#pragma pack()</code> sets the alignment to the one that was in
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effect when compilation started (see also command-line option
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<samp>-fpack-struct[=<var>n</var>]</samp> see <a href="Code-Gen-Options.html#Code-Gen-Options">Code Gen Options</a>).
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</li><li> <code>#pragma pack(push[,<var>n</var>])</code> pushes the current alignment
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setting on an internal stack and then optionally sets the new alignment.
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</li><li> <code>#pragma pack(pop)</code> restores the alignment setting to the one
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saved at the top of the internal stack (and removes that stack entry).
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Note that <code>#pragma pack([<var>n</var>])</code> does not influence this internal
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stack; thus it is possible to have <code>#pragma pack(push)</code> followed by
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multiple <code>#pragma pack(<var>n</var>)</code> instances and finalized by a single
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<code>#pragma pack(pop)</code>.
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</li></ol>
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<p>Some targets, e.g. x86 and PowerPC, support the <code>#pragma ms_struct</code>
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directive which lays out structures and unions subsequently defined as the
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documented <code>__attribute__ ((ms_struct))</code>.
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</p>
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<ol>
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<li> <code>#pragma ms_struct on</code> turns on the Microsoft layout.
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</li><li> <code>#pragma ms_struct off</code> turns off the Microsoft layout.
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</li><li> <code>#pragma ms_struct reset</code> goes back to the default layout.
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</li></ol>
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<p>Most targets also support the <code>#pragma scalar_storage_order</code> directive
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which lays out structures and unions subsequently defined as the documented
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<code>__attribute__ ((scalar_storage_order))</code>.
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</p>
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<ol>
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<li> <code>#pragma scalar_storage_order big-endian</code> sets the storage order
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of the scalar fields to big-endian.
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</li><li> <code>#pragma scalar_storage_order little-endian</code> sets the storage order
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of the scalar fields to little-endian.
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</li><li> <code>#pragma scalar_storage_order default</code> goes back to the endianness
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that was in effect when compilation started (see also command-line option
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<samp>-fsso-struct=<var>endianness</var></samp> see <a href="C-Dialect-Options.html#C-Dialect-Options">C Dialect Options</a>).
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</li></ol>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Weak-Pragmas.html#Weak-Pragmas" accesskey="n" rel="next">Weak Pragmas</a>, Previous: <a href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas" accesskey="p" rel="prev">Symbol-Renaming Pragmas</a>, Up: <a href="Pragmas.html#Pragmas" accesskey="u" rel="up">Pragmas</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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</body>
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</html>
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