<html lang="en">
<head>
<title>Structure-Packing Pragmas - Using the GNU Compiler Collection (GCC)</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="Using the GNU Compiler Collection (GCC)">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Pragmas.html#Pragmas" title="Pragmas">
<link rel="prev" href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas" title="Symbol-Renaming Pragmas">
<link rel="next" href="Weak-Pragmas.html#Weak-Pragmas" title="Weak Pragmas">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1988-2015 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.-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
pre.display { font-family:inherit }
pre.format { font-family:inherit }
pre.smalldisplay { font-family:inherit; font-size:smaller }
pre.smallformat { font-family:inherit; font-size:smaller }
pre.smallexample { font-size:smaller }
pre.smalllisp { font-size:smaller }
span.sc { font-variant:small-caps }
span.roman { font-family:serif; font-weight:normal; }
span.sansserif { font-family:sans-serif; font-weight:normal; }
--></style>
</head>
<body>
<div class="node">
<a name="Structure-Packing-Pragmas"></a>
<a name="Structure_002dPacking-Pragmas"></a>
<p>
Next: <a rel="next" accesskey="n" href="Weak-Pragmas.html#Weak-Pragmas">Weak Pragmas</a>,
Previous: <a rel="previous" accesskey="p" href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas">Symbol-Renaming Pragmas</a>,
Up: <a rel="up" accesskey="u" href="Pragmas.html#Pragmas">Pragmas</a>
<hr>
</div>
<h4 class="subsection">6.60.8 Structure-Packing Pragmas</h4>
<p>For compatibility with Microsoft Windows compilers, GCC supports a
set of <code>#pragma</code> directives that change the maximum alignment of
members of structures (other than zero-width bit-fields), unions, and
classes subsequently defined. The <var>n</var> value below always is required
to be a small power of two and specifies the new alignment in bytes.
<ol type=1 start=1>
<li><code>#pragma pack(</code><var>n</var><code>)</code> simply sets the new alignment.
<li><code>#pragma pack()</code> sets the alignment to the one that was in
effect when compilation started (see also command-line option
<samp><span class="option">-fpack-struct[=</span><var>n</var><span class="option">]</span></samp> see <a href="Code-Gen-Options.html#Code-Gen-Options">Code Gen Options</a>).
<li><code>#pragma pack(push[,</code><var>n</var><code>])</code> pushes the current alignment
setting on an internal stack and then optionally sets the new alignment.
<li><code>#pragma pack(pop)</code> restores the alignment setting to the one
saved at the top of the internal stack (and removes that stack entry).
Note that <code>#pragma pack([</code><var>n</var><code>])</code> does not influence this internal
stack; thus it is possible to have <code>#pragma pack(push)</code> followed by
multiple <code>#pragma pack(</code><var>n</var><code>)</code> instances and finalized by a single
<code>#pragma pack(pop)</code>.
</ol>
<p>Some targets, e.g. x86 and PowerPC, support the <code>ms_struct</code>
<code>#pragma</code> which lays out a structure as the documented
<code>__attribute__ ((ms_struct))</code>.
<ol type=1 start=1>
<li><code>#pragma ms_struct on</code> turns on the layout for structures
declared.
<li><code>#pragma ms_struct off</code> turns off the layout for structures
declared.
<li><code>#pragma ms_struct reset</code> goes back to the default layout.
</ol>
</body></html>