1<html lang="en"> 2<head> 3<title>Structure-Packing Pragmas - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Pragmas.html#Pragmas" title="Pragmas"> 9<link rel="prev" href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas" title="Symbol-Renaming Pragmas"> 10<link rel="next" href="Weak-Pragmas.html#Weak-Pragmas" title="Weak Pragmas"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). A copy of the license is included in the section entitled 23``GNU Free Documentation License''. 24 25(a) The FSF's Front-Cover Text is: 26 27 A GNU Manual 28 29(b) The FSF's Back-Cover Text is: 30 31 You have freedom to copy and modify this GNU Manual, like GNU 32 software. Copies published by the Free Software Foundation raise 33 funds for GNU development.--> 34<meta http-equiv="Content-Style-Type" content="text/css"> 35<style type="text/css"><!-- 36 pre.display { font-family:inherit } 37 pre.format { font-family:inherit } 38 pre.smalldisplay { font-family:inherit; font-size:smaller } 39 pre.smallformat { font-family:inherit; font-size:smaller } 40 pre.smallexample { font-size:smaller } 41 pre.smalllisp { font-size:smaller } 42 span.sc { font-variant:small-caps } 43 span.roman { font-family:serif; font-weight:normal; } 44 span.sansserif { font-family:sans-serif; font-weight:normal; } 45--></style> 46<link rel="stylesheet" type="text/css" href="../cs.css"> 47</head> 48<body> 49<div class="node"> 50<a name="Structure-Packing-Pragmas"></a> 51<a name="Structure_002dPacking-Pragmas"></a> 52<p> 53Next: <a rel="next" accesskey="n" href="Weak-Pragmas.html#Weak-Pragmas">Weak Pragmas</a>, 54Previous: <a rel="previous" accesskey="p" href="Symbol_002dRenaming-Pragmas.html#Symbol_002dRenaming-Pragmas">Symbol-Renaming Pragmas</a>, 55Up: <a rel="up" accesskey="u" href="Pragmas.html#Pragmas">Pragmas</a> 56<hr> 57</div> 58 59<h4 class="subsection">6.56.8 Structure-Packing Pragmas</h4> 60 61<p>For compatibility with Microsoft Windows compilers, GCC supports a 62set of <code>#pragma</code> directives which change the maximum alignment of 63members of structures (other than zero-width bitfields), unions, and 64classes subsequently defined. The <var>n</var> value below always is required 65to be a small power of two and specifies the new alignment in bytes. 66 67 <ol type=1 start=1> 68<li><code>#pragma pack(</code><var>n</var><code>)</code> simply sets the new alignment. 69<li><code>#pragma pack()</code> sets the alignment to the one that was in 70effect when compilation started (see also command-line option 71<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>). 72<li><code>#pragma pack(push[,</code><var>n</var><code>])</code> pushes the current alignment 73setting on an internal stack and then optionally sets the new alignment. 74<li><code>#pragma pack(pop)</code> restores the alignment setting to the one 75saved at the top of the internal stack (and removes that stack entry). 76Note that <code>#pragma pack([</code><var>n</var><code>])</code> does not influence this internal 77stack; thus it is possible to have <code>#pragma pack(push)</code> followed by 78multiple <code>#pragma pack(</code><var>n</var><code>)</code> instances and finalized by a single 79<code>#pragma pack(pop)</code>. 80 </ol> 81 82 <p>Some targets, e.g. i386 and powerpc, support the <code>ms_struct</code> 83<code>#pragma</code> which lays out a structure as the documented 84<code>__attribute__ ((ms_struct))</code>. 85 <ol type=1 start=1> 86<li><code>#pragma ms_struct on</code> turns on the layout for structures 87declared. 88<li><code>#pragma ms_struct off</code> turns off the layout for structures 89declared. 90<li><code>#pragma ms_struct reset</code> goes back to the default layout. 91 </ol> 92 93 </body></html> 94 95