1<html lang="en"> 2<head> 3<title>Alignment - 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="C-Extensions.html#C-Extensions" title="C Extensions"> 9<link rel="prev" href="Type-Attributes.html#Type-Attributes" title="Type Attributes"> 10<link rel="next" href="Inline.html#Inline" title="Inline"> 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="Alignment"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="Inline.html#Inline">Inline</a>, 53Previous: <a rel="previous" accesskey="p" href="Type-Attributes.html#Type-Attributes">Type Attributes</a>, 54Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> 55<hr> 56</div> 57 58<h3 class="section">6.38 Inquiring on Alignment of Types or Variables</h3> 59 60<p><a name="index-alignment-2600"></a><a name="index-type-alignment-2601"></a><a name="index-variable-alignment-2602"></a> 61The keyword <code>__alignof__</code> allows you to inquire about how an object 62is aligned, or the minimum alignment usually required by a type. Its 63syntax is just like <code>sizeof</code>. 64 65 <p>For example, if the target machine requires a <code>double</code> value to be 66aligned on an 8-byte boundary, then <code>__alignof__ (double)</code> is 8. 67This is true on many RISC machines. On more traditional machine 68designs, <code>__alignof__ (double)</code> is 4 or even 2. 69 70 <p>Some machines never actually require alignment; they allow reference to any 71data type even at an odd address. For these machines, <code>__alignof__</code> 72reports the smallest alignment that GCC will give the data type, usually as 73mandated by the target ABI. 74 75 <p>If the operand of <code>__alignof__</code> is an lvalue rather than a type, 76its value is the required alignment for its type, taking into account 77any minimum alignment specified with GCC's <code>__attribute__</code> 78extension (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>). For example, after this 79declaration: 80 81<pre class="smallexample"> struct foo { int x; char y; } foo1; 82</pre> 83 <p class="noindent">the value of <code>__alignof__ (foo1.y)</code> is 1, even though its actual 84alignment is probably 2 or 4, the same as <code>__alignof__ (int)</code>. 85 86 <p>It is an error to ask for the alignment of an incomplete type. 87 88 </body></html> 89 90