<html lang="en"> <head> <title>Alignment - 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="C-Extensions.html#C-Extensions" title="C Extensions"> <link rel="prev" href="Type-Attributes.html#Type-Attributes" title="Type Attributes"> <link rel="next" href="Inline.html#Inline" title="Inline"> <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="Alignment"></a> <p> Next: <a rel="next" accesskey="n" href="Inline.html#Inline">Inline</a>, Previous: <a rel="previous" accesskey="p" href="Type-Attributes.html#Type-Attributes">Type Attributes</a>, Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> <hr> </div> <h3 class="section">6.40 Inquiring on Alignment of Types or Variables</h3> <p><a name="index-alignment-3397"></a><a name="index-type-alignment-3398"></a><a name="index-variable-alignment-3399"></a> The keyword <code>__alignof__</code> allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like <code>sizeof</code>. <p>For example, if the target machine requires a <code>double</code> value to be aligned on an 8-byte boundary, then <code>__alignof__ (double)</code> is 8. This is true on many RISC machines. On more traditional machine designs, <code>__alignof__ (double)</code> is 4 or even 2. <p>Some machines never actually require alignment; they allow reference to any data type even at an odd address. For these machines, <code>__alignof__</code> reports the smallest alignment that GCC gives the data type, usually as mandated by the target ABI. <p>If the operand of <code>__alignof__</code> is an lvalue rather than a type, its value is the required alignment for its type, taking into account any minimum alignment specified with GCC's <code>__attribute__</code> extension (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>). For example, after this declaration: <pre class="smallexample"> struct foo { int x; char y; } foo1; </pre> <p class="noindent">the value of <code>__alignof__ (foo1.y)</code> is 1, even though its actual alignment is probably 2 or 4, the same as <code>__alignof__ (int)</code>. <p>It is an error to ask for the alignment of an incomplete type. </body></html>