<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- Copyright (C) 1988-2018 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. --> <!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ --> <head> <title>Offsetof (Using the GNU Compiler Collection (GCC))</title> <meta name="description" content="Offsetof (Using the GNU Compiler Collection (GCC))"> <meta name="keywords" content="Offsetof (Using the GNU Compiler Collection (GCC))"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="makeinfo"> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <link href="index.html#Top" rel="start" title="Top"> <link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> <link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions"> <link href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" rel="next" title="__sync Builtins"> <link href="Vector-Extensions.html#Vector-Extensions" rel="prev" title="Vector Extensions"> <style type="text/css"> <!-- a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} blockquote.smallquotation {font-size: smaller} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} div.lisp {margin-left: 3.2em} div.smalldisplay {margin-left: 3.2em} div.smallexample {margin-left: 3.2em} div.smalllisp {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} pre.smalldisplay {font-family: inherit; font-size: smaller} pre.smallexample {font-size: smaller} pre.smallformat {font-family: inherit; font-size: smaller} pre.smalllisp {font-size: smaller} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} ul.no-bullet {list-style: none} --> </style> </head> <body lang="en"> <a name="Offsetof"></a> <div class="header"> <p> Next: <a href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" accesskey="n" rel="next">__sync Builtins</a>, Previous: <a href="Vector-Extensions.html#Vector-Extensions" accesskey="p" rel="prev">Vector Extensions</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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> </div> <hr> <a name="Support-for-offsetof"></a> <h3 class="section">6.51 Support for <code>offsetof</code></h3> <a name="index-_005f_005fbuiltin_005foffsetof"></a> <p>GCC implements for both C and C++ a syntactic extension to implement the <code>offsetof</code> macro. </p> <div class="smallexample"> <pre class="smallexample">primary: "__builtin_offsetof" "(" <code>typename</code> "," offsetof_member_designator ")" offsetof_member_designator: <code>identifier</code> | offsetof_member_designator "." <code>identifier</code> | offsetof_member_designator "[" <code>expr</code> "]" </pre></div> <p>This extension is sufficient such that </p> <div class="smallexample"> <pre class="smallexample">#define offsetof(<var>type</var>, <var>member</var>) __builtin_offsetof (<var>type</var>, <var>member</var>) </pre></div> <p>is a suitable definition of the <code>offsetof</code> macro. In C++, <var>type</var> may be dependent. In either case, <var>member</var> may consist of a single identifier, or a sequence of member accesses and array references. </p> </body> </html>