1<html lang="en"> 2<head> 3<title>Offsetof - 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="Vector-Extensions.html#Vector-Extensions" title="Vector Extensions"> 10<link rel="next" href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins" title="__sync Builtins"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988-2013 Free Software Foundation, Inc. 14 15Permission is granted to copy, distribute and/or modify this document 16under the terms of the GNU Free Documentation License, Version 1.3 or 17any later version published by the Free Software Foundation; with the 18Invariant Sections being ``Funding Free Software'', the Front-Cover 19Texts being (a) (see below), and with the Back-Cover Texts being (b) 20(see below). A copy of the license is included in the section entitled 21``GNU Free Documentation License''. 22 23(a) The FSF's Front-Cover Text is: 24 25 A GNU Manual 26 27(b) The FSF's Back-Cover Text is: 28 29 You have freedom to copy and modify this GNU Manual, like GNU 30 software. Copies published by the Free Software Foundation raise 31 funds for GNU development.--> 32<meta http-equiv="Content-Style-Type" content="text/css"> 33<style type="text/css"><!-- 34 pre.display { font-family:inherit } 35 pre.format { font-family:inherit } 36 pre.smalldisplay { font-family:inherit; font-size:smaller } 37 pre.smallformat { font-family:inherit; font-size:smaller } 38 pre.smallexample { font-size:smaller } 39 pre.smalllisp { font-size:smaller } 40 span.sc { font-variant:small-caps } 41 span.roman { font-family:serif; font-weight:normal; } 42 span.sansserif { font-family:sans-serif; font-weight:normal; } 43--></style> 44<link rel="stylesheet" type="text/css" href="../cs.css"> 45</head> 46<body> 47<div class="node"> 48<a name="Offsetof"></a> 49<p> 50Next: <a rel="next" accesskey="n" href="_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins">__sync Builtins</a>, 51Previous: <a rel="previous" accesskey="p" href="Vector-Extensions.html#Vector-Extensions">Vector Extensions</a>, 52Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> 53<hr> 54</div> 55 56<h3 class="section">6.50 Offsetof</h3> 57 58<p><a name="index-g_t_005f_005fbuiltin_005foffsetof-2909"></a> 59GCC implements for both C and C++ a syntactic extension to implement 60the <code>offsetof</code> macro. 61 62<pre class="smallexample"> primary: 63 "__builtin_offsetof" "(" <code>typename</code> "," offsetof_member_designator ")" 64 65 offsetof_member_designator: 66 <code>identifier</code> 67 | offsetof_member_designator "." <code>identifier</code> 68 | offsetof_member_designator "[" <code>expr</code> "]" 69</pre> 70 <p>This extension is sufficient such that 71 72<pre class="smallexample"> #define offsetof(<var>type</var>, <var>member</var>) __builtin_offsetof (<var>type</var>, <var>member</var>) 73</pre> 74 <p class="noindent">is a suitable definition of the <code>offsetof</code> macro. In C++, <var>type</var> 75may be dependent. In either case, <var>member</var> may consist of a single 76identifier, or a sequence of member accesses and array references. 77 78 </body></html> 79 80