1.\" $NetBSD: __alignof__.3,v 1.5 2011/04/14 06:56:28 wiz Exp $ 2.\" 3.\" Copyright (c) 2010 Jukka Ruohonen <jruohonen@iki.fi> 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 16.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 17.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 18.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 19.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 20.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 21.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 22.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 23.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 24.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 25.\" POSSIBILITY OF SUCH DAMAGE. 26.\" 27.Dd December 20, 2010 28.Dt __ALIGNOF__ 3 29.Os 30.Sh NAME 31.Nm __alignof__ 32.Nd GNU extension for alignment of an object 33.Sh SYNOPSIS 34.Ft int 35.Fn __alignof__ "void x" 36.Sh DESCRIPTION 37The 38.Fn __alignof__ 39operator returns the alignment of its operand. 40The operand can be a type or an expression. 41If the operand is a 42.Sq lvalue , 43the return value represents the required alignment of the underlying type, 44not the actual alignment of the specified 45.Sq lvalue . 46.Pp 47The returned value is specific to the architecture and the 48.Tn ABI . 49If the architecture does not impose strict alignment requirements, 50.Fn __alignof__ 51returns the minimum required alignment. 52If 53.Xr __aligned 3 54is used to increase the alignment, 55.Fn __alignof__ 56returns the specified alignment. 57.Sh EXAMPLES 58The syntax is comparable to the 59.Fn sizeof 60operator. 61If the architecture aligns integers along 32-bit address boundaries, 62the following should print the value 4. 63.Bd -literal -offset indent 64(void)printf("%d\en", __alignof__(int)); 65.Ed 66.Pp 67On the other hand, the following example should print the value 1, 68even though this is unlikely to be the actual alignment of the 69structure member. 70.Bd -literal -offset indent 71struct align { 72 int x; 73 char y; 74} a; 75 76(void)printf("%d\en", __alignof__(a.y)); 77.Ed 78.Sh SEE ALSO 79.Xr gcc 1 , 80.Xr attribute 3 , 81.Xr offsetof 3 , 82.Xr typeof 3 83.Sh CAVEATS 84This is a non-standard, compiler-specific extension. 85