.\"	$NetBSD: offsetof.3,v 1.5 2011/04/14 06:56:28 wiz Exp $
.\"
.\"	$OpenBSD: offsetof.3,v 1.2 2010/02/18 18:30:19 jmc Exp $
.\"
.\" Copyright (c) 2010 Thomas Pfaff <tpfaff@tp76.info>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.\"
.Dd April 1, 2011
.Dt OFFSETOF 3
.Os
.Sh NAME
.Nm offsetof
.Nd offset of a structure member
.Sh SYNOPSIS
.In stddef.h
.Ft size_t
.Fn offsetof "type" "member"
.Sh DESCRIPTION
The
.Fn offsetof
macro expands to an integer constant expression of type
.Ft size_t
and yields the offset,
in bytes, of the field
.Ar member
from the start of the structure
.Ar type .
.Pp
A compiler error will result if
.Ar member
is not aligned to a byte boundary (i.e. it is a bit-field).
.Sh EXAMPLES
Regardless of the architecture and the
.Tn ABI ,
the following example prints the value zero for the variable
.Va x .
.Bd -literal -offset indent
struct example {
	double	x;
	int	y;
	char	z;
};

size_t x, y, z;

x = offsetof(struct example, x);
y = offsetof(struct example, y);
z = offsetof(struct example, z);

(void)printf("%zu %zu %zu\en", x, y, z);
.Ed
.Sh SEE ALSO
.Xr __alignof__ 3 ,
.Xr stddef 3 ,
.Xr typeof 3
.Sh STANDARDS
The
.Fn offsetof
macro conforms to
.St -ansiC .