\" $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 <
[email protected]>
\"
\" 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 .
