NetBSD/share/man/man3/offsetof.3

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.\" $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>
.\"
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.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 .