.\"	$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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.\" $FreeBSD$
.\"
.Dd February 18 2010
.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 STANDARDS
The
.Fn offsetof
macro conforms to
.St -ansiC .