1.\" $OpenBSD: offsetof.3,v 1.2 2010/02/18 18:30:19 jmc Exp $ 2.\" 3.\" Copyright (c) 2010 Thomas Pfaff <tpfaff@tp76.info> 4.\" 5.\" Permission to use, copy, modify, and distribute this software for any 6.\" purpose with or without fee is hereby granted, provided that the above 7.\" copyright notice and this permission notice appear in all copies. 8.\" 9.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 10.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 11.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 12.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 13.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 14.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 15.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 16.\" 17.\" $FreeBSD$ 18.\" 19.Dd February 18 2010 20.Dt OFFSETOF 3 21.Os 22.Sh NAME 23.Nm offsetof 24.Nd offset of a structure member 25.Sh SYNOPSIS 26.In stddef.h 27.Ft size_t 28.Fn offsetof "type" "member" 29.Sh DESCRIPTION 30The 31.Fn offsetof 32macro expands to an integer constant expression of type 33.Ft size_t 34and yields the offset, 35in bytes, of the field 36.Ar member 37from the start of the structure 38.Ar type . 39.Pp 40A compiler error will result if 41.Ar member 42is not aligned to a byte boundary (i.e. it is a bit-field). 43.Sh STANDARDS 44The 45.Fn offsetof 46macro conforms to 47.St -ansiC . 48