1.\" $OpenBSD: offsetof.3,v 1.3 2013/06/05 03:42:03 tedu 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.\" 18.Dd $Mdocdate: June 5 2013 $ 19.Dt OFFSETOF 3 20.Os 21.Sh NAME 22.Nm offsetof 23.Nd offset of a structure member 24.Sh SYNOPSIS 25.In stddef.h 26.Ft size_t 27.Fn offsetof "type" "member" 28.Sh DESCRIPTION 29The 30.Fn offsetof 31macro expands to an integer constant expression of type 32.Ft size_t 33and yields the offset, 34in bytes, of the field 35.Ar member 36from the start of the structure 37.Ar type . 38.Pp 39A compiler error will result if 40.Ar member 41is not aligned to a byte boundary (i.e. it is a bit-field). 42.Sh STANDARDS 43The 44.Fn offsetof 45macro conforms to 46.St -ansiC . 47