Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!columbia!rutgers!pyrnj!mirror!ima!haddock!karl
From: karl@haddock.UUCP (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: structure element offsets
Message-ID: <186@haddock.UUCP>
Date: Thu, 4-Dec-86 20:42:53 EST
Article-I.D.: haddock.186
Posted: Thu Dec  4 20:42:53 1986
Date-Received: Fri, 5-Dec-86 06:06:54 EST
References: <1096@spice.cs.cmu.edu> <292@bms-at.UUCP> <720@dg_rtp.UUCP>
Reply-To: karl@haddock.ISC.COM.UUCP (Karl Heuer)
Organization: Interactive Systems, Boston
Lines: 21
Summary: It's available in ANSI

In article <720@dg_rtp.UUCP> throopw@dg_rtp.UUCP (Wayne Throop) writes:
>>bader@spice.cs.cmu.edu (Miles Bader) writes:
>>> Is there any way of finding the offset of a structure element from
>>> the beginning of a structure in a portable AND efficient way?

>stuart@bms-at.UUCP (Stuart D. Gathman) writes:
>> #define spos(s,m)	((char *)&((struct s *)0)->m - (char *)0)
>
>... A couple of points remain.  First, there is no guarantee that pointer
>arithmetic or offset calculations will work for the null pointer.

Note also that the result might not be a legal constant.

However, ANSI seems to have its collective heart set on making offsetof()
part of the standard, which means that on machines where the above construct
doesn't work, *some* hook must be allowed -- even if the implementor has to
make "offsetof" a builtin operator instead of a macro.

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint
(Btw, "sposa(s,m)" is redundant since "spos(s,m[0])" will do, and the macro
would be more useful if the word "struct" were omitted.)