Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!att!cbnewsl!dfp
From: dfp@cbnewsl.ATT.COM (david.f.prosser)
Newsgroups: comp.lang.c
Subject: Re: offsetof() macro
Message-ID: <1629@cbnewsl.ATT.COM>
Date: 25 Aug 89 16:21:38 GMT
References: <70@motto.UUCP>
Reply-To: dfp@cbnewsl.ATT.COM (david.f.prosser)
Organization: AT&T Bell Laboratories
Lines: 47

In article <70@motto.UUCP> dave@motto.UUCP (dave brown) writes:
>My questions are:
>
>1. Am I right about such a macro [offsetof] being proposed?

Yes.

>2. Was it accepted?

Yes.

>3. What arguments does it take?  What value does it return?

	#include <stddef.h>
	size_t offsetof(_type_, _member_);
where
	if given ``static _type_ t;'', then ``&(t._member_)'' must
	be a valid address constant.

>4. Can it be written for all compilers?  Can one version be portable,
>   or would different versions have to be written for different compilers?

There is no known valid C expression that meets all the requirements
for a portable offsetof macro.  This is why it was standardized as
such, instead of giving enough power to constant expressions.

>5. Can it be used in a static initializer, ex:
>
>	size_t mbr_off = offsetof(...);

Yes.

>5. If it could be portable, can you supply a definition.  If not, can you
>   supply one which would work in most cases?

The Rationale suggests some possibilities:

	(size_t)&(((_type_*)0)->_member_)
	(size_t)(char *)&(((_type_*)0)->_member_)
	(size_t)(((char *)&(((_type_*)0)->_member_))-((char *)0))
	(size_t)(((char *)&(((_type_*)&X)->_member_))-((char *)&X))

where in the last X is some predeclared static object address.

None of these are portable.

Dave Prosser	...not an official X3J11 answer...