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From: dbrooks@osf.org (David Brooks)
Newsgroups: comp.lang.c
Subject: Re: finding offset of a member in C structures
Message-ID: <22138@paperboy.OSF.ORG>
Date: 23 May 91 14:34:54 GMT
References: <22051@paperboy.OSF.ORG> <16220@smoke.brl.mil> <22103@paperboy.OSF.ORG> <16236@smoke.brl.mil>
Sender: news@OSF.ORG
Organization: Open Software Foundation
Lines: 23

In article <16236@smoke.brl.mil>, gwyn@smoke.brl.mil (Doug Gwyn) writes:
|> But that's merely a design botch, because in order to ask for the offset
|> of a structure member one HAS to have a particular structure type in mind;
|> to make any use of the information one has to have a suitable pointer at
|> hand.

This is probably dragging the point out too far, but no.  The XtOffset
macro is used to initialize a static array-of-struct.  There is no reason
to have a variable of the struct type in scope.  The table information is
used by a generic procedure compiled separately, to fill in information
from an outside database using code that is probably itself non-portable-
but-invariably-works.

In practice, there are local variables of the same type later in the source
file, but they don't *have* to be there.  They too could be separately
compiled.

As I said, I agree it's a design botch, and it's nice to have ANSI define
offsetof, but it is all done with good intentions, in the interests of
information hiding.
-- 
David Brooks				dbrooks@osf.org
Systems Engineering, OSF		uunet!osf.org!dbrooks