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From: gwyn@brl-smoke.UUCP
Newsgroups: net.lang.c
Subject: Re: (Really addressing inside struct)
Message-ID: <2151@brl-smoke.ARPA>
Date: Sat, 12-Jul-86 09:14:59 EDT
Article-I.D.: brl-smok.2151
Posted: Sat Jul 12 09:14:59 1986
Date-Received: Sun, 13-Jul-86 06:58:06 EDT
References: <165@daisy.UUCP> <334@valid.UUCP> <2206@peora.UUCP>
Reply-To: gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 35

In article <403@anasazi.UUCP> john@anasazi.UUCP (John Moore) writes:
-struct links {
-   struct links *forward;
-   struct links *backward;
-};
-
-struct foo {
-   char bar;
-   struct links snake;
-   struct whocares junk;
-   struct links lizard;
-};
-
-struct links *reptile;
-
-Assume that reptile points to the lizard structure of a "foo" structure.
-What construct allows one to do the following:
-
-struct foo *iwish = (mystery function of reptile);
-
-Example of a wrong solution:
-struct foo *iwish = (struct foo *)((char *)reptile + (char *)0->lizard);

This is rather a strange way to do business, but I can answer the question:
There is no guaranteed way to do what you're trying to do, therefore there
should be no way to keep "lint" from complaining.  However, the following
should work for most C implementations:

struct foo dummy;
#define	LIZ_OFFSET	((char *)&dummy.lizard - (char *)&dummy.bar)
struct foo *iwish = (struct foo *)((char *)reptile - LIZ_OFFSET);

What I would recommend instead is that you keep a pointer to the whole
(struct foo) rather than to the lizard member, if your application permits.
(You might need to add a (struct foo *) member to (struct links).)