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From: jsdy@hadron.UUCP (Joseph S. D. Yao)
Newsgroups: net.unix,net.unix-wizards,net.arch,net.lang.c
Subject: Re: (Really addressing inside struct)Re: get size of malloc'd object
Message-ID: <484@hadron.UUCP>
Date: Thu, 24-Jul-86 01:09:55 EDT
Article-I.D.: hadron.484
Posted: Thu Jul 24 01:09:55 1986
Date-Received: Sun, 27-Jul-86 21:03:07 EDT
References: <165@daisy.UUCP> <334@valid.UUCP> <2206@peora.UUCP>
Reply-To: jsdy@hadron.UUCP (Joseph S. D. Yao)
Organization: Hadron, Inc., Fairfax, VA
Lines: 44
Summary: Put thereptile before the lizard, where it belongs.
Xref: mnetor net.unix:5043 net.unix-wizards:7292 net.arch:2853 net.lang.c:5472

In article <403@anasazi.UUCP> john@anasazi.UUCP (John Moore) writes:
>We have a structure with a number of structures contained within it. 
>Because of linked list processing, we have a pointer to one of the
>structures within, and we want to derive a pointer to the top of
>the structure without LINT complaining, and in a truly portable way.
>struct links {
>   struct links *forward;
>   struct links *backward;
>};
>struct foo {
>   char bar;
>   struct links snake;
>   struct whocares junk;
>   struct links lizard;
>};
	Assume struct foo ssssss;
>struct links *reptile = &ssssss.lizard;
>struct foo *iwish = (mystery function of reptile);
>Example of a wrong solution:
>struct foo *iwish = (struct foo *)((char *)reptile + (char *)0->lizard);

You can't add pointers.

There really isn't a very good and portable solution in C.  Given
your constraints, the best is to replace "+ (char *)0->lizard" with:
"- ((int)(((char *)&ssssss.lizard) - ((char *)&ssssss)))"; and even
this is not guaranteed.  What you need to do is expand your mental
model somewhat.  Assuming that links are always inside foos (to
get out of the necessity of using unions):

struct links {
	struct foo *forward;
	struct foo *backward;
};

Replace mystery-function with identity.
Replace ->forward with ->forward->lizard or ->forward->snake, as
appropriate.  You may find yourself realizing a better yet way to
express yourself, and say what you mean (instead of what you thought
you wanted to cleverly express).
-- 

	Joe Yao		hadron!jsdy@seismo.{CSS.GOV,ARPA,UUCP}
			jsdy@hadron.COM (not yet domainised)