Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!ukma!uflorida!haven!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: address of structure == address of first member?
Message-ID: <14725@mimsy.UUCP>
Date: 26 Nov 88 23:35:51 GMT
References: <2172@iscuva.ISCS.COM> <8954@smoke.BRL.MIL> <2176@iscuva.ISCS.COM> <8976@smoke.BRL.MIL>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 49

[Amazing that no one else has jumped on this yet.]
>In article <2176@iscuva.ISCS.COM> carlp@iscuva.ISCS.COM (Carl Paukstis) writes:
>>86	        code = strcmp (key, *(char **)((char *)table + (m * size)));
[much edited]

In article <8976@smoke.BRL.MIL> gwyn@smoke.BRL.MIL (Doug Gwyn) suggests:
>You already had a (char *).  You then cast it to (char **), and then
>dereferenced that using the * operator.  There was no need for these
>extra steps when the original (char *) was just what you needed.

But the original `char *' is not what he needs, and this does not compile
into an effective no-op (as you suggested in <8954@smoke.brl.mil>).  If
we write:

	char *p;
	
	p = (char *)table + (m * size);

we have p pointing at (not `to': I am reserving that for a pointer of
the appropriate type) an object of `struct <something>'.  This structure
is defined as:

	struct <something> {
		char	*name;
		...
	};

To be completely type-correct, we should convert `p' into a pointer to
the correct structure type-name, and follow that to its `name' field.
The problem, of course, is that in this general-purpose search routine,
we have no idea *which* structure `p' is standing for [to end the
sentence a preposition with].  There are two alternatives: define a
structure that contains only a name; or cheat.

	struct name { char *name; };

	result = strcmp(key, ((struct name *)p)->name);

or

	result = strcmp(key, *(char **)p);

The former is in some sense superiour, particularly under K&R rules,
where any pair of structures that have an initial common set of names
and types are, in that common part, compatible.  (This rule is left
over from the days before unions, and does not appear in the dpANS.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris