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From: wittig@gmdzi.gmd.de (Georg Wittig)
Newsgroups: comp.lang.c
Subject: Re: "void **" : Is it valid and what does it mean?
Message-ID: <4803@gmdzi.gmd.de>
Date: 29 May 91 08:37:30 GMT
References: <AJ3U.91May27213633@jade.cs.virginia.edu> <6001@goanna.cs.rmit.oz.au> <1991May28.115914.19879@email.tuwien.ac.at>
Distribution: comp
Organization: GMD - The German National Research Center for Computer Science
Lines: 37

hp@vmars.tuwien.ac.at (Peter Holzer) writes:

>[...good explanation why void ** isn't a pointer to any pointer

>>Why do you want to do _that_?

>For some reason, which is not relevant here, msg is not a pointer to
>the structure, but a pointer to a pointer to structure. Of course one
>common error is to define a struct foo * mymsg and then pass mymsg
>instead of &mymsg. With the current prototype this error is not
>detected by the compiler, but if some type `pointer to any pointer'
>existed, this type of error could be catched. Unfortunately this is not
>possible in C.

Why not use unions? Example:

	typedef union {	struct struct1 *p_struct1;
			struct struct2 *p_struct2;
			...
		      } my_union_t;

and pass the union address to your function:

	myfunc (&myunion);
	...
	myfunc (my_union_t *p_union){
		...
		p_union -> p_struct2 -> ...

This has the advantage that you MUST define which structures are allowed in
that context, and the compiler can check consistency. ... And you don't
need void **.
-- 
Georg Wittig   GMD-Z1.IT   P.O.Box 1240 | "Freedom's just another word
D-W-5205 St. Augustin 1	   (Germany)	|  for nothing left to lose"
email:		wittig@gmdzi.gmd.de	| (from "Me and Bobby McGee",
telephone:	(+49) 2241 14-2294	|  Janis Joplin, Kris Kristofferson)