Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uwm.edu!spool.mu.edu!munnari.oz.au!goanna!ok
From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: "void **" : Is it valid and what does it mean?
Message-ID: <6001@goanna.cs.rmit.oz.au>
Date: 28 May 91 07:17:22 GMT
References: <AJ3U.91May27213633@jade.cs.virginia.edu>
Distribution: comp
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 45

Yes, void ** is valid, and it means pointer-to-generic pointer.
Note that void ** itself may be represented some other way entirely;
void ** is not itself a generic pointer.

In article <AJ3U.91May27213633@jade.cs.virginia.edu>, aj3u@jade.cs.virginia.edu (Asim Jalis) writes:
> func(void **p)
> { ... }
> 
> main()
> { int *i; float *f; char *c;
> 
>   func(&i); func(&f); func(&c);
> }
> 
> However, the compiler gives me warnings about incompatible type for
> all the function calls.

The compiler is RIGHT.  You promised it (oh, you deceiver) that
you would give func() pointers to generic (void*) pointers
(call the representation of generic pointers REPRESENTATION 0)
But you lied (oh, you scoundrel, you).  You gave it
    -- a pointer to (a pointer to int  -- REPRESENTATION 1)
    -- a pointer to (a pointer to float -- REPRESENTATION 2)
    -- a pointer to (a pointer to char -- REPRESENTATION 0)
char* and void* use the same representation.
But void*, int*, and float* need not even be the same _size_, let
alone use the same physical representation.

> The code itself works fine,

no, there is a bug in there, waiting, like a blood-sucking insect
poised on a bush waiting for a deer to pass by, for an opportunity to bite.

> Basically what I want is a type that would coerce to a pointer to a
> pointer to a type, for any type.  Sort of one level beyond simple
> "void *".

Why do you want to do _that_?  If you _could_ do it, you would be
throwing away information that the receiver would need in order to
be able to _use_ the thing?  What's the context?

-- 
Should you ever intend to dull the wits of a young man and to
incapacitate his brains for any kind of thought whatever, then
you cannot do better than give him Hegel to read.  -- Schopenhauer.