Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!sun!pitstop!sundc!seismo!uunet!vsi!friedl
From: friedl@vsi.COM (Stephen J. Friedl)
Newsgroups: comp.lang.c
Subject: Re: arithmetic on void pointers
Summary: Chapter and verse...
Message-ID: <1067@vsi.COM>
Date: 7 Mar 89 03:45:32 GMT
References: <1527@ncar.ucar.edu>
Organization: V-Systems, Inc. -- Santa Ana, CA
Lines: 52

In article <1527@ncar.ucar.edu>, russ@groucho.ucar.edu (Russ Rew) writes:
> What does the latest dpANS document say?  Is it legal
> to increment a variable of type void* ?

No.

When incrementing a pointer, one multiplies the increment by the
size of the pointed-to object.  Since a `void' has no size, you
can't get to the next one.

The dpANS does not directly say "You can't ++ a void*", so to prove
it you need to string a couple of parts together.

From the May88 dpANS:  (renditions: ITALICS, /Courier/)

	3.1.2.5 - Types

		"The /void/ type comprises an empty set of values; it
		is an incomplete type that cannot be completed"

### /void/ is an incomplete type

	3.3.3.4 - The /sizeof/ operator

		"The /sizeof/ operator shall not be applied to an
		expression that has function type or an incomplete
		type [...]"

### you can't take the size of /void/

	3.3.2.4 - Postfix increment and decrement operators
and	3.3.3.1 - Prefix increment and decrement operators

		"The operand of the prefix increment or decrement
		operator shall have qualified or unqualified scalar
		type and shall be a modifiable lvalue."

### you can't ++ or -- a /void/



     I would guess that those implementations that permit ++ on
void pointers do so by treating them as char pointers instead.

     Steve, the staggering dpANS (with apologies to Karl)

-- 
Stephen J. Friedl / V-Systems, Inc. / Santa Ana, CA / +1 714 545 6442 
3B2-kind-of-guy   / friedl@vsi.com  / {attmail, uunet, etc}!vsi!friedl

"Cold beer, hot women, and fast compilers" - me