Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!caip!topaz!ll-xn!cit-vax!nike!think!ima!haddock!karl
From: karl@haddock
Newsgroups: net.lang.c
Subject: Re: uses of void
Message-ID: <86900012@haddock>
Date: Fri, 15-Aug-86 17:07:00 EDT
Article-I.D.: haddock.86900012
Posted: Fri Aug 15 17:07:00 1986
Date-Received: Sun, 17-Aug-86 07:28:33 EDT
References: <277@sdchema.sdchem.UUCP>
Lines: 22
Nf-ID: #R:sdchema.sdchem.UUCP:277:haddock:86900012:000:883
Nf-From: haddock!karl    Aug 15 17:07:00 1986


sdchem!tps writes:
>sort( (void *)&foo[0], (void *)&foo[20], sizeof(foo), compare_foos );

First, I think you want sizeof(foo_type) or sizeof(foo[0]).

>void *beg, *end;
>int   nelements = (end - beg) / size;

>I (naively?) thought that pointer arithmetic would work with "void *", and
>that it would work in the same abstract units as "sizeof()", so that all
>previous generic "char *" kludges could be replaced by "void *".

The "abstract units" of sizeof() are "char" by definition.  (Too many users
have been assuming sizeof(char)==1, so it's official in C++ and ANSI C.  I
wish it had been measured in BITS in the first place!)  Since "void *" means
"pointer to object of unknown type", you can't do pointer arithmetic with it.

What you really want is "((char *)end - (char *)beg) / size".  (Don't use int
or long!)

Karl W. Z. Heuer (ihnp4!ima!haddock!karl), The Walking Lint