Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!munnari.oz.au!goanna!ok
From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: Pointers and addresses
Message-ID: <4152@goanna.cs.rmit.oz.au>
Date: 30 Oct 90 06:29:19 GMT
References: <15831@mentor.cc.purdue.edu> <4139@goanna.cs.rmit.oz.au> <15847@mentor.cc.purdue.edu>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 42

In article <15847@mentor.cc.purdue.edu>, shepherd@mentor.cc.purdue.edu (Mike Shepherd) writes:
> In article <4139@goanna.cs.rmit.oz.au> ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:
> >In article <15831@mentor.cc.purdue.edu>,
> >  shepherd@mentor.cc.purdue.edu (Mike Shepherd) writes:
> >> Is there a way to change an integer into an address location, vice versa, an
> >> address location into an integer?

>ok> What on earth is an address location?  Do you mean a pointer?
>ok> How about an example of what you want to do?

> 	Well, someone apparently did know what I was talking about because
> someone DID give me help from the Net.  And it worked out very well...
" This is just the compiler's way of saying "You know that it's just 32 bits,"
" and I know that it's just 32 bits, but I'm clever enough to know that it's"
" a *different type* of 32 bits."
" The thing to do is to use a "cast": .."

I have to repeat the negative advice I gave in 4139@goanna.
This bit about "you know it's 32 bits and I know it's 32 bits"
is a disaster waiting to happen.  I gave examples of machines
where pointers aren't 32 bits or integers aren't 32 bits.
It isn't even the case that all pointers are the same size;
there are machines where "data" pointers are 1 word and "function"
pointers are 2 (or even more).  Even on machines where pointers
and ints _are_ the same size, casting may require runtime operations,
and casting some bit patterns from int to pointer may cause a trap.

The advice you were given is ok _if_ you are _consciously_ writing
non-portable code, or at any rate code portable only within a known
range of machines ("32-bit byte-addressed workstations running UNIX", say).
But that's why it was necessary to ask what you _really_ want to do.
Go ahead and make the assumption, but *write it down*, and be prepared
to have someone try to port your code to a PC and come screaming for blood.

I repeat:  how about an example of what you really want to do?
Sure, casts are the *obvious* way of hacking it, they're in every
elementary introduction to C.  But for your actual application,
there may be a more portable alternative.

-- 
The problem about real life is that moving one's knight to QB3
may always be replied to with a lob across the net.  --Alasdair Macintyre.