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From: mouse@mcgill-vision.UUCP
Newsgroups: comp.lang.c,comp.unix.wizards
Subject: Re: pointer alignment when int != char *
Message-ID: <898@mcgill-vision.UUCP>
Date: Tue, 22-Sep-87 01:36:10 EDT
Article-I.D.: mcgill-v.898
Posted: Tue Sep 22 01:36:10 1987
Date-Received: Sat, 3-Oct-87 06:10:18 EDT
References: <493@its63b.ed.ac.uk> <6061@brl-smoke.ARPA> <3812@spool.WISC.EDU> <8024@think.UUCP>
Organization: McGill University, Montreal
Lines: 29
Xref: utgpu comp.lang.c:4433 comp.unix.wizards:4289

In article <8024@think.UUCP>, barmar@think.COM (Barry Margolin) writes:
> In article <588@murphy.UUCP> dave@murphy.UUCP (Dave Cornutt) writes:
[>>> is someone else; Dave is >>]
>>> [Lisp Machines have a garbage collector which moves objects,
>>> resulting in pointers that change behind your back]
>> [but this means a pointer can be invalidated on you]
> [when GC moves something, it updates all pointers.]
> A particular pointer variable will always point to the same object
> (until it is reassigned, of course), although its internal numerical
> value may change.

> I'm not sure how they deal with the fact that a pointer cast into an
> integer and back into a pointer (or is it vice versa?) must maintain
> its value.  My guess is that they maintain a hash table of pointers
> that have been converted into integers.

Lisp doesn't have that sort of cast (well, it usually does, but only as
a documented-to-be-dangerous subprimitive).  Does Symbolics or LMI or
anyone provide a C compiler for a Lisp Machine?  If they do, I would
guess that either they do as you suggest, maintaining some table of
objects which must not be moved, or they have hooks into the garbage
collector permitting the C run-time to lock objects down, or they
simply make the whole C environment one lisp object, which must be
relocated as a whole if it is relocated at all.  (Then of course all
pointers will be relative to the beginning of this area.)

					der Mouse

				(mouse@mcgill-vision.uucp)