Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!dftsrv!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: effect of free()
Message-ID: <19493@mimsy.UUCP>
Date: 9 Sep 89 15:09:39 GMT
References: <319@cubmol.BIO.COLUMBIA.EDU> <3756@buengc.BU.EDU> <2065@munnari.oz.au>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 20

In article <2065@munnari.oz.au> ok@cs.mu.oz.au (Richard O'Keefe) writes:
>Can a capability-based machine support a C implementation at all?
>Might that provide an example where a pointer can't be accessed as data
>(thus making pointer-comparison-via-trapping-methods obligatory)?
>What would that do to pointer subtraction, pointers in unions, and so on?

Some Lisp machines implement pointers as <array, size, index> triples.
The code for `*p = 3', on such a machine, is (in pseudo-assembly)

	load	reg1,p+8	| get index
	load	reg2,p+4	| get size
	compare	reg1,reg2
	jltu	ok		| if (unsigned)index < size, is OK
	load	reg1,<"invalid pointer reference">
	jump	runtime_error_abort
ok:	load	reg2,p+0	| get array
	store	#3,reg2[reg1]	| array[index] = 3
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris