Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!zaphod.mps.ohio-state.edu!rpi!leah!bingvaxu!vu0310
From: vu0310@bingvaxu.cc.binghamton.edu (R. Kym Horsell)
Newsgroups: comp.lang.c
Subject: Re: Array bounds checking with C????
Message-ID: <3890@bingvaxu.cc.binghamton.edu>
Date: 25 Aug 90 17:59:56 GMT
References: <7611@ucdavis.ucdavis.edu>
Reply-To: vu0310@bingvaxu.cc.binghamton.edu.cc.binghamton.edu (R. Kym Horsell)
Organization: SUNY Binghamton, NY
Lines: 24

In article <7611@ucdavis.ucdavis.edu> kuan@iris.ucdavis.edu (Frank [Who me?] Kuan) writes:
>
>	Why is it that most C compilers don't seem to support this
>	nifty little feature?

I guess this isn't usually included because
(a)	array indexing is subsumed by pointer arithmetic & this
	is *much* harder (i.e. impossible in general) to check;
(b)	arrays can be declared with no bounds, i.e.
		extern long arr[];
	which implies either a smart linker and/or runtime
	support for array descriptions -- the antithesis of C
(c)	is is easy enough to do it yourself with macros:
		extern Thingy arr_[];
		#define	arr(i)	arr_[chkbnds(i,0,max_ind_of_arr_)]
		int chkbnds(ind,lwb,upb) {
			if(ind>=lwb && ind<=upb) return ind;
			/* chunder */
			exit(-1);
			}
	(note that we need a routine here so ``ind'', which may
	include side-effects, doesn't get evaluated twice).

-Kym Horsell