Path: utzoo!attcan!uunet!mcvax!ukc!eagle!mtr
From: mtr@eagle.ukc.ac.uk (M.T.Russell)
Newsgroups: comp.lang.c
Subject: Re: "Numerical Recipes in C" is nonportable code
Message-ID: <5514@eagle.ukc.ac.uk>
Date: 15 Sep 88 12:40:58 GMT
References: <664@lindy.Stanford.EDU> <6758@megaron.arizona.edu> <718@gtx.com> <640@drilex.UUCP> <1429@ficc.uu.net> <8470@smoke.ARPA>
Reply-To: mtr@arthur.UUCP (M.T.Russell)
Organization: Computing Lab, University of Kent at Canterbury, UK.
Lines: 26

In article <8470@smoke.ARPA> gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
>In article <1429@ficc.uu.net> peter@ficc.uu.net (Peter da Silva) writes:
>>What's to stop you from doing the following:
>>	Generate code in an array.
>>	Jump to the beginning of the array. *
>>... I can't see how you could write a valid 'C' compiler that wouldn't
>>let you violate this protection.
>
>That's simple.  All the compiler has to do is detect any attempt to
>use a data object as a function.  The only way to even attempt this in
>standard C is via an explicit cast to a function pointer somewhere,
>which is where the compiler would enforce the constraint.

There is another way to treat a data object as a function:

	union foo {
		char *data;
		int (*func)();
	};

The compiler would either have to prohibit unions with both text and
data pointers or do runtime bookkeeping to remember what was last
stored in such unions.

Mark Russell
mtr@ukc.ac.uk