Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!aplcen!samsung!think!snorkelwacker!spdcc!ima!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.std.c
Subject: Re: "expandable" structs with last element declared using [1]
Message-ID: <15474@haddock.ima.isc.com>
Date: 19 Dec 89 05:13:57 GMT
References: <UZTerlu00VcJEDulRd@andrew.cmu.edu> <463@cpsolv.UUCP> <477@longway.TIC.COM>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 28

In article <477@longway.TIC.COM> uunet!sq!msb (Mark Brader) writes:
>A second issue is whether the usage is in conformance with the proposed
>ANSI Standard (pANS) for C.  I claim that it is.

My earlier posting on this topic attempted to prove it rigorously; Doug
acknowledged that I'd proved the legality of strcpy(foo->baz, "x") but
questioned whether explicitly referencing foo->baz[1] is legal.

I claim that there is no difference: if it's illegal to reference foo->baz[1]
directly, for whatever reason, then it cannot become legal simply by using an
auxiliary variable to hide the reference.  A tight-sphinctered implementation
could try to, and should be able to, enforce the bounds-checking at all levels
with run-time checks.  Thus if foo->baz[1]='\0' is illegal, then so is
	char *temp = foo->baz;
	temp[1] = '\0';
and so is
	void hideaway(char *p) { p[1] = '\0'; }
	... hideaway(foo->baz);
and so is
	strcpy(foo->baz, "x");
(all of which are just variations on a theme).

And thus contrapositively, the legality of the strcpy() implies the legality
of the direct reference.

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint
(I don't expect this issue to be settled by anything less than an official
Request for Interpretation, but this is my expert opinion.)