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From: msb@sq.UUCP
Newsgroups: comp.lang.c
Subject: Re: char (*a)[]  (was: Style [++i vs i++])
Message-ID: <1987Aug10.192923.7879@sq.uucp>
Date: Mon, 10-Aug-87 19:29:23 EDT
Article-I.D.: sq.1987Aug10.192923.7879
Posted: Mon Aug 10 19:29:23 1987
Date-Received: Tue, 11-Aug-87 06:25:23 EDT
References: <8298@brl-adm.ARPA> <587@cblpe.ATT.COM> <189@xyzzy.UUCP>
Reply-To: msb@sq.UUCP (Mark Brader)
Organization: SoftQuad Inc., Toronto
Lines: 46
Checksum: 42277

Regarding the code...

> >   main(a)
> >   char (*a)[];
> >   { a = 0; printf("a=0x%x\n", a); a++; printf("a=0x%x\n", a);	}

Wayne Throop writes:

> But the scariest thing about all this is that *none* *of* *my* *tools*
> *caught* *this* *bug*!!!!  Lint happily passed the program ...
> And the compiler didn't complain ...

Same on our machine, by the way.

> (By the way, for those of you who missed it, the program is illegal for
> the obvious reason that it increments a pointer to an object of unknown
> size,

Actually, *declaring* such a pointer is probably illegal.  The language
in K&R appendix A section 8.4 is a bit fuzzy, but seems to imply this;
and section 3.5.3.2 of the (Oct.'86) ANSI draft nails it down clearly.

> but *also* because it performs arithmetic on a null pointer, and
> of course, this is illegal.)

Um, I don't think so, Wayne; it's just that the result, if you indirect
through such a pointer, is undefined.  K&R is silent on this, but ANSI
3.3.6 seems pretty clear.  And here the pointer isn't being indirected through.

The OTHER thing that's wrong with the code is that a "%x" format is used
to print a pointer variable.  "%x" is used to print ints, or at least,
things that printf() can pretend are ints.  Pointers needn't be the
same size as ints.  It's much safer to do this:

	printf ("a=0x%lx\n", (long) a);

Then you get surprised only if the pointers won't even fit in a long.

ANSI has a better solution to this: the new format "%p".  (See 4.9.6.1).
On an ANSI compiler, you would write:

	printf ("a=%p\n", (void *) a);

and be guaranteed reasonable results.  But I don't think "%p" exists yet.

Mark Brader, utzoo!sq!msb			C unions never strike!