Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!rutgers!mcnc!rti!xyzzy!throopw
From: throopw@xyzzy.UUCP (Wayne A. Throop)
Newsgroups: comp.lang.c
Subject: Re: char (*a)[]  (was: Style [++i vs i++])
Message-ID: <196@xyzzy.UUCP>
Date: Thu, 13-Aug-87 09:51:48 EDT
Article-I.D.: xyzzy.196
Posted: Thu Aug 13 09:51:48 1987
Date-Received: Sat, 15-Aug-87 10:42:52 EDT
References: <8298@brl-adm.ARPA> <587@cblpe.ATT.COM> <189@xyzzy.UUCP> <1987Aug10.192923.7879@sq.uucp>
Organization: Data General, RTP NC.
Lines: 48

> msb@sq.uucp (Mark Brader),   >> throopw@xyzzy.UUCP
>>>   main(a)
>>>   char (*a)[];
>>>   { a = 0; printf("a=0x%x\n", a); a++; printf("a=0x%x\n", a);	}
>> 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.  

True.  Draft X3J11 limits arrays of unknown size to formals and
externals.  In my opinion, this is wrong.

>> 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.  

Yes indeed.  Pretty clear:

    For addition, either both operands shall have arithmetic type, or
    one operand shall be a pointer to an object and the other shall have
    integral type.

I repeat: "pointer to an object".  The null pointer doesn't qualify.
Further, K&R are not totally silent on this point either.  I quote from
7.4 in the reference:

    A pointer to an object in an array and a value of any integral type
    may be added.

K&R therefore can be considered to be even *more* restrictive, since
they call out that the object must be a member of an array.  (Actually,
I think draft X3J11 calls this out also, but this result must be peiced
together from several readings of Holy Writ, and I don't want to be
thought to be in competition with The World Tomorrow TV show.)

> The OTHER thing that's wrong with the code is that a "%x" format is used
> to print a pointer variable. [...]
> 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.

Good point.

--
IBM manuals are written by little old ladies in Poughkeepsie who are
instructed to say nothing specific.
                                --- R. T. Lillington