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From: msb@sq.UUCP
Newsgroups: comp.lang.c
Subject: Re: unsigned short vs. negative constant
Message-ID: <1987May8.172821.4163@sq.uucp>
Date: Fri, 8-May-87 17:28:21 EDT
Article-I.D.: sq.1987May8.172821.4163
Posted: Fri May  8 17:28:21 1987
Date-Received: Sun, 10-May-87 02:03:36 EDT
References: <7404@elsie.UUCP>
Reply-To: msb@sq.UUCP (Mark Brader)
Organization: SoftQuad Inc., Toronto
Lines: 38
Checksum: 41829

Arthur David Olson (ado@elsie.UUCP) writes:
> Thanks to everyone who replied to replied to my question about what the code
> 	#include "stdio.h"
> 	main()
> 	{
> 		unsigned short	us;
> 		us = ~0;
> 		(void) printf("%d\n", us == -1);
> 	}
> should print on systems where sizeof short < sizeof int,

But he didn't include some points from my reply, which I think are worth
posting, so I am disregarding his Followup-to and posting this.
(Arthur: did you get my mail at all?)

The first thing is that there is no such thing as a "negative constant"
in C.  Both "~0" and "-1" are int constant-EXPRESSIONS.  They have the same
value on 2's complement machines, but will differ on most or all other
types of machines.  (YES, there are non-2's-complement machines running C.)
This, then, is another nonportability in the above code... and remains one
under the Draft ANSI Standard, unlike the  unsigned short / int interaction.

The second thing is that although it's true that...
> *	K&R can't be used to resolve the issue, since it predates unsigned
> 	shorts.
...nevertheless one may make an inference by considering the interaction
of longs with unsigned ints, which IS specified in K&R.  In THAT case the
corresponding code must print 0.

And a final note.  I hate it when people just post and say "this is the way it
works on MY machine", but since I was posting anyway, this is the way it works
on my machine.  sq is a Sun running SunOS 3.0, and it prints 1 without a
warning.  I guess we don't have the "latest" system, since Arthur said...
> *	The latest Sun systems print 1; issuing a warning during compilation.

Mark Brader, Toronto	    { decvax | ihnp4* | watmath | ... } !utzoo!sq!msb
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