Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!umcp-cs!chris
From: chris@umcp-cs.UUCP (Chris Torek)
Newsgroups: net.lang.c
Subject: Re: Any GOOD reason for C's behaviour in this example?
Message-ID: <2566@umcp-cs.UUCP>
Date: Wed, 23-Jul-86 06:17:23 EDT
Article-I.D.: umcp-cs.2566
Posted: Wed Jul 23 06:17:23 1986
Date-Received: Thu, 24-Jul-86 01:31:25 EDT
References: <1080@ttrdc.UUCP>
Reply-To: chris@maryland.UUCP (Chris Torek)
Organization: University of Maryland, Dept. of Computer Sci.
Lines: 47
Summary: No: nor is your compiler correct

In article <1080@ttrdc.UUCP> levy@ttrdc.UUCP (Daniel R. Levy) writes:
>I recently came across a situation in C which caused me to do a double
>take, at least until I dug out my trusty, dogeared K&R:
>
>main()
>{
>	unsigned short a, b;
>	int i;
>	a=4;
>	b=5;
>	i = ( (int) a) - ( (int) b);
>	(void) printf("%d\n",i);
>	return 0;
>}
>
>... I was half expecting that casting the unsigned short values to
>ints would make them be treated as ints in an expression which takes a
>difference between two of them.  [But] ... this little program produces
>... 65535.
>
>Now deep in the bowels of K&R there is a paragraph explaining that
>indeed this is what is supposed to happen ...

Where?

K&R, p. 42:

  The precise meaning of a cast is in fact as if {\it expression} were
  assigned to a variable of the specified type, which is then used in
  place of the whole construction.

Therefore,

	unsigned short us1, us2;
	int i1, i2;
	int r1, r2;

	us1 = 4; us2 = 5;
	r1 = (int)us1 - (int)us2;
	i1 = us1; i2 = us2; r2 = i1 - i2;

should produce the same values in `r1' and `r2'.  Indeed, the 4.3BSD
C compiler produces -1 for both.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1516)
UUCP:	seismo!umcp-cs!chris
CSNet:	chris@umcp-cs		ARPA:	chris@mimsy.umd.edu