Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!haven!purdue!decwrl!decvax!ima!cfisun!lakart!dg
From: dg@lakart.UUCP (David Goodenough)
Newsgroups: comp.lang.c
Subject: Re: Yet another pcc botch
Message-ID: <398@lakart.UUCP>
Date: 23 Jan 89 14:58:47 GMT
References: <3310@cbnews.ATT.COM>
Organization: Lakart Corporation, Newton, MA
Lines: 40

lvc@cbnews.ATT.COM (Lawrence V. Cipriani) sez:
] This program demonstrates a bug that is in some versions of pcc:
] 
] main()
] {
] 	float f, g;
] 	unsigned short u, v;
] 	short s, t;
] 
] 	u = 250;
] 	s = 250;
] 
] 	t = 100;
] 	v = 100;
] 
] 	f = t - s;
] 	g = v - u;
] 
] 	printf("f = %f, g = %f\n", f, g);
] }
] 
] When compiled and executed the output is:
] 
] f = -150.000000, g = 65386.000000
] 
] The correct value for g is -150.000000.

No. The correct value for g is 65386.0 [1]. If you read the gospel
according to Kernighan and Ritchie [2], it says that when you do
mathematics, and at least one of the operands is unsigned, and the
other is NOT float or better (as above), it is done in an unsigned
manner. Now last time _I_ checked, -150 was a negative number, which
simply doesn't exist in the land of unsigned. HOWEVER this type
conversion DOES NOT OCCUR ACROSS ASSIGNMENTS.

[1] depends on your size of short. 65386.0 is correct for those with
16 bit shorts. Check your system, as your actual mileage may vary.

[2] :-)
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