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From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.lang.c
Subject: Re: Expressions compared to zero in *c*
Message-ID: <14556@topaz.rutgers.edu>
Date: Tue, 8-Sep-87 21:00:02 EDT
Article-I.D.: topaz.14556
Posted: Tue Sep  8 21:00:02 1987
Date-Received: Thu, 10-Sep-87 01:24:48 EDT
References: <374@mcdsun.UUCP>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 29

> Does  (a-b) < 0  imply   a < b   ?  Pick a and b for controversy.

NO.  Not if you overflow the value, the calculations are invalid.
For example.  Define MAXINT to be the largest positive number representable
by type int.  The expression MAXINT+1 will on most machines be the largest
negative number.  Do you insist that it compare ">" 0 ?  Even if the compiler
had the hope of knowing that ( a - b ) caused an overflow that is checkable
by the adjacent > operator, you've destroyed that with the cast in your
example program.  A cast has exactly the same effect as assigning a value
to a variable of the type of the cast and then referencing that variable
That is
	TYPEA	    x;
	TYPEB	    y;

	((TYPEB) x)

is equivelent to

	(y = x)

only without the need for the intermediate "y" value.

Hence (CAST) (a-b) becomes a positive number by virtue of being stored,
(CAST) (MAXINT+1) becomes negative by virtue of being stored.  C doesn't
keep overflow bits in the stored variables.

Overflowing variables (unless you know what will happen) is an error.
This is an EXCEPTION.  You either have to check this carefully or hope
you have hardware that is going to notify you of the error.