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From: jdickson@zook.Jpl.Nasa.GOV (Jeff Dickson)
Newsgroups: comp.lang.c
Subject: MSC C arithmetic
Message-ID: <8792@brl-adm.ARPA>
Date: Thu, 13-Aug-87 22:10:54 EDT
Article-I.D.: brl-adm.8792
Posted: Thu Aug 13 22:10:54 1987
Date-Received: Sat, 15-Aug-87 11:52:57 EDT
Sender: news@brl-adm.ARPA
Lines: 47


	I recently wrote a program for the PC that performed some 
arithmetic. I compiled the code using MSC 'C' 4.0 . The problem I
had, was that simple integer and floating point arithmetic was not
being carried out properly unless I explicitly cast the operands
to the type of the result.

	One of the expressions involved multiplying two integers
and placing the result in a long integer. The code looked something
like this:

	unsigned int repeat_cnt;
	unsigned int units;
	unsigned long n;

	for (n = repeat_cnt * units; n > 0; n--)

	If repeat_cnt equaled 10,000 and units equaled 10, n did
not get set to 100,000! Rather n got set to 34,464 - which is what
the result would be if n were an int not a long (100,000 - 65,536).
However, if I explicitly cast the values as longs then n got assigned
correctly.

	for (n = (long)repeat_cnt * (long)units; n > 0; n--)

	I had a similiar problem with a floating point expression. Not
explicitly casting the operands to float caused a completely bogus
result. In this case, the expression looked something like this:

	double total_bits;
	unsigned int repeat_cnt;
	unsigned int units;
	unsigned int blksiz;

	total_bits = repeat_cnt * units * blksiz * 16.0

	If I interpret K & R correctly, explicit casting of the above
operands and the operands in the first example are not necessary. What
gives? The Sun will run the above code without modification and yield
the expected results. 

	Is this a bug? Or a feature? Or within the spec of the language
that leaves it up to the compiler writer to generate code that works or
breaks? 

Jeff S. Dickson
jdickson@zook.jpl.nasa.gov