Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: checking for overflow in C
Message-ID: <1989May9.182628.1656@utzoo.uucp>
Organization: U of Toronto Zoology
References: <13367@dartvax.Dartmouth.EDU> <472@cbnewsh.ATT.COM>
Date: Tue, 9 May 89 18:26:28 GMT

In article <472@cbnewsh.ATT.COM> rkl@cbnewsh.ATT.COM (kevin.laux) writes:
>	Basically, it is impossible to Overflow when multiplying two integers.

Sorry, wrong.

>If you multiply two 8-bit integers, the result will not exceed 16-bits.
>Likewise, two 16-bit ints won't exceed a 32-bit result.

Except that C multiplication, and the multiply instructions on some (not
all) modern machines, multiply two 16-bit numbers to give a 16-bit number,
or two 32-bit numbers to give a 32-bit number.

>	So, except for the special case above, the CPU's multiply instruction
>(ie. the multiplication algorithm designed into the hardware/firmware/
>microcode)will never Overflow and will return the appropriate result.  It is up to you
>to provide a variable large enough to hold the result...

You are assuming (a) that the CPU *has* a multiply instruction (some don't),
(b) that it gives double-width results, and (c) that it does not trap an
overflow.
-- 
Mars in 1980s:  USSR, 2 tries, |     Henry Spencer at U of Toronto Zoology
2 failures; USA, 0 tries.      | uunet!attcan!utzoo!henry henry@zoo.toronto.edu