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From: mcdaniel@uicsrd.csrd.uiuc.edu (Tim McDaniel)
Newsgroups: comp.lang.c
Subject: Re: How to multiply two ints and check
Message-ID: <1059@garcon.cso.uiuc.edu>
Date: 19 May 89 04:29:52 GMT
References: <434@skye.ed.ac.uk> <4700038@m.cs.uiuc.edu>
Sender: news@garcon.cso.uiuc.edu
Reply-To: mcdaniel@uicsrd.csrd.uiuc.edu (Tim McDaniel)
Organization: Center for Supercomputing R&D (Cedar), U. of Ill.
Lines: 20

In article <4700038@m.cs.uiuc.edu> robison@m.cs.uiuc.edu writes:
>There is a much simpler, faster, and machine-independent way to check
>for multiplication overflow --- check the product by division.  To wit:
...
>			int product = n1*n2;
>			if( n2!=0 && product/n2 != n1 )

(1) The overflow happens before the check.  Bad news if your program
is aborted unrecoverably on overflow.
(2) Divide can overflow.  Consider INT_MIN/-1 on a 2s-complement
machine.

--
"6:20 O Timothy, keep that which is committed to thy trust, avoiding
profane and vain babblings, and oppositions of science falsely so
called: 6:21 Which some professing have erred concerning the faith."

Tim, the Bizarre and Oddly-Dressed Enchanter  |  mcdaniel@uicsrd.csrd.uiuc.edu
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