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From: robison@m.cs.uiuc.edu
Newsgroups: comp.lang.c
Subject: Re: How to multiply two ints and check
Message-ID: <4700038@m.cs.uiuc.edu>
Date: 17 May 89 22:46:00 GMT
References: <434@skye.ed.ac.uk>
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Nf-ID: #R:skye.ed.ac.uk:434:m.cs.uiuc.edu:4700038:000:574
Nf-From: m.cs.uiuc.edu!robison    May 17 17:46:00 1989


Re: "How to multiply two ints and check"

There is a much simpler, faster, and machine-independent way to check
for multiplication overflow --- check the product by division.  To wit:

	int vmul(n1,n2,p_ovf)
		int n1,n2;
		int *p_ovf;
		{
			int product = n1*n2;
			if( n2!=0 && product/n2 != n1 )
				*p_ovf = YES;
			else
				*p_ovf = NO;
			return product;
		}

This code doesn't need to know sizeof(int), and even works on decimal machines!

Arch D. Robison
University of Illinois at Urbana-Champaign

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