Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!usc!snorkelwacker!spdcc!ima!haddock!karl
From: karl@haddock.ima.isc.com (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: Strange multiplication ?
Message-ID: <16905@haddock.ima.isc.com>
Date: 17 Jun 90 01:00:09 GMT
References: <1790@krafla.rhi.hi.is>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 18

In article <1790@krafla.rhi.hi.is> frisk@rhi.hi.is (Fridrik Skulason) writes:
>[On a machine with 8-bit char, 16-bit int, and 32-bit long,
>	unsigned char a = 255;
>	unsigned long i = 256 * a;
>produces 65280 on some compilers, -256 on others.]  I had thought that when
>one operand ("a" in this case) was unsigned, ...

It's an unsigned char.  Since C has no char-typed rvalues, it promotes to
unsigned int (in the unsigned-preserving model) or signed int (in the
value-preserving model).  An ANSI compiler must use the value-preserving
model, in which the computation overflows but probably generates the value
-256.  If the compiler is high-quality, it will also warn about your using a
questionably-signed expression in a signedness-sensitive context.

See also <16836@haddock.ima.isc.com>, where we just finished discussing a
similar problem.

Karl W. Z. Heuer (karl@kelp.ima.isc.com or harvard!ima!karl), The Walking Lint