On Sat, 14 Dec 2002, Ruslan Ermilov wrote:
> On Sat, Dec 14, 2002 at 09:02:40PM +1100, Bruce Evans wrote:
> > For ANSI C, the result of the subtraction only depends on the width
> > of unsigned char. If unsigned char has the same width as int, then
> > the result is UINT_MAX; otherwise the result is -1. This is an example
> > of the brokenness of "value preserving" conversions -- the value is
> > as far as possible from being preserved.
> Hmm, then how you could explain the difference between -traditional
> and -ansi outputs for the following fragment on i386:
>
> int printf(char *, ...);
>
> int
> main(void)
> {
> long long l;
> unsigned char c1 = 1;
> unsigned char c2 = 2;
>
> l = c1 - c2;
> printf("%lld\n", l);
> l = -1;
> printf("%lld\n", l);
> }
>
> Or the same code but with `long' on sparc64.
The first paragraph above is all about the ANSI C case. -traditional
gives signedness-preserving conversions, so c1 is prompted to 1U and
c2 is promoted to 2U. 1U - 2U is UINT_MAX on all machines. The
difference between UINT_MAX and -1 can be seen by converting these
values to a common wider type as in your example. UINT_MAX < LLONG_MAX
on all machines supported by FreeBSD although not in general, so
assigning it to `l' doesn't change its value.
Bruce
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