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| From | Keith Thompson <kst-u@mib.org> |
| Newsgroups | comp.lang.c |
| Subject | Re: "usual arithmetic conversions": please explain something... |
| Date | Tue, 28 Feb 2017 16:03:53 -0800 |
| Organization | None to speak of |
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Lowell Gilbert <lgusenet@be-well.ilk.org> writes:
> Given the following program:
>
> #include <stdio.h>
>
> int main (void)
> {
> unsigned long a = 4000;
> signed long b = -10;
> int c;
>
> c = a * b;
> printf("%d\n", c);
>
> return 0;
> }
>
> I am surprised that I get a negative answer.
[...]
Multiplying an unsigned long by a signed long converts the signed long
operand to unsigned long. Converting -10 to unsigned long yields
ULONG_MAX-9, which when multiplied by 4000 yields some large number that
wraps around in the usual manner. On my system, with 64-bit long, the
result is 18446744073709511616; with 32-bit long, it's 4294927296.
The assignment converts that value from unsigned long to (signed) int.
The behavior is undefined (assuming that value exceeds INT_MAX, which it
almost certainly does, but *typically* it will discard the high-order
bits yielding -40000.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
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"usual arithmetic conversions": please explain something... Lowell Gilbert <lgusenet@be-well.ilk.org> - 2017-02-28 18:33 -0500
Re: "usual arithmetic conversions": please explain something... BartC <bc@freeuk.com> - 2017-02-28 23:45 +0000
Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-02-28 16:59 -0800
Re: "usual arithmetic conversions": please explain something... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-01 02:20 +0000
Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-02-28 18:51 -0800
Re: "usual arithmetic conversions": please explain something... BartC <bc@freeuk.com> - 2017-03-01 12:04 +0000
Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 08:25 -0800
Re: "usual arithmetic conversions": please explain something... BartC <bc@freeuk.com> - 2017-03-01 16:47 +0000
Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 09:43 -0800
Re: "usual arithmetic conversions": please explain something... BartC <bc@freeuk.com> - 2017-03-01 18:00 +0000
Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 10:15 -0800
Re: "usual arithmetic conversions": please explain something... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-03-01 19:08 +0000
Re: "usual arithmetic conversions": please explain something... supercat@casperkitty.com - 2017-03-01 09:01 -0800
Re: "usual arithmetic conversions": please explain something... jameskuyper@verizon.net - 2017-02-28 15:49 -0800
Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-02-28 16:03 -0800
Re: "usual arithmetic conversions": please explain something... Nick Bowler <nbowler@draconx.ca> - 2017-03-01 21:53 +0000
Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 14:18 -0800
Re: "usual arithmetic conversions": please explain something... Lowell Gilbert <lgusenet@be-well.ilk.org> - 2017-03-01 08:29 -0500
Re: "usual arithmetic conversions": please explain something... Lowell Gilbert <lgusenet@be-well.ilk.org> - 2017-03-01 09:11 -0500
Re: "usual arithmetic conversions": please explain something... Keith Thompson <kst-u@mib.org> - 2017-03-01 08:59 -0800
Re: "usual arithmetic conversions": please explain something... scott@slp53.sl.home (Scott Lurndal) - 2017-03-01 14:00 +0000
Re: "usual arithmetic conversions": please explain something... "James R. Kuyper" <jameskuyper@verizon.net> - 2017-03-01 10:58 -0500
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