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Groups > comp.lang.c > #105195
| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: "usual arithmetic conversions": please explain something... |
| Date | 2017-02-28 16:59 -0800 |
| Organization | None to speak of |
| Message-ID | <ln1suhersb.fsf@kst-u.example.com> (permalink) |
| References | <448topoppn.fsf@be-well.ilk.org> <o951vq$ch6$2@dont-email.me> |
BartC <bc@freeuk.com> writes:
> On 28/02/2017 23:33, Lowell Gilbert wrote:
>> 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. From reading the standard,
>> my expectation was that the multiplication would convert the signed
>> operand to unsigned, based on this bit of the standard:
>>
>> Otherwise, if the operand that has unsigned integer type has rank
>> greater or equal to the rank of the type of the other operand,
>> then the operand with signed integer type is converted to the
>> type of the operand with unsigned integer type.
>>
>> They've got the same rank, right?
>>
>> What am I missing? [It must be something, because I don't even get
>> warnings on any of several compilers, with messages cranked up high.]
>
> Try:
>
> printf("%u\n", c);
No, c is an int.
But do try:
printf("%llu\n", a * b);
That will show you the actual value that's converted to int before being
stored in c.
On my system, with the obvious changes to the OP's program, I get:
a * b = 18446744073709511616
c = -40000
or, when I compile in 32-bit mode:
a * b = 4294927296
c = -40000
(Again, the conversion has undefined behavior, but these results is
likely to be typical.)
--
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"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
"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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