Re: Fun with C

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

"Nasser M. Abbasi" <nma@12000.org> writes:
> On 4/16/2011 10:02 AM, George P. wrote:
>> Spend few days chasing bug in embedded C project which can drilled
>> down to this. Consider the following program, and all reasonable men
>> will expect the result to be negative.  Not in C, and it is not a
>> compiler bug :-(
>>
>> And offcorse none of the three comilers I cheched did not bother to
>> issue a warning at least..
>>
>> #include<stdio.h>
>>
>> int main()
>> {
>>       unsigned n = 128;
>>       int i = -2048;
>>       int r;
>>
>>       r = i / n;
>>
>>       printf("R = %d\n", r);
>>
>>       return 0;
>> }
>
>
> $ gcc -Wall  t.c
> $ ./a.out
> R = 33554416

So R = 2**25 - 16, the correct answer is -16, so the actual result is
the correct one except for bit 24, which is 1 when it should be zero.

Out of curiosity, is this actual result a predictable consequence of the
language definition, or is it undefined behavior?

-- 
Ludovic Brenta.