Signed word lengths and indexes

[Walter Bright]

Jérôme M. Berger wrote:
> Jérôme M. Berger wrote:
>> Walter Bright wrote:
>>> Jérôme M. Berger wrote:
>>>>     Actually, that problem already occurs in C. I've had problems when
>>>> porting code from x86 to x86_64 because some unsigned operations
>>>> don't behave the same way on both...
>>> How so? I thought most 64 bit C compilers were specifically designed to
>>> avoid this problem.
>> 	I can't isolate it to a minimal test case, but at my job, we make
>> an image processing library. Since negative image dimensions don't
>> make sense, we decided to define width and height as "unsigned int".
>> Now, we have code that works fine on 32-bit platforms (x86 and arm)
>> but segfaults on x86_64. Simply adding an (int) cast in front of the
>> image dimensions in a couple of places fixes the issue (tested with
>> various versions of gcc on linux and windows).
>>
> 	Gotcha! See the attached test case. I will post the explanation for
> the issue as a reply to give everyone a chance to try and spot the
> error...

Easy. offset should be a size_t, not an unsigned.