Signed word lengths and indexes

["Jérôme M. Berger"]

Walter Bright wrote:
> 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.

	And what about image width and height? Sure, in hindsight they
could probably be made into size_t too. Much easier and safer to
make them into signed ints instead, since we don't manipulate images
bigger than 2_147_483_648 on a side anyway...

	Which is more or less bearophile's point: unless you're *really*
sure that you know what you're doing, use signed ints even if
negative numbers make no sense in a particular context.

		Jerome
-- 
mailto:jeberger at free.fr
http://jeberger.free.fr
Jabber: jeberger at jabber.fr

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 198 bytes
Desc: OpenPGP digital signature
URL: <http://lists.puremagic.com/pipermail/digitalmars-d/attachments/20100616/86d4a9d3/attachment.pgp>