[Coco] Drivewire mixed results...

[Boisy Pitre]

Chuck,

I appreciate you taking the time to enunciate the details.  I'll make  
a note to fix this.

Thanks!

On Sep 25, 2008, at 8:27 AM, Chuck Youse wrote:

> On Thu, 2008-09-25 at 07:56 -0500, Boisy Pitre wrote:
>> On Sep 25, 2008, at 7:52 AM, Chuck Youse wrote:
>>
>>>
>>>>>
>>>>> int calChecksum(char *ptr, int count)
>>>>> {
>>>>>   short Checksum = 0;
>>>>>   while(--count)
>>>>>   {
>>>>>        Checksum += *(ptr++);
>>>>>   }
>>>>>   return (Checksum);
>>>>> }
>
>> I don't see right off-hand how that might be, but I'm open to
>> suggestions.  Feel free to provide an improved function and I'll
>> certainly incorporate it into the next rev of the specification
>> document.
>
> You need to be explicit about the signedness of 'char *ptr'.  Since  
> the
> early days of C this has been an implementation-specific detail - ANSI
> both solved and compounded the problem by adding 'signed char' and
> 'unsigned char', which are distinct from 'char'.  (This is of course  
> not
> orthogonal with respect to integers, which are by default signed.)
>
> In any case, for usual checksum computations this wouldn't matter
> because two's complement and modulo arithmetic is nifty.   
> Unfortunately,
> you're computing a 16-bit checksum for 8-bit data, which means that
> whether or not the addend at *ptr is sign-extended is significant.
>
> And no, I'm not just being pedantic; this actually matters.  Even  
> gcc on
> different architectures will implement the rules differently depending
> upon what is most natural for that architecture, or [where applicable]
> how the native compiler behaves, or what compile-time options are  
> given
> it..
>
> Also, (and this IS me being pedantic), declaring Checksum as 'short'  
> is
> superfluous.  Declaring it as an 'int' would have the same effect, and
> would avoid any penalties that are imposed by dealing with something
> smaller than the natural word-size (which can be significant,  
> especially
> in loops).  Hardly matters on today's machines for something like
> Drivewire, but I only SOMETIMES use that as an excuse to get sloppy :)
>
> C.
>
>
>
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Regards,
Boisy G. Pitre
--
Tee-Boy
Email: boisy at tee-boy.com
Web: http://www.tee-boy.com