unsigned char vs. signed char

[Florian Weimer]

On 07/16/2014 11:00 AM, Jakub Jelinek wrote:
> On Wed, Jul 16, 2014 at 10:55:57AM +0200, Florian Weimer wrote:
>> On 07/15/2014 07:22 PM, Adam Jackson wrote:
>>> On Tue, 2014-07-15 at 10:40 -0600, Orion Poplawski wrote:
>>>> Did you know that "char" defaults to "signed char" on x86 but "unsigned char"
>>>> on ppc and arm?  I didn't.
>>>
>>> Yep, found that out the hard way.  It annoyed me enough that I went
>>> digging in the gcc source to find the answer for all platforms:
>>>
>>> http://ajax.fedorapeople.org/is-char-signed-or-not.txt
>>
>> Any idea why aarch64 and ppc64le got this wrong?
>
> Why is char == unsigned char wrong?  Because i?86/x86_64 do it differently?

Yes, it looks like a totally avoidable portability hazard.  iOS has 
signed chars as well, so even when considering current ARM, the natural 
choice is unclear.  Older ARMs couldn't perform sign-extending 8-bit 
loads in a single instruction, but that's true for historic x86, too, 
and yet even the old 16-bit compilers used signed chars.

History aside, I assumed that the idea behind both architectures 
(aarch64 and ppc64le) is to use it to run current general-purpose server 
loads, and those are predominantly x86 right now.  That's why picking 
unsigned chars puzzles me.

-- 
Florian Weimer / Red Hat Product Security