Why is char initialized to 0xFF ?

[lithium iodate]

On Sunday, 9 June 2019 at 18:27:07 UTC, Patrick Schluter wrote:
> On Sunday, 9 June 2019 at 12:19:43 UTC, Ola Fosheim Grøstad 
> wrote:
>> On Sunday, 9 June 2019 at 08:36:30 UTC, Patrick Schluter wrote:
>>> No, by putting NaN in d you hav e a deterministic error. In C 
>>> and C++ you will have undefined behaviour that will vary with 
>>> compiler, version, options, OS version, architecture, 
>>> position of the moon, etc. and sometimes undetectable bugs.
>>
>> I don't think it is undefined though…
>
> It is undefined behaviour by the definition of the standard. 
> undefined behaviour includes behaviour that can be explained.

To be fair, the C standard (C11) is a bit self-contradicting 
there. Variables of automatic storage duration that are not 
explicitly initialized contain an unspecified value (i. e. any 
valid value) or a trap representation. Types such as integers 
usually don't have any trap representations so reading them 
should be defined on most platforms (unless C permits some sort 
of compile-time-only trap representation? not sure.). Then 
there's informative(!) annex J which explicitly lists it as 
undefined behavior.