> posix advocates using utf-8 as the externel chaset and 32 bit 10646 as internal widechar encoding

But doesn't require it. That's the point.

>
>> As for C, C only has char16_t, char32_t, minimalistic literal support...
>> I'm not sure about standard library. That's about 2% of Unicode support.
>
> also look at iso 14651, 14652

I don't see that in C standard library.

>
> what is missing? i know: quite a lot, but please examplify
>

A full set of std::vector operations on sequences of scalar values.
A full set of std::vector operations on sequences of grapheme clusters.
Querying properties of scalar values.
Normalization.
Case conversion.
Regular expressions.