That's a bit light on the details, and I can't make any sense of "To use stdio even you literally need kernel-level privileges on Console.", C's stdio doesn't require special privileges to use, nor does Crystal's, afaik.

Maybe someone else will know that means.

> their goal is to fit the Ruby lang spec

I honestly thought that "ruby lang spec" was "whatever Matz ruby does." The ISO-IEC-30170 standard cited by mruby's readme seems to be laughably old but I can't obviously tell if mruby is a superset of the ISO or a subset. Their use of the phrase "part of the ISO standard" doesn't help