FFI: calling C code

I am thinking a lot about the design of the runtime support library and the “foreign-function interface”. I’d like to avoid bolting on a big chunk of C++ support code for runtime support; optimally the language would be self-supporting, able to run directly on the kernel API, or possibly on the C stdlib. I haven’t worked out how this could be done yet; building those low-level abstractions into the language seems to conflict with the immutable/functional core semantics.