FFI & callbacks
===============

There are at least 4 cases:

* SLUL calling C (and potentially calling SLUL again)
* C calling SLUL (and potentially calling C again)
* C starting SLUL code (potentially calling C again)
* SLUL starting C code (potentially calling SLUL again)

Problems:

* Passing "user_pointers" back
    - what if there are nested calls in several layers?
      use the latest/"topmost" one?
* Givemes
* Function coloring
    - pure vs mutable vs I/O
    - asynchronous vs synchronous
    - async-cancellable vs not
* Exceptions / synchronous errors in callbacks
* Asynchronous errors/termination in callbacks



User pointers, Givemes
----------------------

Require that all C code passes:
- a "context" parameter (for the SLUL code)
- a "user_param" parameter (for any C callbacks)

Require that SLUL callbacks work through services/entrys?
    - the C code would indirectly (via the SLUL runtime or similar,
      possibly via some context parameter) provide the givemes.

Function coloring, Exceptions in callbacks
------------------------------------------

TODO. Some ideas:

* Run C code out-of-process (slow)
* Provide a virtual implementation of the libc (hard, but fast)
* Proxy some libc functions:
    - do special things on exit(), signal(), etc?
    - track resources?
    - track I/O?

Maybe calling C code should be completely forbidden in pure functions?
Or:

* Run C code in an emulator(!) (very slow)
* Run C code with a fully virtual libc (slow, hard)
    - it must be unable to write to SLUL memory
    - it must be unable to do anything to the OS
    - it's internal state must be copy-on-write
        - and changes discarded on return to SLUL
          (but only the topmost "layer", in case of nested callbacks)