[ocaml-ctypes] Static function callback.

[Romain Beauxis]

Hi guys,

I am playing with a binding for OSX's SecureTransport API. The API
abstracts away the underlying socket implementation and, thus,
requires to pass (static) callbacks, which is where I am having
trouble..

Here are the relevant part of the code:

let fd_read = (...)

let fd_write = (...)

let _set_io_funcs = foreign ~from:lib "SSLSetIOFuncs"
  (ptr void @-> fd_io_typ @-> fd_io_typ @-> returning int)

let _create_context = (...)

let _release_context = (...)

let init s t =
  let ctx =
    _create_context (...)
  in
  Gc.finalise _release_context ctx;
  let read = Root.create fd_read in
  Gc.finalise Root.release read;
  let write = Root.create fd_write in
  Gc.finalise Root.release write;
  check_err (_set_io_funcs ctx fd_read fd_write);
  {ctx;read;write}

The trick above is that read and write callbacks are passed at
initialization and, thus, need to be valid for the life time of the
handler.

I've tried my best to make sure that the passed callback would be
immune to Gc issues. Unfortunately, I am still experiencing issues
with them which, I think, relate to the Gc messing with those.

Basically, if I had some printf inside the fd_* callbacks, the
compiled binding will sometimes work and some times not.

Besides using C stubs (which I am trying to avoid as an exercise
here), is there anything else that I could do to make this work?

Thanks for y'all help!
Romain