[Caml-list] Save callbacks from OCaml to C

["Christoph Höger" <christoph.hoeger@tu-berlin.de>]

Seems this problem bugs me forever...

I have a scenario, where some C-stubs call back into OCaml, so the stack
(growing downwards) looks like this:

caml_foo ...
/* OCaml control ends here */
...
c_stub ...
c_library_routine (user_data; &callback_wrapper, ...)
...
c_callback_wrapper(user_data; ...)
/* OCaml controlled again */
caml_callbackN(user_data->closure, ....)


The pattern should be somewhat familiar to anyone who has ever combined
C and OCaml in a non-trivial way: I store the actual ocaml callbacks in
the heap-allocated user_data structure, which is guaranteed to be passed
to the callbacks by the c-library.

The callback wrapper then wraps all arguments and uses caml_callback on
these closures.

The callback wrapper is a pure C-routine, i.e. no CamlParamN or
CamlLocalN macros are used here. But still, the garbage collector runs
amok. After a while I see the user_data pointer set to 0x0! How can that
be, how can it overwrite a stack value of a calling C-routine?

Here is the real callback:

void g(void* user_data, double t, double const *x, double *g) {
  const struct interface_data* data = ((struct interface_data*)user_data);
  static long count = 0;
  count++;

  /* Wrap the values in fresh big arrays */
  value ml_t = caml_copy_double(t);
  value ml_x = caml_ba_alloc_dims(CAML_BA_FLOAT64 | CAML_BA_C_LAYOUT, 1,
(double*)x, data->qs->n);
  value ml_g = caml_ba_alloc_dims(CAML_BA_FLOAT64 | CAML_BA_C_LAYOUT, 1,
g, data->qs->mc);

  /* call the OCaml callback */
  caml_callback3(data->g, ml_t, ml_x, ml_g);
}

Is there anything obvious, I am doing wrong?

-- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de