Re: [Caml-list] Save callbacks from OCaml to C

[David Sheets <sheets@alum.mit.edu>]

On Wed, Feb 3, 2016 at 12:26 PM, Malcolm Matalka <mmatalka@gmail.com> wrote:
> Jeremie Dimino <jdimino@janestreet.com> writes:
>
>> On Wed, Feb 3, 2016 at 10:54 AM, Christoph Höger <
>> christoph.hoeger@tu-berlin.de> wrote:
>>
>>> 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?
>>>
>>
>> You need to register [ml_t], [ml_x] and [ml_g
>> ] as GC roots. Otherwise if the GC runs in caml_ba_alloc for instance,
>> [ml_t] might ends up containing garbage even before reaching
>> [caml_callback3]. You can use the normal macros for that:
>>
>> void g(...) {
>>   CAMLparam0();
>>   CAMLlocal3(ml_t, ml_x, ml_g);
>>   ...
>>   CAMLreturn0;
>> }
>>
>> Note that &(user_data->g) must be a GC root as well. Are you registering
>> &(user_data->g) with the GC in any way?
>>
>>
>> --
>> Jeremie
>
> If one is using ctypes, is all of this taken care of?  I have a library
> that registers a bunch of Ocaml functions in C code, which the C code
> calls.  I haven't experienced anything bad happening yet, but that
> doesn't mean much...

If you use ctypes and pass OCaml closures to C, you *must* retain a
reference to the closure to avoid it being GCed. If you do not, you
may experience the exception CallToExpiredClosure sporadically.

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