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

[Malcolm Matalka <mmatalka@gmail.com>]

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...