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

[Jeremie Dimino <jdimino@janestreet.com>]

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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

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