Re: [Caml-list] Interfacing C and OCAML using the bigarray library

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

From: Stefano Ballabeni <stefano.ballabeni@yahoo.fr>

> I'm trying to send a dynamically allocated C matrix to
> OCAML.
> 
> I've seen that the bigarray library seems to be what I
> need.
> 
> The problem is that I can't pass a _dynamically_
> allocated matrix to OCAML, the values don't arrive to
> OCAML.

The error is on the C-side: multi-dimensional arrays with bigarray
have a flat representation (which is the usual way to do it in C.)

So your allocation should be:

  unsigned char *my_c_array;
  my_c_array = malloc(sizeof (unsigner char) * 2 * 2);

and assignments should be corrected too.

Jacques Garrigue

> A quick example :
> test.c
> ----
> #include <caml/mlvalues.h>
> #include <caml/bigarray.h>
> 
> value           make_c_matrix(void)
> {
>   long          dims[2];
>   unsigned char **my_c_array;
> 
>   my_c_array = malloc(sizeof (unsigned char *) * 2);
>   my_c_array[0] = malloc(sizeof (unsigned char) * 2);
>   my_c_array[1] = malloc(sizeof (unsigned char) * 2);
> 
>   my_c_array[0][0] = 3;
>   my_c_array[0][1] = 4;
>   my_c_array[1][0] = 5;
>   my_c_array[1][1] = 6;
> 
>   dims[0] = 2;
>   dims[1] = 2;
> 
> 
>   return (alloc_bigarray(BIGARRAY_UINT8 |
> BIGARRAY_C_LAYOUT, 2, my_c_array, dims));
> }