Interfacing C and OCAML using the bigarray library

Interfacing C and OCAML using the bigarray library

[Stefano Ballabeni]

Hi everybody,

First of all, sorry for my english, it's so bad, but
I'll try to be understandable.

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.

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));
}

test.ml
--
external take_matrix  : unit -> (int,
Bigarray.int8_unsigned_elt, Bigarray.c_layout)
Bigarray.Array2.t = "make_c_matrix"

let _ =
  let m = take_matrix () in
    print_string "-- CAML --\n";
    print_int m.{0, 0};
    print_string " ";
    print_int m.{0, 1};
    print_newline ();
    print_int m.{1, 0};
    print_string " ";
    print_int m.{1, 1};
    print_newline ();

Then I compile :
gcc -W -Wall -ansi -pedantic -c test.c
ocamlc -custom bigarray.cma test.o test.ml

I run it :
./a.out

and it displays :
-- CAML --
208 208
6 8

As you can see these values are not exactly the one I
entered in the C code.

So, is that possible to use a dinamically allocated C
matrix in OCAML using bigarray ?
If it is, what do I do wrong ? I have been searching
for days to make this lib work fine...
If not, do you have a pretty solution ?

Thanks in advance.

-- 
Stefano Ballabeni
stefano.ballabeni@yahoo.fr


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

[Jacques Garrigue]

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));
> }