"ocaml_beginners"::[] Interfacing C and OCAML, using the Bigarray library

Hello everybody,

First of all I'd like to apologize for my english which is so bad but
I'm think that I'll have a few more answers than if I was posting in
french.

I'm trying to pass an matrix of unsigned chars that I have allocated
in C to OCAML.
Some people advised me to use the bigarray library. I seems to be
what I needed so I read the documentation of this lib and of the
module.

Currently I can allocate arrays in ocaml without any problem, I can
allocate bigarrays in C, I can give the C allocated bigarray to OCAML
but the values that I gived to the fields of the matrix don't pass.

Two examples :

===================== TEST1 ======================
test.c
----

#include <caml/mlvalues.h&gt;
#include <caml/bigarray.h&gt;
#include <stdio.h>

value caml_get_c_array(void)
{
long dims[2];
unsigned char **my_c_array;

/* I allocate my matrix */
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);

/* I give some values */
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;

/* I return the bigarray */
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 = "caml_get_c_array";;

let _ =
let m = take_matrix () in
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 ();

===
$ cd ../C/ && gcc -W -Wall -ansi -pedantic -I /usr/local/lib/ocaml -c
test.c && cd ../OCAML/ && ocamlc -custom bigarray.cma ../C/test.o
test.ml && ./a.out

208 208
6 8

As you can see these values are no exactly the ones that I gave in
test.c, but if I add :
m.{0, 0} <- 5;
print_int m.{0, 0};
It displays the 5, so I infer that the C array has been allocated.

Example 2 :

===================== TEST2 ======================
test2.c
----

#include <caml/mlvalues.h&gt;
#include <caml/bigarray.h&gt;
#include <stdio.h>

/* this function only displays the bigarray */
value display(value m)
{
unsigned char **t;

t = Data_bigarray_val(m);

printf("-- C --n");
printf(&quot;height : %ld, width %ldn", Bigarray_val(m)->dim[0],
Bigarray_val(m)-&gt;dim[1]);
printf(&quot;%d %dn", t[0][0], t[0][1]);
printf(&quot;%d %dn", t[1][0], t[1][1]);

return Val_unit;
}

/* this one multiplies each field by two */
value bytwo(value m)
{
unsigned char **t;

t = Data_bigarray_val(m);

t[0][0] = t[0][0] * 2;
t[0][1] = t[0][1] * 2;
t[1][0] = t[1][0] * 2;
t[1][1] = t[1][1] * 2;

return Val_unit;
}

test2.ml
----
external print_bigarray : (int, Bigarray.int8_unsigned_elt,
Bigarray.c_layout) Bigarray.Array2.t -> unit = "display"
external double_bigarray : (int, Bigarray.int8_unsigned_elt,
Bigarray.c_layout) Bigarray.Array2.t -> unit = "pardeux"

let _ =
let m = Bigarray.Array2.create Bigarray.int8_unsigned
Bigarray.c_layout 2 2 in (* I create my matrix *)
m.{0, 0} <- 1; (* I give some values *)
m.{0, 1} <- 2;
m.{1, 0} <- 3;
m.{1, 1} <- 4;
print_string "-- OCAML --n"; (* I display the values of the
matrix *)
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 ();
print_bigarray m; (* I ask to a C function to display the values
of the matrix *)
double_bigarray m; (* I ask to a C function to multiply the
values by two *)
print_string "-- OCAML --n"; (* I print the values of the matrix
to be sure that I can see in OCAML the C modifications *)
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 ();
print_bigarray m; (* I ask to C to printf the matrix *)

===
$ cd ../C/ && gcc -W -Wall -ansi -pedantic -I /usr/local/lib/ocaml -c
test2.c && cd ../OCAML/ && ocamlc -custom bigarray.cma ../C/test2.o
test2.ml && ./a.out

-- OCAML --
1 2
3 4
-- C --
height : 2, width 2
Segfault

As you can see no problem from OCAML, the given values are correctly
displayed but the matrix is no passed to C. But I can access to the
fields that give the dimensions of the matrix.

============

My objective is to do what I tryied to in the first example, to
allocate a matrix en C and give the values of the matrix in C and
then give it to OCAML so I can make some modifications with OCAML.
Does anyone know how I can do it ?

If you need more informations, or you have any question, don't be shy
and ask, I'll answer as soon as possible.

Thanks in advance.

--
Stefano Ballabeni

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