Extending a list on the OCAML side that is initially allocated on the C side

Extending a list on the OCAML side that is initially allocated on the C side

[nari]

Hi,

I have an OCAML and C function as follows

**************OCAML side*****************
let simple_list_test m = m @ [100]
let _ =
   Callback.register "simple_list_test" simple_list_test;
***********************************************

*************C side***********************
main(int argc, char *argv[])
{
    int *list, *new_list;

    fprintf (stderr,"Initializing OCAML\n");
   caml_main(argv);
   fprintf (stderr,"Initializing OCAML Done\n");
   list = (int *) calloc(10, sizeof(int));
   new_list = call_caml_simple_list_test(list);
/* I expect eleven elements in list after the call to ocaml and expect
100
     to be printed out */
   printf("The last element is %d\n", new_list[10]);
}

int * call_caml_simple_list_test(int *list)
{
   static value * closure_f = NULL;
   value ret_val;

      if (closure_f == NULL) {
         closure_f = caml_named_value("simple_list_test");
   }
   ret_val = callback(*closure_f, (value) list);
   return (int *)ret_val;
}
**********************End C side*****************

My understanding (from documentation) is that since "list" is allocated
on the "C" side, it is outside the heap of the ocaml garbage collector
and
all I have to do is to cast the pointer to a caml 'value' type.
I get a core dump (segmentation fault) with the above code.
How can I modify or extend the list (or for that matter, any record)
on the OCAML side and then send the C side back a pointer that I can
dereference.
Thanx.

--
**************************************************
Narayanan Krishnamurthy (Nari)
The Somerset PowerPC Design Center,
Architecture and Systems Platforms
Motorola Inc. Semiconductor Products Sector
7700 W. Parmer Lane, Austin, TX 78729

Loc/MD: TX32/PL30         _
email : nari@ibmoto.com /   \ email: r44117@email.mot.com
Ph    : (512) 996-4863  |O O| Fax : (512) 996-7432
************************| I |**********************
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                         ---

Re: Extending a list on the OCAML side that is initially allocated on the C side

[Michel Quercia]

: I have an OCAML and C function as follows
: 
: **************OCAML side*****************
: let simple_list_test m = m @ [100]
: let _ =
:    Callback.register "simple_list_test" simple_list_test;
: ***********************************************

Your function simple_list_test expects a list argument which is represented in
memory as a linked collection of values.

:    list = (int *) calloc(10, sizeof(int));
:    new_list = call_caml_simple_list_test(list);

This cannot work as you give to "simple_list_test" a C array of ints, not a
linked list as expected by the Ocaml compiler.

Have a look at the reference manual, chapter "Interfacing C with Objective
Caml", section 14.2 "The value type". A list cell is a 3 word object containing
a header, the value of the cell, and a pointer to the next cell.

Here is a way to build a legal 10 element list :

void go()
{
  value list, new_list;
  int i;
  
#define nil Val_int(0)
  
  /* builds a list with the 1..10 numbers */
  for(list = nil, i = 10; i > 0; i--) {
    Begin_roots1(list);
    new_list = alloc(2,0);
    Field(new_list,0) = Val_int(i);
    Field(new_list,1) = list;
    End_roots();
    list = new_list;
  }

  /* now calls simple_list_test */
  new_list = call_caml_simple_list_test(list);

  /* prints the last item */
  for (list = new_list; Field(list,1) != nil; list = Field(list,1));
  printf("%d\n", Int_val(Field(list,0)));
  fflush(stdout);

}

: My understanding ...
: all I have to do is to cast the pointer to a caml 'value' type.

Never cast unless you are absolutely sure the datatypes agree.
--
Michel Quercia
9/11 rue du grand rabbin Haguenauer, 54000 Nancy
http://pauillac.inria.fr/~quercia
mailto:quercia@cal.enst.fr