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From: Michel Quercia <quercia@cal.enst.fr>
Reply-To: quercia@cal.enst.fr
To: caml-list@Montchapet.ecole
Subject: Re: Extending a list on the OCAML side that is initially allocated on the
         C side
Date: Mon, 7 Feb 2000 13:30:30 +0000
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: 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