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

From: Michel Quercia (quercia@cal.enst.fr)
Date: Mon Feb 07 2000 - 14:30:30 MET

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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
    



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