[Caml-list] Interfacing with C: bad practice

[Dmitry Bely <dmitry.bely@gmail.com>]

I would like to share my experience of writing bad C bindings. The
following code is wrong, although no "living in harmony with the
garbage collector" rule seems to be violated:

value wrp_ml_cons (value v, value l)
{
  CAMLparam2(v, l);
  CAMLlocal1(cell);
  cell = caml_alloc_small(2, Tag_cons);
  Field(cell, 0) = v;
  Field(cell, 1) = l;
  CAMLreturn(cell);
}

value string_list(const char ** s)
{
    CAMLparam0();
    CAMLlocal1(list);
    list = Val_emptylist;
    while (*s != NULL) {
        list = wrp_ml_cons(caml_copy_string(*s), list); /* bug! */
    }
    CAMLreturn(list);
}

In the line

        list = wrp_ml_cons(caml_copy_string(*s), list); /* bug! */

C compiler first puts "list" pointer on stack and then calls
caml_copy_string(*s), potentially invalidating "list". Of course, the
stack copy of "list" is not registered as a global root so wrp_ml_cons
gets an invalid value.

Maybe Ocaml docs should be updated to warn about pitfalls like that?

- Dmitry Bely