Dmitry Bely wrote:

> I have a C library function with interface like following:
>
> typedef [abstract] void* SomeType;
> typedef int (*CallBack)(int);
> void convert( SomeType* in, SomeType* out, CallBack c);
>
> It would be nice to use it in caml the following way:
>
> external convert: someType->someType->(int->int)->unit = "something"
> ...
> convert stIn stOut ((+) 1)
> ...
>
> Unfortinately CamIDL language does not allow the pointer to function as a
> valid type, but maybe there is some common solution/workaround for this
> problem? Or the only way is writing necessary stubs and conversion
> functions by hands? Is is possible at all?
>

It is possible to call back caml code from a C code which was called from caml, but
you need to write a few lines of code.
here is an example extracted from my application (with a few simplifications).
the resulting caml function iter_tree can be called recursively, it can also raise
exceptions.

Another possible solution was proposed in a previous mail by Thierry Bravier, if
you want to use C++
http://caml.inria.fr/bin/wilma_hiliter/caml-list/199704/msg00029.html

Eric Nassor

-------------------------------------------------------------
in the original C library

typedef tree (*walk_tree_fn) (tree *, void *);

/* walk_tree iters on the tree data structure
  and calls the function func with the current tree value and the data
*/
tree walk_tree (tree *tp,  walk_tree_fn  func,  void *data)

-------------------------------------------------------------
in C_to_ml interface

/*
 * intermediary function called by walk_tree
 *  it calls the caml callback function which is given as argument
 */
static tree iter_tree_func(tree * t,void * fun)
{
  CAMLparam0();
  CAMLlocal1(tr);
/* if you want to stress the memory system */
/*  Garbage_collection_function() ;*/
  tr = camlidl_c2ml_type_tree(t,NULL);
  callback(*(value *)fun, tr);
  CAMLreturn(NULL_TREE);   /* caml exceptions are used if needed instead of  return
values */
}

/*
 * function to apply walk_tree to a CAML function
 */
value iter_tree (value v_function, value v_tree)
{
  CAMLparam2(v_tree, v_function);
  tree t ;
  camlidl_ml2c_type_tree(v_tree,&t,NULL);
  /* v_function is protected but its value may change because of the gc
     so we have to take the address */
  walk_tree(&t, iter_tree_func, &v_function);
  CAMLreturn (Val_unit);
}

camlidl_ml2c_type_tree and camlidl_c2ml_type_tree are  generated by camlidl to
translate tree between C and caml

-------------------------------------------------------------
in the caml code

external iter_tree :     (tree -> unit) -> tree -> unit  = "iter_tree"

-------------------------------------------------------------