Wrapping a callback to OCaml code from C

Wrapping a callback to OCaml code from C

[Richard Jones]

Hi:

I'm currently making some OCaml bindings for some C code.  The C code
which is causing me difficulty provides a callback interface.

The interface, in C, looks like:

  typedef void callback_t (void *data, obj *o1, obj *o2);
  void run (void *data, callback_t *callback);

When 'run' function is called, it will call the callback function
passed several times, passing 'data' as the first parameter.  I want
to provide an equivalent function in OCaml.

My current best attempt is this, which uses the 'data' parameter to
hold the address of the OCaml closure:

  static void
  callback_wrapper (void *fvpv, obj *o1, obj *o2)
  {
    value *fvp = (value *) fvpv;
    value fv = *fvp;
    value o1v, o2v;
    o1v = Val_obj (o1);
    o2v = Val_obj (o2);
    caml_callback2 (fv, o1v, o2v);
  }
  
  CAMLprim value
  run_wrapper (value fv)
  {
    CAMLparam1 (fv);
    value *fvp = &fv;
    caml_register_global_root (fvp);
    run (fvp, callback_wrapper);
    caml_remove_global_root (fvp);
    CAMLreturn (Val_unit);
  }

I suspect that this code is wrong or GC-unsafe in some way.  The
reason is that my program sometimes segfaults, and behaves differently
if I rewrite callback_wrapper in inconsequential ways.

Can someone tell me if I'm doing the right thing here?

Rich.

-- 
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com

Re: [Caml-list] Wrapping a callback to OCaml code from C

[Richard Jones]

On Thu, Jun 23, 2005 at 10:47:33PM +0100, Richard Jones wrote:
[...]

There was quite a delay in this posting appearing.  In the meantime I
think I've fixed the problem by adding CAMLparam0 .. CAMLlocal3 ..
CAMLreturn0 around the callback wrapper function.

Rich.

-- 
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com

Re: [Caml-list] Wrapping a callback to OCaml code from C

[Daniel Bünzli]

Le 24 juin 05 à 12:40, Richard Jones a écrit :

> On Thu, Jun 23, 2005 at 10:47:33PM +0100, Richard Jones wrote:
> [...]
>
> There was quite a delay in this posting appearing.  In the meantime I
> think I've fixed the problem by adding CAMLparam0 .. CAMLlocal3 ..
> CAMLreturn0 around the callback wrapper function.

I would like to understand why this is needed. Did you allocate  
something with the gc in your Val_obj functions ?

Daniel

Re: [Caml-list] Wrapping a callback to OCaml code from C

[Richard Jones]

On Fri, Jun 24, 2005 at 01:15:16PM +0200, Daniel Bünzli wrote:
> 
> Le 24 juin 05 à 12:40, Richard Jones a écrit :
> 
> >On Thu, Jun 23, 2005 at 10:47:33PM +0100, Richard Jones wrote:
> >[...]
> >
> >There was quite a delay in this posting appearing.  In the meantime I
> >think I've fixed the problem by adding CAMLparam0 .. CAMLlocal3 ..
> >CAMLreturn0 around the callback wrapper function.
> 
> I would like to understand why this is needed. Did you allocate  
> something with the gc in your Val_obj functions ?

Yes.  Val_obj is in fact a thin wrapper around this function:

static inline value
Val_voidptr (void *ptr, const char *)
{
  CAMLparam0 ();
  CAMLlocal1 (rv);
  rv = caml_alloc (1, Abstract_tag);
  Field (rv, 0) = (value) ptr;
  CAMLreturn (rv);
}

and of course callback_wrapper calls callback2 which calls into OCaml
code - rather a lot of allocation possibilities there!

Rich.

-- 
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com

Re: [Caml-list] Wrapping a callback to OCaml code from C

[Olivier Andrieu]

Hi,

 Richard Jones [Thursday 23 June 2005] :
 > I'm currently making some OCaml bindings for some C code.  The C
 > code which is causing me difficulty provides a callback interface.
 > 
 > The interface, in C, looks like:
 > 
 >   typedef void callback_t (void *data, obj *o1, obj *o2);
 >   void run (void *data, callback_t *callback);
 > 
 > When 'run' function is called, it will call the callback function
 > passed several times, passing 'data' as the first parameter.  I want
 > to provide an equivalent function in OCaml.
 > 
 > My current best attempt is this, which uses the 'data' parameter to
 > hold the address of the OCaml closure:
 > 
 >   CAMLprim value
 >   run_wrapper (value fv)
 >   {
 >     CAMLparam1 (fv);
 >     value *fvp = &fv;
 >     caml_register_global_root (fvp);
 >     run (fvp, callback_wrapper);
 >     caml_remove_global_root (fvp);
 >     CAMLreturn (Val_unit);
 >   }

you don't need caml_remove_global_root / caml_remove_global_root if
the callback is only called during the run() invocation. A local root
as registered by the CAMLparam macro is enough.

 >   static void
 >   callback_wrapper (void *fvpv, obj *o1, obj *o2)
 >   {
 >     value *fvp = (value *) fvpv;
 >     value fv = *fvp;
 >     value o1v, o2v;
 >     o1v = Val_obj (o1);
 >     o2v = Val_obj (o2);
 >     caml_callback2 (fv, o1v, o2v);
 >   }

you should dereference fvp at the last possible time, after the
Val_obj calls if theses calls are allocating in the caml heap (eg
caml_copy_double, caml_copy_string).

Also you probably should use caml_callback2_exn, otherwise if your
caml callback raises an exception, control will jump back straight to
the ocaml code and skip the end of the C caller code (run), which will
usually result in resource leaks (for instance in your version it would
leak the global root).

-- 
   Olivier