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Wrapping a callback to OCaml code from C
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Date: 2005-06-24 (14:20)
From: Olivier Andrieu <andrieu@i...>
Subject: Re: [Caml-list] Wrapping a callback to OCaml code from C

 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).