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Date: Thu, 23 Jun 2005 22:47:33 +0100
To: caml-list@inria.fr
Subject: Wrapping a callback to OCaml code from C
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From: Richard Jones <rich@annexia.org>
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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