Re: [Caml-list] Calback

[Olivier Andrieu <andrieu@ijm.jussieu.fr>]

 Christophe Raffalli [Tue, 20 Jul 2004]:
 > Thanks for the help on the list. I decided to change Glut.timerFunc
 > for the following code, passing the Caml function and its argument
 > using a C int (transmitted to the times) which is in fact a pointer
 > to a Caml pair. This pointer is registered as a global root and
 > removed by the callback itself. I give the code bellow if someone
 > wants to write something similar.
 > 
 > A small question: why do people use callback trough a name and a
 > hashtable instead of passing the callback inside C variables
 > (global or local) ? It could save a lot of time ?

(caml_named_value uses a linked list, not a hashtable).

 > ----- First the caml code:
 > 
 > 
 > (* timerFunc is non-window-dependent *)
 > external _glutTimerFunc : int->((value:'a->'b) * 'a)->unit = 
 > "ml_glutTimerFunc"
 > let timerFunc ~ms ~cb ~value:v : (unit) =
 >      _glutTimerFunc ms (cb,v);; (* register the callback with GLUT *)

It's just easier not to bother with the argument and have the callback
be of type `unit -> unit' (or `unit -> 'a'), since you can include any
argument you like in the closure.

 > // for timer we can not use Register.callback because it grows 
 > infinitely the
 > // number of global root
 > 
 > static void glutTimerFunc_cb(int fun_arg)
 > {
 >    // fun_arg is a pointer on a caml pair hidden in an integer.
 >    // Moreover we remove the global root because each callback is used once
 >    // by the timer.
 >    value *v = (value) fun_arg;

I'm afraid this will break on plaforms where `sizeof (int) != sizeof (long)'.

-- 
   Olivier

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