Re: [Caml-list] Calback

[Christophe Raffalli <christophe.raffalli@univ-savoie.fr>]

Olivier Andrieu wrote:

 >  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).
 >

This is even worth !

 >  > ----- 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.


This is to follow the same api than GLUT (generalized a bit because in 
glut the argument is always int.

 >  > // 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)'.
 >

It was just a first try. You are right. Do you have such a platform ? 
Could check in that case the prototype of glutTimerFunc in GL/glut.h ?

If it is still int, I could do a test in the C code and go through a C 
array if (sizeof(int) != sizeof(void*))

-- 
Christophe Raffalli
Université de Savoie
Batiment Le Chablais, bureau 21
73376 Le Bourget-du-Lac Cedex

tél: (33) 4 79 75 81 03
fax: (33) 4 79 75 87 42
mail: Christophe.Raffalli@univ-savoie.fr
www: http://www.lama.univ-savoie.fr/~RAFFALLI
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