Re: [PATCH 22/28] libxl: ocaml: event management

[David Scott <dave.scott@eu.citrix.com>]

On 25/03/13 14:45, Rob Hoes wrote:
> +void fd_deregister(void *user, int fd, void *for_app_registration)
> +{
> +	CAMLparam0();
> +	CAMLlocalN(args, 2);
> +	value *func = caml_named_value("fd_deregister");
> +
> +	args[0] = (value) user;
> +	args[1] = Val_int(fd);
> +
> +	caml_callbackN(*func, 2, args);
> +	CAMLreturn0;
> +}

The OCaml manual[*] (S19.7.2) hints that the name lookup is a bit slow:

"The pointer returned by caml_named_value is constant and can safely be 
cached in a C variable to avoid repeated name lookups. On the other 
hand, the value pointed to can change during garbage collection and must 
always be recomputed at the point of use."

The manual suggests caching the value * in a static like this:

   void fd_deregister(void *user, int fd, void *for_app_registration)
   {
     CAMLparam0();
     CAMLlocalN(args, 2);
     static value * func = NULL;
     if (func == NULL) {
       /* First time around, look up by name */
       func = caml_named_value("fd_deregister");
     }
   ...


[*] http://caml.inria.fr/pub/docs/manual-ocaml-4.00/manual033.html