Re: Dispatching callback handed over to C

[Natasha Kerensikova <lithiumcat@gmail.com>]

On 2012-04-05, Randy Brukardt <randy@rrsoftware.com> wrote:
> In particular, we're talking about the mapping of C to Ada here, and not the 
> underlying design of the C API (which is out of our control anyway). The 
> question is how to map "void *" to Ada?
>
> 1) Some convention C access type designating an appropriate Ada type.
>
> 2) Some convention C access type designating a dummy Ada type (use 
> unchecked_conversion as needed on the real types).

What I'm still struggling with is why do I have to settle for #2, or
what is preventing me from going for #1?

   type Callback_Access is access all Event_Callback'Class;
   pragma Convention (C, Callback_Access);

Using Callback_Access in a subprogram imported from C trigged the
following GNAT warning:

        >>> warning: "Imported_Subprogram.Arg" involves a tagged type
which does not correspond to any C type

I guess since it's "only" a warning, I could just silence. However I
can't bring myself to silence something I don't understand.

If it's only about the fact that Callback_Access is a C pointer to an
object with which C subprograms has no way for dealing properly, then
fine (since in that particular case C code considers it as an opaque
pointer).

However it's worded like "warning: you're mixing C and Ada", to which
I'm tempted to answer "breaking news ! writing a binding involves mixing
two languages", but that would be rude and there is no point in telling
a compiler things in a human language anyway.

On the other hand, solution #2 allows the import package to not know
about Event_Callback type, and potentially be used with other Ada
callback implementation.


Thanks for your insights,
Natasha