Re: System.Address to Access to function/procedure conversion

[Tarek Ghaleb <invalid@invalid.org>]

On 2013-07-28, Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
> If you pass anything to C you should better keep it under C convention,
> even if you are certain that C does not touch it. You should pass an
> access-to-procedure/function to C and get it back. The access type will
> have pragma Convention (C, ... set. You don't need address here. E.g.

I agree completely. Using C convention is the way to do it. But is it
*possible* to call a function using its address? (even if not a such a
great idea)

But let me explain how this question came about:

I've been experimenting with writing a thick binding for FLTK for a
couple of days now, just exploring the idea (which deserves a thread
by itself, since it would be interesting to get some
feedback). Anyway, one thing I started working on was callbacks, If I
use C convention the user of the binding will also have to declare his
procedure using C convention. For example (simplified)

  procedure Cb_Button;
  pragma Convention (C, Cb_Button);  -- This is required I guess

and then later register the callback with

  Btn.Callback(Cb_Button'Access);

But if I remove 

  pragma Conversion (C, Cb_Button);

I get 

  ...: subprogram "Cb_Button" has wrong convention
  ...: does not match convention of access type

But I don't want the user to worry about using pragma Convention for
every defined callback procedure or think in terms of interfacing with
C (or in FLTK's case C++). Ideally, Callback() is not the actual
imported callback() but it takes Ada friendly parameters and does any
type conversions needed then calls the imported callback() function. I
thought maybe it works like above but without the convention line and
then Callback() would send the Address instead of the Access to the
actual callback(). But maybe there is a better way to do it.

There are other issues of course, for example: callbacks do take
parameters and some are void* but that would complicate my question
for the time being.

Tarek.

-- 
For fools rush in where angels fear to tread. -- Alexander Pope

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