Re: Address to function pointer conversion

["John G. Volan" <johnvolan@sprintmail.com>]

Robert Dewar wrote (quoting me):
> 
> John says
> ~
> <<Hmm, I think the answer may be that this sort of thing might just be
> beyond the scope of the Ada95 standard. In other words, this may be an
> implementation-specific (maybe even an OS-specific) thing not defined by
> Ada95. There may be no guaranteed portable way to take the address of a
> routine (I assume imported from some other language, such as C) and turn
> it into an access-to-subprogram. In that case, if you have an
> implementation specific solution that works, you may just have to live
> with the lack of portability.
>  >>

Later on in this same post which Robert quoted, I suggested:

>   type Callback_Access_Type is access procedure;
>   pragma Convention (C, Callback_Access_Type); -- ** is this right?
> 
>   function Get_Callback return Callback_Access_Type;
>   pragma Import (C, Get_Callback, "getCallback");
> 
> ** What I don't know here is whether this Convention pragma is correct.
> But if it is, then this would be a guaranteed portable mechanism for
> doing what you want.

In his reply, Robert wrote:

> If the address is coming from C, then I think the following should be quite
> portable:
> 
>    type x is access procedure ....
>    pragma Convention (C, X);
> 
>    function Get_Address return x;
>    pragma Import (C, Get_Address);

Excellent!  Looks just like what I wrote.

> Now your compiler might reject the pragma Convention on X, but it is reasonable
> that it shouold be supported, and if it is supported, then it should work.
> 
> Notice this general approach, instead of importing some foreign gizmo from C
> like an address, import it in properly typed form, but apply pragma Convention
> C to the type.
> 
> This is generally what should be done for access types as well when
> communicating with C.

Thank you Robert, that answers my question.  I didn't know whether this
pragma was available for access-to-subprogram types.  (I was making an
informed guess.)
 
------------------------------------------------------------------------
Internet.Usenet.Put_Signature 
  (Name       => "John G. Volan",
   Employer   => "Texas Instruments Advanced C3I Systems, San Jose, CA",
   Work_Email => "jvolan@ti.com",
   Home_Email => "johnvolan@sprintmail.com",
   Slogan     => "Ada95: World's *FIRST* International-Standard OOPL",
   Disclaimer => "My employer never defined these opinions, so using " & 
                 "them would be totally erroneous...or is that just "  &
                 "nondeterministic behavior now? :-) ");
------------------------------------------------------------------------