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From: "John G. Volan" <johnvolan@sprintmail.com>
Subject: Re: Address to function pointer conversion
Date: 1997/06/17
Message-ID: <33A74403.4B7C@sprintmail.com>#1/1
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References: <5nrq5h$13cm@info4.rus.uni-stuttgart.de>
 <33A1831C.269F@sprintmail.com> <dewar.866334558@merv>
Reply-To: johnvolan@sprintmail.com
Newsgroups: comp.lang.ada
Date: 1997-06-17T00:00:00+00:00
List-Id: <comp.lang.ada>


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.)
 
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