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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: Access to data indirection (newbie)
Date: Tue, 27 Nov 2007 09:11:13 +0100
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Date: 2007-11-27T09:11:13+01:00
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axtens schrieb:
> G'day everyone
> 
> I have a DLL which is receiving a value that is the result of a VARPTR
> in VB6, so the value arriving is the address and the data desired can
> be found at that address.
> 
> So far I've gotten as far as
> 
>    function HandlePointer (
>          Left : in     Win32.LONG)
>      return Win32.LONG is
>      begin
> 
>    end HandlePointer;
> 
> which is, of course, brain-dead. Fact is, though, so far as Ada is
> concerned, I'm a newbie.
> 
> So tell me, what's the syntax?
> 
> Once you've figured that one out, tell me how I'd handle 'Left'
> pointing to a record of two integers, such that I can access both
> elements of the record.

Your next step would be an Unchecked_Conversion:

http://en.wikibooks.org/wiki/Ada_Programming/Type_System#Unchecked_conversion

however - for my taste you are thinking to "Low-Level" and to "C-Like".
In Ada one does not pass pointer or addresses as a pointer. Down that
road lies sorrow not satisfaction.

How about:

type Options is (Long, Short, Whatever);

type Var (Option : Options) is record
   case Option is
	Long =>
	   A_Long : Win32.LONG;
	Short =>
	   A_Short : Win32.SHORT;
        Whatever =>
	   A_Whatever_1 : Win32.LONG;
	   A_Whatever_2 : Win32.LONG;
   end case;
end record:

pragma Unchecked_Union (Var);
pragma Convention (C, Var);

See: http://en.wikibooks.org/wiki/Ada_Programming/Types/record#Union

type VarPtr is access all Var;

pragma Convention (C, VarPtr);

function HandlePointer (
  Left : in VarPtr)
  return Win32.LONG is
begin

end HandlePointer;

pragma Export (C, HandlePointer);

or

function HandlePointer (
  Left : access Var)
  return Win32.LONG is
begin

end HandlePointer;

pragma Export (C, HandlePointer);

or

function HandlePointer (
  Left : in Var)
  return Win32.LONG is
begin

end HandlePointer;

pragma Export (C, HandlePointer);


The later might surprise you bout since the functions is declared
"export c" "in Var" is passed as a pointer.

Martin

-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com