[Ada (GNAT/GCC)] Passing data with VB6's VARPTR to Ada

In the current project, I've been trying to figure out how to pass data to Ada. Because I'll be passing Long, Double (Float), and two Types to emulate a LongLong and a DoubleDouble, I decided I'd use VB's VarPtr and have Ada sort the mess out on its side.

After the loss of a little hair, the reading of bits of Michael B. Feldman's Software Construction and Data Structures with Ada 95, and with help from Martin Krischik and Simon Wright, I came up with the following.

The code was worked out in the context of the afore-posted api project. If you want to play with this, just take the earlier work and stuff this into it.

First up, the additions to api.ads:

function HandlePointer ( Left : in Win32.LONG) return Win32.LONG; ... pragma Export (DLL, HandlePointer );

Next, the additions to api.adb:

function HandlePointer ( Left : in Win32.LONG) return Win32.LONG is type LongRec is record L : Win32.LONG; end record; type PointerToLongRec is access LongRec; function Conv is new Unchecked_Conversion (Win32.LONG, PointerToLongRec ); PLongRec : PointerToLongRec; begin PLongRec := Conv(Left); return PLongRec.All.L; end HandlePointer;

Next, some VB code to demonstrate ...

Declare Function handlepointer Lib "release\api.dll" (ByVal x As Long) As Long Dim i As Long i = 1001 Debug.Print "i="; i Debug.Print "varptr(i)="; VarPtr(i) Debug.Print "what ada thinks is pointed to by varptr(i)="; handlepointer(VarPtr(i))

... and a sample output.

i= 1001 varptr(i)= 1309644 what ada thinks is pointed to by varptr(i)= 1001

I'm in the process now of working out how to access the LongLong and DoubleDouble Types. Shall keep you posted.

By the way, the return value will eventually be a pointer as well. That'll keep the VB programmer on his toes.

© Copyright Bruce M. Axtens, 2007