[Ada (GNAT/GCC)] VARPTR, GNAT and data "transput"

Okay, have got it figured (soli Deo gloria!)

The following listings demonstrate the passing of a two part Type containing two Singles from VB6 into a GNAT/GCC DLL. Once inside, the elements of the Type are swapped and control returns to VB6. VB6 then displays the results.

As before, the code has been worked out in the context of the afore-posted api project. If you want to play with this, just take the earlier work and tweak it appropriately.

First up, the additions to api.ads:

function SwapSingles ( VarPtr : in Win32.LONG) return Win32.BOOL; ... pragma Export (DLL, SwapSingles );

Next, the additions to api.adb:

with Ada.Exceptions; use Ada.Exceptions; ... type Singles is record Part1 : Float; Part2 : Float; end record; type PointerToSingles is access Singles; function To_PointerToSingles is new Ada.Unchecked_Conversion (Win32.LONG, PointerToSingles ); function SwapSingles ( VarPtr : Win32.LONG) return Win32.BOOL is PSingles : PointerToSingles; FTemp1 : Float; FTemp2 : Float; begin PSingles:= To_PointerToSingles(VarPtr); FTemp1 := PSingles.All.Part1; FTemp2 := PSingles.All.Part2; PSingles.All.Part2 := FTemp1; PSingles.All.Part1 := FTemp2; return Win32.TRUE; exception when Constraint_Error => return Win32.FALSE; end SwapSingles;

API.DEF must also be modified, adding swapsingles to the list of EXPORTS.

Next, some VB code to demonstrate ...

Declare Function SwapSingles Lib "api.dll" (ByVal ptr As Long) As Boolean Type SinglesStruct Part1 as Single Part2 as Single End Type ... Dim Singles As SinglesStruct Singles.Part1 = 123.456 Singles.Part2 = 654.321 Debug.Print "Before:" Debug.Print "Singles.Part1 = " & Singles.Part1 Debug.Print "Singles.Part2 = " & Singles.Part2 If swapsingles(VarPtr(Singles)) = False Then Debug.Print "Constraint Error" Else Debug.Print "After:" Debug.Print "Singles.Part1 = " & Singles.Part1 Debug.Print "Singles.Part2 = " & Singles.Part2 End If

... and part of a sample output.

Before: Singles.Part1 = 123.456 Singles.Part2 = 654.321 After: Singles.Part1 = 654.321 Singles.Part2 = 123.456

The project that all this was part of was done in both GNAT/GCC and ObjectAda 7.2.2. Interestingly, ObjectAda's Float is equivalent to VB6's Double, whereas GNAT/GCC's Float is equivalent to VB6's Single. Does anyone know why?

By the way, regarding "transput", Wikipedia's article on Algol states:

ALGOL 60 as officially defined had no I/O facilities; implementations defined their own in ways that were rarely compatible with each other. In contrast, ALGOL 68 offered an extensive library of transput (ALGOL 68 parlance for Input/Output) facilities.

A extensive library of transput facilities is something you'll find in Protium, too.

© Copyright Bruce M. Axtens, 2007