Help with this DLL I am writing in VC++ to be used in VB 6 (Simple Code)

[bignateyk]

ok, I want to write a DLL in visual c++ that just returns the numerical value of a position in an array passed into it thru visual basic...

Here is the C++ DLL Code:

---------------------------------------------------------------------------------------------------
Main.CPP
---------------------------------------------------------------------------------------------------
#define WIN32_LEAN_AND_MEAN // No MFC

#include <windows.h> // Standard Windows Include

INT APIENTRY APITest(int imageArray[])
{
int test = imageArray[4];
return test;
}


---------------------------------------------------------------------------------------------------
definitions.def
---------------------------------------------------------------------------------------------------
EXPORTS
APITest @1





Here is the visual basic code:

Private Declare Function TestMyDll Lib "vbdll.dll" Alias "APITest" (ByRef array2() As Integer) As Long

Private Sub Command1_Click()
On Error Resume Next
Dim x As Integer
Dim a(10) As Integer

For x = 0 To 10
a(x) = x
Next x

x = TestMyDll(a())
msgbox x
End Sub



Whenever I click command1, it either msgboxes "0" or a really big number like "25034745"

I am a beginner at this so I am probably not passing the array right or something...

Any ideas?

Thanks.

 

[Descartes]

An integer in VB is a short in C. You're declaring an array of ints in the C++ side and an array of integers in the VB side. Since an int in C is 32-bits (on most platforms), the 16-bit array of integers in VB will cause each int in the array on the C side to get two extra bytes of the integer in the VB side. Consider this VB array:

[................][................][................][................]

An array of 16-bit integers. Now consider what the C APITest routine expects:

[................................][................................][................................][................................]

Make sense? If you had an array in VB like the following:

Dim nFoo(2) As Integer
nFoo(0) = 1
nFoo(1) = 1

Then you passed nFoo to APITest, the first element in the array passed on the C side would be 0x00010001, or 65537 in decimal: an overflow for a 16-bit integer.

Also note that in most cases, simply passing a short to a routine that expects an int will cause it to zero-extend, thus you're usually safe; however, such is not the case in the context of an array as you can't zero-extend an array composed of arbitrarily defined data types.

I hope that makes at least some sense. Kinda hard to articulate...

 

[bignateyk]

makes sense but it still doesnt work. I changed the int's in the c++ file to shorts.

 

[Kntx]

ew. dlls.

 

[Descartes]

Change your declaration in VB to this:

Private Declare Function TestMyDll Lib "vbdll.dll" Alias "APITest" (ByRef array As Long) As Long

Then call the function like this:

Private Sub Command1_Click()
On Error Resume Next
Dim x As Integer
Dim a(10) As Integer

For x = 0 To 10
a(x) = x
Next x

x = TestMyDll(a(0))
msgbox x
End Sub

Sorry, I focused on the data type mismatch and neglected that part. That *should* get you going, but no guarantees . The point is that you need to send the C++ function the address to the first element in the array. The only way to do this is to pass the first element in ByRef in VB.

 

[bignateyk]

yeah that works now.

My next question is how to make a void that allows me to use several variables from it.

For example,


C++ Code:

void _stdcall imageSubtraction()
{

long number1 = 20;
long number 2 = 20;
bool q = true;
}


VB Code:

dim test as long
dim booltest as boolean
Call imageSubtraction()

booltest = q
if booltest = true then
test = number 1
end if

---------------------------------------------------

There isnt really a point to that code, but I need to know how I can use the variables from a void function.

I tried using the method above, but it wont return those variables to VB.

 

[Descartes]

You can't alter the value of a variable if you don't pass it a pointer to its value. You need to declare a pointer to the variable in your function declaration. e.g.:

void foo(int *p)
{
*p = 1;
}

Then on the VB side you need to pass in the appropriate data type ByRef:

Declare Sub Foo Lib "vbdll.dll" (ByRef n As Long)

Dim i As Integer
i = 9
Call Foo(i)
' i now == 1

Do the equivalent for whatever data type you want to alter within the function you call.

 

[bignateyk]

cool. thanks. Also, is there a c++ equivalent to "Exit Sub" ? I tried "return 0" but that only works in functions not voids.

 

[MrChad]

Just "return;" should work for void methods