Problems with callback from DLL invoked from VBA

[Jag Man]

I have to invoke a C++ function from VBA/Excel that does a callback. I've
isolated the problem I'm having in the code below.

In the C++ (complied & linked to a DLL) I have:

long __stdcall tryCB3(long n, long (*cbFunc)(long m))
{
long r;
r = cbFunc(n); // call back
return r;
}

In the VBA I have:

Private Declare Function tryCB3 Lib "SparkExUtilsDll.dll" _
(ByVal n As Long, ByVal theFunct As Any) As Long

Public Function theCBFunc3(ByVal m As Long) As Long
Debug.Print "In theCBFunc3"
theCBFunc3 = m * m
End Function

Public Function tryCBDrv3()
Dim n As Long
Dim q As Long
n = 2
q = tryCB3(2, AddressOf theCBFunc3)
Debug.Print "n= " & CStr(n)
Debug.Print "q= " & CStr(q)

End Function

When I step through tryCB3Drv it calls tryCB3, and then steps through
theCBFunc3, all working as expected, but then crashes with a C++
error dialog saying the ESP was not preserved through a function call etc.

Note that the it works without crashing if the callback function
has no arguments, so it must be some problem with the way VBA is setting
up the argument to theCBFunc3, vs. the way it's declared in C++.

Any ideas?

TIA

Ed

 

[Chip Pearson]

Change your C++ function to

long __stdcall tryCB3(long n,long(__stdcall *cbFunc)(long m))


--
Cordially,
Chip Pearson
Microsoft MVP - Excel
Pearson Software Consulting, LLC
www.cpearson.com

 

[Jag Man]

Yes, that works. Thanks, Chip.

Now, moving closer to what I really need to do, I need to add an
array argument to the callback function:

long __stdcall tryCB3(long n,long(__stdcall *cbFunc)(long m, long *))
{
....
long* x= new long[n];
....
r = cbFunc(n, x);
...

delete [] x;
return r;
}

And in the VB:

Public Function theCBFunc4(ByVal m As Long, ByRef x() As Long) As Long
Dim i As Integer
Dim s As Long
s = 0
For i = 0 To m - 1
s = s + x(i)
Next i
theCBFunc4 = s
End Function

But, it doesn't work. It crashes Excel when I call the tryCB3 function from
VB.

Any ideas?

TIA

Ed

 

[Chip Pearson]

I'm not sure what the problem is. I suspect that it is because
VB/VBA uses SAFEARRAY structures rather than C-type arrays. You
might have more luck posting this in one of the Visual C
newsgroups.


--
Cordially,
Chip Pearson
Microsoft MVP - Excel
Pearson Software Consulting, LLC
www.cpearson.com

 

[Jag Man]

Thanks, Chip. When I get to the bottom of it I'll post the solution.

Ed