I would like to wrap a C DLL in a type library for use in VB. Some of
the functions register callbacks. I have not been able to wrap the
callback registering functions in the type library. The following code
gives the error "Can't find dll entry point fnCTestCB in CTest.dll", in
VB when I try to call fnCTestCB. In the IDL file I tried void*, long,
and the fnptr typedef, nothing seems to work.

Thanks

CTest.h -------

#include "windows.h"

typedef void (__stdcall* fnptr)(int i);

int WINAPI fnCTestCB(fnptr cb);

TypeLib.idl ----------

#define CDTAPI __stdcall

// C DLL Test Interface
[
uuid(29FB2B91-F89B-4776-A4B9-67B67C95ABFB),
helpstring("C DLL Test Interface"),
lcid (0x00000000),
version(1.0)
]

library CDLLTest
{
    importlib("stdole32.tlb");
    importlib("stdole2.tlb");

    [
    uuid(41072DE3-C829-4813-A4A5-6BDC84399133),
    helpstring("C DLL Test Functions"),
    dllname("CTest.dll")
    ]
    module CDTFunctions
    {
      [
      entry("fnCTestCB"),
      helpstring("Function that registers a callback.")
      ]
      int CDTAPI fnCTestCB(void* cb);
    }

Public Function vbCTestCB(ByVal n As Long)
      Debug.Print "In VB callback n=" & n
End Function

VB: Form1.frm ---------

Private Sub Command1_Click()
      Call fnCTestCB(AddressOf vbCTestCB)
End Sub

Add extern "c" to your function declaration

kp

<rant>
COM is great for what it does but, it's dependency on the registry makes
it extremely volatile, MS should really give more support. This whole
thing came about because of the difference in struct alignment in VB and
C/C++, VB has to 4-byte while C/C++ defaults to 8-byte, this requires VB
applications to hack into it's type defintions with padding Byte arrays
or use a TypeLib. The TypeLib solutions seems the right way to go, but
it is turning into a small nightmare to wrap the whole thing and not
just the type defintions. I am surprised that MS has not given more
information on how to wrap a C DLL in a TypeLib, since that seems to be
the most reliable way to interface to C DLLs.

About this VB problem:
1) View dependencies doesn't see fnCTestCB
2) OLE/COM Viewer doesn't see fnCTestCB
3) VB returns ""Can't find dll entry point fnCTestCB in CTest.dll" even
when the function does not exist.
</rant>

Thanks

CTest.def -------
; DEF file

EXPORTS
        fnCTest
        ;fnCTestCB