C++ DLL program called from VB6 cannot be found

Question

We compiled a C++ program as DLL and want to use it from VB6. The program has subroutines like

int __stdcall setup(int exposure_time, double shutter, double gain, int numImages) {
....
}

int __stdcall test() {
  return 8;
}

And the Def file is defined as:

LIBRARY
EXPORTS
   setup=setup
   test=test

And we are declaring them in VB6 like this:

Public Declare Function setup Lib "C:\MyDll.dll" () As Long

Public Declare Function test Lib "C:\MyDll.dll" () As Long

And trying to access then in a form:

Private Sub Form_Load()

     Debug.Print (test())

End Sub

But we are getting "File not found" in VB, when the execution hits to very first function call! The MyDll.dll program is in the declared location and it is not to be registered. What is missing to declare?

Hello Bathsheba,

I followed your suggestions but the VB program still could not find the dll.

Declarations in VB:

 Public Declare Function setup Lib "C:\Math\FlyCapture2\bin\PGLCTrigger.dll" ( _
     ByVal exposure_time As Long, _
     ByVal shutter As Double, _
     ByVal gain As Double, _
     ByVal numImages As Long) As Long

 Public Declare Function test Lib "C:\Math\FlyCapture2\bin\PGLCTrigger.dll" () As Long

Def File:

 LIBRARY
 EXPORTS
    setup=@1
    test=@2

C++ program:

 __declspec(dllexport) int __stdcall setup(int exposure_time, double shutter, double gain,  int numImages) {
 ....
}

 __declspec(dllexport) int __stdcall test() {
    return 8;
}

And the VB calling program:

 Private Sub Form_Load()

      setup 12, 24#, 1#, 10
      test

 End Sub

As soon as the execution hits the setup line in the program above, the "dll not found" error comes.

I defined the following in a .def file as suggested by Compile a DLL in C/C++, then call it from another program:

 //DLL Export-Import definitions
#ifdef BUILD_DLL
#define EXPORT __declspec(dllexport)
#else
#define EXPORT __declspec(dllimport)
#endif

So that I can reference the functions in the DLL as

EXPORT int __stdcall setup(int exposure_time, double shutter, double gain, int numImages)

But VS2010 produces an error message for the import.

So I am stuck. Any further help will be appreciated a lot. Thank you.

Answer 1

Others have told you that you must declare the parameters for the function. If the DLL will not load, and you are sure it is there then it is likely missing a dependency. Debug this with Dependency Walker. Load up the executable and run it in profile mode from the Profile menu. This will log loader events and you'll see exactly the cause for failure.

Answer 2

You need to tell VB6 about the function arguments for setup:

Public Declare Function setup Lib "C:\MyDll.dll" ( _
    ByVal exposure_time As Long, _
    ByVal shutter As Double, _
    ByVal gain As Double, _
    ByVal numImages A Long) As long

And your .def file, I think, is incorrect. I use

EXPORTS
   setup @1
   test @2

Where 1 and 2 are arbitrary but distinct positive integers called ordinals. A couple of remarks:

A Long in VB is an int in C++.

You can use __declspec(dllexport) and extern "C" {/*your function here*/} in place of the .def file.