Trying to call a C DLL from VB. Can't get one of the parameters working

Question

Trying to set up a USB power strip.

Here\'s the documentation:

Initializes the Power USB API.

Name: InitPowerUSB 

Parameters: model:returns the mod         


        

Answer 1

I can't answer the rest of your function signature woes since I don't have any documentation for your PwrUSBDll.dll.

However "Bad DLL calling convention" errors generally mean you have a CDecl entrypoint and VB6 can only call those with some help.

There are a couple of fixes.

The obvious one is to modifiy the source and recompile that DLL using StdCall instead.

Another is to create a type library for that DLL, which helps inform VB6 about the issue and resolves it.

Then you have the option of using VB6's undocumented CDecl decorator:

Public Declare Function InitPowerUSB CDecl Lib "PwrUSBDll.dll" ( _
    ByRef model As Integer, _
    ByVal firmware As String) As Integer

However the downside is that this will not work when run within the IDE, nor will it work when compiled to p-code. The p-code interpreter doesn't process this keyword.

So you could just bypass it in IDE runs and supply dummy results for testing, or you can create a small wrapper DLL in VB6 that you separately compile to native code.

Caveats:

For this to solve your problem we'd have to assume you are passing correct data types in that argument list. A C++ int is a VB6 Long. You are probably better off passing a VB6 Byte array ByRef for that char[8] unless this is a Unicode DLL entrypoint. The function return value is also most likely Long.