Question

I’m trying to call a DLL written in C++ from a VB6 application.

Here’s the C++ example code for calling the DLL.

char firmware[32];
int maxUnits = InitPowerDevice(firmware);

However, when I try to call it from VB6 I get the error bad DLL calling convention.

Public Declare Function InitPowerDevice Lib "PwrDeviceDll.dll" (ByRef firmware() As Byte) As Long

Dim firmware(32) As Byte
InitPowerDevice(firmware)

Edit: The C++ Prototype:

Name: InitPowerDevice
Parameters: firmware: returns firmware version in ?.? format in a character string (major revision and minor revision)
Return: >0 if successful. Returns number of Power devices connected

CLASS_DECLSPEC int InitPowerDevice(char firmware[]);

Answer 1

Been a long time, but I think you also need to change your C function to be stdcall.

// In the C code when compiling to build the dll
CLASS_DECLSPEC int __stdcall InitPowerDevice(char firmware[]);

' VB Declaration
Public Declare Function InitPowerDevice Lib "PwrDeviceDll.dll" _
        (ByVal firmware As String) As Long

' VB Call
Dim fmware as String
Dim r  as Long
fmware = Space(32)
r = InitPowerDevice(fmware)

I don’t think VB6 supports calling cdecl functions in any normal way – there may be hacks for doing it. May be you can write a wrapper dll which wraps the cdecl function with a stdcall function and just forwards the call.

These are some hacks – but I haven’t tried it.

http://planet-source-code.com/vb/scripts/ShowCode.asp?txtCodeId=49776&lngWId=1

http://planet-source-code.com/vb/scripts/ShowCode.asp?txtCodeId=62014&lngWId=1