Question

I created c++ dll (using mingw) from code I wrote on linux (gcc), but somehow have difficulties using it in VC++. The dll basically exposes just one class, I created pure virtual interface for it and also factory function which creates the object (the only export) which looks like this:

extern "C" __declspec(dllexport) DeviceDriverApi* GetX5Driver(); 

I added extern “C” to prevent name mangling, dllexport is replaced by dllimport in actual code where I want to use the dll, DeviceDriverApi is the pure virtual interface.

Now I wrote simple code in VC++ which just call the factory function and then just tries to delete the pointer. It compiles without any problems but when I try to run it I get access violation error. If I try to call any method of the object I get access violation again.

When I compile the same code in MinGW (gcc) and use the same library, it runs without any problems. So there must be something (hehe, I guess many differences actually :)) between how VC++ code uses the library and gcc code.

Any ideas what?

Cheers,
Tom

Edit:
The code is:

    DeviceDriverApi* x5Driver = GetX5Driver();


if (x5Driver->isConnected())
    Console::WriteLine(L"Hello World");

delete x5Driver;

It’s crashing when I try to call the method and when I try to delete the pointer as well. The object is created correctly though (the first line). There are some debug outputs when the object is created and I can see them before I get the access violation error.

Answer 1

• You’re using one compiler (mingw) for the DLL, and another (VC++) for the calling code.
• You’re calling a ‘C’ function, but returning a pointer to a C++ Object.

That will never work, because VTable layouts are almost guranteed to be incompatible. And, the DLL and app are probably using different memory managers, so you’re doing new() with one and delete() with the other. Again, it just won’t work.

For this to work the two compilers need to both support a standard ABI (Application Binary Interface). I don’t think such a thing exists for Windows.

The best option is to expose all you DLL object methods and properties via C functions (including one to delete the object). You can the re-wrap into a C++ object on the calling end.