How to implement rundll2-callable function?

Question

I'm trying to write an exe that also exports functions which can be called with rundll32. Is this possible and if so, why isn't it working like this?

I closely followed Microsoft's advice on this.

#define RUNDLL32(func) extern "C" __declspec(dllexport) void CALLBACK func(HWND hWnd, HINSTANCE hInst, LPSTR lpszCmdLine, int nCmdShow)

RUNDLL32(MyFunc)
{
  MessageBox(0, 0, 0, 0);
}

But when called with

rundll32 myprog.exe,_MyFunc@16

rundll32 crashes/DEP kicks in.

Why are you using the mangled name of the function? Dont `rundll32 myprog.exe,MyFunc` works? — Nicolas Repiquet, Nov 04 '10 at 11:56
That's because the function is exported under that name. I could make a .def file and export it under a different name, but I don't care if it looks a little weird (rundll32 finds it, that's not the problem) plus I don't really know how to tell it to use the .def file. — flacs, Nov 04 '10 at 12:18

score 3 · Accepted Answer · answered Nov 04 '10 at 13:28

Rundll32.exe uses LoadLibrary() to load the executable image. This is not likely to work out well for an EXE, it doesn't expect to get loaded on an address that's that not its default. Which is guaranteed to happen, rundll32.exe already occupies that default address. Not sure if you could tinker with the linker so that it doesn't omit the relocation records.

But don't bother with this approach, just create a DLL instead of an EXE. And pass real arguments to MessageBox(). And, yes, use a .def file to rename the exported function.

How to implement rundll2-callable function?

1 Answers1