I'd like to access some function in win32 dll

Sample dll's source is like the belows.

-- part of source --

#ifndef
TESTDLL
#define TESTDLL extern "C"
__declspec(dllimport)
#endif


TESTDLL float  _stdcall AddNum(float a, int b);
TESTDLL int    _stdcall MulNum(int a, int b);


Firstly, I succeed accessing functions with "Win32API"

But, I met the limitation of accessing 'float' or 'string' variable,
so I'd like to access functions using "ruby/dl'

Please let me know the ways and samples.

Thank you for reading my questions.

Then, Thank you in advance.