I think what you might be missing is to declare your functions as being exported.
You need to do something like the following (which I borrow from what I was told in a VC++ forum):
* Add this code before the class declaration (in the header file):
#ifdef IN_DLL
#define WHICHWAY __declspec (dllexport)
#else
#define WHICHWAY __declspec (dllimport)
#endif
* Then you would add this variable to your class as follows:
class WHICHWAY ClassNameHere
{
};
* Now inside your source file (.cpp) you put the following at the top of file:
#define IN_DLL
He was showing me how to export a class, but I must say I never tried it like that... Instead I export functions using the same method.
You might need to adapt the code above, since I don`t know if it is 100% C (could be some C++ there, aside from the "class"

. Anyway, it`s a good staring point.
Vincent