Question

e.g. In a static library I had

  void function (void);

  function();

And function() existed in the main application.

But If I build it as a DLL the linker complains that the function is undefined on the DLL.

Answer 1

Yes, but this is hackish.

In the dll :

typedef void(*funcPtr)(void);
// Declare a variable named "ptrFunction" which is a pointer to a function which takes void as parameter and returns void
funcPtr ptrFunction;
// setter for ptrFunction; will be called from the .exe
void DLL_EXPORT setFunction(funcPtr f){ptrFunction f;}
// You can now use function() through ptrFunction
foo(){
   ptrFunction();
}

and you call setFunction from the .exe.

void function(){
    // implementation here
}    
int main(){
    setFunction(&function);  // now the dll knows the adress of function()
    foo(); // calls foo() in dll, which calls function() in the .exe
    ....
}

Great, huh ? :/
You should probably refactor your code so that function() is in yet another DLL, but it depends.