this
Code:
error: unresolved external symbol
means that you prototyped a function that doesn't exist in your project. It literally means: "hey, it's the linker here. The C compiler gave me a compiled C file in which there is a call for a function that's in another file, but I don't know where in hell I can find it, and it didn't told me either, so I don't know what to do!"
Check if that function exists the way the linker expects to find it. And ignore that warning, it will get away when the problem will be solved.