extern and SDL_Surface

If i declare a variable using the extern keyword in a header file I am able to define it in another c++ file. However when i use the extern keyword for SDL_Surface,TTF_Font or any other SDL library I cannot define it anywhere else and the linker throws a multiple definition error at me. Example: I have a header file and I declare like this Then in a separate file c++ source file I define it like this If I declare a const or any other datatype it works fine.Is there something else I must do when using abstract datatypes from the SDL Library(if i am correct SDL_Surface can be known as abstract?)