SDL joystick support.
This is the lower-level joystick handling. If you want the simpler option, where what buttons does what is well-defined, you should use the gamepad API instead.
The term "instance_id" is the current instantiation of a joystick device in the system, if the joystick is removed and then re-inserted then it will get a new instance_id, instance_id's are monotonically increasing identifiers of a joystick plugged in.
The term "player_index" is the number assigned to a player on a specific controller. For XInput controllers this returns the XInput user index. Many joysticks will not be able to supply this information.
The term SDL_JoystickGUID is a stable 128-bit identifier for a joystick device that does not change over time, it identifies class of the device (a X360 wired controller for example). This identifier is platform dependent.