Use this function to get the number of trackballs on a joystick.
int SDL_JoystickNumBalls(SDL_Joystick* joystick)
an SDL_Joystick structure containing joystick information
Returns the number of trackballs on success or a negative error code on failure; call SDL_GetError() for more information.
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Joystick trackballs have only relative motion events associated with them and their state cannot be polled.