SDL_GetRelativeMouseState (3) - Linux Manuals
SDL_GetRelativeMouseState: Retrieve the current state of the mouse
NAMESDL_GetRelativeMouseState - Retrieve the current state of the mouse
The current button state is returned as a button bitmask, which can be tested using the SDL_BUTTON(X) macros, and x and y are set to the change in the mouse position since the last call to SDL_GetRelativeMouseState or since event initialization. You can pass NULL for either x or y.