SDL_GetRelativeMouseState man page
SDL_GetRelativeMouseState — Retrieve the current state of the mouse
Uint8 SDL_GetRelativeMouseState(int *x, int *y);
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.