SDL_GetMouseState man page
SDL_GetMouseState — Retrieve the current state of the mouse
Uint8 SDL_GetMouseState(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 current mouse cursor position. You can pass NULL for either x or y.
SDL_PumpEvents(); if(SDL_GetMouseState(NULL, NULL)&SDL_BUTTON(1)) printf("Mouse Button 1(left) is pressed. ");