extern int (*gui_mouse_x)();
extern int (*gui_mouse_y)();
extern int (*gui_mouse_z)();
extern int (*gui_mouse_b)();
Hook functions, used by the GUI routines whenever they need to access the mouse state. By default these just return copies of the mouse_x, mouse_y, mouse_z, and mouse_b variables, but they could be used to offset or scale the mouse position, or read input from a different source entirely.
The man pages gui_mouse_b(3), gui_mouse_y(3) and gui_mouse_z(3) are aliases of gui_mouse_x(3).