x.lua

local ffi = require 'ffi'
ffi.cdef[[
typedef struct _XDisplay Display;
typedef struct Screen Screen;
extern Display *XOpenDisplay(
    const char*	/* display_name */
);
extern int XDefaultScreen(
    Display*		/* display */
);
extern int XDisplayWidth(
    Display*		/* display */,
    int			/* screen_number */
);
extern int XDisplayHeight(
    Display*		/* display */,
    int			/* screen_number */
);
]]
local X = ffi.load('X11')

local dpy = X.XOpenDisplay(nil)
local n = X.XDefaultScreen(dpy)
local width = X.XDisplayWidth(dpy, 0)
local height = X.XDisplayHeight(dpy, 0)
print(string.format("%d x %d", width, height))