Xlib，如何让应用程序直接在根窗口上绘图？

Display *display = XOpenDisplay(NULL);
if (display == NULL) {
    fprintf(stderr, "无法连接到X服务器\n");
    exit(1);
}

Window root = DefaultRootWindow(display);

GC gc = XCreateGC(display, root, 0, NULL);

XSetForeground(display, gc, BlackPixel(display, DefaultScreen(display)));

XDrawLine(display, root, gc, x1, y1, x2, y2);

XFlush(display);

XCloseDisplay(display);