##The following program creates a window with a little black square in it.

     /*
       Simple Xlib application drawing a box in a window.
       gcc input.c -o output -lX11
     */
     
     #include <X11/Xlib.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>
     
     int main(void) {
       Display *d;
       Window w;
       XEvent e;
       char *msg = "Hello, World!";
       int s;
     
                            /* open connection with the server */
       d = XOpenDisplay(NULL);
       if (d == NULL) {
         fprintf(stderr, "Cannot open display\n");
         exit(1);
       }
     
       s = DefaultScreen(d);
     
                            /* create window */
       w = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 200, 200, 1,
                               BlackPixel(d, s), WhitePixel(d, s));
     
                            /* select kind of events we are interested in */
       XSelectInput(d, w, ExposureMask | KeyPressMask);
     
                            /* map (show) the window */
       XMapWindow(d, w);
     
                            /* event loop */
       while (1) {
         XNextEvent(d, &e);
                            /* draw or redraw the window */
         if (e.type == Expose) {
           XFillRectangle(d, w, DefaultGC(d, s), 20, 20, 10, 10);
           XDrawString(d, w, DefaultGC(d, s), 50, 50, msg, strlen(msg));
         }
                            /* exit on key press */
         if (e.type == KeyPress)
           break;
       }
     
                            /* close connection to server */
       XCloseDisplay(d);
     
       return 0;
     }