Xlib

/*   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> #include <stdbool.h>  int main(void) {   Display *d;   Window w;   XEvent e;   char *msg = "Hello, World!";   int s;    bool done = false;    /* 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, 640, 480, 0, 			  BlackPixel(d, s), WhitePixel(d, s));    /* register interest in the delete window message */   Atom wmDeleteMessage = XInternAtom(d, "WM_DELETE_WINDOW", False);   XSetWMProtocols(d, w, &wmDeleteMessage, 1);       /* select kind of events we are interested in */   XSelectInput(d, w, ExposureMask | KeyPressMask | StructureNotifyMask);    /* map (show) the window */   XMapWindow(d, w);    /* event loop */   while (!done) {     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 */     switch(e.type){            case KeyPress:       XDestroyWindow(d, w);       break;      case DestroyNotify:       done = true;       break;      case ClientMessage:       if (e.xclient.data.l[0] == wmDeleteMessage){ 	done = true;       }       break;           }   }    /* close connection to server */   XCloseDisplay(d);    return 0; }