#include <X11/Xlib.h>
#include <stdio.h>
#define TRUE 1
#define FALSE 0

main(argc,argv)
int argc;
char **argv;
{
  int i,j;
  Display *display;
  Window root, window;
  long fgcolor, bgcolor;
  int screen, pointx, pointy;
  int eventmask = KeyPressMask|ExposureMask|ButtonPressMask;
  XEvent event;
  XGCValues gcval;
  GC draw;
  int FirstPt = TRUE;
    if (!(display = XOpenDisplay(argv[1]))) {
      perror("XOpenDisplay");
      exit(1);
    }

    root = RootWindow(display,screen = DefaultScreen(display));
    fgcolor = BlackPixel(display,screen);
    bgcolor = WhitePixel(display,screen);

    window = XCreateSimpleWindow(display,root,0,0,500,500,2,
        fgcolor,bgcolor);

    gcval.foreground = fgcolor;
    gcval.background = bgcolor;
    draw = XCreateGC(display,window,GCForeground|GCBackground,&gcval);

    XSelectInput(display,window,eventmask);
    XMapWindow(display,window);

  for (;;) {
    XWindowEvent(display,window,eventmask,&event);
    switch (event.type) {
    case Expose:
      XClearWindow(display,window);
      break;
    case ButtonPress:
      if (FirstPt) {
        FirstPt=FALSE;
        pointx = event.xbutton.x;
        pointy = event.xbutton.y;
        XDrawPoint(display,window,draw,pointx,pointy);
        break;
      }
      else {
        FirstPt=TRUE;
        XDrawLine(display,window,draw,pointx,pointy,
              event.xbutton.x, event.xbutton.y);
        break;
      }
    case KeyPress:
        XDestroyWindow(display, window);
      exit(0);
    default:
      fprintf(stderr,"Unexpected event: %d\n",event.type);
    }
  }
}