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


main(argc,argv)
int argc;
char **argv;
{
    Display *display;
    Window root, window;
    long fgcolor, bgcolor;
    int screen, pointx, pointy;
    int  eventmask = KeyPressMask|ExposureMask|ButtonPressMask;
    int curx, cury;
    XEvent event;
    XGCValues gcval;
    GC draw, erase;
    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,0,0,200,200,2,fgcolor,bgcolor);

    gcval.foreground = fgcolor;
    gcval.background = bgcolor;
/* use XOR instead of erase */
    gcval.function = GXxor;
    draw = XCreateGC(display,window,GCForeground|GCBackground|GCFunction,&gcval);

/*
    gcval.foreground = bgcolor;
    gcval.background = fgcolor;
    erase= 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;
                curx = pointx = event.xbutton.x;
                cury = pointy = event.xbutton.y;
                XDrawPoint(display,window,draw,pointx,pointy);
                XSelectInput(display,window,eventmask |= PointerMotionMask);
            }
            else {
                FirstPt=TRUE;
		XDrawLine(display,window,draw,pointx,pointy,curx,cury);
		XDrawLine(display,window,draw,pointx,pointy,event.xbutton.x,
			  event.xbutton.y);
		XSelectInput(display,window,eventmask &= ~PointerMotionMask);
	    }
	    break;
	  case MotionNotify:
	    XDrawLine(display,window,draw,pointx,pointy,curx,cury);
	    XDrawLine(display,window,draw,pointx,pointy,curx = event.xmotion.x,
		      cury = event.xmotion.y);
	    break;
          case KeyPress:
            exit(0);
	  default:
	    fprintf(stderr,"Unexpected event: %d\n",event.type);
	}
    }
}