Trying to use an xlib GXxor

[Alan Corey]

This probably isn't exactly the right list for this, but maybe
somebody lingering can answer.  I'm trying to use xor to put a line on
the screen then erase it again when I want.  With the XSetFunction()
to xor uncommented I get no line.  If I comment it out I get a line,
but no erase.  If I move it between the draws I get no erase.

xor (exclusive or) should set every bit where there's one and only one
of the inputs true.  If you xor something to the screen twice the 2nd
time erases what you wrote the first time.  Except I'm doing something
wrong or there's something I didn't consider.

/*
  My first recreational X activity. Based on Tronche's tutorial.
*/

#include <X11/Xlib.h> // Every Xlib program must include this
#include <assert.h>   // I include this to test return values the lazy way
#include <unistd.h>   // So we got the profile for 10 seconds
#include <stdio.h>
#include <stdlib.h> // for exit

Display *dpy;
int blackcolor,whitecolor;
Window w;
GC gc;
XEvent e; // a struct

#define WWIDTH 320
#define WHEIGHT 240

int main(void) {
  dpy = XOpenDisplay(NULL);
  assert(dpy);  // be sure we've got dpy
  blackcolor = BlackPixel(dpy, DefaultScreen(dpy));
  whitecolor = WhitePixel(dpy, DefaultScreen(dpy));
  w = XCreateSimpleWindow(dpy,DefaultRootWindow(dpy),0,0,WWIDTH,WHEIGHT,0,\
    blackcolor,whitecolor);
  XSelectInput(dpy,w,StructureNotifyMask);  // watch notifications
  XMapWindow(dpy,w);
  gc = XCreateGC(dpy,w,0,NULL);
  XSetFunction(dpy,gc,GXxor);  // sets xor mode
  XSetForeground(dpy,gc,blackcolor);
  // MapNotify wait loop, wait for a MapNotify event before drawing
  // confirms that XMapWindow worked.
  for (;;) {
    XNextEvent(dpy,&e);  // fetch an event
    if (e.type == MapNotify)  // done when we find one
      break;
  }
  XDrawLine(dpy, w, gc, 10, 60, 180, 20);  // draw a line
  XFlush(dpy);
printf("after draw\n");  // to stdout
  getchar();  // wait for keypress
  XDrawLine(dpy, w, gc, 10, 60, 180, 20);    // should erase it
  XFlush(dpy);  // sends request to server
printf("After erase\n");
  getchar();
  return 0;
}

-- 
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