Trying to use an xlib GXxor

Alan Corey alan01346 at
Mon Feb 6 01:18:41 UTC 2017

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,\
  XSelectInput(dpy,w,StructureNotifyMask);  // watch notifications
  gc = XCreateGC(dpy,w,0,NULL);
  XSetFunction(dpy,gc,GXxor);  // sets xor mode
  // 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
  XDrawLine(dpy, w, gc, 10, 60, 180, 20);  // draw a line
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");
  return 0;

No, I won't  call it "climate change", do you have a "reality problem"? - AB1JX
Impeach  Impeach  Impeach  Impeach  Impeach  Impeach  Impeach  Impeach

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