XOpenDisplay != X11Info::display()

[hasanen]

Hi
I was trying to do some core X11 operations from Qt , but nothing works until I replaced XOpenDisplay(NULL) with X11Info::display() , then all operations worked fine.
I printed both pointers received from functions and they are not the same at all.

display=NULL;
// display=XOpenDisplay(NULL);   <<-- NOT WORKING !!
 display=QX11Info::display(); 
 printf("display=%d\n",display);
 if (!display) { qDebug("unable to open local display"); return;}
 window=RootWindow(display,0);

display=NULL;
// display=XOpenDisplay(NULL);   <<-- NOT WORKING !!
 display=QX11Info::display(); 
 printf("display=%d\n",display);
 if (!display) { qDebug("unable to open local display"); return;}
 window=RootWindow(display,0);

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How can I use XOpenDisplay under QT?

thanx.