There is a bug that calling XGetExtensionVersion() after XInitThreads() always   
causes a deadlock.   
   
I do not really use XGetExtensionVersion() myself, but I found a deadlock in  
the Qt library in the QApplication constructor, and this seems to be caused by   
XGetExtensionVersion() and happens only if XInitThreads() is called before.   
According to this forum posting (which is probably the same bug):   
http://www.qt-forum.org/thread.php?threadid=14988&sid=e99b9990f5fbbec62dc5e650d36b160f&hilight=XInitThreads   
this did not happen in XFree 4.3 - but I have not tried this myself, so I  
cannot confirm this. The bug can be reproduced with X.org 6.8.2 and X.org 6.9  
with the following test program:  
   
Simple test program test.c to reproduce this bug:     
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#include <X11/Xlib.h>      
#include <X11/extensions/XInput.h>      
      
int main(int argc, char * argv[]) {      
        XInitThreads();      
        Display * dpy = XOpenDisplay(NULL);      
        XGetExtensionVersion (dpy, "XInputExtension");      
        return 0;      
}      
     
---------------------------------------------     
     
gcc -I/usr/include/X11 -L/usr/X11R6/lib -lXs11 -lXi -lpthread test.c -o test