Given two(2) separate files that will be compiled and linked together.

In file 1 several windows are opened in the main().  All the windows are defined globally. Example 

Fl_Window *win;

Fl_Window *w1;

Fl_Window *w2;

I define and show() main window "win" and also define and show() "w1" in the main() program in File 1.  I define but hide() "w2" in File 1.

In File 2 I declare the following globally

extern Fl_Window *win;

extern Fl_Window *w1;

extern Fl_Window *w2;. 

Now in File 2 I wish to hide() "w1"[w1,hide()] and show() "w2"[w2.show()]

After a successful compile and link it gives a segmentation fault(core dumped) response?

Can anyone comment on how to globally pass defined windows from file to file?