problem to get screen height

Hello again. 

I started a C script using Xlib, and I cannot get the screen
height using DisplayHeight or XDisplayHeight, like this:
display_height = DisplayHeight(display, screen_num);

The C script comes from a tutorial I found at the following url: 
http://users.actcom.co.il/~choo/lupg/tutorials/xlib-programming/simple-window.c

I compiled it with following command
cc simple-window.c -o simple-window -L/usr/X11R6/lib -lX11

Return value is "nothing", event not 0, while screen width is
set to 1400. 

I tried this on a dell inspiron laptop, with nvidia graphic
driver, on gnome and redhat9. 

Could someone inform me where the problem comes from ??


Cheers, Xavier.