contol the position of Gtk_Window related the X Display

[WAN Hing Wah <50191914 ee cityu edu hk>]

How can I control the position of the Gtk_Window?
I wanna do that coz in my application I have 3 gtk_window
created with gtk_window_new(GTK_WINDOW_TOP_LEVEL),and they simply overlap
each other when I run my program...I wanna align them....
I try gtk_window_reposition() but seem nothing happen.. 
and gtk_window_set_position() don't allow me to set a x,y position..