glutGetWindow man page
glutGetWindow — Return the current window identifier, 0 if undefined
OpenGLUT - window
glutGetWindow() returns the
. This is useful, e.g., if you have a generic function that is used with several windows and it needs to temporarily change to another window. (There is no window stack for you to use with pushes and pops. Do not be confused by glutPushWindow() and glutPopWindow(); those pushes and pops are
One cause for the function to return 0 is if you have called glutDestroyWindow() on the
and have done nothing to set a new window as current.
Explore man page connections for glutGetWindow(3).