finding the position of a (just created) widget

[Julian Bradfield <jcb+gtk dcs ed ac uk>]

I want to show a widget, several layers down in a hierarchy
of containers, and then know immediately where it is, thus:
  gtk_widget_show(w); /* w is a previously unshown child of a box */
  /* here I want to know its position */
How can I do this?

The only thing I can think of so far is to iterate through the main
loop until I find some sensible values in the allocation record, but
that is unclean and requires me to disable all my other event handlers
that I don't want dispatched at that time.
Is there a clean way to get a synchronous resize calculation done?