scrolled window viewport area?

[Deborah Swayne <dfs research att com>]

How do I get the dimensions of the viewing area of
a scrolled window?  By that I mean the size of the
area that's currently visible, not the total possible
size.  The scrolled_window holds an event box, which
holds a table.

Is this what I need?  It doesn't seem to work the
way I thought it should, but I'm not sure.

    GTK_BIN (scrolled_window)->child->allocation.height,
    GTK_BIN (scrolled_window)->child->allocation.width

Debby