How to get the first line number of the GtkTextview

[searun <searun gmail com>]

Hi all:
    In my application I want to get the first line number of the
GtkTextview in the visible area, but I can't find any solution to it.
Now I can only use the build-in GtkTextMark "insert" to get the line
number.
     Any one can help me? Thank you.