creating an image or a in memory buffer from a cairo context

hi,
i have a cairo context created from a gdk window (GDKDrawable). i
would like to get this in a buffer or store it to a png. i could not
create an api to create a cairo surface from a cairo context based on
gdkdrawable.
iam new to cairo and hence may be missng something basic.
Any inputs would be greatly helpful

thanks,
Zaheer
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