how to make a widget background transparent?

["Chandra Shekhar Sengupta" <sayan801 gmail com>]

Hi,
I am writing a custom widget where i need the background of the widget to be
transparent. i have taken drawing area as base and drawing a transparent png
image from file there using cairo. i am setting gtk_widget_set_app_paintable
for drawing area  as TRUE and drawing the background using cairo. But the
background is coming black. Could you help me how to make widget backgound
transparent.

Thanks & Regards,
Chandra Shekhar Sengupta