[cairo] How to export image_surface pixel data?

[Steve Chaplin]

I know you can import/export an entire image in png format using
  cairo_surface_write_to_png() and 
  cairo_image_surface_create_from_png(). 
But what if you wish to work directly with pixel data in memory instead
of reading/writing to files?

To import pixel data into cairo you can use
  cairo_image_surface_create_for_data()

There does not seem to be a corresponding function to allow the export
of pixel data from cairo. I'd expect there to be something like
  "unsigned char *data = cairo_image_surface_get_data()"

I know you could create the surface yourself with
  cairo_image_surface_create_for_data() rather than
  cairo_image_surface_create() 
and keep the 'data' pointer around to use later. But what if the surface
was created by another library and the 'data' pointer is not available?

Steve

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