When Should You Call cairo_surface_flush?

Does cairo_surface_flush
<http://cairographics.org/manual/cairo-cairo-surface-t.html#cairo-surface-flush>
need to be called before making non-Cairo accesses to an image surface?
The docs say “This function must be called before switching from
drawing on the surface with cairo to drawing on it directly with native
APIs” -- of course “drawing ... with native APIs” has to mean “any kind
of memory access”, so my guess is “yes”.

Currently in my Qahirah examples
<https://github.com/ldo/qahirah_examples>, I am doing a flush before
writing the images to PNG files. I would have thought Cairo would do
this automatically, but I can find no sign of that in the source code
(src/cairo-png.c).

Thanks for any clarification.
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