buffer -- a gtk:text-buffer object for which the format is registered
content -- a gtk:text-buffer object to serialize
start -- a gtk:text-iter start iterator of the block of text to serialize
end -- a gtk:text-iter end iterator of the block of text to serialize
result -- a newly allocated array of guint8 which contains the serialized data, or nil if an error occured

A function that is called to serialize the content of a text buffer. It must return the serialized form of the content.