(glib:define-gboxed-opaque selection-data "GtkSelectionData"
  :export t
  :type-initializer "gtk_selection_data_get_type"
  :alloc (error "GtkSelectionData cannot be created from the Lisp side."))  

The gtk:selection-data structure is used to store a chunk of data along with the data type and other associated information. The fields of the gtk:selection-data structure are private and can only be retrieved with the corresponding accessor functions.