The setters receive a pdfout_data argument.
The getters return a pdfout_data object.
E.g.
void
pdfout_page_labels_set (fz_context *ctx, pdf_document *doc, pdfout_data *labels);
pdfout_data *
pdfout_page_labels_get (fz_context *ctx, pdf_document *doc);
Remove the data if the setter is given a NULL
argument. Return a zero-length pdfout_data object in the getter, if the data is not contained in the PDF.