int (*init) (struct output *o);
int (*data) (struct output *o, char *data_in, uint64_t length_in,
char **data_out, uint64_t *length_out);
int (*init) (struct output *o);
int (*data) (struct output *o, char *data_in, uint64_t length_in,
char **data_out, uint64_t *length_out);