md_get_int64
error = md_get_int64(mdp, &v);
error = md_get_int64(mdp, &v);
int md_get_int64(struct mdchain *mdp, int64_t *x);
md_get_int64(mbp, NULL); /* creation time */
md_get_int64(mbp, NULL); /* real size (should use) */