gzseek
gzseek(file, 1L, SEEK_CUR); /* add one zero byte */
pos = gzseek(file, -8L, SEEK_CUR);
ZEXTERN z_off_t ZEXPORT gzseek(gzFile, z_off_t, int);
ZEXTERN z_off_t ZEXPORT gzseek(gzFile, z_off_t, int);