grp, found, errno = _C_getgrgid_r(_C_gid_t(gid),
			(*_C_char)(unsafe.Pointer(&buf[0])), _C_size_t(len(buf)))
		return syscall.Errno(errno)
	})
	if err == syscall.ENOENT || (err == nil && !found) {
		return nil, UnknownGroupIdError(strconv.Itoa(gid))
	}
	if err != nil {
		return nil, fmt.Errorf("user: lookup groupid %d: %v", gid, err)
	}
	return buildGroup(&grp), nil
}

func buildGroup(grp *_C_struct_group) *Group {