func lookupUnixGid(gid int) (*Group, error) {
	var grp _C_struct_group
	var found bool

	err := retryWithBuffer(groupBuffer, func(buf []byte) syscall.Errno {
		var errno syscall.Errno
		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))