Constant ZXIO_SOCKET_MARK_UID 

pub const ZXIO_SOCKET_MARK_UID: u8 = ZXIO_SOCKET_MARK_DOMAIN_2 /* 2u8 */;

Fuchsia does not have uids, we use the ZXIO_SOCKET_MARK_DOMAIN_2 on the socket to store the UID for the sockets created by starnix.