* indirect driver for controlling tty.
(*cdevsw
[tp
->t_dev
.d_major
].d_open
)(tp
->t_dev
, flag
);
(*cdevsw
[tp
->t_dev
.d_major
].d_read
)(tp
->t_dev
);
(*cdevsw
[tp
->t_dev
.d_major
].d_write
)(tp
->t_dev
);
(*cdevsw
[tp
->t_dev
.d_major
].d_sgtty
)(tp
->t_dev
, flag
);