* indirect driver for controlling tty.
(*cdevsw
[major(u
.u_ttyd
)].d_open
)(u
.u_ttyd
, flag
);
(*cdevsw
[major(u
.u_ttyd
)].d_read
)(u
.u_ttyd
);
(*cdevsw
[major(u
.u_ttyd
)].d_write
)(u
.u_ttyd
);
syioctl(dev
, cmd
, addr
, flag
)
(*cdevsw
[major(u
.u_ttyd
)].d_ioctl
)(u
.u_ttyd
, cmd
, addr
, flag
);