NG_TTY(4) MidnightBSD Kernel Interfaces Manual NG_TTY(4)


ng_tty — netgraph node type that is also a line discipline


#include <sys/types.h>
#include <sys/ttycom.h>
#include <netgraph/ng_tty.h>


The tty node type is both a netgraph node type and a line discipline. A new node is created when the corresponding line discipline, NETGRAPHDISC, is registered on a tty device (see tty(4)).

The node has a single hook called hook. Incoming bytes received on the tty device are sent out on this hook, and frames received on hook are transmitted out on the tty device. No modification to the data is performed in either direction. While the line discipline is installed on a tty, the normal read and write operations are unavailable, returning EIO.

The node supports an optional ‘‘hot character’’. If set to non-zero, incoming data from the tty device is queued until this character is seen. This avoids sending lots of mbufs containing a small number of bytes, but introduces potentially infinite latency. The default hot character is 0x7e, consistent with hook being connected to a ng_async(4) type node. The hot character has no effect on the transmission of data.

The node will attempt to give itself the same netgraph name as the name of the tty device. In any case, information about the node is available via the netgraph ioctl(2) command NGIOCGINFO. This command returns a struct nodeinfo similar to the NGM_NODEINFO netgraph control message.


This node type supports the following hooks:


tty(4) serial data contained in mbuf structures, with arbitrary inter-frame boundaries.


This node type supports the generic control messages, plus the following:


This command takes an integer argument and sets the hot character from the lower 8 bits. A hot character of zero disables queueing, so that all received data is forwarded immediately.


Returns an integer containing the current hot character in the lower eight bits.


This node shuts down when the corresponding device is closed (or the line discipline is uninstalled on the device). The NGM_SHUTDOWN control message is not valid, and always returns the error EOPNOTSUPP.


ioctl(2), netgraph(4), ng_async(4), tty(4), ngctl(8)


The ng_tty node type was implemented in FreeBSD 4.0.


Archie Cobbs 〈〉


The serial driver code also has a notion of a ‘‘hot character’’. Unfortunately, this value is statically defined in terms of the line discipline and cannot be changed. Therefore, if a hot character other than 0x7e (the default) is set for the ng_tty node, the node has no way to convey this information to the serial driver, and sub-optimal performance may result.

MidnightBSD 0.3 January 19, 1999 MidnightBSD 0.3