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ng_tty - netgraph node type that is also a line discipline
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
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:
hook tty(4) serial data contained in mbuf structures, with arbitrary
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
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 〈archie@FreeBSD.org〉
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.