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
Er 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_async4
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.
HOOKS
This node type supports the following hooks:
hook
tty(4)
serial data contained in
mbuf
structures, with arbitrary inter-frame boundaries.
CONTROL MESSAGES
This node type supports the generic control messages, plus the following:
NGM_TTY_SET_HOTCHAR
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.
NGM_TTY_GET_HOTCHAR
Returns an integer containing the current hot character in the lower
eight bits.
SHUTDOWN
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
Er EOPNOTSUPP .
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
node, the node has no way to convey this information to the
serial driver, and sub-optimal performance may result.
