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ng_tty — netgraph node type that is also a TTY hook
The tty node type is both a netgraph node type and a TTY hook.
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 hook is installed on a tty, the
normal read and write operations are unavailable, returning EIO.
Incoming data is delivered directly to ng_tty via the tty bypass hook as
a buffer pointer and length, this is converted to a mbuf and passed to
The node supports an optional “hot character”. If the driver can not
deliver data directly to the tty bypass hook then each character is input
one at a time. If set to non-zero and bypass mode is unavailable,
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.
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 command takes integer process ID and file descriptor of open
tty and registers the tty hooks.
This node shuts down when the corresponding device is closed.
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⟩
Andrew Thompson ⟨thompsa@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.