Provided by: freebsd-manpages_12.2-1_all bug


     ng_tty — netgraph node type that is also a TTY hook


     #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 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 peer.

     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 inter-frame


     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 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 <>
     Andrew Thompson <>


     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.