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NAME

     ng_tty - netgraph node type that is also a line discipline

SYNOPSIS

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

DESCRIPTION

     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.

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 EOPNOTSUPP.

SEE ALSO

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

HISTORY

     The ng_tty node type was implemented in FreeBSD 4.0.

AUTHORS

     Archie Cobbs 〈archie@FreeBSD.org

BUGS

     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.