NAME

     ng_tty — netgraph node type that is also a TTY hook

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 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.

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.

     NGM_TTY_SET_TTY
          This command takes integer process ID and file descriptor of open tty and registers the tty hooks.

SHUTDOWN

     This node shuts down when the corresponding device is closed.

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>
     Andrew Thompson <thompsa@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.