sxt(7)                           UNIX System V                           sxt(7)


NAME
      sxt - pseudo-device driver

DESCRIPTION
      The special file /dev/sxt is a pseudo-device driver that interposes a
      discipline between the standard tty line disciplines and a real device
      driver.  The standard disciplines manipulate virtual tty structures
      (channels) declared by the /dev/sxt driver.  /dev/sxt acts as a
      discipline manipulating a real tty structure declared by a real device
      driver.  The /dev/sxt driver is currently only used by the shl(1)
      command.

      Virtual ttys are named by inodes in the subdirectory /dev/sxt and are
      allocated in groups of up to eight.  To allocate a group, a program
      should exclusively open a file with a name of the form /dev/sxt/??0
      (channel 0) and then execute a SXTIOCLINK ioctl call to initiate the
      multiplexing.

      Only one channel, the controlling channel, can receive input from the
      keyboard at a time; others attempting to read will be blocked.

      There are two groups of ioctl(2) commands supported by sxt.  The first
      group contains the standard ioctl commands described in termio(7), with
      the addition of the following:

      TIOCEXCL            Set exclusive use mode: no further opens are
                          permitted until the file has been closed.

      TIOCNXCL            Reset exclusive use mode: further opens are once
                          again permitted.

      The second group are commands to sxt itself.  Some of these may only be
      executed on channel 0.

      SXTIOCLINK          Allocate a channel group and multiplex the virtual
                          ttys onto the real tty.  The argument is the number
                          of channels to allocate.  This command may only be
                          executed on channel 0.  Possible errors include:

                          EINVAL    The argument is out of range.

                          ENOTTY    The command was not issued from a real tty.

                          ENXIO     linesw is not configured with sxt.

                          EBUSY     An SXTIOCLINK command has already been
                                    issued for this real tty.

                          ENOMEM    There is no system memory available for
                                    allocating the virtual tty structures.




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sxt(7)                           UNIX System V                           sxt(7)


                          EBADF     Channel 0 was not opened before this call.

      SXTIOCSWTCH         Set the controlling channel.  Possible errors
                          include:

                          EINVAL    An invalid channel number was given.

                          EPERM     The command was not executed from channel
                                    0.

      SXTIOCWF            Cause a channel to wait until it is the controlling
                          channel.  This command will return the error, EINVAL,
                          if an invalid channel number is given.

      SXTIOCUBLK          Turn off the loblk control flag in the virtual tty of
                          the indicated channel.  The error EINVAL will be
                          returned if an invalid number or channel 0 is given.

      SXTIOCSTAT          Get the status (blocked on input or output) of each
                          channel and store in the sxtblock structure
                          referenced by the argument.  The error EFAULT will be
                          returned if the structure cannot be written.

      SXTIOCTRACE         Enable tracing.  Tracing information is written to
                          the console on the 3B2 Computer.  This command has no
                          effect if tracing is not configured.

      SXTIOCNOTRACE       Disable tracing.  This command has no effect if
                          tracing is not configured.

FILES
      /dev/sxt/??[0-7]    Virtual tty devices

SEE ALSO
      shl(1), stty(1) ioctl(2), open(2), termio(7)



















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