sxt(M)                          19 June 1992                          sxt(M)


 Name

    sxt - pseudo-device driver

 Description

    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
    sxt driver.  sxt acts as a discipline manipulating a real tty structure
    declared by a real device driver.  The sxt driver is currently only used
    by the shl(C) command.

    Virtual ttys are named /dev/sxt?? or /dev/sxt/?? (where ?? is a combina-
    tion of two digits, each in the range 0 ..7 ) and are allocated in groups
    of up to eight.  Filenames end in three digits, where the first two
    digits represent the group and the last digit represents the virtual tty
    number of the group.  The /dev/sxt form of the name increases the size of
    /dev, which adversely affects some commands; the /dev/sxt/ form is not
    understood by most commands.  To allocate a group, a program should
    exclusively open a file with a name of the form /dev/sxt??0 (channel 0)
    or /dev/sxt/??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(S) commands supported by sxt.  The first
    group contains the standard ioctl commands described in termio(M), with
    the addition of the following:

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

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

    The second group are directives 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.

                      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.  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
    /dev/sxt/??[0-7]
    /usr/include/sys/sxt.h  driver specific definitions


 See also

    ioctl(S), open(S), shl(C), stty(C), termio(M)