Sharing X Applications With XMX
A Quick Tutorial
John Bazik
jsb@cs.brown.edu
The Virtual Root Window
X client applications draw on a root window, which is just the
background upon which all other windows appear. XMX creates a virtual
root window, which is a root-window-in-a-window. The virtual root
window encapsulates an entire X session.
Run xmx
% xmx
Assuming you are in an X session and your DISPLAY environment variable
is set correctly, you should see a virtual root window appear that is a
third the size of your screen and has the familiar "root weave" pattern
on the background. There are no X client applications running in the
session, so there is nothing you can do. Kill it and try this
% xmxinit
Again, you should see the root weave virtual root window, but this time
you will see a lone xterm in the upper left corner. Experienced X
users will recognize that this is exactly what xinit does when a user
lacks a .xinitrc file. xinit is used to start an X session. xmxinit
is used to start an XMX session. A .xinitrc file is a per-user shell
script that runs all the X client applications the user wants to see
when starting an X session. Similarly, a .xmxinitrc file runs all the
X client applications a user wants to see when starting up an XMX
session.
The virtual X session is completely separate and self-contained. From
the xterm in the virtual root window, run a window manager. Try
running some of your favorite applications. Create a .xmxinitrc file.
Use your .xinitrc file as a model, but avoid doing things that don't
make sense in a nested X session, like running xmodmap or redirecting
console output (by running xconsole or xterm -C). To exit the XMX
session, type "exit" in the xterm. Rerun xmxinit to try out your new
session.
Multiple Displays
A nested X server is a neat trick, but XMX is really about application
sharing. Create a shared X session
% xmxinit -- -floor $DISPLAY
Two identical virtual root windows appear on your screen. Notice that
as your pointer tracks through one, a telepointer echos the movement in
the other. Anything you do in one appears in both. XMX provides a
WYSIWIS (What You See Is What I See) shared environment.
Log onto a different host and start an X session (or ask a friend who
is logged in to help). On the remote host, issue an xhost command
remote% xhost +myhost
Now, on myhost, run a shared session
myhost% xmxinit -- -floor remote:0
Again, two virtual root windows appear, but this time there is one on
each of the two machines involved. Both users have control over the
virtual X session.
Controlling It
From within a shared X session, run
% xmtg
xmtg is an XMC (X Multiplexor Control) client application. It allows
you to control XMX. The people icons represent participants in the
session. Here you see two virtual root windows on a machine named "no"
(xmtg added -a to disambiguate the second one).
Select an icon and click on the Drop button. The virtual root window
disappears. Click on Add and enter the display string for the
just-dropped display, and it reappears. Try adding a new display. Try
clicking on Telepointer. Telepointers are optional. The buttons named
Floor, Seat and View allow you to control which displays may provide
keyboard and mouse input to shared applications.
Floor, Seat, View
Xmx provides three input modes for participants. These input modes
dictate what happens when a user types or moves the mouse in a virtual
root window. They are named for the roles one might have at a
relatively formal meeting. The only mode we have encountered so far is
Floor. When you have the Floor, your keyboard and mouse inputs control
the shared applications in the virtual X session. Having the floor is
like running a meeting, you're in control and everyone is watching what
you do.
View mode is the exact opposite of floor mode. When a display is in
view mode, the user may only watch passively what goes on in the
session. All mouse and keyboard input is ignored. Think of view mode
as being in the spectator's gallery.
Seat mode is inbetween floor and view modes. When a display is in seat
mode, the user's mouse and keyboard input is ignored by the X clients
in the shared session, just like in view mode. But an XMC client, like
xmtg, can choose to see input events from displays in seat mode. Think
of seat mode as being a member of some organization that is having a
meeting - it gives you the right to be recognized and participate,
though you are not running things at the moment.
Try running a session with multiple displays (even if they are all on
one machine). Try changing the input modes of the virtual root
windows. First select an icon in xmtg, then click on either the Floor,
Seat or View button to change it's mode. The icon changes to reflect
the input mode: displays in floor mode have a gavel and a hat, displays
in seat mode have only a hat, displays in view mode are bareheaded.
Xmtg insists that you have at least one display in floor mode -
otherwise you can do nothing!
When you have a display (a virtual root window) in seat mode, try
clicking on the xmtg window there. Xmtg will see the mouse click and
raise the hand of the corresponding seat-mode icon. All the other X
clients ignore your inputs. In this way, an XMC client can coordinate
giving the floor to, or sharing the floor among participants in the
shared session.
DISPLAY Magic
X clients need to know what X server to talk to. They determine this
from either the -display option provided on the command line, or from
the DISPLAY environment variable. The display string has several
components.
All running X servers can be located by a display string. So, too, can
a running XMX. By convention, X servers usually default to display
zero, as shown above. If there is more than one X server on a
particular machine, it must have a different display number, so XMX,
when it is actiing like an X server, defaults to display number one to
avoid conflicting with the local X server.
Note that X clients use a display string to find XMX, but XMX also uses
display strings to find the X servers to which it connects.
% xmx -display :0 &
% xterm -display :1
In this example, XMX connects to the X server that is running on the
local host at display number zero (since no screen number is given, it
is assumed to be zero). Then an xterm connects to XMX, thinking it is
an X server, and is displayed within XMX's virtual root window. These
two commands are exactly what xmxinit did when it found no .xmxinitrc
file.
How Does XMX Work?
X applications or clients, like xterm, do not draw on your screen or
watch your keyboard and mouse. They talk to an X server which does
that for them. X clients talk to an X server over a data pipeline
called a socket. A single X server does all the drawing for all the
applications displayed on your machine, and it watches your keyboard
and mouse for all the events that any of those applications are
interested in.
An X client does not need to be running on the same machine as the X
server to which it is talking. You can run X clients on faraway
machines, and display them on your desktop, and vice versa. And though
few clients do, an X client can connect to, and draw on, and receive
events from, as many displays as it likes. These are all features of
X. X is a network-transparent windowing sytem.
XMX is an X client. But instead of connecting to only one X server, it
connects to many X servers, drawing exactly the same thing on each of
them. XMX also pretends to be an X server, so that other X clients can
talk to it. In this way, it allows many people to share the same
client applications, without those clients knowing that there are many
people.
Because XMX is interposed between X clients and X servers, it cannot
jump into an existing client-server session and begin sharing it. XMX
can only share X sessions it was involved in from the start.
When sharing an X session like this, it is helpful to have participants
(X servers) join and leave the session and to control which
participant(s) can interact with the X clients while the others watch.
This control is provided by a separate XMC (X Multiplexor Control)
connection.
Because the XMC connection is completely separate from the X
connection, an XMC client does not have to be displayed in the shared
session. It can be displayed in a different, non-shared X session, or
it can have no graphical user interface at all. In fact, there is a
unix command-line XMC application called xmc, which is included in the
xmx distribution.
Like the X Window System itself, XMX does not set any policy. The way
a shared session works, the way that input modes are assigned, and
users join or leave, is entirely up to the XMC clients that control
it. Xmtg is merely a sample.
This document: http://www.cs.brown.edu/software/xmx/tutorial/
PDF version:
http://downloads.openchannelfoundation.org/XMX/tutorial.pdf
October 7, 1999
