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Xnest - a nested X server
Xnest [ options ]
Xnest is both an X client and an X server. Xnest is a client of the
real server which manages windows and graphics requests on its behalf.
Xnest is a server to its own clients. Xnest manages windows and graph-
ics requests on their behalf. To these clients, Xnest appears to be a
Xnest supports all standard options of the sample server implementa-
tion. For more details, please see Xserver(1). The following addi-
tional arguments are supported as well.
This option specifies the display name of the real server that
Xnest should try to connect to. If it is not provided on the
command line, Xnest will read the DISPLAY environment variable
in order to find out this information.
-sync This option tells Xnest to synchronize its window and graphics
operations with the real server. This is a useful option for
debugging, but it will slow down Xnest's performance consider-
ably. It should not be used unless absolutely necessary.
-full This option tells Xnest to utilize full regeneration of real
server objects and reopen a new connection to the real server
each time the nested server regenerates. The sample server
implementation regenerates all objects in the server when the
last client of this server terminates. When this happens, Xnest
by default maintains the same top-level window and the same real
server connection in each new generation. If the user selects
full regeneration, even the top-level window and the connection
to the real server will be regenerated for each server genera-
This option specifies the default visual class of the nested
server. It is similar to the -cc option from the set of stan-
dard options except that it will accept a string rather than a
number for the visual class specification. The string must be
one of the following six values: StaticGray, GrayScale, Static-
Color, PseudoColor, TrueColor, or DirectColor. If both the
-class and -cc options are specified, the last instance of
either option takes precedence. The class of the default visual
of the nested server need not be the same as the class of the
default visual of the real server, but it must be supported by
the real server. Use xdpyinfo(1) to obtain a list of supported
visual classes on the real server before starting Xnest. If the
user chooses a static class, all the colors in the default color
map will be preallocated. If the user chooses a dynamic class,
colors in the default color map will be available to individual
clients for allocation.
This option specifies the default visual depth of the nested
server. The depth of the default visual of the nested server
need not be the same as the depth of the default visual of the
real server, but it must be supported by the real server. Use
xdpyinfo(1) to obtain a list of supported visual depths on the
real server before starting Xnest.
-sss This option tells Xnest to use the software screen saver. By
default, Xnest will use the screen saver that corresponds to the
hardware screen saver in the real server. Of course, even this
screen saver is software-generated since Xnest does not control
any actual hardware. However, it is treated as a hardware
screen saver within the sample server code.
This option specifies the geometry parameters for the top-level
Xnest window. See "GEOMETRY SPECIFICATIONS" in X(7) for a dis-
cussion of this option's syntax. This window corresponds to the
root window of the nested server. The width W and height H
specified with this option will be the maximum width and height
of each top-level Xnest window. Xnest will allow the user to
make any top-level window smaller, but it will not actually
change the size of the nested server root window. Xnest does
not yet support the RANDR extension for resizing, rotation, and
reflection of the root window. If this option is not specified,
Xnest will choose W and H to be 3/4ths the dimensions of the
root window of the real server.
This option specifies the border width of the top-level Xnest
window. The integer parameter int must be positive. The
default border width is 1.
This option specifies the name of the top-level Xnest window as
string. The default value is the program name.
This option specifies the number of screens to create in the
nested server. For each screen, Xnest will create a separate
top-level window. Each screen is referenced by the number after
the dot in the client display name specification. For example,
xterm -display :1.1 will open an xterm(1) client in the nested
server with the display number :1 on the second screen. The
number of screens is limited by the hard-coded constant in the
server sample code, which is usually 3.
This option tells Xnest to do its own color map installation by
bypassing the real window manager. For it to work properly, the
user will probably have to temporarily quit the real window man-
ager. By default, Xnest will keep the nested client window
whose color map should be installed in the real server in the
WM_COLORMAP_WINDOWS property of the top-level Xnest window. If
this color map is of the same visual type as the root window of
the nested server, Xnest will associate this color map with the
top-level Xnest window as well. Since this does not have to be
the case, window managers should look primarily at the WM_COL-
ORMAP_WINDOWS property rather than the color map associated with
the top-level Xnest window. Unfortunately, window managers are
not very good at doing that yet so this option might come in
This option tells Xnest to use window_id as the root window
instead of creating a window.
Starting up Xnest is just as simple as starting up xclock(1) from a
terminal emulator. If a user wishes to run Xnest on the same worksta-
tion as the real server, it is important that the nested server is
given its own listening socket address. Therefore, if there is a
server already running on the user's workstation, Xnest will have to be
started up with a new display number. Since there is usually no more
than one server running on a workstation, specifying `Xnest :1' on the
command line will be sufficient for most users. For each server run-
ning on the workstation, the display number needs to be incremented by
one. Thus, if you wish to start another Xnest, you will need to type
`Xnest :2' on the command line.
To run clients in the nested server, each client needs to be given the
same display number as the nested server. For example, `xterm -display
:1' will start up an xterm process in the first nested server and
`xterm -display :2' will start an xterm in the second nested server
from the example above. Additional clients can be started from these
xterms in each nested server.
Xnest as a client
Xnest behaves and looks to the real server and other real clients as
another real client. It is a rather demanding client, however, since
almost any window or graphics request from a nested client will result
in a window or graphics request from Xnest to the real server. There-
fore, it is desirable that Xnest and the real server are on a local
network, or even better, on the same machine. Xnest assumes that the
real server supports the SHAPE extension. There is no way to turn off
this assumption dynamically. Xnest can be compiled without the SHAPE
extension built in, in which case the real server need not support it.
Dynamic SHAPE extension selection support may be considered in further
development of Xnest.
Since Xnest need not use the same default visual as the the real
server, the top-level window of the Xnest client always has its own
color map. This implies that other windows' colors will not be dis-
played properly while the keyboard or pointer focus is in the Xnest
window, unless the real server has support for more than one installed
color map at any time. The color map associated with the top window of
the Xnest client need not be the appropriate color map that the nested
server wants installed in the real server. In the case that a nested
client attempts to install a color map of a different visual from the
default visual of the nested server, Xnest will put the top window of
this nested client and all other top windows of the nested clients that
use the same color map into the WM_COLORMAP_WINDOWS property of the
top-level Xnest window on the real server. Thus, it is important that
the real window manager that manages the Xnest top-level window looks
at the WM_COLORMAP_WINDOWS property rather than the color map associ-
ated with the top-level Xnest window. Since most window managers don't
yet appear to implement this convention properly, Xnest can optionally
do direct installation of color maps into the real server bypassing the
real window manager. If the user chooses this option, it is usually
necessary to temporarily disable the real window manager since it will
interfere with the Xnest scheme of color map installation.
Keyboard and pointer control procedures of the nested server change the
keyboard and pointer control parameters of the real server. Therefore,
after Xnest is started up, it will change the keyboard and pointer con-
trols of the real server to its own internal defaults.
Xnest as a server
Xnest as a server looks exactly like a real server to its own clients.
For the clients, there is no way of telling if they are running on a
real or a nested server.
As already mentioned, Xnest is a very user-friendly server when it
comes to customization. Xnest will pick up a number of command-line
arguments that can configure its default visual class and depth, number
of screens, etc.
The only apparent intricacy from the users' perspective about using
Xnest as a server is the selection of fonts. Xnest manages fonts by
loading them locally and then passing the font name to the real server
and asking it to load that font remotely. This approach avoids the
overload of sending the glyph bits across the network for every text
operation, although it is really a bug. The consequence of this
approach is that the user will have to worry about two different font
paths -- a local one for the nested server and a remote one for the
real server -- since Xnest does not propagate its font path to the real
server. The reason for this is because real and nested servers need
not run on the same file system which makes the two font paths mutually
incompatible. Thus, if there is a font in the local font path of the
nested server, there is no guarantee that this font exists in the
remote font path of the real server. The xlsfonts(1) client, if run on
the nested server, will list fonts in the local font path and, if run
on the real server, will list fonts in the remote font path. Before a
font can be successfully opened by the nested server, it has to exist
in local and remote font paths. It is the users' responsibility to
make sure that this is the case.
Make dynamic the requirement for the SHAPE extension in the real
server, rather than having to recompile Xnest to turn this requirement
on and off.
Perhaps there should be a command-line option to tell Xnest to inherit
the keyboard and pointer control parameters from the real server rather
than imposing its own.
Xnest should read a customization input file to provide even greater
freedom and simplicity in selecting the desired layout.
There is no support for backing store and save unders, but this should
also be considered.
The proper implementation of fonts should be moved into the os layer.
Doesn't run well on servers supporting different visual depths.
Still crashes randomly.
Probably has some memory leaks.
Davor Matic, MIT X Consortium
Xserver(1), xdpyinfo(1), X(7)
X Version 11 Xnest(1)