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Xnest - a nested X server
Xnest is a client and a 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 graphics requests
on their behalf. To these clients Xnest appears to be a conventional server.
Xnest supports all standard options of the sample server implementation.
For more details, please see the manual page on your system for Xserver.
The following additional arguments are supported as well.
Starting up Xnest is as simple as starting up
xclock from a terminal emulator. If a user wishes to run Xnest on the same
workstation 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 running 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.
- -display string
- This option specifies the display name of the real server that Xnest should
try to connect with. If it is not provided on the command line Xnest will
read the DISPLAY environment variable in order to find out the same information.
- 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 the performance considerably. It should not be used unless absolutely
- 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 generation.
- This option specifies the default visual class of the nested server.
It is similar to the -cc option from the set of standard 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, StaticColor, PseudoColor, TrueColor, or DirectColor. If both,
-class and -cc options are specified, the last instance of either option
assumes 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;
although, it has to be supported by the real server. See xdpyinfo for 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 colormap
will be preallocated. If the user chooses a dynamic class, colors in the
default colormap will be available to individual clients for allocation.
- -depth int
- 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; although,
it has to be supported by the real server. See xdpyinfo for a list of supported
visual depths on the real server before starting Xnest.
- 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.
- -geometry W+H+X+Y
- This option
specifies geometry parameters for the top level Xnest windows. These windows
corresponds to the root windows of the nested server. The width and height
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. As of yet, there is no mechanism within the sample
server implementation to change the size of the root window after screen
initialization. In order to do so, one would probably need to extend the
X protocol. Therefore, it is not likely that this will be available any
time soon. If this option is not specified Xnest will choose width and
height to be 3/4 of the dimensions of the root window of the real server.
- -bw int
- This option specifies the border width of the top level Xnest window.
The integer parameter must be a positive number. The default border width
- -name string
- This option specifies the name of the top level Xnest
window. The default value is the program name.
- -scrns int
- 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 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
- This option tells Xnest to do its own colormap installation
by bypassing the real window manager. For it to work properly the user
will probably have to temporarily quit the real window manager. By default
Xnest will keep the nested client window whose colormap should be installed
in the real server in the WM_COLORMAP_WINDOWS property of the top level
Xnest window. If this colormap is of the same visual type as the root window
of the nested server, Xnest will associate this colormap 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_COLORMAP_WINDOWS property rather than the
colormap associated with the top level Xnest window. Unfortunately, window
managers are not very good at doing that yet so this option might come
- -parent window_id
- This option tells Xnest to use the window_id
as the root window instead of creating a window. This option is used by
the xrx xnestplugin.
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 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 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. Therefore,
it is desirable that Xnest and the real server are on a local network,
or even better, on the same machine. As of now, 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, and in that case the real server need not support it. The dynamic
shape extension selection support should be considered in further development
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 colormap.
This implies that other windows' colors will not be displayed properly
while the keyboard or pointer focus is in the Xnest window, unless the
real server has support for more than one installed colormap at any time.
The colormap associated with the top window of the Xnest client need not
be the appropriate colormap that the nested server wants installed in the
real server. In the case that a nested client attempts to install a colormap
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 colormap 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 colormap
associated with the top level Xnest window. Since most window managers
appear to not implement this convention properly as of yet, Xnest can optionally
do direct installation of colormaps 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 colormap 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 controls of the real server to its
own internal defaults. 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. This is a future consideration.
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. In the future, Xnest should read
a customization input file to provide even greater freedom and simplicity
in selecting the desired layout. Unfortunately, there is no support for
backing store and save under as of yet, but this should also be considered
in the future development of Xnest.
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 proper implementation
of fonts should be moved into the os layer. 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. Xlsfonts
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.
Won't run well on servers supporting
different visual depths. Still crashes randomly. Probably has some memory
Davor Matic, MIT X Consortium
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