An application that can handle many types of data is very complex. As
a result, it is difficult to use, time-consuming to write and
impossible to extend.
What if one could write applications that do a single thing well,
and invoke external applications for everything else? Want to display
an image? Sure! Just tell the main application what plugin can handle
that image type and off you go. Want to display a table in your
word processor? Plug in a spreadsheet!
In order to integrate the two applications well, they need to know about
each other. There must be a protocol that allows them to communicate.
The protocol used by Siag Office is very simple and uses standard
Unix pipes for the communication. The plugin reads commands from
stdin and prints replies on stdout. All of this is of course completely
transparent to the user. Plugins can even be nested, so that a
plugin can itself have plugins.
All of the main applications in Siag Office can load and display
plugins, and are also able to themselves function as a plugin.
This is the way to put a table into a PW document: plug in Siag.
There are also a few additional plugins.
Before Siag can use a plugin, it must know that the plugin exists.
To do this, there is a file called
which contains the necessary Scheme code. Additional plugins can
be registered by adding them to plugin.scm. A user can add his own
plugins by registering them in the personal configuration files
for the respective applications ($HOME/.siag/siag.scm,
$HOME/.siag/pw.scm and $HOME/.siag/egon.scm),
The Image Plugin
One of the plugins that are supplied with Siag Office is the image
plugin. It displays most common image formats if the NETPBM
collection of graphics converters is installed. Otherwise it will
be able to display XPM images.
The plugin has no facilities for editing or otherwise manipulating
the images; for that you have to use another application.
The Clipart Plugin
The Clipart plugin displays vector graphics. It was originally
developed for images in Postscript (ps) or Encapsulated Postscript
(eps) format, but can also display LaTeX and DVI documents.
The Dummy Plugin
Of course, most existing applications have no understanding of the
communication protocol. The dummy application works as a "shim"
between Siag and another application. It speaks the plugin
protocol with Siag and does its best to manage the external
Due to the fact that the "kidnapped" applications do not speak
the communications protocol for plugins used by the Siag Office
suite, there may be no way to do things that would be expected
from normal plugins, such as loading or saving files, printing
or even terminating gracefully. Still most applications work
fine, including xterm, Emacs and Netscape, as well as simple
ones like xclock or xeyes.
Configuring the Dummy Plugin
The dummy plugin has its own configuration files. There is a
system-wide configuration file in
and a personal configuration file in $HOME/.siag/dummy.scm.
The Hello Plugin
This one is for demonstrational purposes only: it displays the
message "Hello, World" in its window. It is suitable as an
example of a simple plugin, in that it contains all the necessary
code to make a plugin of a normal X program.
Loading and Saving
There are two way to include a plugin in a document: linking
or importing. A plugin that is imported is saved as part of the
main document. A linked plugin stays in a separate file and can
be shared between several applications. As long as the document
stays on the same machine, it doesn't matter much which way it
is plugged in, but importing is better for documents that will
be copied to other locations.
The file format is based on tar, so it can easily be dissected
and contents inspected. Secret file formats are bad.
A plugin can be exported to a separate document. This requires that the
plugin has saving capabilities; that is not necessarily the case for
the dummy plugin.
Ulric Eriksson - January 2002 - email@example.com