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<h3 class="section">6.1 How External Modules Interface with Geomview</h3>
<p>External modules appear in the <em>Modules</em> browser in Geomview's
<em>Main</em> panel. To run a module, click the left mouse button on
the module's entry in the browser. While the module is running, an
additional line for that module will appear in red in the browser.
This line begins with a number in brackets, which indicates the
<em>instace</em> number of the module. (For some modules it makes sense
to have more than one instance of the module running at the same
time.) You can kill an external module by clicking on its red
instance entry.
<p>By default when Geomview starts, it displays all the modules that have
been installed on your system.
<p>For instructions on installing a module on your system so that it will
appear in the <em>Modules</em> browser every time Geomview is run by
anyone on your system, See <a href="Module-Installation.html#Module-Installation">Module Installation</a>.
<p>When Geomview invokes an external module, it creates pipes connected
to the module's standard input and output. (Pipes are like files
except they are used for communication between programs rather than
for storing things on a disk.) Geomview interprets anything that the
module writes to its standard output as a GCL command.
Likewise, if the external module requests any data from Geomview,
Geomview writes that data to the module's standard input. Thus all a
module has to do in order to communicate with Geomview is write
commands to standard output and (optionally) receive data on standard
input. Note that this means that the module cannot use standard input
and output for communicating with the user. If a module needs to
communicate with the user it can do so either through a control
panel of its own or else by responding to certain events that it finds
out about from Geomview.
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