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<h1>12. Web Application Development</h1>
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<h2>12.10. Perl Hosting</h2>
<p>Virtuoso functionality can be enhanced through external libraries by
loading shared objects or DLLs. The new functions are written in a language of the developer's
choice and compiled to produce a shared library appropriate to the operating
system. The path to the shared library must be declared in the Virtuoso INI
file and the server restarted before it can be used.</p>
<p>The Perl language is hosted within Virtuoso in this way. hosting_perl.so
is the library used.</p>
<p>The Virtuoso INI file uses a [Plugins] configuration section for listing
shared libraries for the server to load upon startup. An example of this is:</p>
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<pre class="programlisting">
[Plugins]
LoadPath = /home/virtuoso/hosting
Load1 = Hosting, hosting_perl.so
..
</pre>
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<p>The "Hosting" type defines the entry points for initialization of the runtime
hosting environment, destruction of the user environment and execution of a
file or string containing commands in the hosted language. It also returns a
list of file extensions that it is capable of processing. Virtuoso
dynamically defines memory-resident (no disk image) HTTP server handlers for
each specified type. </p>
<p>The Perl hosting plugin supports 'pl' extension. Hence, upon
initialization of the hosting plugin, Virtuoso defines the
<span class="computeroutput">__http_handler_pl(..)</span> function according to the API for
file type handlers in the Virtuoso HTTP server. With this handler in place,
each hit on a .pl file (file system or WebDAV) with appropriate execute
permissions will cause the HTTP server to execute the code within it and return
the result instead of simply the file's contents.</p>
<p>The handler will call the __hosting_http_handler VSE with a special set
of parameters to represent the HTTP environment correctly.</p>
<p>Virtuoso will normally call the plugin to memory as required, and expel
it when finished. This behavior can be controlled by the INI file parameter:</p>
<div>
<pre class="programlisting">
[HTTPServer]
PersistentHostingModules = 1/0 default 0
</pre>
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<p>Setting <span class="computeroutput">PersistentHostingModules</span> to "1"
prevents Virtuoso from removing the interpreters from the HTTP threads after
each request.</p>
<div class="tip">
<div class="tiptitle">See Also:</div>
<p>
<a href="vseplugins.html">VSEI Plugins</a>
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<a name="ex_hostingsharedobjects" />
<div class="example">
<div class="exampletitle">Using the Perl Plugin</div>
<p>Executing Perl code directly:</p>
<div>
<pre class="programlisting">
select __hosting_http_handler ('pl', 'print "hello world"; ', '', vector (), 'helloworld.pl');
</pre>
</div>
<div>
<pre class="programlisting">
returns : hello world
</pre>
</div>
<p>Executing a perl script file (perl/test_print.pl in the Virtuoso
working directory):</p>
<div>
<pre class="programlisting">
/perl/test_print.pl
-------------------
#!/usr/bin/perl
print "hello world file";
</pre>
</div>
<div>
<pre class="programlisting">
select __hosting_http_handler ('pl', 'perl/test_print.pl');
</pre>
</div>
<div>
<pre class="programlisting">
returns : hello world file
</pre>
</div>
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<div class="note">
<div class="notetitle">Note:</div>
<p>The hosting_perl hosting module uses the perl <span class="computeroutput">tie()</span>
function to "tie" up the STDIN, STDOUT, STDERR, exit() and %ENV perl objects.
Untying any of these may lead to unpredictable results.</p>
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