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<th colspan="3" align="center">LPRng Reference Manual: 24
Sep 2004 (For LPRng-3.8.28)</th>
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<h1 class="SECT1"><a name="AEN8434">16.9. <b class=
"APPLICATION">LPRng</b> Supported Filters</a></h1>
<p>There already exists a large library of ready-to-use
filters. Some of them have <b class=
"APPLICATION">LPRng</b>-specific versions, which can be found
at the <a href="secftp.htm"><b class="APPLICATION">LPRng</b>
ftp mirror sites</a>.</p>
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<h2 class="SECT2"><a name="AEN8441">16.9.1. Filter Support
Conventions</a></h2>
<p>By convention, most filters are either totally
standalone (very rare) or require a set of support files or
configuration files. There are two types of configuration
files: per print queue configuration information and global
configuration information.</p>
<p>Since a print filter executes with the spool queue
directory as the current directory, most filters put per
print queue configuration information in a file in the
spool directory. Some <span class="emphasis"><i class=
"EMPHASIS">vintage</i></span> filters insist on having
these files <span class="emphasis"><i class=
"EMPHASIS">hidden</i></span> with names such as <span
class="emphasis"><i class="EMPHASIS"><tt class=
"FILENAME">.setup</tt></i></span>. This can make it
difficult for administrators to determine where the
configuration files are.</p>
<p>Global configuration files are usually placed in
commonly accessible directories such as <span class=
"emphasis"><i class="EMPHASIS"><tt class=
"FILENAME">/usr/local/libexec/filters</tt></i></span> and
its subdirectories. This allows the <b class=
"APPLICATION">LPRng</b> administrator to set the privileges
on these directories such that only the <b class=
"APPLICATION">lpd</b> process can access them.</p>
<p>When a filter is invoked, it is passed a large number of
options, many of which are totally ignored in filter
operation. However, for many purposes it is necessary to
provide options to the filters to tailor their operation to
the particular spool queue needs.</p>
<p>An alternative to using information in a file is to
place options in the printcap entry and have the filter
extract them from the <var class=
"ENVAR">PRINTCAP_ENTRY</var> environment variable value.
This is much easier to implement, but is specific to <b
class="APPLICATION">LPRng</b>.</p>
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