LPRng Reference Manual: 24 Sep 2004 (For LPRng-3.8.28)
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# 16.9. LPRng Supported Filters

There already exists a large library of ready-to-use filters. Some of them have LPRng-specific versions, which can be found at the LPRng ftp mirror sites.

## 16.9.1. Filter Support Conventions

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.

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 vintage filters insist on having these files hidden with names such as `.setup`. This can make it difficult for administrators to determine where the configuration files are.

Global configuration files are usually placed in commonly accessible directories such as `/usr/local/libexec/filters` and its subdirectories. This allows the LPRng administrator to set the privileges on these directories such that only the lpd process can access them.

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.

An alternative to using information in a file is to place options in the printcap entry and have the filter extract them from the PRINTCAP_ENTRY environment variable value. This is much easier to implement, but is specific to LPRng.

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