System Administration Guide: Printing

Writing a Print Filter Program

The LP print service provides filter programs in the /usr/lib/lp/postscript directory. These filters cover most PostScript printing situations, where the destination printer requires the data to be in PostScript format. A print filter program must be a binary executable file.

Types of Filters

There are two types of print filters: fast filters and slow filters.

Fast filters do not require much processing time to prepare a file for printing. They must have access to the printer when they run. To be capable of detecting printer faults, a print filter must be a fast filter. Any filter that uses the PRINTER keyword as a filter option must be installed as a fast filter.

Slow filters require a great deal of processing time to prepare a file for printing. They do not require access to the printer when they run. Slow filters are run in the background so they do not tie up the printer, allowing other files that do not need slow filtering to be printed.

Converting Files

The LP print service uses print filters to convert files from one content type to another content type. You can specify the accepted file content types for each printer. The user specifies the file content type when submitting a print request. Then, the LP print service finds a printer that can print files of that content type. Because many applications can generate files for various printers, this method is often sufficient. However, some applications can generate files that cannot be printed on any available printers.

Each time the LP print service receives a request to print a type of file that is in a format that cannot be accepted directly by a printer, the LP print service tries to match the content type of the print request with the content type of the available (or specified) printer. If there is a match, the file can be sent directly to the printer without filtering. If no match is found, or if the content type specifies that a filter be used, the LP print service tries to match the content type of the file with the input content type of available filters. In addition, the LP print service tries to match the output type of the filter with the content type of the printer. When an appropriate filter is found, the print request is passed through the filter.

Handling Special Printing Modes

A print filter handles special printing modes and requests to print specific pages. A special printing mode is needed to print any characteristics of print requests that require a customized filter.

Filters handle the following characteristics:

The LP print service provides default settings for these characteristics. However, a print filter can handle some characteristics more efficiently. For example, some printers can handle multiple copies more efficiently than the LP print service. So, in this case, you can provide a filter for multiple-copy page control.

Detecting Printer Faults

Each printer has its own way of detecting printer faults and transmitting fault signals to the LP print service. The LP print service only checks for hang-ups (loss of carrier) and excessive delays in printing.

Some printers provide good fault coverage and can send a message that describes the reason for a fault. Other printers indicate a fault by using signals other than the signals for loss of carrier signal or shut off of data flow. A filter is required to interpret this additional printer fault information.

A filter can also put a print request on hold, wait for a printer fault to clear, and then resume printing. With this capability, the print request that was interrupted does not need to be reprinted in its entirety. Only a filter that knows the control sequences used by a printer can determine where to break a file into pages. Consequently, only such a filter can find the place in the file where printing should start after a fault is cleared.

When a print filter generates messages, those messages are handled by the LP print service. If alerts are enabled, the LP print service sends alerts to the system administrator. For further information, see Setting Up Printer Fault Alerts by Using LP Print Commands.

Requirements for a Print Filter Program

A print filter can be simple or complex, but it has to meet the following requirements:

If you want the filter to detect printer faults, it should also meet the following requirements: