The filters supplied with 4.3BSD handle printing and accounting for most common line printers, the Benson-Varian, the wide (36") and narrow (11") Versatec printer/plotters. For other devices or accounting methods, it may be necessary to create a new filter.

Filters are spawned by lpd with their standard input the data to be printed, and standard output the printer. The standard error is attached to the lf file for logging errors or syslogd may be used for logging errors. A filter must return a 0 exit code if there were no errors, 1 if the job should be reprinted, and 2 if the job should be thrown away. When lprm sends a kill signal to the lpd process controlling printing, it sends a SIGINT signal to all filters and descendents of filters. This signal can be trapped by filters that need to do cleanup operations such as deleting temporary files.

Arguments passed to a filter depend on its type. The of filter is called with the following arguments. filter -wwidth -llength The width and length values come from the pw and pl entries in the printcap database. The if filter is passed the following parameters. filter [\|-c\|] -wwidth -llength -iindent -n login -h host accounting_file The -c flag is optional, and only supplied when control characters are to be passed uninterpreted to the printer (when using the -l option of lpr to print the file). The -w and -l parameters are the same as for the of filter. The -n and -h parameters specify the login name and host name of the job owner. The last argument is the name of the accounting file from printcap .

All other filters are called with the following arguments: filter -xwidth -ylength -n login -h host accounting_file The -x and -y options specify the horizontal and vertical page size in pixels (from the px and py entries in the printcap file). The rest of the arguments are the same as for the if filter.