FILTER(2) System Calls Manual FILTER(2)

filter - data processing interface

include "filter.m";
filter := load Filter filterpath;

Rq: adt {
	pick {
	Start =>
		pid: int;
	Fill or Result =>
		buf: array of byte;
		reply: chan of int;
	Finished =>
		buf: array of byte;
	Info =>
		msg: string;
	Error =>
		e: string;
	}
};
init:  fn();
start: fn(param: string): chan of ref Rq;

Filter defines a general module interface for byte-stream processing. This manual page documents how to use the interface, and by implication how a Filter module should behave. There is a different implementation module for each filter type and algorithm (eg, for compression or line encoding). All implementations are instances of type Filter, loaded from the Dis file filterpath, given in the manual page for each standard filter (or you can write your own to match this specification). For details of each existing filter module, see filter-deflate (2) and following manual pages.

Init must be called before any other operation of a filter module.

Start sets the filter going; param can be used to pass any filter-specific information to the processor. Start spawns a new thread to do the processing; it returns a channel that is used to receive requests from the filter. The first message sent is always Rq.Start; pid is the process id of the new process spawned.

Subsequent messages are:

A request by the filter to fill buf with data. The number of bytes that have actually been placed in the buffer should be sent on reply. If -1 is sent, the filter will terminate. If the value is 0, the filter will terminate once it has processed all its input.
Buf contains data from the filter. Receipt of the the data must be acknowledged by sending a value on reply. If the value is -1, the filter will terminate.
The filter has finished processing. Buf contains any data that was not consumed by the filter. The filter terminates after sending this message.
This message is used to send a string of arbitrary information from the filter during the course of its processing.
The filter has encountered an error when processing. E is a string describing the error. The filter terminates after sending this message.

/module/filter.m

gzip (1), filter-deflate (2), filter-slip (2)