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;

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:

Rq.Fill

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.

Rq.Result

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.

Rq.Finished

The filter has finished processing. Buf contains any data that was not consumed by the filter. The filter terminates after sending this message.

Rq.Info

This message is used to send a string of arbitrary information from the filter during the course of its processing.

Rq.Error

The filter has encountered an error when processing. E is a string describing the error. The filter terminates after sending this message.