ref: 3efb5bbb4061056e523858b134c555949591efe2
dir: /man/2/filter/
.TH FILTER 2
.SH NAME
filter \- data processing interface
.SH SYNOPSIS
.B
include "filter.m";
.br
.BI "filter := load Filter " filterpath ";"
.EX
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;
.EE
.SH DESCRIPTION
.B Filter
defines a general module interface for byte-stream processing.
This manual page documents how to use the interface, and
by implication how a
.B 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
.BR Filter ,
loaded from the Dis file
.IR 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
.IR filter-deflate (2)
and following manual pages.
.PP
.B Init
must be called before any other operation of a filter module.
.PP
.B Start
sets the filter going;
.I param
can be used to pass any filter-specific information
to the processor.
.B 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
.BR Rq.Start ;
.I pid
is the process id of the new process spawned.
.PP
Subsequent messages are:
.TF Rq.Finished
.PD
.TP
.B Rq.Fill
A request by the filter to fill
.I buf
with data.
The number of bytes that have actually
been placed in the buffer should be sent
on
.IR 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.
.TP
.B Rq.Result
.I Buf
contains data from the filter.
Receipt of the the data must be acknowledged
by sending a value on
.IR reply .
If the value is \-1, the filter will terminate.
.TP
.B Rq.Finished
The filter has finished processing.
.I Buf
contains any data that was not consumed
by the filter. The filter terminates after
sending this message.
.TP
.B Rq.Info
This message is used to send a string of
arbitrary information from the filter
during the course of its processing.
.TP
.B Rq.Error
The filter has encountered an error when processing.
.I E
is a string describing the error. The filter terminates
after sending this message.
.SH SOURCE
.B /module/filter.m
.SH SEE ALSO
.IR gzip (1),
.IR filter-deflate (2),
.IR filter-slip (2)