FILTER(2) FILTER(2)
NAME
filter - data processing interface
SYNOPSIS
include "filter.m";
filter = load Filter FilterModulePath;
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;
DESCRIPTION
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. For details of the filter modules that exist, see
filters(2).
Init must be called when a filter module has been loaded, to
initialise any internal data structures. 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. Other 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
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FILTER(2) FILTER(2)
the data must be acknowledged by sending a value
on reply. If the value is -1, the filter will ter-
minate.
Rq.Finished
The filter has finished processing. Buf contains
the data that remains unconsumed by the filter, if
any. The filter terminates after sending this mes-
sage.
Rq.Info This message is used to communicate a string of
arbitrary information from the filter during the
course of its processing.
Rq.Error The filter has encountered an error when process-
ing. E is a string describing the error. The fil-
ter terminates after sending this message.
SOURCE
/module/filter.m
SEE ALSO
filters(2), gzip(1)
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