NAME

POE::Filter::Block - translate data between streams and blocks

SYNOPSIS

  #!perl

  use warnings;
  use strict;
  use POE::Filter::Block;

  my $filter = POE::Filter::Block->new( BlockSize => 8 );

  # Prints three lines: abcdefgh, ijklmnop, qrstuvwx.
  # Bytes "y" and "z" remain in the buffer and await completion of the
  # next 8-byte block.

  $filter->get_one_start([ "abcdefghijklmnopqrstuvwxyz" ]);
  while (1) {
    my $block = $filter->get_one();
    last unless @$block;
    print $block->[0], "\n";
  }

  # Print one line: yz123456

  $filter->get_one_start([ "123456" ]);
  while (1) {
    my $block = $filter->get_one();
    last unless @$block;
    print $block->[0], "\n";
  }

DESCRIPTION

POE::Filter::Block translates data between serial streams and blocks. It can handle fixed-length and length-prepended blocks, and it may be extended to handle other block types.

Fixed-length blocks are used when Block's constructor is called with a BlockSize value. Otherwise the Block filter uses length-prepended blocks.

Users who specify block sizes less than one deserve what they get.

In variable-length mode, a LengthCodec parameter may be specified. The LengthCodec value should be a reference to a list of two functions: the length encoder, and the length decoder:

  LengthCodec => [ \&encoder, \&decoder ]

The encoder takes a reference to a buffer and prepends the buffer's length to it. The default encoder prepends the ASCII representation of the buffer's length and a chr(0) byte to separate the length from the actual data:

  sub _default_encoder {
    my $stuff = shift;
    substr($$stuff, 0, 0) = length($$stuff) . "\0";
    return;
  }

The corresponding decoder returns the block length after removing it and the separator from the buffer. It returns nothing if no length can be determined.

  sub _default_decoder {
    my $stuff = shift;
    unless ($$stuff =~ s/^(\d+)\0//s) {
      warn length($1), " strange bytes removed from stream"
        if $$stuff =~ s/^(\D+)//s;
      return;
    }
    return $1;
  }

This filter holds onto incomplete blocks until they are completed in a framing buffer. To control memory usage, a maximum framing buffer size is imposed. This maximum size defaults to 512 MB (512*1024*1024 octets). You may change this size limit with the "MaxBuffer" parameter.

    MaxBuffer => 1099511627776  # One terabyte!

The size of each individual block is also limited. By default, each block may be no more then 64 MB. You may change this size limit with the "MaxLength" parameter.

    MaxLength => 10             # small blocks

Remember that MaxBuffer needs to be larger then MaxLength. What's more, it needs to have room for the length prefix.

If either the "MaxLength" or "MaxBuffer" constraint is exceeded, "POE::Filter::Bock" will throw an exception.

PUBLIC FILTER METHODS

POE::Filter::Block has no additional public methods.

BUGS

The put() method doesn't verify block sizes.

AUTHORS & COPYRIGHTS

The Block filter was contributed by Dieter Pearcey, with changes by Rocco Caputo.