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.\" ========================================================================
.\"
.IX Title "POE::Filter::Block 3pm"
.TH POE::Filter::Block 3pm "2014-10-01" "perl v5.20.1" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH "NAME"
POE::Filter::Block \- translate data between streams and blocks
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  #!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], "\en";
\&  }
\&
\&  # Print one line: yz123456
\&
\&  $filter\->get_one_start([ "123456" ]);
\&  while (1) {
\&    my $block = $filter\->get_one();
\&    last unless @$block;
\&    print $block\->[0], "\en";
\&  }
.Ve
.SH "DESCRIPTION"
.IX Header "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.
.PP
Fixed-length blocks are used when Block's constructor is called with a
BlockSize value.  Otherwise the Block filter uses length-prepended
blocks.
.PP
Users who specify block sizes less than one deserve what they get.
.PP
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:
.PP
.Vb 1
\&  LengthCodec => [ \e&encoder, \e&decoder ]
.Ve
.PP
The encoder takes a reference to a buffer and prepends the buffer's
length to it.  The default encoder prepends the \s-1ASCII\s0 representation
of the buffer's length and a \fIchr\fR\|(0) byte to separate the length from
the actual data:
.PP
.Vb 5
\&  sub _default_encoder {
\&    my $stuff = shift;
\&    substr($$stuff, 0, 0) = length($$stuff) . "\e0";
\&    return;
\&  }
.Ve
.PP
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.
.PP
.Vb 9
\&  sub _default_decoder {
\&    my $stuff = shift;
\&    unless ($$stuff =~ s/^(\ed+)\e0//s) {
\&      warn length($1), " strange bytes removed from stream"
\&        if $$stuff =~ s/^(\eD+)//s;
\&      return;
\&    }
\&    return $1;
\&  }
.Ve
.PP
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 \s-1MB \s0(512*1024*1024 octets).  You
may change this size limit with the \f(CW\*(C`MaxBuffer\*(C'\fR parameter.
.PP
.Vb 1
\&    MaxBuffer => 1099511627776  # One terabyte!
.Ve
.PP
The size of each individual block is also limited.  By default, each block
may be no more then 64 \s-1MB. \s0 You may change this size limit with the
\&\f(CW\*(C`MaxLength\*(C'\fR parameter.
.PP
.Vb 1
\&    MaxLength => 10             # small blocks
.Ve
.PP
Remember that MaxBuffer needs to be larger then MaxLength.  What's more, it
needs to have room for the length prefix.
.PP
If either the \f(CW\*(C`MaxLength\*(C'\fR or \f(CW\*(C`MaxBuffer\*(C'\fR constraint is exceeded,
\&\f(CW\*(C`POE::Filter::Bock\*(C'\fR will throw an exception.
.SH "PUBLIC FILTER METHODS"
.IX Header "PUBLIC FILTER METHODS"
POE::Filter::Block has no additional public methods.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
Please see POE::Filter for documentation regarding the base
interface.
.PP
The \s-1SEE ALSO\s0 section in \s-1POE\s0 contains a table of contents covering
the entire \s-1POE\s0 distribution.
.SH "BUGS"
.IX Header "BUGS"
The \fIput()\fR method doesn't verify block sizes.
.SH "AUTHORS & COPYRIGHTS"
.IX Header "AUTHORS & COPYRIGHTS"
The Block filter was contributed by Dieter Pearcey, with changes by
Rocco Caputo.
.PP
Please see \s-1POE\s0 for more information about authors and contributors.