MIME::Decoder(3User Contributed Perl DocumentatioMIME::Decoder(3)NAMEMIME::Decoder - an object for decoding the body part of a
MIME stream
WARNING: This code is in an evaluation phase until 1
August 1996. Depending on any comments/complaints
received before this cutoff date, the interface may change
in a non-backwards-compatible manner.
DESCRIPTION
This abstract class, and its private concrete subclasses
(see below) provide an OO front end to the action of
decoding a MIME-encoded stream.
The constructor for MIME::Decoder takes the name of an
encoding (base64, 7bit, etc.), and returns an instance of
a subclass of MIME::Decoder whose decode() method will
perform the appropriate decoding action.
You can even create your own subclasses and "install" them
so that MIME::Decoder will know about them, via the
install() method
Want to know if a given encoding is currently supported?
Use the supported() class method.
SYNOPSIS
Here's a simple filter program to read quoted-printable
data from STDIN and write the decoded data to STDOUT:
#!/usr/bin/perl
use MIME::Decoder;
$encoding = 'quoted-printable';
$decoder = new MIME::Decoder $encoding or die "$encoding unsupported";
$decoder->decode(\*STDIN, \*STDOUT);
The decode() method will always eat up all input to the
end of file.
PUBLIC INTERFACE
If all you are doing is using this class, here's all
you'll need...
new ENCODING
Class method. Create and return a new decoder object
which can handle the given ENCODING.
my $decoder = new MIME::Decoder "7bit";
Returns the undefined value if no known decoders are
appropriate.
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decode INSTREAM,OUTSTREAM
Decode the document waiting in the MIME input
filehandle INSTREAM, writing the decoded information
to the output filehandle OUTSTREAM.
supported [ENCODING]
Class method. With one arg (an ENCODING name),
returns truth if that encoding is currently handled,
and falsity otherwise. The ENCODING will be
automatically coerced to lowercase:
if (MIME::Decoder->supported('7BIT')) {
# yes, we can handle it...
}
else {
# drop back six and punt...
}
With no args, returns all the available decoders as a
hash reference... where the key is the encoding name
(all lowercase, like '7bit'), and the associated value
is true (it happens to be the name of the class that
handles the decoding, but you probably shouldn't rely
on that). Hence:
my $supported = MIME::Decoder->supported;
if ($supported->{7bit}) {
# yes, we can handle it...
}
elsif ($supported->{8bit}) {
# yes, we can handle it...
}
You may safely modify this hash; it will not change
the way the module performs its lookups. Only install
can do that.
Thanks to Achim Bohnet for suggesting this method.
SUBCLASS INTERFACE
If you are writing/installing a new decoder, here's some
other stuff you'll need:
install ENCODING
Class method. Install this class so that ENCODING is
handled by it.
PRIVATE BUILT-IN DECODERS
You don't need to "use" any other Perl modules; the
following are included as part of MIME::Decoder.
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MIME::Decoder::Base64
The built-in decoder for the "base64" encoding.
The name was chosen to jibe with the pre-existing
MIME::Base64 utility package, which this class actually
uses to translate each line.
MIME::Decoder::Binary
The built-in decoder for a "binary" encoding (in other
words, no encoding).
The "binary" decoder is a special case, since it's ill-
advised to read the input line-by-line: after all, an
uncompressed image file might conceivably have
loooooooooong stretches of bytes without a "\n" among
them, and we don't want to risk blowing out our core. So,
we read-and-write fixed-size chunks.
MIME::Decoder::QuotedPrint
The built-in decoder the for "quoted-printable" encoding.
The name was chosen to jibe with the pre-existing
MIME::QuotedPrint utility package, which this class
actually uses to translate each line.
MIME::Decoder::Xbit
The built-in decoder for both "7bit" and "8bit" encodings,
which guarantee short lines (a maximum of 1000 characters
per line) of US-ASCII data compatible with RFC-821.
This decoder does a line-by-line pass-through from input
to output, leaving the data unchanged except that an end-
of-line sequence of CRLF is converted to a newline "\n".
WRITING A DECODER
If you're experimenting with your own encodings, you'll
probably want to write a decoder. Here are the basics:
1. Create a module, like "MyDecoder::", for your decoder.
Declare it to be a subclass of MIME::Decoder.
2. Create the instance method MyDecoder::decode_it(), as
follows:
Your method should take as arguments the $self object
(natch), a filehandle opened for input, called $in,
and a filehandle opened for output, called $out.
Your method should read from the input filehandle,
decode this input, and print its decoded output to the
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$out filehandle. You may do this however you see fit,
so long as the end result is the same.
Your method must return either undef (to indicate
failure), or 1 (to indicate success).
3. In your application program, activate your decoder for
one or more encodings like this:
require MyDecoder;
install MyDecoder "7bit"; # use MyDecoder to decode "7bit"
install MyDecoder "x-foo"; # also, use MyDecoder to decode "x-foo"
To illustrate, here's a custom decoder class for the
base64 encoding:
package MyBase64Decoder;
@ISA = qw(MIME::Decoder);
use MIME::Decoder;
use MIME::Base64;
# decode_it - the private decoding method
sub decode_it {
my ($self, $in, $out) = @_;
while (<$in>) {
my $decoded = decode_base64($_);
print $out $decoded;
}
1;
}
That's it.
The task was pretty simple because the "base64" encoding
can easily and efficiently be parsed line-by-line... as
can "quoted-printable", and even "7bit" and "8bit" (since
all these encodings guarantee short lines, with a max of
1000 characters). The good news is: it is very likely
that it will be similarly-easy to write a MIME::Decoder
for any future standard encodings.
The "binary" decoder, however, really required block reads
and writes: see the section on MIME::Decoder::Binary for
details.
SEE ALSO
MIME::Decoder, MIME::Entity, MIME::Head, MIME::Parser.
AUTHOR
Copyright (c) 1996 by Eryq / eryq@rhine.gsfc.nasa.gov
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All rights reserved. This program is free software; you
can redistribute it and/or modify it under the same terms
as Perl itself.
VERSION
$Revision: 1.9 $ $Date: 1996/06/24 19:02:31 $
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