Mini Shell

=head1 NAME

Encode::PerlIO -- a detailed document on Encode and PerlIO

=head1 Overview

It is very common to want to do encoding transformations when
reading or writing files, network connections, pipes etc.
If Perl is configured to use the new 'perlio' IO system then
C<Encode> provides a "layer" (see L<PerlIO>) which can transform
data as it is read or written.

Here is how the blind poet would modernise the encoding:

    use Encode;
    open(my $iliad,'<:encoding(iso-8859-7)','iliad.greek');
    open(my $utf8,'>:utf8','iliad.utf8');
    my @epic = <$iliad>;
    print $utf8 @epic;
    close($utf8);
    close($illiad);

In addition, the new IO system can also be configured to read/write
UTF-8 encoded characters (as noted above, this is efficient):

    open(my $fh,'>:utf8','anything');
    print $fh "Any \x{0021} string \N{SMILEY FACE}\n";

Either of the above forms of "layer" specifications can be made the default
for a lexical scope with the C<use open ...> pragma. See L<open>.

Once a handle is open, its layers can be altered using C<binmode>.

Without any such configuration, or if Perl itself is built using the
system's own IO, then write operations assume that the file handle
accepts only I<bytes> and will C<die> if a character larger than 255 is
written to the handle. When reading, each octet from the handle becomes
a byte-in-a-character. Note that this default is the same behaviour
as bytes-only languages (including Perl before v5.6) would have,
and is sufficient to handle native 8-bit encodings e.g. iso-8859-1,
EBCDIC etc. and any legacy mechanisms for handling other encodings
and binary data.

In other cases, it is the program's responsibility to transform
characters into bytes using the API above before doing writes, and to
transform the bytes read from a handle into characters before doing
"character operations" (e.g. C<lc>, C</\W+/>, ...).

You can also use PerlIO to convert larger amounts of data you don't
want to bring into memory.  For example, to convert between ISO-8859-1
(Latin 1) and UTF-8 (or UTF-EBCDIC in EBCDIC machines):

    open(F, "<:encoding(iso-8859-1)", "data.txt") or die $!;
    open(G, ">:utf8",                 "data.utf") or die $!;
    while (<F>) { print G }

    # Could also do "print G <F>" but that would pull
    # the whole file into memory just to write it out again.

More examples:

    open(my $f, "<:encoding(cp1252)")
    open(my $g, ">:encoding(iso-8859-2)")
    open(my $h, ">:encoding(latin9)")       # iso-8859-15

See also L<encoding> for how to change the default encoding of the
data in your script.

=head1 How does it work?

Here is a crude diagram of how filehandle, PerlIO, and Encode
interact.

  filehandle <-> PerlIO        PerlIO <-> scalar (read/printed)
                       \      /
                        Encode   

When PerlIO receives data from either direction, it fills a buffer
(currently with 1024 bytes) and passes the buffer to Encode.
Encode tries to convert the valid part and passes it back to PerlIO,
leaving invalid parts (usually a partial character) in the buffer.
PerlIO then appends more data to the buffer, calls Encode again,
and so on until the data stream ends.

To do so, PerlIO always calls (de|en)code methods with CHECK set to 1.
This ensures that the method stops at the right place when it
encounters partial character.  The following is what happens when
PerlIO and Encode tries to encode (from utf8) more than 1024 bytes
and the buffer boundary happens to be in the middle of a character.

   A   B   C   ....   ~     \x{3000}    ....
  41  42  43   ....  7E   e3   80   80  ....
  <- buffer --------------->
  << encoded >>>>>>>>>>
                       <- next buffer ------

Encode converts from the beginning to \x7E, leaving \xe3 in the buffer
because it is invalid (partial character).

Unfortunately, this scheme does not work well with escape-based
encodings such as ISO-2022-JP.

=head1 Line Buffering

Now let's see what happens when you try to decode from ISO-2022-JP and
the buffer ends in the middle of a character.

              JIS208-ESC   \x{5f3e}
   A   B   C   ....   ~   \e   $   B  |DAN | ....
  41  42  43   ....  7E   1b  24  41  43  46 ....
  <- buffer --------------------------->
  << encoded >>>>>>>>>>>>>>>>>>>>>>>

As you see, the next buffer begins with \x43.  But \x43 is 'C' in
ASCII, which is wrong in this case because we are now in JISX 0208
area so it has to convert \x43\x46, not \x43.  Unlike utf8 and EUC,
in escape-based encodings you can't tell if a given octet is a whole
character or just part of it.

Fortunately PerlIO also supports line buffer if you tell PerlIO to use
one instead of fixed buffer.  Since ISO-2022-JP is guaranteed to revert to ASCII at the end of the line, partial
character will never happen when line buffer is used.

To tell PerlIO to use line buffer, implement -E<gt>needs_lines method
for your encoding object.  See  L<Encode::Encoding> for details.

Thanks to these efforts most encodings that come with Encode support
PerlIO but that still leaves following encodings.

  iso-2022-kr
  MIME-B
  MIME-Header
  MIME-Q

Fortunately iso-2022-kr is hardly used (according to Jungshik) and
MIME-* are very unlikely to be fed to PerlIO because they are for mail
headers.  See L<Encode::MIME::Header> for details.

=head2 How can I tell whether my encoding fully supports PerlIO ?

As of this writing, any encoding whose class belongs to Encode::XS and
Encode::Unicode works.  The Encode module has a C<perlio_ok> method
which you can use before applying PerlIO encoding to the filehandle.
Here is an example:

  my $use_perlio = perlio_ok($enc);
  my $layer = $use_perlio ? "<:raw" : "<:encoding($enc)";
  open my $fh, $layer, $file or die "$file : $!";
  while(<$fh>){
    $_ = decode($enc, $_) unless $use_perlio;
    # .... 
  }

=head1 SEE ALSO

L<Encode::Encoding>,
L<Encode::Supported>,
L<Encode::PerlIO>, 
L<encoding>,
L<perlebcdic>, 
L<perlfunc/open>, 
L<perlunicode>, 
L<utf8>, 
the Perl Unicode Mailing List E<lt>perl-unicode@perl.orgE<gt>

=cut