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Encode::PerlIO(3)      Perl Programmers Reference Guide      Encode::PerlIO(3)



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

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 "Encode" provides a "layer" (see Per-
       lIO) 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 "use
       open ..." pragma. See open.

       Once a handle is open, its layers can be altered using "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 bytes and will "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. "lc", "/\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 encoding for how to change the default encoding of the data in your script.

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 char-
       acter.

          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 charac-
       ter).

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

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 char-
       acter.

                                 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 encod-
       ings 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 ->needs_lines method for your encoding object.  See
       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 Encode::MIME::Header for details.

       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 mod-
       ule has a "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;
           # ....
         }

SEE ALSO
       Encode::Encoding, Encode::Supported, Encode::PerlIO, encoding, perlebcdic, "open" in perlfunc, perlunicode,
       utf8, the Perl Unicode Mailing List <perl-unicodeATperl.org>



perl v5.8.8                       2001-09-21                 Encode::PerlIO(3)