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

Provided by: perl-doc_5.26.1-6ubuntu0.7_all

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 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 "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 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.

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 ->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 module 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;
           # ....
         }

NAME

Overview

How does it work?

Line Buffering

SEE ALSO