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




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;
           # ....
         }


SEE ALSO

       Encode::Encoding(3), Encode::Supported(3), Encode::PerlIO(3),
       encoding(3), perlebcdic(1), "open" in perlfunc(1), perlunicode(1), 
       utf8(3), the Perl Unicode Mailing List <perl-unicode@perl.org>



perl v5.26.1                      2017-07-18               Encode::PerlIO(3pm)

perl 5.26.1 - Generated Sat Nov 4 14:28:56 CDT 2017
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