MirOS Manual: Encode::PerlIO(3p)


ext::Encode::lib:PerloProgrammeexrts::REencode::lib::Encode::PerlIO(3p)

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 "bin-
     mode".

     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 char-
     acter larger than 255 is written to the handle. When read-
     ing, 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+/", ...).

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ext::Encode::lib:PerloProgrammeexrts::REencode::lib::Encode::PerlIO(3p)

     You can also use PerlIO to convert larger amounts of data
     you don't want to bring into memory.  For example, to con-
     vert 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).

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ext::Encode::lib:PerloProgrammeexrts::REencode::lib::Encode::PerlIO(3p)

     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 charac-
     ter.

                               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 Per-
     lIO encoding to the filehandle. Here is an example:

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ext::Encode::lib:PerloProgrammeexrts::REencode::lib::Encode::PerlIO(3p)

       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, encod-
     ing, perlebcdic, "open" in perlfunc, perlunicode, utf8, the
     Perl Unicode Mailing List <perl-unicode@perl.org>

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