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Riley Schneider
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database/perl/lib/Encode/PerlIO.pod
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database/perl/lib/Encode/PerlIO.pod
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=head1 NAME
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Encode::PerlIO -- a detailed document on Encode and PerlIO
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=head1 Overview
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It is very common to want to do encoding transformations when
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reading or writing files, network connections, pipes etc.
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If Perl is configured to use the new 'perlio' IO system then
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C<Encode> provides a "layer" (see L<PerlIO>) which can transform
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data as it is read or written.
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Here is how the blind poet would modernise the encoding:
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use Encode;
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open(my $iliad,'<:encoding(iso-8859-7)','iliad.greek');
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open(my $utf8,'>:utf8','iliad.utf8');
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my @epic = <$iliad>;
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print $utf8 @epic;
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close($utf8);
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close($illiad);
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In addition, the new IO system can also be configured to read/write
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UTF-8 encoded characters (as noted above, this is efficient):
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open(my $fh,'>:utf8','anything');
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print $fh "Any \x{0021} string \N{SMILEY FACE}\n";
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Either of the above forms of "layer" specifications can be made the default
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for a lexical scope with the C<use open ...> pragma. See L<open>.
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Once a handle is open, its layers can be altered using C<binmode>.
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Without any such configuration, or if Perl itself is built using the
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system's own IO, then write operations assume that the file handle
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accepts only I<bytes> and will C<die> if a character larger than 255 is
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written to the handle. When reading, each octet from the handle becomes
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a byte-in-a-character. Note that this default is the same behaviour
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as bytes-only languages (including Perl before v5.6) would have,
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and is sufficient to handle native 8-bit encodings e.g. iso-8859-1,
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EBCDIC etc. and any legacy mechanisms for handling other encodings
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and binary data.
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In other cases, it is the program's responsibility to transform
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characters into bytes using the API above before doing writes, and to
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transform the bytes read from a handle into characters before doing
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"character operations" (e.g. C<lc>, C</\W+/>, ...).
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You can also use PerlIO to convert larger amounts of data you don't
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want to bring into memory. For example, to convert between ISO-8859-1
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(Latin 1) and UTF-8 (or UTF-EBCDIC in EBCDIC machines):
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open(F, "<:encoding(iso-8859-1)", "data.txt") or die $!;
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open(G, ">:utf8", "data.utf") or die $!;
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while (<F>) { print G }
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# Could also do "print G <F>" but that would pull
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# the whole file into memory just to write it out again.
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More examples:
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open(my $f, "<:encoding(cp1252)")
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open(my $g, ">:encoding(iso-8859-2)")
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open(my $h, ">:encoding(latin9)") # iso-8859-15
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See also L<encoding> for how to change the default encoding of the
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data in your script.
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=head1 How does it work?
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Here is a crude diagram of how filehandle, PerlIO, and Encode
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interact.
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filehandle <-> PerlIO PerlIO <-> scalar (read/printed)
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\ /
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Encode
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When PerlIO receives data from either direction, it fills a buffer
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(currently with 1024 bytes) and passes the buffer to Encode.
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Encode tries to convert the valid part and passes it back to PerlIO,
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leaving invalid parts (usually a partial character) in the buffer.
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PerlIO then appends more data to the buffer, calls Encode again,
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and so on until the data stream ends.
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To do so, PerlIO always calls (de|en)code methods with CHECK set to 1.
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This ensures that the method stops at the right place when it
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encounters partial character. The following is what happens when
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PerlIO and Encode tries to encode (from utf8) more than 1024 bytes
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and the buffer boundary happens to be in the middle of a character.
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A B C .... ~ \x{3000} ....
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41 42 43 .... 7E e3 80 80 ....
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<- buffer --------------->
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<< encoded >>>>>>>>>>
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<- next buffer ------
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Encode converts from the beginning to \x7E, leaving \xe3 in the buffer
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because it is invalid (partial character).
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Unfortunately, this scheme does not work well with escape-based
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encodings such as ISO-2022-JP.
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=head1 Line Buffering
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Now let's see what happens when you try to decode from ISO-2022-JP and
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the buffer ends in the middle of a character.
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JIS208-ESC \x{5f3e}
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A B C .... ~ \e $ B |DAN | ....
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41 42 43 .... 7E 1b 24 41 43 46 ....
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<- buffer --------------------------->
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<< encoded >>>>>>>>>>>>>>>>>>>>>>>
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As you see, the next buffer begins with \x43. But \x43 is 'C' in
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ASCII, which is wrong in this case because we are now in JISX 0208
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area so it has to convert \x43\x46, not \x43. Unlike utf8 and EUC,
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in escape-based encodings you can't tell if a given octet is a whole
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character or just part of it.
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Fortunately PerlIO also supports line buffer if you tell PerlIO to use
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one instead of fixed buffer. Since ISO-2022-JP is guaranteed to revert to ASCII at the end of the line, partial
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character will never happen when line buffer is used.
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To tell PerlIO to use line buffer, implement -E<gt>needs_lines method
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for your encoding object. See L<Encode::Encoding> for details.
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Thanks to these efforts most encodings that come with Encode support
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PerlIO but that still leaves following encodings.
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iso-2022-kr
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MIME-B
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MIME-Header
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MIME-Q
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Fortunately iso-2022-kr is hardly used (according to Jungshik) and
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MIME-* are very unlikely to be fed to PerlIO because they are for mail
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headers. See L<Encode::MIME::Header> for details.
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=head2 How can I tell whether my encoding fully supports PerlIO ?
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As of this writing, any encoding whose class belongs to Encode::XS and
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Encode::Unicode works. The Encode module has a C<perlio_ok> method
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which you can use before applying PerlIO encoding to the filehandle.
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Here is an example:
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my $use_perlio = perlio_ok($enc);
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my $layer = $use_perlio ? "<:raw" : "<:encoding($enc)";
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open my $fh, $layer, $file or die "$file : $!";
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while(<$fh>){
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$_ = decode($enc, $_) unless $use_perlio;
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# ....
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}
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=head1 SEE ALSO
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L<Encode::Encoding>,
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L<Encode::Supported>,
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L<Encode::PerlIO>,
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L<encoding>,
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L<perlebcdic>,
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L<perlfunc/open>,
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L<perlunicode>,
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L<utf8>,
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the Perl Unicode Mailing List E<lt>perl-unicode@perl.orgE<gt>
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=cut
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