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Diffstat (limited to 'Master/tlpkg/tlperl0/lib/Encode/PerlIO.pod')
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diff --git a/Master/tlpkg/tlperl0/lib/Encode/PerlIO.pod b/Master/tlpkg/tlperl0/lib/Encode/PerlIO.pod new file mode 100755 index 00000000000..1a9269a9b0c --- /dev/null +++ b/Master/tlpkg/tlperl0/lib/Encode/PerlIO.pod @@ -0,0 +1,167 @@ +=head1 NAME + +Encode::PerlIO -- a detailed document on Encode and PerlIO + +=head1 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 +C<Encode> provides a "layer" (see L<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 C<use open ...> pragma. See L<open>. + +Once a handle is open, its layers can be altered using C<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 I<bytes> and will C<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. C<lc>, C</\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 L<encoding> for how to change the default encoding of the +data in your script. + +=head1 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. + +=head1 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 -E<gt>needs_lines method +for your encoding object. See L<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 L<Encode::MIME::Header> for details. + +=head2 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 C<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; + # .... + } + +=head1 SEE ALSO + +L<Encode::Encoding>, +L<Encode::Supported>, +L<Encode::PerlIO>, +L<encoding>, +L<perlebcdic>, +L<perlfunc/open>, +L<perlunicode>, +L<utf8>, +the Perl Unicode Mailing List E<lt>perl-unicode@perl.orgE<gt> + +=cut + |