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1 =head1 NAME 2 3 Encode::PerlIO -- a detailed document on Encode and PerlIO 4 5 =head1 Overview 6 7 It is very common to want to do encoding transformations when 8 reading or writing files, network connections, pipes etc. 9 If Perl is configured to use the new 'perlio' IO system then 10 C<Encode> provides a "layer" (see L<PerlIO>) which can transform 11 data as it is read or written. 12 13 Here is how the blind poet would modernise the encoding: 14 15 use Encode; 16 open(my $iliad,'<:encoding(iso-8859-7)','iliad.greek'); 17 open(my $utf8,'>:utf8','iliad.utf8'); 18 my @epic = <$iliad>; 19 print $utf8 @epic; 20 close($utf8); 21 close($illiad); 22 23 In addition, the new IO system can also be configured to read/write 24 UTF-8 encoded characters (as noted above, this is efficient): 25 26 open(my $fh,'>:utf8','anything'); 27 print $fh "Any \x{0021} string \N{SMILEY FACE}\n"; 28 29 Either of the above forms of "layer" specifications can be made the default 30 for a lexical scope with the C<use open ...> pragma. See L<open>. 31 32 Once a handle is open, its layers can be altered using C<binmode>. 33 34 Without any such configuration, or if Perl itself is built using the 35 system's own IO, then write operations assume that the file handle 36 accepts only I<bytes> and will C<die> if a character larger than 255 is 37 written to the handle. When reading, each octet from the handle becomes 38 a byte-in-a-character. Note that this default is the same behaviour 39 as bytes-only languages (including Perl before v5.6) would have, 40 and is sufficient to handle native 8-bit encodings e.g. iso-8859-1, 41 EBCDIC etc. and any legacy mechanisms for handling other encodings 42 and binary data. 43 44 In other cases, it is the program's responsibility to transform 45 characters into bytes using the API above before doing writes, and to 46 transform the bytes read from a handle into characters before doing 47 "character operations" (e.g. C<lc>, C</\W+/>, ...). 48 49 You can also use PerlIO to convert larger amounts of data you don't 50 want to bring into memory. For example, to convert between ISO-8859-1 51 (Latin 1) and UTF-8 (or UTF-EBCDIC in EBCDIC machines): 52 53 open(F, "<:encoding(iso-8859-1)", "data.txt") or die $!; 54 open(G, ">:utf8", "data.utf") or die $!; 55 while (<F>) { print G } 56 57 # Could also do "print G <F>" but that would pull 58 # the whole file into memory just to write it out again. 59 60 More examples: 61 62 open(my $f, "<:encoding(cp1252)") 63 open(my $g, ">:encoding(iso-8859-2)") 64 open(my $h, ">:encoding(latin9)") # iso-8859-15 65 66 See also L<encoding> for how to change the default encoding of the 67 data in your script. 68 69 =head1 How does it work? 70 71 Here is a crude diagram of how filehandle, PerlIO, and Encode 72 interact. 73 74 filehandle <-> PerlIO PerlIO <-> scalar (read/printed) 75 \ / 76 Encode 77 78 When PerlIO receives data from either direction, it fills a buffer 79 (currently with 1024 bytes) and passes the buffer to Encode. 80 Encode tries to convert the valid part and passes it back to PerlIO, 81 leaving invalid parts (usually a partial character) in the buffer. 82 PerlIO then appends more data to the buffer, calls Encode again, 83 and so on until the data stream ends. 84 85 To do so, PerlIO always calls (de|en)code methods with CHECK set to 1. 86 This ensures that the method stops at the right place when it 87 encounters partial character. The following is what happens when 88 PerlIO and Encode tries to encode (from utf8) more than 1024 bytes 89 and the buffer boundary happens to be in the middle of a character. 90 91 A B C .... ~ \x{3000} .... 92 41 42 43 .... 7E e3 80 80 .... 93 <- buffer ---------------> 94 << encoded >>>>>>>>>> 95 <- next buffer ------ 96 97 Encode converts from the beginning to \x7E, leaving \xe3 in the buffer 98 because it is invalid (partial character). 99 100 Unfortunately, this scheme does not work well with escape-based 101 encodings such as ISO-2022-JP. 102 103 =head1 Line Buffering 104 105 Now let's see what happens when you try to decode from ISO-2022-JP and 106 the buffer ends in the middle of a character. 107 108 JIS208-ESC \x{5f3e} 109 A B C .... ~ \e $ B |DAN | .... 110 41 42 43 .... 7E 1b 24 41 43 46 .... 111 <- buffer ---------------------------> 112 << encoded >>>>>>>>>>>>>>>>>>>>>>> 113 114 As you see, the next buffer begins with \x43. But \x43 is 'C' in 115 ASCII, which is wrong in this case because we are now in JISX 0208 116 area so it has to convert \x43\x46, not \x43. Unlike utf8 and EUC, 117 in escape-based encodings you can't tell if a given octet is a whole 118 character or just part of it. 119 120 Fortunately PerlIO also supports line buffer if you tell PerlIO to use 121 one instead of fixed buffer. Since ISO-2022-JP is guaranteed to revert to ASCII at the end of the line, partial 122 character will never happen when line buffer is used. 123 124 To tell PerlIO to use line buffer, implement -E<gt>needs_lines method 125 for your encoding object. See L<Encode::Encoding> for details. 126 127 Thanks to these efforts most encodings that come with Encode support 128 PerlIO but that still leaves following encodings. 129 130 iso-2022-kr 131 MIME-B 132 MIME-Header 133 MIME-Q 134 135 Fortunately iso-2022-kr is hardly used (according to Jungshik) and 136 MIME-* are very unlikely to be fed to PerlIO because they are for mail 137 headers. See L<Encode::MIME::Header> for details. 138 139 =head2 How can I tell whether my encoding fully supports PerlIO ? 140 141 As of this writing, any encoding whose class belongs to Encode::XS and 142 Encode::Unicode works. The Encode module has a C<perlio_ok> method 143 which you can use before applying PerlIO encoding to the filehandle. 144 Here is an example: 145 146 my $use_perlio = perlio_ok($enc); 147 my $layer = $use_perlio ? "<:raw" : "<:encoding($enc)"; 148 open my $fh, $layer, $file or die "$file : $!"; 149 while(<$fh>){ 150 $_ = decode($enc, $_) unless $use_perlio; 151 # .... 152 } 153 154 =head1 SEE ALSO 155 156 L<Encode::Encoding>, 157 L<Encode::Supported>, 158 L<Encode::PerlIO>, 159 L<encoding>, 160 L<perlebcdic>, 161 L<perlfunc/open>, 162 L<perlunicode>, 163 L<utf8>, 164 the Perl Unicode Mailing List E<lt>perl-unicode@perl.orgE<gt> 165 166 =cut 167
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