ext::Encode::lib:PerloProgramext::Encode::lib::Encode::PerlIO(3p)NAMEEncode::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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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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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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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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