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Encode::PerlIO(3)
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 "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 bytes and will "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. "lc", "/\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 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).
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 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 ->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 appling 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;
# ....
}
SEE ALSO
Encode::Encoding, Encode::Supported, Encode::PerlIO, encoding, perlebcdic,
"open" in perlfunc, perlunicode, utf8, the Perl Unicode Mailing List
<perl-unicode@perl.org>
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