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Encode::PerlIO(3pm) Perl Programmers Reference Guide Encode::PerlIO(3pm)
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 ’per-
lio’ 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 charac-
ter) 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 charac-
ter. 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::Uni-
code 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-uni-
code AT perl.org>
perl v5.8.6 2001-09-21 Encode::PerlIO(3pm)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-11-22 05:49 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)