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RedHat 9 (Linux i386) - man page for encode::perlio (redhat section 3pm)

Encode::PerlIO(3pm)		 Perl Programmers Reference Guide	      Encode::PerlIO(3pm)

       Encode::PerlIO -- a detailed document on Encode and PerlIO

       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

       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;

       In addition, the new IO system can also be configured to read/write UTF-8 encoded charac-
       ters (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 char-
       acter 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)
			      \      /

       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.  Let's see what happens in that case in the next chapter.

       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.

       There are actually several ways to solve this problem but none of them is fast enough to
       be practical.  From Encode's point of view, the easiest solution is for PerlIO to imple-
       ment a line buffer instead of a fixed-length buffer, but that makes PerlIO really compli-

       So for the time being, using escape-based encodings in the ":encoding()" layer of PerlIO
       does not work well.


       If you still insist, you can at least use ":encoding()" by making sure the buffer never
       gets full.  Here is an example.

	 use FileHandle;
	 binmode(STDOUT, ":encoding(7bit-jis)");
	 STDOUT->autoflush(1); # don't forget this!
	 for my $l (@lines){   # $l cannot be longer than 1023 bytes
	   print $l;

       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 : $!";
	   $_ = decode($enc, $_) unless $use_perlio;
	   # ....

       Encode::Encoding, Encode::Supported, Encode::PerlIO, encoding, perlebcdic, "open" in perl-
       func, perlunicode, utf8, the Perl Unicode Mailing List <perl-unicode@perl.org>

perl v5.8.0				    2002-06-01			      Encode::PerlIO(3pm)

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