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

       PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space

	 open($fh,"<:crlf", "my.txt"); # portably open a text file for reading

	 open($fh,"<","his.jpg");      # portably open a binary file for reading

	   PERLIO=perlio perl ....

       When an undefined layer 'foo' is encountered in an "open" or "binmode" layer specification
       then C code performs the equivalent of:

	 use PerlIO 'foo';

       The perl code in PerlIO.pm then attempts to locate a layer by doing

	 require PerlIO::foo;

       Otherwise the "PerlIO" package is a place holder for additional PerlIO related functions.

       The following layers are currently defined:

	   Low level layer which calls "read", "write" and "lseek" etc.

	   Layer which calls "fread", "fwrite" and "fseek"/"ftell" etc.  Note that as this is
	   "real" stdio it will ignore any layers beneath it and got straight to the operating
	   system via the C library as usual.

	   This is a re-implementation of "stdio-like" buffering written as a PerlIO "layer".  As
	   such it will call whatever layer is below it for its operations.

	   A layer which does CRLF to "\n" translation distinguishing "text" and "binary" files
	   in the manner of MS-DOS and similar operating systems.  (It currently does not mimic
	   MS-DOS as far as treating of Control-Z as being an end-of-file marker.)

	   Declares that the stream accepts perl's internal encoding of characters.  (Which
	   really is UTF-8 on ASCII machines, but is UTF-EBCDIC on EBCDIC machines.)  This allows
	   any character perl can represent to be read from or written to the stream. The UTF-X
	   encoding is chosen to render simple text parts (i.e.  non-accented letters, digits and
	   common punctuation) human readable in the encoded file.

	   Here is how to write your native data out using UTF-8 (or UTF-EBCDIC) and then read it
	   back in.

		   open(F, ">:utf8", "data.utf");
		   print F $out;

		   open(F, "<:utf8", "data.utf");
		   $in = <F>;

	   This is the inverse of ":utf8" layer. It turns off the flag on the layer below so that
	   data read from it is considered to be "octets" i.e. characters in range 0..255 only.
	   Likewise on output perl will warn if a "wide" character is written to a such a stream.

       raw The ":raw" layer is defined as being identical to calling "binmode($fh)" - the stream
	   is made suitable for passing binary data i.e. each byte is passed as-is. The stream
	   will still be buffered. Unlike earlier versions of perl ":raw" is not just the inverse
	   of ":crlf" - other layers which would affect the binary nature of the stream are also
	   removed or disabled.

	   The implementation of ":raw" is as a pseudo-layer which when "pushed" pops itself and
	   then any layers which do not declare themselves as suitable for binary data. (Undoing
	   :utf8 and :crlf are implemented by clearing flags rather than poping layers but that
	   is an implementation detail.)

	   As a consequence of the fact that ":raw" normally pops layers it usually only makes
	   sense to have it as the only or first element in a layer specification.  When used as
	   the first element it provides a known base on which to build e.g.


	   will construct a "binary" stream, but then enable UTF-8 translation.

       pop A pseudo layer that removes the top-most layer. Gives perl code a way to manipulate
	   the layer stack. Should be considered as experimental. Note that ":pop" only works on
	   real layers and will not undo the effects of pseudo layers like ":utf8".  An example
	   of a possible use might be:

	       binmode($fh,":encoding(...)");  # next chunk is encoded
	       binmode($fh,":pop");	       # back to un-encocded

	   A more elegant (and safer) interface is needed.

       Alternatives to raw

       To get a binary stream an alternate method is to use:


       this has advantage of being backward compatible with how such things have had to be coded
       on some platforms for years.

       To get an un-buffered stream specify an unbuffered layer (e.g. ":unix") in the open call:


       Defaults and how to override them

       If the platform is MS-DOS like and normally does CRLF to "\n" translation for text files
       then the default layers are :

	 unix crlf

       (The low level "unix" layer may be replaced by a platform specific low level layer.)

       Otherwise if "Configure" found out how to do "fast" IO using system's stdio, then the
       default layers are :

	 unix stdio

       Otherwise the default layers are

	 unix perlio

       These defaults may change once perlio has been better tested and tuned.

       The default can be overridden by setting the environment variable PERLIO to a space sepa-
       rated list of layers (unix or platform low level layer is always pushed first).

       This can be used to see the effect of/bugs in the various layers e.g.

	 cd .../perl/t
	 PERLIO=stdio  ./perl harness
	 PERLIO=perlio ./perl harness

       Nick Ing-Simmons <nick@ing-simmons.net>

       "binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode

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