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

NAME

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

SYNOPSIS

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

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

	 Shell:
	   PERLIO=perlio perl ....

DESCRIPTION

       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:

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

       stdio
	   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.

       perlio
	   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.

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

       utf8
	   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 cho-
	   sen 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;
		   close(F);

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

       bytes
	   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 imple-
	   mentation 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.

	       open($fh,":raw:utf8",...)

	   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 experimen-
	   tal. 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:

	       open($fh,...)
	       ...
	       binmode($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:

	   open($fh,"whatever")
	   binmode($fh);

       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:

	   open($fh,"<:unix",$path)

       Defaults and how to override them

       If the platform is MS-DOS like and normally does CRLF to "
" 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 separated 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

AUTHOR

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

SEE ALSO

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

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