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

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

	 open($fh, "<:crlf", "my.txt");	# support platform-native and
					# CRLF text files

	 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 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:

	   Lowest level	layer which provides basic PerlIO operations in	terms
	   of UNIX/POSIX numeric file descriptor calls (open(),	read(),
	   write(), lseek(), close()).

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

	   A from scratch implementation of buffering for PerlIO. Provides
	   fast	access to the buffer for "sv_gets" which implements perl's
	   readline/<> and in general attempts to minimize data	copying.

	   ":perlio" will insert a ":unix" layer below itself to do low	level

	   A layer that	implements DOS/Windows like CRLF line endings.	On
	   read	converts pairs of CR,LF	to a single "\n" newline character.
	   On write converts each "\n" to a CR,LF pair.	 Note that this	layer
	   will	silently refuse	to be pushed on	top of itself.

	   It currently	does not mimic MS-DOS as far as	treating of Control-Z
	   as being an end-of-file marker.

	   Based on the	":perlio" layer.

	   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.

	   (CAUTION: This layer	does not validate byte sequences.  For reading
	   input, you should instead use ":encoding(UTF-8)" instead of bare

	   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 the ":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 the range 0..255	only. Likewise on
	   output perl will warn if a "wide" character is written to a such a

	   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

	   In Perl 5.6 and some	books the ":raw" layer (previously sometimes
	   also	referred to as a "discipline") is documented as	the inverse of
	   the ":crlf" layer. That is no longer	the case - other layers	which
	   would alter the binary nature of the	stream are also	disabled.  If
	   you want UNIX line endings on a platform that normally does CRLF
	   translation,	but still want UTF-8 or	encoding defaults, the
	   appropriate thing to	do is to add ":perlio" to the PERLIO
	   environment variable.

	   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 popping 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

	   A pseudo layer that removes the top-most layer. Gives perl code a
	   way to manipulate the layer stack.  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-encoded

	   A more elegant (and safer) interface	is needed.

	   On Win32 platforms this experimental	layer uses the native "handle"
	   IO rather than the unix-like	numeric	file descriptor	layer. Known
	   to be buggy as of perl 5.8.2.

   Custom Layers
       It is possible to write custom layers in	addition to the	above builtin
       ones, both in C/XS and Perl.  Two such layers (and one example written
       in Perl using the latter) come with the Perl distribution.

	   Use ":encoding(ENCODING)" either in open() or binmode() to install
	   a layer that	transparently does character set and encoding
	   transformations, for	example	from Shift-JIS to Unicode.  Note that
	   under "stdio" an ":encoding"	also enables ":utf8".  See
	   PerlIO::encoding for	more information.

	   A layer which implements "reading" of files by using	"mmap()" to
	   make	a (whole) file appear in the process's address space, and then
	   using that as PerlIO's "buffer". This may be	faster in certain
	   circumstances for large files, and may result in less physical
	   memory use when multiple processes are reading the same file.

	   Files which are not "mmap()"-able revert to behaving	like the
	   ":perlio" layer. Writes also	behave like the	":perlio" layer, as
	   "mmap()" for	write needs extra house-keeping	(to extend the file)
	   which negates any advantage.

	   The ":mmap" layer will not exist if the platform does not support

	   Use ":via(MODULE)" either in	open() or binmode() to install a layer
	   that	does whatever transformation (for example compression /
	   decompression, encryption / decryption) to the filehandle.  See
	   PerlIO::via for more	information.

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


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

       To get an unbuffered 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 the
       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

       For the various values of PERLIO	see "PERLIO" in	perlrun.

   Querying the	layers of filehandles
       The following returns the names of the PerlIO layers on a filehandle.

	  my @layers = PerlIO::get_layers($fh);	# Or FH, *FH, "FH".

       The layers are returned in the order an open() or binmode() call	would
       use them.  Note that the	"default stack"	depends	on the operating
       system and on the Perl version, and both	the compile-time and runtime
       configurations of Perl.

       The following table summarizes the default layers on UNIX-like and DOS-
       like platforms and depending on the setting of $ENV{PERLIO}:

	PERLIO	   UNIX-like		       DOS-like
	------	   ---------		       --------
	unset /	"" unix	perlio / stdio [1]     unix crlf
	stdio	   unix	perlio / stdio [1]     stdio
	perlio	   unix	perlio		       unix perlio

	# [1] "stdio" if Configure found out how to do "fast stdio" (depends
	# on the stdio implementation) and in Perl 5.8,	otherwise "unix	perlio"

       By default the layers from the input side of the	filehandle are
       returned; to get	the output side, use the optional "output" argument:

	  my @layers = PerlIO::get_layers($fh, output => 1);

       (Usually	the layers are identical on either side	of a filehandle	but
       for example with	sockets	there may be differences, or if	you have been
       using the "open"	pragma.)

       There is	no set_layers(), nor does get_layers() return a	tied array
       mirroring the stack, or anything	fancy like that.  This is not
       accidental or unintentional.  The PerlIO	layer stack is a bit more
       complicated than	just a stack (see for example the behaviour of
       ":raw").	 You are supposed to use open()	and binmode() to manipulate
       the stack.

       Implementation details follow, please close your	eyes.

       The arguments to	layers are by default returned in parentheses after
       the name	of the layer, and certain layers (like "utf8") are not real
       layers but instead flags	on real	layers;	to get all of these returned
       separately, use the optional "details" argument:

	  my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);

       The result will be up to	be three times the number of layers: the first
       element will be a name, the second element the arguments	(unspecified
       arguments will be "undef"), the third element the flags,	the fourth
       element a name again, and so forth.

       You may open your eyes now.

       Nick Ing-Simmons	<>

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

perl v5.28.3			  2020-05-14			     PerlIO(3)


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