package open;
use warnings;
use Carp;
$open::hint_bits = 0x20000; # HINT_LOCALIZE_HH

our $VERSION = '1.04';

require 5.008001; # for PerlIO::get_layers()

my $locale_encoding;

sub _get_encname {
    return ($1, Encode::resolve_alias($1)) if $_[0] =~ /^:?encoding\((.+)\)$/;

sub _drop_oldenc {
    # If by the time we arrive here there already is at the top of the
    # perlio layer stack an encoding identical to what we would like
    # to push via this open pragma, we will pop away the old encoding
    # (+utf8) so that we can push ourselves in place (this is easier
    # than ignoring pushing ourselves because of the way how ${^OPEN}
    # works).  So we are looking for something like
    #   stdio encoding(xxx) utf8
    # in the existing layer stack, and in the new stack chunk for
    #   :encoding(xxx)
    # If we find a match, we pop the old stack (once, since
    # the utf8 is just a flag on the encoding layer)
    my ($h, @new) = @_;
    return unless @new >= 1 && $new[-1] =~ /^:encoding\(.+\)$/;
    my @old = PerlIO::get_layers($h);
    return unless @old >= 3 &&
	          $old[-1] eq 'utf8' &&
                  $old[-2] =~ /^encoding\(.+\)$/;
    require Encode;
    my ($loname, $lcname) = _get_encname($old[-2]);
    unless (defined $lcname) { # Should we trust get_layers()?
	require Carp;
	Carp::croak("open: Unknown encoding '$loname'");
    my ($voname, $vcname) = _get_encname($new[-1]);
    unless (defined $vcname) {
	require Carp;
	Carp::croak("open: Unknown encoding '$voname'");
    if ($lcname eq $vcname) {
	binmode($h, ":pop"); # utf8 is part of the encoding layer

sub import {
    my ($class,@args) = @_;
    croak("open: needs explicit list of PerlIO layers") unless @args;
    my $std;
    $^H |= $open::hint_bits;
    my ($in,$out) = split(/\0/,(${^OPEN} || "\0"), -1);
    while (@args) {
	my $type = shift(@args);
	my $dscp;
	if ($type =~ /^:?(utf8|locale|encoding\(.+\))$/) {
	    $type = 'IO';
	    $dscp = ":$1";
	} elsif ($type eq ':std') {
	    $std = 1;
	} else {
	    $dscp = shift(@args) || '';
	my @val;
	foreach my $layer (split(/\s+/,$dscp)) {
            $layer =~ s/^://;
	    if ($layer eq 'locale') {
		require Encode;
		require encoding;
		$locale_encoding = encoding::_get_locale_encoding()
		    unless defined $locale_encoding;
		(warnings::warnif("layer", "Cannot figure out an encoding to use"), last)
		    unless defined $locale_encoding;
		if ($locale_encoding =~ /^utf-?8$/i) {
		    $layer = "utf8";
		} else {
		    $layer = "encoding($locale_encoding)";
		$std = 1;
	    } else {
		my $target = $layer;		# the layer name itself
		$target =~ s/^(\w+)\(.+\)$/$1/;	# strip parameters

		unless(PerlIO::Layer::->find($target,1)) {
		    warnings::warnif("layer", "Unknown PerlIO layer '$target'");
	    if ($layer =~ /^(crlf|raw)$/) {
		$^H{"open_$type"} = $layer;
	if ($type eq 'IN') {
	    _drop_oldenc(*STDIN, @val);
	    $in  = join(' ', @val);
	elsif ($type eq 'OUT') {
	    _drop_oldenc(*STDOUT, @val);
	    $out = join(' ', @val);
	elsif ($type eq 'IO') {
	    _drop_oldenc(*STDIN,  @val);
	    _drop_oldenc(*STDOUT, @val);
	    $in = $out = join(' ', @val);
	else {
	    croak "Unknown PerlIO layer class '$type'";
    ${^OPEN} = join("\0", $in, $out);
    if ($std) {
	if ($in) {
	    if ($in =~ /:utf8\b/) {
		    binmode(STDIN,  ":utf8");
		} elsif ($in =~ /(\w+\(.+\))/) {
		    binmode(STDIN,  ":$1");
	if ($out) {
	    if ($out =~ /:utf8\b/) {
		binmode(STDOUT,  ":utf8");
		binmode(STDERR,  ":utf8");
	    } elsif ($out =~ /(\w+\(.+\))/) {
		binmode(STDOUT,  ":$1");
		binmode(STDERR,  ":$1");


=head1 NAME

open - perl pragma to set default PerlIO layers for input and output


    use open IN  => ":crlf", OUT => ":bytes";
    use open OUT => ':utf8';
    use open IO  => ":encoding(iso-8859-7)";

    use open IO  => ':locale';

    use open ':utf8';
    use open ':locale';
    use open ':encoding(iso-8859-7)';

    use open ':std';


Full-fledged support for I/O layers is now implemented provided
Perl is configured to use PerlIO as its IO system (which is now the

The C<open> pragma serves as one of the interfaces to declare default
"layers" (also known as "disciplines") for all I/O. Any two-argument
open(), readpipe() (aka qx//) and similar operators found within the
lexical scope of this pragma will use the declared defaults.
Three-argument opens are not affected by this pragma since there you
(can) explicitly specify the layers and are supposed to know what you
are doing.

With the C<IN> subpragma you can declare the default layers
of input streams, and with the C<OUT> subpragma you can declare
the default layers of output streams.  With the C<IO>  subpragma
you can control both input and output streams simultaneously.

If you have a legacy encoding, you can use the C<:encoding(...)> tag.

if you want to set your encoding layers based on your
locale environment variables, you can use the C<:locale> tag.
For example:

    $ENV{LANG} = 'ru_RU.KOI8-R';
    # the :locale will probe the locale environment variables like LANG
    use open OUT => ':locale';
    open(O, ">koi8");
    print O chr(0x430); # Unicode CYRILLIC SMALL LETTER A = KOI8-R 0xc1
    close O;
    open(I, "<koi8");
    printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1
    close I;

These are equivalent

    use open ':utf8';
    use open IO => ':utf8';

as are these

    use open ':locale';
    use open IO => ':locale';

and these

    use open ':encoding(iso-8859-7)';
    use open IO => ':encoding(iso-8859-7)';

The matching of encoding names is loose: case does not matter, and
many encodings have several aliases.  See L<Encode::Supported> for
details and the list of supported locales.

Note that C<:utf8> PerlIO layer must always be specified exactly like
that, it is not subject to the loose matching of encoding names.

When open() is given an explicit list of layers they are appended to
the list declared using this pragma.

The C<:std> subpragma on its own has no effect, but if combined with
the C<:utf8> or C<:encoding> subpragmas, it converts the standard
filehandles (STDIN, STDOUT, STDERR) to comply with encoding selected
for input/output handles.  For example, if both input and out are
chosen to be C<:utf8>, a C<:std> will mean that STDIN, STDOUT, and
STDERR are also in C<:utf8>.  On the other hand, if only output is
chosen to be in C<< :encoding(koi8r) >>, a C<:std> will cause only the
STDOUT and STDERR to be in C<koi8r>.  The C<:locale> subpragma
implicitly turns on C<:std>.

The logic of C<:locale> is described in full in L</encoding>,
but in short it is first trying nl_langinfo(CODESET) and then
guessing from the LC_ALL and LANG locale environment variables.

Directory handles may also support PerlIO layers in the future.


If Perl is not built to use PerlIO as its IO system then only the two
pseudo-layers C<:bytes> and C<:crlf> are available.

The C<:bytes> layer corresponds to "binary mode" and the C<:crlf>
layer corresponds to "text mode" on platforms that distinguish
between the two modes when opening files (which is many DOS-like
platforms, including Windows).  These two layers are no-ops on
platforms where binmode() is a no-op, but perform their functions
everywhere if PerlIO is enabled.


There is a class method in C<PerlIO::Layer> C<find> which is
implemented as XS code.  It is called by C<import> to validate the


The return value (if defined) is a Perl object, of class
C<PerlIO::Layer> which is created by the C code in F<perlio.c>.  As
yet there is nothing useful you can do with the object at the perl

=head1 SEE ALSO

L<perlfunc/"binmode">, L<perlfunc/"open">, L<perlunicode>, L<PerlIO>,