open(3)

NAME

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

SYNOPSIS

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';

DESCRIPTION

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 default).

The "open" pragma serves as one of the interfaces to
declare default "layers" (also known as "disciplines") for
all I/O. Any open(), readpipe() (aka qx//) and similar operators found within the lexical scope of this pragma
will use the declared defaults.

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

If you have a legacy encoding, you can use the ":encod
ing(...)" tag.

if you want to set your encoding layers based on your
locale environment variables, you can use the ":locale"
tag. For example:
$ENV{LANG} = 'ru_RU.KOI8-R';
# the :locale will probe the locale environment vari
ables 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 "%#x0, ord(<I>), "0; # 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
Encode::Supported for details and the list of supported
locales.
Note that ":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 ":std" subpragma on its own has no effect, but if com
bined with the ":utf8" or ":encoding" subpragmas, it con
verts 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
":utf8", a ":std" will mean that STDIN, STDOUT, and STDERR
are also in ":utf8". On the other hand, if only output is
chosen to be in ":encoding(koi8r)", a ":std" will cause
only the STDOUT and STDERR to be in "koi8r". The
":locale" subpragma implicitly turns on ":std".
The logic of ":locale" is as follows:
1. If the platform supports the langinfo(CODESET) inter
face, the codeset returned is used as the default
encoding for the open pragma.
2. If 1. didn't work but we are under the locale pragma,
the environment variables LC_ALL and LANG (in that
order) are matched for encodings (the part after ".",
if any), and if any found, that is used as the default
encoding for the open pragma.
3. If 1. and 2. didn't work, the environment variables
LC_ALL and LANG (in that order) are matched for any
thing looking like UTF-8, and if any found, ":utf8" is
used as the default encoding for the open pragma.
If your locale environment variables (LANGUAGE, LC_ALL,
LC_CTYPE, LANG) contain the strings 'UTF-8' or 'UTF8'
(case-insensitive matching), the default encoding of your
STDIN, STDOUT, and STDERR, and of any subsequent file open, is UTF-8.
Directory handles may also support PerlIO layers in the
future.

NONPERLIO FUNCTIONALITY

If Perl is not built to use PerlIO as its IO system then
only the two pseudo-layers ":bytes" and ":crlf" are avail
able.

The ":bytes" layer corresponds to "binary mode" and the
":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 Per
lIO is enabled.

IMPLEMENTATION DETAILS

There is a class method in "PerlIO::Layer" "find" which is
implemented as XS code. It is called by "import" to vali
date the layers:
PerlIO::Layer::->find("perlio")
The return value (if defined) is a Perl object, of class
"PerlIO::Layer" which is created by the C code in per_
lio.c. As yet there is nothing useful you can do with the
object at the perl level.

SEE ALSO

"binmode" in perlfunc, "open" in perlfunc, perlunicode,
PerlIO, encoding
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