open(3p) Perl Programmers Reference Guide open(3p)NAMEopen - 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 pro-
vided 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 two-argument open(), readpipe() (aka qx//) and similar
operators found within the lexical scope of this pragma will
use the declared defaults. Even three-argument opens may be
affected by this pragma when they don't specify IO layers in
MODE.
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 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;
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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 (with the
three-arg syntax), they override 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 impli-
citly turns on ":std".
The logic of ":locale" is described in full in encoding, but
in short it is first trying nl_langinfo(CODESET) and then
guessing from the LC_ALL and LANG locale environment vari-
ables.
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.
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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 PerlIO 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 perlio.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, Per-
lIO, encoding
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