open - perl pragma to set default PerlIO layers for input and output
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 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 :encoding(...)
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;
These are equivalent
as are these
and these
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.
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 combined with
the :utf8
or :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 :encoding(utf8)
, a :std
will mean that STDIN, STDOUT,
and STDERR are also in :encoding(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 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 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 :bytes
and :crlf
are available.
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.
There is a class method in PerlIO::Layer
find
which is
implemented as XS code. It is called by import to validate 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.