open(3pm) Perl Programmers Reference Guide open(3pm)
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(), read-
pipe() (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 lay-
ers 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
", ord(<I>), "
"; # 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 combined with the ":utf8" or ":encoding" subpragmas, it converts the standard file-
handles (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) interface, 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 encod-
ings (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 anything looking like UTF-8, and if
any found, ":utf8" is used as the default encoding for the open pragma.
If your locale environment variables (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 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 bin-
mode() 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 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 noth-
ing useful you can do with the object at the perl level.
SEE ALSO
"binmode" in perlfunc, "open" in perlfunc, perlunicode, PerlIO, encoding
perl v5.8.0 2002-06-01 open(3pm)