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 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

    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 the Encode::Supported manpage 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 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 :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 described in full in the encoding manpage, 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:


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.


binmode in the perlfunc manpage, open in the perlfunc manpage, the perlunicode manpage, the PerlIO manpage, the encoding manpage