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Jarkko Hietaniemi


open - perl pragma to set default disciplines for input and output


    use open IN  => ":crlf", OUT => ":raw";
    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)';


Full-fledged support for I/O disciplines 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" (aka disciplines) for all I/O.

The open pragma is used to declare one or more default layers for I/O operations. 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 :encoding(...) tag.

if you want to set your encoding disciplines 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)';

When open() is given an explicit list of layers they are appended to the list declared using this pragma.

Directory handles may also support disciplines in future.


If Perl is not built to use PerlIO as its IO system then only the two pseudo-disciplines ":raw" and ":crlf" are available.

The ":raw" discipline corresponds to "binary mode" and the ":crlf" discipline 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 disciplines 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 perlfunc, "open" in perlfunc, perlunicode, PerlIO, encoding