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# open

Perl 5 version 10.1 documentation
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# open

- NAME
- SYNOPSIS
- DESCRIPTION
- NONPERLIO FUNCTIONALITY
- IMPLEMENTATION DETAILS
- SEE ALSO

# 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 ':encoding(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 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 ':encoding(utf8)';    use open IO => ':encoding(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.

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.

# 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 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 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.

# SEE ALSO

binmode, open, perlunicode, PerlIO, encoding

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- IMPLEMENTATION DETAILS
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