open(3)          Perl Programmers Reference Guide         open(3)



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 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 ":encod-
       ing(...)" 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, "), "\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
       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 (with the
       three-arg syntax), they override the list declared using
       this pragma.

       The ":std" subpragma on its own has no effect, but if com-
       bined with the ":utf8" or ":encoding" subpragmas, it con-
       verts 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 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 avail-
       able.

       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 Per-
       lIO 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 vali-
       date 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 per-
       lio.c.  As yet there is nothing useful you can do with the
       object at the perl level.

SEE ALSO
       "binmode" in perlfunc, "open" in perlfunc, perlunicode,
       PerlIO, encoding