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 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 environ-
       ment 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 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 combined with the
       ":utf8" or ":encoding" subpragmas, it converts the standard filehandles
       (STDIN, STDOUT, STDERR) to comply with encoding selected for input/out-
       put 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 ":encod-
       ing(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.


       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, includ-
       ing Windows).  These two layers are no-ops on platforms where binmode()
       is a no-op, but perform their functions everywhere if PerlIO is


       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 "Per-
       lIO::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, encod-

perl v5.8.8                       2006-06-14                           open(3)

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