MirOS Manual: open(3p)


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

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 pro-
     vided 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, "<koi8");
         printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1
         close I;

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

     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 impli-
     citly 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 vari-
     ables.

     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.

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

     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 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 perlio.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, Per-
     lIO, encoding

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