open

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 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 file-
       handles (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 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 "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.

SEE ALSO
       "binmode" in perlfunc, "open" in perlfunc, perlunicode, PerlIO, encod-
       ing



perl v5.8.8			  2001-09-21			      open(3)