open

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

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(UTF-8)';
           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(UTF-8)';
           use open IO => ':encoding(UTF-8)';

       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.  open() can
       also be given a single colon (:) for a layer name, to override this
       pragma and use the default (":raw" on Unix, ":crlf" on Windows).

       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(UTF-8)", a ":std" will mean that STDIN, STDOUT, and
       STDERR are also in ":encoding(UTF-8)".  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" in perlfunc, "open" in perlfunc, perlunicode, PerlIO,
       encoding

perl v5.30.0                      2023-11-23                       open(3perl)