open(3pm)

1open(3pm)              Perl Programmers Reference Guide              open(3pm)
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NAME

6       open - perl pragma to set default PerlIO layers for input and output
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SYNOPSIS

9           use open IN  => ":crlf", OUT => ":bytes";
10           use open OUT => ':utf8';
11           use open IO  => ":encoding(iso-8859-7)";
12
13           use open IO  => ':locale';
14
15           use open ':encoding(utf8)';
16           use open ':locale';
17           use open ':encoding(iso-8859-7)';
18
19           use open ':std';
20

DESCRIPTION

22       Full-fledged support for I/O layers is now implemented provided Perl is
23       configured to use PerlIO as its IO system (which is now the default).
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25       The "open" pragma serves as one of the interfaces to declare default
26       "layers" (also known as "disciplines") for all I/O. Any two-argument
27       open(), readpipe() (aka qx//) and similar operators found within the
28       lexical scope of this pragma will use the declared defaults.  Even
29       three-argument opens may be affected by this pragma when they don't
30       specify IO layers in MODE.
31
32       With the "IN" subpragma you can declare the default layers of input
33       streams, and with the "OUT" subpragma you can declare the default
34       layers of output streams.  With the "IO"  subpragma you can control
35       both input and output streams simultaneously.
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37       If you have a legacy encoding, you can use the ":encoding(...)" tag.
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39       If you want to set your encoding layers based on your locale
40       environment variables, you can use the ":locale" tag.  For example:
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42           $ENV{LANG} = 'ru_RU.KOI8-R';
43           # the :locale will probe the locale environment variables like LANG
44           use open OUT => ':locale';
45           open(O, ">koi8");
46           print O chr(0x430); # Unicode CYRILLIC SMALL LETTER A = KOI8-R 0xc1
47           close O;
48           open(I, "<koi8");
49           printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1
50           close I;
51
52       These are equivalent
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54           use open ':encoding(utf8)';
55           use open IO => ':encoding(utf8)';
56
57       as are these
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59           use open ':locale';
60           use open IO => ':locale';
61
62       and these
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64           use open ':encoding(iso-8859-7)';
65           use open IO => ':encoding(iso-8859-7)';
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67       The matching of encoding names is loose: case does not matter, and many
68       encodings have several aliases.  See Encode::Supported for details and
69       the list of supported locales.
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71       When open() is given an explicit list of layers (with the three-arg
72       syntax), they override the list declared using this pragma.  open() can
73       also be given a single colon (:) for a layer name, to override this
74       pragma and use the default (":raw" on Unix, ":crlf" on Windows).
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76       The ":std" subpragma on its own has no effect, but if combined with the
77       ":utf8" or ":encoding" subpragmas, it converts the standard filehandles
78       (STDIN, STDOUT, STDERR) to comply with encoding selected for
79       input/output handles.  For example, if both input and out are chosen to
80       be ":encoding(utf8)", a ":std" will mean that STDIN, STDOUT, and STDERR
81       are also in ":encoding(utf8)".  On the other hand, if only output is
82       chosen to be in ":encoding(koi8r)", a ":std" will cause only the STDOUT
83       and STDERR to be in "koi8r".  The ":locale" subpragma implicitly turns
84       on ":std".
85
86       The logic of ":locale" is described in full in encoding, but in short
87       it is first trying nl_langinfo(CODESET) and then guessing from the
88       LC_ALL and LANG locale environment variables.
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90       Directory handles may also support PerlIO layers in the future.
91

NONPERLIO FUNCTIONALITY

93       If Perl is not built to use PerlIO as its IO system then only the two
94       pseudo-layers ":bytes" and ":crlf" are available.
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96       The ":bytes" layer corresponds to "binary mode" and the ":crlf" layer
97       corresponds to "text mode" on platforms that distinguish between the
98       two modes when opening files (which is many DOS-like platforms,
99       including Windows).  These two layers are no-ops on platforms where
100       binmode() is a no-op, but perform their functions everywhere if PerlIO
101       is enabled.
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IMPLEMENTATION DETAILS

104       There is a class method in "PerlIO::Layer" "find" which is implemented
105       as XS code.  It is called by "import" to validate the layers:
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107          PerlIO::Layer::->find("perlio")
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109       The return value (if defined) is a Perl object, of class
110       "PerlIO::Layer" which is created by the C code in perlio.c.  As yet
111       there is nothing useful you can do with the object at the perl level.
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NAME

SYNOPSIS

DESCRIPTION

NONPERLIO FUNCTIONALITY

IMPLEMENTATION DETAILS

SEE ALSO