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PerlIO(3pm)            Perl Programmers Reference Guide            PerlIO(3pm)

       PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name

         open($fh,"<:crlf", "my.txt"); # support platform-native and CRLF text files

         open($fh,"<","his.jpg");      # portably open a binary file for reading

           PERLIO=perlio perl ....

       When an undefined layer 'foo' is encountered in an "open" or "binmode"
       layer specification then C code performs the equivalent of:

         use PerlIO 'foo';

       The perl code in PerlIO.pm then attempts to locate a layer by doing

         require PerlIO::foo;

       Otherwise the "PerlIO" package is a place holder for additional PerlIO
       related functions.

       The following layers are currently defined:

           Lowest level layer which provides basic PerlIO operations in terms
           of UNIX/POSIX numeric file descriptor calls (open(), read(),
           write(), lseek(), close()).

           Layer which calls "fread", "fwrite" and "fseek"/"ftell" etc.  Note
           that as this is "real" stdio it will ignore any layers beneath it
           and go straight to the operating system via the C library as usual.

           A from scratch implementation of buffering for PerlIO. Provides
           fast access to the buffer for "sv_gets" which implements perl's
           readline/<> and in general attempts to minimize data copying.

           ":perlio" will insert a ":unix" layer below itself to do low level

           A layer that implements DOS/Windows like CRLF line endings.  On
           read converts pairs of CR,LF to a single "\n" newline character.
           On write converts each "\n" to a CR,LF pair.  Note that this layer
           likes to be one of its kind: it silently ignores attempts to be
           pushed into the layer stack more than once.

           It currently does not mimic MS-DOS as far as treating of Control-Z
           as being an end-of-file marker.

           (Gory details follow) To be more exact what happens is this: after
           pushing itself to the stack, the ":crlf" layer checks all the
           layers below itself to find the first layer that is capable of
           being a CRLF layer but is not yet enabled to be a CRLF layer.  If
           it finds such a layer, it enables the CRLFness of that other deeper
           layer, and then pops itself off the stack.  If not, fine, use the
           one we just pushed.

           The end result is that a ":crlf" means "please enable the first
           CRLF layer you can find, and if you can't find one, here would be a
           good spot to place a new one."

           Based on the ":perlio" layer.

           A layer which implements "reading" of files by using "mmap()" to
           make a (whole) file appear in the process's address space, and then
           using that as PerlIO's "buffer". This may be faster in certain
           circumstances for large files, and may result in less physical
           memory use when multiple processes are reading the same file.

           Files which are not "mmap()"-able revert to behaving like the
           ":perlio" layer. Writes also behave like the ":perlio" layer, as
           "mmap()" for write needs extra house-keeping (to extend the file)
           which negates any advantage.

           The ":mmap" layer will not exist if the platform does not support

           Declares that the stream accepts perl's internal encoding of
           characters.  (Which really is UTF-8 on ASCII machines, but is UTF-
           EBCDIC on EBCDIC machines.)  This allows any character perl can
           represent to be read from or written to the stream. The UTF-X
           encoding is chosen to render simple text parts (i.e.  non-accented
           letters, digits and common punctuation) human readable in the
           encoded file.

           Here is how to write your native data out using UTF-8 (or UTF-
           EBCDIC) and then read it back in.

                   open(F, ">:utf8", "data.utf");
                   print F $out;

                   open(F, "<:utf8", "data.utf");
                   $in = <F>;

           Note that this layer does not validate byte sequences. For reading
           input, using ":encoding(utf8)" instead of bare ":utf8" is strongly

           This is the inverse of the ":utf8" layer. It turns off the flag on
           the layer below so that data read from it is considered to be
           "octets" i.e. characters in the range 0..255 only. Likewise on
           output perl will warn if a "wide" character is written to a such a

           The ":raw" layer is defined as being identical to calling
           "binmode($fh)" - the stream is made suitable for passing binary
           data, i.e. each byte is passed as-is. The stream will still be

           In Perl 5.6 and some books the ":raw" layer (previously sometimes
           also referred to as a "discipline") is documented as the inverse of
           the ":crlf" layer. That is no longer the case - other layers which
           would alter the binary nature of the stream are also disabled.  If
           you want UNIX line endings on a platform that normally does CRLF
           translation, but still want UTF-8 or encoding defaults, the
           appropriate thing to do is to add ":perlio" to the PERLIO
           environment variable.

           The implementation of ":raw" is as a pseudo-layer which when
           "pushed" pops itself and then any layers which do not declare
           themselves as suitable for binary data. (Undoing :utf8 and :crlf
           are implemented by clearing flags rather than popping layers but
           that is an implementation detail.)

           As a consequence of the fact that ":raw" normally pops layers, it
           usually only makes sense to have it as the only or first element in
           a layer specification.  When used as the first element it provides
           a known base on which to build e.g.


           will construct a "binary" stream, but then enable UTF-8

           A pseudo layer that removes the top-most layer. Gives perl code a
           way to manipulate the layer stack. Should be considered as
           experimental. Note that ":pop" only works on real layers and will
           not undo the effects of pseudo layers like ":utf8".  An example of
           a possible use might be:

               binmode($fh,":encoding(...)");  # next chunk is encoded
               binmode($fh,":pop");            # back to un-encoded

           A more elegant (and safer) interface is needed.

           On Win32 platforms this experimental layer uses the native "handle"
           IO rather than the unix-like numeric file descriptor layer. Known
           to be buggy as of perl 5.8.2.

   Custom Layers
       It is possible to write custom layers in addition to the above builtin
       ones, both in C/XS and Perl.  Two such layers (and one example written
       in Perl using the latter) come with the Perl distribution.

           Use ":encoding(ENCODING)" either in open() or binmode() to install
           a layer that transparently does character set and encoding
           transformations, for example from Shift-JIS to Unicode.  Note that
           under "stdio" an ":encoding" also enables ":utf8".  See
           PerlIO::encoding for more information.

           Use ":via(MODULE)" either in open() or binmode() to install a layer
           that does whatever transformation (for example compression /
           decompression, encryption / decryption) to the filehandle.  See
           PerlIO::via for more information.

   Alternatives to raw
       To get a binary stream an alternate method is to use:


       this has the advantage of being backward compatible with how such
       things have had to be coded on some platforms for years.

       To get an unbuffered stream specify an unbuffered layer (e.g. ":unix")
       in the open call:


   Defaults and how to override them
       If the platform is MS-DOS like and normally does CRLF to "\n"
       translation for text files then the default layers are :

         unix crlf

       (The low level "unix" layer may be replaced by a platform specific low
       level layer.)

       Otherwise if "Configure" found out how to do "fast" IO using the
       system's stdio, then the default layers are:

         unix stdio

       Otherwise the default layers are

         unix perlio

       These defaults may change once perlio has been better tested and tuned.

       The default can be overridden by setting the environment variable
       PERLIO to a space separated list of layers ("unix" or platform low
       level layer is always pushed first).

       This can be used to see the effect of/bugs in the various layers e.g.

         cd .../perl/t
         PERLIO=stdio  ./perl harness
         PERLIO=perlio ./perl harness

       For the various values of PERLIO see "PERLIO" in perlrun.

   Querying the layers of filehandles
       The following returns the names of the PerlIO layers on a filehandle.

          my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".

       The layers are returned in the order an open() or binmode() call would
       use them.  Note that the "default stack" depends on the operating
       system and on the Perl version, and both the compile-time and runtime
       configurations of Perl.

       The following table summarizes the default layers on UNIX-like and DOS-
       like platforms and depending on the setting of $ENV{PERLIO}:

        PERLIO     UNIX-like                   DOS-like
        ------     ---------                   --------
        unset / "" unix perlio / stdio [1]     unix crlf
        stdio      unix perlio / stdio [1]     stdio
        perlio     unix perlio                 unix perlio
        mmap       unix mmap                   unix mmap

        # [1] "stdio" if Configure found out how to do "fast stdio" (depends
        # on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"

       By default the layers from the input side of the filehandle are
       returned; to get the output side, use the optional "output" argument:

          my @layers = PerlIO::get_layers($fh, output => 1);

       (Usually the layers are identical on either side of a filehandle but
       for example with sockets there may be differences, or if you have been
       using the "open" pragma.)

       There is no set_layers(), nor does get_layers() return a tied array
       mirroring the stack, or anything fancy like that.  This is not
       accidental or unintentional.  The PerlIO layer stack is a bit more
       complicated than just a stack (see for example the behaviour of
       ":raw").  You are supposed to use open() and binmode() to manipulate
       the stack.

       Implementation details follow, please close your eyes.

       The arguments to layers are by default returned in parentheses after
       the name of the layer, and certain layers (like "utf8") are not real
       layers but instead flags on real layers; to get all of these returned
       separately, use the optional "details" argument:

          my @layer_and_args_and_flags = PerlIO::get_layers($fh, details => 1);

       The result will be up to be three times the number of layers: the first
       element will be a name, the second element the arguments (unspecified
       arguments will be "undef"), the third element the flags, the fourth
       element a name again, and so forth.

       You may open your eyes now.

       Nick Ing-Simmons <nick@ing-simmons.net>

       "binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode

perl v5.12.1                      2010-04-26                       PerlIO(3pm)

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