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



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

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

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

         Shell:
           PERLIO=perlio perl ....

DESCRIPTION
       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:

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

       :stdio
           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.

       :perlio
           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 IO.

       :crlf
           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 will silently refuse to be pushed on top of itself.

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

           Based on the ":perlio" layer.

       :utf8
           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;
                   close(F);

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

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

       :bytes
           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 stream.

       :raw
           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
           buffered.

           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 caseother
           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.

               open($fh,":raw:utf8",...)

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

       :pop
           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:

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

           A more elegant (and safer) interface is needed.

       :win32
           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.

       :encoding
           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.

       :mmap
           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 "mmap()".

       :via
           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:

           open($fh,"whatever")
           binmode($fh);

       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:

           open($fh,"<:unix",$path)

   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

        # [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.

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

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



perl v5.16.2                                     2012-10-11                                      PerlIO(3pm)