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+package PerlIO;
+
+our $VERSION = '1.06';
+
+# Map layer name to package that defines it
+our %alias;
+
+sub import
+{
+ my $class = shift;
+ while (@_)
+ {
+ my $layer = shift;
+ if (exists $alias{$layer})
+ {
+ $layer = $alias{$layer}
+ }
+ else
+ {
+ $layer = "${class}::$layer";
+ }
+ eval "require $layer";
+ warn $@ if $@;
+ }
+}
+
+sub F_UTF8 () { 0x8000 }
+
+1;
+__END__
+
+=head1 NAME
+
+PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
+
+=head1 SYNOPSIS
+
+ open($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 ....
+
+=head1 DESCRIPTION
+
+When an undefined layer 'foo' is encountered in an C<open> or
+C<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 C<PerlIO> package is a place holder for additional
+PerlIO related functions.
+
+The following layers are currently defined:
+
+=over 4
+
+=item :unix
+
+Lowest level layer which provides basic PerlIO operations in terms of
+UNIX/POSIX numeric file descriptor calls
+(open(), read(), write(), lseek(), close()).
+
+=item :stdio
+
+Layer which calls C<fread>, C<fwrite> and C<fseek>/C<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.
+
+=item :perlio
+
+A from scratch implementation of buffering for PerlIO. Provides fast
+access to the buffer for C<sv_gets> which implements perl's readline/E<lt>E<gt>
+and in general attempts to minimize data copying.
+
+C<:perlio> will insert a C<:unix> layer below itself to do low level IO.
+
+=item :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 likes to be
+one of its kind: it silently ignores attempts to be pushed into the
+layer stack more than once.
+
+It currently does I<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 C<: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 C<: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 C<:perlio> layer.
+
+=item :mmap
+
+A layer which implements "reading" of files by using C<mmap()> to
+make a (whole) file appear in the process's address space, and then
+using that as PerlIO's "buffer". This I<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 C<mmap()>-able revert to behaving like the C<:perlio>
+layer. Writes also behave like the C<:perlio> layer, as C<mmap()> for write
+needs extra house-keeping (to extend the file) which negates any advantage.
+
+The C<:mmap> layer will not exist if the platform does not support C<mmap()>.
+
+=item :utf8
+
+Declares that the stream accepts perl's I<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");
+ $in = <F>;
+ close(F);
+
+Note that this layer does not validate byte sequences. For reading
+input, using C<:encoding(utf8)> instead of bare C<:utf8> is strongly
+recommended.
+
+=item :bytes
+
+This is the inverse of the C<:utf8> layer. It turns off the flag
+on the layer below so that data read from it is considered to
+be "octets" i.e. characters in the range 0..255 only. Likewise
+on output perl will warn if a "wide" character is written
+to a such a stream.
+
+=item :raw
+
+The C<:raw> layer is I<defined> as being identical to calling
+C<binmode($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 C<:raw> layer (previously sometimes also
+referred to as a "discipline") is documented as the inverse of the
+C<: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
+C<:perlio> to the PERLIO environment variable.
+
+The implementation of C<: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 C<: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($fh,":raw:utf8",...)
+
+will construct a "binary" stream, but then enable UTF-8 translation.
+
+=item :pop
+
+A pseudo layer that removes the top-most layer. Gives perl code
+a way to manipulate the layer stack. Should be considered
+as experimental. Note that C<:pop> only works on real layers
+and will not undo the effects of pseudo layers like C<:utf8>.
+An example of a possible use might be:
+
+ open($fh,...)
+ ...
+ binmode($fh,":encoding(...)"); # next chunk is encoded
+ ...
+ binmode($fh,":pop"); # back to un-encoded
+
+A more elegant (and safer) interface is needed.
+
+=item :win32
+
+On Win32 platforms this I<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.
+
+=back
+
+=head2 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.
+
+=over 4
+
+=item :encoding
+
+Use C<: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 C<stdio>
+an C<:encoding> also enables C<:utf8>. See L<PerlIO::encoding>
+for more information.
+
+=item :via
+
+Use C<: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 L<PerlIO::via> for more information.
+
+=back
+
+=head2 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. C<:unix>)
+in the open call:
+
+ open($fh,"<:unix",$path)
+
+=head2 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 C<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 (C<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 L<perlrun/PERLIO>.
+
+=head2 Querying the layers of filehandles
+
+The following returns the B<names> of the PerlIO layers on a filehandle.
+
+ my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".
+
+The layers are returned in the order an open() or binmode() call would
+use them. Note that the "default stack" depends on the operating
+system and on the Perl version, and both the compile-time and
+runtime configurations of Perl.
+
+The following table summarizes the default layers on UNIX-like and
+DOS-like platforms and depending on the setting of C<$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 C<output> argument:
+
+ my @layers = PerlIO::get_layers($fh, output => 1);
+
+(Usually the layers are identical on either side of a filehandle but
+for example with sockets there may be differences, or if you have
+been using the C<open> pragma.)
+
+There is no set_layers(), nor does get_layers() return a tied array
+mirroring the stack, or anything fancy like that. This is not
+accidental or unintentional. The PerlIO layer stack is a bit more
+complicated than just a stack (see for example the behaviour of C<:raw>).
+You are supposed to use open() and binmode() to manipulate the stack.
+
+B<Implementation details follow, please close your eyes.>
+
+The arguments to layers are by default returned in parentheses after
+the name of the layer, and certain layers (like C<utf8>) are not real
+layers but instead flags on real layers; to get all of these returned
+separately, use the optional C<details> argument:
+
+ my @layer_and_args_and_flags = PerlIO::get_layers($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 C<undef>), the third element the flags,
+the fourth element a name again, and so forth.
+
+B<You may open your eyes now.>
+
+=head1 AUTHOR
+
+Nick Ing-Simmons E<lt>nick@ing-simmons.netE<gt>
+
+=head1 SEE ALSO
+
+L<perlfunc/"binmode">, L<perlfunc/"open">, L<perlunicode>, L<perliol>,
+L<Encode>
+
+=cut