package PerlIO;
+our $VERSION = '1.01';
+
# Map layer name to package that defines it
-my %alias = (encoding => 'Encode');
+our %alias;
sub import
{
1;
__END__
+
+=head1 NAME
+
+PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
+
+=head1 SYNOPSIS
+
+ open($fh,"<:crlf", "my.txt"); # portably open a text file for reading
+
+ 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
+
+Low level layer which calls C<read>, C<write> and C<lseek> etc.
+
+=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
+got straight to the operating system via the C library as usual.
+
+=item perlio
+
+This is a re-implementation of "stdio-like" buffering written as a
+PerlIO "layer". As such it will call whatever layer is below it for
+its operations.
+
+=item crlf
+
+A layer which does CRLF to "\n" translation distinguishing "text" and
+"binary" files in the manner of MS-DOS and similar operating systems.
+(It currently does I<not> mimic MS-DOS as far as treating of Control-Z
+as being an end-of-file marker.)
+
+=item 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");
+ $in = <F>;
+ close(F);
+
+=item bytes
+
+This is the inverse of C<:utf8> layer. It turns off the flag
+on the layer below so that data read from it is considered to
+be "octets" i.e. characters in range 0..255 only. Likewise
+on output perl will warn if a "wide" character is written
+to a such a stream.
+
+=item raw
+
+The C<:raw> layer is I<defined> as being identical to calling
+C<binmode($fh)> - the stream is made suitable for passing binary
+data i.e. each byte is passed as-is. The stream will still be
+buffered. Unlike earlier versions of perl C<:raw> is I<not> just the
+inverse of C<:crlf> - other layers which would affect the binary nature of
+the stream are also removed or disabled.
+
+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 poping 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-encocded
+
+A more elegant (and safer) interface is needed.
+
+=back
+
+=head2 Alternatives to raw
+
+To get a binary stream an alternate method is to use:
+
+ open($fh,"whatever")
+ binmode($fh);
+
+this has advantage of being backward compatible with how such things have
+had to be coded on some platforms for years.
+
+To get an un-buffered 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 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
+
+=head1 AUTHOR
+
+Nick Ing-Simmons E<lt>nick@ing-simmons.netE<gt>
+
+=head1 SEE ALSO
+
+L<perlfunc/"binmode">, L<perlfunc/"open">, L<perlunicode>, L<Encode>
+
+=cut
+
projects / p5sagit/p5-mst-13.2.git / blobdiff