package PerlIO;
-our $VERSION = '1.01';
+our $VERSION = '1.04';
# Map layer name to package that defines it
our %alias;
=head1 SYNOPSIS
- open($fh,"<:crlf", "my.txt"); # portably open a text file for reading
+ 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);
=over 4
-=item unix
+=item :unix
-Low level layer which calls C<read>, C<write> and C<lseek> etc.
+Lowest level layer which provides basic PerlIO operations in terms of
+UNIX/POSIX numeric file descriptor calls
+(open(), read(), write(), lseek(), close()).
-=item stdio
+=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
+=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.
+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.
-=item crlf
+C<:perlio> will insert a C<:unix> layer below itself to do low level IO.
-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 :crlf
-=item utf8
+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.
-Declares that the stream accepts perl's internal encoding of
+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 (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 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 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
$in = <F>;
close(F);
-=item bytes
+Note that this layer does not validate byte sequences. For reading
+input, using C<:encoding(utf8)> instead of bare C<:utf8>, is strongly
+recommended.
+
+=item :bytes
This is the inverse of C<:utf8> layer. It turns off the flag
on the layer below so that data read from it is considered to
on output perl will warn if a "wide" character is written
to a such a stream.
-=item raw
+=item :raw
The C<:raw> layer is I<defined> as being identical to calling
-C<binmode($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.
+C<binmode($fh)> - the stream is made suitable for passing binary data
+i.e. each byte is passed as-is. The stream will still be
+buffered.
+
+In Perl 5.6 and some books the C<:raw> layer (previously sometimes also
+referred to as a "discipline") is documented as the inverse of the
+C<:crlf> layer. That is no longer the case - other layers which would
+alter binary nature of the stream are also disabled. If you want UNIX
+line endings on a platform that normally does CRLF translation, but still
+want UTF-8 or encoding defaults the appropriate thing to do is to add
+C<:perlio> to PERLIO environment variable.
The implementation of C<:raw> is as a pseudo-layer which when "pushed"
pops itself and then any layers which do not declare themselves as suitable
will construct a "binary" stream, but then enable UTF-8 translation.
-=item pop
+=item :pop
A pseudo layer that removes the top-most layer. Gives perl code
a way to manipulate the layer stack. Should be considered
...
binmode($fh,":encoding(...)"); # next chunk is encoded
...
- binmode($fh,":pop"); # back to un-encocded
+ binmode($fh,":pop"); # back to un-encoded
A more elegant (and safer) interface is needed.
+=item :win32
+
+On Win32 platforms this I<experimental> layer uses native "handle" IO
+rather than unix-like numeric file descriptor layer. Known to be
+buggy as of perl 5.8.2.
+
=back
=head2 Custom Layers
Use C<:encoding(ENCODING)> either in open() or binmode() to install
a layer that does transparently character set and encoding transformations,
-for example from Shift-JIS to Unicode. Note that an C<:encoding> also
-enables C<:utf8>. See L<PerlIO::encoding> for more information.
+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
level layer.)
Otherwise if C<Configure> found out how to do "fast" IO using system's
-stdio, then the default layers are :
+stdio, then the default layers are:
unix stdio
PERLIO=stdio ./perl harness
PERLIO=perlio ./perl harness
-=head2 Querying the layers of filehandle
+For the various value 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);
+ my @layers = PerlIO::get_layers($fh); # Or FH, *FH, "FH".
The layers are returned in the order an open() or binmode() call would
-use them. Note that the stack begings (normally) from C<stdio>, the
-platform specific low-level I/O (like C<unix>) is not part of the stack.
+use them. Note that the "default stack" depends on the operating
+system and on the Perl version, and both the compile-time and
+runtime configurations of Perl.
+
+The following table summarizes the default layers on UNIX-like and
+DOS-like platforms and depending on the setting of the C<$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 is
returned, to get the output side use the optional C<output> argument:
- my @layers = PerlIO::get_layers(FH, output => 1);
+ my @layers = PerlIO::get_layers($fh, output => 1);
(Usually the layers are identical on either side of a filehandle but
-for example with sockets there may be differences.)
+for example with sockets there may be differences, or if you have
+been using the C<open> pragma.)
+
+There is no set_layers(), nor does get_layers() return a tied array
+mirroring the stack, or anything fancy like that. This is not
+accidental or unintentional. The PerlIO layer stack is a bit more
+complicated than just a stack (see for example the behaviour of C<:raw>).
+You are supposed to use open() and binmode() to manipulate the stack.
B<Implementation details follow, please close your eyes.>
The arguments to layers are by default returned in parenthesis after
the name of the layer, and certain layers (like C<utf8>) are not real
layers but instead flags on real layers: to get all of these returned
-separately use the optional C<separate> argument:
+separately use the optional C<details> argument:
- my @layer_and_args_and_flags = PerlIO::get_layers(FH, details => 1);
+ my @layer_and_args_and_flags = PerlIO::get_layers($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
L<Encode>
=cut
-
https://perl5.git.perl.org / perl5.git / blobdiff