=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.
+
+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 internal encoding of
characters. (Which really is UTF-8 on ASCII machines, but is
$in = <F>;
close(F);
-=item bytes
+=item :bytes
This is the inverse of C<:utf8> layer. It turns off the flag
on the layer below so that data read from it is considered to
on output perl will warn if a "wide" character is written
to a such a stream.
-=item raw
+=item :raw
The C<:raw> layer is I<defined> as being identical to calling
C<binmode($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 in the 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.
+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
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
encodings as defaults.
The layers that it makes sense to include in the PERLIO environment
-variable are summarised below. For more details see L<PerlIO>.
+variable are
briefly summarised below. For more details see L<PerlIO>.
=over 8
=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.
-
-(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.
+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 :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()>.
+using that as PerlIO's "buffer".
=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.
+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 (typically C<:unix>).
=item :pop
An experimental pseudolayer that removes the topmost layer.
-Use with the same care as is reserved for nitroglyserin.
+Use with the same care as is reserved for nitroglycerin.
=item :raw
pass each byte as-is without any translation. In particular CRLF
translation, and/or :utf8 intuited from locale are disabled.
-Arranges for all accesses go straight to the lowest buffered layer provided
-by the configration. That is it strips off any layers above that layer.
-
-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.
+Unlike in the 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.
=item :stdio
=item :unix
-Lowest level layer which provides basic PerlIO operations in terms of
-UNIX/POSIX numeric file descriptor calls
-C<open(), read(), write(), lseek(), close()>
+Low level layer which calls C<read>, C<write> and C<lseek> etc.
=item :utf8
A pseudolayer that turns on a flag on the layer below to tell perl
-that data sent to the stream should be converted to perl internal
-"utf8" form and that data from the stream should be considered as so
-encoded. On ASCII based platforms the encoding is UTF-8 and on EBCDIC
-platforms UTF-EBCDIC. May be useful in PERLIO environment variable to
-make UTF-8 the default. (To turn off that behaviour use C<:bytes>
-layer.)
+that output should be in utf8 and that input should be regarded as
+already in utf8 form. May be useful in PERLIO environment
+variable to make UTF-8 the default. (To turn off that behaviour
+use C<:bytes> layer.)
=item :win32
This release uses C<unix> as the bottom layer on Win32 and so still uses C
compiler's numeric file descriptor routines. There is an experimental native
-C<win32> layer which is expected to be enhanced and should eventually replace
-the C<unix> layer.
+C<win32> layer which is expected to be enhanced and should eventually be
+the default under Win32.
=item PERLIO_DEBUG
projects / platform / upstream / perl.git / commitdiff