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

NAME

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

SYNOPSIS

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

         # append UTF-8 encoded text
         open(my $fh, ">>:encoding(UTF-8)", "some.log")
           or die "open failed: $!";

         # portably open a binary file for reading
         open(my $fh, "<", "his.jpg") or die "open failed: $!";
         binmode($fh) or die "binmode failed: $!";

         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.

Layers
Generally speaking, PerlIO layers (previously sometimes referred to as "disciplines") are
an ordered stack applied to a filehandle (specified as a space- or colon-separated list,
conventionally written with a leading colon). Each layer performs some operation on any
input or output, except when bypassed such as with "sysread" or "syswrite". Read
operations go through the stack in the order they are set (left to right), and write
operations in the reverse order.

There are also layers which actually just set flags on lower layers, or layers that modify
the current stack but don't persist on the stack themselves; these are referred to as
pseudo-layers.

When opening a handle, it will be opened with any layers specified explicitly in the
open() call (or the platform defaults, if specified as a colon with no following layers).

If layers are not explicitly specified, the handle will be opened with the layers
specified by the ${^OPEN} variable (usually set by using the open pragma for a lexical
scope, or the "-C" command-line switch or "PERL_UNICODE" environment variable for the main
program scope).

If layers are not specified in the open() call or "${^OPEN}" variable, the handle will be
opened with the default layer stack configured for that architecture; see "Defaults and
how to override them".

Some layers will automatically insert required lower level layers if not present; for
example ":perlio" will insert ":unix" below itself for low level IO, and ":encoding" will
insert the platform defaults for buffered IO.

The "binmode" function can be called on an opened handle to push additional layers onto
the stack, which may also modify the existing layers. "binmode" called with no layers
will remove or unset any existing layers which transform the byte stream, making the
handle suitable for binary data.

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()). It is used
even on non-Unix architectures, and most other layers operate on top of it.

: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. This layer implements both low level IO and
buffering, but is rarely used on modern architectures.

: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.

On DOS/Windows like architectures where this layer is part of the defaults, it also
acts like the ":perlio" layer, and removing the CRLF translation (such as with ":raw")
will only unset the CRLF translation flag. Since Perl 5.14, you can also apply
another ":crlf" layer later, such as when the CRLF translation must occur after an
encoding layer. On other architectures, it is a mundane CRLF translation layer and
can be added and removed normally.

# translate CRLF after encoding on Perl 5.14 or newer
binmode $fh, ":raw:encoding(UTF-16LE):crlf"
or die "binmode failed: $!";

:utf8
Pseudo-layer that declares that the stream accepts Perl's internal upgraded encoding
of characters, which is approximately UTF-8 on ASCII machines, but UTF-EBCDIC on
EBCDIC machines. This allows any character Perl can represent to be read from or
written to the stream.

This layer (which actually sets a flag on the preceding layer, and is implicitly set
by any ":encoding" layer) does not translate or validate byte sequences. It instead
indicates that the byte stream will have been arranged by other layers to be provided
in Perl's internal upgraded encoding, which Perl code (and correctly written XS code)
will interpret as decoded Unicode characters.

CAUTION: Do not use this layer to translate from UTF-8 bytes, as invalid UTF-8 or
binary data will result in malformed Perl strings. It is unlikely to produce invalid
UTF-8 when used for output, though it will instead produce UTF-EBCDIC on EBCDIC
systems. The ":encoding(UTF-8)" layer (hyphen is significant) is preferred as it will
ensure translation between valid UTF-8 bytes and valid Unicode characters.

:bytes
This is the inverse of the ":utf8" pseudo-layer. It turns off the flag on the layer
below so that data read from it is considered to be Perl's internal downgraded
encoding, thus interpreted as the native single-byte encoding of Latin-1 or EBCDIC.
Likewise on output Perl will warn if a "wide" character (a codepoint not in the range
0..255) is written to a such a stream.

This is very dangerous to push on a handle using an ":encoding" layer, as such a layer
assumes to be working with Perl's internal upgraded encoding, so you will likely get a
mangled result. Instead use ":raw" or ":pop" to remove encoding layers.

:raw
The ":raw" pseudo-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 (but this was not always true before Perl 5.14).

In Perl 5.6 and some books the ":raw" layer is documented as the inverse of the
":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 ":perlio" to the PERLIO environment variable, or
open the handle explicitly with that layer, to replace the platform default of
":crlf".

The implementation of ":raw" is as a pseudo-layer which when "pushed" pops itself and
then any layers which would modify the binary data stream. (Undoing ":utf8" and
":crlf" may be 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(my $fh,">:raw:encoding(UTF-8)",...)
or die "open failed: $!";

will construct a "binary" stream regardless of the platform defaults, 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. Note that ":pop" only works on real layers and will not undo the
effects of pseudo-layers or flags like ":utf8". An example of a possible use might
be:

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

A more elegant (and safer) interface is needed.

Custom Layers
It is possible to write custom layers in addition to the above builtin ones, both in C/XS
and Perl, as a module named "PerlIO::<layer name>". Some custom layers come with the Perl
distribution.

:encoding
Use ":encoding(ENCODING)" to transparently do character set and encoding
transformations, for example from Shift-JIS to Unicode. Note that 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()". See
PerlIO::mmap for more information.

:via
":via(MODULE)" allows a transformation to be applied by an arbitrary Perl module, for
example compression / decompression, encryption / decryption. See PerlIO::via for
more information.

:scalar
A layer implementing "in memory" files using scalar variables, automatically used in
place of the platform defaults for IO when opening such a handle. As such, the scalar
is expected to act like a file, only containing or storing bytes. See PerlIO::scalar
for more information.

Alternatives to raw
To get a binary stream an alternate method is to use:

open(my $fh,"<","whatever") or die "open failed: $!";
binmode($fh) or die "binmode failed: $!";

This has the advantage of being backward compatible with older versions of Perl that did
not use PerlIO or where ":raw" was buggy (as it was before Perl 5.14).

To get an unbuffered stream specify an unbuffered layer (e.g. ":unix") in the open call:

open(my $fh,"<:unix",$path) or die "open failed: $!";

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

Otherwise if "Configure" found out how to do "fast" IO using the system's stdio (not
common on modern architectures), then the default layers are:

:stdio

Otherwise the default layers are

:unix:perlio

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 default can be overridden
by setting the environment variable PERLIO to a space or colon separated list of layers,
however this cannot be used to set layers that require loading modules like ":encoding".

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.

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 :stdio :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, else ":unix:perlio"

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, and
without colons.

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.)

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>

NAME

SYNOPSIS

DESCRIPTION

AUTHOR

SEE ALSO