PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
- open($fh,"<:crlf", "my.txt"); # support platform-native and CRLF text files
- open($fh,"<","his.jpg"); # portably open a binary file for reading
- PERLIO=perlio perl ....
- use PerlIO 'foo';
The perl code in PerlIO.pm then attempts to locate a layer by doing
- require PerlIO::foo;
package is a place holder for additional
PerlIO related functions.
The following layers are currently defined:
Lowest level layer which provides basic PerlIO operations in terms of UNIX/POSIX numeric file descriptor calls (open(), read(), write(), lseek(), close()).
Layer which calls
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.
A from scratch implementation of buffering for PerlIO. Provides fast
access to the buffer for
which implements perl's readline/<>
and in general attempts to minimize data copying.
will insert a
layer below itself to do low level IO.
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.
Based on the
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.
Note that this layer does not validate byte sequences. For reading
instead of bare
This is the inverse of the
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.
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
In Perl 5.6 and some books the
layer (previously sometimes also
referred to as a "discipline") is documented as the inverse of the
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
to the PERLIO environment variable.
The implementation of
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
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.
will construct a "binary" stream, but then enable UTF-8 translation.
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
only works on real layers
and will not undo the effects of pseudo layers like
An example of a possible use might be:
A more elegant (and safer) interface is needed.
On Win32 platforms this 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.
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.
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
. See PerlIO::encoding
for more information.
A layer which implements "reading" of files by using
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
-able revert to behaving like the
layer. Writes also behave like the
needs extra house-keeping (to extend the file) which negates any advantage.
layer will not exist if the platform does not support
either in open() or binmode() to install a layer
that does whatever transformation (for example compression /
decompression, encryption / decryption) to the filehandle.
See PerlIO::via for more information.
To get a binary stream an alternate method is to use:
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.
in the open call:
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.)
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 (
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 PERLIO in perlrun.
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. 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
- PERLIO UNIX-like DOS-like
- ------ --------- --------
- unset / "" unix perlio / stdio  unix crlf
- stdio unix perlio / stdio  stdio
- perlio unix perlio unix perlio
- #  "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
- my @layers = PerlIO::get_layers($fh, output => 1);
(Usually the layers are identical on either side of a filehandle but
for example with sockets there may be differences, or if you have
been using the
There is no set_layers(), nor does get_layers() return a tied array
mirroring the stack, or anything fancy like that. This is not
accidental or unintentional. The PerlIO layer stack is a bit more
complicated than just a stack (see for example the behaviour of
You are supposed to use open() and binmode() to manipulate the stack.
Implementation details follow, please close your eyes.
The arguments to layers are by default returned in parentheses after
the name of the layer, and certain layers (like
) are not real
layers but instead flags on real layers; to get all of these returned
separately, use the optional
- my @layer_and_args_and_flags = PerlIO::get_layers($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.
Nick Ing-Simmons <email@example.com>