NAME¶
PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
SYNOPSIS¶
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);
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.
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()).
- :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.
- :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.
Based on the ":perlio" layer.
- :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.
( CAUTION: This layer does not validate byte sequences. For reading
input, you should instead use ":encoding(utf8)" instead of bare
":utf8".)
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");
$in = <F>;
close(F);
- :bytes
- This is the inverse of the ":utf8" layer. It turns off the flag
on the layer below so that data read from it is considered to be
"octets" i.e. characters in the range 0..255 only. Likewise on
output perl will warn if a "wide" character is written to a such
a stream.
- :raw
- The ":raw" layer is defined as being identical to calling
"binmode($fh)" - the stream is made suitable for passing binary
data, i.e. each byte is passed as-is. The stream will still be buffered.
In Perl 5.6 and some books the ":raw" layer (previously sometimes
also referred to as a "discipline") 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.
The implementation of ":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 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($fh,":raw:utf8",...)
will construct a "binary" stream, but then enable UTF-8
translation.
- :pop
- A pseudo layer that removes the top-most layer. Gives perl code a way to
manipulate the layer stack. Note that ":pop" only works on real
layers and will not undo the effects of pseudo layers like
":utf8". An example of a possible use might be:
open($fh,...)
...
binmode($fh,":encoding(...)"); # next chunk is encoded
...
binmode($fh,":pop"); # back to un-encoded
A more elegant (and safer) interface is needed.
- :win32
- 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.
Custom Layers¶
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.
- :encoding
- Use ":encoding(ENCODING)" 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 "stdio" 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()".
- :via
- Use ":via(MODULE)" 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.
Alternatives to raw¶
To get a binary stream an alternate method is to use:
open($fh,"whatever")
binmode($fh);
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. ":unix")
in the open call:
open($fh,"<:unix",$path)
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 "Configure" 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 ("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
For the various values of PERLIO see "PERLIO" in perlrun.
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. 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 $ENV{PERLIO}:
PERLIO UNIX-like DOS-like
------ --------- --------
unset / "" unix perlio / stdio [1] unix crlf
stdio unix perlio / stdio [1] stdio
perlio unix perlio unix perlio
# [1] "stdio" if Configure found out how to do "fast stdio" (depends
# on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"
By default the layers from the input side of the filehandle are returned; to get
the output side, use the optional "output" argument:
my @layers = PerlIO::get_layers($fh, output => 1);
(Usually the layers are identical on either side of a filehandle but for example
with sockets there may be differences, or if you have been using the
"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 ":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>
SEE ALSO¶
"binmode" in perlfunc, "open" in perlfunc, perlunicode,
perliol, Encode