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PerlIO(3perl) Perl Programmers Reference Guide PerlIO(3perl)
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 got 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 likes to be one of its kind: it silently ignores attempts
to be pushed into the layer stack more than once.
It currently does 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 ":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 ":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 ":perlio" layer.
:mmap
A layer which implements "reading" of files by using "mmap()" to make (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 ":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 platform does not support "mmap()".
: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.
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");
$in = <F>;
close(F);
Note that this layer does not validate byte sequences. For reading input, using
":encoding(utf8)" instead of bare ":utf8", is strongly recommended.
:bytes
This is the inverse of ":utf8" layer. It turns off the flag on the layer below so that
data read from it is considered to be "octets" i.e. characters in range 0..255 only.
Likewise on output perl will warn if a "wide" character is written to a such a stream.
:raw
The ":raw" layer is defined as being identical to calling "binmode($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 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 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($fh,":raw:utf8",...)
will construct a "binary" stream, but then enable UTF-8 translation.
:pop
A pseudo layer that removes the top-most layer. Gives perl code a way to manipulate
the layer stack. Should be considered as experimental. Note that ":pop" only works on
real layers and will not undo the effects of pseudo layers like ":utf8". An example
of a possible use might be:
open($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 native "handle" IO rather than 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 does
transparently 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.
: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 advantage of being backward compatible with how such things have had to be coded
on some platforms for years.
To get an un-buffered 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 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 value 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 the $ENV{PERLIO}:
PERLIO UNIX-like DOS-like
------ --------- --------
unset / "" unix perlio / stdio [1] unix crlf
stdio unix perlio / stdio [1] stdio
perlio unix perlio unix perlio
mmap unix mmap unix mmap
# [1] "stdio" if Configure found out how to do "fast stdio" (depends
# on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"
By default the layers from the input side of the filehandle is returned, to get the output
side use the optional "output" argument:
my @layers = PerlIO::get_layers($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 parenthesis 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 AT ing-simmons.net>
SEE ALSO
"binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode
perl v5.10.0 2011-06-30 PerlIO(3perl)
Generated by $Id: phpMan.php,v 4.49 2006/02/26 13:18:18 chedong Exp $ Author: Che Dong
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2012-05-25 00:38 @38.107.179.238 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)