PerlIO.pdf

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Perl version 5.10.0 documentation - PerlIO
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");
binmode($fh);
Shell:
PERLIO=perlio perl ....
# portably open a binary file for reading
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.
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Perl version 5.10.0 documentation - PerlIO
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
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Perl version 5.10.0 documentation - PerlIO
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($fh,":encoding(...)");
...
binmode($fh,":pop");
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.
# next chunk is encoded
# back to un-encoded
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
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Perl version 5.10.0 documentation - PerlIO
(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
$ENV{PERLIO}:
PERLIO
UNIX-like
------
---------
unset / "" unix perlio / stdio [1]
stdio
unix perlio / stdio [1]
perlio
unix perlio
mmap
unix mmap
DOS-like
--------
unix crlf
stdio
unix perlio
unix mmap
# [1] "stdio" if Configure found out how to do "fast stdio" (depends
# on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"
By default the layers from the input side of the filehandle is returned, to get the output side use the
optional
output
argument:
my @layers = PerlIO::get_layers($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
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Perl version 5.10.0 documentation - PerlIO
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@ing-simmons.net>
SEE ALSO
"binmode" in perlfunc, "open" in perlfunc, perlunicode, perliol, Encode
http://perldoc.perl.org
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