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man : PerlIO(3p)
PerlIO(3p) Perl Programmers Reference Guide PerlIO(3p)
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 addi-
tional PerlIO related functions.
The following layers are currently defined:
:unix
Lowest level layer which provides basic PerlIO opera-
tions 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
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"\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 mem-
ory 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.
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open(F, ">:utf8", "data.utf");
print F $out;
close(F);
open(F, "<:utf8", "data.utf");
$in = <F>;
close(F);
:bytes
This is the inverse of ":utf8" layer. It turns off the
flag on the layer below so that data read from it is
considered to be "octets" i.e. characters in range
0..255 only. Likewise on output perl will warn if a
"wide" character is written to a such a stream.
:raw
The ":raw" layer is defined as being identical to
calling "binmode($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 (previ-
ously 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 encod-
ing 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($fh,":raw:utf8",...)
will construct a "binary" stream, but then enable
UTF-8 translation.
:pop
A pseudo layer that removes the top-most layer. Gives
perl code a way to manipulate the layer stack. Should
be considered as experimental. Note that ":pop" only
works on real layers and will not undo the effects of
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pseudo layers like ":utf8". An example of a possible
use might be:
open($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 lay-
ers (and one example written in Perl using the latter)
come with the Perl distribution.
:encoding
Use ":encoding(ENCODING)" either in open() or bin-
mode() to install a layer that does transparently
character set and encoding transformations, for exam-
ple from Shift-JIS to Unicode. Note that under
"stdio" an ":encoding" also enables ":utf8". See Per-
lIO::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)
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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 bin-
mode() 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 $ENV{PERLIO}:
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PERLIO UNIX-like DOS-like
------ --------- --------
unset / "" unix perlio / stdio [1] unix crlf
stdio unix perlio / stdio [1] stdio
perlio unix perlio unix perlio
mmap unix mmap unix mmap
# [1] "stdio" if Configure found out how to do "fast stdio" (depends
# on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"
By default the layers from the input side of the filehan-
dle is returned, to get the output side use the optional
"output" argument:
my @layers = PerlIO::get_layers($fh, output => 1);
(Usually the layers are identical on either side of a
filehandle but for example with sockets there may be dif-
ferences, 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 Per-
lIO 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 paren-
thesis 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 lay-
ers: 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 <nickATing-simmons.net>
SEE ALSO
"binmode" in perlfunc, "open" in perlfunc, perlunicode,
perliol, Encode
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