PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
open($fh,"<:crlf", "my.txt"); # portably open a text file for reading
open($fh,"<","his.jpg"); # portably open a binary file for reading binmode($fh);
Shell: PERLIO=perlio perl ....
When an undefined layer 'foo' is encountered in an
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
PerlIO package is a place holder for additional
PerlIO related functions.
The following layers are currently defined:
ftelletc. 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.
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"); $in = <F>; close(F);
:utf8layer. It turns off the flag on the layer below so that data read from it is considered to be "octets" i.e. characters in range 0..255 only. Likewise on output perl will warn if a "wide" character is written to a such a stream.
:raw layer is defined as being identical to calling
binmode($fh) - the stream is made suitable for passing binary
data i.e. each byte is passed as-is. The stream will still be
buffered. Unlike earlier versions of perl
:raw is not just the
:crlf - other layers which would affect the binary nature of
the stream are also removed or disabled.
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 poping 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.
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
:pop only works on real layers
and will not undo the effects of pseudo layers like
An example of a possible use might be:
open($fh,...) ... binmode($fh,":encoding(...)"); # next chunk is encoded ... binmode($fh,":pop"); # back to un-encocded
A more elegant (and safer) interface is needed.
To get a binary stream an alternate method is to use:
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.
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 :
(The low level "unix" layer may be replaced by a platform specific low level layer.)
Configure found out how to do "fast" IO using system's
stdio, then the default layers are :
Otherwise the default layers are
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
Nick Ing-Simmons <email@example.com>
perlfunc/"binmode", perlfunc/"open", perlunicode, Encode