PerlIO - On demand loader for PerlIO layers and root of PerlIO::* name space
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 ....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:
Lowest level layer which provides basic PerlIO operations in terms of UNIX/POSIX numeric file descriptor calls (open(), read(), write(), lseek(), close()).
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.
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.
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.
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");
$in = <F>;
close(F);This is the inverse of the :utf8 layer. It turns off the flag on the layer below so that data read from it is considered to be "octets" i.e. characters in the range 0..255 only. Likewise on output perl will warn if a "wide" character is written to a such a stream.
The :raw layer is defined as being identical to calling binmode($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($fh,":raw:utf8",...)will construct a "binary" stream, but then enable UTF-8 translation.
A pseudo layer that removes the top-most layer. Gives perl code a way to manipulate the layer stack. Note that :pop only works on real layers and will not undo the effects of pseudo layers like :utf8. An example of a possible use might be:
open($fh,...)
...
binmode($fh,":encoding(...)"); # next chunk is encoded
...
binmode($fh,":pop"); # back to un-encodedA more elegant (and safer) interface is needed.
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.
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.
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