PerlIO 0.02

PerlIO is a Perl module created to load on demand 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 ....

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 perls 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 cant 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 processs address space, and then using that as PerlIOs "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 perls 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 = ;
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 (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 this experimental layer uses native "handle" IO rather than unix-like numeric file descriptor layer. Known to be buggy as of perl 5.8