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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 ....
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:
":perlio" will insert a ":unix" layer below itself to do low level IO.
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.
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()".
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");
$in = <F>;
close(F);
In Perl 5.6 and some books the ":raw" layer (previously sometimes also referred to as a ``discipline'') is documented as the inverse of the ":crlf" layer. That is no longer the case - other layers which would alter binary nature of the stream are also disabled. If you want UNIX line endings on a platform that normally does CRLF translation, but still want UTF-8 or encoding defaults the appropriate thing to do is to add ":perlio" to PERLIO environment variable.
The implementation of ":raw" is as a pseudo-layer which when ``pushed'' pops itself and then any layers which do not declare themselves as suitable for binary data. (Undoing :utf8 and :crlf are implemented by clearing flags rather than popping layers but that is an implementation detail.)
As a consequence of the fact that ":raw" normally pops layers it usually only makes sense to have it as the only or first element in a layer specification. When used as the first element it provides a known base on which to build e.g.
open($fh,":raw:utf8",...)
will construct a ``binary'' stream, but then enable UTF-8 translation.
open($fh,...)
...
binmode($fh,":encoding(...)"); # next chunk is encoded
...
binmode($fh,":pop"); # back to un-encoded
A more elegant (and safer) interface is needed.
open($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",$path)
unix crlf
(The low level ``unix'' layer may be replaced by a platform specific low level layer.)
Otherwise if "Configure" found out how to do ``fast'' IO using system's stdio, then the default layers are:
unix stdio
Otherwise the default layers are
unix perlio
These defaults may change once perlio has been better tested and tuned.
The default can be overridden by setting the environment variable PERLIO to a space separated list of layers ("unix" or platform low level layer is always pushed first).
This can be used to see the effect of/bugs in the various layers e.g.
cd .../perl/t PERLIO=stdio ./perl harness PERLIO=perlio ./perl harness
For the various value of PERLIO see ``PERLIO'' in perlrun.
my @layers = PerlIO::get_layers($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 $ENV{PERLIO}:
PERLIO UNIX-like DOS-like ------ --------- -------- unset / "" unix perlio / stdio [1] unix crlf stdio unix perlio / stdio [1] stdio perlio unix perlio unix perlio mmap unix mmap unix mmap
# [1] "stdio" if Configure found out how to do "fast stdio" (depends # on the stdio implementation) and in Perl 5.8, otherwise "unix perlio"
By default the layers from the input side of the filehandle is returned, to get the output side use the optional "output" argument:
my @layers = PerlIO::get_layers($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 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.
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