perlio::via(3)

NAME

PerlIO::via - Helper class for PerlIO layers implemented
in perl

SYNOPSIS

use PerlIO::via::Layer;
open($fh,"<:via(Layer)",...);
use Some::Other::Package;
open($fh,">:via(Some::Other::Package)",...);

DESCRIPTION

The PerlIO::via module allows you to develop PerlIO layers
in Perl, without having to go into the nitty gritty of
programming C with XS as the interface to Perl.

One example module, PerlIO::via::QuotedPrint, is included with Perl 5.8.0, and more example modules are available from CPAN, such as PerlIO::via::StripHTML and Per lIO::via::Base64. The PerlIO::via::StripHTML module for instance, allows you to say:: use PerlIO::via::StripHTML;
open( my $fh, "<:via(StripHTML)", "index.html" );
my @line = <$fh>;
to obtain the text of an HTML-file in an array with all the HTML-tags automagically removed.
Please note that if the layer is created in the Per
lIO::via:: namespace, it does not have to be fully quali
fied. The PerlIO::via module will prefix the Per
lIO::via:: namespace if the specified modulename does not
exist as a fully qualified module name.

EXPECTED METHODS

To create a Perl module that implements a PerlIO layer in
Perl (as opposed to in C using XS as the interface to
Perl), you need to supply some of the following subrou
tines. It is recommended to create these Perl modules in
the PerlIO::via:: namespace, so that they can easily be
located on CPAN and use the default namespace feature of
the PerlIO::via module itself.

Please note that this is an area of recent development in
Perl and that the interface described here is therefore
still subject to change (and hopefully will have better
documentation and more examples).

In the method descriptions below $fh will be a reference
to a glob which can be treated as a perl file handle. It
refers to the layer below. $fh is not passed if the layer
is at the bottom of the stack, for this reason and to
maintain some level of "compatibility" with TIEHANDLE
classes it is passed last.

$class->PUSHED([$mode[,$fh]]): Should return an object or the class, or -1 on fail
ure. (Compare TIEHANDLE.) The arguments are an
optional mode string ("r", "w", "w+", ...) and a file
handle for the PerlIO layer below. Mandatory.; When layer is pushed as part of an "open" call,
"PUSHED" will be called before the actual open occurs
whether than be via "OPEN", "SYSOPEN", "FDOPEN" or by
letting lower layer do the open.
$obj->POPPED([$fh]): Optional - layer is about to be removed.
$obj->OPEN($path,$mode[,$fh]): Optional - if not present lower layer does open. If
present called for normal opens after layer is pushed.
This function is subject to change as there is no easy
way to get lower layer to do open and then regain con
trol.
$obj->BINMODE([,$fh]): Optional - if not available layer is popped on bin
mode($fh) or when ":raw" is pushed. If present it
should return 0 on success -1 on error and undef to
pop the layer.
$obj->FDOPEN($fd[,$fh]): Optional - if not present lower layer does open. If
present called for opens which pass a numeric file
descriptor after layer is pushed. This function is
subject to change as there is no easy way to get lower
layer to do open and then regain control.
$obj->SYSOPEN($path,$imode,$perm,[,$fh]): Optional - if not present lower layer does open. If
present called for sysopen style opens which pass a
numeric mode and permissions after layer is pushed.
This function is subject to change as there is no easy
way to get lower layer to do open and then regain con
trol.
$obj->FILENO($fh): Returns a numeric value for Unix-like file descriptor.
Return -1 if there isn't one. Optional. Default is
fileno($fh).
$obj->READ($buffer,$len,$fh): Returns the number of octets placed in $buffer (must
be less than or equal to $len). Optional. Default is
to use FILL instead.
$obj->WRITE($buffer,$fh): Returns the number of octets from buffer that have
been sucessfully written.
$obj->FILL($fh): Should return a string to be placed in the buffer.
Optional. If not provided must provide READ or reject
handles open for reading in PUSHED.
$obj->CLOSE($fh): Should return 0 on success, -1 on error. Optional.
$obj->SEEK($posn,$whence,$fh): Should return 0 on success, -1 on error. Optional.
Default is to fail, but that is likely to be changed
in future.
$obj->TELL($fh): Returns file postion. Optional. Default to be deter
mined.
$obj->UNREAD($buffer,$fh): Returns the number of octets from buffer that have
been sucessfully saved to be returned on future
FILL/READ calls. Optional. Default is to push data
into a temporary layer above this one.
$obj->FLUSH($fh): Flush any buffered write data. May possibly be called
on readable handles too. Should return 0 on success,
-1 on error.
$obj->SETLINEBUF($fh): Optional. No return.
$obj->CLEARERR($fh): Optional. No return.
$obj->ERROR($fh): Optional. Returns error state. Default is no error
until a mechanism to signal error (die?) is worked
out.
$obj->EOF($fh): Optional. Returns end-of-file state. Default is func
tion of return value of FILL or READ.

EXAMPLES

Check the PerlIO::via:: namespace on CPAN for examples of
PerlIO layers implemented in Perl. To give you an idea
how simple the implementation of a PerlIO layer can look,
as simple example is included here.

Example - a Hexadecimal Handle

Given the following module, PerlIO::via::Hex :: package PerlIO::via::Hex;; sub PUSHED
{
my ($class,$mode,$fh) = @_;
# When writing we buffer the data
my $buf = '';
return bless uf,$class;; }; sub FILL
{
my ($obj,$fh) = @_;
my $line = <$fh>;
return (defined $line) ? pack("H*", $line) : undef;; }; sub WRITE
{
my ($obj,$buf,$fh) = @_;
$$obj .= unpack("H*", $buf);
return length($buf);; }; sub FLUSH
{
my ($obj,$fh) = @_;
print $fh $$obj or return -1;
$$obj = '';
return 0;; }; 1;
the following code opens up an output handle that will convert any output to hexadecimal dump of the output bytes: for example "A" will be converted to "41" (on ASCII-based machines, on EBCDIC platforms the "A" will become "c1"): use PerlIO::via::Hex;
open(my $fh, ">:via(Hex)", "foo.hex");
and the following code will read the hexdump in and con vert it on the fly back into bytes:: open(my $fh, "<:via(Hex)", "foo.hex");

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