PerlIO::via(3perl) Perl Programmers Reference Guide PerlIO::via(3perl)
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 PerlIO::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 PerlIO::via:: namespace,
it does not have to be fully qualified. The PerlIO::via module will
prefix the PerlIO::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 subroutines. 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 failure. (Compare
TIEHANDLE.) The arguments are an optional mode string ("r", "w",
"w+", ...) and a filehandle for the PerlIO layer below. Mandatory.
When the layer is pushed as part of an "open" call, "PUSHED" will be
called before the actual open occurs, whether that be via "OPEN",
"SYSOPEN", "FDOPEN" or by letting a lower layer do the open.
$obj->POPPED([$fh])
Optional - called when the layer is about to be removed.
$obj->UTF8($belowFlag,[$fh])
Optional - if present it will be called immediately after PUSHED has
returned. It should return a true value if the layer expects data to
be UTF-8 encoded. If it returns true, the result is as if the caller
had done
":via(YourClass):utf8"
If not present or if it returns false, then the stream is left with
the UTF-8 flag clear. The $belowFlag argument will be true if there
is a layer below and that layer was expecting UTF-8.
$obj->OPEN($path,$mode,[$fh])
Optional - if not present a lower layer does the open. If present,
called for normal opens after the layer is pushed. This function is
subject to change as there is no easy way to get a lower layer to do
the open and then regain control.
$obj->BINMODE([$fh])
Optional - if not present the layer is popped on binmode($fh) or
when ":raw" is pushed. If present it should return 0 on success, -1
on error, or undef to pop the layer.
$obj->FDOPEN($fd,[$fh])
Optional - if not present a lower layer does the open. If present,
called after the layer is pushed for opens which pass a numeric file
descriptor. This function is subject to change as there is no easy
way to get a lower layer to do the open and then regain control.
$obj->SYSOPEN($path,$imode,$perm,[$fh])
Optional - if not present a lower layer does the open. If present,
called after the layer is pushed for sysopen style opens which pass
a numeric mode and permissions. This function is subject to change
as there is no easy way to get a lower layer to do the open and then
regain control.
$obj->FILENO($fh)
Returns a numeric value for a Unix-like file descriptor. Returns -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 undef to
indicate an error or a non-negative integer less than or equal to
the minimum of $len and the length of the updated $buffer).
Optional. Default is to use FILL instead.
$obj->WRITE($buffer,$fh)
Returns the number of octets from $buffer that have been
successfully 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 position. Optional. Default to be determined.
$obj->UNREAD($buffer,$fh)
Returns the number of octets from $buffer that have been
successfully 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 a function of the
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, a 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 $buf = '';
return bless \$buf,$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 a 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 convert it on the
fly back into bytes:
open(my $fh, "<:via(Hex)", "foo.hex");
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