PerlIO::via(3pm) Perl Programmers Reference Guide PerlIO::via(3pm)
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 Per-lIO::via::StripHTML PerlIO::via::StripHTML
lIO::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 module-name modulename
name 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 cre-ate create
ate 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 exam-ples). examples).
ples).
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 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.
$obj->UTF8($bellowFlag,[$fh])
Optional - if present it will be called immediately after PUSHED has returned. It should return
true value if the layer expects data to be UTF-8 encoded. If it returns true result is as if
caller had done
":via(YourClass):utf8"
If not present of it it returns false, then stream is left with flag clear. The $bellowFlag
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 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 control.
$obj->BINMODE([,$fh])
Optional - if not available layer is popped on binmode($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 control.
$obj->FILENO($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 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 postion. 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 function 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 $buf = '';
return bless \$buf,$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 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");
perl v5.8.8 2001-09-21 PerlIO::via(3pm)
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