PerlIO::via - phpMan

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 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 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");