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RedHat 9 (Linux i386) - man page for perlio::via (redhat section 3pm)

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 exam-
       ple 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 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 docu-
       mentation 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 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 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 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 determined.

       $obj->UNREAD($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 tempo-
	   rary 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 v5.8.0				    2002-06-01				 PerlIO::via(3pm)


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