PerlIO::via man page on MirBSD

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ext::PerlIO::via:Perl(Programmers Refereext::PerlIO::via::via(3p)

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 pro-
     gramming 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 quali-
     fied.  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

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ext::PerlIO::via:Perl(Programmers Refereext::PerlIO::via::via(3p)

     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 con-
	 trol.

     $obj->BINMODE([,$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 sub-
	 ject 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])

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ext::PerlIO::via:Perl(Programmers Refereext::PerlIO::via::via(3p)

	 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 deter-
	 mined.

     $obj->UNREAD($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 tem-
	 porary 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)

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ext::PerlIO::via:Perl(Programmers Refereext::PerlIO::via::via(3p)

	 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;
	 }

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ext::PerlIO::via:Perl(Programmers Refereext::PerlIO::via::via(3p)

	 1;

     the following code opens up an output handle that will con-
     vert 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		   2005-02-05				5

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