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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
       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 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 Per-
       lIO::via:: namespace, it does not have to be fully quali-
       fied.  The PerlIO::via module will prefix the Per-
       lIO::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 subrou-
       tines.  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



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


       classes it is passed last.

       $class->PUSHED([$mode[,$fh]])
           Should return an object or the class, or -1 on fail-
           ure.  (Compare TIEHANDLE.)  The arguments are an
           optional mode string ("r", "w", "w+", ...) and a file-
           handle 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 bin-
           mode($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



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


           way to get lower layer to do open and then regain con-
           trol.

       $obj->FILENO($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 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



perl v5.8.5                 2002-11-06                          3





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


           out.

       $obj->EOF($fh)
           Optional. Returns end-of-file state. Default is func-
           tion 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



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


       become "c1")

           use PerlIO::via::Hex;
           open(my $fh, ">:via(Hex)", "foo.hex");

       and the following code will read the hexdump in and con-
       vert it on the fly back into bytes:

           open(my $fh, "<:via(Hex)", "foo.hex");
















































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