


ext::PerlIO-via::Perl3Programmers Referenext::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
       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



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ext::PerlIO-via::Perl3Programmers Referenext::PerlIO-via::via(3p)


       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.

       $obj->UTF8($bellowFlag,[$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 $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 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



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ext::PerlIO-via::Perl3Programmers Referenext::PerlIO-via::via(3p)


           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.





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ext::PerlIO-via::Perl3Programmers Referenext::PerlIO-via::via(3p)


       $obj->ERROR($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



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ext::PerlIO-via::Perl3Programmers Referenext::PerlIO-via::via(3p)


       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");














































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