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 example modules are available from CPAN, such as Per-
       lIO::via::StripHTML and PerlIO::via::Base64.  The Per-
       lIO::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:: names-
       pace, it does nnoott 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.

EEXXPPEECCTTEEDD MMEETTHHOODDSS
       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 Per-
       lIO::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 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 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 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 success-
           fully 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 success-
           fully 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 mecha-
           nism 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.

EEXXAAMMPPLLEESS
       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.

       EExxaammppllee - aa HHeexxaaddeecciimmaall HHaannddllee

       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 out-
       put 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                       2001-09-21                  PerlIO::via(3pm)