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PerlIO::via(3)         Perl Programmers Reference Guide         PerlIO::via(3)



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 avail-
       able 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 there-
       fore 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 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 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 suc-
           cess, -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 v5.8.8                       2001-09-21                    PerlIO::via(3)