MirOS Manual: PerlIO::via(3p)


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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     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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         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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         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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         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");

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