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PerlIO::Via - Helper class for PerlIO layers implemented in perl


   use Some::Package;



The package to be used as a layer should implement at least some of the following methods. 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.

As an example, in Perl release 5.8.0 the included MIME::QuotedPrint module defines the required TIEHANDLE methods so that you can say

        use MIME::QuotedPrint;
        open(my $fh, ">Via(MIME::QuotedPrint)", "qp");

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.


Optional - layer is about to be removed.


Not yet in use.


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.


Not yet in use.


Not yet in use.


Returns a numeric value for Unix-like file descriptor. Return -1 if there isn't one. Optional. Default is fileno($fh).


Returns the number of octets placed in $buffer (must be less than or equal to $len). Optional. Default is to use FILL instead.


Returns the number of octets from buffer that have been sucessfully written.


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.


Should return 0 on success, -1 on error. Optional.


Should return 0 on success, -1 on error. Optional. Default is to fail, but that is likely to be changed in future.


Returns file postion. Optional. Default to be determined.


Returns the number of octets from buffer that have been sucessfully saved to be returned on future FILL/READ calls. Optional. Default is to push data into a temporary layer above this one.


Flush any buffered write data. May possibly be called on readable handles too. Should return 0 on success, -1 on error.


Optional. No return.


Optional. No return.


Optional. Returns error state. Default is no error until a mechanism to signal error (die?) is worked out.


Optional. Returns end-of-file state. Default is function of return value of FILL or READ.

Example - a Hexadecimal Handle

Given the following module,

    package 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;


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 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");