4 XSLoader::load 'PerlIO::via';
10 PerlIO::via - Helper class for PerlIO layers implemented in perl
14 use PerlIO::via::Layer;
15 open($fh,"<:via(Layer)",...);
17 use Some::Other::Package;
18 open($fh,">:via(Some::Other::Package)",...);
22 The PerlIO::via module allows you to develop PerlIO layers in Perl, without
23 having to go into the nitty gritty of programming C with XS as the interface
26 One example module, L<PerlIO::via::QuotedPrint>, is included with Perl
27 5.8.0, and more example modules are available from CPAN, such as
28 L<PerlIO::via::StripHTML> and L<PerlIO::via::Base64>. The
29 PerlIO::via::StripHTML for instance, allows you to say:
31 use PerlIO::via::StripHTML;
32 open( my $fh, "<:via(StripHTML)", "index.html" );
35 to obtain the text of an HTML-file in an array with all the HTML-tags
36 automagically removed.
38 Please note that if the layer is created in the PerlIO::via:: namespace, it
39 does B<not> have to be fully qualified. The PerlIO::via module will prefix
40 the PerlIO::via:: namespace if the specified modulename does not exist as a
41 fully qualified module name.
43 =head1 EXPECTED METHODS
45 To create a Perl module that implements a PerlIO layer in Perl (as opposed to
46 in C using XS as the interface to Perl), you need to supply some of the
47 following subroutines. It is recommended to create these Perl modules in the
48 PerlIO::via:: namespace, so that they can easily be located on CPAN and use
49 the default namespace feature of the PerlIO::via module itself.
51 Please note that this is an area of recent development in Perl and that the
52 interface described here is therefor still subject to change (and hopefully
53 better documentation and more examples).
55 In the method descriptions below I<$fh> will be
56 a reference to a glob which can be treated as a perl file handle.
57 It refers to the layer below. I<$fh> is not passed if the layer
58 is at the bottom of the stack, for this reason and to maintain
59 some level of "compatibility" with TIEHANDLE classes it is passed last.
63 =item $class->PUSHED([$mode[,$fh]])
65 Should return an object or the class, or -1 on failure. (Compare
66 TIEHANDLE.) The arguments are an optional mode string ("r", "w",
67 "w+", ...) and a filehandle for the PerlIO layer below. Mandatory.
69 When layer is pushed as part of an C<open> call, C<PUSHED> will be called
70 I<before> the actual open occurs whether than be via C<OPEN>, C<SYSOPEN>,
71 C<FDOPEN> or by letting lower layer do the open.
73 =item $obj->POPPED([$fh])
75 Optional - layer is about to be removed.
77 =item $obj->OPEN($path,$mode[,$fh])
79 Optional - if not present lower layer does open.
80 If present called for normal opens after layer is pushed.
81 This function is subject to change as there is no easy way
82 to get lower layer to do open and then regain control.
84 =item $obj->BINMODE([,$fh])
86 Optional - if not available layer is popped on binmode($fh) or when C<:raw>
87 is pushed. If present it should return 0 on success -1 on error and undef
90 =item $obj->FDOPEN($fd[,$fh])
92 Optional - if not present lower layer does open.
93 If present called for opens which pass a numeric file
94 descriptor after layer is pushed.
95 This function is subject to change as there is no easy way
96 to get lower layer to do open and then regain control.
98 =item $obj->SYSOPEN($path,$imode,$perm,[,$fh])
100 Optional - if not present lower layer does open.
101 If present called for sysopen style opens which pass a numeric mode
102 and permissions after layer is pushed.
103 This function is subject to change as there is no easy way
104 to get lower layer to do open and then regain control.
106 =item $obj->FILENO($fh)
108 Returns a numeric value for Unix-like file descriptor. Return -1 if
109 there isn't one. Optional. Default is fileno($fh).
111 =item $obj->READ($buffer,$len,$fh)
113 Returns the number of octets placed in $buffer (must be less than or
114 equal to $len). Optional. Default is to use FILL instead.
116 =item $obj->WRITE($buffer,$fh)
118 Returns the number of octets from buffer that have been sucessfully written.
120 =item $obj->FILL($fh)
122 Should return a string to be placed in the buffer. Optional. If not
123 provided must provide READ or reject handles open for reading in
126 =item $obj->CLOSE($fh)
128 Should return 0 on success, -1 on error.
131 =item $obj->SEEK($posn,$whence,$fh)
133 Should return 0 on success, -1 on error.
134 Optional. Default is to fail, but that is likely to be changed
137 =item $obj->TELL($fh)
139 Returns file postion.
140 Optional. Default to be determined.
142 =item $obj->UNREAD($buffer,$fh)
144 Returns the number of octets from buffer that have been sucessfully
145 saved to be returned on future FILL/READ calls. Optional. Default is
146 to push data into a temporary layer above this one.
148 =item $obj->FLUSH($fh)
150 Flush any buffered write data. May possibly be called on readable
151 handles too. Should return 0 on success, -1 on error.
153 =item $obj->SETLINEBUF($fh)
157 =item $obj->CLEARERR($fh)
161 =item $obj->ERROR($fh)
163 Optional. Returns error state. Default is no error until a mechanism
164 to signal error (die?) is worked out.
168 Optional. Returns end-of-file state. Default is function of return
169 value of FILL or READ.
175 Check the PerlIO::via:: namespace on CPAN for examples of PerlIO layers
176 implemented in Perl. To give you an idea how simple the implementation of
177 a PerlIO layer can look, as simple example is included here.
179 =head2 Example - a Hexadecimal Handle
181 Given the following module, PerlIO::via::Hex.pm:
183 package PerlIO::via::Hex;
187 my ($class,$mode,$fh) = @_;
188 # When writing we buffer the data
190 return bless \$buf,$class;
197 return (defined $line) ? pack("H*", $line) : undef;
202 my ($obj,$buf,$fh) = @_;
203 $$obj .= unpack("H*", $buf);
210 print $fh $$obj or return -1;
217 the following code opens up an output handle that will convert any
218 output to hexadecimal dump of the output bytes: for example "A" will
219 be converted to "41" (on ASCII-based machines, on EBCDIC platforms
220 the "A" will become "c1")
222 use PerlIO::via::Hex;
223 open(my $fh, ">:via(Hex)", "foo.hex");
225 and the following code will read the hexdump in and convert it
226 on the fly back into bytes:
228 open(my $fh, "<:via(Hex)", "foo.hex");