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 module for instance, allows you to say:
31 use PerlIO::via::StripHTML;
32 open( my $fh, "<:via(StripHTML)", "index.html" );
35 to obtain the text of an HTML-file in an array with all the HTML-tags
36 automagically removed.
38 Please note that if the layer is created in the PerlIO::via:: namespace, it
39 does B<not> have to be fully qualified. The PerlIO::via module will prefix
40 the PerlIO::via:: namespace if the specified modulename does not exist as a
41 fully qualified module name.
43 =head1 EXPECTED METHODS
45 To create a Perl module that implements a PerlIO layer in Perl (as opposed to
46 in C using XS as the interface to Perl), you need to supply some of the
47 following subroutines. It is recommended to create these Perl modules in the
48 PerlIO::via:: namespace, so that they can easily be located on CPAN and use
49 the default namespace feature of the PerlIO::via module itself.
51 Please note that this is an area of recent development in Perl and that the
52 interface described here is therefore still subject to change (and hopefully
53 will have better documentation and more examples).
55 In the method descriptions below I<$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 the layer is pushed as part of an C<open> call, C<PUSHED> will be called
70 I<before> the actual open occurs, whether that be via C<OPEN>, C<SYSOPEN>,
71 C<FDOPEN> or by letting a lower layer do the open.
73 =item $obj->POPPED([$fh])
75 Optional - called when the layer is about to be removed.
77 =item $obj->UTF8($bellowFlag,[$fh])
79 Optional - if present it will be called immediately after PUSHED has
80 returned. It should return a true value if the layer expects data to be
81 UTF-8 encoded. If it returns true, the result is as if the caller had done
83 ":via(YourClass):utf8"
85 If not present or if it returns false, then the stream is left with
87 The I<$bellowFlag> argument will be true if there is a layer below
88 and that layer was expecting UTF-8.
90 =item $obj->OPEN($path,$mode,[$fh])
92 Optional - if not present a lower layer does the open.
93 If present, called for normal opens after the layer is pushed.
94 This function is subject to change as there is no easy way
95 to get a lower layer to do the open and then regain control.
97 =item $obj->BINMODE([$fh])
99 Optional - if not present the layer is popped on binmode($fh) or when C<:raw>
100 is pushed. If present it should return 0 on success, -1 on error, or undef
103 =item $obj->FDOPEN($fd,[$fh])
105 Optional - if not present a lower layer does the open.
106 If present, called after the layer is pushed for opens which pass
107 a numeric file descriptor.
108 This function is subject to change as there is no easy way
109 to get a lower layer to do the open and then regain control.
111 =item $obj->SYSOPEN($path,$imode,$perm,[$fh])
113 Optional - if not present a lower layer does the open.
114 If present, called after the layer is pushed for sysopen style opens
115 which pass a numeric mode and permissions.
116 This function is subject to change as there is no easy way
117 to get a lower layer to do the open and then regain control.
119 =item $obj->FILENO($fh)
121 Returns a numeric value for a Unix-like file descriptor. Returns -1 if
122 there isn't one. Optional. Default is fileno($fh).
124 =item $obj->READ($buffer,$len,$fh)
126 Returns the number of octets placed in $buffer (must be less than or
127 equal to $len). Optional. Default is to use FILL instead.
129 =item $obj->WRITE($buffer,$fh)
131 Returns the number of octets from $buffer that have been successfully written.
133 =item $obj->FILL($fh)
135 Should return a string to be placed in the buffer. Optional. If not
136 provided, must provide READ or reject handles open for reading in
139 =item $obj->CLOSE($fh)
141 Should return 0 on success, -1 on error.
144 =item $obj->SEEK($posn,$whence,$fh)
146 Should return 0 on success, -1 on error.
147 Optional. Default is to fail, but that is likely to be changed
150 =item $obj->TELL($fh)
152 Returns file position.
153 Optional. Default to be determined.
155 =item $obj->UNREAD($buffer,$fh)
157 Returns the number of octets from $buffer that have been successfully
158 saved to be returned on future FILL/READ calls. Optional. Default is
159 to push data into a temporary layer above this one.
161 =item $obj->FLUSH($fh)
163 Flush any buffered write data. May possibly be called on readable
164 handles too. Should return 0 on success, -1 on error.
166 =item $obj->SETLINEBUF($fh)
170 =item $obj->CLEARERR($fh)
174 =item $obj->ERROR($fh)
176 Optional. Returns error state. Default is no error until a mechanism
177 to signal error (die?) is worked out.
181 Optional. Returns end-of-file state. Default is a function of the return
182 value of FILL or READ.
188 Check the PerlIO::via:: namespace on CPAN for examples of PerlIO layers
189 implemented in Perl. To give you an idea how simple the implementation of
190 a PerlIO layer can look, a simple example is included here.
192 =head2 Example - a Hexadecimal Handle
194 Given the following module, PerlIO::via::Hex :
196 package PerlIO::via::Hex;
200 my ($class,$mode,$fh) = @_;
201 # When writing we buffer the data
203 return bless \$buf,$class;
210 return (defined $line) ? pack("H*", $line) : undef;
215 my ($obj,$buf,$fh) = @_;
216 $$obj .= unpack("H*", $buf);
223 print $fh $$obj or return -1;
230 The following code opens up an output handle that will convert any
231 output to a hexadecimal dump of the output bytes: for example "A" will
232 be converted to "41" (on ASCII-based machines, on EBCDIC platforms
233 the "A" will become "c1")
235 use PerlIO::via::Hex;
236 open(my $fh, ">:via(Hex)", "foo.hex");
238 and the following code will read the hexdump in and convert it
239 on the fly back into bytes:
241 open(my $fh, "<:via(Hex)", "foo.hex");