1PerlIO::via(3pm) Perl Programmers Reference Guide PerlIO::via(3pm)
2
6 PerlIO::via - Helper class for PerlIO layers implemented in perl
7
9 use PerlIO::via::Layer;
10 open($fh,"<:via(Layer)",...);
11 use Some::Other::Package;
13 open($fh,">:via(Some::Other::Package)",...);
14
16 The PerlIO::via module allows you to develop PerlIO layers in Perl,
17 without having to go into the nitty gritty of programming C with XS as
18 the interface to Perl.
19 One example module, PerlIO::via::QuotedPrint, is included with Perl
21 5.8.0, and more example modules are available from CPAN, such as
22 PerlIO::via::StripHTML and PerlIO::via::Base64. The
23 PerlIO::via::StripHTML module for instance, allows you to say:
24 use PerlIO::via::StripHTML;
26 open( my $fh, "<:via(StripHTML)", "index.html" );
27 my @line = <$fh>;
28 to obtain the text of an HTML-file in an array with all the HTML-tags
30 automagically removed.
31 Please note that if the layer is created in the PerlIO::via::
33 namespace, it does not have to be fully qualified. The PerlIO::via
34 module will prefix the PerlIO::via:: namespace if the specified
35 modulename does not exist as a fully qualified module name.
36
38 To create a Perl module that implements a PerlIO layer in Perl (as
39 opposed to in C using XS as the interface to Perl), you need to supply
40 some of the following subroutines. It is recommended to create these
41 Perl modules in the PerlIO::via:: namespace, so that they can easily be
42 located on CPAN and use the default namespace feature of the
43 PerlIO::via module itself.
44 Please note that this is an area of recent development in Perl and that
46 the interface described here is therefore still subject to change (and
47 hopefully will have better documentation and more examples).
48 In the method descriptions below $fh will be a reference to a glob
50 which can be treated as a perl file handle. It refers to the layer
51 below. $fh is not passed if the layer is at the bottom of the stack,
52 for this reason and to maintain some level of "compatibility" with
53 TIEHANDLE classes it is passed last.
54 $class->PUSHED([$mode,[$fh]])
56 Should return an object or the class, or -1 on failure. (Compare
57 TIEHANDLE.) The arguments are an optional mode string ("r", "w",
58 "w+", ...) and a filehandle for the PerlIO layer below. Mandatory.
59 When the layer is pushed as part of an "open" call, "PUSHED" will
61 be called before the actual open occurs, whether that be via
62 "OPEN", "SYSOPEN", "FDOPEN" or by letting a lower layer do the
63 open.
64 $obj->POPPED([$fh])
66 Optional - called when the layer is about to be removed.
67 $obj->UTF8($belowFlag,[$fh])
69 Optional - if present it will be called immediately after PUSHED
70 has returned. It should return a true value if the layer expects
71 data to be UTF-8 encoded. If it returns true, the result is as if
72 the caller had done
73 ":via(YourClass):utf8"
75 If not present or if it returns false, then the stream is left with
77 the UTF-8 flag clear. The $belowFlag argument will be true if
78 there is a layer below and that layer was expecting UTF-8.
79 $obj->OPEN($path,$mode,[$fh])
81 Optional - if not present a lower layer does the open. If present,
82 called for normal opens after the layer is pushed. This function
83 is subject to change as there is no easy way to get a lower layer
84 to do the open and then regain control.
85 $obj->BINMODE([$fh])
87 Optional - if not present the layer is popped on binmode($fh) or
88 when ":raw" is pushed. If present it should return 0 on success, -1
89 on error, or undef to pop the layer.
90 $obj->FDOPEN($fd,[$fh])
92 Optional - if not present a lower layer does the open. If present,
93 called after the layer is pushed for opens which pass a numeric
94 file descriptor. This function is subject to change as there is no
95 easy way to get a lower layer to do the open and then regain
96 control.
97 $obj->SYSOPEN($path,$imode,$perm,[$fh])
99 Optional - if not present a lower layer does the open. If present,
100 called after the layer is pushed for sysopen style opens which pass
101 a numeric mode and permissions. This function is subject to change
102 as there is no easy way to get a lower layer to do the open and
103 then regain control.
104 $obj->FILENO($fh)
106 Returns a numeric value for a Unix-like file descriptor. Returns -1
107 if there isn't one. Optional. Default is fileno($fh).
108 $obj->READ($buffer,$len,$fh)
110 Returns the number of octets placed in $buffer (must be less than
111 or equal to $len). Optional. Default is to use FILL instead.
112 $obj->WRITE($buffer,$fh)
114 Returns the number of octets from $buffer that have been
115 successfully written.
116 $obj->FILL($fh)
118 Should return a string to be placed in the buffer. Optional. If
119 not provided, must provide READ or reject handles open for reading
120 in PUSHED.
121 $obj->CLOSE($fh)
123 Should return 0 on success, -1 on error. Optional.
124 $obj->SEEK($posn,$whence,$fh)
126 Should return 0 on success, -1 on error. Optional. Default is to
127 fail, but that is likely to be changed in future.
128 $obj->TELL($fh)
130 Returns file position. Optional. Default to be determined.
131 $obj->UNREAD($buffer,$fh)
133 Returns the number of octets from $buffer that have been
134 successfully saved to be returned on future FILL/READ calls.
135 Optional. Default is to push data into a temporary layer above
136 this one.
137 $obj->FLUSH($fh)
139 Flush any buffered write data. May possibly be called on readable
140 handles too. Should return 0 on success, -1 on error.
141 $obj->SETLINEBUF($fh)
143 Optional. No return.
144 $obj->CLEARERR($fh)
146 Optional. No return.
147 $obj->ERROR($fh)
149 Optional. Returns error state. Default is no error until a
150 mechanism to signal error (die?) is worked out.
151 $obj->EOF($fh)
153 Optional. Returns end-of-file state. Default is a function of the
154 return value of FILL or READ.
155
157 Check the PerlIO::via:: namespace on CPAN for examples of PerlIO layers
158 implemented in Perl. To give you an idea how simple the implementation
159 of a PerlIO layer can look, a simple example is included here.
160 Example - a Hexadecimal Handle
162 Given the following module, PerlIO::via::Hex :
163 package PerlIO::via::Hex;
165 sub PUSHED
167 {
168 my ($class,$mode,$fh) = @_;
169 # When writing we buffer the data
170 my $buf = '';
171 return bless \$buf,$class;
172 }
173 sub FILL
175 {
176 my ($obj,$fh) = @_;
177 my $line = <$fh>;
178 return (defined $line) ? pack("H*", $line) : undef;
179 }
180 sub WRITE
182 {
183 my ($obj,$buf,$fh) = @_;
184 $$obj .= unpack("H*", $buf);
185 return length($buf);
186 }
187 sub FLUSH
189 {
190 my ($obj,$fh) = @_;
191 print $fh $$obj or return -1;
192 $$obj = '';
193 return 0;
194 }
195 1;
197 The following code opens up an output handle that will convert any
199 output to a hexadecimal dump of the output bytes: for example "A" will
200 be converted to "41" (on ASCII-based machines, on EBCDIC platforms the
201 "A" will become "c1")
202 use PerlIO::via::Hex;
204 open(my $fh, ">:via(Hex)", "foo.hex");
205 and the following code will read the hexdump in and convert it on the
207 fly back into bytes:
208 open(my $fh, "<:via(Hex)", "foo.hex");
210perl v5.26.3 2018-03-23 PerlIO::via(3pm)