1PerlIO::via(3pm) Perl Programmers Reference Guide PerlIO::via(3pm)
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6 PerlIO::via - Helper class for PerlIO layers implemented in perl
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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 Per‐
22 lIO::via::StripHTML and PerlIO::via::Base64. The Per‐
23 lIO::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:: names‐
33 pace, it does not have to be fully qualified. The PerlIO::via module
34 will prefix the PerlIO::via:: namespace if the specified modulename
35 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 Per‐
43 lIO::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 layer is pushed as part of an "open" call, "PUSHED" will be
61 called before the actual open occurs whether than be via "OPEN",
62 "SYSOPEN", "FDOPEN" or by letting lower layer do the open.
63 $obj->POPPED([$fh])
65 Optional - layer is about to be removed.
66 $obj->UTF8($bellowFlag,[$fh])
68 Optional - if present it will be called immediately after PUSHED
69 has returned. It should return true value if the layer expects data
70 to be UTF-8 encoded. If it returns true result is as if caller had
71 done
72 ":via(YourClass):utf8"
74 If not present of it it returns false, then stream is left with
76 flag clear. The $bellowFlag argument will be true if there is a
77 layer below and that layer was expecting UTF-8.
78 $obj->OPEN($path,$mode[,$fh])
80 Optional - if not present lower layer does open. If present called
81 for normal opens after layer is pushed. This function is subject
82 to change as there is no easy way to get lower layer to do open and
83 then regain control.
84 $obj->BINMODE([,$fh])
86 Optional - if not available layer is popped on binmode($fh) or when
87 ":raw" is pushed. If present it should return 0 on success -1 on
88 error and undef to pop the layer.
89 $obj->FDOPEN($fd[,$fh])
91 Optional - if not present lower layer does open. If present called
92 for opens which pass a numeric file descriptor after layer is
93 pushed. This function is subject to change as there is no easy way
94 to get lower layer to do open and then regain control.
95 $obj->SYSOPEN($path,$imode,$perm,[,$fh])
97 Optional - if not present lower layer does open. If present called
98 for sysopen style opens which pass a numeric mode and permissions
99 after layer is pushed. This function is subject to change as there
100 is no easy way to get lower layer to do open and then regain con‐
101 trol.
102 $obj->FILENO($fh)
104 Returns a numeric value for Unix-like file descriptor. Return -1 if
105 there isn't one. Optional. Default is fileno($fh).
106 $obj->READ($buffer,$len,$fh)
108 Returns the number of octets placed in $buffer (must be less than
109 or equal to $len). Optional. Default is to use FILL instead.
110 $obj->WRITE($buffer,$fh)
112 Returns the number of octets from buffer that have been success‐
113 fully written.
114 $obj->FILL($fh)
116 Should return a string to be placed in the buffer. Optional. If
117 not provided must provide READ or reject handles open for reading
118 in PUSHED.
119 $obj->CLOSE($fh)
121 Should return 0 on success, -1 on error. Optional.
122 $obj->SEEK($posn,$whence,$fh)
124 Should return 0 on success, -1 on error. Optional. Default is to
125 fail, but that is likely to be changed in future.
126 $obj->TELL($fh)
128 Returns file postion. Optional. Default to be determined.
129 $obj->UNREAD($buffer,$fh)
131 Returns the number of octets from buffer that have been success‐
132 fully saved to be returned on future FILL/READ calls. Optional.
133 Default is to push data into a temporary layer above this one.
134 $obj->FLUSH($fh)
136 Flush any buffered write data. May possibly be called on readable
137 handles too. Should return 0 on success, -1 on error.
138 $obj->SETLINEBUF($fh)
140 Optional. No return.
141 $obj->CLEARERR($fh)
143 Optional. No return.
144 $obj->ERROR($fh)
146 Optional. Returns error state. Default is no error until a mecha‐
147 nism to signal error (die?) is worked out.
148 $obj->EOF($fh)
150 Optional. Returns end-of-file state. Default is function of return
151 value of FILL or READ.
152
154 Check the PerlIO::via:: namespace on CPAN for examples of PerlIO layers
155 implemented in Perl. To give you an idea how simple the implementation
156 of a PerlIO layer can look, as simple example is included here.
157 Example - a Hexadecimal Handle
159 Given the following module, PerlIO::via::Hex :
161 package PerlIO::via::Hex;
163 sub PUSHED
165 {
166 my ($class,$mode,$fh) = @_;
167 # When writing we buffer the data
168 my $buf = '';
169 return bless \$buf,$class;
170 }
171 sub FILL
173 {
174 my ($obj,$fh) = @_;
175 my $line = <$fh>;
176 return (defined $line) ? pack("H*", $line) : undef;
177 }
178 sub WRITE
180 {
181 my ($obj,$buf,$fh) = @_;
182 $$obj .= unpack("H*", $buf);
183 return length($buf);
184 }
185 sub FLUSH
187 {
188 my ($obj,$fh) = @_;
189 print $fh $$obj or return -1;
190 $$obj = '';
191 return 0;
192 }
193 1;
195 the following code opens up an output handle that will convert any out‐
197 put to hexadecimal dump of the output bytes: for example "A" will be
198 converted to "41" (on ASCII-based machines, on EBCDIC platforms the "A"
199 will become "c1")
200 use PerlIO::via::Hex;
202 open(my $fh, ">:via(Hex)", "foo.hex");
203 and the following code will read the hexdump in and convert it on the
205 fly back into bytes:
206 open(my $fh, "<:via(Hex)", "foo.hex");
208perl v5.8.8 2001-09-21 PerlIO::via(3pm)