1PERLFILTER(1) Perl Programmers Reference Guide PERLFILTER(1)
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6 perlfilter - Source Filters
7
9 This article is about a little-known feature of Perl called source fil‐
10 ters. Source filters alter the program text of a module before Perl
11 sees it, much as a C preprocessor alters the source text of a C program
12 before the compiler sees it. This article tells you more about what
13 source filters are, how they work, and how to write your own.
14 The original purpose of source filters was to let you encrypt your pro‐
16 gram source to prevent casual piracy. This isn't all they can do, as
17 you'll soon learn. But first, the basics.
18
20 Before the Perl interpreter can execute a Perl script, it must first
21 read it from a file into memory for parsing and compilation. If that
22 script itself includes other scripts with a "use" or "require" state‐
23 ment, then each of those scripts will have to be read from their
24 respective files as well.
25 Now think of each logical connection between the Perl parser and an
27 individual file as a source stream. A source stream is created when the
28 Perl parser opens a file, it continues to exist as the source code is
29 read into memory, and it is destroyed when Perl is finished parsing the
30 file. If the parser encounters a "require" or "use" statement in a
31 source stream, a new and distinct stream is created just for that file.
32 The diagram below represents a single source stream, with the flow of
34 source from a Perl script file on the left into the Perl parser on the
35 right. This is how Perl normally operates.
36 file -------> parser
38 There are two important points to remember:
40 1. Although there can be any number of source streams in existence at
42 any given time, only one will be active.
43 2. Every source stream is associated with only one file.
45 A source filter is a special kind of Perl module that intercepts and
47 modifies a source stream before it reaches the parser. A source filter
48 changes our diagram like this:
49 file ----> filter ----> parser
51 If that doesn't make much sense, consider the analogy of a command
53 pipeline. Say you have a shell script stored in the compressed file
54 trial.gz. The simple pipeline command below runs the script without
55 needing to create a temporary file to hold the uncompressed file.
56 gunzip -c trial.gz ⎪ sh
58 In this case, the data flow from the pipeline can be represented as
60 follows:
61 trial.gz ----> gunzip ----> sh
63 With source filters, you can store the text of your script compressed
65 and use a source filter to uncompress it for Perl's parser:
66 compressed gunzip
68 Perl program ---> source filter ---> parser
69
71 So how do you use a source filter in a Perl script? Above, I said that
72 a source filter is just a special kind of module. Like all Perl mod‐
73 ules, a source filter is invoked with a use statement.
74 Say you want to pass your Perl source through the C preprocessor before
76 execution. You could use the existing "-P" command line option to do
77 this, but as it happens, the source filters distribution comes with a C
78 preprocessor filter module called Filter::cpp. Let's use that instead.
79 Below is an example program, "cpp_test", which makes use of this fil‐
81 ter. Line numbers have been added to allow specific lines to be refer‐
82 enced easily.
83 1: use Filter::cpp;
85 2: #define TRUE 1
86 3: $a = TRUE;
87 4: print "a = $a\n";
88 When you execute this script, Perl creates a source stream for the
90 file. Before the parser processes any of the lines from the file, the
91 source stream looks like this:
92 cpp_test ---------> parser
94 Line 1, "use Filter::cpp", includes and installs the "cpp" filter mod‐
96 ule. All source filters work this way. The use statement is compiled
97 and executed at compile time, before any more of the file is read, and
98 it attaches the cpp filter to the source stream behind the scenes. Now
99 the data flow looks like this:
100 cpp_test ----> cpp filter ----> parser
102 As the parser reads the second and subsequent lines from the source
104 stream, it feeds those lines through the "cpp" source filter before
105 processing them. The "cpp" filter simply passes each line through the
106 real C preprocessor. The output from the C preprocessor is then
107 inserted back into the source stream by the filter.
108 .-> cpp --.
110 ⎪ ⎪
111 ⎪ ⎪
112 ⎪ <-'
113 cpp_test ----> cpp filter ----> parser
114 The parser then sees the following code:
116 use Filter::cpp;
118 $a = 1;
119 print "a = $a\n";
120 Let's consider what happens when the filtered code includes another
122 module with use:
123 1: use Filter::cpp;
125 2: #define TRUE 1
126 3: use Fred;
127 4: $a = TRUE;
128 5: print "a = $a\n";
129 The "cpp" filter does not apply to the text of the Fred module, only to
131 the text of the file that used it ("cpp_test"). Although the use state‐
132 ment on line 3 will pass through the cpp filter, the module that gets
133 included ("Fred") will not. The source streams look like this after
134 line 3 has been parsed and before line 4 is parsed:
135 cpp_test ---> cpp filter ---> parser (INACTIVE)
137 Fred.pm ----> parser
139 As you can see, a new stream has been created for reading the source
141 from "Fred.pm". This stream will remain active until all of "Fred.pm"
142 has been parsed. The source stream for "cpp_test" will still exist, but
143 is inactive. Once the parser has finished reading Fred.pm, the source
144 stream associated with it will be destroyed. The source stream for
145 "cpp_test" then becomes active again and the parser reads line 4 and
146 subsequent lines from "cpp_test".
147 You can use more than one source filter on a single file. Similarly,
149 you can reuse the same filter in as many files as you like.
150 For example, if you have a uuencoded and compressed source file, it is
152 possible to stack a uudecode filter and an uncompression filter like
153 this:
154 use Filter::uudecode; use Filter::uncompress;
156 M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/
157 M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[>
158 ...
159 Once the first line has been processed, the flow will look like this:
161 file ---> uudecode ---> uncompress ---> parser
163 filter filter
164 Data flows through filters in the same order they appear in the source
166 file. The uudecode filter appeared before the uncompress filter, so the
167 source file will be uudecoded before it's uncompressed.
168
170 There are three ways to write your own source filter. You can write it
171 in C, use an external program as a filter, or write the filter in Perl.
172 I won't cover the first two in any great detail, so I'll get them out
173 of the way first. Writing the filter in Perl is most convenient, so
174 I'll devote the most space to it.
175
177 The first of the three available techniques is to write the filter com‐
178 pletely in C. The external module you create interfaces directly with
179 the source filter hooks provided by Perl.
180 The advantage of this technique is that you have complete control over
182 the implementation of your filter. The big disadvantage is the
183 increased complexity required to write the filter - not only do you
184 need to understand the source filter hooks, but you also need a reason‐
185 able knowledge of Perl guts. One of the few times it is worth going to
186 this trouble is when writing a source scrambler. The "decrypt" filter
187 (which unscrambles the source before Perl parses it) included with the
188 source filter distribution is an example of a C source filter (see
189 Decryption Filters, below).
190 Decryption Filters
192 All decryption filters work on the principle of "security through
193 obscurity." Regardless of how well you write a decryption filter
194 and how strong your encryption algorithm, anyone determined enough
195 can retrieve the original source code. The reason is quite simple
196 - once the decryption filter has decrypted the source back to its
197 original form, fragments of it will be stored in the computer's
198 memory as Perl parses it. The source might only be in memory for a
199 short period of time, but anyone possessing a debugger, skill, and
200 lots of patience can eventually reconstruct your program.
201 That said, there are a number of steps that can be taken to make
203 life difficult for the potential cracker. The most important:
204 Write your decryption filter in C and statically link the decryp‐
205 tion module into the Perl binary. For further tips to make life
206 difficult for the potential cracker, see the file decrypt.pm in
207 the source filters module.
208
210 An alternative to writing the filter in C is to create a separate exe‐
211 cutable in the language of your choice. The separate executable reads
212 from standard input, does whatever processing is necessary, and writes
213 the filtered data to standard output. "Filter:cpp" is an example of a
214 source filter implemented as a separate executable - the executable is
215 the C preprocessor bundled with your C compiler.
216 The source filter distribution includes two modules that simplify this
218 task: "Filter::exec" and "Filter::sh". Both allow you to run any exter‐
219 nal executable. Both use a coprocess to control the flow of data into
220 and out of the external executable. (For details on coprocesses, see
221 Stephens, W.R. "Advanced Programming in the UNIX Environment." Addi‐
222 son-Wesley, ISBN 0-210-56317-7, pages 441-445.) The difference between
223 them is that "Filter::exec" spawns the external command directly, while
224 "Filter::sh" spawns a shell to execute the external command. (Unix uses
225 the Bourne shell; NT uses the cmd shell.) Spawning a shell allows you
226 to make use of the shell metacharacters and redirection facilities.
227 Here is an example script that uses "Filter::sh":
229 use Filter::sh 'tr XYZ PQR';
231 $a = 1;
232 print "XYZ a = $a\n";
233 The output you'll get when the script is executed:
235 PQR a = 1
237 Writing a source filter as a separate executable works fine, but a
239 small performance penalty is incurred. For example, if you execute the
240 small example above, a separate subprocess will be created to run the
241 Unix "tr" command. Each use of the filter requires its own subprocess.
242 If creating subprocesses is expensive on your system, you might want to
243 consider one of the other options for creating source filters.
244
246 The easiest and most portable option available for creating your own
247 source filter is to write it completely in Perl. To distinguish this
248 from the previous two techniques, I'll call it a Perl source filter.
249 To help understand how to write a Perl source filter we need an example
251 to study. Here is a complete source filter that performs rot13 decod‐
252 ing. (Rot13 is a very simple encryption scheme used in Usenet postings
253 to hide the contents of offensive posts. It moves every letter forward
254 thirteen places, so that A becomes N, B becomes O, and Z becomes M.)
255 package Rot13;
257 use Filter::Util::Call;
259 sub import {
261 my ($type) = @_;
262 my ($ref) = [];
263 filter_add(bless $ref);
264 }
265 sub filter {
267 my ($self) = @_;
268 my ($status);
269 tr/n-za-mN-ZA-M/a-zA-Z/
271 if ($status = filter_read()) > 0;
272 $status;
273 }
274 1;
276 All Perl source filters are implemented as Perl classes and have the
278 same basic structure as the example above.
279 First, we include the "Filter::Util::Call" module, which exports a num‐
281 ber of functions into your filter's namespace. The filter shown above
282 uses two of these functions, "filter_add()" and "filter_read()".
283 Next, we create the filter object and associate it with the source
285 stream by defining the "import" function. If you know Perl well enough,
286 you know that "import" is called automatically every time a module is
287 included with a use statement. This makes "import" the ideal place to
288 both create and install a filter object.
289 In the example filter, the object ($ref) is blessed just like any other
291 Perl object. Our example uses an anonymous array, but this isn't a
292 requirement. Because this example doesn't need to store any context
293 information, we could have used a scalar or hash reference just as
294 well. The next section demonstrates context data.
295 The association between the filter object and the source stream is made
297 with the "filter_add()" function. This takes a filter object as a
298 parameter ($ref in this case) and installs it in the source stream.
299 Finally, there is the code that actually does the filtering. For this
301 type of Perl source filter, all the filtering is done in a method
302 called "filter()". (It is also possible to write a Perl source filter
303 using a closure. See the "Filter::Util::Call" manual page for more
304 details.) It's called every time the Perl parser needs another line of
305 source to process. The "filter()" method, in turn, reads lines from the
306 source stream using the "filter_read()" function.
307 If a line was available from the source stream, "filter_read()" returns
309 a status value greater than zero and appends the line to $_. A status
310 value of zero indicates end-of-file, less than zero means an error. The
311 filter function itself is expected to return its status in the same
312 way, and put the filtered line it wants written to the source stream in
313 $_. The use of $_ accounts for the brevity of most Perl source filters.
314 In order to make use of the rot13 filter we need some way of encoding
316 the source file in rot13 format. The script below, "mkrot13", does just
317 that.
318 die "usage mkrot13 filename\n" unless @ARGV;
320 my $in = $ARGV[0];
321 my $out = "$in.tmp";
322 open(IN, "<$in") or die "Cannot open file $in: $!\n";
323 open(OUT, ">$out") or die "Cannot open file $out: $!\n";
324 print OUT "use Rot13;\n";
326 while (<IN>) {
327 tr/a-zA-Z/n-za-mN-ZA-M/;
328 print OUT;
329 }
330 close IN;
332 close OUT;
333 unlink $in;
334 rename $out, $in;
335 If we encrypt this with "mkrot13":
337 print " hello fred \n";
339 the result will be this:
341 use Rot13;
343 cevag "uryyb serq\a";
344 Running it produces this output:
346 hello fred
348
350 The rot13 example was a trivial example. Here's another demonstration
351 that shows off a few more features.
352 Say you wanted to include a lot of debugging code in your Perl script
354 during development, but you didn't want it available in the released
355 product. Source filters offer a solution. In order to keep the example
356 simple, let's say you wanted the debugging output to be controlled by
357 an environment variable, "DEBUG". Debugging code is enabled if the
358 variable exists, otherwise it is disabled.
359 Two special marker lines will bracket debugging code, like this:
361 ## DEBUG_BEGIN
363 if ($year > 1999) {
364 warn "Debug: millennium bug in year $year\n";
365 }
366 ## DEBUG_END
367 When the "DEBUG" environment variable exists, the filter ensures that
369 Perl parses only the code between the "DEBUG_BEGIN" and "DEBUG_END"
370 markers. That means that when "DEBUG" does exist, the code above should
371 be passed through the filter unchanged. The marker lines can also be
372 passed through as-is, because the Perl parser will see them as comment
373 lines. When "DEBUG" isn't set, we need a way to disable the debug code.
374 A simple way to achieve that is to convert the lines between the two
375 markers into comments:
376 ## DEBUG_BEGIN
378 #if ($year > 1999) {
379 # warn "Debug: millennium bug in year $year\n";
380 #}
381 ## DEBUG_END
382 Here is the complete Debug filter:
384 package Debug;
386 use strict;
388 use warnings;
389 use Filter::Util::Call;
390 use constant TRUE => 1;
392 use constant FALSE => 0;
393 sub import {
395 my ($type) = @_;
396 my (%context) = (
397 Enabled => defined $ENV{DEBUG},
398 InTraceBlock => FALSE,
399 Filename => (caller)[1],
400 LineNo => 0,
401 LastBegin => 0,
402 );
403 filter_add(bless \%context);
404 }
405 sub Die {
407 my ($self) = shift;
408 my ($message) = shift;
409 my ($line_no) = shift ⎪⎪ $self->{LastBegin};
410 die "$message at $self->{Filename} line $line_no.\n"
411 }
412 sub filter {
414 my ($self) = @_;
415 my ($status);
416 $status = filter_read();
417 ++ $self->{LineNo};
418 # deal with EOF/error first
420 if ($status <= 0) {
421 $self->Die("DEBUG_BEGIN has no DEBUG_END")
422 if $self->{InTraceBlock};
423 return $status;
424 }
425 if ($self->{InTraceBlock}) {
427 if (/^\s*##\s*DEBUG_BEGIN/ ) {
428 $self->Die("Nested DEBUG_BEGIN", $self->{LineNo})
429 } elsif (/^\s*##\s*DEBUG_END/) {
430 $self->{InTraceBlock} = FALSE;
431 }
432 # comment out the debug lines when the filter is disabled
434 s/^/#/ if ! $self->{Enabled};
435 } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
436 $self->{InTraceBlock} = TRUE;
437 $self->{LastBegin} = $self->{LineNo};
438 } elsif ( /^\s*##\s*DEBUG_END/ ) {
439 $self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo});
440 }
441 return $status;
442 }
443 1;
445 The big difference between this filter and the previous example is the
447 use of context data in the filter object. The filter object is based on
448 a hash reference, and is used to keep various pieces of context infor‐
449 mation between calls to the filter function. All but two of the hash
450 fields are used for error reporting. The first of those two, Enabled,
451 is used by the filter to determine whether the debugging code should be
452 given to the Perl parser. The second, InTraceBlock, is true when the
453 filter has encountered a "DEBUG_BEGIN" line, but has not yet encoun‐
454 tered the following "DEBUG_END" line.
455 If you ignore all the error checking that most of the code does, the
457 essence of the filter is as follows:
458 sub filter {
460 my ($self) = @_;
461 my ($status);
462 $status = filter_read();
463 # deal with EOF/error first
465 return $status if $status <= 0;
466 if ($self->{InTraceBlock}) {
467 if (/^\s*##\s*DEBUG_END/) {
468 $self->{InTraceBlock} = FALSE
469 }
470 # comment out debug lines when the filter is disabled
472 s/^/#/ if ! $self->{Enabled};
473 } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
474 $self->{InTraceBlock} = TRUE;
475 }
476 return $status;
477 }
478 Be warned: just as the C-preprocessor doesn't know C, the Debug filter
480 doesn't know Perl. It can be fooled quite easily:
481 print <<EOM;
483 ##DEBUG_BEGIN
484 EOM
485 Such things aside, you can see that a lot can be achieved with a modest
487 amount of code.
488
490 You now have better understanding of what a source filter is, and you
491 might even have a possible use for them. If you feel like playing with
492 source filters but need a bit of inspiration, here are some extra fea‐
493 tures you could add to the Debug filter.
494 First, an easy one. Rather than having debugging code that is
496 all-or-nothing, it would be much more useful to be able to control
497 which specific blocks of debugging code get included. Try extending the
498 syntax for debug blocks to allow each to be identified. The contents of
499 the "DEBUG" environment variable can then be used to control which
500 blocks get included.
501 Once you can identify individual blocks, try allowing them to be
503 nested. That isn't difficult either.
504 Here is an interesting idea that doesn't involve the Debug filter.
506 Currently Perl subroutines have fairly limited support for formal
507 parameter lists. You can specify the number of parameters and their
508 type, but you still have to manually take them out of the @_ array
509 yourself. Write a source filter that allows you to have a named parame‐
510 ter list. Such a filter would turn this:
511 sub MySub ($first, $second, @rest) { ... }
513 into this:
515 sub MySub($$@) {
517 my ($first) = shift;
518 my ($second) = shift;
519 my (@rest) = @_;
520 ...
521 }
522 Finally, if you feel like a real challenge, have a go at writing a
524 full-blown Perl macro preprocessor as a source filter. Borrow the use‐
525 ful features from the C preprocessor and any other macro processors you
526 know. The tricky bit will be choosing how much knowledge of Perl's syn‐
527 tax you want your filter to have.
528
530 Some Filters Clobber the "DATA" Handle
531 Some source filters use the "DATA" handle to read the calling pro‐
532 gram. When using these source filters you cannot rely on this
533 handle, nor expect any particular kind of behavior when operating
534 on it. Filters based on Filter::Util::Call (and therefore Fil‐
535 ter::Simple) do not alter the "DATA" filehandle.
536
538 The Source Filters distribution is available on CPAN, in
539 CPAN/modules/by-module/Filter
541 Starting from Perl 5.8 Filter::Util::Call (the core part of the Source
543 Filters distribution) is part of the standard Perl distribution. Also
544 included is a friendlier interface called Filter::Simple, by Damian
545 Conway.
546
548 Paul Marquess <Paul.Marquess@btinternet.com>
549
551 This article originally appeared in The Perl Journal #11, and is copy‐
552 right 1998 The Perl Journal. It appears courtesy of Jon Orwant and The
553 Perl Journal. This document may be distributed under the same terms as
554 Perl itself.
555perl v5.8.8 2006-01-07 PERLFILTER(1)