3 perlfilter - Source Filters
7 This article is about a little-known feature of Perl called
8 I<source filters>. Source filters alter the program text of a module
9 before Perl sees it, much as a C preprocessor alters the source text of
10 a C program before the compiler sees it. This article tells you more
11 about what source filters are, how they work, and how to write your
14 The original purpose of source filters was to let you encrypt your
15 program source to prevent casual piracy. This isn't all they can do, as
16 you'll soon learn. But first, the basics.
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 C<use> or C<require>
23 statement, then each of those scripts will have to be read from their
24 respective files as well.
26 Now think of each logical connection between the Perl parser and an
27 individual file as a I<source stream>. A source stream is created when
28 the Perl parser opens a file, it continues to exist as the source code
29 is read into memory, and it is destroyed when Perl is finished parsing
30 the file. If the parser encounters a C<require> or C<use> statement in
31 a source stream, a new and distinct stream is created just for that
34 The diagram below represents a single source stream, with the flow of
35 source from a Perl script file on the left into the Perl parser on the
36 right. This is how Perl normally operates.
40 There are two important points to remember:
46 Although there can be any number of source streams in existence at any
47 given time, only one will be active.
51 Every source stream is associated with only one file.
55 A source filter is a special kind of Perl module that intercepts and
56 modifies a source stream before it reaches the parser. A source filter
57 changes our diagram like this:
59 file ----> filter ----> parser
61 If that doesn't make much sense, consider the analogy of a command
62 pipeline. Say you have a shell script stored in the compressed file
63 I<trial.gz>. The simple pipeline command below runs the script without
64 needing to create a temporary file to hold the uncompressed file.
66 gunzip -c trial.gz | sh
68 In this case, the data flow from the pipeline can be represented as follows:
70 trial.gz ----> gunzip ----> sh
72 With source filters, you can store the text of your script compressed and use a source filter to uncompress it for Perl's parser:
75 Perl program ---> source filter ---> parser
79 So how do you use a source filter in a Perl script? Above, I said that
80 a source filter is just a special kind of module. Like all Perl
81 modules, a source filter is invoked with a use statement.
83 Say you want to pass your Perl source through the C preprocessor before
84 execution. As it happens, the source filters distribution comes with a C
85 preprocessor filter module called Filter::cpp.
87 Below is an example program, C<cpp_test>, which makes use of this filter.
88 Line numbers have been added to allow specific lines to be referenced
96 When you execute this script, Perl creates a source stream for the
97 file. Before the parser processes any of the lines from the file, the
98 source stream looks like this:
100 cpp_test ---------> parser
102 Line 1, C<use Filter::cpp>, includes and installs the C<cpp> filter
103 module. All source filters work this way. The use statement is compiled
104 and executed at compile time, before any more of the file is read, and
105 it attaches the cpp filter to the source stream behind the scenes. Now
106 the data flow looks like this:
108 cpp_test ----> cpp filter ----> parser
110 As the parser reads the second and subsequent lines from the source
111 stream, it feeds those lines through the C<cpp> source filter before
112 processing them. The C<cpp> filter simply passes each line through the
113 real C preprocessor. The output from the C preprocessor is then
114 inserted back into the source stream by the filter.
120 cpp_test ----> cpp filter ----> parser
122 The parser then sees the following code:
128 Let's consider what happens when the filtered code includes another
137 The C<cpp> filter does not apply to the text of the Fred module, only
138 to the text of the file that used it (C<cpp_test>). Although the use
139 statement on line 3 will pass through the cpp filter, the module that
140 gets included (C<Fred>) will not. The source streams look like this
141 after line 3 has been parsed and before line 4 is parsed:
143 cpp_test ---> cpp filter ---> parser (INACTIVE)
147 As you can see, a new stream has been created for reading the source
148 from C<Fred.pm>. This stream will remain active until all of C<Fred.pm>
149 has been parsed. The source stream for C<cpp_test> will still exist,
150 but is inactive. Once the parser has finished reading Fred.pm, the
151 source stream associated with it will be destroyed. The source stream
152 for C<cpp_test> then becomes active again and the parser reads line 4
153 and subsequent lines from C<cpp_test>.
155 You can use more than one source filter on a single file. Similarly,
156 you can reuse the same filter in as many files as you like.
158 For example, if you have a uuencoded and compressed source file, it is
159 possible to stack a uudecode filter and an uncompression filter like
162 use Filter::uudecode; use Filter::uncompress;
163 M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/
164 M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[>
167 Once the first line has been processed, the flow will look like this:
169 file ---> uudecode ---> uncompress ---> parser
172 Data flows through filters in the same order they appear in the source
173 file. The uudecode filter appeared before the uncompress filter, so the
174 source file will be uudecoded before it's uncompressed.
176 =head1 WRITING A SOURCE FILTER
178 There are three ways to write your own source filter. You can write it
179 in C, use an external program as a filter, or write the filter in Perl.
180 I won't cover the first two in any great detail, so I'll get them out
181 of the way first. Writing the filter in Perl is most convenient, so
182 I'll devote the most space to it.
184 =head1 WRITING A SOURCE FILTER IN C
186 The first of the three available techniques is to write the filter
187 completely in C. The external module you create interfaces directly
188 with the source filter hooks provided by Perl.
190 The advantage of this technique is that you have complete control over
191 the implementation of your filter. The big disadvantage is the
192 increased complexity required to write the filter - not only do you
193 need to understand the source filter hooks, but you also need a
194 reasonable knowledge of Perl guts. One of the few times it is worth
195 going to this trouble is when writing a source scrambler. The
196 C<decrypt> filter (which unscrambles the source before Perl parses it)
197 included with the source filter distribution is an example of a C
198 source filter (see Decryption Filters, below).
203 =item B<Decryption Filters>
205 All decryption filters work on the principle of "security through
206 obscurity." Regardless of how well you write a decryption filter and
207 how strong your encryption algorithm is, anyone determined enough can
208 retrieve the original source code. The reason is quite simple - once
209 the decryption filter has decrypted the source back to its original
210 form, fragments of it will be stored in the computer's memory as Perl
211 parses it. The source might only be in memory for a short period of
212 time, but anyone possessing a debugger, skill, and lots of patience can
213 eventually reconstruct your program.
215 That said, there are a number of steps that can be taken to make life
216 difficult for the potential cracker. The most important: Write your
217 decryption filter in C and statically link the decryption module into
218 the Perl binary. For further tips to make life difficult for the
219 potential cracker, see the file I<decrypt.pm> in the source filters
224 =head1 CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE
226 An alternative to writing the filter in C is to create a separate
227 executable in the language of your choice. The separate executable
228 reads from standard input, does whatever processing is necessary, and
229 writes the filtered data to standard output. C<Filter::cpp> is an
230 example of a source filter implemented as a separate executable - the
231 executable is the C preprocessor bundled with your C compiler.
233 The source filter distribution includes two modules that simplify this
234 task: C<Filter::exec> and C<Filter::sh>. Both allow you to run any
235 external executable. Both use a coprocess to control the flow of data
236 into and out of the external executable. (For details on coprocesses,
237 see Stephens, W.R. "Advanced Programming in the UNIX Environment."
238 Addison-Wesley, ISBN 0-210-56317-7, pages 441-445.) The difference
239 between them is that C<Filter::exec> spawns the external command
240 directly, while C<Filter::sh> spawns a shell to execute the external
241 command. (Unix uses the Bourne shell; NT uses the cmd shell.) Spawning
242 a shell allows you to make use of the shell metacharacters and
243 redirection facilities.
245 Here is an example script that uses C<Filter::sh>:
247 use Filter::sh 'tr XYZ PQR';
249 print "XYZ a = $a\n";
251 The output you'll get when the script is executed:
255 Writing a source filter as a separate executable works fine, but a
256 small performance penalty is incurred. For example, if you execute the
257 small example above, a separate subprocess will be created to run the
258 Unix C<tr> command. Each use of the filter requires its own subprocess.
259 If creating subprocesses is expensive on your system, you might want to
260 consider one of the other options for creating source filters.
262 =head1 WRITING A SOURCE FILTER IN PERL
264 The easiest and most portable option available for creating your own
265 source filter is to write it completely in Perl. To distinguish this
266 from the previous two techniques, I'll call it a Perl source filter.
268 To help understand how to write a Perl source filter we need an example
269 to study. Here is a complete source filter that performs rot13
270 decoding. (Rot13 is a very simple encryption scheme used in Usenet
271 postings to hide the contents of offensive posts. It moves every letter
272 forward thirteen places, so that A becomes N, B becomes O, and Z
278 use Filter::Util::Call;
283 filter_add(bless $ref);
290 tr/n-za-mN-ZA-M/a-zA-Z/
291 if ($status = filter_read()) > 0;
297 All Perl source filters are implemented as Perl classes and have the
298 same basic structure as the example above.
300 First, we include the C<Filter::Util::Call> module, which exports a
301 number of functions into your filter's namespace. The filter shown
302 above uses two of these functions, C<filter_add()> and
305 Next, we create the filter object and associate it with the source
306 stream by defining the C<import> function. If you know Perl well
307 enough, you know that C<import> is called automatically every time a
308 module is included with a use statement. This makes C<import> the ideal
309 place to both create and install a filter object.
311 In the example filter, the object (C<$ref>) is blessed just like any
312 other Perl object. Our example uses an anonymous array, but this isn't
313 a requirement. Because this example doesn't need to store any context
314 information, we could have used a scalar or hash reference just as
315 well. The next section demonstrates context data.
317 The association between the filter object and the source stream is made
318 with the C<filter_add()> function. This takes a filter object as a
319 parameter (C<$ref> in this case) and installs it in the source stream.
321 Finally, there is the code that actually does the filtering. For this
322 type of Perl source filter, all the filtering is done in a method
323 called C<filter()>. (It is also possible to write a Perl source filter
324 using a closure. See the C<Filter::Util::Call> manual page for more
325 details.) It's called every time the Perl parser needs another line of
326 source to process. The C<filter()> method, in turn, reads lines from
327 the source stream using the C<filter_read()> function.
329 If a line was available from the source stream, C<filter_read()>
330 returns a status value greater than zero and appends the line to C<$_>.
331 A status value of zero indicates end-of-file, less than zero means an
332 error. The filter function itself is expected to return its status in
333 the same way, and put the filtered line it wants written to the source
334 stream in C<$_>. The use of C<$_> accounts for the brevity of most Perl
337 In order to make use of the rot13 filter we need some way of encoding
338 the source file in rot13 format. The script below, C<mkrot13>, does
341 die "usage mkrot13 filename\n" unless @ARGV;
344 open(IN, "<$in") or die "Cannot open file $in: $!\n";
345 open(OUT, ">$out") or die "Cannot open file $out: $!\n";
347 print OUT "use Rot13;\n";
349 tr/a-zA-Z/n-za-mN-ZA-M/;
358 If we encrypt this with C<mkrot13>:
360 print " hello fred \n";
362 the result will be this:
365 cevag "uryyb serq\a";
367 Running it produces this output:
371 =head1 USING CONTEXT: THE DEBUG FILTER
373 The rot13 example was a trivial example. Here's another demonstration
374 that shows off a few more features.
376 Say you wanted to include a lot of debugging code in your Perl script
377 during development, but you didn't want it available in the released
378 product. Source filters offer a solution. In order to keep the example
379 simple, let's say you wanted the debugging output to be controlled by
380 an environment variable, C<DEBUG>. Debugging code is enabled if the
381 variable exists, otherwise it is disabled.
383 Two special marker lines will bracket debugging code, like this:
387 warn "Debug: millennium bug in year $year\n";
391 When the C<DEBUG> environment variable exists, the filter ensures that
392 Perl parses only the code between the C<DEBUG_BEGIN> and C<DEBUG_END>
393 markers. That means that when C<DEBUG> does exist, the code above
394 should be passed through the filter unchanged. The marker lines can
395 also be passed through as-is, because the Perl parser will see them as
396 comment lines. When C<DEBUG> isn't set, we need a way to disable the
397 debug code. A simple way to achieve that is to convert the lines
398 between the two markers into comments:
402 # warn "Debug: millennium bug in year $year\n";
406 Here is the complete Debug filter:
412 use Filter::Util::Call;
414 use constant TRUE => 1;
415 use constant FALSE => 0;
420 Enabled => defined $ENV{DEBUG},
421 InTraceBlock => FALSE,
422 Filename => (caller)[1],
426 filter_add(bless \%context);
431 my ($message) = shift;
432 my ($line_no) = shift || $self->{LastBegin};
433 die "$message at $self->{Filename} line $line_no.\n"
439 $status = filter_read();
442 # deal with EOF/error first
444 $self->Die("DEBUG_BEGIN has no DEBUG_END")
445 if $self->{InTraceBlock};
449 if ($self->{InTraceBlock}) {
450 if (/^\s*##\s*DEBUG_BEGIN/ ) {
451 $self->Die("Nested DEBUG_BEGIN", $self->{LineNo})
452 } elsif (/^\s*##\s*DEBUG_END/) {
453 $self->{InTraceBlock} = FALSE;
456 # comment out the debug lines when the filter is disabled
457 s/^/#/ if ! $self->{Enabled};
458 } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
459 $self->{InTraceBlock} = TRUE;
460 $self->{LastBegin} = $self->{LineNo};
461 } elsif ( /^\s*##\s*DEBUG_END/ ) {
462 $self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo});
469 The big difference between this filter and the previous example is the
470 use of context data in the filter object. The filter object is based on
471 a hash reference, and is used to keep various pieces of context
472 information between calls to the filter function. All but two of the
473 hash fields are used for error reporting. The first of those two,
474 Enabled, is used by the filter to determine whether the debugging code
475 should be given to the Perl parser. The second, InTraceBlock, is true
476 when the filter has encountered a C<DEBUG_BEGIN> line, but has not yet
477 encountered the following C<DEBUG_END> line.
479 If you ignore all the error checking that most of the code does, the
480 essence of the filter is as follows:
485 $status = filter_read();
487 # deal with EOF/error first
488 return $status if $status <= 0;
489 if ($self->{InTraceBlock}) {
490 if (/^\s*##\s*DEBUG_END/) {
491 $self->{InTraceBlock} = FALSE
494 # comment out debug lines when the filter is disabled
495 s/^/#/ if ! $self->{Enabled};
496 } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
497 $self->{InTraceBlock} = TRUE;
502 Be warned: just as the C-preprocessor doesn't know C, the Debug filter
503 doesn't know Perl. It can be fooled quite easily:
509 Such things aside, you can see that a lot can be achieved with a modest
514 You now have better understanding of what a source filter is, and you
515 might even have a possible use for them. If you feel like playing with
516 source filters but need a bit of inspiration, here are some extra
517 features you could add to the Debug filter.
519 First, an easy one. Rather than having debugging code that is
520 all-or-nothing, it would be much more useful to be able to control
521 which specific blocks of debugging code get included. Try extending the
522 syntax for debug blocks to allow each to be identified. The contents of
523 the C<DEBUG> environment variable can then be used to control which
526 Once you can identify individual blocks, try allowing them to be
527 nested. That isn't difficult either.
529 Here is an interesting idea that doesn't involve the Debug filter.
530 Currently Perl subroutines have fairly limited support for formal
531 parameter lists. You can specify the number of parameters and their
532 type, but you still have to manually take them out of the C<@_> array
533 yourself. Write a source filter that allows you to have a named
534 parameter list. Such a filter would turn this:
536 sub MySub ($first, $second, @rest) { ... }
542 my ($second) = shift;
547 Finally, if you feel like a real challenge, have a go at writing a
548 full-blown Perl macro preprocessor as a source filter. Borrow the
549 useful features from the C preprocessor and any other macro processors
550 you know. The tricky bit will be choosing how much knowledge of Perl's
551 syntax you want your filter to have.
553 =head1 THINGS TO LOOK OUT FOR
557 =item Some Filters Clobber the C<DATA> Handle
559 Some source filters use the C<DATA> handle to read the calling program.
560 When using these source filters you cannot rely on this handle, nor expect
561 any particular kind of behavior when operating on it. Filters based on
562 Filter::Util::Call (and therefore Filter::Simple) do not alter the C<DATA>
569 The Source Filters distribution is available on CPAN, in
571 CPAN/modules/by-module/Filter
573 Starting from Perl 5.8 Filter::Util::Call (the core part of the
574 Source Filters distribution) is part of the standard Perl distribution.
575 Also included is a friendlier interface called Filter::Simple, by
580 Paul Marquess E<lt>Paul.Marquess@btinternet.comE<gt>
584 This article originally appeared in The Perl Journal #11, and is
585 copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and
586 The Perl Journal. This document may be distributed under the same terms