3 perlfilter - Source Filters
8 This article is about a little-known feature of Perl called
9 I<source filters>. Source filters alter the program text of a module
10 before Perl sees it, much as a C preprocessor alters the source text of
11 a C program before the compiler sees it. This article tells you more
12 about what source filters are, how they work, and how to write your
15 The original purpose of source filters was to let you encrypt your
16 program source to prevent casual piracy. This isn't all they can do, as
17 you'll soon learn. But first, the basics.
21 Before the Perl interpreter can execute a Perl script, it must first
22 read it from a file into memory for parsing and compilation. (Even
23 scripts specified on the command line with the C<-e> option are stored in
24 a temporary file for the parser to process.) If that script itself
25 includes other scripts with a C<use> or C<require> statement, then each
26 of those scripts will have to be read from their respective files as
29 Now think of each logical connection between the Perl parser and an
30 individual file as a I<source stream>. A source stream is created when
31 the Perl parser opens a file, it continues to exist as the source code
32 is read into memory, and it is destroyed when Perl is finished parsing
33 the file. If the parser encounters a C<require> or C<use> statement in
34 a source stream, a new and distinct stream is created just for that
37 The diagram below represents a single source stream, with the flow of
38 source from a Perl script file on the left into the Perl parser on the
39 right. This is how Perl normally operates.
43 There are two important points to remember:
49 Although there can be any number of source streams in existence at any
50 given time, only one will be active.
54 Every source stream is associated with only one file.
58 A source filter is a special kind of Perl module that intercepts and
59 modifies a source stream before it reaches the parser. A source filter
60 changes our diagram like this:
62 file ----> filter ----> parser
64 If that doesn't make much sense, consider the analogy of a command
65 pipeline. Say you have a shell script stored in the compressed file
66 I<trial.gz>. The simple pipeline command below runs the script without
67 needing to create a temporary file to hold the uncompressed file.
69 gunzip -c trial.gz | sh
71 In this case, the data flow from the pipeline can be represented as follows:
73 trial.gz ----> gunzip ----> sh
75 With source filters, you can store the text of your script compressed and use a source filter to uncompress it for Perl's parser:
78 Perl program ---> source filter ---> parser
82 So how do you use a source filter in a Perl script? Above, I said that
83 a source filter is just a special kind of module. Like all Perl
84 modules, a source filter is invoked with a use statement.
86 Say you want to pass your Perl source through the C preprocessor before
87 execution. You could use the existing C<-P> command line option to do
88 this, but as it happens, the source filters distribution comes with a C
89 preprocessor filter module called Filter::cpp. Let's use that instead.
91 Below is an example program, C<cpp_test>, which makes use of this filter.
92 Line numbers have been added to allow specific lines to be referenced
100 When you execute this script, Perl creates a source stream for the
101 file. Before the parser processes any of the lines from the file, the
102 source stream looks like this:
104 cpp_test ---------> parser
106 Line 1, C<use Filter::cpp>, includes and installs the C<cpp> filter
107 module. All source filters work this way. The use statement is compiled
108 and executed at compile time, before any more of the file is read, and
109 it attaches the cpp filter to the source stream behind the scenes. Now
110 the data flow looks like this:
112 cpp_test ----> cpp filter ----> parser
114 As the parser reads the second and subsequent lines from the source
115 stream, it feeds those lines through the C<cpp> source filter before
116 processing them. The C<cpp> filter simply passes each line through the
117 real C preprocessor. The output from the C preprocessor is then
118 inserted back into the source stream by the filter.
124 cpp_test ----> cpp filter ----> parser
126 The parser then sees the following code:
132 Let's consider what happens when the filtered code includes another
139 5: print "a = $a\n" ;
141 The C<cpp> filter does not apply to the text of the Fred module, only
142 to the text of the file that used it (C<cpp_test>). Although the use
143 statement on line 3 will pass through the cpp filter, the module that
144 gets included (C<Fred>) will not. The source streams look like this
145 after line 3 has been parsed and before line 4 is parsed:
147 cpp_test ---> cpp filter ---> parser (INACTIVE)
151 As you can see, a new stream has been created for reading the source
152 from C<Fred.pm>. This stream will remain active until all of C<Fred.pm>
153 has been parsed. The source stream for C<cpp_test> will still exist,
154 but is inactive. Once the parser has finished reading Fred.pm, the
155 source stream associated with it will be destroyed. The source stream
156 for C<cpp_test> then becomes active again and the parser reads line 4
157 and subsequent lines from C<cpp_test>.
159 You can use more than one source filter on a single file. Similarly,
160 you can reuse the same filter in as many files as you like.
162 For example, if you have a uuencoded and compressed source file, it is
163 possible to stack a uudecode filter and an uncompression filter like
166 use Filter::uudecode ; use Filter::uncompress ;
167 M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/
168 M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[>
171 Once the first line has been processed, the flow will look like this:
173 file ---> uudecode ---> uncompress ---> parser
176 Data flows through filters in the same order they appear in the source
177 file. The uudecode filter appeared before the uncompress filter, so the
178 source file will be uudecoded before it's uncompressed.
180 =head1 WRITING A SOURCE FILTER
182 There are three ways to write your own source filter. You can write it
183 in C, use an external program as a filter, or write the filter in Perl.
184 I won't cover the first two in any great detail, so I'll get them out
185 of the way first. Writing the filter in Perl is most convenient, so
186 I'll devote the most space to it.
188 =head1 WRITING A SOURCE FILTER IN C
190 The first of the three available techniques is to write the filter
191 completely in C. The external module you create interfaces directly
192 with the source filter hooks provided by Perl.
194 The advantage of this technique is that you have complete control over
195 the implementation of your filter. The big disadvantage is the
196 increased complexity required to write the filter - not only do you
197 need to understand the source filter hooks, but you also need a
198 reasonable knowledge of Perl guts. One of the few times it is worth
199 going to this trouble is when writing a source scrambler. The
200 C<decrypt> filter (which unscrambles the source before Perl parses it)
201 included with the source filter distribution is an example of a C
202 source filter (see Decryption Filters, below).
207 =item B<Decryption Filters>
209 All decryption filters work on the principle of "security through
210 obscurity." Regardless of how well you write a decryption filter and
211 how strong your encryption algorithm, anyone determined enough can
212 retrieve the original source code. The reason is quite simple - once
213 the decryption filter has decrypted the source back to its original
214 form, fragments of it will be stored in the computer's memory as Perl
215 parses it. The source might only be in memory for a short period of
216 time, but anyone possessing a debugger, skill, and lots of patience can
217 eventually reconstruct your program.
219 That said, there are a number of steps that can be taken to make life
220 difficult for the potential cracker. The most important: Write your
221 decryption filter in C and statically link the decryption module into
222 the Perl binary. For further tips to make life difficult for the
223 potential cracker, see the file I<decrypt.pm> in the source filters
228 =head1 CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE
230 An alternative to writing the filter in C is to create a separate
231 executable in the language of your choice. The separate executable
232 reads from standard input, does whatever processing is necessary, and
233 writes the filtered data to standard output. C<Filter:cpp> is an
234 example of a source filter implemented as a separate executable - the
235 executable is the C preprocessor bundled with your C compiler.
237 The source filter distribution includes two modules that simplify this
238 task: C<Filter::exec> and C<Filter::sh>. Both allow you to run any
239 external executable. Both use a coprocess to control the flow of data
240 into and out of the external executable. (For details on coprocesses,
241 see Stephens, W.R. "Advanced Programming in the UNIX Environment."
242 Addison-Wesley, ISBN 0-210-56317-7, pages 441-445.) The difference
243 between them is that C<Filter::exec> spawns the external command
244 directly, while C<Filter::sh> spawns a shell to execute the external
245 command. (Unix uses the Bourne shell; NT uses the cmd shell.) Spawning
246 a shell allows you to make use of the shell metacharacters and
247 redirection facilities.
249 Here is an example script that uses C<Filter::sh>:
251 use Filter::sh 'tr XYZ PQR' ;
253 print "XYZ a = $a\n" ;
255 The output you'll get when the script is executed:
259 Writing a source filter as a separate executable works fine, but a
260 small performance penalty is incurred. For example, if you execute the
261 small example above, a separate subprocess will be created to run the
262 Unix C<tr> command. Each use of the filter requires its own subprocess.
263 If creating subprocesses is expensive on your system, you might want to
264 consider one of the other options for creating source filters.
266 =head1 WRITING A SOURCE FILTER IN PERL
268 The easiest and most portable option available for creating your own
269 source filter is to write it completely in Perl. To distinguish this
270 from the previous two techniques, I'll call it a Perl source filter.
272 To help understand how to write a Perl source filter we need an example
273 to study. Here is a complete source filter that performs rot13
274 decoding. (Rot13 is a very simple encryption scheme used in Usenet
275 postings to hide the contents of offensive posts. It moves every letter
276 forward thirteen places, so that A becomes N, B becomes O, and Z
282 use Filter::Util::Call ;
287 filter_add(bless $ref) ;
294 tr/n-za-mN-ZA-M/a-zA-Z/
295 if ($status = filter_read()) > 0 ;
301 All Perl source filters are implemented as Perl classes and have the
302 same basic structure as the example above.
304 First, we include the C<Filter::Util::Call> module, which exports a
305 number of functions into your filter's namespace. The filter shown
306 above uses two of these functions, C<filter_add()> and
309 Next, we create the filter object and associate it with the source
310 stream by defining the C<import> function. If you know Perl well
311 enough, you know that C<import> is called automatically every time a
312 module is included with a use statement. This makes C<import> the ideal
313 place to both create and install a filter object.
315 In the example filter, the object (C<$ref>) is blessed just like any
316 other Perl object. Our example uses an anonymous array, but this isn't
317 a requirement. Because this example doesn't need to store any context
318 information, we could have used a scalar or hash reference just as
319 well. The next section demonstrates context data.
321 The association between the filter object and the source stream is made
322 with the C<filter_add()> function. This takes a filter object as a
323 parameter (C<$ref> in this case) and installs it in the source stream.
325 Finally, there is the code that actually does the filtering. For this
326 type of Perl source filter, all the filtering is done in a method
327 called C<filter()>. (It is also possible to write a Perl source filter
328 using a closure. See the C<Filter::Util::Call> manual page for more
329 details.) It's called every time the Perl parser needs another line of
330 source to process. The C<filter()> method, in turn, reads lines from
331 the source stream using the C<filter_read()> function.
333 If a line was available from the source stream, C<filter_read()>
334 returns a status value greater than zero and appends the line to C<$_>.
335 A status value of zero indicates end-of-file, less than zero means an
336 error. The filter function itself is expected to return its status in
337 the same way, and put the filtered line it wants written to the source
338 stream in C<$_>. The use of C<$_> accounts for the brevity of most Perl
341 In order to make use of the rot13 filter we need some way of encoding
342 the source file in rot13 format. The script below, C<mkrot13>, does
345 die "usage mkrot13 filename\n" unless @ARGV ;
347 my $out = "$in.tmp" ;
348 open(IN, "<$in") or die "Cannot open file $in: $!\n";
349 open(OUT, ">$out") or die "Cannot open file $out: $!\n";
351 print OUT "use Rot13;\n" ;
353 tr/a-zA-Z/n-za-mN-ZA-M/ ;
362 If we encrypt this with C<mkrot13>:
364 print " hello fred \n" ;
366 the result will be this:
369 cevag "uryyb serq\a" ;
371 Running it produces this output:
375 =head1 USING CONTEXT: THE DEBUG FILTER
377 The rot13 example was a trivial example. Here's another demonstration
378 that shows off a few more features.
380 Say you wanted to include a lot of debugging code in your Perl script
381 during development, but you didn't want it available in the released
382 product. Source filters offer a solution. In order to keep the example
383 simple, let's say you wanted the debugging output to be controlled by
384 an environment variable, C<DEBUG>. Debugging code is enabled if the
385 variable exists, otherwise it is disabled.
387 Two special marker lines will bracket debugging code, like this:
391 warn "Debug: millennium bug in year $year\n" ;
395 When the C<DEBUG> environment variable exists, the filter ensures that
396 Perl parses only the code between the C<DEBUG_BEGIN> and C<DEBUG_END>
397 markers. That means that when C<DEBUG> does exist, the code above
398 should be passed through the filter unchanged. The marker lines can
399 also be passed through as-is, because the Perl parser will see them as
400 comment lines. When C<DEBUG> isn't set, we need a way to disable the
401 debug code. A simple way to achieve that is to convert the lines
402 between the two markers into comments:
406 # warn "Debug: millennium bug in year $year\n" ;
410 Here is the complete Debug filter:
415 use Filter::Util::Call ;
417 use constant TRUE => 1 ;
418 use constant FALSE => 0 ;
423 Enabled => defined $ENV{DEBUG},
424 InTraceBlock => FALSE,
425 Filename => (caller)[1],
429 filter_add(bless \%context) ;
434 my ($message) = shift ;
435 my ($line_no) = shift || $self->{LastBegin} ;
436 die "$message at $self->{Filename} line $line_no.\n"
442 $status = filter_read() ;
445 # deal with EOF/error first
447 $self->Die("DEBUG_BEGIN has no DEBUG_END")
448 if $self->{InTraceBlock} ;
452 if ($self->{InTraceBlock}) {
453 if (/^\s*##\s*DEBUG_BEGIN/ ) {
454 $self->Die("Nested DEBUG_BEGIN", $self->{LineNo})
455 } elsif (/^\s*##\s*DEBUG_END/) {
456 $self->{InTraceBlock} = FALSE ;
459 # comment out the debug lines when the filter is disabled
460 s/^/#/ if ! $self->{Enabled} ;
461 } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
462 $self->{InTraceBlock} = TRUE ;
463 $self->{LastBegin} = $self->{LineNo} ;
464 } elsif ( /^\s*##\s*DEBUG_END/ ) {
465 $self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo});
472 The big difference between this filter and the previous example is the
473 use of context data in the filter object. The filter object is based on
474 a hash reference, and is used to keep various pieces of context
475 information between calls to the filter function. All but two of the
476 hash fields are used for error reporting. The first of those two,
477 Enabled, is used by the filter to determine whether the debugging code
478 should be given to the Perl parser. The second, InTraceBlock, is true
479 when the filter has encountered a C<DEBUG_BEGIN> line, but has not yet
480 encountered the following C<DEBUG_END> line.
482 If you ignore all the error checking that most of the code does, the
483 essence of the filter is as follows:
488 $status = filter_read() ;
490 # deal with EOF/error first
491 return $status if $status <= 0 ;
492 if ($self->{InTraceBlock}) {
493 if (/^\s*##\s*DEBUG_END/) {
494 $self->{InTraceBlock} = FALSE
497 # comment out debug lines when the filter is disabled
498 s/^/#/ if ! $self->{Enabled} ;
499 } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
500 $self->{InTraceBlock} = TRUE ;
505 Be warned: just as the C-preprocessor doesn't know C, the Debug filter
506 doesn't know Perl. It can be fooled quite easily:
512 Such things aside, you can see that a lot can be achieved with a modest
517 You now have better understanding of what a source filter is, and you
518 might even have a possible use for them. If you feel like playing with
519 source filters but need a bit of inspiration, here are some extra
520 features you could add to the Debug filter.
522 First, an easy one. Rather than having debugging code that is
523 all-or-nothing, it would be much more useful to be able to control
524 which specific blocks of debugging code get included. Try extending the
525 syntax for debug blocks to allow each to be identified. The contents of
526 the C<DEBUG> environment variable can then be used to control which
529 Once you can identify individual blocks, try allowing them to be
530 nested. That isn't difficult either.
532 Here is a interesting idea that doesn't involve the Debug filter.
533 Currently Perl subroutines have fairly limited support for formal
534 parameter lists. You can specify the number of parameters and their
535 type, but you still have to manually take them out of the C<@_> array
536 yourself. Write a source filter that allows you to have a named
537 parameter list. Such a filter would turn this:
539 sub MySub ($first, $second, @rest) { ... }
544 my ($first) = shift ;
545 my ($second) = shift ;
550 Finally, if you feel like a real challenge, have a go at writing a
551 full-blown Perl macro preprocessor as a source filter. Borrow the
552 useful features from the C preprocessor and any other macro processors
553 you know. The tricky bit will be choosing how much knowledge of Perl's
554 syntax you want your filter to have.
558 The Source Filters distribution is available on CPAN, in
560 CPAN/modules/by-module/Filter
564 Paul Marquess E<lt>Paul.Marquess@btinternet.comE<gt>
568 This article originally appeared in The Perl Journal #11, and is
569 copyright 1998 The Perl Journal. It appears courtesy of Jon Orwant and
570 The Perl Journal. This document may be distributed under the same terms