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1=head1 NAME
2
3perlfilter - Source Filters
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5=head1 DESCRIPTION
6
7This article is about a little-known feature of Perl called
8I<source filters>. Source filters alter the program text of a module
9before Perl sees it, much as a C preprocessor alters the source text of
10a C program before the compiler sees it. This article tells you more
11about what source filters are, how they work, and how to write your
12own.
13
14The original purpose of source filters was to let you encrypt your
15program source to prevent casual piracy. This isn't all they can do, as
16you'll soon learn. But first, the basics.
17
18=head1 CONCEPTS
19
20Before the Perl interpreter can execute a Perl script, it must first
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21read it from a file into memory for parsing and compilation. If that
22script itself includes other scripts with a C<use> or C<require>
23statement, then each of those scripts will have to be read from their
24respective files as well.
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25
26Now think of each logical connection between the Perl parser and an
27individual file as a I<source stream>. A source stream is created when
28the Perl parser opens a file, it continues to exist as the source code
29is read into memory, and it is destroyed when Perl is finished parsing
30the file. If the parser encounters a C<require> or C<use> statement in
31a source stream, a new and distinct stream is created just for that
32file.
33
34The diagram below represents a single source stream, with the flow of
35source from a Perl script file on the left into the Perl parser on the
36right. This is how Perl normally operates.
37
38 file -------> parser
39
40There are two important points to remember:
41
42=over 5
43
44=item 1.
45
46Although there can be any number of source streams in existence at any
47given time, only one will be active.
48
49=item 2.
50
51Every source stream is associated with only one file.
52
53=back
54
55A source filter is a special kind of Perl module that intercepts and
56modifies a source stream before it reaches the parser. A source filter
40b7eeef 57changes our diagram like this:
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58
59 file ----> filter ----> parser
60
61If that doesn't make much sense, consider the analogy of a command
62pipeline. Say you have a shell script stored in the compressed file
63I<trial.gz>. The simple pipeline command below runs the script without
64needing to create a temporary file to hold the uncompressed file.
65
66 gunzip -c trial.gz | sh
67
68In this case, the data flow from the pipeline can be represented as follows:
69
70 trial.gz ----> gunzip ----> sh
71
72With source filters, you can store the text of your script compressed and use a source filter to uncompress it for Perl's parser:
73
74 compressed gunzip
75 Perl program ---> source filter ---> parser
76
77=head1 USING FILTERS
78
79So how do you use a source filter in a Perl script? Above, I said that
80a source filter is just a special kind of module. Like all Perl
81modules, a source filter is invoked with a use statement.
82
83Say you want to pass your Perl source through the C preprocessor before
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84execution. As it happens, the source filters distribution comes with a C
85preprocessor filter module called Filter::cpp.
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86
87Below is an example program, C<cpp_test>, which makes use of this filter.
88Line numbers have been added to allow specific lines to be referenced
89easily.
90
4358a253 91 1: use Filter::cpp;
c7c04614 92 2: #define TRUE 1
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93 3: $a = TRUE;
94 4: print "a = $a\n";
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95
96When you execute this script, Perl creates a source stream for the
97file. Before the parser processes any of the lines from the file, the
98source stream looks like this:
99
100 cpp_test ---------> parser
101
102Line 1, C<use Filter::cpp>, includes and installs the C<cpp> filter
103module. All source filters work this way. The use statement is compiled
104and executed at compile time, before any more of the file is read, and
105it attaches the cpp filter to the source stream behind the scenes. Now
106the data flow looks like this:
107
108 cpp_test ----> cpp filter ----> parser
109
110As the parser reads the second and subsequent lines from the source
111stream, it feeds those lines through the C<cpp> source filter before
112processing them. The C<cpp> filter simply passes each line through the
113real C preprocessor. The output from the C preprocessor is then
114inserted back into the source stream by the filter.
115
116 .-> cpp --.
117 | |
118 | |
119 | <-'
120 cpp_test ----> cpp filter ----> parser
121
122The parser then sees the following code:
123
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124 use Filter::cpp;
125 $a = 1;
126 print "a = $a\n";
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127
128Let's consider what happens when the filtered code includes another
129module with use:
130
4358a253 131 1: use Filter::cpp;
c7c04614 132 2: #define TRUE 1
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133 3: use Fred;
134 4: $a = TRUE;
135 5: print "a = $a\n";
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136
137The C<cpp> filter does not apply to the text of the Fred module, only
138to the text of the file that used it (C<cpp_test>). Although the use
139statement on line 3 will pass through the cpp filter, the module that
140gets included (C<Fred>) will not. The source streams look like this
141after line 3 has been parsed and before line 4 is parsed:
142
143 cpp_test ---> cpp filter ---> parser (INACTIVE)
144
145 Fred.pm ----> parser
146
147As you can see, a new stream has been created for reading the source
148from C<Fred.pm>. This stream will remain active until all of C<Fred.pm>
149has been parsed. The source stream for C<cpp_test> will still exist,
150but is inactive. Once the parser has finished reading Fred.pm, the
151source stream associated with it will be destroyed. The source stream
152for C<cpp_test> then becomes active again and the parser reads line 4
153and subsequent lines from C<cpp_test>.
154
155You can use more than one source filter on a single file. Similarly,
156you can reuse the same filter in as many files as you like.
157
158For example, if you have a uuencoded and compressed source file, it is
159possible to stack a uudecode filter and an uncompression filter like
160this:
161
4358a253 162 use Filter::uudecode; use Filter::uncompress;
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163 M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/
164 M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[>
165 ...
166
167Once the first line has been processed, the flow will look like this:
168
169 file ---> uudecode ---> uncompress ---> parser
170 filter filter
171
172Data flows through filters in the same order they appear in the source
173file. The uudecode filter appeared before the uncompress filter, so the
174source file will be uudecoded before it's uncompressed.
175
176=head1 WRITING A SOURCE FILTER
177
178There are three ways to write your own source filter. You can write it
179in C, use an external program as a filter, or write the filter in Perl.
180I won't cover the first two in any great detail, so I'll get them out
181of the way first. Writing the filter in Perl is most convenient, so
182I'll devote the most space to it.
183
184=head1 WRITING A SOURCE FILTER IN C
185
186The first of the three available techniques is to write the filter
187completely in C. The external module you create interfaces directly
188with the source filter hooks provided by Perl.
189
190The advantage of this technique is that you have complete control over
191the implementation of your filter. The big disadvantage is the
192increased complexity required to write the filter - not only do you
193need to understand the source filter hooks, but you also need a
194reasonable knowledge of Perl guts. One of the few times it is worth
195going to this trouble is when writing a source scrambler. The
196C<decrypt> filter (which unscrambles the source before Perl parses it)
197included with the source filter distribution is an example of a C
198source filter (see Decryption Filters, below).
c47ff5f1 199
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200
201=over 5
202
203=item B<Decryption Filters>
204
205All decryption filters work on the principle of "security through
206obscurity." Regardless of how well you write a decryption filter and
e8c6d8ab 207how strong your encryption algorithm is, anyone determined enough can
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208retrieve the original source code. The reason is quite simple - once
209the decryption filter has decrypted the source back to its original
210form, fragments of it will be stored in the computer's memory as Perl
211parses it. The source might only be in memory for a short period of
212time, but anyone possessing a debugger, skill, and lots of patience can
213eventually reconstruct your program.
214
215That said, there are a number of steps that can be taken to make life
216difficult for the potential cracker. The most important: Write your
217decryption filter in C and statically link the decryption module into
218the Perl binary. For further tips to make life difficult for the
219potential cracker, see the file I<decrypt.pm> in the source filters
e8c6d8ab 220distribution.
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221
222=back
223
224=head1 CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE
225
226An alternative to writing the filter in C is to create a separate
227executable in the language of your choice. The separate executable
228reads from standard input, does whatever processing is necessary, and
e8c6d8ab 229writes the filtered data to standard output. C<Filter::cpp> is an
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230example of a source filter implemented as a separate executable - the
231executable is the C preprocessor bundled with your C compiler.
232
233The source filter distribution includes two modules that simplify this
234task: C<Filter::exec> and C<Filter::sh>. Both allow you to run any
235external executable. Both use a coprocess to control the flow of data
236into and out of the external executable. (For details on coprocesses,
e8c6d8ab 237see Stephens, W.R., "Advanced Programming in the UNIX Environment."
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238Addison-Wesley, ISBN 0-210-56317-7, pages 441-445.) The difference
239between them is that C<Filter::exec> spawns the external command
240directly, while C<Filter::sh> spawns a shell to execute the external
241command. (Unix uses the Bourne shell; NT uses the cmd shell.) Spawning
242a shell allows you to make use of the shell metacharacters and
243redirection facilities.
244
245Here is an example script that uses C<Filter::sh>:
246
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247 use Filter::sh 'tr XYZ PQR';
248 $a = 1;
249 print "XYZ a = $a\n";
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250
251The output you'll get when the script is executed:
252
253 PQR a = 1
254
255Writing a source filter as a separate executable works fine, but a
256small performance penalty is incurred. For example, if you execute the
257small example above, a separate subprocess will be created to run the
258Unix C<tr> command. Each use of the filter requires its own subprocess.
259If creating subprocesses is expensive on your system, you might want to
260consider one of the other options for creating source filters.
261
262=head1 WRITING A SOURCE FILTER IN PERL
263
264The easiest and most portable option available for creating your own
265source filter is to write it completely in Perl. To distinguish this
266from the previous two techniques, I'll call it a Perl source filter.
267
268To help understand how to write a Perl source filter we need an example
269to study. Here is a complete source filter that performs rot13
270decoding. (Rot13 is a very simple encryption scheme used in Usenet
271postings to hide the contents of offensive posts. It moves every letter
272forward thirteen places, so that A becomes N, B becomes O, and Z
273becomes M.)
274
275
4358a253 276 package Rot13;
c7c04614 277
4358a253 278 use Filter::Util::Call;
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279
280 sub import {
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281 my ($type) = @_;
282 my ($ref) = [];
283 filter_add(bless $ref);
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284 }
285
286 sub filter {
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287 my ($self) = @_;
288 my ($status);
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289
290 tr/n-za-mN-ZA-M/a-zA-Z/
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291 if ($status = filter_read()) > 0;
292 $status;
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293 }
294
295 1;
296
297All Perl source filters are implemented as Perl classes and have the
298same basic structure as the example above.
299
300First, we include the C<Filter::Util::Call> module, which exports a
301number of functions into your filter's namespace. The filter shown
302above uses two of these functions, C<filter_add()> and
303C<filter_read()>.
304
305Next, we create the filter object and associate it with the source
306stream by defining the C<import> function. If you know Perl well
307enough, you know that C<import> is called automatically every time a
308module is included with a use statement. This makes C<import> the ideal
309place to both create and install a filter object.
310
311In the example filter, the object (C<$ref>) is blessed just like any
312other Perl object. Our example uses an anonymous array, but this isn't
313a requirement. Because this example doesn't need to store any context
314information, we could have used a scalar or hash reference just as
315well. The next section demonstrates context data.
316
317The association between the filter object and the source stream is made
318with the C<filter_add()> function. This takes a filter object as a
319parameter (C<$ref> in this case) and installs it in the source stream.
320
321Finally, there is the code that actually does the filtering. For this
322type of Perl source filter, all the filtering is done in a method
323called C<filter()>. (It is also possible to write a Perl source filter
324using a closure. See the C<Filter::Util::Call> manual page for more
325details.) It's called every time the Perl parser needs another line of
326source to process. The C<filter()> method, in turn, reads lines from
327the source stream using the C<filter_read()> function.
328
329If a line was available from the source stream, C<filter_read()>
330returns a status value greater than zero and appends the line to C<$_>.
331A status value of zero indicates end-of-file, less than zero means an
332error. The filter function itself is expected to return its status in
333the same way, and put the filtered line it wants written to the source
334stream in C<$_>. The use of C<$_> accounts for the brevity of most Perl
335source filters.
336
337In order to make use of the rot13 filter we need some way of encoding
338the source file in rot13 format. The script below, C<mkrot13>, does
339just that.
340
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341 die "usage mkrot13 filename\n" unless @ARGV;
342 my $in = $ARGV[0];
343 my $out = "$in.tmp";
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344 open(IN, "<$in") or die "Cannot open file $in: $!\n";
345 open(OUT, ">$out") or die "Cannot open file $out: $!\n";
346
4358a253 347 print OUT "use Rot13;\n";
c7c04614 348 while (<IN>) {
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349 tr/a-zA-Z/n-za-mN-ZA-M/;
350 print OUT;
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351 }
352
353 close IN;
354 close OUT;
355 unlink $in;
356 rename $out, $in;
357
358If we encrypt this with C<mkrot13>:
359
4358a253 360 print " hello fred \n";
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361
362the result will be this:
363
364 use Rot13;
4358a253 365 cevag "uryyb serq\a";
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366
367Running it produces this output:
368
369 hello fred
370
371=head1 USING CONTEXT: THE DEBUG FILTER
372
373The rot13 example was a trivial example. Here's another demonstration
374that shows off a few more features.
375
376Say you wanted to include a lot of debugging code in your Perl script
377during development, but you didn't want it available in the released
378product. Source filters offer a solution. In order to keep the example
379simple, let's say you wanted the debugging output to be controlled by
380an environment variable, C<DEBUG>. Debugging code is enabled if the
381variable exists, otherwise it is disabled.
382
383Two special marker lines will bracket debugging code, like this:
384
385 ## DEBUG_BEGIN
386 if ($year > 1999) {
4358a253 387 warn "Debug: millennium bug in year $year\n";
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388 }
389 ## DEBUG_END
390
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391The filter ensures that Perl parses the code between the <DEBUG_BEGIN>
392and C<DEBUG_END> markers only when the C<DEBUG> environment variable
393exists. That means that when C<DEBUG> does exist, the code above
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394should be passed through the filter unchanged. The marker lines can
395also be passed through as-is, because the Perl parser will see them as
396comment lines. When C<DEBUG> isn't set, we need a way to disable the
397debug code. A simple way to achieve that is to convert the lines
398between the two markers into comments:
399
400 ## DEBUG_BEGIN
401 #if ($year > 1999) {
4358a253 402 # warn "Debug: millennium bug in year $year\n";
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403 #}
404 ## DEBUG_END
405
406Here is the complete Debug filter:
407
408 package Debug;
409
410 use strict;
9f1b1f2d 411 use warnings;
4358a253 412 use Filter::Util::Call;
c7c04614 413
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414 use constant TRUE => 1;
415 use constant FALSE => 0;
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416
417 sub import {
4358a253 418 my ($type) = @_;
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419 my (%context) = (
420 Enabled => defined $ENV{DEBUG},
421 InTraceBlock => FALSE,
422 Filename => (caller)[1],
423 LineNo => 0,
424 LastBegin => 0,
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425 );
426 filter_add(bless \%context);
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427 }
428
429 sub Die {
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430 my ($self) = shift;
431 my ($message) = shift;
432 my ($line_no) = shift || $self->{LastBegin};
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433 die "$message at $self->{Filename} line $line_no.\n"
434 }
435
436 sub filter {
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437 my ($self) = @_;
438 my ($status);
439 $status = filter_read();
440 ++ $self->{LineNo};
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441
442 # deal with EOF/error first
443 if ($status <= 0) {
444 $self->Die("DEBUG_BEGIN has no DEBUG_END")
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445 if $self->{InTraceBlock};
446 return $status;
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447 }
448
449 if ($self->{InTraceBlock}) {
450 if (/^\s*##\s*DEBUG_BEGIN/ ) {
451 $self->Die("Nested DEBUG_BEGIN", $self->{LineNo})
452 } elsif (/^\s*##\s*DEBUG_END/) {
4358a253 453 $self->{InTraceBlock} = FALSE;
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454 }
455
456 # comment out the debug lines when the filter is disabled
4358a253 457 s/^/#/ if ! $self->{Enabled};
c7c04614 458 } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
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459 $self->{InTraceBlock} = TRUE;
460 $self->{LastBegin} = $self->{LineNo};
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461 } elsif ( /^\s*##\s*DEBUG_END/ ) {
462 $self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo});
463 }
4358a253 464 return $status;
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465 }
466
4358a253 467 1;
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468
469The big difference between this filter and the previous example is the
470use of context data in the filter object. The filter object is based on
471a hash reference, and is used to keep various pieces of context
472information between calls to the filter function. All but two of the
473hash fields are used for error reporting. The first of those two,
474Enabled, is used by the filter to determine whether the debugging code
475should be given to the Perl parser. The second, InTraceBlock, is true
476when the filter has encountered a C<DEBUG_BEGIN> line, but has not yet
477encountered the following C<DEBUG_END> line.
478
479If you ignore all the error checking that most of the code does, the
480essence of the filter is as follows:
481
482 sub filter {
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483 my ($self) = @_;
484 my ($status);
485 $status = filter_read();
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486
487 # deal with EOF/error first
4358a253 488 return $status if $status <= 0;
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489 if ($self->{InTraceBlock}) {
490 if (/^\s*##\s*DEBUG_END/) {
491 $self->{InTraceBlock} = FALSE
492 }
493
494 # comment out debug lines when the filter is disabled
4358a253 495 s/^/#/ if ! $self->{Enabled};
c7c04614 496 } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
4358a253 497 $self->{InTraceBlock} = TRUE;
c7c04614 498 }
4358a253 499 return $status;
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500 }
501
502Be warned: just as the C-preprocessor doesn't know C, the Debug filter
503doesn't know Perl. It can be fooled quite easily:
504
505 print <<EOM;
506 ##DEBUG_BEGIN
507 EOM
508
509Such things aside, you can see that a lot can be achieved with a modest
40b7eeef 510amount of code.
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511
512=head1 CONCLUSION
513
514You now have better understanding of what a source filter is, and you
515might even have a possible use for them. If you feel like playing with
516source filters but need a bit of inspiration, here are some extra
517features you could add to the Debug filter.
518
519First, an easy one. Rather than having debugging code that is
520all-or-nothing, it would be much more useful to be able to control
521which specific blocks of debugging code get included. Try extending the
522syntax for debug blocks to allow each to be identified. The contents of
523the C<DEBUG> environment variable can then be used to control which
524blocks get included.
525
526Once you can identify individual blocks, try allowing them to be
527nested. That isn't difficult either.
528
d1be9408 529Here is an interesting idea that doesn't involve the Debug filter.
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530Currently Perl subroutines have fairly limited support for formal
531parameter lists. You can specify the number of parameters and their
532type, but you still have to manually take them out of the C<@_> array
533yourself. Write a source filter that allows you to have a named
534parameter list. Such a filter would turn this:
535
536 sub MySub ($first, $second, @rest) { ... }
537
538into this:
539
540 sub MySub($$@) {
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541 my ($first) = shift;
542 my ($second) = shift;
543 my (@rest) = @_;
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544 ...
545 }
546
547Finally, if you feel like a real challenge, have a go at writing a
548full-blown Perl macro preprocessor as a source filter. Borrow the
549useful features from the C preprocessor and any other macro processors
550you know. The tricky bit will be choosing how much knowledge of Perl's
551syntax you want your filter to have.
552
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553=head1 LIMITATIONS
554
555Source filters only work on the string level, thus are highly limited
556in its ability to change source code on the fly. It cannot detect
557comments, quoted strings, heredocs, it is no replacement for a real
558parser.
559The only stable usage for source filters are encryption, compression,
560or the byteloader, to translate binary code back to source code.
561
356231b0 562See for example the limitations in L<Switch>, which uses source filters,
f686c54e 563and thus is does not work inside a string eval, the presence of
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564regexes with embedded newlines that are specified with raw C</.../>
565delimiters and don't have a modifier C<//x> are indistinguishable from
566code chunks beginning with the division operator C</>. As a workaround
567you must use C<m/.../> or C<m?...?> for such patterns. Also, the presence of
568regexes specified with raw C<?...?> delimiters may cause mysterious
569errors. The workaround is to use C<m?...?> instead. See
570L<http://search.cpan.org/perldoc?Switch#LIMITATIONS>
571
572Currently the content of the C<__DATA__> block is not filtered.
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573
574Currently internal buffer lengths are limited to 32-bit only.
575
576
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577=head1 THINGS TO LOOK OUT FOR
578
579=over 5
580
581=item Some Filters Clobber the C<DATA> Handle
582
583Some source filters use the C<DATA> handle to read the calling program.
584When using these source filters you cannot rely on this handle, nor expect
585any particular kind of behavior when operating on it. Filters based on
586Filter::Util::Call (and therefore Filter::Simple) do not alter the C<DATA>
356231b0 587filehandle, but on the other hand totally ignore the text after C<__DATA__>.
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588
589=back
590
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591=head1 REQUIREMENTS
592
593The Source Filters distribution is available on CPAN, in
594
595 CPAN/modules/by-module/Filter
596
83df6a1d
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597Starting from Perl 5.8 Filter::Util::Call (the core part of the
598Source Filters distribution) is part of the standard Perl distribution.
599Also included is a friendlier interface called Filter::Simple, by
600Damian Conway.
601
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602=head1 AUTHOR
603
604Paul Marquess E<lt>Paul.Marquess@btinternet.comE<gt>
605
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606Reini Urban E<lt>rurban@cpan.orgE<gt>
607
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608=head1 Copyrights
609
356231b0
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610The first version of this article originally appeared in The Perl
611Journal #11, and is copyright 1998 The Perl Journal. It appears
612courtesy of Jon Orwant and The Perl Journal. This document may be
613distributed under the same terms as Perl itself.