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1 | =head1 NAME |
2 | |
3 | perlfilter - Source Filters |
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4 | |
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5 | =head1 DESCRIPTION |
6 | |
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 |
12 | own. |
13 | |
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. |
17 | |
18 | =head1 CONCEPTS |
19 | |
20 | Before the Perl interpreter can execute a Perl script, it must first |
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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. |
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25 | |
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 |
32 | file. |
33 | |
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. |
37 | |
38 | file -------> parser |
39 | |
40 | There are two important points to remember: |
41 | |
42 | =over 5 |
43 | |
44 | =item 1. |
45 | |
46 | Although there can be any number of source streams in existence at any |
47 | given time, only one will be active. |
48 | |
49 | =item 2. |
50 | |
51 | Every source stream is associated with only one file. |
52 | |
53 | =back |
54 | |
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 |
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57 | changes our diagram like this: |
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58 | |
59 | file ----> filter ----> parser |
60 | |
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. |
65 | |
66 | gunzip -c trial.gz | sh |
67 | |
68 | In this case, the data flow from the pipeline can be represented as follows: |
69 | |
70 | trial.gz ----> gunzip ----> sh |
71 | |
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: |
73 | |
74 | compressed gunzip |
75 | Perl program ---> source filter ---> parser |
76 | |
77 | =head1 USING FILTERS |
78 | |
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. |
82 | |
83 | Say you want to pass your Perl source through the C preprocessor before |
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84 | execution. As it happens, the source filters distribution comes with a C |
85 | preprocessor filter module called Filter::cpp. |
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86 | |
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 |
89 | easily. |
90 | |
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91 | 1: use Filter::cpp; |
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92 | 2: #define TRUE 1 |
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93 | 3: $a = TRUE; |
94 | 4: print "a = $a\n"; |
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95 | |
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: |
99 | |
100 | cpp_test ---------> parser |
101 | |
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: |
107 | |
108 | cpp_test ----> cpp filter ----> parser |
109 | |
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. |
115 | |
116 | .-> cpp --. |
117 | | | |
118 | | | |
119 | | <-' |
120 | cpp_test ----> cpp filter ----> parser |
121 | |
122 | The 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 | |
128 | Let's consider what happens when the filtered code includes another |
129 | module with use: |
130 | |
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131 | 1: use Filter::cpp; |
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132 | 2: #define TRUE 1 |
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133 | 3: use Fred; |
134 | 4: $a = TRUE; |
135 | 5: print "a = $a\n"; |
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136 | |
137 | The C<cpp> filter does not apply to the text of the Fred module, only |
138 | to the text of the file that used it (C<cpp_test>). Although the use |
139 | statement on line 3 will pass through the cpp filter, the module that |
140 | gets included (C<Fred>) will not. The source streams look like this |
141 | after line 3 has been parsed and before line 4 is parsed: |
142 | |
143 | cpp_test ---> cpp filter ---> parser (INACTIVE) |
144 | |
145 | Fred.pm ----> parser |
146 | |
147 | As you can see, a new stream has been created for reading the source |
148 | from C<Fred.pm>. This stream will remain active until all of C<Fred.pm> |
149 | has been parsed. The source stream for C<cpp_test> will still exist, |
150 | but is inactive. Once the parser has finished reading Fred.pm, the |
151 | source stream associated with it will be destroyed. The source stream |
152 | for C<cpp_test> then becomes active again and the parser reads line 4 |
153 | and subsequent lines from C<cpp_test>. |
154 | |
155 | You can use more than one source filter on a single file. Similarly, |
156 | you can reuse the same filter in as many files as you like. |
157 | |
158 | For example, if you have a uuencoded and compressed source file, it is |
159 | possible to stack a uudecode filter and an uncompression filter like |
160 | this: |
161 | |
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162 | use Filter::uudecode; use Filter::uncompress; |
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163 | M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/ |
164 | M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[> |
165 | ... |
166 | |
167 | Once the first line has been processed, the flow will look like this: |
168 | |
169 | file ---> uudecode ---> uncompress ---> parser |
170 | filter filter |
171 | |
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. |
175 | |
176 | =head1 WRITING A SOURCE FILTER |
177 | |
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. |
183 | |
184 | =head1 WRITING A SOURCE FILTER IN C |
185 | |
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. |
189 | |
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). |
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199 | |
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200 | |
201 | =over 5 |
202 | |
203 | =item B<Decryption Filters> |
204 | |
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, 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. |
214 | |
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 |
220 | module. |
221 | |
222 | =back |
223 | |
224 | =head1 CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE |
225 | |
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. |
232 | |
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. |
244 | |
245 | Here 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 = $a\n"; |
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250 | |
251 | The output you'll get when the script is executed: |
252 | |
253 | PQR a = 1 |
254 | |
255 | Writing a source filter as a separate executable works fine, but a |
256 | small performance penalty is incurred. For example, if you execute the |
257 | small example above, a separate subprocess will be created to run the |
258 | Unix C<tr> command. Each use of the filter requires its own subprocess. |
259 | If creating subprocesses is expensive on your system, you might want to |
260 | consider one of the other options for creating source filters. |
261 | |
262 | =head1 WRITING A SOURCE FILTER IN PERL |
263 | |
264 | The easiest and most portable option available for creating your own |
265 | source filter is to write it completely in Perl. To distinguish this |
266 | from the previous two techniques, I'll call it a Perl source filter. |
267 | |
268 | To help understand how to write a Perl source filter we need an example |
269 | to study. Here is a complete source filter that performs rot13 |
270 | decoding. (Rot13 is a very simple encryption scheme used in Usenet |
271 | postings to hide the contents of offensive posts. It moves every letter |
272 | forward thirteen places, so that A becomes N, B becomes O, and Z |
273 | becomes M.) |
274 | |
275 | |
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276 | package Rot13; |
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277 | |
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278 | use Filter::Util::Call; |
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279 | |
280 | sub import { |
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281 | my ($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 | $status; |
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293 | } |
294 | |
295 | 1; |
296 | |
297 | All Perl source filters are implemented as Perl classes and have the |
298 | same basic structure as the example above. |
299 | |
300 | First, we include the C<Filter::Util::Call> module, which exports a |
301 | number of functions into your filter's namespace. The filter shown |
302 | above uses two of these functions, C<filter_add()> and |
303 | C<filter_read()>. |
304 | |
305 | Next, we create the filter object and associate it with the source |
306 | stream by defining the C<import> function. If you know Perl well |
307 | enough, you know that C<import> is called automatically every time a |
308 | module is included with a use statement. This makes C<import> the ideal |
309 | place to both create and install a filter object. |
310 | |
311 | In the example filter, the object (C<$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. |
316 | |
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<$ref> in this case) and installs it in the source stream. |
320 | |
321 | Finally, there is the code that actually does the filtering. For this |
322 | type of Perl source filter, all the filtering is done in a method |
323 | called C<filter()>. (It is also possible to write a Perl source filter |
324 | using a closure. See the C<Filter::Util::Call> manual page for more |
325 | details.) It's called every time the Perl parser needs another line of |
326 | source to process. The C<filter()> method, in turn, reads lines from |
327 | the source stream using the C<filter_read()> function. |
328 | |
329 | If a line was available from the source stream, C<filter_read()> |
330 | returns a status value greater than zero and appends the line to C<$_>. |
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 |
335 | source filters. |
336 | |
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 |
339 | just 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 | |
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347 | print OUT "use Rot13;\n"; |
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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, $in; |
357 | |
358 | If we encrypt this with C<mkrot13>: |
359 | |
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360 | print " hello fred \n"; |
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361 | |
362 | the result will be this: |
363 | |
364 | use Rot13; |
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365 | cevag "uryyb serq\a"; |
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366 | |
367 | Running it produces this output: |
368 | |
369 | hello fred |
370 | |
371 | =head1 USING CONTEXT: THE DEBUG FILTER |
372 | |
373 | The rot13 example was a trivial example. Here's another demonstration |
374 | that shows off a few more features. |
375 | |
376 | Say you wanted to include a lot of debugging code in your Perl script |
377 | during development, but you didn't want it available in the released |
378 | product. Source filters offer a solution. In order to keep the example |
379 | simple, let's say you wanted the debugging output to be controlled by |
380 | an environment variable, C<DEBUG>. Debugging code is enabled if the |
381 | variable exists, otherwise it is disabled. |
382 | |
383 | Two special marker lines will bracket debugging code, like this: |
384 | |
385 | ## DEBUG_BEGIN |
386 | if ($year > 1999) { |
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387 | warn "Debug: millennium bug in year $year\n"; |
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388 | } |
389 | ## DEBUG_END |
390 | |
391 | When the C<DEBUG> environment variable exists, the filter ensures that |
392 | Perl parses only the code between the C<DEBUG_BEGIN> and C<DEBUG_END> |
393 | markers. That means that when C<DEBUG> does exist, the code above |
394 | should be passed through the filter unchanged. The marker lines can |
395 | also be passed through as-is, because the Perl parser will see them as |
396 | comment lines. When C<DEBUG> isn't set, we need a way to disable the |
397 | debug code. A simple way to achieve that is to convert the lines |
398 | between the two markers into comments: |
399 | |
400 | ## DEBUG_BEGIN |
401 | #if ($year > 1999) { |
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402 | # warn "Debug: millennium bug in year $year\n"; |
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403 | #} |
404 | ## DEBUG_END |
405 | |
406 | Here is the complete Debug filter: |
407 | |
408 | package Debug; |
409 | |
410 | use strict; |
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411 | use warnings; |
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412 | use Filter::Util::Call; |
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413 | |
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414 | use constant TRUE => 1; |
415 | use constant FALSE => 0; |
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416 | |
417 | sub import { |
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418 | my ($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/) { |
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453 | $self->{InTraceBlock} = FALSE; |
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454 | } |
455 | |
456 | # comment out the debug lines when the filter is disabled |
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457 | s/^/#/ if ! $self->{Enabled}; |
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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 | } |
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464 | return $status; |
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465 | } |
466 | |
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467 | 1; |
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468 | |
469 | The big difference between this filter and the previous example is the |
470 | use of context data in the filter object. The filter object is based on |
471 | a hash reference, and is used to keep various pieces of context |
472 | information between calls to the filter function. All but two of the |
473 | hash fields are used for error reporting. The first of those two, |
474 | Enabled, is used by the filter to determine whether the debugging code |
475 | should be given to the Perl parser. The second, InTraceBlock, is true |
476 | when the filter has encountered a C<DEBUG_BEGIN> line, but has not yet |
477 | encountered the following C<DEBUG_END> line. |
478 | |
479 | If you ignore all the error checking that most of the code does, the |
480 | essence of the filter is as follows: |
481 | |
482 | sub filter { |
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483 | my ($self) = @_; |
484 | my ($status); |
485 | $status = filter_read(); |
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486 | |
487 | # deal with EOF/error first |
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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 |
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495 | s/^/#/ if ! $self->{Enabled}; |
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496 | } elsif ( /^\s*##\s*DEBUG_BEGIN/ ) { |
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497 | $self->{InTraceBlock} = TRUE; |
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498 | } |
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499 | return $status; |
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500 | } |
501 | |
502 | Be warned: just as the C-preprocessor doesn't know C, the Debug filter |
503 | doesn't know Perl. It can be fooled quite easily: |
504 | |
505 | print <<EOM; |
506 | ##DEBUG_BEGIN |
507 | EOM |
508 | |
509 | Such things aside, you can see that a lot can be achieved with a modest |
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510 | amount of code. |
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511 | |
512 | =head1 CONCLUSION |
513 | |
514 | You now have better understanding of what a source filter is, and you |
515 | might even have a possible use for them. If you feel like playing with |
516 | source filters but need a bit of inspiration, here are some extra |
517 | features you could add to the Debug filter. |
518 | |
519 | First, an easy one. Rather than having debugging code that is |
520 | all-or-nothing, it would be much more useful to be able to control |
521 | which specific blocks of debugging code get included. Try extending the |
522 | syntax for debug blocks to allow each to be identified. The contents of |
523 | the C<DEBUG> environment variable can then be used to control which |
524 | blocks get included. |
525 | |
526 | Once you can identify individual blocks, try allowing them to be |
527 | nested. That isn't difficult either. |
528 | |
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529 | Here is an interesting idea that doesn't involve the Debug filter. |
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530 | Currently Perl subroutines have fairly limited support for formal |
531 | parameter lists. You can specify the number of parameters and their |
532 | type, but you still have to manually take them out of the C<@_> array |
533 | yourself. Write a source filter that allows you to have a named |
534 | parameter list. Such a filter would turn this: |
535 | |
536 | sub MySub ($first, $second, @rest) { ... } |
537 | |
538 | into 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 | |
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. |
552 | |
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553 | =head1 THINGS TO LOOK OUT FOR |
554 | |
555 | =over 5 |
556 | |
557 | =item Some Filters Clobber the C<DATA> Handle |
558 | |
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> |
563 | filehandle. |
564 | |
565 | =back |
566 | |
c7c04614 |
567 | =head1 REQUIREMENTS |
568 | |
569 | The Source Filters distribution is available on CPAN, in |
570 | |
571 | CPAN/modules/by-module/Filter |
572 | |
83df6a1d |
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 |
576 | Damian Conway. |
577 | |
c7c04614 |
578 | =head1 AUTHOR |
579 | |
580 | Paul Marquess E<lt>Paul.Marquess@btinternet.comE<gt> |
581 | |
582 | =head1 Copyrights |
583 | |
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 |
587 | as Perl itself. |