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10862624 RF |
1 | =head1 NAME |
2 | ||
3 | perldebtut - Perl debugging tutorial | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
7 | A (very) lightweight introduction in the use of the perl debugger, and a | |
8 | pointer to existing, deeper sources of information on the subject of debugging | |
7218dffe | 9 | perl programs. |
10862624 RF |
10 | |
11 | There's an extraordinary number of people out there who don't appear to know | |
12 | anything about using the perl debugger, though they use the language every | |
13 | day. | |
14 | This is for them. | |
15 | ||
16 | ||
17 | =head1 use strict | |
18 | ||
7218dffe JH |
19 | First of all, there's a few things you can do to make your life a lot more |
20 | straightforward when it comes to debugging perl programs, without using the | |
6a8e4891 EM |
21 | debugger at all. To demonstrate, here's a simple script, named "hello", with |
22 | a problem: | |
10862624 RF |
23 | |
24 | #!/usr/bin/perl | |
cea6626f | 25 | |
10862624 RF |
26 | $var1 = 'Hello World'; # always wanted to do that :-) |
27 | $var2 = "$varl\n"; | |
cea6626f | 28 | |
10862624 RF |
29 | print $var2; |
30 | exit; | |
31 | ||
32 | While this compiles and runs happily, it probably won't do what's expected, | |
33 | namely it doesn't print "Hello World\n" at all; It will on the other hand do | |
34 | exactly what it was told to do, computers being a bit that way inclined. That | |
35 | is, it will print out a newline character, and you'll get what looks like a | |
36 | blank line. It looks like there's 2 variables when (because of the typo) | |
37 | there's really 3: | |
38 | ||
6a8e4891 EM |
39 | $var1 = 'Hello World'; |
40 | $varl = undef; | |
41 | $var2 = "\n"; | |
10862624 RF |
42 | |
43 | To catch this kind of problem, we can force each variable to be declared | |
44 | before use by pulling in the strict module, by putting 'use strict;' after the | |
45 | first line of the script. | |
46 | ||
47 | Now when you run it, perl complains about the 3 undeclared variables and we | |
48 | get four error messages because one variable is referenced twice: | |
cea6626f | 49 | |
10862624 RF |
50 | Global symbol "$var1" requires explicit package name at ./t1 line 4. |
51 | Global symbol "$var2" requires explicit package name at ./t1 line 5. | |
52 | Global symbol "$varl" requires explicit package name at ./t1 line 5. | |
53 | Global symbol "$var2" requires explicit package name at ./t1 line 7. | |
7218dffe | 54 | Execution of ./hello aborted due to compilation errors. |
10862624 RF |
55 | |
56 | Luvverly! and to fix this we declare all variables explicitly and now our | |
57 | script looks like this: | |
58 | ||
59 | #!/usr/bin/perl | |
60 | use strict; | |
cea6626f | 61 | |
10862624 | 62 | my $var1 = 'Hello World'; |
6a8e4891 | 63 | my $varl = undef; |
10862624 | 64 | my $var2 = "$varl\n"; |
cea6626f | 65 | |
10862624 RF |
66 | print $var2; |
67 | exit; | |
68 | ||
69 | We then do (always a good idea) a syntax check before we try to run it again: | |
70 | ||
71 | > perl -c hello | |
72 | hello syntax OK | |
73 | ||
74 | And now when we run it, we get "\n" still, but at least we know why. Just | |
cb0b211a | 75 | getting this script to compile has exposed the '$varl' (with the letter 'l') |
10862624 RF |
76 | variable, and simply changing $varl to $var1 solves the problem. |
77 | ||
78 | ||
492652be | 79 | =head1 Looking at data and -w and v |
10862624 RF |
80 | |
81 | Ok, but how about when you want to really see your data, what's in that | |
82 | dynamic variable, just before using it? | |
83 | ||
84 | #!/usr/bin/perl | |
85 | use strict; | |
86 | ||
87 | my $key = 'welcome'; | |
88 | my %data = ( | |
89 | 'this' => qw(that), | |
90 | 'tom' => qw(and jerry), | |
91 | 'welcome' => q(Hello World), | |
92 | 'zip' => q(welcome), | |
93 | ); | |
94 | my @data = keys %data; | |
95 | ||
96 | print "$data{$key}\n"; | |
97 | exit; | |
98 | ||
99 | Looks OK, after it's been through the syntax check (perl -c scriptname), we | |
100 | run it and all we get is a blank line again! Hmmmm. | |
cea6626f | 101 | |
10862624 RF |
102 | One common debugging approach here, would be to liberally sprinkle a few print |
103 | statements, to add a check just before we print out our data, and another just | |
104 | after: | |
105 | ||
106 | print "All OK\n" if grep($key, keys %data); | |
107 | print "$data{$key}\n"; | |
108 | print "done: '$data{$key}'\n"; | |
109 | ||
110 | And try again: | |
cea6626f | 111 | |
10862624 RF |
112 | > perl data |
113 | All OK | |
cea6626f | 114 | |
10862624 RF |
115 | done: '' |
116 | ||
117 | After much staring at the same piece of code and not seeing the wood for the | |
118 | trees for some time, we get a cup of coffee and try another approach. That | |
7218dffe | 119 | is, we bring in the cavalry by giving perl the 'B<-d>' switch on the command |
10862624 RF |
120 | line: |
121 | ||
122 | > perl -d data | |
123 | Default die handler restored. | |
124 | ||
125 | Loading DB routines from perl5db.pl version 1.07 | |
126 | Editor support available. | |
127 | ||
128 | Enter h or `h h' for help, or `man perldebug' for more help. | |
129 | ||
130 | main::(./data:4): my $key = 'welcome'; | |
131 | ||
132 | Now, what we've done here is to launch the built-in perl debugger on our | |
133 | script. It's stopped at the first line of executable code and is waiting for | |
134 | input. | |
135 | ||
136 | Before we go any further, you'll want to know how to quit the debugger: use | |
7218dffe | 137 | just the letter 'B<q>', not the words 'quit' or 'exit': |
10862624 RF |
138 | |
139 | DB<1> q | |
140 | > | |
cea6626f | 141 | |
10862624 RF |
142 | That's it, you're back on home turf again. |
143 | ||
7218dffe JH |
144 | |
145 | =head1 help | |
146 | ||
10862624 | 147 | Fire the debugger up again on your script and we'll look at the help menu. |
492652be RF |
148 | There's a couple of ways of calling help: a simple 'B<h>' will get the summary |
149 | help list, 'B<|h>' (pipe-h) will pipe the help through your pager (which is | |
150 | (probably 'more' or 'less'), and finally, 'B<h h>' (h-space-h) will give you | |
151 | the entire help screen. Here is the summary page: | |
152 | ||
153 | DB<1>h | |
6a8e4891 EM |
154 | |
155 | List/search source lines: Control script execution: | |
492652be RF |
156 | l [ln|sub] List source code T Stack trace |
157 | - or . List previous/current line s [expr] Single step [in expr] | |
158 | v [line] View around line n [expr] Next, steps over subs | |
159 | f filename View source in file <CR/Enter> Repeat last n or s | |
160 | /pattern/ ?patt? Search forw/backw r Return from subroutine | |
161 | M Show module versions c [ln|sub] Continue until position | |
947cb114 | 162 | Debugger controls: L List break/watch/actions |
492652be RF |
163 | o [...] Set debugger options t [expr] Toggle trace [trace expr] |
164 | <[<]|{[{]|>[>] [cmd] Do pre/post-prompt b [ln|event|sub] [cnd] Set breakpoint | |
165 | ! [N|pat] Redo a previous command B ln|* Delete a/all breakpoints | |
166 | H [-num] Display last num commands a [ln] cmd Do cmd before line | |
167 | = [a val] Define/list an alias A ln|* Delete a/all actions | |
168 | h [db_cmd] Get help on command w expr Add a watch expression | |
947cb114 | 169 | h h Complete help page W expr|* Delete a/all watch exprs |
492652be RF |
170 | |[|]db_cmd Send output to pager ![!] syscmd Run cmd in a subprocess |
171 | q or ^D Quit R Attempt a restart | |
6a8e4891 | 172 | Data Examination: expr Execute perl code, also see: s,n,t expr |
492652be RF |
173 | x|m expr Evals expr in list context, dumps the result or lists methods. |
174 | p expr Print expression (uses script's current package). | |
175 | S [[!]pat] List subroutine names [not] matching pattern | |
176 | V [Pk [Vars]] List Variables in Package. Vars can be ~pattern or !pattern. | |
177 | X [Vars] Same as "V current_package [Vars]". | |
947cb114 | 178 | y [n [Vars]] List lexicals in higher scope <n>. Vars same as V. |
6a8e4891 | 179 | For more help, type h cmd_letter, or run man perldebug for all docs. |
cea6626f | 180 | |
10862624 RF |
181 | More confusing options than you can shake a big stick at! It's not as bad as |
182 | it looks and it's very useful to know more about all of it, and fun too! | |
183 | ||
7218dffe | 184 | There's a couple of useful ones to know about straight away. You wouldn't |
492652be RF |
185 | think we're using any libraries at all at the moment, but 'B<M>' will show |
186 | which modules are currently loaded, and their version number, while 'B<m>' | |
187 | will show the methods, and 'B<S>' shows all subroutines (by pattern) as | |
188 | shown below. 'B<V>' and 'B<X>' show variables in the program by package | |
189 | scope and can be constrained by pattern. | |
10862624 RF |
190 | |
191 | DB<2>S str | |
192 | dumpvar::stringify | |
193 | strict::bits | |
194 | strict::import | |
195 | strict::unimport | |
7218dffe JH |
196 | |
197 | Using 'X' and cousins requires you not to use the type identifiers ($@%), just | |
198 | the 'name': | |
199 | ||
200 | DM<3>X ~err | |
201 | FileHandle(stderr) => fileno(2) | |
cea6626f | 202 | |
7218dffe | 203 | Remember we're in our tiny program with a problem, we should have a look at |
492652be RF |
204 | where we are, and what our data looks like. First of all let's view some code |
205 | at our present position (the first line of code in this case), via 'B<v>': | |
10862624 | 206 | |
492652be | 207 | DB<4> v |
10862624 RF |
208 | 1 #!/usr/bin/perl |
209 | 2: use strict; | |
210 | 3 | |
211 | 4==> my $key = 'welcome'; | |
212 | 5: my %data = ( | |
213 | 6 'this' => qw(that), | |
214 | 7 'tom' => qw(and jerry), | |
215 | 8 'welcome' => q(Hello World), | |
216 | 9 'zip' => q(welcome), | |
217 | 10 ); | |
218 | ||
219 | At line number 4 is a helpful pointer, that tells you where you are now. To | |
492652be | 220 | see more code, type 'v' again: |
cea6626f | 221 | |
492652be | 222 | DB<4> v |
10862624 RF |
223 | 8 'welcome' => q(Hello World), |
224 | 9 'zip' => q(welcome), | |
225 | 10 ); | |
226 | 11: my @data = keys %data; | |
227 | 12: print "All OK\n" if grep($key, keys %data); | |
228 | 13: print "$data{$key}\n"; | |
229 | 14: print "done: '$data{$key}'\n"; | |
230 | 15: exit; | |
231 | ||
7218dffe | 232 | And if you wanted to list line 5 again, type 'l 5', (note the space): |
10862624 RF |
233 | |
234 | DB<4> l 5 | |
235 | 5: my %data = ( | |
cea6626f | 236 | |
10862624 | 237 | In this case, there's not much to see, but of course normally there's pages of |
7218dffe JH |
238 | stuff to wade through, and 'l' can be very useful. To reset your view to the |
239 | line we're about to execute, type a lone period '.': | |
10862624 | 240 | |
7218dffe | 241 | DB<5> . |
10862624 | 242 | main::(./data_a:4): my $key = 'welcome'; |
cea6626f | 243 | |
10862624 | 244 | The line shown is the one that is about to be executed B<next>, it hasn't |
7218dffe JH |
245 | happened yet. So while we can print a variable with the letter 'B<p>', at |
246 | this point all we'd get is an empty (undefined) value back. What we need to | |
247 | do is to step through the next executable statement with an 'B<s>': | |
cea6626f | 248 | |
10862624 RF |
249 | DB<6> s |
250 | main::(./data_a:5): my %data = ( | |
251 | main::(./data_a:6): 'this' => qw(that), | |
252 | main::(./data_a:7): 'tom' => qw(and jerry), | |
253 | main::(./data_a:8): 'welcome' => q(Hello World), | |
254 | main::(./data_a:9): 'zip' => q(welcome), | |
255 | main::(./data_a:10): ); | |
256 | ||
257 | Now we can have a look at that first ($key) variable: | |
258 | ||
259 | DB<7> p $key | |
260 | welcome | |
261 | ||
262 | line 13 is where the action is, so let's continue down to there via the letter | |
7218dffe JH |
263 | 'B<c>', which by the way, inserts a 'one-time-only' breakpoint at the given |
264 | line or sub routine: | |
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265 | |
266 | DB<8> c 13 | |
267 | All OK | |
268 | main::(./data_a:13): print "$data{$key}\n"; | |
cea6626f | 269 | |
10862624 RF |
270 | We've gone past our check (where 'All OK' was printed) and have stopped just |
271 | before the meat of our task. We could try to print out a couple of variables | |
272 | to see what is happening: | |
273 | ||
274 | DB<9> p $data{$key} | |
cea6626f | 275 | |
7218dffe | 276 | Not much in there, lets have a look at our hash: |
cea6626f | 277 | |
10862624 RF |
278 | DB<10> p %data |
279 | Hello Worldziptomandwelcomejerrywelcomethisthat | |
280 | ||
281 | DB<11> p keys %data | |
282 | Hello Worldtomwelcomejerrythis | |
cea6626f | 283 | |
7218dffe JH |
284 | Well, this isn't very easy to read, and using the helpful manual (B<h h>), the |
285 | 'B<x>' command looks promising: | |
10862624 RF |
286 | |
287 | DB<12> x %data | |
288 | 0 'Hello World' | |
289 | 1 'zip' | |
290 | 2 'tom' | |
291 | 3 'and' | |
292 | 4 'welcome' | |
293 | 5 undef | |
294 | 6 'jerry' | |
295 | 7 'welcome' | |
296 | 8 'this' | |
297 | 9 'that' | |
298 | ||
b1866b2d | 299 | That's not much help, a couple of welcomes in there, but no indication of |
7218dffe | 300 | which are keys, and which are values, it's just a listed array dump and, in |
10862624 RF |
301 | this case, not particularly helpful. The trick here, is to use a B<reference> |
302 | to the data structure: | |
303 | ||
304 | DB<13> x \%data | |
305 | 0 HASH(0x8194bc4) | |
306 | 'Hello World' => 'zip' | |
307 | 'jerry' => 'welcome' | |
308 | 'this' => 'that' | |
309 | 'tom' => 'and' | |
310 | 'welcome' => undef | |
311 | ||
312 | The reference is truly dumped and we can finally see what we're dealing with. | |
313 | Our quoting was perfectly valid but wrong for our purposes, with 'and jerry' | |
314 | being treated as 2 separate words rather than a phrase, thus throwing the | |
315 | evenly paired hash structure out of alignment. | |
316 | ||
7218dffe | 317 | The 'B<-w>' switch would have told us about this, had we used it at the start, |
10862624 RF |
318 | and saved us a lot of trouble: |
319 | ||
320 | > perl -w data | |
321 | Odd number of elements in hash assignment at ./data line 5. | |
322 | ||
323 | We fix our quoting: 'tom' => q(and jerry), and run it again, this time we get | |
324 | our expected output: | |
325 | ||
326 | > perl -w data | |
327 | Hello World | |
328 | ||
329 | ||
7218dffe | 330 | While we're here, take a closer look at the 'B<x>' command, it's really useful |
10862624 | 331 | and will merrily dump out nested references, complete objects, partial objects |
a31a806a | 332 | - just about whatever you throw at it: |
10862624 | 333 | |
da75cd15 | 334 | Let's make a quick object and x-plode it, first we'll start the debugger: |
10862624 RF |
335 | it wants some form of input from STDIN, so we give it something non-commital, |
336 | a zero: | |
337 | ||
338 | > perl -de 0 | |
339 | Default die handler restored. | |
340 | ||
341 | Loading DB routines from perl5db.pl version 1.07 | |
342 | Editor support available. | |
343 | ||
344 | Enter h or `h h' for help, or `man perldebug' for more help. | |
345 | ||
346 | main::(-e:1): 0 | |
347 | ||
348 | Now build an on-the-fly object over a couple of lines (note the backslash): | |
349 | ||
350 | DB<1> $obj = bless({'unique_id'=>'123', 'attr'=> \ | |
351 | cont: {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class') | |
352 | ||
353 | And let's have a look at it: | |
cea6626f | 354 | |
10862624 RF |
355 | DB<2> x $obj |
356 | 0 MY_class=HASH(0x828ad98) | |
357 | 'attr' => HASH(0x828ad68) | |
358 | 'col' => 'black' | |
359 | 'things' => ARRAY(0x828abb8) | |
360 | 0 'this' | |
361 | 1 'that' | |
362 | 2 'etc' | |
363 | 'unique_id' => 123 | |
364 | DB<3> | |
365 | ||
366 | Useful, huh? You can eval nearly anything in there, and experiment with bits | |
367 | of code or regexes until the cows come home: | |
368 | ||
369 | DB<3> @data = qw(this that the other atheism leather theory scythe) | |
cea6626f | 370 | |
10862624 RF |
371 | DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data)) |
372 | atheism | |
373 | leather | |
374 | other | |
375 | scythe | |
376 | the | |
377 | theory | |
378 | saw -> 6 | |
379 | ||
7218dffe | 380 | If you want to see the command History, type an 'B<H>': |
10862624 RF |
381 | |
382 | DB<5> H | |
383 | 4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data)) | |
384 | 3: @data = qw(this that the other atheism leather theory scythe) | |
385 | 2: x $obj | |
386 | 1: $obj = bless({'unique_id'=>'123', 'attr'=> | |
387 | {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class') | |
388 | DB<5> | |
cea6626f | 389 | |
7218dffe | 390 | And if you want to repeat any previous command, use the exclamation: 'B<!>': |
10862624 RF |
391 | |
392 | DB<5> !4 | |
393 | p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data)) | |
394 | atheism | |
395 | leather | |
396 | other | |
397 | scythe | |
398 | the | |
399 | theory | |
400 | saw -> 12 | |
401 | ||
7218dffe JH |
402 | For more on references see L<perlref> and L<perlreftut> |
403 | ||
10862624 RF |
404 | |
405 | =head1 Stepping through code | |
406 | ||
d1f7ad93 | 407 | Here's a simple program which converts between Celsius and Fahrenheit, it too |
10862624 RF |
408 | has a problem: |
409 | ||
410 | #!/usr/bin/perl -w | |
411 | use strict; | |
412 | ||
413 | my $arg = $ARGV[0] || '-c20'; | |
414 | ||
415 | if ($arg =~ /^\-(c|f)((\-|\+)*\d+(\.\d+)*)$/) { | |
416 | my ($deg, $num) = ($1, $2); | |
417 | my ($in, $out) = ($num, $num); | |
418 | if ($deg eq 'c') { | |
419 | $deg = 'f'; | |
420 | $out = &c2f($num); | |
421 | } else { | |
422 | $deg = 'c'; | |
423 | $out = &f2c($num); | |
424 | } | |
425 | $out = sprintf('%0.2f', $out); | |
426 | $out =~ s/^((\-|\+)*\d+)\.0+$/$1/; | |
427 | print "$out $deg\n"; | |
428 | } else { | |
429 | print "Usage: $0 -[c|f] num\n"; | |
430 | } | |
431 | exit; | |
432 | ||
433 | sub f2c { | |
434 | my $f = shift; | |
435 | my $c = 5 * $f - 32 / 9; | |
436 | return $c; | |
437 | } | |
438 | ||
439 | sub c2f { | |
440 | my $c = shift; | |
441 | my $f = 9 * $c / 5 + 32; | |
442 | return $f; | |
443 | } | |
444 | ||
445 | ||
d1f7ad93 | 446 | For some reason, the Fahrenheit to Celsius conversion fails to return the |
10862624 RF |
447 | expected output. This is what it does: |
448 | ||
449 | > temp -c0.72 | |
450 | 33.30 f | |
cea6626f | 451 | |
10862624 RF |
452 | > temp -f33.3 |
453 | 162.94 c | |
cea6626f | 454 | |
10862624 RF |
455 | Not very consistent! We'll set a breakpoint in the code manually and run it |
456 | under the debugger to see what's going on. A breakpoint is a flag, to which | |
a31a806a | 457 | the debugger will run without interruption, when it reaches the breakpoint, it |
10862624 RF |
458 | will stop execution and offer a prompt for further interaction. In normal |
459 | use, these debugger commands are completely ignored, and they are safe - if a | |
460 | little messy, to leave in production code. | |
cea6626f | 461 | |
10862624 RF |
462 | my ($in, $out) = ($num, $num); |
463 | $DB::single=2; # insert at line 9! | |
464 | if ($deg eq 'c') | |
465 | ... | |
cea6626f | 466 | |
10862624 RF |
467 | > perl -d temp -f33.3 |
468 | Default die handler restored. | |
469 | ||
470 | Loading DB routines from perl5db.pl version 1.07 | |
471 | Editor support available. | |
472 | ||
473 | Enter h or `h h' for help, or `man perldebug' for more help. | |
474 | ||
475 | main::(temp:4): my $arg = $ARGV[0] || '-c100'; | |
476 | ||
7218dffe | 477 | We'll simply continue down to our pre-set breakpoint with a 'B<c>': |
10862624 RF |
478 | |
479 | DB<1> c | |
480 | main::(temp:10): if ($deg eq 'c') { | |
481 | ||
492652be | 482 | Followed by a view command to see where we are: |
cea6626f | 483 | |
492652be | 484 | DB<1> v |
10862624 RF |
485 | 7: my ($deg, $num) = ($1, $2); |
486 | 8: my ($in, $out) = ($num, $num); | |
487 | 9: $DB::single=2; | |
488 | 10==> if ($deg eq 'c') { | |
489 | 11: $deg = 'f'; | |
490 | 12: $out = &c2f($num); | |
491 | 13 } else { | |
492 | 14: $deg = 'c'; | |
493 | 15: $out = &f2c($num); | |
494 | 16 } | |
495 | ||
496 | And a print to show what values we're currently using: | |
497 | ||
7218dffe | 498 | DB<1> p $deg, $num |
10862624 | 499 | f33.3 |
13a2d996 | 500 | |
10862624 RF |
501 | We can put another break point on any line beginning with a colon, we'll use |
502 | line 17 as that's just as we come out of the subroutine, and we'd like to | |
503 | pause there later on: | |
cea6626f | 504 | |
7218dffe | 505 | DB<2> b 17 |
cea6626f | 506 | |
10862624 RF |
507 | There's no feedback from this, but you can see what breakpoints are set by |
508 | using the list 'L' command: | |
509 | ||
7218dffe | 510 | DB<3> L |
10862624 RF |
511 | temp: |
512 | 17: print "$out $deg\n"; | |
513 | break if (1) | |
514 | ||
515 | Note that to delete a breakpoint you use 'd' or 'D'. | |
516 | ||
517 | Now we'll continue down into our subroutine, this time rather than by line | |
492652be | 518 | number, we'll use the subroutine name, followed by the now familiar 'v': |
10862624 | 519 | |
7218dffe | 520 | DB<3> c f2c |
10862624 RF |
521 | main::f2c(temp:30): my $f = shift; |
522 | ||
492652be | 523 | DB<4> v |
7218dffe JH |
524 | 24: exit; |
525 | 25 | |
526 | 26 sub f2c { | |
527 | 27==> my $f = shift; | |
528 | 28: my $c = 5 * $f - 32 / 9; | |
529 | 29: return $c; | |
530 | 30 } | |
531 | 31 | |
532 | 32 sub c2f { | |
533 | 33: my $c = shift; | |
534 | ||
535 | ||
536 | Note that if there was a subroutine call between us and line 29, and we wanted | |
537 | to B<single-step> through it, we could use the 'B<s>' command, and to step | |
538 | over it we would use 'B<n>' which would execute the sub, but not descend into | |
539 | it for inspection. In this case though, we simply continue down to line 29: | |
540 | ||
541 | DB<4> c 29 | |
542 | main::f2c(temp:29): return $c; | |
13a2d996 | 543 | |
10862624 RF |
544 | And have a look at the return value: |
545 | ||
7218dffe | 546 | DB<5> p $c |
10862624 RF |
547 | 162.944444444444 |
548 | ||
549 | This is not the right answer at all, but the sum looks correct. I wonder if | |
550 | it's anything to do with operator precedence? We'll try a couple of other | |
551 | possibilities with our sum: | |
552 | ||
7218dffe | 553 | DB<6> p (5 * $f - 32 / 9) |
10862624 | 554 | 162.944444444444 |
cea6626f | 555 | |
7218dffe | 556 | DB<7> p 5 * $f - (32 / 9) |
10862624 | 557 | 162.944444444444 |
cea6626f | 558 | |
7218dffe | 559 | DB<8> p (5 * $f) - 32 / 9 |
10862624 | 560 | 162.944444444444 |
cea6626f | 561 | |
7218dffe | 562 | DB<9> p 5 * ($f - 32) / 9 |
10862624 RF |
563 | 0.722222222222221 |
564 | ||
565 | :-) that's more like it! Ok, now we can set our return variable and we'll | |
566 | return out of the sub with an 'r': | |
567 | ||
7218dffe | 568 | DB<10> $c = 5 * ($f - 32) / 9 |
cea6626f | 569 | |
7218dffe | 570 | DB<11> r |
10862624 | 571 | scalar context return from main::f2c: 0.722222222222221 |
cea6626f | 572 | |
10862624 RF |
573 | Looks good, let's just continue off the end of the script: |
574 | ||
7218dffe | 575 | DB<12> c |
10862624 RF |
576 | 0.72 c |
577 | Debugged program terminated. Use q to quit or R to restart, | |
578 | use O inhibit_exit to avoid stopping after program termination, | |
579 | h q, h R or h O to get additional info. | |
580 | ||
581 | A quick fix to the offending line (insert the missing parentheses) in the | |
582 | actual program and we're finished. | |
583 | ||
584 | ||
585 | =head1 Placeholder for a, w, t, T | |
586 | ||
7218dffe | 587 | Actions, watch variables, stack traces etc.: on the TODO list. |
10862624 RF |
588 | |
589 | a | |
cea6626f | 590 | |
492652be | 591 | w |
cea6626f | 592 | |
10862624 | 593 | t |
cea6626f | 594 | |
10862624 RF |
595 | T |
596 | ||
597 | ||
7218dffe | 598 | =head1 REGULAR EXPRESSIONS |
10862624 RF |
599 | |
600 | Ever wanted to know what a regex looked like? You'll need perl compiled with | |
601 | the DEBUGGING flag for this one: | |
cea6626f | 602 | |
10862624 RF |
603 | > perl -Dr -e '/^pe(a)*rl$/i' |
604 | Compiling REx `^pe(a)*rl$' | |
605 | size 17 first at 2 | |
606 | rarest char | |
607 | at 0 | |
608 | 1: BOL(2) | |
609 | 2: EXACTF <pe>(4) | |
610 | 4: CURLYN[1] {0,32767}(14) | |
611 | 6: NOTHING(8) | |
612 | 8: EXACTF <a>(0) | |
613 | 12: WHILEM(0) | |
614 | 13: NOTHING(14) | |
615 | 14: EXACTF <rl>(16) | |
616 | 16: EOL(17) | |
617 | 17: END(0) | |
618 | floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF <pe>' | |
619 | anchored(BOL) minlen 4 | |
620 | Omitting $` $& $' support. | |
13a2d996 | 621 | |
10862624 RF |
622 | EXECUTING... |
623 | ||
624 | Freeing REx: `^pe(a)*rl$' | |
625 | ||
626 | Did you really want to know? :-) | |
7218dffe JH |
627 | For more gory details on getting regular expressions to work, have a look at |
628 | L<perlre>, L<perlretut>, and to decode the mysterious labels (BOL and CURLYN, | |
629 | etc. above), see L<perldebguts>. | |
10862624 RF |
630 | |
631 | ||
7218dffe | 632 | =head1 OUTPUT TIPS |
10862624 RF |
633 | |
634 | To get all the output from your error log, and not miss any messages via | |
635 | helpful operating system buffering, insert a line like this, at the start of | |
636 | your script: | |
637 | ||
638 | $|=1; | |
639 | ||
640 | To watch the tail of a dynamically growing logfile, (from the command line): | |
641 | ||
642 | tail -f $error_log | |
643 | ||
644 | Wrapping all die calls in a handler routine can be useful to see how, and from | |
645 | where, they're being called, L<perlvar> has more information: | |
646 | ||
7218dffe | 647 | BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } } |
10862624 RF |
648 | |
649 | Various useful techniques for the redirection of STDOUT and STDERR filehandles | |
7218dffe | 650 | are explained in L<perlopentut> and L<perlfaq8>. |
10862624 RF |
651 | |
652 | ||
653 | =head1 CGI | |
654 | ||
7218dffe JH |
655 | Just a quick hint here for all those CGI programmers who can't figure out how |
656 | on earth to get past that 'waiting for input' prompt, when running their CGI | |
657 | script from the command-line, try something like this: | |
10862624 RF |
658 | |
659 | > perl -d my_cgi.pl -nodebug | |
660 | ||
13a2d996 | 661 | Of course L<CGI> and L<perlfaq9> will tell you more. |
10862624 RF |
662 | |
663 | ||
664 | =head1 GUIs | |
665 | ||
666 | The command line interface is tightly integrated with an B<emacs> extension | |
667 | and there's a B<vi> interface too. | |
668 | ||
669 | You don't have to do this all on the command line, though, there are a few GUI | |
670 | options out there. The nice thing about these is you can wave a mouse over a | |
3958b146 | 671 | variable and a dump of its data will appear in an appropriate window, or in a |
10862624 RF |
672 | popup balloon, no more tiresome typing of 'x $varname' :-) |
673 | ||
674 | In particular have a hunt around for the following: | |
675 | ||
676 | B<ptkdb> perlTK based wrapper for the built-in debugger | |
677 | ||
678 | B<ddd> data display debugger | |
cea6626f | 679 | |
10862624 RF |
680 | B<PerlDevKit> and B<PerlBuilder> are NT specific |
681 | ||
682 | NB. (more info on these and others would be appreciated). | |
683 | ||
684 | ||
7218dffe | 685 | =head1 SUMMARY |
10862624 RF |
686 | |
687 | We've seen how to encourage good coding practices with B<use strict> and | |
688 | B<-w>. We can run the perl debugger B<perl -d scriptname> to inspect your | |
689 | data from within the perl debugger with the B<p> and B<x> commands. You can | |
690 | walk through your code, set breakpoints with B<b> and step through that code | |
691 | with B<s> or B<n>, continue with B<c> and return from a sub with B<r>. Fairly | |
692 | intuitive stuff when you get down to it. | |
693 | ||
694 | There is of course lots more to find out about, this has just scratched the | |
695 | surface. The best way to learn more is to use perldoc to find out more about | |
696 | the language, to read the on-line help (L<perldebug> is probably the next | |
697 | place to go), and of course, experiment. | |
698 | ||
699 | ||
700 | =head1 SEE ALSO | |
701 | ||
702 | L<perldebug>, | |
703 | L<perldebguts>, | |
704 | L<perldiag>, | |
705 | L<dprofpp>, | |
706 | L<perlrun> | |
707 | ||
708 | ||
709 | =head1 AUTHOR | |
710 | ||
711 | Richard Foley <richard@rfi.net> Copyright (c) 2000 | |
712 | ||
713 | ||
714 | =head1 CONTRIBUTORS | |
715 | ||
716 | Various people have made helpful suggestions and contributions, in particular: | |
717 | ||
718 | Ronald J Kimball <rjk@linguist.dartmouth.edu> | |
719 | ||
7218dffe JH |
720 | Hugo van der Sanden <hv@crypt0.demon.co.uk> |
721 | ||
10c10266 | 722 | Peter Scott <Peter@PSDT.com> |
10862624 | 723 |