3 perldebtut - Perl debugging tutorial
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
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
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
21 debugger at all. To demonstrate, here's a simple script, named "hello", with
26 $var1 = 'Hello World'; # always wanted to do that :-)
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)
39 $var1 = 'Hello World';
43 To catch this kind of problem, we can force each variable to be declared
44 before use by pulling in the strict module, by putting 'use strict;' after the
45 first line of the script.
47 Now when you run it, perl complains about the 3 undeclared variables and we
48 get four error messages because one variable is referenced twice:
50 Global symbol "$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.
54 Execution of ./hello aborted due to compilation errors.
56 Luvverly! and to fix this we declare all variables explicitly and now our
57 script looks like this:
62 my $var1 = 'Hello World';
69 We then do (always a good idea) a syntax check before we try to run it again:
74 And now when we run it, we get "\n" still, but at least we know why. Just
75 getting this script to compile has exposed the '$varl' (with the letter 'l')
76 variable, and simply changing $varl to $var1 solves the problem.
79 =head1 Looking at data and -w and v
81 Ok, but how about when you want to really see your data, what's in that
82 dynamic variable, just before using it?
90 'tom' => qw(and jerry),
91 'welcome' => q(Hello World),
94 my @data = keys %data;
96 print "$data{$key}\n";
99 Looks OK, after it's been through the syntax check (perl -c scriptname), we
100 run it and all we get is a blank line again! Hmmmm.
102 One common debugging approach here, would be to liberally sprinkle a few print
103 statements, to add a check just before we print out our data, and another just
106 print "All OK\n" if grep($key, keys %data);
107 print "$data{$key}\n";
108 print "done: '$data{$key}'\n";
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
119 is, we bring in the cavalry by giving perl the 'B<-d>' switch on the command
123 Default die handler restored.
125 Loading DB routines from perl5db.pl version 1.07
126 Editor support available.
128 Enter h or `h h' for help, or `man perldebug' for more help.
130 main::(./data:4): my $key = 'welcome';
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
136 Before we go any further, you'll want to know how to quit the debugger: use
137 just the letter 'B<q>', not the words 'quit' or 'exit':
142 That's it, you're back on home turf again.
147 Fire the debugger up again on your script and we'll look at the help menu.
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:
155 List/search source lines: Control script execution:
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
162 Debugger controls: L List break/watch/actions
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
169 h h Complete help page W expr|* Delete a/all watch expressions
170 |[|]db_cmd Send output to pager ![!] syscmd Run cmd in a subprocess
171 q or ^D Quit R Attempt a restart
172 Data Examination: expr Execute perl code, also see: s,n,t expr
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]".
178 For more help, type h cmd_letter, or run man perldebug for all docs.
180 More confusing options than you can shake a big stick at! It's not as bad as
181 it looks and it's very useful to know more about all of it, and fun too!
183 There's a couple of useful ones to know about straight away. You wouldn't
184 think we're using any libraries at all at the moment, but 'B<M>' will show
185 which modules are currently loaded, and their version number, while 'B<m>'
186 will show the methods, and 'B<S>' shows all subroutines (by pattern) as
187 shown below. 'B<V>' and 'B<X>' show variables in the program by package
188 scope and can be constrained by pattern.
196 Using 'X' and cousins requires you not to use the type identifiers ($@%), just
200 FileHandle(stderr) => fileno(2)
202 Remember we're in our tiny program with a problem, we should have a look at
203 where we are, and what our data looks like. First of all let's view some code
204 at our present position (the first line of code in this case), via 'B<v>':
210 4==> my $key = 'welcome';
212 6 'this' => qw(that),
213 7 'tom' => qw(and jerry),
214 8 'welcome' => q(Hello World),
215 9 'zip' => q(welcome),
218 At line number 4 is a helpful pointer, that tells you where you are now. To
219 see more code, type 'v' again:
222 8 'welcome' => q(Hello World),
223 9 'zip' => q(welcome),
225 11: my @data = keys %data;
226 12: print "All OK\n" if grep($key, keys %data);
227 13: print "$data{$key}\n";
228 14: print "done: '$data{$key}'\n";
231 And if you wanted to list line 5 again, type 'l 5', (note the space):
236 In this case, there's not much to see, but of course normally there's pages of
237 stuff to wade through, and 'l' can be very useful. To reset your view to the
238 line we're about to execute, type a lone period '.':
241 main::(./data_a:4): my $key = 'welcome';
243 The line shown is the one that is about to be executed B<next>, it hasn't
244 happened yet. So while we can print a variable with the letter 'B<p>', at
245 this point all we'd get is an empty (undefined) value back. What we need to
246 do is to step through the next executable statement with an 'B<s>':
249 main::(./data_a:5): my %data = (
250 main::(./data_a:6): 'this' => qw(that),
251 main::(./data_a:7): 'tom' => qw(and jerry),
252 main::(./data_a:8): 'welcome' => q(Hello World),
253 main::(./data_a:9): 'zip' => q(welcome),
254 main::(./data_a:10): );
256 Now we can have a look at that first ($key) variable:
261 line 13 is where the action is, so let's continue down to there via the letter
262 'B<c>', which by the way, inserts a 'one-time-only' breakpoint at the given
267 main::(./data_a:13): print "$data{$key}\n";
269 We've gone past our check (where 'All OK' was printed) and have stopped just
270 before the meat of our task. We could try to print out a couple of variables
271 to see what is happening:
275 Not much in there, lets have a look at our hash:
278 Hello Worldziptomandwelcomejerrywelcomethisthat
281 Hello Worldtomwelcomejerrythis
283 Well, this isn't very easy to read, and using the helpful manual (B<h h>), the
284 'B<x>' command looks promising:
298 That's not much help, a couple of welcomes in there, but no indication of
299 which are keys, and which are values, it's just a listed array dump and, in
300 this case, not particularly helpful. The trick here, is to use a B<reference>
301 to the data structure:
305 'Hello World' => 'zip'
311 The reference is truly dumped and we can finally see what we're dealing with.
312 Our quoting was perfectly valid but wrong for our purposes, with 'and jerry'
313 being treated as 2 separate words rather than a phrase, thus throwing the
314 evenly paired hash structure out of alignment.
316 The 'B<-w>' switch would have told us about this, had we used it at the start,
317 and saved us a lot of trouble:
320 Odd number of elements in hash assignment at ./data line 5.
322 We fix our quoting: 'tom' => q(and jerry), and run it again, this time we get
329 While we're here, take a closer look at the 'B<x>' command, it's really useful
330 and will merrily dump out nested references, complete objects, partial objects
331 - just about whatever you throw at it:
333 Let's make a quick object and x-plode it, first we'll start the debugger:
334 it wants some form of input from STDIN, so we give it something non-commital,
338 Default die handler restored.
340 Loading DB routines from perl5db.pl version 1.07
341 Editor support available.
343 Enter h or `h h' for help, or `man perldebug' for more help.
347 Now build an on-the-fly object over a couple of lines (note the backslash):
349 DB<1> $obj = bless({'unique_id'=>'123', 'attr'=> \
350 cont: {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
352 And let's have a look at it:
355 0 MY_class=HASH(0x828ad98)
356 'attr' => HASH(0x828ad68)
358 'things' => ARRAY(0x828abb8)
365 Useful, huh? You can eval nearly anything in there, and experiment with bits
366 of code or regexes until the cows come home:
368 DB<3> @data = qw(this that the other atheism leather theory scythe)
370 DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
379 If you want to see the command History, type an 'B<H>':
382 4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
383 3: @data = qw(this that the other atheism leather theory scythe)
385 1: $obj = bless({'unique_id'=>'123', 'attr'=>
386 {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
389 And if you want to repeat any previous command, use the exclamation: 'B<!>':
392 p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data))
401 For more on references see L<perlref> and L<perlreftut>
404 =head1 Stepping through code
406 Here's a simple program which converts between Celsius and Fahrenheit, it too
412 my $arg = $ARGV[0] || '-c20';
414 if ($arg =~ /^\-(c|f)((\-|\+)*\d+(\.\d+)*)$/) {
415 my ($deg, $num) = ($1, $2);
416 my ($in, $out) = ($num, $num);
424 $out = sprintf('%0.2f', $out);
425 $out =~ s/^((\-|\+)*\d+)\.0+$/$1/;
428 print "Usage: $0 -[c|f] num\n";
434 my $c = 5 * $f - 32 / 9;
440 my $f = 9 * $c / 5 + 32;
445 For some reason, the Fahrenheit to Celsius conversion fails to return the
446 expected output. This is what it does:
454 Not very consistent! We'll set a breakpoint in the code manually and run it
455 under the debugger to see what's going on. A breakpoint is a flag, to which
456 the debugger will run without interruption, when it reaches the breakpoint, it
457 will stop execution and offer a prompt for further interaction. In normal
458 use, these debugger commands are completely ignored, and they are safe - if a
459 little messy, to leave in production code.
461 my ($in, $out) = ($num, $num);
462 $DB::single=2; # insert at line 9!
466 > perl -d temp -f33.3
467 Default die handler restored.
469 Loading DB routines from perl5db.pl version 1.07
470 Editor support available.
472 Enter h or `h h' for help, or `man perldebug' for more help.
474 main::(temp:4): my $arg = $ARGV[0] || '-c100';
476 We'll simply continue down to our pre-set breakpoint with a 'B<c>':
479 main::(temp:10): if ($deg eq 'c') {
481 Followed by a view command to see where we are:
484 7: my ($deg, $num) = ($1, $2);
485 8: my ($in, $out) = ($num, $num);
487 10==> if ($deg eq 'c') {
489 12: $out = &c2f($num);
492 15: $out = &f2c($num);
495 And a print to show what values we're currently using:
500 We can put another break point on any line beginning with a colon, we'll use
501 line 17 as that's just as we come out of the subroutine, and we'd like to
502 pause there later on:
506 There's no feedback from this, but you can see what breakpoints are set by
507 using the list 'L' command:
511 17: print "$out $deg\n";
514 Note that to delete a breakpoint you use 'd' or 'D'.
516 Now we'll continue down into our subroutine, this time rather than by line
517 number, we'll use the subroutine name, followed by the now familiar 'v':
520 main::f2c(temp:30): my $f = shift;
527 28: my $c = 5 * $f - 32 / 9;
535 Note that if there was a subroutine call between us and line 29, and we wanted
536 to B<single-step> through it, we could use the 'B<s>' command, and to step
537 over it we would use 'B<n>' which would execute the sub, but not descend into
538 it for inspection. In this case though, we simply continue down to line 29:
541 main::f2c(temp:29): return $c;
543 And have a look at the return value:
548 This is not the right answer at all, but the sum looks correct. I wonder if
549 it's anything to do with operator precedence? We'll try a couple of other
550 possibilities with our sum:
552 DB<6> p (5 * $f - 32 / 9)
555 DB<7> p 5 * $f - (32 / 9)
558 DB<8> p (5 * $f) - 32 / 9
561 DB<9> p 5 * ($f - 32) / 9
564 :-) that's more like it! Ok, now we can set our return variable and we'll
565 return out of the sub with an 'r':
567 DB<10> $c = 5 * ($f - 32) / 9
570 scalar context return from main::f2c: 0.722222222222221
572 Looks good, let's just continue off the end of the script:
576 Debugged program terminated. Use q to quit or R to restart,
577 use O inhibit_exit to avoid stopping after program termination,
578 h q, h R or h O to get additional info.
580 A quick fix to the offending line (insert the missing parentheses) in the
581 actual program and we're finished.
584 =head1 Placeholder for a, w, t, T
586 Actions, watch variables, stack traces etc.: on the TODO list.
597 =head1 REGULAR EXPRESSIONS
599 Ever wanted to know what a regex looked like? You'll need perl compiled with
600 the DEBUGGING flag for this one:
602 > perl -Dr -e '/^pe(a)*rl$/i'
603 Compiling REx `^pe(a)*rl$'
609 4: CURLYN[1] {0,32767}(14)
617 floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF <pe>'
618 anchored(BOL) minlen 4
619 Omitting $` $& $' support.
623 Freeing REx: `^pe(a)*rl$'
625 Did you really want to know? :-)
626 For more gory details on getting regular expressions to work, have a look at
627 L<perlre>, L<perlretut>, and to decode the mysterious labels (BOL and CURLYN,
628 etc. above), see L<perldebguts>.
633 To get all the output from your error log, and not miss any messages via
634 helpful operating system buffering, insert a line like this, at the start of
639 To watch the tail of a dynamically growing logfile, (from the command line):
643 Wrapping all die calls in a handler routine can be useful to see how, and from
644 where, they're being called, L<perlvar> has more information:
646 BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } }
648 Various useful techniques for the redirection of STDOUT and STDERR filehandles
649 are explained in L<perlopentut> and L<perlfaq8>.
654 Just a quick hint here for all those CGI programmers who can't figure out how
655 on earth to get past that 'waiting for input' prompt, when running their CGI
656 script from the command-line, try something like this:
658 > perl -d my_cgi.pl -nodebug
660 Of course L<CGI> and L<perlfaq9> will tell you more.
665 The command line interface is tightly integrated with an B<emacs> extension
666 and there's a B<vi> interface too.
668 You don't have to do this all on the command line, though, there are a few GUI
669 options out there. The nice thing about these is you can wave a mouse over a
670 variable and a dump of its data will appear in an appropriate window, or in a
671 popup balloon, no more tiresome typing of 'x $varname' :-)
673 In particular have a hunt around for the following:
675 B<ptkdb> perlTK based wrapper for the built-in debugger
677 B<ddd> data display debugger
679 B<PerlDevKit> and B<PerlBuilder> are NT specific
681 NB. (more info on these and others would be appreciated).
686 We've seen how to encourage good coding practices with B<use strict> and
687 B<-w>. We can run the perl debugger B<perl -d scriptname> to inspect your
688 data from within the perl debugger with the B<p> and B<x> commands. You can
689 walk through your code, set breakpoints with B<b> and step through that code
690 with B<s> or B<n>, continue with B<c> and return from a sub with B<r>. Fairly
691 intuitive stuff when you get down to it.
693 There is of course lots more to find out about, this has just scratched the
694 surface. The best way to learn more is to use perldoc to find out more about
695 the language, to read the on-line help (L<perldebug> is probably the next
696 place to go), and of course, experiment.
710 Richard Foley <richard@rfi.net> Copyright (c) 2000
715 Various people have made helpful suggestions and contributions, in particular:
717 Ronald J Kimball <rjk@linguist.dartmouth.edu>
719 Hugo van der Sanden <hv@crypt0.demon.co.uk>
721 Peter Scott <Peter@PSDT.com>