perldebtut - Perl debugging tutorial
A (very) lightweight introduction in the use of the perl
debugger, and a pointer to existing, deeper sources of
information on the subject of debugging perl programs.
There's an extraordinary number of people out there who
don't appear to know anything about using the perl debugger,
though they use the language every day. This is for
them.
use strict
First of all, there's a few things you can do to make your
life a lot more straightforward when it comes to debugging
perl programs, without using the debugger at all. To
demonstrate, here's a simple script, named "hello", with a
problem:
#!/usr/bin/perl
$var1 = 'Hello World'; # always wanted to do that
:-)
$var2 = "$varl0;
print $var2;
exit;
While this compiles and runs happily, it probably won't do
what's expected, namely it doesn't print "Hello World0
at all; It will on the other hand do exactly what it was
told to do, computers being a bit that way inclined. That
is, it will print out a newline character, and you'll get
what looks like a blank line. It looks like there's 2
variables when (because of the typo) there's really 3:
$var1 = 'Hello World';
$varl = undef;
$var2 = "0;
To catch this kind of problem, we can force each variable
to be declared before use by pulling in the strict module,
by putting 'use strict;' after the first line of the
script.
Now when you run it, perl complains about the 3 undeclared
variables and we get four error messages because one variable
is referenced twice:
Global symbol "$var1" requires explicit package name at
./t1 line 4.
Global symbol "$var2" requires explicit package name at
./t1 line 5.
Global symbol "$varl" requires explicit package name at
./t1 line 5.
Global symbol "$var2" requires explicit package name at
./t1 line 7.
Execution of ./hello aborted due to compilation errors.
Luvverly! and to fix this we declare all variables explicitly
and now our script looks like this:
#!/usr/bin/perl
use strict;
my $var1 = 'Hello World';
my $varl = undef;
my $var2 = "$varl0;
print $var2;
exit;
We then do (always a good idea) a syntax check before we
try to run it again:
> perl -c hello
hello syntax OK
And now when we run it, we get "0 still, but at least we
know why. Just getting this script to compile has exposed
the '$varl' (with the letter 'l') variable, and simply
changing $varl to $var1 solves the problem.
Looking at data and -w and v [Toc] [Back] Ok, but how about when you want to really see your data,
what's in that dynamic variable, just before using it?
#!/usr/bin/perl
use strict;
my $key = 'welcome';
my %data = (
'this' => qw(that),
'tom' => qw(and jerry),
'welcome' => q(Hello World),
'zip' => q(welcome),
);
my @data = keys %data;
print "$data{$key}0;
exit;
Looks OK, after it's been through the syntax check (perl
-c scriptname), we run it and all we get is a blank line
again! Hmmmm.
One common debugging approach here, would be to liberally
sprinkle a few print statements, to add a check just
before we print out our data, and another just after:
print "All OK0 if grep($key, keys %data);
print "$data{$key}0;
print "done: '$data{$key}'0;
And try again:
> perl data
All OK
done: ''
After much staring at the same piece of code and not seeing
the wood for the trees for some time, we get a cup of
coffee and try another approach. That is, we bring in the
cavalry by giving perl the '-d' switch on the command
line:
> perl -d data
Default die handler restored.
Loading DB routines from perl5db.pl version 1.07
Editor support available.
Enter h or `h h' for help, or `man perldebug' for
more help.
main::(./data:4): my $key = 'welcome';
Now, what we've done here is to launch the built-in perl
debugger on our script. It's stopped at the first line of
executable code and is waiting for input.
Before we go any further, you'll want to know how to quit
the debugger: use just the letter 'q', not the words
'quit' or 'exit':
DB<1> q
>
That's it, you're back on home turf again.
help
Fire the debugger up again on your script and we'll look
at the help menu. There's a couple of ways of calling
help: a simple 'h' will get the summary help list, '|h'
(pipe-h) will pipe the help through your pager (which is
(probably 'more' or 'less'), and finally, 'h h'
(h-space-h) will give you the entire help screen. Here is
the summary page:
D1h
List/search source lines: Control script
execution:
l [ln|sub] List source code T
Stack trace
- or . List previous/current line s [expr] Single step [in expr]
v [line] View around line n [expr]
Next, steps over subs
f filename View source in file <CR/Enter> Repeat last n or s
/pattern/ ?patt? Search forw/backw r Return from subroutine
M Show module versions c [ln|sub] Continue until position
Debugger controls: L List
break/watch/actions
o [...] Set debugger options t [expr] Toggle trace [trace expr]
<[<]|{[{]|>[>] [cmd] Do pre/post-prompt b [ln|event|sub]
[cnd] Set breakpoint
! [N|pat] Redo a previous command B ln|*
Delete a/all breakpoints
H [-num] Display last num commands a [ln] cmd Do
cmd before line
= [a val] Define/list an alias A ln|*
Delete a/all actions
h [db_cmd] Get help on command w expr Add
a watch expression
h h Complete help page W expr|*
Delete a/all watch exprs
|[|]db_cmd Send output to pager ![!] syscmd Run
cmd in a subprocess
q or ^D Quit R Attempt a restart
Data Examination: expr Execute perl code, also
see: s,n,t expr
x|m expr Evals expr in list context, dumps the result or lists methods.
p expr Print expression (uses script's current
package).
S [[!]pat] List subroutine names [not] matching pattern
V [Pk [Vars]] List Variables in Package. Vars can be
~pattern or !pattern.
X [Vars] Same as "V current_package [Vars]".
y [n [Vars]] List lexicals in higher scope <n>. Vars
same as V.
For more help, type h cmd_letter, or run man perldebug
for all docs.
More confusing options than you can shake a big stick at!
It's not as bad as it looks and it's very useful to know
more about all of it, and fun too!
There's a couple of useful ones to know about straight
away. You wouldn't think we're using any libraries at all
at the moment, but 'M' will show which modules are currently
loaded, and their version number, while 'm' will
show the methods, and 'S' shows all subroutines (by pattern)
as shown below. 'V' and 'X' show variables in the
program by package scope and can be constrained by pattern.
DB<2>S str
dumpvar::stringify
strict::bits
strict::import
strict::unimport
Using 'X' and cousins requires you not to use the type
identifiers ($@%), just the 'name':
DM<3>X ~err
FileHandle(stderr) => fileno(2)
Remember we're in our tiny program with a problem, we
should have a look at where we are, and what our data
looks like. First of all let's view some code at our
present position (the first line of code in this case),
via 'v':
DB<4> v
1 #!/usr/bin/perl
2: use strict;
3
4==> my $key = 'welcome';
5: my %data = (
6 'this' => qw(that),
7 'tom' => qw(and jerry),
8 'welcome' => q(Hello World),
9 'zip' => q(welcome),
10 );
At line number 4 is a helpful pointer, that tells you
where you are now. To see more code, type 'v' again:
DB<4> v
8 'welcome' => q(Hello World),
9 'zip' => q(welcome),
10 );
11: my @data = keys %data;
12: print "All OK0 if grep($key, keys %data);
13: print "$data{$key}0;
14: print "done: '$data{$key}'0;
15: exit;
And if you wanted to list line 5 again, type 'l 5', (note
the space):
DB<4> l 5
5: my %data = (
In this case, there's not much to see, but of course normally
there's pages of stuff to wade through, and 'l' can
be very useful. To reset your view to the line we're
about to execute, type a lone period '.':
DB<5> .
main::(./data_a:4): my $key = 'welcome';
The line shown is the one that is about to be executed
next, it hasn't happened yet. So while we can print a
variable with the letter 'p', at this point all we'd get
is an empty (undefined) value back. What we need to do is
to step through the next executable statement with an 's':
DB<6> s
main::(./data_a:5): my %data = (
main::(./data_a:6): 'this' =>
qw(that),
main::(./data_a:7): 'tom' => qw(and
jerry),
main::(./data_a:8): 'welcome' =>
q(Hello World),
main::(./data_a:9): 'zip' => q(welcome),
main::(./data_a:10): );
Now we can have a look at that first ($key) variable:
DB<7> p $key
welcome
line 13 is where the action is, so let's continue down to
there via the letter 'c', which by the way, inserts a
'one-time-only' breakpoint at the given line or sub routine:
DB<8> c 13
All OK
main::(./data_a:13): print "$data{$key}0;
We've gone past our check (where 'All OK' was printed) and
have stopped just before the meat of our task. We could
try to print out a couple of variables to see what is happening:
DB<9> p $data{$key}
Not much in there, lets have a look at our hash:
DB<10> p %data
Hello Worldziptomandwelcomejerrywelcomethisthat
DB<11> p keys %data
Hello Worldtomwelcomejerrythis
Well, this isn't very easy to read, and using the helpful
manual (h h), the 'x' command looks promising:
DB<12> x %data
0 'Hello World'
1 'zip'
2 'tom'
3 'and'
4 'welcome'
5 undef
6 'jerry'
7 'welcome'
8 'this'
9 'that'
That's not much help, a couple of welcomes in there, but
no indication of which are keys, and which are values,
it's just a listed array dump and, in this case, not particularly
helpful. The trick here, is to use a reference
to the data structure:
DB<13> x data
0 HASH(0x8194bc4)
'Hello World' => 'zip'
'jerry' => 'welcome'
'this' => 'that'
'tom' => 'and'
'welcome' => undef
The reference is truly dumped and we can finally see what
we're dealing with. Our quoting was perfectly valid but
wrong for our purposes, with 'and jerry' being treated as
2 separate words rather than a phrase, thus throwing the
evenly paired hash structure out of alignment.
The '-w' switch would have told us about this, had we used
it at the start, and saved us a lot of trouble:
> perl -w data
Odd number of elements in hash assignment at ./data line 5.
We fix our quoting: 'tom' => q(and jerry), and run it
again, this time we get our expected output:
> perl -w data
Hello World
While we're here, take a closer look at the 'x' command,
it's really useful and will merrily dump out nested references,
complete objects, partial objects - just about
whatever you throw at it:
Let's make a quick object and x-plode it, first we'll
start the debugger: it wants some form of input from
STDIN, so we give it something non-commital, a zero:
> perl -de 0
Default die handler restored.
Loading DB routines from perl5db.pl version 1.07
Editor support available.
Enter h or `h h' for help, or `man perldebug' for
more help.
main::(-e:1): 0
Now build an on-the-fly object over a couple of lines
(note the backslash):
DB<1> $obj = bless({'unique_id'=>'123', 'attr'=>
cont: {'col' => 'black', 'things' => [qw(this that etc)]}},
'MY_class')
And let's have a look at it:
DB<2> x $obj
0 MY_class=HASH(0x828ad98)
'attr' => HASH(0x828ad68)
'col' => 'black'
'things' => ARRAY(0x828abb8)
0 'this'
1 'that'
2 'etc'
'unique_id' => 123
DB<3>
Useful, huh? You can eval nearly anything in there, and
experiment with bits of code or regexes until the cows
come home:
DB<3> @data = qw(this that the other atheism
leather theory scythe)
DB<4> p 'saw -> '.($cnt += map { print ":$_0 }
grep(/the/, sort @data))
atheism
leather
other
scythe
the
theory
saw -> 6
If you want to see the command History, type an 'H':
DB<5> H
4: p 'saw -> '.($cnt += map { print ":$_0 }
grep(/the/, sort @data))
3: @data = qw(this that the other atheism leather
theory scythe)
2: x $obj
1: $obj = bless({'unique_id'=>'123', 'attr'=>
{'col' => 'black', 'things' => [qw(this that
etc)]}}, 'MY_class')
DB<5>
And if you want to repeat any previous command, use the
exclamation: '!':
DB<5> !4
p 'saw -> '.($cnt += map { print "$_0 }
grep(/the/, sort @data))
atheism
leather
other
scythe
the
theory
saw -> 12
For more on references see perlref and perlreftutStepping through code [Toc] [Back] Here's a simple program which converts between Celsius and
Fahrenheit, it too has a problem:
#!/usr/bin/perl -w
use strict;
my $arg = $ARGV[0] || '-c20';
if ($arg =~ /^-(c|f)((-|+)*+(.+)*)$/) {
my ($deg, $num) = ($1, $2);
my ($in, $out) = ($num, $num);
if ($deg eq 'c') {
$deg = 'f';
$out = &c2f($num);
} else {
$deg = 'c';
$out = &f2c($num);
}
$out = sprintf('%0.2f', $out);
$out =~ s/^((-|+)*+).0+$/$1/;
print "$out $deg0;
} else {
print "Usage: $0 -[c|f] num0;
}
exit;
sub f2c {
my $f = shift;
my $c = 5 * $f - 32 / 9;
return $c;
}
sub c2f {
my $c = shift;
my $f = 9 * $c / 5 + 32;
return $f;
}
For some reason, the Fahrenheit to Celsius conversion
fails to return the expected output. This is what it
does:
> temp -c0.72
33.30 f
> temp -f33.3
162.94 c
Not very consistent! We'll set a breakpoint in the code
manually and run it under the debugger to see what's going
on. A breakpoint is a flag, to which the debugger will
run without interruption, when it reaches the breakpoint,
it will stop execution and offer a prompt for further
interaction. In normal use, these debugger commands are
completely ignored, and they are safe - if a little messy,
to leave in production code.
my ($in, $out) = ($num, $num);
$DB::single=2; # insert at line 9!
if ($deg eq 'c')
...
> perl -d temp -f33.3
Default die handler restored.
Loading DB routines from perl5db.pl version 1.07
Editor support available.
Enter h or `h h' for help, or `man perldebug' for
more help.
main::(temp:4): my $arg = $ARGV[0] || '-c100';
We'll simply continue down to our pre-set breakpoint with
a 'c':
DB<1> c
main::(temp:10): if ($deg eq 'c') {
Followed by a view command to see where we are:
DB<1> v
7: my ($deg, $num) = ($1, $2);
8: my ($in, $out) = ($num, $num);
9: $DB::single=2;
10==> if ($deg eq 'c') {
11: $deg = 'f';
12: $out = &c2f($num);
13 } else {
14: $deg = 'c';
15: $out = &f2c($num);
16 }
And a print to show what values we're currently using:
DB<1> p $deg, $num
f33.3
We can put another break point on any line beginning with
a colon, we'll use line 17 as that's just as we come out
of the subroutine, and we'd like to pause there later on:
DB<2> b 17
There's no feedback from this, but you can see what breakpoints
are set by using the list 'L' command:
DB<3> L
temp:
17: print "$out $deg0;
break if (1)
Note that to delete a breakpoint you use 'd' or 'D'.
Now we'll continue down into our subroutine, this time
rather than by line number, we'll use the subroutine name,
followed by the now familiar 'v':
DB<3> c f2c
main::f2c(temp:30): my $f = shift;
DB<4> v
24: exit;
25
26 sub f2c {
27==> my $f = shift;
28: my $c = 5 * $f - 32 / 9;
29: return $c;
30 }
31
32 sub c2f {
33: my $c = shift;
Note that if there was a subroutine call between us and
line 29, and we wanted to single-step through it, we could
use the 's' command, and to step over it we would use 'n'
which would execute the sub, but not descend into it for
inspection. In this case though, we simply continue down
to line 29:
DB<4> c 29
main::f2c(temp:29): return $c;
And have a look at the return value:
DB<5> p $c
162.944444444444
This is not the right answer at all, but the sum looks
correct. I wonder if it's anything to do with operator
precedence? We'll try a couple of other possibilities
with our sum:
DB<6> p (5 * $f - 32 / 9)
162.944444444444
DB<7> p 5 * $f - (32 / 9)
162.944444444444
DB<8> p (5 * $f) - 32 / 9
162.944444444444
DB<9> p 5 * ($f - 32) / 9
0.722222222222221
:-) that's more like it! Ok, now we can set our return
variable and we'll return out of the sub with an 'r':
DB<10> $c = 5 * ($f - 32) / 9
DB<11> r
scalar context return from main::f2c:
0.722222222222221
Looks good, let's just continue off the end of the script:
DB<12> c
0.72 c
Debugged program terminated. Use q to quit or R
to restart,
use O inhibit_exit to avoid stopping after program
termination,
h q, h R or h O to get additional info.
A quick fix to the offending line (insert the missing
parentheses) in the actual program and we're finished.
Placeholder for a, w, t, T
Actions, watch variables, stack traces etc.: on the TODO
list.
a
w
t
T
Ever wanted to know what a regex looked like? You'll need
perl compiled with the DEBUGGING flag for this one:
> perl -Dr -e '/^pe(a)*rl$/i'
Compiling REx `^pe(a)*rl$'
size 17 first at 2
rarest char
at 0
1: BOL(2)
2: EXACTF <pe>(4)
4: CURLYN[1] {0,32767}(14)
6: NOTHING(8)
8: EXACTF <a>(0)
12: WHILEM(0)
13: NOTHING(14)
14: EXACTF <rl>(16)
16: EOL(17)
17: END(0)
floating `'$ at 4..2147483647 (checking floating)
stclass `EXACTF <pe>'
anchored(BOL) minlen 4
Omitting $` $& $' support.
EXECUTING...
Freeing REx: `^pe(a)*rl$'
Did you really want to know? :-) For more gory details on
getting regular expressions to work, have a look at
perlre, perlretut, and to decode the mysterious labels
(BOL and CURLYN, etc. above), see perldebguts.
To get all the output from your error log, and not miss
any messages via helpful operating system buffering,
insert a line like this, at the start of your script:
$|=1;
To watch the tail of a dynamically growing logfile, (from
the command line):
tail -f $error_log
Wrapping all die calls in a handler routine can be useful
to see how, and from where, they're being called, perlvar
has more information:
BEGIN { $SIG{__DIE__} = sub { require Carp;
Carp::confess(@_) } }
Various useful techniques for the redirection of STDOUT
and STDERR filehandles are explained in perlopentut and
perlfaq8.
Just a quick hint here for all those CGI programmers who
can't figure out how on earth to get past that 'waiting
for input' prompt, when running their CGI script from the
command-line, try something like this:
> perl -d my_cgi.pl -nodebug
Of course CGI and perlfaq9 will tell you more.
The command line interface is tightly integrated with an
emacs extension and there's a vi interface too.
You don't have to do this all on the command line, though,
there are a few GUI options out there. The nice thing
about these is you can wave a mouse over a variable and a
dump of its data will appear in an appropriate window, or
in a popup balloon, no more tiresome typing of 'x $varname'
:-)
In particular have a hunt around for the following:
ptkdb perlTK based wrapper for the built-in debugger
ddd data display debugger
PerlDevKit and PerlBuilder are NT specific
NB. (more info on these and others would be appreciated).
We've seen how to encourage good coding practices with use
strict and -w. We can run the perl debugger perl -d
scriptname to inspect your data from within the perl
debugger with the p and x commands. You can walk through
your code, set breakpoints with b and step through that
code with s or n, continue with c and return from a sub
with r. Fairly intuitive stuff when you get down to it.
There is of course lots more to find out about, this has
just scratched the surface. The best way to learn more is
to use perldoc to find out more about the language, to
read the on-line help (perldebug is probably the next
place to go), and of course, experiment.
perldebug, perldebguts, perldiag, dprofpp, perlrun
Richard Foley <[email protected]> Copyright (c) 2000
Various people have made helpful suggestions and contributions,
in particular:
Ronald J Kimball <[email protected]>
Hugo van der Sanden <[email protected]>
Peter Scott <[email protected]>
perl v5.8.5 2002-11-06 15 [ Back ] |
