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 with a problem:
+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 :-)
$var2 = "$varl\n";
-
+
print $var2;
exit;
blank line. It looks like there's 2 variables when (because of the typo)
there's really 3:
- $var1 = 'Hello World'
- $varl = undef
- $var2 = "\n"
+ $var1 = 'Hello World';
+ $varl = undef;
+ $var2 = "\n";
To catch this kind of problem, we can force each variable to be declared
before use by pulling in the strict module, by putting 'use strict;' after the
Now when you run it, perl complains about the 3 undeclared variables and we
get four error messages because one variable is referenced twice:
-
+
Global symbol "$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.
#!/usr/bin/perl
use strict;
-
+
my $var1 = 'Hello World';
- my $varl = '';
+ my $varl = undef;
my $var2 = "$varl\n";
-
+
print $var2;
exit;
hello syntax OK
And now when we run it, we get "\n" still, but at least we know why. Just
-getting this script to compile has exposed the '$varl' (with the letter 'l)
+getting this script to compile has exposed the '$varl' (with the letter 'l')
variable, and simply changing $varl to $var1 solves the problem.
-=head1 Looking at data and -w and w
+=head1 Looking at data and -w and v
Ok, but how about when you want to really see your data, what's in that
dynamic variable, just before using it?
Looks OK, after it's been through the syntax check (perl -c scriptname), we
run it and all we get is a blank line again! Hmmmm.
-
+
One common debugging approach here, would be to liberally sprinkle a few print
statements, to add a check just before we print out our data, and another just
after:
print "done: '$data{$key}'\n";
And try again:
-
+
> perl data
All OK
-
+
done: ''
After much staring at the same piece of code and not seeing the wood for the
DB<1> q
>
-
+
That's it, you're back on home turf again.
=head1 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 'B<h>' will get you a long
-scrolled list of help, 'B<|h>' (pipe-h) will pipe the help through your pager
-('more' or 'less' probably), and finally, 'B<h h>' (h-space-h) will give you a
-helpful mini-screen snapshot:
+There's a couple of ways of calling help: a simple 'B<h>' will get the summary
+help list, 'B<|h>' (pipe-h) will pipe the help through your pager (which is
+(probably 'more' or 'less'), and finally, 'B<h h>' (h-space-h) will give you
+the entire help screen. Here is the summary page:
+
+DB<1>h
- DB<1> h h
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]
- w [line] List 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
- v Show versions of modules 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 d [ln] or D Delete a/all breakpoints
- H [-num] Display last num commands a [ln] cmd Do cmd before line
- = [a val] Define/list an alias W expr Add a watch expression
- h [db_cmd] Get help on command A or W Delete all actions/watch
- |[|]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 array 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]".
- For more help, type h cmd_letter, or run man perldebug for all docs.
-
+ 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 'B<v>' will show
-which modules are currently loaded, by the debugger as well your script.
-'B<V>' and 'B<X>' show variables in the program by package scope and can be
-constrained by pattern. 'B<m>' shows methods and 'B<S>' shows all subroutines
-(by pattern):
+think we're using any libraries at all at the moment, but 'B<M>' will show
+which modules are currently loaded, and their version number, while 'B<m>'
+will show the methods, and 'B<S>' shows all subroutines (by pattern) as
+shown below. 'B<V>' and 'B<X>' show variables in the program by package
+scope and can be constrained by pattern.
DB<2>S str
dumpvar::stringify
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 have a window
-on our present position (the first line of code in this case), via the letter
-'B<w>':
+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 'B<v>':
- DB<4> w
+ DB<4> v
1 #!/usr/bin/perl
2: use strict;
3
10 );
At line number 4 is a helpful pointer, that tells you where you are now. To
-see more code, type 'w' again:
-
- DB<4> w
+see more code, type 'v' again:
+
+ DB<4> v
8 'welcome' => q(Hello World),
9 'zip' => q(welcome),
10 );
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 $key = 'welcome';
-
+
The line shown is the one that is about to be executed B<next>, it hasn't
happened yet. So while we can print a variable with the letter 'B<p>', at
this point all we'd get is an empty (undefined) value back. What we need to
do is to step through the next executable statement with an 'B<s>':
-
+
DB<6> s
main::(./data_a:5): my %data = (
main::(./data_a:6): 'this' => qw(that),
DB<8> c 13
All OK
main::(./data_a:13): print "$data{$key}\n";
-
+
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 (B<h h>), the
'B<x>' command looks promising:
8 'this'
9 'that'
-That's not much help, a couple of welcome's in there, but no indication of
+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 B<reference>
to the data structure:
While we're here, take a closer look at the 'B<x>' command, it's really useful
and will merrily dump out nested references, complete objects, partial objects
-- justabout whatever you throw at it:
+- just about whatever you throw at it:
-Let's make a quick object and x-plode it, first we'll start the the debugger:
+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:
cont: {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
And let's have a look at it:
-
+
DB<2> x $obj
0 MY_class=HASH(0x828ad98)
'attr' => HASH(0x828ad68)
of code or regexes until the cows come home:
DB<3> @data = qw(this that the other atheism leather theory scythe)
-
+
DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
atheism
leather
1: $obj = bless({'unique_id'=>'123', 'attr'=>
{'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
DB<5>
-
+
And if you want to repeat any previous command, use the exclamation: 'B<!>':
DB<5> !4
> temp -c0.72
33.30 f
-
+
> temp -f33.3
162.94 c
-
+
Not very consistent! We'll set a breakpoint in the code manually and run it
under the debugger to see what's going on. A breakpoint is a flag, to which
-the debugger will run without interuption, when it reaches the breakpoint, it
+the debugger will run without interruption, when it reaches the breakpoint, it
will stop execution and offer a prompt for further interaction. In normal
use, these debugger commands are completely ignored, and they are safe - if a
little messy, to leave in production code.
-
+
my ($in, $out) = ($num, $num);
$DB::single=2; # insert at line 9!
if ($deg eq 'c')
...
-
+
> perl -d temp -f33.3
Default die handler restored.
DB<1> c
main::(temp:10): if ($deg eq 'c') {
-Followed by a window command to see where we are:
-
- DB<1> w
+Followed by a view command to see where we are:
+
+ DB<1> v
7: my ($deg, $num) = ($1, $2);
8: my ($in, $out) = ($num, $num);
9: $DB::single=2;
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:
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 'w':
+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> w
+ DB<4> v
24: exit;
25
26 sub f2c {
DB<4> c 29
main::f2c(temp:29): return $c;
-
+
And have a look at the return value:
DB<5> p $c
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
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
Actions, watch variables, stack traces etc.: on the TODO list.
a
-
- W
-
+
+ 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
floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF <pe>'
anchored(BOL) minlen 4
Omitting $` $& $' support.
-
+
EXECUTING...
Freeing REx: `^pe(a)*rl$'
> perl -d my_cgi.pl -nodebug
-Of course
'L<perldoc CGI>' and L<perlfaq9> will tell you more.
+Of course
L<CGI> and L<perlfaq9> will tell you more.
=head1 GUIs
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 it's data will appear in an appropriate window, or in 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:
B<ptkdb> perlTK based wrapper for the built-in debugger
B<ddd> data display debugger
-
+
B<PerlDevKit> and B<PerlBuilder> are NT specific
NB. (more info on these and others would be appreciated).
https://perl5.git.perl.org / perl5.git / blobdiff