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.
+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
=head1 use strict
-There's a few things you can do to make your life a lot more straightforward
-when it comes to debugging perl programs. To demonstrate, here's a simple
-script with a problem:
+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:
#!/usr/bin/perl
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 ./t1 aborted due to compilation errors.
+ 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:
variable, and simply changing $varl to $var1 solves the problem.
-=head1 Looking at data and -w
+=head1 Looking at data and -w and w
Ok, but how about when you want to really see your data, what's in that
dynamic variable, just before using it?
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 C<-d> switch on the command
+is, we bring in the cavalry by giving perl the 'B<-d>' switch on the command
line:
> perl -d data
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':
+just the letter 'B<q>', not the words 'quit' or 'exit':
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 'h' will get you a long
-scrolled list of help, '|h' (pipe-h) will pipe the help through your pager
-('more' or 'less' probably), and finally, 'h h' (h-space-h) will give you a
+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:
- 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
+ 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.
+ 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.
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 'v'
-will show which modules are currently loaded, by the debugger as well your
-script. 'V' and 'X' show variables in the program by package scope and can be
-constrained by pattern. 'S' shows all subroutines (by pattern):
+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):
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 want to have a look at
+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), via the letter 'w':
+on our present position (the first line of code in this case), via the letter
+'B<w>':
- DB<3> w
+ DB<4> w
1 #!/usr/bin/perl
2: use strict;
3
At line number 4 is a helpful pointer, that tells you where you are now. To
see more code, type 'w' again:
- DB<3> w
+ DB<4> w
8 'welcome' => q(Hello World),
9 'zip' => q(welcome),
10 );
14: print "done: '$data{$key}'\n";
15: exit;
-And if you wanted to list line 5 again, type 'l 5', note the space:
+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. To reset your view to the line we're about to execute,
-type a lone period '.':
+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<6> .
+ 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 'p', at this
-point all we'd get is an empty (undefined) value back. What we need to do is
-to step to the next executable statement with an 's':
+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 = (
welcome
line 13 is where the action is, so let's continue down to there via the letter
-'c':
+'B<c>', which by the way, inserts a 'one-time-only' breakpoint at the given
+line or sub routine:
DB<8> c 13
All OK
DB<9> p $data{$key}
-Nothing!
+Not much in there, lets have a look at our hash:
DB<10> p %data
Hello Worldziptomandwelcomejerrywelcomethisthat
DB<11> p keys %data
Hello Worldtomwelcomejerrythis
-Reading the helpful manual (h h), the 'x' command looks promising:
+Well, this isn't very easy to read, and using the helpful manual (B<h h>), the
+'B<x>' command looks promising:
DB<12> x %data
0 'Hello World'
9 'that'
That's not much help, a couple of welcome's in there, but no indication of
-which are keys, and which are values, it's just a straight array dump and, in
+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:
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,
+The 'B<-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
Hello World
-While we're here, take a closer look at the 'x' command, it's really useful
+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:
theory
saw -> 6
-If you want to see all the command history, an 'H':
+If you want to see the command History, type an 'B<H>':
DB<5> H
4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
{'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
DB<5>
-And if you want to repeat any previous command, use the exclamation: '!':
+And if you want to repeat any previous command, use the exclamation: 'B<!>':
DB<5> !4
p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data))
theory
saw -> 12
+For more on references see L<perlref> and L<perlreftut>
+
=head1 Stepping through code
main::(temp:4): my $arg = $ARGV[0] || '-c100';
-We'll simply continue down to our pre-set breakpoint with a 'c':
+We'll simply continue down to our pre-set breakpoint with a 'B<c>':
DB<1> c
main::(temp:10): if ($deg eq 'c') {
Followed by a window command to see where we are:
- DB<2> w
+ DB<1> w
7: my ($deg, $num) = ($1, $2);
8: my ($in, $out) = ($num, $num);
9: $DB::single=2;
And a print to show what values we're currently using:
- DB<3> p $deg, $num
+ DB<1> p $deg, $num
f33.3
We can put another break point on any line beginning with a colon, we'll use
line 17 as that's just as we come out of the subroutine, and we'd like to
pause there later on:
- DB<4> b 17
+ DB<2> b 17
There's no feedback from this, but you can see what breakpoints are set by
using the list 'L' command:
- DB<5> L
+ DB<3> L
temp:
17: print "$out $deg\n";
break if (1)
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':
- DB<6> c f2c
+ DB<3> c f2c
main::f2c(temp:30): my $f = shift;
- DB<7> w
- 27 }
- 28
- 29 sub f2c {
- 30==> my $f = shift;
- 31: my $c = 5 * $f - 32 / 9;
- 32: return $c;
- 33 }
- 34
-
-
-Note that if there was a subroutine call between us and line 32, and we didn't
-want to single-step through it, we could use the next command 'n', which would
-execute the sub, but not descend into it for inspection. In this case though,
-we simply single step down to line 32:
-
- DB<8> s 32
- main::f2c(temp:28): return $c;
-
+ DB<4> w
+ 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 B<single-step> through it, we could use the 'B<s>' command, and to step
+over it we would use 'B<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<9> p $c
+ 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<10> p (5 * $f - 32 / 9)
+ DB<6> p (5 * $f - 32 / 9)
162.944444444444
- DB<11> p 5 * $f - (32 / 9)
+ DB<7> p 5 * $f - (32 / 9)
162.944444444444
- DB<12> p (5 * $f) - 32 / 9
+ DB<8> p (5 * $f) - 32 / 9
162.944444444444
- DB<13> p 5 * ($f - 32) / 9
+ 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<14> $c = 5 * ($f - 32) / 9
+ DB<10> $c = 5 * ($f - 32) / 9
- DB<15> r
+ DB<11> r
scalar context return from main::f2c: 0.722222222222221
Looks good, let's just continue off the end of the script:
- DB<16> c
+ 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,
=head1 Placeholder for a, w, t, T
-Actions, watch variables, stack traces on the TODO list.
+Actions, watch variables, stack traces etc.: on the TODO list.
a
T
-=head1 Regular expressions
+=head1 REGULAR EXPRESSIONS
Ever wanted to know what a regex looked like? You'll need perl compiled with
the DEBUGGING flag for this one:
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
+L<perlre>, L<perlretut>, and to decode the mysterious labels (BOL and CURLYN,
+etc. above), see L<perldebguts>.
-=head1 Some ideas for output
+=head1 OUTPUT TIPS
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
Wrapping all die calls in a handler routine can be useful to see how, and from
where, they're being called, L<perlvar> has more information:
- BEGIN { $SIG{__DIE__} = sub { use Carp; Carp::confess(@_) } }
+ BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } }
Various useful techniques for the redirection of STDOUT and STDERR filehandles
-are explained in L<perlfunc> and L<perlopentut> and L<perlfaq8>
+are explained in L<perlopentut> and L<perlfaq8>.
=head1 CGI
-Just a hint here for all those CGI programmers who can't figure out how on
-earth to get past that 'waiting for input' prompt, try something like this:
+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
NB. (more info on these and others would be appreciated).
-=head1 Summary
+=head1 SUMMARY
We've seen how to encourage good coding practices with B<use strict> and
B<-w>. We can run the perl debugger B<perl -d scriptname> to inspect your
Ronald J Kimball <rjk@linguist.dartmouth.edu>
-Hugo <hv@crypt.compulink.co.uk>
+Hugo van der Sanden <hv@crypt0.demon.co.uk>
+
+Peter Scott <Peter.J.Scott@jpl.nasa.gov>
projects / platform / upstream / perl.git / commitdiff