perldebtut
(1)
Name
perldebtut - Perl debugging tutorial
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLDEBTUT(1)
NAME
perldebtut - Perl debugging tutorial
DESCRIPTION
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 :-)
$var2 = "$varl\n";
print $var2;
exit;
While this compiles and runs happily, it probably won't do
what's expected, namely it doesn't print "Hello World\n" 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;
$var2 = "\n";
To catch this kind of problem, we can force each variable to
be declared before use by pulling in the strict module, by
putting 'use strict;' after the first line of the script.
Now when you run it, perl complains about the 3 undeclared
variables and we get four error messages because one
variable is referenced twice:
Global symbol "$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.
perl v5.12.5 Last change: 2012-11-03 1
Perl Programmers Reference Guide PERLDEBTUT(1)
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 $var2 = "$varl\n";
print $var2;
exit;
We then do (always a good idea) a syntax check before we try
to run it again:
> perl -c hello
hello syntax OK
And now when we run it, we get "\n" still, but at least we
know why. Just getting this script to compile has exposed
the '$varl' (with the letter 'l') variable, and simply
changing $varl to $var1 solves the problem.
Looking at data and -w and v
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{$key}\n";
exit;
Looks OK, after it's been through the syntax check (perl -c
scriptname), we run it and all we get is a blank line again!
Hmmmm.
One common debugging approach here, would be to liberally
sprinkle a few print statements, to add a check just before
we print out our data, and another just after:
perl v5.12.5 Last change: 2012-11-03 2
Perl Programmers Reference Guide PERLDEBTUT(1)
print "All OK\n" if grep($key, keys %data);
print "$data{$key}\n";
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 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
perl v5.12.5 Last change: 2012-11-03 3
Perl Programmers Reference Guide PERLDEBTUT(1)
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.
perl v5.12.5 Last change: 2012-11-03 4
Perl Programmers Reference Guide PERLDEBTUT(1)
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 $key = 'welcome';
5: my %data = (
6 'this' => qw(that),
7 'tom' => qw(and jerry),
8 'welcome' => q(Hello World),
9 'zip' => q(welcome),
10 );
At line number 4 is a helpful pointer, that tells you where
you are now. To see more code, type 'v' again:
DB<4> v
8 'welcome' => q(Hello World),
9 'zip' => q(welcome),
10 );
11: my @data = keys %data;
12: print "All OK\n" if grep($key, keys %data);
13: print "$data{$key}\n";
14: print "done: '$data{$key}'\n";
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 $key = 'welcome';
The line shown is the one that is about to be executed next,
it hasn't happened yet. So while we can print a variable
with the letter 'p', at this point all we'd get is an empty
(undefined) value back. What we need to do is to step
through the next executable statement with an 's':
perl v5.12.5 Last change: 2012-11-03 5
Perl Programmers Reference Guide PERLDEBTUT(1)
DB<6> s
main::(./data_a:5): my %data = (
main::(./data_a:6): 'this' => qw(that),
main::(./data_a:7): 'tom' => qw(and jerry),
main::(./data_a:8): 'welcome' => q(Hello World),
main::(./data_a:9): 'zip' => q(welcome),
main::(./data_a:10): );
Now we can have a look at that first ($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}\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 (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
perl v5.12.5 Last change: 2012-11-03 6
Perl Programmers Reference Guide PERLDEBTUT(1)
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-committal, 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):
perl v5.12.5 Last change: 2012-11-03 7
Perl Programmers Reference Guide PERLDEBTUT(1)
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 "\t:\t$_\n" } grep(/the/, sort @data))
atheism
leather
other
scythe
the
theory
saw -> 6
If you want to see the command History, type an 'H':
DB<5> H
4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))
3: @data = qw(this that the other atheism leather theory scythe)
2: x $obj
1: $obj = bless({'unique_id'=>'123', 'attr'=>
{'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')
DB<5>
And if you want to repeat any previous command, use the
exclamation: '!':
perl v5.12.5 Last change: 2012-11-03 8
Perl Programmers Reference Guide PERLDEBTUT(1)
DB<5> !4
p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data))
atheism
leather
other
scythe
the
theory
saw -> 12
For more on references see perlref and perlreftut
Stepping through code
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)((\-|\+)*\d+(\.\d+)*)$/) {
my ($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);
$out =~ s/^((\-|\+)*\d+)\.0+$/$1/;
print "$out $deg\n";
} else {
print "Usage: $0 -[c|f] num\n";
}
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 $f;
}
perl v5.12.5 Last change: 2012-11-03 9
Perl Programmers Reference Guide PERLDEBTUT(1)
For some reason, the Fahrenheit to Celsius conversion fails
to return the expected output. This is what it does:
> temp -c0.72
33.30 f
> temp -f33.3
162.94 c
Not very consistent! We'll set a breakpoint in the code
manually and run it under the debugger to see what's going
on. A breakpoint is a flag, to which the debugger will run
without interruption, when it reaches the breakpoint, it
will stop execution and offer a prompt for further
interaction. In normal use, these debugger commands are
completely ignored, and they are safe - if a little messy,
to leave in production code.
my ($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 }
perl v5.12.5 Last change: 2012-11-03 10
Perl Programmers Reference Guide PERLDEBTUT(1)
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 $deg\n";
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:
perl v5.12.5 Last change: 2012-11-03 11
Perl Programmers Reference Guide PERLDEBTUT(1)
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
perl v5.12.5 Last change: 2012-11-03 12
Perl Programmers Reference Guide PERLDEBTUT(1)
REGULAR EXPRESSIONS
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.
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 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
perl v5.12.5 Last change: 2012-11-03 13
Perl Programmers Reference Guide PERLDEBTUT(1)
perlfaq8.
CGI
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.
GUIs
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).
SUMMARY
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.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
perl v5.12.5 Last change: 2012-11-03 14
Perl Programmers Reference Guide PERLDEBTUT(1)
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
perldebug, perldebguts, perldiag, dprofpp, perlrun
AUTHOR
Richard Foley <[email protected]> Copyright (c) 2000
CONTRIBUTORS
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]>
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
perl v5.12.5 Last change: 2012-11-03 15