PERLDEBTUT(1) 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.
Theres an extraordinary number of people out there who dont appear to
know anything about using the perl debugger, though they use the lan
guage every day. This is for them.
use strict
First of all, theres 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, heres 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 wont do whats
expected, namely it doesnt 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
youll get what looks like a blank line. It looks like theres 2 vari
ables when (because of the typo) theres 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.
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, whats 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{$key}\n";
exit;
Looks OK, after its been through the syntax check (perl -c script
name), we run it and all we get is a blank line again! Hmmmm.
One common debugging approach here, would be to liberally sprinkle a
few print statements, to add a check just before we print out our data,
and another just after:
print "All OK\n" if grep($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 weve done here is to launch the built-in perl debugger on
our script. Its stopped at the first line of executable code and is
waiting for input.
Before we go any further, youll want to know how to quit the debugger:
use just the letter q, not the words quit or exit:
DB<1> q
>
Thats it, youre back on home turf again.
help
Fire the debugger up again on your script and well look at the help
menu. Theres 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 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 scripts 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 . 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! Its not as
bad as it looks and its very useful to know more about all of it, and
fun too!
Theres a couple of useful ones to know about straight away. You
wouldnt think were 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 pat
tern) 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 were 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 lets
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, theres not much to see, but of course normally theres
pages of stuff to wade through, and l can be very useful. To reset
your view to the line were about to execute, type a lone period .:
DB<5> .
main::(./data_a:4): my $key = welcome;
The line shown is the one that is about to be executed next, it hasnt
happened yet. So while we can print a variable with the letter p, at
this point all wed get is an empty (undefined) value back. What we
need to do is to step through the next executable statement with an
s:
DB<6> s
main::(./data_a:5): my %data = (
main::(./data_a:6): this => qw(that),
main::(./data_a:7): tom => qw(and jerry),
main::(./data_a:8): welcome => q(Hello World),
main::(./data_a:9): zip => q(welcome),
main::(./data_a:10): );
Now we can have a look at that first ($key) variable:
DB<7> p $key
welcome
line 13 is where the action is, so lets continue down to there via the
letter c, which by the way, inserts a one-time-only breakpoint at
the given line or sub routine:
DB<8> c 13
All OK
main::(./data_a:13): print "$data{$key}\n";
Weve 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 isnt 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
Thats not much help, a couple of welcomes in there, but no indication
of which are keys, and which are values, its 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 were 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 were here, take a closer look at the x command, its really
useful and will merrily dump out nested references, complete objects,
partial objects - just about whatever you throw at it:
Lets make a quick object and x-plode it, first well start the debug
ger: 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 back
slash):
DB<1> $obj = bless({unique_id=>123, attr=> \
cont: {col => black, things => [qw(this that etc)]}}, MY_class)
And lets 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: $obj = bless({unique_id=>123, attr=>
{col => black, things => [qw(this that etc)]}}, MY_class)
DB<5>
And if you want to repeat any previous command, use the exclamation:
!:
DB<5> !4
p saw -> .($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
Heres 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 $f;
}
For some reason, the Fahrenheit to Celsius conversion fails to return
the expected output. This is what it does:
> temp -c0.72
33.30 f
> temp -f33.3
162.94 c
Not very consistent! Well set a breakpoint in the code manually and
run it under the debugger to see whats going on. A breakpoint is a
flag, to which the debugger will run without interruption, when it
reaches the breakpoint, it will stop execution and offer a prompt for
further interaction. In normal use, these debugger commands are com
pletely ignored, and they are safe - if a little messy, to leave in
production code.
my ($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 = $ARGV[0] || -c100;
Well simply continue down to our pre-set breakpoint with a c:
DB<1> c
main::(temp:10): if ($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 were currently using:
DB<1> p $deg, $num
f33.3
We can put another break point on any line beginning with a colon,
well use line 17 as thats just as we come out of the subroutine, and
wed like to pause there later on:
DB<2> b 17
Theres no feedback from this, but you can see what breakpoints are set
by using the list L command:
DB<3> L
temp:
17: print "$out $deg\n";
break if (1)
Note that to delete a breakpoint you use d or D.
Now well continue down into our subroutine, this time rather than by
line number, well use the subroutine name, followed by the now famil
iar 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 con
tinue 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 won
der if its anything to do with operator precedence? Well try a cou
ple 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
:-) thats more like it! Ok, now we can set our return variable and
well 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, lets 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 were finished.
Placeholder for a, w, t, T
Actions, watch variables, stack traces etc.: on the TODO list.
a
w
t
T
REGULAR EXPRESSIONS
Ever wanted to know what a regex looked like? Youll need perl com
piled 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 (4)
4: CURLYN[1] {0,32767}(14)
6: NOTHING(8)
8: EXACTF (0)
12: WHILEM(0)
13: NOTHING(14)
14: EXACTF (16)
16: EOL(17)
17: END(0)
floating $ at 4..2147483647 (checking floating) stclass EXACTF
anchored(BOL) minlen 4
Omitting $ $& $ support.
EXECUTING...
Freeing REx: ^pe(a)*rl$
Did you really want to know? :-) For more gory details on getting regu
lar expressions to work, have a look at perlre, perlretut, and to
decode the mysterious labels (BOL and CURLYN, etc. above), see perlde
bguts.
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, theyre 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.
CGI
Just a quick hint here for all those CGI programmers who cant figure
out how on earth to get past that waiting for input prompt, when run
ning 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 exten
sion and theres a vi interface too.
You dont 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 appro
priate 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
Weve 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 proba
bly the next place to go), and of course, experiment.
SEE ALSO
perldebug, perldebguts, perldiag, dprofpp, perlrun
AUTHOR
Richard Foley Copyright (c) 2000
CONTRIBUTORS
Various people have made helpful suggestions and contributions, in par
ticular:
Ronald J Kimball
Hugo van der Sanden
Peter Scott
perl v5.8.8 2008-04-25 PERLDEBTUT(1)
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