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NAME    [Toc]    [Back]

       perldebtut - Perl debugging tutorial

DESCRIPTION    [Toc]    [Back]

       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

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


               $var1  = 'Hello World'; # always wanted to do that
               $var2 = "$varl0;

               print $var2;

       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

       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:

               use strict;

               my $var1 = 'Hello World';
               my $varl = undef;
               my $var2 = "$varl0;

               print $var2;

       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?

               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;

       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

               > 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.

       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:

        List/search  source  lines:                Control script
         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
         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
         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

       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;
               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'     =>
               main::(./data_a:7):              'tom'  =>  qw(and
               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

       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)]}},
       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

       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))
               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')

       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))
               saw -> 12

       For more on references see perlref and perlreftut

Stepping 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;

               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

               > 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

       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
                       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;
               26      sub f2c {
               27==>           my $f = shift;
               28:             my $c = 5 * $f - 32 / 9;
               29:             return $c;
               30      }
               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

       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)

               DB<7> p 5 * $f - (32 / 9)

               DB<8> p (5 * $f) - 32 / 9
               DB<9> p 5 * ($f - 32) / 9

       :-) 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:

       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
               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






       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.


               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    [Toc]    [Back]

       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:


       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

CGI    [Toc]    [Back]

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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.

SEE ALSO    [Toc]    [Back]

       perldebug, perldebguts, perldiag, dprofpp, perlrun

AUTHOR    [Toc]    [Back]

       Richard Foley <richard@rfi.net> Copyright (c) 2000

CONTRIBUTORS    [Toc]    [Back]

       Various people have made helpful suggestions and contributions,
 in particular:

       Ronald J Kimball <rjk@linguist.dartmouth.edu>
       Hugo van der Sanden <hv@crypt0.demon.co.uk>

       Peter Scott <Peter@PSDT.com>

perl v5.8.5                 2002-11-06                         15
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