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       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 debug­
       ger, 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 :-)
               $var2 = "$varl\n";

               print $var2;

       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

       Now when you run it, perl complains about the 3 undeclared
       variables and we get four error messages because one vari­
       able 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.

       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?

               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";

       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 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 see­
       ing the wood for the trees for some time, we get a cup of
       coffee and try another approach.  That is, we bring in the
       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 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

       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 pre­
       sent position (the first line of code in this case), via

               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 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 nor­
       mally 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: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 rou­

               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 hap­

               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 par­
       ticularly helpful.  The trick here, is to use a reference
       to the data structure:

       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 refer­
       ences, 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)]}}, '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

       Useful, huh?  You can eval nearly anything in there, and
       experiment with bits of code or regexes until the cows
               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')

       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))
               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";

       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);

       points are set by using the list 'L' command:

               DB<3> L
                       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;
               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)
               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






       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$'

               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


       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.


       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 $var­
       name' :-)

       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).


       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.
       Various people have made helpful suggestions and contribu­
       tions, in particular:

       Ronald J Kimball <rjk@linguist.dartmouth.edu>

       Hugo van der Sanden <hv@crypt0.demon.co.uk>

       Peter Scott <Peter@PSDT.com>

perl v5.8.1                 2003-09-02              PERLDEBTUT(1)

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