MirBSD manpage: perldebtut(1)

PERLDEBTUT(1)   Perl Programmers Reference Guide    PERLDEBTUT(1)


     perldebtut - Perl debugging tutorial


     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 demon-
     strate, here's a simple script, named "hello", with a prob-


             $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 script.

     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.

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     Luvverly! and to fix this we declare all variables expli-
     citly and now our script looks like this:

             use strict;

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

             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 "\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!

     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:

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             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.


     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:


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      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

     Using 'X' and cousins requires you not to use the type iden-
     tifiers ($@%), 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.

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     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 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: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':

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             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

     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

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     just a listed array dump and, in this case, not particularly
     helpful.  The trick here, is to use a reference to the data

             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 refer-
     ences, complete objects, partial objects - just about what-
     ever 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):

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             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 come

             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))
             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 exc-
     lamation: '!':

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             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";

             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;

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     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 interac-
     tion.  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

             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              }

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     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 break-
     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 inspec-
     tion.  In this case though, we simply continue down to line

             DB<4> c 29
             main::f2c(temp:29):             return $c;

     And have a look at the return value:

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             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 pre-
     cedence?  We'll try a couple of other possibilities with our

             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: 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





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     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 CUR-
     LYN, etc. above), see perldebguts.


     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

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     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 $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).


     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.


     perldebug, perldebguts, perldiag, dprofpp, perlrun


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

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     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>

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