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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.

       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:


	       $var1 = Hello World; # always wanted to do that :-)
	       $var2 = "$varl\n";

	       print $var2;

       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

	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 $var2 = "$varl\n";

	       print $var2;

       We then do (always a good idea) a syntax check before we try to run it

	       > 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

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?

	       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 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


       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.

       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


	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

       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;
	       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

	       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 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

	       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

       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))
	       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
       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";

	       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

       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
		       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;
	       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 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

       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)

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

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

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

       :-) 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.





       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.


	       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

       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

	       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.

       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.

       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).

       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.

       perldebug, perldebguts, perldiag, dprofpp, perlrun

       Richard Foley  Copyright (c) 2000

       Various people have made helpful suggestions and contributions, in par

       Ronald J Kimball 

       Hugo van der Sanden 

       Peter Scott 

perl v5.8.8			  2008-04-25			 PERLDEBTUT(1)

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