perldebug - Perl debugging


First of all, have you tried using the -w switch?

The Perl Debugger

If you invoke Perl with the -d switch, your script runs under the Perl source debugger. This works like an interactive Perl environment, prompting for debugger commands that let you examine source code, set breakpoints, get stack back-traces, change the values of variables, etc. This is so convenient that you often fire up the debugger all by itself just to test out Perl constructs interactively to see what they do. For example:

    perl -d -e 42

In Perl, the debugger is not a separate program as it usually is in the typical compiled environment. Instead, the -d flag tells the compiler to insert source information into the parse trees it's about to hand off to the interpreter. That means your code must first compile correctly for the debugger to work on it. Then when the interpreter starts up, it pre-loads a Perl library file containing the debugger itself.

The program will halt right before the first run-time executable statement (but see below regarding compile-time statements) and ask you to enter a debugger command. Contrary to popular expectations, whenever the debugger halts and shows you a line of code, it always displays the line it's about to execute, rather than the one it has just executed.

Any command not recognized by the debugger is directly executed (eval'd) as Perl code in the current package. (The debugger uses the DB package for its own state information.)

Leading white space before a command would cause the debugger to think it's NOT a debugger command but for Perl, so be careful not to do that.

Debugger Commands

The debugger understands the following commands:

h [command]
Prints out a help message.

If you supply another debugger command as an argument to the h command, it prints out the description for just that command. The special argument of h h produces a more compact help listing, designed to fit together on one screen.

If the output the h command (or any command, for that matter) scrolls past your screen, either precede the command with a leading pipe symbol so it's run through your pager, as in

    DB> |h

You may change the pager which is used via O pager=... command.

p expr
Same as print {$DB::OUT} expr in the current package. In particular, because this is just Perl's own print function, this means that nested data structures and objects are not dumped, unlike with the x command.

The DB::OUT filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to.

x expr
Evaluates its expression in list context and dumps out the result in a pretty-printed fashion. Nested data structures are printed out recursively, unlike the print function.

The details of printout are governed by multiple Options.

V [pkg [vars]]
Display all (or some) variables in package (defaulting to the main package) using a data pretty-printer (hashes show their keys and values so you see what's what, control characters are made printable, etc.). Make sure you don't put the type specifier (like $) there, just the symbol names, like this:

    V DB filename line

Use ~pattern and !pattern for positive and negative regexps.

Nested data structures are printed out in a legible fashion, unlike the print function.

The details of printout are governed by multiple Options.

X [vars]
Same as V currentpackage [vars].

Produce a stack back-trace. See below for details on its output.

s [expr]
Single step. Executes until it reaches the beginning of another statement, descending into subroutine calls. If an expression is supplied that includes function calls, it too will be single-stepped.

n [expr]
Next. Executes over subroutine calls, until it reaches the beginning of the next statement. If an expression is supplied that includes function calls, those functions will be executed with stops before each statement.

Repeat last n or s command.

c [line|sub]
Continue, optionally inserting a one-time-only breakpoint at the specified line or subroutine.

List next window of lines.

l min+incr
List incr+1 lines starting at min.

l min-max
List lines min through max. l - is synonymous to -.

l line
List a single line.

l subname
List first window of lines from subroutine.

List previous window of lines.

w [line]
List window (a few lines) around the current line.

Return debugger pointer to the last-executed line and print it out.

f filename
Switch to viewing a different file or eval statement. If filename is not a full filename as found in values of %INC, it is considered as a regexp.

Search forwards for pattern; final / is optional.

Search backwards for pattern; final ? is optional.

List all breakpoints and actions.

S [[!]pattern]
List subroutine names [not] matching pattern.

Toggle trace mode (see also AutoTrace Option).

t expr
Trace through execution of expr. For example:

 $ perl -de 42
 Stack dump during die enabled outside of evals.

 Loading DB routines from patch level 0.94
 Emacs support available.

 Enter h or `h h' for help.

 main::(-e:1):   0
   DB<1> sub foo { 14 }

   DB<2> sub bar { 3 }

   DB<3> t print foo() * bar()
 main::((eval 172):3):   print foo() + bar();
 main::foo((eval 168):2):
 main::bar((eval 170):2):

or, with the Option frame=2 set,

   DB<4> O f=2
                frame = '2'
   DB<5> t print foo() * bar()
 3:      foo() * bar()
 entering main::foo
  2:     sub foo { 14 };
 exited main::foo
 entering main::bar
  2:     sub bar { 3 };
 exited main::bar

b [line] [condition]
Set a breakpoint. If line is omitted, sets a breakpoint on the line that is about to be executed. If a condition is specified, it's evaluated each time the statement is reached and a breakpoint is taken only if the condition is true. Breakpoints may be set on only lines that begin an executable statement. Conditions don't use if:

    b 237 $x > 30
    b 237 ++$count237 < 11
    b 33 /pattern/i

b subname [condition]
Set a breakpoint at the first line of the named subroutine.

b postpone subname [condition]
Set breakpoint at first line of subroutine after it is compiled.

b load filename
Set breakpoint at the first executed line of the file. Filename should be a full name as found in values of %INC.

b compile subname
Sets breakpoint at the first statement executed after the subroutine is compiled.

d [line]
Delete a breakpoint at the specified line. If line is omitted, deletes the breakpoint on the line that is about to be executed.

Delete all installed breakpoints.

a [line] command
Set an action to be done before the line is executed. The sequence of steps taken by the debugger is

  1. check for a breakpoint at this line
  2. print the line if necessary (tracing)
  3. do any actions associated with that line
  4. prompt user if at a breakpoint or in single-step
  5. evaluate line

For example, this will print out $foo every time line 53 is passed:

    a 53 print "DB FOUND $foo\n"

Delete all installed actions.

O [opt[=val]] [opt"val"] [opt?]...
Set or query values of options. val defaults to 1. opt can be abbreviated. Several options can be listed.

recallCommand, ShellBang
The characters used to recall command or spawn shell. By default, these are both set to !.

Program to use for output of pager-piped commands (those beginning with a | character.) By default, $ENV{PAGER} will be used.

Run Tk while prompting (with ReadLine).

signalLevel, warnLevel, dieLevel
Level of verbosity. By default the debugger is in a sane verbose mode, thus it will print backtraces on all the warnings and die-messages which are going to be printed out, and will print a message when interesting uncaught signals arrive.

To disable this behaviour, set these values to 0. If dieLevel is 2, then the messages which will be caught by surrounding eval are also printed.

Trace mode (similar to t command, but can be put into PERLDB_OPTS).

File or pipe to print line number info to. If it is a pipe (say, |visual_perl_db), then a short, ``emacs like'' message is used.

If 0, allows stepping off the end of the script.

affects printing of return value after r command.

affects printing messages on entry and exit from subroutines. If frame & 2 is false, messages are printed on entry only. (Printing on exit may be useful if interspersed with other messages.)

If frame & 4, arguments to functions are printed as well as the context and caller info. If frame & 8, overloaded stringify and tied FETCH are enabled on the printed arguments. The length at which the argument list is truncated is governed by the next option:

length at which the argument list is truncated when frame option's bit 4 is set.

The following options affect what happens with V, X, and x commands:

arrayDepth, hashDepth
Print only first N elements ('' for all).

compactDump, veryCompact
Change style of array and hash dump. If compactDump, short array may be printed on one line.

Whether to print contents of globs.

Dump arrays holding debugged files.

Dump symbol tables of packages.

quote, HighBit, undefPrint
Change style of string dump. Default value of quote is auto, one can enable either double-quotish dump, or single-quotish by setting it to " or '. By default, characters with high bit set are printed as is.

very rudimentally per-package memory usage dump. Calculates total size of strings in variables in the package.

During startup options are initialized from $ENV{PERLDB_OPTS}. You can put additional initialization options TTY, noTTY, ReadLine, and NonStop there.

Example rc file:

  &parse_options("NonStop=1 LineInfo=db.out AutoTrace");

The script will run without human intervention, putting trace information into the file db.out. (If you interrupt it, you would better reset LineInfo to something ``interactive''!)

The TTY to use for debugging I/O.

If set, goes in NonStop mode, and would not connect to a TTY. If interrupt (or if control goes to debugger via explicit setting of $DB::signal or $DB::single from the Perl script), connects to a TTY specified by the TTY option at startup, or to a TTY found at runtime using Term::Rendezvous module of your choice.

This module should implement a method new which returns an object with two methods: IN and OUT, returning two filehandles to use for debugging input and output correspondingly. Method new may inspect an argument which is a value of $ENV{PERLDB_NOTTY} at startup, or is "/tmp/perldbtty$$" otherwise.

If false, readline support in debugger is disabled, so you can debug ReadLine applications.

If set, debugger goes into non-interactive mode until interrupted, or programmatically by setting $DB::signal or $DB::single.

Here's an example of using the $ENV{PERLDB_OPTS} variable:

  $ PERLDB_OPTS="N f=2" perl -d myprogram

will run the script myprogram without human intervention, printing out the call tree with entry and exit points. Note that N f=2 is equivalent to NonStop=1 frame=2. Note also that at the moment when this documentation was written all the options to the debugger could be uniquely abbreviated by the first letter (with exception of Dump* options).

Other examples may include

  $ PERLDB_OPTS="N f A L=listing" perl -d myprogram

- runs script non-interactively, printing info on each entry into a subroutine and each executed line into the file listing. (If you interrupt it, you would better reset LineInfo to something ``interactive''!)

  $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram

may be useful for debugging a program which uses Term::ReadLine itself. Do not forget detach shell from the TTY in the window which corresponds to /dev/ttyc, say, by issuing a command like

  $ sleep 1000000

See Debugger Internals below for more details.

lt [ command ]
Set an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines. If command is missing, resets the list of actions.

ltlt command
Add an action (Perl command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.

gt command
Set an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines. If command is missing, resets the list of actions.

gtgt command
Adds an action (Perl command) to happen after the prompt when you've just given a command to return to executing the script. A multi-line command may be entered by backslashing the newlines.

{ [ command ]
Set an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines. If command is missing, resets the list of actions.

{{ command
Add an action (debugger command) to happen before every debugger prompt. A multi-line command may be entered by backslashing the newlines.

! number
Redo a previous command (default previous command).

! -number
Redo number'th-to-last command.

! pattern
Redo last command that started with pattern. See O recallCommand, too.

!! cmd
Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See O shellBang too.

H -number
Display last n commands. Only commands longer than one character are listed. If number is omitted, lists them all.

q or ^D
Quit. (``quit'' doesn't work for this.) This is the only supported way to exit the debugger, though typing exit twice may do it too.

Set an Option inhibit_exit to 0 if you want to be able to step off the end the script. You may also need to set $finished to 0 at some moment if you want to step through global destruction.

Restart the debugger by execing a new session. It tries to maintain your history across this, but internal settings and command line options may be lost.

Currently the following setting are preserved: history, breakpoints, actions, debugger Options, and the following command-line options: -w, -I, and -e.

Run debugger command, piping DB::OUT to current pager.

Same as |dbcmd but DB::OUT is temporarily selected as well. Often used with commands that would otherwise produce long output, such as

    |V main

= [alias value]
Define a command alias, like

    = quit q

or list current aliases.

Execute command as a Perl statement. A missing semicolon will be supplied.

m expr
The expression is evaluated, and the methods which may be applied to the result are listed.

m package
The methods which may be applied to objects in the package are listed.

Debugger input/output

The debugger prompt is something like


or even


where that number is the command number, which you'd use to access with the built-in csh-like history mechanism, e.g., !17 would repeat command number 17. The number of angle brackets indicates the depth of the debugger. You could get more than one set of brackets, for example, if you'd already at a breakpoint and then printed out the result of a function call that itself also has a breakpoint, or you step into an expression via s/n/t expression command.

Multi-line commands
If you want to enter a multi-line command, such as a subroutine definition with several statements, or a format, you may escape the newline that would normally end the debugger command with a backslash. Here's an example:

      DB<1> for (1..4) {         \
      cont:     print "ok\n";   \
      cont: }

Note that this business of escaping a newline is specific to interactive commands typed into the debugger.

Stack backtrace
Here's an example of what a stack back-trace via T command might look like:

    $ = main::infested called from file `' line 10
    @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
    $ = main::pests('bactrian', 4) called from file `camel_flea' line 4

The left-hand character up there tells whether the function was called in a scalar or list context (we bet you can tell which is which). What that says is that you were in the function main::infested when you ran the stack dump, and that it was called in a scalar context from line 10 of the file, but without any arguments at all, meaning it was called as &infested. The next stack frame shows that the function Ambulation::legs was called in a list context from the camel_flea file with four arguments. The last stack frame shows that main::pests was called in a scalar context, also from camel_flea, but from line 4.

Note that if you execute T command from inside an active use statement, the backtrace will contain both require frame and an eval EXPR) frame.

Listing given via different flavors of l command looks like this:

    DB<<13>> l
  101:                @i{@i} = ();
  102:b               @isa{@i,$pack} = ()
  103                     if(exists $i{$prevpack} || exists $isa{$pack});
  104             }
  106             next
  107==>              if(exists $isa{$pack});
  109:a           if ($extra-- > 0) {
  110:                %isa = ($pack,1);

Note that the breakable lines are marked with :, lines with breakpoints are marked by b, with actions by a, and the next executed line is marked by ==>.

Frame listing
When frame option is set, debugger would print entered (and optionally exited) subroutines in different styles.

What follows is the start of the listing of

  env "PERLDB_OPTS=f=1 N" perl -d -V

  1.   entering main::BEGIN
       entering Config::BEGIN
        Package lib/
        Package lib/
       Package lib/
       entering Config::TIEHASH
       entering Exporter::import
        entering Exporter::export
      entering Config::myconfig
       entering Config::FETCH
       entering Config::FETCH
       entering Config::FETCH
       entering Config::FETCH

  2.   entering main::BEGIN
       entering Config::BEGIN
        Package lib/
        Package lib/
       exited Config::BEGIN
       Package lib/
       entering Config::TIEHASH
       exited Config::TIEHASH
       entering Exporter::import
        entering Exporter::export
        exited Exporter::export
       exited Exporter::import
      exited main::BEGIN
      entering Config::myconfig
       entering Config::FETCH
       exited Config::FETCH
       entering Config::FETCH
       exited Config::FETCH
       entering Config::FETCH

  3.   in  $=main::BEGIN() from /dev/nul:0
       in  $=Config::BEGIN() from lib/
        Package lib/
        Package lib/
       Package lib/
       in  $=Config::TIEHASH('Config') from lib/
       in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
        in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
      in  @=Config::myconfig() from /dev/nul:0
       in  $=Config::FETCH(ref(Config), 'package') from lib/
       in  $=Config::FETCH(ref(Config), 'baserev') from lib/
       in  $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/
       in  $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/
       in  $=Config::FETCH(ref(Config), 'osname') from lib/
       in  $=Config::FETCH(ref(Config), 'osvers') from lib/

  4.   in  $=main::BEGIN() from /dev/nul:0
       in  $=Config::BEGIN() from lib/
        Package lib/
        Package lib/
       out $=Config::BEGIN() from lib/
       Package lib/
       in  $=Config::TIEHASH('Config') from lib/
       out $=Config::TIEHASH('Config') from lib/
       in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
        in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
        out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
       out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
      out $=main::BEGIN() from /dev/nul:0
      in  @=Config::myconfig() from /dev/nul:0
       in  $=Config::FETCH(ref(Config), 'package') from lib/
       out $=Config::FETCH(ref(Config), 'package') from lib/
       in  $=Config::FETCH(ref(Config), 'baserev') from lib/
       out $=Config::FETCH(ref(Config), 'baserev') from lib/
       in  $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/
       out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/
       in  $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/

  5.   in  $=main::BEGIN() from /dev/nul:0
       in  $=Config::BEGIN() from lib/
        Package lib/
        Package lib/
       out $=Config::BEGIN() from lib/
       Package lib/
       in  $=Config::TIEHASH('Config') from lib/
       out $=Config::TIEHASH('Config') from lib/
       in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
        in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
        out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
       out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
      out $=main::BEGIN() from /dev/nul:0
      in  @=Config::myconfig() from /dev/nul:0
       in  $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/
       out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/
       in  $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/
       out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/

In all the cases indentation of lines shows the call tree, if bit 2 of frame is set, then a line is printed on exit from a subroutine as well, if bit 4 is set, then the arguments are printed as well as the caller info, if bit 8 is set, the arguments are printed even if they are tied or references.

When a package is compiled, a line like this

    Package lib/

is printed with proper indentation.

Debugging compile-time statements

If you have any compile-time executable statements (code within a BEGIN block or a use statement), these will NOT be stopped by debugger, although requires will (and compile-time statements can be traced with AutoTrace option set in PERLDB_OPTS). From your own Perl code, however, you can transfer control back to the debugger using the following statement, which is harmless if the debugger is not running:

    $DB::single = 1;

If you set $DB::single to the value 2, it's equivalent to having just typed the n command, whereas a value of 1 means the s command. The $DB::trace variable should be set to 1 to simulate having typed the t command.

Another way to debug compile-time code is to start debugger, set a breakpoint on load of some module thusly

    DB<7> b load f:/perllib/lib/
  Will stop on load of `f:/perllib/lib/'.

and restart debugger by R command (if possible). One can use b compile subname for the same purpose.

Debugger Customization

Most probably you not want to modify the debugger, it contains enough hooks to satisfy most needs. You may change the behaviour of debugger from the debugger itself, using Options, from the command line via PERLDB_OPTS environment variable, and from customization files.

You can do some customization by setting up a .perldb file which contains initialization code. For instance, you could make aliases like these (the last one is one people expect to be there):

    $DB::alias{'len'}  = 's/^len(.*)/p length($1)/';
    $DB::alias{'stop'} = 's/^stop (at|in)/b/';
    $DB::alias{'ps'}   = 's/^ps\b/p scalar /';
    $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';

One changes options from .perldb file via calls like this one;

    parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");

(the code is executed in the package DB). Note that .perldb is processed before processing PERLDB_OPTS. If .perldb defines the subroutine afterinit, it is called after all the debugger initialization ends. .perldb may be contained in the current directory, or in the LOGDIR/HOME directory.

If you want to modify the debugger, copy from the Perl library to another name and modify it as necessary. You'll also want to set your PERL5DB environment variable to say something like this:

    BEGIN { require "" }

As the last resort, one can use PERL5DB to customize debugger by directly setting internal variables or calling debugger functions.

Readline Support

As shipped, the only command line history supplied is a simplistic one that checks for leading exclamation points. However, if you install the Term::ReadKey and Term::ReadLine modules from CPAN, you will have full editing capabilities much like GNU readline(3) provides. Look for these in the modules/by-module/Term directory on CPAN.

A rudimentary command-line completion is also available. Unfortunately, the names of lexical variables are not available for completion.

Editor Support for Debugging

If you have GNU emacs installed on your system, it can interact with the Perl debugger to provide an integrated software development environment reminiscent of its interactions with C debuggers.

Perl is also delivered with a start file for making emacs act like a syntax-directed editor that understands (some of) Perl's syntax. Look in the emacs directory of the Perl source distribution.

(Historically, a similar setup for interacting with vi and the X11 window system had also been available, but at the time of this writing, no debugger support for vi currently exists.)

The Perl Profiler

If you wish to supply an alternative debugger for Perl to run, just invoke your script with a colon and a package argument given to the -d flag. One of the most popular alternative debuggers for Perl is DProf, the Perl profiler. As of this writing, DProf is not included with the standard Perl distribution, but it is expected to be included soon, for certain values of ``soon''.

Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming it's properly installed on your system, to profile your Perl program in the file, just type:

    perl -d:DProf

When the script terminates the profiler will dump the profile information to a file called tmon.out. A tool like dprofpp (also supplied with the Devel::DProf package) can be used to interpret the information which is in that profile.

Debugger support in perl

When you call the caller function (see caller) from the package DB, Perl sets the array @DB::args to contain the arguments the corresponding stack frame was called with.

If perl is run with -d option, the following additional features are enabled:

Note that no subroutine call is possible until &DB::sub is defined (for subroutines outside of package DB). (This restriction is recently lifted.)

(In fact, for the standard debugger the same is true if $DB::deep (how many levels of recursion deep into the debugger you can go before a mandatory break) is not defined.)

With the recent updates the minimal possible debugger consists of one line

  sub DB::DB {}

which is quite handy as contents of PERL5DB environment variable:

  env "PERL5DB=sub DB::DB {}" perl -d your-script

Another (a little bit more useful) minimal debugger can be created with the only line being

  sub DB::DB {print ++$i; scalar <STDIN>}

This debugger would print the sequential number of encountered statement, and would wait for your CR to continue.

The following debugger is quite functional:

    package DB; 
    sub DB  {} 
    sub sub {print ++$i, " $sub\n"; &$sub}

It prints the sequential number of subroutine call and the name of the called subroutine. Note that &DB::sub should be compiled into the package DB.

Debugger Internals

At the start, the debugger reads your rc file (./.perldb or ~/.perldb under UNIX), which can set important options. This file may define a subroutine &afterinit to be executed after the debugger is initialized.

After the rc file is read, the debugger reads environment variable PERLDB_OPTS and parses it as a rest of O ... line in debugger prompt.

It also maintains magical internal variables, such as @DB::dbline, %DB::dbline, which are aliases for @{"::_ %{"::_. Here current_file is the currently selected (with the debugger's f command, or by flow of execution) file.

Some functions are provided to simplify customization. See Debugger Customization for description of DB::parse_options. The function DB::dump_trace skips the specified number of frames, and returns an array containing info about the caller frames (all if count is missing). Each entry is a hash with keys context ($ or @), sub (subroutine name, or info about eval), args (undef or a reference to an array), file, and line.

The function DB::print_trace prints formatted info about caller frames. The last two functions may be convenient as arguments to <, << commands.

Other resources

You did try the -w switch, didn't you?


You cannot get the stack frame information or otherwise debug functions that were not compiled by Perl, such as C or C++ extensions.

If you alter your @_ arguments in a subroutine (such as with shift or pop, the stack back-trace will not show the original values.