path: root/pod/perldebug.pod

=head1 NAME

perldebug - Perl debugging

=head1 DESCRIPTION

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

=head1 The Perl Debugger

"As soon as we started programming, we found to our
surprise that it wasn't as easy to get programs right
as we had thought.  Debugging had to be discovered.
I can remember the exact instant when I realized that
a large part of my life from then on was going to be
spent in finding mistakes in my own programs."

I<  --Maurice Wilkes, 1949>

If you invoke Perl with the B<-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 backtraces, 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 B<-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
preloads a Perl library file containing the debugger itself.

The program will halt I<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 I<about> to execute, rather than the one it has just executed.

Any command not recognized by the debugger is directly executed
(C<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 I<NOT> a debugger command but for Perl, so be careful not to do
that.

=head2 Debugger Commands

The debugger understands the following commands:

=over 12

=item h [command]

Prints out a help message.

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

If the output of the C<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 C<O pager=...> command.

=item p expr

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

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

=item 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 C<print> function.

The details of printout are governed by multiple C<O>ptions.

=item V [pkg [vars]]

Display all (or some) variables in package (defaulting to the C<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 C<$>) there, just the symbol
names, like this:

    V DB filename line

Use C<~pattern> and C<!pattern> for positive and negative regexps.

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

The details of printout are governed by multiple C<O>ptions.

=item X [vars]

Same as C<V currentpackage [vars]>.

=item T

Produce a stack backtrace.  See below for details on its output.

=item 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.

=item 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.

=item E<lt>CRE<gt>

Repeat last C<n> or C<s> command.

=item c [line|sub]

Continue, optionally inserting a one-time-only breakpoint
at the specified line or subroutine.

=item l

List next window of lines.

=item l min+incr

List C<incr+1> lines starting at C<min>.

=item l min-max

List lines C<min> through C<max>.  C<l -> is synonymous to C<->.

=item l line

List a single line.

=item l subname

List first window of lines from subroutine.

=item -

List previous window of lines.

=item w [line]

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

=item .

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

=item f filename

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

=item /pattern/

Search forwards for pattern; final / is optional.

=item ?pattern?

Search backwards for pattern; final ? is optional.

=item L

List all breakpoints and actions.

=item S [[!]pattern]

List subroutine names [not] matching pattern.

=item t

Toggle trace mode (see also C<AutoTrace> C<O>ption).

=item t expr

Trace through execution of expr.  For example:

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

 Loading DB routines from perl5db.pl 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):
 42

or, with the C<O>ption C<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
 42

=item 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 B<if>:

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

=item b subname [condition]

Set a breakpoint at the first line of the named subroutine.

=item b postpone subname [condition]

Set breakpoint at first line of subroutine after it is compiled.

=item 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.

=item b compile subname

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

=item 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.

=item D

Delete all installed breakpoints.

=item 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"

=item A

Delete all installed actions.

=item W [expr]

Add a global watch-expression.

=item W

Delete all watch-expressions.

=item 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.

=over 12

=item C<recallCommand>, C<ShellBang>

The characters used to recall command or spawn shell.  By
default, these are both set to C<!>.

=item C<pager>

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

=item C<tkRunning>

Run Tk while prompting (with ReadLine).

=item C<signalLevel>, C<warnLevel>, C<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 C<dieLevel> is 2,
then the messages which will be caught by surrounding C<eval> are also
printed.

=item C<AutoTrace>

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

=item C<LineInfo>

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

=item C<inhibit_exit>

If 0, allows I<stepping off> the end of the script.

=item C<PrintRet>

affects printing of return value after C<r> command.

=item C<ornaments>

affects screen appearance of the command line (see L<Term::ReadLine>).

=item C<frame>

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

If C<frame & 4>, arguments to functions are printed as well as the
context and caller info.  If C<frame & 8>, overloaded C<stringify> and
C<tie>d C<FETCH> are enabled on the printed arguments. If C<frame &
16>, the return value from the subroutine is printed as well.

The length at which the argument list is truncated is governed by the
next option:

=item C<maxTraceLen>

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

=back

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

=over 12

=item C<arrayDepth>, C<hashDepth>

Print only first N elements ('' for all).

=item C<compactDump>, C<veryCompact>

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

=item C<globPrint>

Whether to print contents of globs.

=item C<DumpDBFiles>

Dump arrays holding debugged files.

=item C<DumpPackages>

Dump symbol tables of packages.

=item C<DumpReused>

Dump contents of "reused" addresses.

=item C<quote>, C<HighBit>, C<undefPrint>

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

=item C<UsageOnly>

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

=back

During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
You can put additional initialization options C<TTY>, C<noTTY>,
C<ReadLine>, and C<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 I<db.out>.  (If you interrupt it, you would better reset
C<LineInfo> to something "interactive"!)

=over 12

=item C<TTY>

The TTY to use for debugging I/O.

=item C<noTTY>

If set, goes in C<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 C<TTY> option at startup, or to a TTY found at
runtime using C<Term::Rendezvous> module of your choice.

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

=item C<ReadLine>

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

=item C<NonStop>

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

=back

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

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

will run the script C<myprogram> without human intervention, printing
out the call tree with entry and exit points.  Note that C<N f=2> is
equivalent to C<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
C<Dump*> options).

Other examples may include

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

- runs script noninteractively, printing info on each entry into a
subroutine and each executed line into the file F<listing>. (If you
interrupt it, you would better reset C<LineInfo> to something
"interactive"!)


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

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

  $ sleep 1000000

See L<"Debugger Internals"> below for more details.

=item E<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
C<command> is missing, resets the list of actions.

=item E<lt>E<lt> command

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

=item E<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 C<command> is
missing, resets the list of actions.

=item E<gt>E<gt> 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.

=item { [ command ]

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

=item {{ command

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

=item ! number

Redo a previous command (default previous command).

=item ! -number

Redo number'th-to-last command.

=item ! pattern

Redo last command that started with pattern.
See C<O recallCommand>, too.

=item !! cmd

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

=item H -number

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

=item q or ^D

Quit.  ("quit" doesn't work for this.)  This is the only supported way
to exit the debugger, though typing C<exit> twice may do it too.

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

=item R

Restart the debugger by B<exec>ing 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 C<O>ptions, and the following command line
options: B<-w>, B<-I>, and B<-e>.

=item |dbcmd

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

=item ||dbcmd

Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
Often used with commands that would otherwise produce long
output, such as

    |V main

=item = [alias value]

Define a command alias, like

    = quit q

or list current aliases.

=item command

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

=item m expr

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

=item m package

The methods which may be applied to objects in the C<package> are listed.

=back

=head2 Debugger input/output

=over 8

=item Prompt

The debugger prompt is something like

    DB<8>

or even

    DB<<17>>

where that number is the command number, which you'd use to access with
the builtin B<csh>-like history mechanism, e.g., C<!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 C<s/n/t expression> command.

=item Multiline 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: }
      ok
      ok
      ok
      ok

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

=item Stack backtrace

Here's an example of what a stack backtrace via C<T> command might
look like:

    $ = main::infested called from file `Ambulation.pm' 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 C<main::infested> when you ran
the stack dump, and that it was called in a scalar context from line 10
of the file I<Ambulation.pm>, but without any arguments at all, meaning
it was called as C<&infested>.  The next stack frame shows that the
function C<Ambulation::legs> was called in a list context from the
I<camel_flea> file with four arguments.  The last stack frame shows that
C<main::pests> was called in a scalar context, also from I<camel_flea>,
but from line 4.

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

=item Listing

Listing given via different flavors of C<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             }
  105
  106             next
  107==>              if(exists $isa{$pack});
  108
  109:a           if ($extra-- > 0) {
  110:                %isa = ($pack,1);

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

=item Frame listing

When C<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=n N" perl -d -V

for different values of C<n>:

=over 4

=item 1

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

=item 2

  entering main::BEGIN
   entering Config::BEGIN
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   exited Config::BEGIN
   Package lib/Config.pm.
   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

=item 4

  in  $=main::BEGIN() from /dev/nul:0
   in  $=Config::BEGIN() from lib/Config.pm:2
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   Package lib/Config.pm.
   in  $=Config::TIEHASH('Config') from lib/Config.pm:644
   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/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574

=item 6

  in  $=main::BEGIN() from /dev/nul:0
   in  $=Config::BEGIN() from lib/Config.pm:2
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   out $=Config::BEGIN() from lib/Config.pm:0
   Package lib/Config.pm.
   in  $=Config::TIEHASH('Config') from lib/Config.pm:644
   out $=Config::TIEHASH('Config') from lib/Config.pm:644
   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/Config.pm:574
   out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
   out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
   out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574

=item 14

  in  $=main::BEGIN() from /dev/nul:0
   in  $=Config::BEGIN() from lib/Config.pm:2
    Package lib/Exporter.pm.
    Package lib/Carp.pm.
   out $=Config::BEGIN() from lib/Config.pm:0
   Package lib/Config.pm.
   in  $=Config::TIEHASH('Config') from lib/Config.pm:644
   out $=Config::TIEHASH('Config') from lib/Config.pm:644
   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/Config.pm:574
   out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
   in  $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
   out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574

=item 30

  in  $=CODE(0x15eca4)() from /dev/null:0
   in  $=CODE(0x182528)() from lib/Config.pm:2
    Package lib/Exporter.pm.
   out $=CODE(0x182528)() from lib/Config.pm:0
   scalar context return from CODE(0x182528): undef
   Package lib/Config.pm.
   in  $=Config::TIEHASH('Config') from lib/Config.pm:628
   out $=Config::TIEHASH('Config') from lib/Config.pm:628
   scalar context return from Config::TIEHASH:   empty hash
   in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
    out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
    scalar context return from Exporter::export: ''
   out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
   scalar context return from Exporter::import: ''


=back

In all the cases indentation of lines shows the call tree, if bit 2 of
C<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, if bit 16 is set, the return value is printed
as well.

When a package is compiled, a line like this

    Package lib/Carp.pm.

is printed with proper indentation.

=back

=head2 Debugging compile-time statements

If you have any compile-time executable statements (code within a BEGIN
block or a C<use> statement), these will C<NOT> be stopped by debugger,
although C<require>s will (and compile-time statements can be traced
with C<AutoTrace> option set in C<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 C<$DB::single> to the value 2, it's equivalent to having
just typed the C<n> command, whereas a value of 1 means the C<s>
command.  The C<$DB::trace>  variable should be set to 1 to simulate
having typed the C<t> command.

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

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

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

=head2 Debugger Customization

Most probably you do 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 C<O>ptions, from the command line via
C<PERLDB_OPTS> environment variable, and from I<customization files>.

You can do some customization by setting up a F<.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 F<.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 C<DB>).  Note that F<.perldb> is
processed before processing C<PERLDB_OPTS>.  If F<.perldb> defines the
subroutine C<afterinit>, it is called after all the debugger
initialization ends.  F<.perldb> may be contained in the current
directory, or in the C<LOGDIR>/C<HOME> directory.

If you want to modify the debugger, copy F<perl5db.pl> from the Perl
library to another name and modify it as necessary.  You'll also want
to set your C<PERL5DB> environment variable to say something like this:

    BEGIN { require "myperl5db.pl" }

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

=head2 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 I<readline>(3) provides.
Look for these in the F<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.

=head2 Editor Support for Debugging

If you have GNU B<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 B<emacs> act like a
syntax-directed editor that understands (some of) Perl's syntax.  Look in
the I<emacs> directory of the Perl source distribution.

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

=head2 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 B<-d>
flag.  One of the most popular alternative debuggers for Perl is
B<DProf>, the Perl profiler.   As of this writing, B<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 F<mycode.pl>, just type:

    perl -d:DProf mycode.pl

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

=head2 Debugger support in perl

When you call the B<caller> function (see L<perlfunc/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 B<-d> option, the following additional features
are enabled (cf. L<perlvar/$^P>):

=over

=item *

Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
'perl5db.pl'}> if not present) before the first line of the
application.

=item *

The array C<@{"_E<lt>$filename"}> is the line-by-line contents of
$filename for all the compiled files.  Same for C<eval>ed strings which
contain subroutines, or which are currently executed.  The C<$filename>
for C<eval>ed strings looks like C<(eval 34)>.

=item *

The hash C<%{"_E<lt>$filename"}> contains breakpoints and action (it is
keyed by line number), and individual entries are settable (as opposed
to the whole hash).  Only true/false is important to Perl, though the
values used by F<perl5db.pl> have the form
C<"$break_condition\0$action">.  Values are magical in numeric context:
they are zeros if the line is not breakable.

Same for evaluated strings which contain subroutines, or which are
currently executed.  The $filename for C<eval>ed strings looks like
C<(eval 34)>.

=item *

The scalar C<${"_E<lt>$filename"}> contains C<"_E<lt>$filename">.  Same for
evaluated strings which contain subroutines, or which are currently
executed.  The $filename for C<eval>ed strings looks like C<(eval
34)>.

=item *

After each C<require>d file is compiled, but before it is executed,
C<DB::postponed(*{"_E<lt>$filename"})> is called (if subroutine
C<DB::postponed> exists).  Here the $filename is the expanded name of
the C<require>d file (as found in values of %INC).

=item *

After each subroutine C<subname> is compiled existence of
C<$DB::postponed{subname}> is checked.  If this key exists,
C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
exists).

=item *

A hash C<%DB::sub> is maintained, with keys being subroutine names,
values having the form C<filename:startline-endline>.  C<filename> has
the form C<(eval 31)> for subroutines defined inside C<eval>s.

=item *

When execution of the application reaches a place that can have
a breakpoint, a call to C<DB::DB()> is performed if any one of
variables $DB::trace, $DB::single, or $DB::signal is true. (Note that
these variables are not C<local>izable.) This feature is disabled when
the control is inside C<DB::DB()> or functions called from it (unless
C<$^D & (1E<lt>E<lt>30)>).

=item *

When execution of the application reaches a subroutine call, a call
to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
the name of the called subroutine. (Unless the subroutine is compiled
in the package C<DB>.)

=back

Note that if C<&DB::sub> needs some external data to be setup for it
to work, no subroutine call is possible until this is done.  For the
standard debugger C<$DB::deep> (how many levels of recursion deep into
the debugger you can go before a mandatory break) gives an example of
such a dependency.

The minimal working debugger consists of one line

  sub DB::DB {}

which is quite handy as contents of C<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 C<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 C<&DB::sub> should be compiled into the
package C<DB>.

=head2 Debugger Internals

At the start, the debugger reads your rc file (F<./.perldb> or
F<~/.perldb> under Unix), which can set important options.  This file may
define a subroutine C<&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 C<O ...> line in debugger prompt.

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

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

The function C<DB::print_trace(FH, skip[, count[, short]])> prints
formatted info about caller frames.  The last two functions may be
convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.

=head2 Other resources

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

=head1 BUGS

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 B<shift>
or B<pop>, the stack backtrace will not show the original values.