README.vms and related updates (from Peter Prymmer <pvhp@best.com>)

[p5sagit/p5-mst-13.2.git] / 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 E<45>> is synonymous to C<E<45>>.

=item l line

List a single line.

=item l subname

List first window of lines from subroutine.  I<subname> may
be a variable which contains a code reference.

=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.

C<eval>ed strings (when accessible) are considered to be filenames:
C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
(in the order of execution).  The bodies of currently executed C<eval>
and of C<eval>ed strings which define subroutines are saved, thus are
accessible by this mechanism.

=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.  I<subname> may
be a variable which contains a code reference (in this case I<condition>
is not supported).

=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 $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), 'PERL_VERSION') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'PERL_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), 'PERL_VERSION') from lib/Config.pm:574
   out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
   in  $=Config::FETCH(ref(Config), 'PERL_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 $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 a list 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?

=head2 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.

=head1 Debugging Perl memory usage

Perl is I<very> frivolous with memory.  There is a saying that to
estimate memory usage of Perl, assume a reasonable algorithm of
allocation, and multiply your estimates by 10.  This is not absolutely
true, but may give you a good grasp of what happens.

Say, an integer cannot take less than 20 bytes of memory, a float
cannot take less than 24 bytes, a string cannot take less than 32
bytes (all these examples assume 32-bit architectures, the result are
much worse on 64-bit architectures).  If a variable is accessed in two
of three different ways (which require an integer, a float, or a
string), the memory footprint may increase by another 20 bytes.  A
sloppy malloc() implementation will make these numbers yet more.

On the opposite end of the scale, a declaration like

  sub foo;

may take (on some versions of perl) up to 500 bytes of memory.

Off-the-cuff anecdotal estimates of a code bloat give a factor around
8.  This means that the compiled form of reasonable (commented
indented etc.)  code will take approximately 8 times more than the
disk space the code takes.

There are two Perl-specific ways to analyze the memory usage:
$ENV{PERL_DEBUG_MSTATS} and B<-DL> switch.  First one is available
only if perl is compiled with Perl's malloc(), the second one only if
Perl compiled with C<-DDEBUGGING> (as with giving C<-D optimise=-g>
option to F<Configure>).

=head2 Using C<$ENV{PERL_DEBUG_MSTATS}>

If your perl is using Perl's malloc(), and compiled with correct
switches (this is the default), then it will print memory usage
statistics after compiling your code (if C<$ENV{PERL_DEBUG_MSTATS}> >
1), and before termination of the script (if
C<$ENV{PERL_DEBUG_MSTATS}> >= 1).  The report format is similar to one
in the following example:

  env PERL_DEBUG_MSTATS=2 perl -e "require Carp"
  Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
     14216 free:   130   117    28     7     9   0   2     2   1 0 0
                437    61    36     0     5
     60924 used:   125   137   161    55     7   8   6    16   2 0 1
                 74   109   304    84    20
  Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
  Memory allocation statistics after execution:   (buckets 4(4)..8188(8192)
     30888 free:   245    78    85    13     6   2   1     3   2 0 1
                315   162    39    42    11
    175816 used:   265   176  1112   111    26  22  11    27   2 1 1
                196   178  1066   798    39
  Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.

It is possible to ask for such a statistic at arbitrary moment by
using Devel::Peek::mstats() (module Devel::Peek is available on CPAN).

Here is the explanation of different parts of the format:

=over

=item C<buckets SMALLEST(APPROX)..GREATEST(APPROX)>

Perl's malloc() uses bucketed allocations.  Every request is rounded
up to the closest bucket size available, and a bucket of these size is
taken from the pool of the buckets of this size.

The above line describes limits of buckets currently in use.  Each
bucket has two sizes: memory footprint, and the maximal size of user
data which may be put into this bucket.  Say, in the above example the
smallest bucket is both sizes 4.  The biggest bucket has usable size
8188, and the memory footprint 8192.  

With debugging Perl some buckets may have negative usable size.  This
means that these buckets cannot (and will not) be used.  For greater
buckets the memory footprint may be one page greater than a power of
2.  In such a case the corresponding power of two is printed instead
in the C<APPROX> field above.

=item Free/Used

The following 1 or 2 rows of numbers correspond to the number of
buckets of each size between C<SMALLEST> and C<GREATEST>.  In the
first row the sizes (memory footprints) of buckets are powers of two
(or possibly one page greater).  In the second row (if present) the
memory footprints of the buckets are between memory footprints of two
buckets "above".  

Say, with the above example the memory footprints are (with current
algorithm)

     free:    8     16    32    64    128  256 512 1024 2048 4096 8192
           4     12    24    48    80

With non-C<DEBUGGING> perl the buckets starting from C<128>-long ones
have 4-byte overhead, thus 8192-long bucket may take up to
8188-byte-long allocations.

=item C<Total sbrk(): SBRKed/SBRKs:CONTINUOUS>

The first two fields give the total amount of memory perl sbrk()ed,
and number of sbrk()s used.  The third number is what perl thinks
about continuity of returned chunks.  As far as this number is
positive, malloc() will assume that it is probable that sbrk() will
provide continuous memory.

The amounts sbrk()ed by external libraries is not counted.

=item C<pad: 0>

The amount of sbrk()ed memory needed to keep buckets aligned.

=item C<heads: 2192>

While memory overhead of bigger buckets is kept inside the bucket, for
smaller buckets it is kept in separate areas.  This field gives the
total size of these areas.

=item C<chain: 0>

malloc() may want to subdivide a bigger bucket into smaller buckets.
If only a part of the deceased-bucket is left non-subdivided, the rest
is kept as an element of a linked list.  This field gives the total
size of these chunks.

=item C<tail: 6144>

To minimize amount of sbrk()s malloc() asks for more memory.  This
field gives the size of the yet-unused part, which is sbrk()ed, but
never touched.

=back

=head2 Example of using B<-DL> switch

Below we show how to analyse memory usage by 

  do 'lib/auto/POSIX/autosplit.ix';

The file in question contains a header and 146 lines similar to

  sub getcwd ;

B<Note:> I<the discussion below supposes 32-bit architecture.  In the
newer versions of perl the memory usage of the constructs discussed
here is much improved, but the story discussed below is a real-life
story.  This story is very terse, and assumes more than cursory
knowledge of Perl internals.>

Here is the itemized list of Perl allocations performed during parsing
of this file:

 !!! "after" at test.pl line 3.
    Id  subtot   4   8  12  16  20  24  28  32  36  40  48  56  64  72  80 80+
  0 02   13752   .   .   .   . 294   .   .   .   .   .   .   .   .   .   .   4
  0 54    5545   .   .   8 124  16   .   .   .   1   1   .   .   .   .   .   3
  5 05      32   .   .   .   .   .   .   .   1   .   .   .   .   .   .   .   .
  6 02    7152   .   .   .   .   .   .   .   .   .   . 149   .   .   .   .   .
  7 02    3600   .   .   .   .   . 150   .   .   .   .   .   .   .   .   .   .
  7 03      64   .  -1   .   1   .   .   2   .   .   .   .   .   .   .   .   .
  7 04    7056   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   7
  7 17   38404   .   .   .   .   .   .   .   1   .   . 442 149   .   . 147   .
  9 03    2078  17 249  32   .   .   .   .   2   .   .   .   .   .   .   .   .


To see this list insert two C<warn('!...')> statements around the call:

  warn('!');
  do 'lib/auto/POSIX/autosplit.ix';
  warn('!!! "after"');

and run it with B<-DL> option.  The first warn() will print memory
allocation info before the parsing of the file, and will memorize the
statistics at this point (we ignore what it prints). The second warn()
will print increments w.r.t. this memorized statistics.  This is the
above printout.

Different I<Id>s on the left correspond to different subsystems of
perl interpreter, they are just first argument given to perl memory
allocation API New().  To find what C<9 03> means C<grep> the perl
source for C<903>.  You will see that it is F<util.c>, function
savepvn().  This function is used to store a copy of existing chunk of
memory.  Using C debugger, one can see that it is called either
directly from gv_init(), or via sv_magic(), and gv_init() is called
from gv_fetchpv() - which is called from newSUB().

B<Note:> to reach this place in debugger and skip all the calls to
savepvn during the compilation of the main script, set a C breakpoint
in Perl_warn(), C<continue> this point is reached, I<then> set
breakpoint in Perl_savepvn().  Note that you may need to skip a
handful of Perl_savepvn() which do not correspond to mass production
of CVs (there are more C<903> allocations than 146 similar lines of
F<lib/auto/POSIX/autosplit.ix>).  Note also that C<Perl_> prefixes are
added by macroization code in perl header files to avoid conflicts
with external libraries.

Anyway, we see that C<903> ids correspond to creation of globs, twice
per glob - for glob name, and glob stringification magic.

Here are explanations for other I<Id>s above: 

=over

=item C<717> 

is for creation of bigger C<XPV*> structures.  In the above case it
creates 3 C<AV> per subroutine, one for a list of lexical variable
names, one for a scratchpad (which contains lexical variables and
C<targets>), and one for the array of scratchpads needed for
recursion.  

It also creates a C<GV> and a C<CV> per subroutine (all called from
start_subparse()).

=item C<002>

Creates C array corresponding to the C<AV> of scratchpads, and the
scratchpad itself (the first fake entry of this scratchpad is created
though the subroutine itself is not defined yet).

It also creates C arrays to keep data for the stash (this is one HV,
but it grows, thus there are 4 big allocations: the big chunks are not
freed, but are kept as additional arenas for C<SV> allocations).

=item C<054>

creates a C<HEK> for the name of the glob for the subroutine (this
name is a key in a I<stash>).

Big allocations with this I<Id> correspond to allocations of new
arenas to keep C<HE>.

=item C<602>

creates a C<GP> for the glob for the subroutine.

=item C<702>

creates the C<MAGIC> for the glob for the subroutine.

=item C<704>

creates I<arenas> which keep SVs.

=back

=head2 B<-DL> details

If Perl is run with B<-DL> option, then warn()s which start with `!'
behave specially.  They print a list of I<categories> of memory
allocations, and statistics of allocations of different sizes for
these categories.

If warn() string starts with

=over

=item C<!!!> 

print changed categories only, print the differences in counts of allocations;

=item C<!!> 

print grown categories only; print the absolute values of counts, and totals;

=item C<!>

print nonempty categories, print the absolute values of counts and totals.

=back

=head2 Limitations of B<-DL> statistic

If an extension or an external library does not use Perl API to
allocate memory, these allocations are not counted.

=head1 Debugging regular expressions

There are two ways to enable debugging output for regular expressions.

If your perl is compiled with C<-DDEBUGGING>, you may use the
B<-Dr> flag on the command line.

Otherwise, one can C<use re 'debug'>, which has effects both at
compile time, and at run time (and is I<not> lexically scoped).

=head2 Compile-time output

The debugging output for the compile time looks like this:

  compiling RE `[bc]d(ef*g)+h[ij]k$'
  size 43 first at 1
     1: ANYOF(11)
    11: EXACT <d>(13)
    13: CURLYX {1,32767}(27)
    15:   OPEN1(17)
    17:     EXACT <e>(19)
    19:     STAR(22)
    20:       EXACT <f>(0)
    22:     EXACT <g>(24)
    24:   CLOSE1(26)
    26:   WHILEM(0)
    27: NOTHING(28)
    28: EXACT <h>(30)
    30: ANYOF(40)
    40: EXACT <k>(42)
    42: EOL(43)
    43: END(0)
  anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
                                    stclass `ANYOF' minlen 7

The first line shows the pre-compiled form of the regexp, and the
second shows the size of the compiled form (in arbitrary units,
usually 4-byte words) and the label I<id> of the first node which
does a match.

The last line (split into two lines in the above) contains the optimizer
info.  In the example shown, the optimizer found that the match 
should contain a substring C<de> at the offset 1, and substring C<gh>
at some offset between 3 and infinity.  Moreover, when checking for
these substrings (to abandon impossible matches quickly) it will check
for the substring C<gh> before checking for the substring C<de>.  The
optimizer may also use the knowledge that the match starts (at the
C<first> I<id>) with a character class, and the match cannot be
shorter than 7 chars.

The fields of interest which may appear in the last line are

=over

=item C<anchored> I<STRING> C<at> I<POS>

=item C<floating> I<STRING> C<at> I<POS1..POS2>

see above;

=item C<matching floating/anchored>

which substring to check first;

=item C<minlen>

the minimal length of the match;

=item C<stclass> I<TYPE>

The type of the first matching node.

=item C<noscan>

which advises to not scan for the found substrings;

=item C<isall>

which says that the optimizer info is in fact all that the regular
expression contains (thus one does not need to enter the RE engine at
all);

=item C<GPOS>

if the pattern contains C<\G>;

=item C<plus> 

if the pattern starts with a repeated char (as in C<x+y>);

=item C<implicit>

if the pattern starts with C<.*>;

=item C<with eval> 

if the pattern contain eval-groups (see L<perlre/(?{ code })>);

=item C<anchored(TYPE)>

if the pattern may
match only at a handful of places  (with C<TYPE> being
C<BOL>, C<MBOL>, or C<GPOS>, see the table below).

=back

If a substring is known to match at end-of-line only, it may be
followed by C<$>, as in C<floating `k'$>.

The optimizer-specific info is used to avoid entering (a slow) RE
engine on strings which will definitely not match.  If C<isall> flag
is set, a call to the RE engine may be avoided even when optimizer
found an appropriate place for the match.

The rest of the output contains the list of I<nodes> of the compiled
form of the RE.  Each line has format 

C<   >I<id>: I<TYPE> I<OPTIONAL-INFO> (I<next-id>)

=head2 Types of nodes

Here is the list of possible types with short descriptions:

    # TYPE arg-description [num-args] [longjump-len] DESCRIPTION

    # Exit points
    END         no      End of program.
    SUCCEED     no      Return from a subroutine, basically.

    # Anchors:
    BOL         no      Match "" at beginning of line.
    MBOL        no      Same, assuming multiline.
    SBOL        no      Same, assuming singleline.
    EOS         no      Match "" at end of string.
    EOL         no      Match "" at end of line.
    MEOL        no      Same, assuming multiline.
    SEOL        no      Same, assuming singleline.
    BOUND       no      Match "" at any word boundary
    BOUNDL      no      Match "" at any word boundary
    NBOUND      no      Match "" at any word non-boundary
    NBOUNDL     no      Match "" at any word non-boundary
    GPOS        no      Matches where last m//g left off.

    # [Special] alternatives
    ANY         no      Match any one character (except newline).
    SANY        no      Match any one character.
    ANYOF       sv      Match character in (or not in) this class.
    ALNUM       no      Match any alphanumeric character
    ALNUML      no      Match any alphanumeric char in locale
    NALNUM      no      Match any non-alphanumeric character
    NALNUML     no      Match any non-alphanumeric char in locale
    SPACE       no      Match any whitespace character
    SPACEL      no      Match any whitespace char in locale
    NSPACE      no      Match any non-whitespace character
    NSPACEL     no      Match any non-whitespace char in locale
    DIGIT       no      Match any numeric character
    NDIGIT      no      Match any non-numeric character

    # BRANCH    The set of branches constituting a single choice are hooked
    #           together with their "next" pointers, since precedence prevents
    #           anything being concatenated to any individual branch.  The
    #           "next" pointer of the last BRANCH in a choice points to the
    #           thing following the whole choice.  This is also where the
    #           final "next" pointer of each individual branch points; each
    #           branch starts with the operand node of a BRANCH node.
    #
    BRANCH      node    Match this alternative, or the next...

    # BACK      Normal "next" pointers all implicitly point forward; BACK
    #           exists to make loop structures possible.
    # not used
    BACK        no      Match "", "next" ptr points backward.

    # Literals
    EXACT       sv      Match this string (preceded by length).
    EXACTF      sv      Match this string, folded (prec. by length).
    EXACTFL     sv      Match this string, folded in locale (w/len).

    # Do nothing
    NOTHING     no      Match empty string.
    # A variant of above which delimits a group, thus stops optimizations
    TAIL        no      Match empty string. Can jump here from outside.

    # STAR,PLUS '?', and complex '*' and '+', are implemented as circular
    #           BRANCH structures using BACK.  Simple cases (one character
    #           per match) are implemented with STAR and PLUS for speed
    #           and to minimize recursive plunges.
    #
    STAR        node    Match this (simple) thing 0 or more times.
    PLUS        node    Match this (simple) thing 1 or more times.

    CURLY       sv 2    Match this simple thing {n,m} times.
    CURLYN      no 2    Match next-after-this simple thing 
    #                   {n,m} times, set parenths.
    CURLYM      no 2    Match this medium-complex thing {n,m} times.
    CURLYX      sv 2    Match this complex thing {n,m} times.

    # This terminator creates a loop structure for CURLYX
    WHILEM      no      Do curly processing and see if rest matches.

    # OPEN,CLOSE,GROUPP ...are numbered at compile time.
    OPEN        num 1   Mark this point in input as start of #n.
    CLOSE       num 1   Analogous to OPEN.

    REF         num 1   Match some already matched string
    REFF        num 1   Match already matched string, folded
    REFFL       num 1   Match already matched string, folded in loc.

    # grouping assertions
    IFMATCH     off 1 2 Succeeds if the following matches.
    UNLESSM     off 1 2 Fails if the following matches.
    SUSPEND     off 1 1 "Independent" sub-RE.
    IFTHEN      off 1 1 Switch, should be preceeded by switcher .
    GROUPP      num 1   Whether the group matched.

    # Support for long RE
    LONGJMP     off 1 1 Jump far away.
    BRANCHJ     off 1 1 BRANCH with long offset.

    # The heavy worker
    EVAL        evl 1   Execute some Perl code.

    # Modifiers
    MINMOD      no      Next operator is not greedy.
    LOGICAL     no      Next opcode should set the flag only.

    # This is not used yet
    RENUM       off 1 1 Group with independently numbered parens.

    # This is not really a node, but an optimized away piece of a "long" node.
    # To simplify debugging output, we mark it as if it were a node
    OPTIMIZED   off     Placeholder for dump.

=head2 Run-time output

First of all, when doing a match, one may get no run-time output even
if debugging is enabled.  this means that the RE engine was never
entered, all of the job was done by the optimizer.

If RE engine was entered, the output may look like this:

  Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
    Setting an EVAL scope, savestack=3
     2 <ab> <cdefg__gh_>    |  1: ANYOF
     3 <abc> <defg__gh_>    | 11: EXACT <d>
     4 <abcd> <efg__gh_>    | 13: CURLYX {1,32767}
     4 <abcd> <efg__gh_>    | 26:   WHILEM
                                0 out of 1..32767  cc=effff31c
     4 <abcd> <efg__gh_>    | 15:     OPEN1
     4 <abcd> <efg__gh_>    | 17:     EXACT <e>
     5 <abcde> <fg__gh_>    | 19:     STAR
                             EXACT <f> can match 1 times out of 32767...
    Setting an EVAL scope, savestack=3
     6 <bcdef> <g__gh__>    | 22:       EXACT <g>
     7 <bcdefg> <__gh__>    | 24:       CLOSE1
     7 <bcdefg> <__gh__>    | 26:       WHILEM
                                    1 out of 1..32767  cc=effff31c
    Setting an EVAL scope, savestack=12
     7 <bcdefg> <__gh__>    | 15:         OPEN1
     7 <bcdefg> <__gh__>    | 17:         EXACT <e>
       restoring \1 to 4(4)..7
                                    failed, try continuation...
     7 <bcdefg> <__gh__>    | 27:         NOTHING
     7 <bcdefg> <__gh__>    | 28:         EXACT <h>
                                    failed...
                                failed...

The most significant information in the output is about the particular I<node>
of the compiled RE which is currently being tested against the target string.
The format of these lines is

C<    >I<STRING-OFFSET> <I<PRE-STRING>> <I<POST-STRING>>   |I<ID>:  I<TYPE>

The I<TYPE> info is indented with respect to the backtracking level.
Other incidental information appears interspersed within.

=cut