PERLDEBUG(1)     Perl Programmers Reference Guide    PERLDEBUG(1)

NNAAMMEE
       perldebug - Perl debugging

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

TThhee PPeerrll DDeebbuuggggeerr
       "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."

         --Maurice Wilkes, 1949

       If you invoke Perl with the --dd 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 --dd 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 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.

       DDeebbuuggggeerr CCoommmmaannddss

       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 of 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 pprriinntt 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].

       T           Produce a stack backtrace.  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.

       <CR>        Repeat last n or s command.

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

       l           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.

       /pattern/   Search forwards for pattern; final / is
                   optional.

       ?pattern?   Search backwards for pattern; final ? is
                   optional.

       L           List all breakpoints and actions.

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

       t           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 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 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
                    42

       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 iiff:

                       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.

       D           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"

       A           Delete all installed actions.

       W [expr]    Add a global watch-expression.

       W           Delete all watch-expressions.

       O [opt[=val]] [op'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 !.

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

       tkRunning               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.

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

       LineInfo                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.

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

       PrintRet                affects printing of return value
                               after r command.

       ornaments               affects screen appearance of the
                               command line (see the
                               Term::ReadLine manpage).

       frame                   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
                               inter(di)spersed 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. If 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:

       maxTraceLen             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.

       globPrint               Whether to print contents of
                               globs.

       DumpDBFiles             Dump arrays holding debugged
                               files.

       DumpPackages            Dump symbol tables of packages.

       DumpReused              Dump contents of "reused"
                               addresses.

       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.

       UsageOnly               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"!)

       TTY                     The TTY to use for debugging I/O.

       noTTY                   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.

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

       NonStop                 If set, debugger goes into
                               noninteractive 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 noninteractively,
                               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 the section on Debugger
                               Internals below for more details.

       < [ 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.

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

       > 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.

       >> 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.

       R           Restart the debugger by eexxeeccing 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: --ww, --II, and --ee.

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

       ||dbcmd     Same as |dbcmd but DB::OUT is temporarily
                   sseelleecctted 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.

       command     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.

       DDeebbuuggggeerr iinnppuutt//oouuttppuutt

       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 ccsshh-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.

       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.

       Stack backtrace
               Here's an example of what a stack backtrace via 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
               main::infested when you ran the stack dump, and
               that it was called in a scalar context from line
               10 of the file Ambulation.pm, 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) frame.

       Listing 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             }
                 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 :,
               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=n N" perl -d -V

               for different values of n:

       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

       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

       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

       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

       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

       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: ''

                   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, 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.

       DDeebbuuggggiinngg ccoommppiillee--ttiimmee ssttaatteemmeennttss

       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/Carp.pm
         Will stop on load of `f:/perllib/lib/Carp.pm'.

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

       DDeebbuuggggeerr CCuussttoommiizzaattiioonn

       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 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 perl5db.pl 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 "myperl5db.pl" }

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

       RReeaaddlliinnee SSuuppppoorrtt

       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.

       EEddiittoorr SSuuppppoorrtt ffoorr DDeebbuuggggiinngg

       If you have GNU eemmaaccss 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 eemmaaccss
       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 vvii and
       the X11 window system had also been available, but at the
       time of this writing, no debugger support for vvii currently
       exists.)

       TThhee PPeerrll PPrrooffiilleerr

       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 --dd flag.  One of the most popular
       alternative debuggers for Perl is DDPPrrooff, the Perl
       profiler.   As of this writing, DDPPrrooff 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 mycode.pl, just
       type:

           perl -d:DProf mycode.pl

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

       DDeebbuuggggeerr ssuuppppoorrtt iinn ppeerrll

       When you call the ccaalllleerr function (see the caller entry in
       the perlfunc manpage) 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 --dd option, the following additional
       features are enabled (cf. the section on $^P in the
       perlvar manpage):

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

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

       o    The hash %{"_<$filename"} contains breakpoints and
            action (it is keyed by line number), and individual
            entries are settable (as opposed to the whole hash).
            Only true/false is important to Perl, though the
            values used by perl5db.pl have the form
            "$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
            evaled strings looks like (eval 34).

       o    The scalar ${"_<$filename"} contains "_<$filename".
            Same for evaluated strings which contain subroutines,
            or which are currently executed.  The $filename for
            evaled strings looks like (eval 34).

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

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

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

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

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

       Note that if &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 $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 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.

       DDeebbuuggggeerr IInntteerrnnaallss

       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
       @{"::_<current_file"} %{"::_<current_file"}.  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 the section on Debugger Customization for description
       of DB::parse_options(string).  The function
       DB::dump_trace(skip[, count]) skips the specified number
       of frames, and returns a list 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(FH, skip[, count[, short]])
       prints formatted info about caller frames.  The last two
       functions may be convenient as arguments to <, <<
       commands.

       OOtthheerr rreessoouurrcceess

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

       BBUUGGSS

       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 sshhiifftt or ppoopp, the stack backtrace will not show the
       original values.

DDeebbuuggggiinngg PPeerrll mmeemmoorryy uussaaggee
       Perl is 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 --DDLL switch.  First one
       is available only if perl is compiled with Perl's
       malloc(), the second one only if Perl compiled with
       -DDEBUGGING (as with giving -D optimise=-g option to
       Configure).

       UUssiinngg $$EENNVV{{PPEERRLL_<i>_DDEEBBUUGG_<i>_MMSSTTAATTSS}}

       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
       $ENV{PERL_DEBUG_MSTATS} > 1), and before termination of
       the script (if $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:

       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 APPROX field above.

       Free/Used
            The following 1 or 2 rows of numbers correspond to
            the number of buckets of each size between SMALLEST
            and 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-DEBUGGING perl the buckets starting from
            128-long ones have 4-byte overhead, thus 8192-long
            bucket may take up to 8188-byte-long allocations.

       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.

       pad: 0
            The amount of sbrk()ed memory needed to keep buckets
            aligned.

       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.

       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.

       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.

       EExxaammppllee ooff uussiinngg --DDLL 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 ;

       NNoottee:: 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 warn('!...') statements around
       the call:

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

       and run it with --DDLL 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 Ids 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 9 03 means grep the perl source for 903.  You
       will see that it is 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().

       NNoottee:: 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(), continue this
       point is reached, 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 903 allocations than 146 similar lines of
       lib/auto/POSIX/autosplit.ix).  Note also that Perl_
       prefixes are added by macroization code in perl header
       files to avoid conflicts with external libraries.

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

       Here are explanations for other Ids above:

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

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

       002  Creates C array corresponding to the 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 SV allocations).

       054  creates a HEK for the name of the glob for the
            subroutine (this name is a key in a stash).

            Big allocations with this Id correspond to
            allocations of new arenas to keep HE.

       602  creates a GP for the glob for the subroutine.

       702  creates the MAGIC for the glob for the subroutine.

       704  creates arenas which keep SVs.

       --DDLL details

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

       If warn() string starts with

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

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

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

       LLiimmiittaattiioonnss ooff --DDLL statistic

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

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

       If your perl is compiled with -DDEBUGGING, you may use the
       --DDrr flag on the command line.

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

       CCoommppiillee--ttiimmee oouuttppuutt

       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 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 de at the
       offset 1, and substring 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 gh before checking for the substring de.
       The optimizer may also use the knowledge that the match
       starts (at the first 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

       anchored STRING at POS

       floating STRING at POS1..POS2
            see above;

       matching floating/anchored
            which substring to check first;

       minlen
            the minimal length of the match;

       stclass TYPE
            The type of the first matching node.

       noscan
            which advises to not scan for the found substrings;

       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);

       GPOS if the pattern contains \G;

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

       implicit
            if the pattern starts with .*;

       with eval
            if the pattern contain eval-groups (see the section
            on (?{ code }) in the perlre manpage);

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

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

       The optimizer-specific info is used to avoid entering (a
       slow) RE engine on strings which will definitely not
       match.  If 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 nodes of the
       compiled form of the RE.  Each line has format

          id: TYPE OPTIONAL-INFO (next-id)

       TTyyppeess ooff nnooddeess

       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.

       RRuunn--ttiimmee oouuttppuutt

       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 node of the compiled RE which is currently
       being tested against the target string.  The format of
       these lines is

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

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

28/Mar/1999            perl 5.005, patch 03                     1