3 perldebug - Perl debugging
7 First of all, have you tried using the B<-w> switch?
9 =head1 The Perl Debugger
11 "As soon as we started programming, we found to our
12 surprise that it wasn't as easy to get programs right
13 as we had thought. Debugging had to be discovered.
14 I can remember the exact instant when I realized that
15 a large part of my life from then on was going to be
16 spent in finding mistakes in my own programs."
18 I< --Maurice Wilkes, 1949>
20 If you invoke Perl with the B<-d> switch, your script runs under the
21 Perl source debugger. This works like an interactive Perl
22 environment, prompting for debugger commands that let you examine
23 source code, set breakpoints, get stack backtraces, change the values of
24 variables, etc. This is so convenient that you often fire up
25 the debugger all by itself just to test out Perl constructs
26 interactively to see what they do. For example:
30 In Perl, the debugger is not a separate program as it usually is in the
31 typical compiled environment. Instead, the B<-d> flag tells the compiler
32 to insert source information into the parse trees it's about to hand off
33 to the interpreter. That means your code must first compile correctly
34 for the debugger to work on it. Then when the interpreter starts up, it
35 preloads a Perl library file containing the debugger itself.
37 The program will halt I<right before> the first run-time executable
38 statement (but see below regarding compile-time statements) and ask you
39 to enter a debugger command. Contrary to popular expectations, whenever
40 the debugger halts and shows you a line of code, it always displays the
41 line it's I<about> to execute, rather than the one it has just executed.
43 Any command not recognized by the debugger is directly executed
44 (C<eval>'d) as Perl code in the current package. (The debugger uses the
45 DB package for its own state information.)
47 Leading white space before a command would cause the debugger to think
48 it's I<NOT> a debugger command but for Perl, so be careful not to do
51 =head2 Debugger Commands
53 The debugger understands the following commands:
59 Prints out a help message.
61 If you supply another debugger command as an argument to the C<h> command,
62 it prints out the description for just that command. The special
63 argument of C<h h> produces a more compact help listing, designed to fit
64 together on one screen.
66 If the output of the C<h> command (or any command, for that matter) scrolls
67 past your screen, either precede the command with a leading pipe symbol so
68 it's run through your pager, as in
72 You may change the pager which is used via C<O pager=...> command.
76 Same as C<print {$DB::OUT} expr> in the current package. In particular,
77 because this is just Perl's own B<print> function, this means that nested
78 data structures and objects are not dumped, unlike with the C<x> command.
80 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
81 where STDOUT may be redirected to.
85 Evaluates its expression in list context and dumps out the result
86 in a pretty-printed fashion. Nested data structures are printed out
87 recursively, unlike the C<print> function.
89 The details of printout are governed by multiple C<O>ptions.
93 Display all (or some) variables in package (defaulting to the C<main>
94 package) using a data pretty-printer (hashes show their keys and values so
95 you see what's what, control characters are made printable, etc.). Make
96 sure you don't put the type specifier (like C<$>) there, just the symbol
101 Use C<~pattern> and C<!pattern> for positive and negative regexps.
103 Nested data structures are printed out in a legible fashion, unlike
104 the C<print> function.
106 The details of printout are governed by multiple C<O>ptions.
110 Same as C<V currentpackage [vars]>.
114 Produce a stack backtrace. See below for details on its output.
118 Single step. Executes until it reaches the beginning of another
119 statement, descending into subroutine calls. If an expression is
120 supplied that includes function calls, it too will be single-stepped.
124 Next. Executes over subroutine calls, until it reaches the beginning
125 of the next statement. If an expression is supplied that includes
126 function calls, those functions will be executed with stops before
131 Repeat last C<n> or C<s> command.
135 Continue, optionally inserting a one-time-only breakpoint
136 at the specified line or subroutine.
140 List next window of lines.
144 List C<incr+1> lines starting at C<min>.
148 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
156 List first window of lines from subroutine.
160 List previous window of lines.
164 List window (a few lines) around the current line.
168 Return debugger pointer to the last-executed line and
173 Switch to viewing a different file or eval statement. If C<filename>
174 is not a full filename as found in values of %INC, it is considered as
177 C<eval>ed strings (when accessible) are considered to be filenames:
178 C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
179 (in the order of execution). The bodies of currently executed C<eval>
180 and of C<eval>ed strings which define subroutines are saved, thus are
181 accessible by this mechanism.
185 Search forwards for pattern; final / is optional.
189 Search backwards for pattern; final ? is optional.
193 List all breakpoints and actions.
197 List subroutine names [not] matching pattern.
201 Toggle trace mode (see also C<AutoTrace> C<O>ption).
205 Trace through execution of expr. For example:
208 Stack dump during die enabled outside of evals.
210 Loading DB routines from perl5db.pl patch level 0.94
211 Emacs support available.
213 Enter h or `h h' for help.
220 DB<3> t print foo() * bar()
221 main::((eval 172):3): print foo() + bar();
222 main::foo((eval 168):2):
223 main::bar((eval 170):2):
226 or, with the C<O>ption C<frame=2> set,
230 DB<5> t print foo() * bar()
240 =item b [line] [condition]
242 Set a breakpoint. If line is omitted, sets a breakpoint on the line
243 that is about to be executed. If a condition is specified, it's
244 evaluated each time the statement is reached and a breakpoint is taken
245 only if the condition is true. Breakpoints may be set on only lines
246 that begin an executable statement. Conditions don't use B<if>:
249 b 237 ++$count237 < 11
252 =item b subname [condition]
254 Set a breakpoint at the first line of the named subroutine.
256 =item b postpone subname [condition]
258 Set breakpoint at first line of subroutine after it is compiled.
260 =item b load filename
262 Set breakpoint at the first executed line of the file. Filename should
263 be a full name as found in values of %INC.
265 =item b compile subname
267 Sets breakpoint at the first statement executed after the subroutine
272 Delete a breakpoint at the specified line. If line is omitted, deletes
273 the breakpoint on the line that is about to be executed.
277 Delete all installed breakpoints.
279 =item a [line] command
281 Set an action to be done before the line is executed.
282 The sequence of steps taken by the debugger is
284 1. check for a breakpoint at this line
285 2. print the line if necessary (tracing)
286 3. do any actions associated with that line
287 4. prompt user if at a breakpoint or in single-step
290 For example, this will print out $foo every time line
293 a 53 print "DB FOUND $foo\n"
297 Delete all installed actions.
301 Add a global watch-expression.
305 Delete all watch-expressions.
307 =item O [opt[=val]] [opt"val"] [opt?]...
309 Set or query values of options. val defaults to 1. opt can
310 be abbreviated. Several options can be listed.
314 =item C<recallCommand>, C<ShellBang>
316 The characters used to recall command or spawn shell. By
317 default, these are both set to C<!>.
321 Program to use for output of pager-piped commands (those
322 beginning with a C<|> character.) By default,
323 C<$ENV{PAGER}> will be used.
327 Run Tk while prompting (with ReadLine).
329 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
331 Level of verbosity. By default the debugger is in a sane verbose mode,
332 thus it will print backtraces on all the warnings and die-messages
333 which are going to be printed out, and will print a message when
334 interesting uncaught signals arrive.
336 To disable this behaviour, set these values to 0. If C<dieLevel> is 2,
337 then the messages which will be caught by surrounding C<eval> are also
342 Trace mode (similar to C<t> command, but can be put into
347 File or pipe to print line number info to. If it is a pipe (say,
348 C<|visual_perl_db>), then a short, "emacs like" message is used.
350 =item C<inhibit_exit>
352 If 0, allows I<stepping off> the end of the script.
356 affects printing of return value after C<r> command.
360 affects screen appearance of the command line (see L<Term::ReadLine>).
364 affects printing messages on entry and exit from subroutines. If
365 C<frame & 2> is false, messages are printed on entry only. (Printing
366 on exit may be useful if inter(di)spersed with other messages.)
368 If C<frame & 4>, arguments to functions are printed as well as the
369 context and caller info. If C<frame & 8>, overloaded C<stringify> and
370 C<tie>d C<FETCH> are enabled on the printed arguments. If C<frame &
371 16>, the return value from the subroutine is printed as well.
373 The length at which the argument list is truncated is governed by the
378 length at which the argument list is truncated when C<frame> option's
383 The following options affect what happens with C<V>, C<X>, and C<x>
388 =item C<arrayDepth>, C<hashDepth>
390 Print only first N elements ('' for all).
392 =item C<compactDump>, C<veryCompact>
394 Change style of array and hash dump. If C<compactDump>, short array
395 may be printed on one line.
399 Whether to print contents of globs.
403 Dump arrays holding debugged files.
405 =item C<DumpPackages>
407 Dump symbol tables of packages.
411 Dump contents of "reused" addresses.
413 =item C<quote>, C<HighBit>, C<undefPrint>
415 Change style of string dump. Default value of C<quote> is C<auto>, one
416 can enable either double-quotish dump, or single-quotish by setting it
417 to C<"> or C<'>. By default, characters with high bit set are printed
422 I<very> rudimentally per-package memory usage dump. Calculates total
423 size of strings in variables in the package.
427 During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
428 You can put additional initialization options C<TTY>, C<noTTY>,
429 C<ReadLine>, and C<NonStop> there.
433 &parse_options("NonStop=1 LineInfo=db.out AutoTrace");
435 The script will run without human intervention, putting trace information
436 into the file I<db.out>. (If you interrupt it, you would better reset
437 C<LineInfo> to something "interactive"!)
443 The TTY to use for debugging I/O.
447 If set, goes in C<NonStop> mode, and would not connect to a TTY. If
448 interrupt (or if control goes to debugger via explicit setting of
449 $DB::signal or $DB::single from the Perl script), connects to a TTY
450 specified by the C<TTY> option at startup, or to a TTY found at
451 runtime using C<Term::Rendezvous> module of your choice.
453 This module should implement a method C<new> which returns an object
454 with two methods: C<IN> and C<OUT>, returning two filehandles to use
455 for debugging input and output correspondingly. Method C<new> may
456 inspect an argument which is a value of C<$ENV{PERLDB_NOTTY}> at
457 startup, or is C<"/tmp/perldbtty$$"> otherwise.
461 If false, readline support in debugger is disabled, so you can debug
462 ReadLine applications.
466 If set, debugger goes into noninteractive mode until interrupted, or
467 programmatically by setting $DB::signal or $DB::single.
471 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
473 $ PERLDB_OPTS="N f=2" perl -d myprogram
475 will run the script C<myprogram> without human intervention, printing
476 out the call tree with entry and exit points. Note that C<N f=2> is
477 equivalent to C<NonStop=1 frame=2>. Note also that at the moment when
478 this documentation was written all the options to the debugger could
479 be uniquely abbreviated by the first letter (with exception of
482 Other examples may include
484 $ PERLDB_OPTS="N f A L=listing" perl -d myprogram
486 - runs script noninteractively, printing info on each entry into a
487 subroutine and each executed line into the file F<listing>. (If you
488 interrupt it, you would better reset C<LineInfo> to something
492 $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
494 may be useful for debugging a program which uses C<Term::ReadLine>
495 itself. Do not forget detach shell from the TTY in the window which
496 corresponds to F</dev/ttyc>, say, by issuing a command like
500 See L<"Debugger Internals"> below for more details.
502 =item E<lt> [ command ]
504 Set an action (Perl command) to happen before every debugger prompt.
505 A multi-line command may be entered by backslashing the newlines. If
506 C<command> is missing, resets the list of actions.
508 =item E<lt>E<lt> command
510 Add an action (Perl command) to happen before every debugger prompt.
511 A multi-line command may be entered by backslashing the newlines.
515 Set an action (Perl command) to happen after the prompt when you've
516 just given a command to return to executing the script. A multi-line
517 command may be entered by backslashing the newlines. If C<command> is
518 missing, resets the list of actions.
520 =item E<gt>E<gt> command
522 Adds an action (Perl command) to happen after the prompt when you've
523 just given a command to return to executing the script. A multi-line
524 command may be entered by backslashing the newlines.
528 Set an action (debugger command) to happen before every debugger prompt.
529 A multi-line command may be entered by backslashing the newlines. If
530 C<command> is missing, resets the list of actions.
534 Add an action (debugger command) to happen before every debugger prompt.
535 A multi-line command may be entered by backslashing the newlines.
539 Redo a previous command (default previous command).
543 Redo number'th-to-last command.
547 Redo last command that started with pattern.
548 See C<O recallCommand>, too.
552 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT)
553 See C<O shellBang> too.
557 Display last n commands. Only commands longer than one character are
558 listed. If number is omitted, lists them all.
562 Quit. ("quit" doesn't work for this.) This is the only supported way
563 to exit the debugger, though typing C<exit> twice may do it too.
565 Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
566 off> the end the script. You may also need to set $finished to 0 at
567 some moment if you want to step through global destruction.
571 Restart the debugger by B<exec>ing a new session. It tries to maintain
572 your history across this, but internal settings and command line options
575 Currently the following setting are preserved: history, breakpoints,
576 actions, debugger C<O>ptions, and the following command line
577 options: B<-w>, B<-I>, and B<-e>.
581 Run debugger command, piping DB::OUT to current pager.
585 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
586 Often used with commands that would otherwise produce long
591 =item = [alias value]
593 Define a command alias, like
597 or list current aliases.
601 Execute command as a Perl statement. A missing semicolon will be
606 The expression is evaluated, and the methods which may be applied to
607 the result are listed.
611 The methods which may be applied to objects in the C<package> are listed.
615 =head2 Debugger input/output
621 The debugger prompt is something like
629 where that number is the command number, which you'd use to access with
630 the builtin B<csh>-like history mechanism, e.g., C<!17> would repeat
631 command number 17. The number of angle brackets indicates the depth of
632 the debugger. You could get more than one set of brackets, for example, if
633 you'd already at a breakpoint and then printed out the result of a
634 function call that itself also has a breakpoint, or you step into an
635 expression via C<s/n/t expression> command.
637 =item Multiline commands
639 If you want to enter a multi-line command, such as a subroutine
640 definition with several statements, or a format, you may escape the
641 newline that would normally end the debugger command with a backslash.
645 cont: print "ok\n"; \
652 Note that this business of escaping a newline is specific to interactive
653 commands typed into the debugger.
655 =item Stack backtrace
657 Here's an example of what a stack backtrace via C<T> command might
660 $ = main::infested called from file `Ambulation.pm' line 10
661 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
662 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
664 The left-hand character up there tells whether the function was called
665 in a scalar or list context (we bet you can tell which is which). What
666 that says is that you were in the function C<main::infested> when you ran
667 the stack dump, and that it was called in a scalar context from line 10
668 of the file I<Ambulation.pm>, but without any arguments at all, meaning
669 it was called as C<&infested>. The next stack frame shows that the
670 function C<Ambulation::legs> was called in a list context from the
671 I<camel_flea> file with four arguments. The last stack frame shows that
672 C<main::pests> was called in a scalar context, also from I<camel_flea>,
675 Note that if you execute C<T> command from inside an active C<use>
676 statement, the backtrace will contain both C<require>
677 frame and an C<eval>) frame.
681 Listing given via different flavors of C<l> command looks like this:
685 102:b @isa{@i,$pack} = ()
686 103 if(exists $i{$prevpack} || exists $isa{$pack});
690 107==> if(exists $isa{$pack});
692 109:a if ($extra-- > 0) {
693 110: %isa = ($pack,1);
695 Note that the breakable lines are marked with C<:>, lines with
696 breakpoints are marked by C<b>, with actions by C<a>, and the
697 next executed line is marked by C<==E<gt>>.
701 When C<frame> option is set, debugger would print entered (and
702 optionally exited) subroutines in different styles.
704 What follows is the start of the listing of
706 env "PERLDB_OPTS=f=n N" perl -d -V
708 for different values of C<n>:
715 entering Config::BEGIN
716 Package lib/Exporter.pm.
718 Package lib/Config.pm.
719 entering Config::TIEHASH
720 entering Exporter::import
721 entering Exporter::export
722 entering Config::myconfig
723 entering Config::FETCH
724 entering Config::FETCH
725 entering Config::FETCH
726 entering Config::FETCH
731 entering Config::BEGIN
732 Package lib/Exporter.pm.
735 Package lib/Config.pm.
736 entering Config::TIEHASH
737 exited Config::TIEHASH
738 entering Exporter::import
739 entering Exporter::export
740 exited Exporter::export
741 exited Exporter::import
743 entering Config::myconfig
744 entering Config::FETCH
746 entering Config::FETCH
748 entering Config::FETCH
752 in $=main::BEGIN() from /dev/nul:0
753 in $=Config::BEGIN() from lib/Config.pm:2
754 Package lib/Exporter.pm.
756 Package lib/Config.pm.
757 in $=Config::TIEHASH('Config') from lib/Config.pm:644
758 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
759 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
760 in @=Config::myconfig() from /dev/nul:0
761 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
762 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
763 in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
764 in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
765 in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
766 in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
770 in $=main::BEGIN() from /dev/nul:0
771 in $=Config::BEGIN() from lib/Config.pm:2
772 Package lib/Exporter.pm.
774 out $=Config::BEGIN() from lib/Config.pm:0
775 Package lib/Config.pm.
776 in $=Config::TIEHASH('Config') from lib/Config.pm:644
777 out $=Config::TIEHASH('Config') from lib/Config.pm:644
778 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
779 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
780 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
781 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
782 out $=main::BEGIN() from /dev/nul:0
783 in @=Config::myconfig() from /dev/nul:0
784 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
785 out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
786 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
787 out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
788 in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
789 out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
790 in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
794 in $=main::BEGIN() from /dev/nul:0
795 in $=Config::BEGIN() from lib/Config.pm:2
796 Package lib/Exporter.pm.
798 out $=Config::BEGIN() from lib/Config.pm:0
799 Package lib/Config.pm.
800 in $=Config::TIEHASH('Config') from lib/Config.pm:644
801 out $=Config::TIEHASH('Config') from lib/Config.pm:644
802 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
803 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
804 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
805 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
806 out $=main::BEGIN() from /dev/nul:0
807 in @=Config::myconfig() from /dev/nul:0
808 in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
809 out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
810 in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
811 out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
815 in $=CODE(0x15eca4)() from /dev/null:0
816 in $=CODE(0x182528)() from lib/Config.pm:2
817 Package lib/Exporter.pm.
818 out $=CODE(0x182528)() from lib/Config.pm:0
819 scalar context return from CODE(0x182528): undef
820 Package lib/Config.pm.
821 in $=Config::TIEHASH('Config') from lib/Config.pm:628
822 out $=Config::TIEHASH('Config') from lib/Config.pm:628
823 scalar context return from Config::TIEHASH: empty hash
824 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
825 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
826 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
827 scalar context return from Exporter::export: ''
828 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
829 scalar context return from Exporter::import: ''
834 In all the cases indentation of lines shows the call tree, if bit 2 of
835 C<frame> is set, then a line is printed on exit from a subroutine as
836 well, if bit 4 is set, then the arguments are printed as well as the
837 caller info, if bit 8 is set, the arguments are printed even if they
838 are tied or references, if bit 16 is set, the return value is printed
841 When a package is compiled, a line like this
845 is printed with proper indentation.
849 =head2 Debugging compile-time statements
851 If you have any compile-time executable statements (code within a BEGIN
852 block or a C<use> statement), these will C<NOT> be stopped by debugger,
853 although C<require>s will (and compile-time statements can be traced
854 with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
855 code, however, you can
856 transfer control back to the debugger using the following statement,
857 which is harmless if the debugger is not running:
861 If you set C<$DB::single> to the value 2, it's equivalent to having
862 just typed the C<n> command, whereas a value of 1 means the C<s>
863 command. The C<$DB::trace> variable should be set to 1 to simulate
864 having typed the C<t> command.
866 Another way to debug compile-time code is to start debugger, set a
867 breakpoint on I<load> of some module thusly
869 DB<7> b load f:/perllib/lib/Carp.pm
870 Will stop on load of `f:/perllib/lib/Carp.pm'.
872 and restart debugger by C<R> command (if possible). One can use C<b
873 compile subname> for the same purpose.
875 =head2 Debugger Customization
877 Most probably you do not want to modify the debugger, it contains enough
878 hooks to satisfy most needs. You may change the behaviour of debugger
879 from the debugger itself, using C<O>ptions, from the command line via
880 C<PERLDB_OPTS> environment variable, and from I<customization files>.
882 You can do some customization by setting up a F<.perldb> file which
883 contains initialization code. For instance, you could make aliases
884 like these (the last one is one people expect to be there):
886 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
887 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
888 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
889 $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
891 One changes options from F<.perldb> file via calls like this one;
893 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
895 (the code is executed in the package C<DB>). Note that F<.perldb> is
896 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
897 subroutine C<afterinit>, it is called after all the debugger
898 initialization ends. F<.perldb> may be contained in the current
899 directory, or in the C<LOGDIR>/C<HOME> directory.
901 If you want to modify the debugger, copy F<perl5db.pl> from the Perl
902 library to another name and modify it as necessary. You'll also want
903 to set your C<PERL5DB> environment variable to say something like this:
905 BEGIN { require "myperl5db.pl" }
907 As the last resort, one can use C<PERL5DB> to customize debugger by
908 directly setting internal variables or calling debugger functions.
910 =head2 Readline Support
912 As shipped, the only command line history supplied is a simplistic one
913 that checks for leading exclamation points. However, if you install
914 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
915 have full editing capabilities much like GNU I<readline>(3) provides.
916 Look for these in the F<modules/by-module/Term> directory on CPAN.
918 A rudimentary command line completion is also available.
919 Unfortunately, the names of lexical variables are not available for
922 =head2 Editor Support for Debugging
924 If you have GNU B<emacs> installed on your system, it can interact with
925 the Perl debugger to provide an integrated software development
926 environment reminiscent of its interactions with C debuggers.
928 Perl is also delivered with a start file for making B<emacs> act like a
929 syntax-directed editor that understands (some of) Perl's syntax. Look in
930 the I<emacs> directory of the Perl source distribution.
932 (Historically, a similar setup for interacting with B<vi> and the
933 X11 window system had also been available, but at the time of this
934 writing, no debugger support for B<vi> currently exists.)
936 =head2 The Perl Profiler
938 If you wish to supply an alternative debugger for Perl to run, just
939 invoke your script with a colon and a package argument given to the B<-d>
940 flag. One of the most popular alternative debuggers for Perl is
941 B<DProf>, the Perl profiler. As of this writing, B<DProf> is not
942 included with the standard Perl distribution, but it is expected to
943 be included soon, for certain values of "soon".
945 Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming
946 it's properly installed on your system, to profile your Perl program in
947 the file F<mycode.pl>, just type:
949 perl -d:DProf mycode.pl
951 When the script terminates the profiler will dump the profile information
952 to a file called F<tmon.out>. A tool like B<dprofpp> (also supplied with
953 the Devel::DProf package) can be used to interpret the information which is
956 =head2 Debugger support in perl
958 When you call the B<caller> function (see L<perlfunc/caller>) from the
959 package DB, Perl sets the array @DB::args to contain the arguments the
960 corresponding stack frame was called with.
962 If perl is run with B<-d> option, the following additional features
963 are enabled (cf. L<perlvar/$^P>):
969 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
970 'perl5db.pl'}> if not present) before the first line of the
975 The array C<@{"_E<lt>$filename"}> is the line-by-line contents of
976 $filename for all the compiled files. Same for C<eval>ed strings which
977 contain subroutines, or which are currently executed. The $filename
978 for C<eval>ed strings looks like C<(eval 34)>.
982 The hash C<%{"_E<lt>$filename"}> contains breakpoints and action (it is
983 keyed by line number), and individual entries are settable (as opposed
984 to the whole hash). Only true/false is important to Perl, though the
985 values used by F<perl5db.pl> have the form
986 C<"$break_condition\0$action">. Values are magical in numeric context:
987 they are zeros if the line is not breakable.
989 Same for evaluated strings which contain subroutines, or which are
990 currently executed. The $filename for C<eval>ed strings looks like
995 The scalar C<${"_E<lt>$filename"}> contains C<"_E<lt>$filename">. Same for
996 evaluated strings which contain subroutines, or which are currently
997 executed. The $filename for C<eval>ed strings looks like C<(eval
1002 After each C<require>d file is compiled, but before it is executed,
1003 C<DB::postponed(*{"_E<lt>$filename"})> is called (if subroutine
1004 C<DB::postponed> exists). Here the $filename is the expanded name of
1005 the C<require>d file (as found in values of %INC).
1009 After each subroutine C<subname> is compiled existence of
1010 C<$DB::postponed{subname}> is checked. If this key exists,
1011 C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
1016 A hash C<%DB::sub> is maintained, with keys being subroutine names,
1017 values having the form C<filename:startline-endline>. C<filename> has
1018 the form C<(eval 31)> for subroutines defined inside C<eval>s.
1022 When execution of the application reaches a place that can have
1023 a breakpoint, a call to C<DB::DB()> is performed if any one of
1024 variables $DB::trace, $DB::single, or $DB::signal is true. (Note that
1025 these variables are not C<local>izable.) This feature is disabled when
1026 the control is inside C<DB::DB()> or functions called from it (unless
1027 C<$^D & (1E<lt>E<lt>30)>).
1031 When execution of the application reaches a subroutine call, a call
1032 to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
1033 the name of the called subroutine. (Unless the subroutine is compiled
1034 in the package C<DB>.)
1038 Note that if C<&DB::sub> needs some external data to be setup for it
1039 to work, no subroutine call is possible until this is done. For the
1040 standard debugger C<$DB::deep> (how many levels of recursion deep into
1041 the debugger you can go before a mandatory break) gives an example of
1044 The minimal working debugger consists of one line
1048 which is quite handy as contents of C<PERL5DB> environment
1051 env "PERL5DB=sub DB::DB {}" perl -d your-script
1053 Another (a little bit more useful) minimal debugger can be created
1054 with the only line being
1056 sub DB::DB {print ++$i; scalar <STDIN>}
1058 This debugger would print the sequential number of encountered
1059 statement, and would wait for your C<CR> to continue.
1061 The following debugger is quite functional:
1066 sub sub {print ++$i, " $sub\n"; &$sub}
1069 It prints the sequential number of subroutine call and the name of the
1070 called subroutine. Note that C<&DB::sub> should be compiled into the
1073 =head2 Debugger Internals
1075 At the start, the debugger reads your rc file (F<./.perldb> or
1076 F<~/.perldb> under Unix), which can set important options. This file may
1077 define a subroutine C<&afterinit> to be executed after the debugger is
1080 After the rc file is read, the debugger reads environment variable
1081 PERLDB_OPTS and parses it as a rest of C<O ...> line in debugger prompt.
1083 It also maintains magical internal variables, such as C<@DB::dbline>,
1084 C<%DB::dbline>, which are aliases for C<@{"::_<current_file"}>
1085 C<%{"::_<current_file"}>. Here C<current_file> is the currently
1086 selected (with the debugger's C<f> command, or by flow of execution)
1089 Some functions are provided to simplify customization. See L<"Debugger
1090 Customization"> for description of C<DB::parse_options(string)>. The
1091 function C<DB::dump_trace(skip[, count])> skips the specified number
1092 of frames, and returns a list containing info about the caller
1093 frames (all if C<count> is missing). Each entry is a hash with keys
1094 C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
1095 eval), C<args> (C<undef> or a reference to an array), C<file>, and
1098 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
1099 formatted info about caller frames. The last two functions may be
1100 convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.
1102 =head2 Other resources
1104 You did try the B<-w> switch, didn't you?
1108 You cannot get the stack frame information or otherwise debug functions
1109 that were not compiled by Perl, such as C or C++ extensions.
1111 If you alter your @_ arguments in a subroutine (such as with B<shift>
1112 or B<pop>, the stack backtrace will not show the original values.
1114 =head1 Debugging Perl memory usage
1116 Perl is I<very> frivolous with memory. There is a saying that to
1117 estimate memory usage of Perl, assume a reasonable algorithm of
1118 allocation, and multiply your estimates by 10. This is not absolutely
1119 true, but may give you a good grasp of what happens.
1121 Say, an integer cannot take less than 20 bytes of memory, a float
1122 cannot take less than 24 bytes, a string cannot take less than 32
1123 bytes (all these examples assume 32-bit architectures, the result are
1124 much worse on 64-bit architectures). If a variable is accessed in two
1125 of three different ways (which require an integer, a float, or a
1126 string), the memory footprint may increase by another 20 bytes. A
1127 sloppy malloc() implementation will make these numbers yet more.
1129 On the opposite end of the scale, a declaration like
1133 may take (on some versions of perl) up to 500 bytes of memory.
1135 Off-the-cuff anecdotal estimates of a code bloat give a factor around
1136 8. This means that the compiled form of reasonable (commented
1137 indented etc.) code will take approximately 8 times more than the
1138 disk space the code takes.
1140 There are two Perl-specific ways to analyze the memory usage:
1141 $ENV{PERL_DEBUG_MSTATS} and B<-DL> switch. First one is available
1142 only if perl is compiled with Perl's malloc(), the second one only if
1143 Perl compiled with C<-DDEBUGGING> (as with giving C<-D optimise=-g>
1144 option to F<Configure>).
1146 =head2 Using C<$ENV{PERL_DEBUG_MSTATS}>
1148 If your perl is using Perl's malloc(), and compiled with correct
1149 switches (this is the default), then it will print memory usage
1150 statistics after compiling your code (if C<$ENV{PERL_DEBUG_MSTATS}> >
1151 1), and before termination of the script (if
1152 C<$ENV{PERL_DEBUG_MSTATS}> >= 1). The report format is similar to one
1153 in the following example:
1155 env PERL_DEBUG_MSTATS=2 perl -e "require Carp"
1156 Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
1157 14216 free: 130 117 28 7 9 0 2 2 1 0 0
1159 60924 used: 125 137 161 55 7 8 6 16 2 0 1
1161 Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
1162 Memory allocation statistics after execution: (buckets 4(4)..8188(8192)
1163 30888 free: 245 78 85 13 6 2 1 3 2 0 1
1165 175816 used: 265 176 1112 111 26 22 11 27 2 1 1
1167 Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.
1169 It is possible to ask for such a statistic at arbitrary moment by
1170 using Devel::Peek::mstats() (module Devel::Peek is available on CPAN).
1172 Here is the explanation of different parts of the format:
1176 =item C<buckets SMALLEST(APPROX)..GREATEST(APPROX)>
1178 Perl's malloc() uses bucketed allocations. Every request is rounded
1179 up to the closest bucket size available, and a bucket of these size is
1180 taken from the pool of the buckets of this size.
1182 The above line describes limits of buckets currently in use. Each
1183 bucket has two sizes: memory footprint, and the maximal size of user
1184 data which may be put into this bucket. Say, in the above example the
1185 smallest bucket is both sizes 4. The biggest bucket has usable size
1186 8188, and the memory footprint 8192.
1188 With debugging Perl some buckets may have negative usable size. This
1189 means that these buckets cannot (and will not) be used. For greater
1190 buckets the memory footprint may be one page greater than a power of
1191 2. In such a case the corresponding power of two is printed instead
1192 in the C<APPROX> field above.
1196 The following 1 or 2 rows of numbers correspond to the number of
1197 buckets of each size between C<SMALLEST> and C<GREATEST>. In the
1198 first row the sizes (memory footprints) of buckets are powers of two
1199 (or possibly one page greater). In the second row (if present) the
1200 memory footprints of the buckets are between memory footprints of two
1203 Say, with the above example the memory footprints are (with current
1206 free: 8 16 32 64 128 256 512 1024 2048 4096 8192
1209 With non-C<DEBUGGING> perl the buckets starting from C<128>-long ones
1210 have 4-byte overhead, thus 8192-long bucket may take up to
1211 8188-byte-long allocations.
1213 =item C<Total sbrk(): SBRKed/SBRKs:CONTINUOUS>
1215 The first two fields give the total amount of memory perl sbrk()ed,
1216 and number of sbrk()s used. The third number is what perl thinks
1217 about continuity of returned chunks. As far as this number is
1218 positive, malloc() will assume that it is probable that sbrk() will
1219 provide continuous memory.
1221 The amounts sbrk()ed by external libraries is not counted.
1225 The amount of sbrk()ed memory needed to keep buckets aligned.
1227 =item C<heads: 2192>
1229 While memory overhead of bigger buckets is kept inside the bucket, for
1230 smaller buckets it is kept in separate areas. This field gives the
1231 total size of these areas.
1235 malloc() may want to subdivide a bigger bucket into smaller buckets.
1236 If only a part of the deceased-bucket is left non-subdivided, the rest
1237 is kept as an element of a linked list. This field gives the total
1238 size of these chunks.
1242 To minimize amount of sbrk()s malloc() asks for more memory. This
1243 field gives the size of the yet-unused part, which is sbrk()ed, but
1248 =head2 Example of using B<-DL> switch
1250 Below we show how to analyse memory usage by
1252 do 'lib/auto/POSIX/autosplit.ix';
1254 The file in question contains a header and 146 lines similar to
1258 B<Note:> I<the discussion below supposes 32-bit architecture. In the
1259 newer versions of perl the memory usage of the constructs discussed
1260 here is much improved, but the story discussed below is a real-life
1261 story. This story is very terse, and assumes more than cursory
1262 knowledge of Perl internals.>
1264 Here is the itemized list of Perl allocations performed during parsing
1267 !!! "after" at test.pl line 3.
1268 Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+
1269 0 02 13752 . . . . 294 . . . . . . . . . . 4
1270 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3
1271 5 05 32 . . . . . . . 1 . . . . . . . .
1272 6 02 7152 . . . . . . . . . . 149 . . . . .
1273 7 02 3600 . . . . . 150 . . . . . . . . . .
1274 7 03 64 . -1 . 1 . . 2 . . . . . . . . .
1275 7 04 7056 . . . . . . . . . . . . . . . 7
1276 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 .
1277 9 03 2078 17 249 32 . . . . 2 . . . . . . . .
1280 To see this list insert two C<warn('!...')> statements around the call:
1283 do 'lib/auto/POSIX/autosplit.ix';
1284 warn('!!! "after"');
1286 and run it with B<-DL> option. The first warn() will print memory
1287 allocation info before the parsing of the file, and will memorize the
1288 statistics at this point (we ignore what it prints). The second warn()
1289 will print increments w.r.t. this memorized statistics. This is the
1292 Different I<Id>s on the left correspond to different subsystems of
1293 perl interpreter, they are just first argument given to perl memory
1294 allocation API New(). To find what C<9 03> means C<grep> the perl
1295 source for C<903>. You will see that it is F<util.c>, function
1296 savepvn(). This function is used to store a copy of existing chunk of
1297 memory. Using C debugger, one can see that it is called either
1298 directly from gv_init(), or via sv_magic(), and gv_init() is called
1299 from gv_fetchpv() - which is called from newSUB().
1301 B<Note:> to reach this place in debugger and skip all the calls to
1302 savepvn during the compilation of the main script, set a C breakpoint
1303 in Perl_warn(), C<continue> this point is reached, I<then> set
1304 breakpoint in Perl_savepvn(). Note that you may need to skip a
1305 handful of Perl_savepvn() which do not correspond to mass production
1306 of CVs (there are more C<903> allocations than 146 similar lines of
1307 F<lib/auto/POSIX/autosplit.ix>). Note also that C<Perl_> prefixes are
1308 added by macroization code in perl header files to avoid conflicts
1309 with external libraries.
1311 Anyway, we see that C<903> ids correspond to creation of globs, twice
1312 per glob - for glob name, and glob stringification magic.
1314 Here are explanations for other I<Id>s above:
1320 is for creation of bigger C<XPV*> structures. In the above case it
1321 creates 3 C<AV> per subroutine, one for a list of lexical variable
1322 names, one for a scratchpad (which contains lexical variables and
1323 C<targets>), and one for the array of scratchpads needed for
1326 It also creates a C<GV> and a C<CV> per subroutine (all called from
1331 Creates C array corresponding to the C<AV> of scratchpads, and the
1332 scratchpad itself (the first fake entry of this scratchpad is created
1333 though the subroutine itself is not defined yet).
1335 It also creates C arrays to keep data for the stash (this is one HV,
1336 but it grows, thus there are 4 big allocations: the big chunks are not
1337 freed, but are kept as additional arenas for C<SV> allocations).
1341 creates a C<HEK> for the name of the glob for the subroutine (this
1342 name is a key in a I<stash>).
1344 Big allocations with this I<Id> correspond to allocations of new
1345 arenas to keep C<HE>.
1349 creates a C<GP> for the glob for the subroutine.
1353 creates the C<MAGIC> for the glob for the subroutine.
1357 creates I<arenas> which keep SVs.
1361 =head2 B<-DL> details
1363 If Perl is run with B<-DL> option, then warn()s which start with `!'
1364 behave specially. They print a list of I<categories> of memory
1365 allocations, and statistics of allocations of different sizes for
1368 If warn() string starts with
1374 print changed categories only, print the differences in counts of allocations;
1378 print grown categories only; print the absolute values of counts, and totals;
1382 print nonempty categories, print the absolute values of counts and totals.
1386 =head2 Limitations of B<-DL> statistic
1388 If an extension or an external library does not use Perl API to
1389 allocate memory, these allocations are not counted.
1391 =head1 Debugging regular expressions
1393 There are two ways to enable debugging output for regular expressions.
1395 If your perl is compiled with C<-DDEBUGGING>, you may use the
1396 B<-Dr> flag on the command line.
1398 Otherwise, one can C<use re 'debug'>, which has effects both at
1399 compile time, and at run time (and is I<not> lexically scoped).
1401 =head2 Compile-time output
1403 The debugging output for the compile time looks like this:
1405 compiling RE `[bc]d(ef*g)+h[ij]k$'
1409 13: CURLYX {1,32767}(27)
1423 anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
1424 stclass `ANYOF' minlen 7
1426 The first line shows the pre-compiled form of the regexp, and the
1427 second shows the size of the compiled form (in arbitrary units,
1428 usually 4-byte words) and the label I<id> of the first node which
1431 The last line (split into two lines in the above) contains the optimizer
1432 info. In the example shown, the optimizer found that the match
1433 should contain a substring C<de> at the offset 1, and substring C<gh>
1434 at some offset between 3 and infinity. Moreover, when checking for
1435 these substrings (to abandon impossible matches quickly) it will check
1436 for the substring C<gh> before checking for the substring C<de>. The
1437 optimizer may also use the knowledge that the match starts (at the
1438 C<first> I<id>) with a character class, and the match cannot be
1439 shorter than 7 chars.
1441 The fields of interest which may appear in the last line are
1445 =item C<anchored> I<STRING> C<at> I<POS>
1447 =item C<floating> I<STRING> C<at> I<POS1..POS2>
1451 =item C<matching floating/anchored>
1453 which substring to check first;
1457 the minimal length of the match;
1459 =item C<stclass> I<TYPE>
1461 The type of the first matching node.
1465 which advises to not scan for the found substrings;
1469 which says that the optimizer info is in fact all that the regular
1470 expression contains (thus one does not need to enter the RE engine at
1475 if the pattern contains C<\G>;
1479 if the pattern starts with a repeated char (as in C<x+y>);
1483 if the pattern starts with C<.*>;
1487 if the pattern contain eval-groups (see L<perlre/(?{ code })>);
1489 =item C<anchored(TYPE)>
1492 match only at a handful of places (with C<TYPE> being
1493 C<BOL>, C<MBOL>, or C<GPOS>, see the table below).
1497 If a substring is known to match at end-of-line only, it may be
1498 followed by C<$>, as in C<floating `k'$>.
1500 The optimizer-specific info is used to avoid entering (a slow) RE
1501 engine on strings which will definitely not match. If C<isall> flag
1502 is set, a call to the RE engine may be avoided even when optimizer
1503 found an appropriate place for the match.
1505 The rest of the output contains the list of I<nodes> of the compiled
1506 form of the RE. Each line has format
1508 C< >I<id>: I<TYPE> I<OPTIONAL-INFO> (I<next-id>)
1510 =head2 Types of nodes
1512 Here is the list of possible types with short descriptions:
1514 # TYPE arg-description [num-args] [longjump-len] DESCRIPTION
1517 END no End of program.
1518 SUCCEED no Return from a subroutine, basically.
1521 BOL no Match "" at beginning of line.
1522 MBOL no Same, assuming multiline.
1523 SBOL no Same, assuming singleline.
1524 EOS no Match "" at end of string.
1525 EOL no Match "" at end of line.
1526 MEOL no Same, assuming multiline.
1527 SEOL no Same, assuming singleline.
1528 BOUND no Match "" at any word boundary
1529 BOUNDL no Match "" at any word boundary
1530 NBOUND no Match "" at any word non-boundary
1531 NBOUNDL no Match "" at any word non-boundary
1532 GPOS no Matches where last m//g left off.
1534 # [Special] alternatives
1535 ANY no Match any one character (except newline).
1536 SANY no Match any one character.
1537 ANYOF sv Match character in (or not in) this class.
1538 ALNUM no Match any alphanumeric character
1539 ALNUML no Match any alphanumeric char in locale
1540 NALNUM no Match any non-alphanumeric character
1541 NALNUML no Match any non-alphanumeric char in locale
1542 SPACE no Match any whitespace character
1543 SPACEL no Match any whitespace char in locale
1544 NSPACE no Match any non-whitespace character
1545 NSPACEL no Match any non-whitespace char in locale
1546 DIGIT no Match any numeric character
1547 NDIGIT no Match any non-numeric character
1549 # BRANCH The set of branches constituting a single choice are hooked
1550 # together with their "next" pointers, since precedence prevents
1551 # anything being concatenated to any individual branch. The
1552 # "next" pointer of the last BRANCH in a choice points to the
1553 # thing following the whole choice. This is also where the
1554 # final "next" pointer of each individual branch points; each
1555 # branch starts with the operand node of a BRANCH node.
1557 BRANCH node Match this alternative, or the next...
1559 # BACK Normal "next" pointers all implicitly point forward; BACK
1560 # exists to make loop structures possible.
1562 BACK no Match "", "next" ptr points backward.
1565 EXACT sv Match this string (preceded by length).
1566 EXACTF sv Match this string, folded (prec. by length).
1567 EXACTFL sv Match this string, folded in locale (w/len).
1570 NOTHING no Match empty string.
1571 # A variant of above which delimits a group, thus stops optimizations
1572 TAIL no Match empty string. Can jump here from outside.
1574 # STAR,PLUS '?', and complex '*' and '+', are implemented as circular
1575 # BRANCH structures using BACK. Simple cases (one character
1576 # per match) are implemented with STAR and PLUS for speed
1577 # and to minimize recursive plunges.
1579 STAR node Match this (simple) thing 0 or more times.
1580 PLUS node Match this (simple) thing 1 or more times.
1582 CURLY sv 2 Match this simple thing {n,m} times.
1583 CURLYN no 2 Match next-after-this simple thing
1584 # {n,m} times, set parenths.
1585 CURLYM no 2 Match this medium-complex thing {n,m} times.
1586 CURLYX sv 2 Match this complex thing {n,m} times.
1588 # This terminator creates a loop structure for CURLYX
1589 WHILEM no Do curly processing and see if rest matches.
1591 # OPEN,CLOSE,GROUPP ...are numbered at compile time.
1592 OPEN num 1 Mark this point in input as start of #n.
1593 CLOSE num 1 Analogous to OPEN.
1595 REF num 1 Match some already matched string
1596 REFF num 1 Match already matched string, folded
1597 REFFL num 1 Match already matched string, folded in loc.
1599 # grouping assertions
1600 IFMATCH off 1 2 Succeeds if the following matches.
1601 UNLESSM off 1 2 Fails if the following matches.
1602 SUSPEND off 1 1 "Independent" sub-RE.
1603 IFTHEN off 1 1 Switch, should be preceeded by switcher .
1604 GROUPP num 1 Whether the group matched.
1606 # Support for long RE
1607 LONGJMP off 1 1 Jump far away.
1608 BRANCHJ off 1 1 BRANCH with long offset.
1611 EVAL evl 1 Execute some Perl code.
1614 MINMOD no Next operator is not greedy.
1615 LOGICAL no Next opcode should set the flag only.
1617 # This is not used yet
1618 RENUM off 1 1 Group with independently numbered parens.
1620 # This is not really a node, but an optimized away piece of a "long" node.
1621 # To simplify debugging output, we mark it as if it were a node
1622 OPTIMIZED off Placeholder for dump.
1624 =head2 Run-time output
1626 First of all, when doing a match, one may get no run-time output even
1627 if debugging is enabled. this means that the RE engine was never
1628 entered, all of the job was done by the optimizer.
1630 If RE engine was entered, the output may look like this:
1632 Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
1633 Setting an EVAL scope, savestack=3
1634 2 <ab> <cdefg__gh_> | 1: ANYOF
1635 3 <abc> <defg__gh_> | 11: EXACT <d>
1636 4 <abcd> <efg__gh_> | 13: CURLYX {1,32767}
1637 4 <abcd> <efg__gh_> | 26: WHILEM
1638 0 out of 1..32767 cc=effff31c
1639 4 <abcd> <efg__gh_> | 15: OPEN1
1640 4 <abcd> <efg__gh_> | 17: EXACT <e>
1641 5 <abcde> <fg__gh_> | 19: STAR
1642 EXACT <f> can match 1 times out of 32767...
1643 Setting an EVAL scope, savestack=3
1644 6 <bcdef> <g__gh__> | 22: EXACT <g>
1645 7 <bcdefg> <__gh__> | 24: CLOSE1
1646 7 <bcdefg> <__gh__> | 26: WHILEM
1647 1 out of 1..32767 cc=effff31c
1648 Setting an EVAL scope, savestack=12
1649 7 <bcdefg> <__gh__> | 15: OPEN1
1650 7 <bcdefg> <__gh__> | 17: EXACT <e>
1651 restoring \1 to 4(4)..7
1652 failed, try continuation...
1653 7 <bcdefg> <__gh__> | 27: NOTHING
1654 7 <bcdefg> <__gh__> | 28: EXACT <h>
1658 The most significant information in the output is about the particular I<node>
1659 of the compiled RE which is currently being tested against the target string.
1660 The format of these lines is
1662 C< >I<STRING-OFFSET> <I<PRE-STRING>> <I<POST-STRING>> |I<ID>: I<TYPE>
1664 The I<TYPE> info is indented with respect to the backtracking level.
1665 Other incidental information appears interspersed within.