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 Continue until return from the current subroutine. Dump the return
132 value, if the PrintRet option is set (default).
136 Repeat last C<n> or C<s> command.
140 Continue, optionally inserting a one-time-only breakpoint
141 at the specified line or subroutine.
145 List next window of lines.
149 List C<incr+1> lines starting at C<min>.
153 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
161 List first window of lines from subroutine. I<subname> may
162 be a variable which contains a code reference.
166 List previous window of lines.
170 List window (a few lines) around the current line.
174 Return debugger pointer to the last-executed line and
179 Switch to viewing a different file or eval statement. If C<filename>
180 is not a full filename as found in values of %INC, it is considered as
183 C<eval>ed strings (when accessible) are considered to be filenames:
184 C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
185 (in the order of execution). The bodies of currently executed C<eval>
186 and of C<eval>ed strings which define subroutines are saved, thus are
187 accessible by this mechanism.
191 Search forwards for pattern; final / is optional.
195 Search backwards for pattern; final ? is optional.
199 List all breakpoints and actions.
203 List subroutine names [not] matching pattern.
207 Toggle trace mode (see also C<AutoTrace> C<O>ption).
211 Trace through execution of expr. For example:
214 Stack dump during die enabled outside of evals.
216 Loading DB routines from perl5db.pl patch level 0.94
217 Emacs support available.
219 Enter h or `h h' for help.
226 DB<3> t print foo() * bar()
227 main::((eval 172):3): print foo() + bar();
228 main::foo((eval 168):2):
229 main::bar((eval 170):2):
232 or, with the C<O>ption C<frame=2> set,
236 DB<5> t print foo() * bar()
246 =item b [line] [condition]
248 Set a breakpoint. If line is omitted, sets a breakpoint on the line
249 that is about to be executed. If a condition is specified, it's
250 evaluated each time the statement is reached and a breakpoint is taken
251 only if the condition is true. Breakpoints may be set on only lines
252 that begin an executable statement. Conditions don't use B<if>:
255 b 237 ++$count237 < 11
258 =item b subname [condition]
260 Set a breakpoint at the first line of the named subroutine. I<subname> may
261 be a variable which contains a code reference (in this case I<condition>
264 =item b postpone subname [condition]
266 Set breakpoint at first line of subroutine after it is compiled.
268 =item b load filename
270 Set breakpoint at the first executed line of the file. Filename should
271 be a full name as found in values of %INC.
273 =item b compile subname
275 Sets breakpoint at the first statement executed after the subroutine
280 Delete a breakpoint at the specified line. If line is omitted, deletes
281 the breakpoint on the line that is about to be executed.
285 Delete all installed breakpoints.
287 =item a [line] command
289 Set an action to be done before the line is executed. If line is
290 omitted, sets an action on the line that is about to be executed.
291 The sequence of steps taken by the debugger is
293 1. check for a breakpoint at this line
294 2. print the line if necessary (tracing)
295 3. do any actions associated with that line
296 4. prompt user if at a breakpoint or in single-step
299 For example, this will print out $foo every time line
302 a 53 print "DB FOUND $foo\n"
306 Delete an action at the specified line. If line is omitted, deletes
307 the action on the line that is about to be executed.
311 Delete all installed actions.
315 Add a global watch-expression.
319 Delete all watch-expressions.
321 =item O [opt[=val]] [opt"val"] [opt?]...
323 Set or query values of options. val defaults to 1. opt can
324 be abbreviated. Several options can be listed.
328 =item C<recallCommand>, C<ShellBang>
330 The characters used to recall command or spawn shell. By
331 default, these are both set to C<!>.
335 Program to use for output of pager-piped commands (those
336 beginning with a C<|> character.) By default,
337 C<$ENV{PAGER}> will be used.
341 Run Tk while prompting (with ReadLine).
343 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
345 Level of verbosity. By default the debugger is in a sane verbose mode,
346 thus it will print backtraces on all the warnings and die-messages
347 which are going to be printed out, and will print a message when
348 interesting uncaught signals arrive.
350 To disable this behaviour, set these values to 0. If C<dieLevel> is 2,
351 then the messages which will be caught by surrounding C<eval> are also
356 Trace mode (similar to C<t> command, but can be put into
361 File or pipe to print line number info to. If it is a pipe (say,
362 C<|visual_perl_db>), then a short, "emacs like" message is used.
364 =item C<inhibit_exit>
366 If 0, allows I<stepping off> the end of the script.
370 Print return value after C<r> command if set (default).
374 affects screen appearance of the command line (see L<Term::ReadLine>).
378 affects printing messages on entry and exit from subroutines. If
379 C<frame & 2> is false, messages are printed on entry only. (Printing
380 on exit may be useful if inter(di)spersed with other messages.)
382 If C<frame & 4>, arguments to functions are printed as well as the
383 context and caller info. If C<frame & 8>, overloaded C<stringify> and
384 C<tie>d C<FETCH> are enabled on the printed arguments. If C<frame &
385 16>, the return value from the subroutine is printed as well.
387 The length at which the argument list is truncated is governed by the
392 length at which the argument list is truncated when C<frame> option's
397 The following options affect what happens with C<V>, C<X>, and C<x>
402 =item C<arrayDepth>, C<hashDepth>
404 Print only first N elements ('' for all).
406 =item C<compactDump>, C<veryCompact>
408 Change style of array and hash dump. If C<compactDump>, short array
409 may be printed on one line.
413 Whether to print contents of globs.
417 Dump arrays holding debugged files.
419 =item C<DumpPackages>
421 Dump symbol tables of packages.
425 Dump contents of "reused" addresses.
427 =item C<quote>, C<HighBit>, C<undefPrint>
429 Change style of string dump. Default value of C<quote> is C<auto>, one
430 can enable either double-quotish dump, or single-quotish by setting it
431 to C<"> or C<'>. By default, characters with high bit set are printed
436 I<very> rudimentally per-package memory usage dump. Calculates total
437 size of strings in variables in the package.
441 During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
442 You can put additional initialization options C<TTY>, C<noTTY>,
443 C<ReadLine>, and C<NonStop> there.
447 &parse_options("NonStop=1 LineInfo=db.out AutoTrace");
449 The script will run without human intervention, putting trace information
450 into the file I<db.out>. (If you interrupt it, you would better reset
451 C<LineInfo> to something "interactive"!)
457 The TTY to use for debugging I/O.
461 If set, goes in C<NonStop> mode, and would not connect to a TTY. If
462 interrupt (or if control goes to debugger via explicit setting of
463 $DB::signal or $DB::single from the Perl script), connects to a TTY
464 specified by the C<TTY> option at startup, or to a TTY found at
465 runtime using C<Term::Rendezvous> module of your choice.
467 This module should implement a method C<new> which returns an object
468 with two methods: C<IN> and C<OUT>, returning two filehandles to use
469 for debugging input and output correspondingly. Method C<new> may
470 inspect an argument which is a value of C<$ENV{PERLDB_NOTTY}> at
471 startup, or is C<"/tmp/perldbtty$$"> otherwise.
475 If false, readline support in debugger is disabled, so you can debug
476 ReadLine applications.
480 If set, debugger goes into noninteractive mode until interrupted, or
481 programmatically by setting $DB::signal or $DB::single.
485 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
487 $ PERLDB_OPTS="N f=2" perl -d myprogram
489 will run the script C<myprogram> without human intervention, printing
490 out the call tree with entry and exit points. Note that C<N f=2> is
491 equivalent to C<NonStop=1 frame=2>. Note also that at the moment when
492 this documentation was written all the options to the debugger could
493 be uniquely abbreviated by the first letter (with exception of
496 Other examples may include
498 $ PERLDB_OPTS="N f A L=listing" perl -d myprogram
500 - runs script noninteractively, printing info on each entry into a
501 subroutine and each executed line into the file F<listing>. (If you
502 interrupt it, you would better reset C<LineInfo> to something
506 $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
508 may be useful for debugging a program which uses C<Term::ReadLine>
509 itself. Do not forget detach shell from the TTY in the window which
510 corresponds to F</dev/ttyc>, say, by issuing a command like
514 See L<"Debugger Internals"> below for more details.
518 Set an action (Perl command) to happen before every debugger prompt.
519 A multi-line command may be entered by backslashing the newlines. If
520 C<command> is missing, resets the list of actions.
524 Add an action (Perl command) to happen before every debugger prompt.
525 A multi-line command may be entered by backslashing the newlines.
529 Set an action (Perl command) to happen after the prompt when you've
530 just given a command to return to executing the script. A multi-line
531 command may be entered by backslashing the newlines. If C<command> is
532 missing, resets the list of actions.
536 Adds an action (Perl command) to happen after the prompt when you've
537 just given a command to return to executing the script. A multi-line
538 command may be entered by backslashing the newlines.
542 Set an action (debugger command) to happen before every debugger prompt.
543 A multi-line command may be entered by backslashing the newlines. If
544 C<command> is missing, resets the list of actions.
548 Add an action (debugger command) to happen before every debugger prompt.
549 A multi-line command may be entered by backslashing the newlines.
553 Redo a previous command (default previous command).
557 Redo number'th-to-last command.
561 Redo last command that started with pattern.
562 See C<O recallCommand>, too.
566 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT)
567 See C<O shellBang> too.
571 Display last n commands. Only commands longer than one character are
572 listed. If number is omitted, lists them all.
576 Quit. ("quit" doesn't work for this.) This is the only supported way
577 to exit the debugger, though typing C<exit> twice may do it too.
579 Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
580 off> the end the script. You may also need to set $finished to 0 at
581 some moment if you want to step through global destruction.
585 Restart the debugger by B<exec>ing a new session. It tries to maintain
586 your history across this, but internal settings and command line options
589 Currently the following setting are preserved: history, breakpoints,
590 actions, debugger C<O>ptions, and the following command line
591 options: B<-w>, B<-I>, and B<-e>.
595 Run debugger command, piping DB::OUT to current pager.
599 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
600 Often used with commands that would otherwise produce long
605 =item = [alias value]
607 Define a command alias, like
611 or list current aliases.
615 Execute command as a Perl statement. A missing semicolon will be
620 The expression is evaluated, and the methods which may be applied to
621 the result are listed.
625 The methods which may be applied to objects in the C<package> are listed.
629 =head2 Debugger input/output
635 The debugger prompt is something like
643 where that number is the command number, which you'd use to access with
644 the builtin B<csh>-like history mechanism, e.g., C<!17> would repeat
645 command number 17. The number of angle brackets indicates the depth of
646 the debugger. You could get more than one set of brackets, for example, if
647 you'd already at a breakpoint and then printed out the result of a
648 function call that itself also has a breakpoint, or you step into an
649 expression via C<s/n/t expression> command.
651 =item Multiline commands
653 If you want to enter a multi-line command, such as a subroutine
654 definition with several statements, or a format, you may escape the
655 newline that would normally end the debugger command with a backslash.
659 cont: print "ok\n"; \
666 Note that this business of escaping a newline is specific to interactive
667 commands typed into the debugger.
669 =item Stack backtrace
671 Here's an example of what a stack backtrace via C<T> command might
674 $ = main::infested called from file `Ambulation.pm' line 10
675 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
676 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
678 The left-hand character up there tells whether the function was called
679 in a scalar or list context (we bet you can tell which is which). What
680 that says is that you were in the function C<main::infested> when you ran
681 the stack dump, and that it was called in a scalar context from line 10
682 of the file I<Ambulation.pm>, but without any arguments at all, meaning
683 it was called as C<&infested>. The next stack frame shows that the
684 function C<Ambulation::legs> was called in a list context from the
685 I<camel_flea> file with four arguments. The last stack frame shows that
686 C<main::pests> was called in a scalar context, also from I<camel_flea>,
689 Note that if you execute C<T> command from inside an active C<use>
690 statement, the backtrace will contain both C<require>
691 frame and an C<eval>) frame.
695 Listing given via different flavors of C<l> command looks like this:
699 102:b @isa{@i,$pack} = ()
700 103 if(exists $i{$prevpack} || exists $isa{$pack});
704 107==> if(exists $isa{$pack});
706 109:a if ($extra-- > 0) {
707 110: %isa = ($pack,1);
709 Note that the breakable lines are marked with C<:>, lines with
710 breakpoints are marked by C<b>, with actions by C<a>, and the
711 next executed line is marked by C<< ==> >>.
715 When C<frame> option is set, debugger would print entered (and
716 optionally exited) subroutines in different styles.
718 What follows is the start of the listing of
720 env "PERLDB_OPTS=f=n N" perl -d -V
722 for different values of C<n>:
729 entering Config::BEGIN
730 Package lib/Exporter.pm.
732 Package lib/Config.pm.
733 entering Config::TIEHASH
734 entering Exporter::import
735 entering Exporter::export
736 entering Config::myconfig
737 entering Config::FETCH
738 entering Config::FETCH
739 entering Config::FETCH
740 entering Config::FETCH
745 entering Config::BEGIN
746 Package lib/Exporter.pm.
749 Package lib/Config.pm.
750 entering Config::TIEHASH
751 exited Config::TIEHASH
752 entering Exporter::import
753 entering Exporter::export
754 exited Exporter::export
755 exited Exporter::import
757 entering Config::myconfig
758 entering Config::FETCH
760 entering Config::FETCH
762 entering Config::FETCH
766 in $=main::BEGIN() from /dev/nul:0
767 in $=Config::BEGIN() from lib/Config.pm:2
768 Package lib/Exporter.pm.
770 Package lib/Config.pm.
771 in $=Config::TIEHASH('Config') from lib/Config.pm:644
772 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
773 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
774 in @=Config::myconfig() from /dev/nul:0
775 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
776 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
777 in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
778 in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
779 in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
780 in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
784 in $=main::BEGIN() from /dev/nul:0
785 in $=Config::BEGIN() from lib/Config.pm:2
786 Package lib/Exporter.pm.
788 out $=Config::BEGIN() from lib/Config.pm:0
789 Package lib/Config.pm.
790 in $=Config::TIEHASH('Config') from lib/Config.pm:644
791 out $=Config::TIEHASH('Config') from lib/Config.pm:644
792 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
793 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
794 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
795 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
796 out $=main::BEGIN() from /dev/nul:0
797 in @=Config::myconfig() from /dev/nul:0
798 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
799 out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
800 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
801 out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
802 in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
803 out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
804 in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
808 in $=main::BEGIN() from /dev/nul:0
809 in $=Config::BEGIN() from lib/Config.pm:2
810 Package lib/Exporter.pm.
812 out $=Config::BEGIN() from lib/Config.pm:0
813 Package lib/Config.pm.
814 in $=Config::TIEHASH('Config') from lib/Config.pm:644
815 out $=Config::TIEHASH('Config') from lib/Config.pm:644
816 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
817 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
818 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
819 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
820 out $=main::BEGIN() from /dev/nul:0
821 in @=Config::myconfig() from /dev/nul:0
822 in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
823 out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
824 in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
825 out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
829 in $=CODE(0x15eca4)() from /dev/null:0
830 in $=CODE(0x182528)() from lib/Config.pm:2
831 Package lib/Exporter.pm.
832 out $=CODE(0x182528)() from lib/Config.pm:0
833 scalar context return from CODE(0x182528): undef
834 Package lib/Config.pm.
835 in $=Config::TIEHASH('Config') from lib/Config.pm:628
836 out $=Config::TIEHASH('Config') from lib/Config.pm:628
837 scalar context return from Config::TIEHASH: empty hash
838 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
839 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
840 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
841 scalar context return from Exporter::export: ''
842 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
843 scalar context return from Exporter::import: ''
848 In all the cases indentation of lines shows the call tree, if bit 2 of
849 C<frame> is set, then a line is printed on exit from a subroutine as
850 well, if bit 4 is set, then the arguments are printed as well as the
851 caller info, if bit 8 is set, the arguments are printed even if they
852 are tied or references, if bit 16 is set, the return value is printed
855 When a package is compiled, a line like this
859 is printed with proper indentation.
863 =head2 Debugging compile-time statements
865 If you have any compile-time executable statements (code within a BEGIN
866 block or a C<use> statement), these will C<NOT> be stopped by debugger,
867 although C<require>s will (and compile-time statements can be traced
868 with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
869 code, however, you can
870 transfer control back to the debugger using the following statement,
871 which is harmless if the debugger is not running:
875 If you set C<$DB::single> to the value 2, it's equivalent to having
876 just typed the C<n> command, whereas a value of 1 means the C<s>
877 command. The C<$DB::trace> variable should be set to 1 to simulate
878 having typed the C<t> command.
880 Another way to debug compile-time code is to start debugger, set a
881 breakpoint on I<load> of some module thusly
883 DB<7> b load f:/perllib/lib/Carp.pm
884 Will stop on load of `f:/perllib/lib/Carp.pm'.
886 and restart debugger by C<R> command (if possible). One can use C<b
887 compile subname> for the same purpose.
889 =head2 Debugger Customization
891 Most probably you do not want to modify the debugger, it contains enough
892 hooks to satisfy most needs. You may change the behaviour of debugger
893 from the debugger itself, using C<O>ptions, from the command line via
894 C<PERLDB_OPTS> environment variable, and from I<customization files>.
896 You can do some customization by setting up a F<.perldb> file which
897 contains initialization code. For instance, you could make aliases
898 like these (the last one is one people expect to be there):
900 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
901 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
902 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
903 $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
905 One changes options from F<.perldb> file via calls like this one;
907 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
909 (the code is executed in the package C<DB>). Note that F<.perldb> is
910 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
911 subroutine C<afterinit>, it is called after all the debugger
912 initialization ends. F<.perldb> may be contained in the current
913 directory, or in the C<LOGDIR>/C<HOME> directory.
915 If you want to modify the debugger, copy F<perl5db.pl> from the Perl
916 library to another name and modify it as necessary. You'll also want
917 to set your C<PERL5DB> environment variable to say something like this:
919 BEGIN { require "myperl5db.pl" }
921 As the last resort, one can use C<PERL5DB> to customize debugger by
922 directly setting internal variables or calling debugger functions.
924 =head2 Readline Support
926 As shipped, the only command line history supplied is a simplistic one
927 that checks for leading exclamation points. However, if you install
928 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
929 have full editing capabilities much like GNU I<readline>(3) provides.
930 Look for these in the F<modules/by-module/Term> directory on CPAN.
932 A rudimentary command line completion is also available.
933 Unfortunately, the names of lexical variables are not available for
936 =head2 Editor Support for Debugging
938 If you have GNU B<emacs> installed on your system, it can interact with
939 the Perl debugger to provide an integrated software development
940 environment reminiscent of its interactions with C debuggers.
942 Perl is also delivered with a start file for making B<emacs> act like a
943 syntax-directed editor that understands (some of) Perl's syntax. Look in
944 the I<emacs> directory of the Perl source distribution.
946 (Historically, a similar setup for interacting with B<vi> and the
947 X11 window system had also been available, but at the time of this
948 writing, no debugger support for B<vi> currently exists.)
950 =head2 The Perl Profiler
952 If you wish to supply an alternative debugger for Perl to run, just
953 invoke your script with a colon and a package argument given to the B<-d>
954 flag. One of the most popular alternative debuggers for Perl is
955 B<DProf>, the Perl profiler. As of this writing, B<DProf> is not
956 included with the standard Perl distribution, but it is expected to
957 be included soon, for certain values of "soon".
959 Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming
960 it's properly installed on your system, to profile your Perl program in
961 the file F<mycode.pl>, just type:
963 perl -d:DProf mycode.pl
965 When the script terminates the profiler will dump the profile information
966 to a file called F<tmon.out>. A tool like B<dprofpp> (also supplied with
967 the Devel::DProf package) can be used to interpret the information which is
970 =head2 Debugger support in perl
972 When you call the B<caller> function (see L<perlfunc/caller>) from the
973 package DB, Perl sets the array @DB::args to contain the arguments the
974 corresponding stack frame was called with.
976 If perl is run with B<-d> option, the following additional features
977 are enabled (cf. L<perlvar/$^P>):
983 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
984 'perl5db.pl'}> if not present) before the first line of the
989 The array C<@{"_<$filename"}> is the line-by-line contents of
990 $filename for all the compiled files. Same for C<eval>ed strings which
991 contain subroutines, or which are currently executed. The $filename
992 for C<eval>ed strings looks like C<(eval 34)>.
996 The hash C<%{"_<$filename"}> contains breakpoints and action (it is
997 keyed by line number), and individual entries are settable (as opposed
998 to the whole hash). Only true/false is important to Perl, though the
999 values used by F<perl5db.pl> have the form
1000 C<"$break_condition\0$action">. Values are magical in numeric context:
1001 they are zeros if the line is not breakable.
1003 Same for evaluated strings which contain subroutines, or which are
1004 currently executed. The $filename for C<eval>ed strings looks like
1009 The scalar C<${"_<$filename"}> contains C<"_<$filename">. Same for
1010 evaluated strings which contain subroutines, or which are currently
1011 executed. The $filename for C<eval>ed strings looks like C<(eval
1016 After each C<require>d file is compiled, but before it is executed,
1017 C<DB::postponed(*{"_<$filename"})> is called (if subroutine
1018 C<DB::postponed> exists). Here the $filename is the expanded name of
1019 the C<require>d file (as found in values of %INC).
1023 After each subroutine C<subname> is compiled existence of
1024 C<$DB::postponed{subname}> is checked. If this key exists,
1025 C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
1030 A hash C<%DB::sub> is maintained, with keys being subroutine names,
1031 values having the form C<filename:startline-endline>. C<filename> has
1032 the form C<(eval 31)> for subroutines defined inside C<eval>s.
1036 When execution of the application reaches a place that can have
1037 a breakpoint, a call to C<DB::DB()> is performed if any one of
1038 variables $DB::trace, $DB::single, or $DB::signal is true. (Note that
1039 these variables are not C<local>izable.) This feature is disabled when
1040 the control is inside C<DB::DB()> or functions called from it (unless
1045 When execution of the application reaches a subroutine call, a call
1046 to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
1047 the name of the called subroutine. (Unless the subroutine is compiled
1048 in the package C<DB>.)
1052 Note that if C<&DB::sub> needs some external data to be setup for it
1053 to work, no subroutine call is possible until this is done. For the
1054 standard debugger C<$DB::deep> (how many levels of recursion deep into
1055 the debugger you can go before a mandatory break) gives an example of
1058 The minimal working debugger consists of one line
1062 which is quite handy as contents of C<PERL5DB> environment
1065 env "PERL5DB=sub DB::DB {}" perl -d your-script
1067 Another (a little bit more useful) minimal debugger can be created
1068 with the only line being
1070 sub DB::DB {print ++$i; scalar <STDIN>}
1072 This debugger would print the sequential number of encountered
1073 statement, and would wait for your C<CR> to continue.
1075 The following debugger is quite functional:
1080 sub sub {print ++$i, " $sub\n"; &$sub}
1083 It prints the sequential number of subroutine call and the name of the
1084 called subroutine. Note that C<&DB::sub> should be compiled into the
1087 =head2 Debugger Internals
1089 At the start, the debugger reads your rc file (F<./.perldb> or
1090 F<~/.perldb> under Unix), which can set important options. This file may
1091 define a subroutine C<&afterinit> to be executed after the debugger is
1094 After the rc file is read, the debugger reads environment variable
1095 PERLDB_OPTS and parses it as a rest of C<O ...> line in debugger prompt.
1097 It also maintains magical internal variables, such as C<@DB::dbline>,
1098 C<%DB::dbline>, which are aliases for C<@{"::_<current_file"}>
1099 C<%{"::_<current_file"}>. Here C<current_file> is the currently
1100 selected (with the debugger's C<f> command, or by flow of execution)
1103 Some functions are provided to simplify customization. See L<"Debugger
1104 Customization"> for description of C<DB::parse_options(string)>. The
1105 function C<DB::dump_trace(skip[, count])> skips the specified number
1106 of frames, and returns a list containing info about the caller
1107 frames (all if C<count> is missing). Each entry is a hash with keys
1108 C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
1109 eval), C<args> (C<undef> or a reference to an array), C<file>, and
1112 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
1113 formatted info about caller frames. The last two functions may be
1114 convenient as arguments to C<< < >>, C<< << >> commands.
1116 =head2 Other resources
1118 You did try the B<-w> switch, didn't you?
1122 You cannot get the stack frame information or otherwise debug functions
1123 that were not compiled by Perl, such as C or C++ extensions.
1125 If you alter your @_ arguments in a subroutine (such as with B<shift>
1126 or B<pop>, the stack backtrace will not show the original values.
1128 =head1 Debugging Perl memory usage
1130 Perl is I<very> frivolous with memory. There is a saying that to
1131 estimate memory usage of Perl, assume a reasonable algorithm of
1132 allocation, and multiply your estimates by 10. This is not absolutely
1133 true, but may give you a good grasp of what happens.
1135 Say, an integer cannot take less than 20 bytes of memory, a float
1136 cannot take less than 24 bytes, a string cannot take less than 32
1137 bytes (all these examples assume 32-bit architectures, the result are
1138 much worse on 64-bit architectures). If a variable is accessed in two
1139 of three different ways (which require an integer, a float, or a
1140 string), the memory footprint may increase by another 20 bytes. A
1141 sloppy malloc() implementation will make these numbers yet more.
1143 On the opposite end of the scale, a declaration like
1147 may take (on some versions of perl) up to 500 bytes of memory.
1149 Off-the-cuff anecdotal estimates of a code bloat give a factor around
1150 8. This means that the compiled form of reasonable (commented
1151 indented etc.) code will take approximately 8 times more than the
1152 disk space the code takes.
1154 There are two Perl-specific ways to analyze the memory usage:
1155 $ENV{PERL_DEBUG_MSTATS} and B<-DL> switch. First one is available
1156 only if perl is compiled with Perl's malloc(), the second one only if
1157 Perl compiled with C<-DDEBUGGING> (as with giving C<-D optimise=-g>
1158 option to F<Configure>).
1160 =head2 Using C<$ENV{PERL_DEBUG_MSTATS}>
1162 If your perl is using Perl's malloc(), and compiled with correct
1163 switches (this is the default), then it will print memory usage
1164 statistics after compiling your code (if C<$ENV{PERL_DEBUG_MSTATS}> >
1165 1), and before termination of the script (if
1166 C<$ENV{PERL_DEBUG_MSTATS}> >= 1). The report format is similar to one
1167 in the following example:
1169 env PERL_DEBUG_MSTATS=2 perl -e "require Carp"
1170 Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
1171 14216 free: 130 117 28 7 9 0 2 2 1 0 0
1173 60924 used: 125 137 161 55 7 8 6 16 2 0 1
1175 Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
1176 Memory allocation statistics after execution: (buckets 4(4)..8188(8192)
1177 30888 free: 245 78 85 13 6 2 1 3 2 0 1
1179 175816 used: 265 176 1112 111 26 22 11 27 2 1 1
1181 Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.
1183 It is possible to ask for such a statistic at arbitrary moment by
1184 using Devel::Peek::mstats() (module Devel::Peek is available on CPAN).
1186 Here is the explanation of different parts of the format:
1190 =item C<buckets SMALLEST(APPROX)..GREATEST(APPROX)>
1192 Perl's malloc() uses bucketed allocations. Every request is rounded
1193 up to the closest bucket size available, and a bucket of these size is
1194 taken from the pool of the buckets of this size.
1196 The above line describes limits of buckets currently in use. Each
1197 bucket has two sizes: memory footprint, and the maximal size of user
1198 data which may be put into this bucket. Say, in the above example the
1199 smallest bucket is both sizes 4. The biggest bucket has usable size
1200 8188, and the memory footprint 8192.
1202 With debugging Perl some buckets may have negative usable size. This
1203 means that these buckets cannot (and will not) be used. For greater
1204 buckets the memory footprint may be one page greater than a power of
1205 2. In such a case the corresponding power of two is printed instead
1206 in the C<APPROX> field above.
1210 The following 1 or 2 rows of numbers correspond to the number of
1211 buckets of each size between C<SMALLEST> and C<GREATEST>. In the
1212 first row the sizes (memory footprints) of buckets are powers of two
1213 (or possibly one page greater). In the second row (if present) the
1214 memory footprints of the buckets are between memory footprints of two
1217 Say, with the above example the memory footprints are (with current
1220 free: 8 16 32 64 128 256 512 1024 2048 4096 8192
1223 With non-C<DEBUGGING> perl the buckets starting from C<128>-long ones
1224 have 4-byte overhead, thus 8192-long bucket may take up to
1225 8188-byte-long allocations.
1227 =item C<Total sbrk(): SBRKed/SBRKs:CONTINUOUS>
1229 The first two fields give the total amount of memory perl sbrk()ed,
1230 and number of sbrk()s used. The third number is what perl thinks
1231 about continuity of returned chunks. As far as this number is
1232 positive, malloc() will assume that it is probable that sbrk() will
1233 provide continuous memory.
1235 The amounts sbrk()ed by external libraries is not counted.
1239 The amount of sbrk()ed memory needed to keep buckets aligned.
1241 =item C<heads: 2192>
1243 While memory overhead of bigger buckets is kept inside the bucket, for
1244 smaller buckets it is kept in separate areas. This field gives the
1245 total size of these areas.
1249 malloc() may want to subdivide a bigger bucket into smaller buckets.
1250 If only a part of the deceased-bucket is left non-subdivided, the rest
1251 is kept as an element of a linked list. This field gives the total
1252 size of these chunks.
1256 To minimize amount of sbrk()s malloc() asks for more memory. This
1257 field gives the size of the yet-unused part, which is sbrk()ed, but
1262 =head2 Example of using B<-DL> switch
1264 Below we show how to analyse memory usage by
1266 do 'lib/auto/POSIX/autosplit.ix';
1268 The file in question contains a header and 146 lines similar to
1272 B<Note:> I<the discussion below supposes 32-bit architecture. In the
1273 newer versions of perl the memory usage of the constructs discussed
1274 here is much improved, but the story discussed below is a real-life
1275 story. This story is very terse, and assumes more than cursory
1276 knowledge of Perl internals.>
1278 Here is the itemized list of Perl allocations performed during parsing
1281 !!! "after" at test.pl line 3.
1282 Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+
1283 0 02 13752 . . . . 294 . . . . . . . . . . 4
1284 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3
1285 5 05 32 . . . . . . . 1 . . . . . . . .
1286 6 02 7152 . . . . . . . . . . 149 . . . . .
1287 7 02 3600 . . . . . 150 . . . . . . . . . .
1288 7 03 64 . -1 . 1 . . 2 . . . . . . . . .
1289 7 04 7056 . . . . . . . . . . . . . . . 7
1290 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 .
1291 9 03 2078 17 249 32 . . . . 2 . . . . . . . .
1294 To see this list insert two C<warn('!...')> statements around the call:
1297 do 'lib/auto/POSIX/autosplit.ix';
1298 warn('!!! "after"');
1300 and run it with B<-DL> option. The first warn() will print memory
1301 allocation info before the parsing of the file, and will memorize the
1302 statistics at this point (we ignore what it prints). The second warn()
1303 will print increments w.r.t. this memorized statistics. This is the
1306 Different I<Id>s on the left correspond to different subsystems of
1307 perl interpreter, they are just first argument given to perl memory
1308 allocation API New(). To find what C<9 03> means C<grep> the perl
1309 source for C<903>. You will see that it is F<util.c>, function
1310 savepvn(). This function is used to store a copy of existing chunk of
1311 memory. Using C debugger, one can see that it is called either
1312 directly from gv_init(), or via sv_magic(), and gv_init() is called
1313 from gv_fetchpv() - which is called from newSUB().
1315 B<Note:> to reach this place in debugger and skip all the calls to
1316 savepvn during the compilation of the main script, set a C breakpoint
1317 in Perl_warn(), C<continue> this point is reached, I<then> set
1318 breakpoint in Perl_savepvn(). Note that you may need to skip a
1319 handful of Perl_savepvn() which do not correspond to mass production
1320 of CVs (there are more C<903> allocations than 146 similar lines of
1321 F<lib/auto/POSIX/autosplit.ix>). Note also that C<Perl_> prefixes are
1322 added by macroization code in perl header files to avoid conflicts
1323 with external libraries.
1325 Anyway, we see that C<903> ids correspond to creation of globs, twice
1326 per glob - for glob name, and glob stringification magic.
1328 Here are explanations for other I<Id>s above:
1334 is for creation of bigger C<XPV*> structures. In the above case it
1335 creates 3 C<AV> per subroutine, one for a list of lexical variable
1336 names, one for a scratchpad (which contains lexical variables and
1337 C<targets>), and one for the array of scratchpads needed for
1340 It also creates a C<GV> and a C<CV> per subroutine (all called from
1345 Creates C array corresponding to the C<AV> of scratchpads, and the
1346 scratchpad itself (the first fake entry of this scratchpad is created
1347 though the subroutine itself is not defined yet).
1349 It also creates C arrays to keep data for the stash (this is one HV,
1350 but it grows, thus there are 4 big allocations: the big chunks are not
1351 freed, but are kept as additional arenas for C<SV> allocations).
1355 creates a C<HEK> for the name of the glob for the subroutine (this
1356 name is a key in a I<stash>).
1358 Big allocations with this I<Id> correspond to allocations of new
1359 arenas to keep C<HE>.
1363 creates a C<GP> for the glob for the subroutine.
1367 creates the C<MAGIC> for the glob for the subroutine.
1371 creates I<arenas> which keep SVs.
1375 =head2 B<-DL> details
1377 If Perl is run with B<-DL> option, then warn()s which start with `!'
1378 behave specially. They print a list of I<categories> of memory
1379 allocations, and statistics of allocations of different sizes for
1382 If warn() string starts with
1388 print changed categories only, print the differences in counts of allocations;
1392 print grown categories only; print the absolute values of counts, and totals;
1396 print nonempty categories, print the absolute values of counts and totals.
1400 =head2 Limitations of B<-DL> statistic
1402 If an extension or an external library does not use Perl API to
1403 allocate memory, these allocations are not counted.
1405 =head1 Debugging regular expressions
1407 There are two ways to enable debugging output for regular expressions.
1409 If your perl is compiled with C<-DDEBUGGING>, you may use the
1410 B<-Dr> flag on the command line.
1412 Otherwise, one can C<use re 'debug'>, which has effects both at
1413 compile time, and at run time (and is I<not> lexically scoped).
1415 =head2 Compile-time output
1417 The debugging output for the compile time looks like this:
1419 compiling RE `[bc]d(ef*g)+h[ij]k$'
1423 13: CURLYX {1,32767}(27)
1437 anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
1438 stclass `ANYOF' minlen 7
1440 The first line shows the pre-compiled form of the regexp, and the
1441 second shows the size of the compiled form (in arbitrary units,
1442 usually 4-byte words) and the label I<id> of the first node which
1445 The last line (split into two lines in the above) contains the optimizer
1446 info. In the example shown, the optimizer found that the match
1447 should contain a substring C<de> at the offset 1, and substring C<gh>
1448 at some offset between 3 and infinity. Moreover, when checking for
1449 these substrings (to abandon impossible matches quickly) it will check
1450 for the substring C<gh> before checking for the substring C<de>. The
1451 optimizer may also use the knowledge that the match starts (at the
1452 C<first> I<id>) with a character class, and the match cannot be
1453 shorter than 7 chars.
1455 The fields of interest which may appear in the last line are
1459 =item C<anchored> I<STRING> C<at> I<POS>
1461 =item C<floating> I<STRING> C<at> I<POS1..POS2>
1465 =item C<matching floating/anchored>
1467 which substring to check first;
1471 the minimal length of the match;
1473 =item C<stclass> I<TYPE>
1475 The type of the first matching node.
1479 which advises to not scan for the found substrings;
1483 which says that the optimizer info is in fact all that the regular
1484 expression contains (thus one does not need to enter the RE engine at
1489 if the pattern contains C<\G>;
1493 if the pattern starts with a repeated char (as in C<x+y>);
1497 if the pattern starts with C<.*>;
1501 if the pattern contain eval-groups (see L<perlre/(?{ code })>);
1503 =item C<anchored(TYPE)>
1506 match only at a handful of places (with C<TYPE> being
1507 C<BOL>, C<MBOL>, or C<GPOS>, see the table below).
1511 If a substring is known to match at end-of-line only, it may be
1512 followed by C<$>, as in C<floating `k'$>.
1514 The optimizer-specific info is used to avoid entering (a slow) RE
1515 engine on strings which will definitely not match. If C<isall> flag
1516 is set, a call to the RE engine may be avoided even when optimizer
1517 found an appropriate place for the match.
1519 The rest of the output contains the list of I<nodes> of the compiled
1520 form of the RE. Each line has format
1522 C< >I<id>: I<TYPE> I<OPTIONAL-INFO> (I<next-id>)
1524 =head2 Types of nodes
1526 Here is the list of possible types with short descriptions:
1528 # TYPE arg-description [num-args] [longjump-len] DESCRIPTION
1531 END no End of program.
1532 SUCCEED no Return from a subroutine, basically.
1535 BOL no Match "" at beginning of line.
1536 MBOL no Same, assuming multiline.
1537 SBOL no Same, assuming singleline.
1538 EOS no Match "" at end of string.
1539 EOL no Match "" at end of line.
1540 MEOL no Same, assuming multiline.
1541 SEOL no Same, assuming singleline.
1542 BOUND no Match "" at any word boundary
1543 BOUNDL no Match "" at any word boundary
1544 NBOUND no Match "" at any word non-boundary
1545 NBOUNDL no Match "" at any word non-boundary
1546 GPOS no Matches where last m//g left off.
1548 # [Special] alternatives
1549 ANY no Match any one character (except newline).
1550 SANY no Match any one character.
1551 ANYOF sv Match character in (or not in) this class.
1552 ALNUM no Match any alphanumeric character
1553 ALNUML no Match any alphanumeric char in locale
1554 NALNUM no Match any non-alphanumeric character
1555 NALNUML no Match any non-alphanumeric char in locale
1556 SPACE no Match any whitespace character
1557 SPACEL no Match any whitespace char in locale
1558 NSPACE no Match any non-whitespace character
1559 NSPACEL no Match any non-whitespace char in locale
1560 DIGIT no Match any numeric character
1561 NDIGIT no Match any non-numeric character
1563 # BRANCH The set of branches constituting a single choice are hooked
1564 # together with their "next" pointers, since precedence prevents
1565 # anything being concatenated to any individual branch. The
1566 # "next" pointer of the last BRANCH in a choice points to the
1567 # thing following the whole choice. This is also where the
1568 # final "next" pointer of each individual branch points; each
1569 # branch starts with the operand node of a BRANCH node.
1571 BRANCH node Match this alternative, or the next...
1573 # BACK Normal "next" pointers all implicitly point forward; BACK
1574 # exists to make loop structures possible.
1576 BACK no Match "", "next" ptr points backward.
1579 EXACT sv Match this string (preceded by length).
1580 EXACTF sv Match this string, folded (prec. by length).
1581 EXACTFL sv Match this string, folded in locale (w/len).
1584 NOTHING no Match empty string.
1585 # A variant of above which delimits a group, thus stops optimizations
1586 TAIL no Match empty string. Can jump here from outside.
1588 # STAR,PLUS '?', and complex '*' and '+', are implemented as circular
1589 # BRANCH structures using BACK. Simple cases (one character
1590 # per match) are implemented with STAR and PLUS for speed
1591 # and to minimize recursive plunges.
1593 STAR node Match this (simple) thing 0 or more times.
1594 PLUS node Match this (simple) thing 1 or more times.
1596 CURLY sv 2 Match this simple thing {n,m} times.
1597 CURLYN no 2 Match next-after-this simple thing
1598 # {n,m} times, set parenths.
1599 CURLYM no 2 Match this medium-complex thing {n,m} times.
1600 CURLYX sv 2 Match this complex thing {n,m} times.
1602 # This terminator creates a loop structure for CURLYX
1603 WHILEM no Do curly processing and see if rest matches.
1605 # OPEN,CLOSE,GROUPP ...are numbered at compile time.
1606 OPEN num 1 Mark this point in input as start of #n.
1607 CLOSE num 1 Analogous to OPEN.
1609 REF num 1 Match some already matched string
1610 REFF num 1 Match already matched string, folded
1611 REFFL num 1 Match already matched string, folded in loc.
1613 # grouping assertions
1614 IFMATCH off 1 2 Succeeds if the following matches.
1615 UNLESSM off 1 2 Fails if the following matches.
1616 SUSPEND off 1 1 "Independent" sub-RE.
1617 IFTHEN off 1 1 Switch, should be preceeded by switcher .
1618 GROUPP num 1 Whether the group matched.
1620 # Support for long RE
1621 LONGJMP off 1 1 Jump far away.
1622 BRANCHJ off 1 1 BRANCH with long offset.
1625 EVAL evl 1 Execute some Perl code.
1628 MINMOD no Next operator is not greedy.
1629 LOGICAL no Next opcode should set the flag only.
1631 # This is not used yet
1632 RENUM off 1 1 Group with independently numbered parens.
1634 # This is not really a node, but an optimized away piece of a "long" node.
1635 # To simplify debugging output, we mark it as if it were a node
1636 OPTIMIZED off Placeholder for dump.
1638 =head2 Run-time output
1640 First of all, when doing a match, one may get no run-time output even
1641 if debugging is enabled. this means that the RE engine was never
1642 entered, all of the job was done by the optimizer.
1644 If RE engine was entered, the output may look like this:
1646 Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
1647 Setting an EVAL scope, savestack=3
1648 2 <ab> <cdefg__gh_> | 1: ANYOF
1649 3 <abc> <defg__gh_> | 11: EXACT <d>
1650 4 <abcd> <efg__gh_> | 13: CURLYX {1,32767}
1651 4 <abcd> <efg__gh_> | 26: WHILEM
1652 0 out of 1..32767 cc=effff31c
1653 4 <abcd> <efg__gh_> | 15: OPEN1
1654 4 <abcd> <efg__gh_> | 17: EXACT <e>
1655 5 <abcde> <fg__gh_> | 19: STAR
1656 EXACT <f> can match 1 times out of 32767...
1657 Setting an EVAL scope, savestack=3
1658 6 <bcdef> <g__gh__> | 22: EXACT <g>
1659 7 <bcdefg> <__gh__> | 24: CLOSE1
1660 7 <bcdefg> <__gh__> | 26: WHILEM
1661 1 out of 1..32767 cc=effff31c
1662 Setting an EVAL scope, savestack=12
1663 7 <bcdefg> <__gh__> | 15: OPEN1
1664 7 <bcdefg> <__gh__> | 17: EXACT <e>
1665 restoring \1 to 4(4)..7
1666 failed, try continuation...
1667 7 <bcdefg> <__gh__> | 27: NOTHING
1668 7 <bcdefg> <__gh__> | 28: EXACT <h>
1672 The most significant information in the output is about the particular I<node>
1673 of the compiled RE which is currently being tested against the target string.
1674 The format of these lines is
1676 C< >I<STRING-OFFSET> <I<PRE-STRING>> <I<POST-STRING>> |I<ID>: I<TYPE>
1678 The I<TYPE> info is indented with respect to the backtracking level.
1679 Other incidental information appears interspersed within.