3 perldebug - Perl debugging
7 First of all, have you tried using the B<-w> switch?
9 =head1 The Perl Debugger
11 If you invoke Perl with the B<-d> switch, your script runs under the
12 Perl source debugger. This works like an interactive Perl
13 environment, prompting for debugger commands that let you examine
14 source code, set breakpoints, get stack backtraces, change the values of
15 variables, etc. This is so convenient that you often fire up
16 the debugger all by itself just to test out Perl constructs
17 interactively to see what they do. For example:
21 In Perl, the debugger is not a separate program the way it usually is in the
22 typical compiled environment. Instead, the B<-d> flag tells the compiler
23 to insert source information into the parse trees it's about to hand off
24 to the interpreter. That means your code must first compile correctly
25 for the debugger to work on it. Then when the interpreter starts up, it
26 preloads a special Perl library file containing the debugger.
28 The program will halt I<right before> the first run-time executable
29 statement (but see below regarding compile-time statements) and ask you
30 to enter a debugger command. Contrary to popular expectations, whenever
31 the debugger halts and shows you a line of code, it always displays the
32 line it's I<about> to execute, rather than the one it has just executed.
34 Any command not recognized by the debugger is directly executed
35 (C<eval>'d) as Perl code in the current package. (The debugger
36 uses the DB package for keeping its own state information.)
38 For any text entered at the debugger prompt, leading and trailing whitespace
39 is first stripped before further processing. If a debugger command
40 coincides with some function in your own program, merely precede the
41 function with something that doesn't look like a debugger command, such
42 as a leading C<;> or perhaps a C<+>, or by wrapping it with parentheses
45 =head2 Debugger Commands
47 The debugger understands the following commands:
53 Prints out a help message.
55 If you supply another debugger command as an argument to the C<h> command,
56 it prints out the description for just that command. The special
57 argument of C<h h> produces a more compact help listing, designed to fit
58 together on one screen.
60 If the output of the C<h> command (or any command, for that matter) scrolls
61 past your screen, precede the command with a leading pipe symbol so
62 that it's run through your pager, as in
66 You may change the pager which is used via C<O pager=...> command.
70 Same as C<print {$DB::OUT} expr> in the current package. In particular,
71 because this is just Perl's own B<print> function, this means that nested
72 data structures and objects are not dumped, unlike with the C<x> command.
74 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
75 where STDOUT may be redirected to.
79 Evaluates its expression in list context and dumps out the result
80 in a pretty-printed fashion. Nested data structures are printed out
81 recursively, unlike the real C<print> function in Perl.
82 See L<Dumpvalue> if you'd like to do this yourself.
84 The output format is governed by multiple options described under
89 Display all (or some) variables in package (defaulting to C<main>)
90 using a data pretty-printer (hashes show their keys and values so
91 you see what's what, control characters are made printable, etc.).
92 Make sure you don't put the type specifier (like C<$>) there, just
93 the symbol names, like this:
97 Use C<~pattern> and C<!pattern> for positive and negative regexes.
99 This is similar to calling the C<x> command on each applicable var.
103 Same as C<V currentpackage [vars]>.
107 Produce a stack backtrace. See below for details on its output.
111 Single step. Executes until the beginning of another
112 statement, descending into subroutine calls. If an expression is
113 supplied that includes function calls, it too will be single-stepped.
117 Next. Executes over subroutine calls, until the beginning
118 of the next statement. If an expression is supplied that includes
119 function calls, those functions will be executed with stops before
124 Continue until the return from the current subroutine.
125 Dump the return value if the C<PrintRet> option is set (default).
129 Repeat last C<n> or C<s> command.
133 Continue, optionally inserting a one-time-only breakpoint
134 at the specified line or subroutine.
138 List next window of lines.
142 List C<incr+1> lines starting at C<min>.
146 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
154 List first window of lines from subroutine. I<subname> may
155 be a variable that contains a code reference.
159 List previous window of lines.
163 List window (a few lines) around the current line.
167 Return the internal debugger pointer to the line last
168 executed, and print out that line.
172 Switch to viewing a different file or C<eval> statement. If I<filename>
173 is not a full pathname found in the values of %INC, it is considered
176 C<eval>ed strings (when accessible) are considered to be filenames:
177 C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
178 (in the order of execution). The bodies of the currently executed C<eval>
179 and of C<eval>ed strings that define subroutines are saved and thus
184 Search forwards for pattern (a Perl regex); final / is optional.
188 Search backwards for pattern; final ? is optional.
192 List all breakpoints and actions.
196 List subroutine names [not] matching the regex.
200 Toggle trace mode (see also the C<AutoTrace> option).
204 Trace through execution of C<expr>.
205 See L<perldebguts/"Frame Listing Output Examples"> for examples.
207 =item b [line] [condition]
209 Set a breakpoint before the given line. If I<line> is omitted, set a
210 breakpoint on the line about to be executed. If a condition
211 is specified, it's evaluated each time the statement is reached: a
212 breakpoint is taken only if the condition is true. Breakpoints may
213 only be set on lines that begin an executable statement. Conditions
217 b 237 ++$count237 < 11
220 =item b subname [condition]
222 Set a breakpoint before the first line of the named subroutine. I<subname> may
223 be a variable containing a code reference (in this case I<condition>
226 =item b postpone subname [condition]
228 Set a breakpoint at first line of subroutine after it is compiled.
230 =item b load filename
232 Set a breakpoint before the first executed line of the I<filename>,
233 which should be a full pathname found amongst the %INC values.
235 =item b compile subname
237 Sets a breakpoint before the first statement executed after the specified
238 subroutine is compiled.
242 Delete a breakpoint from the specified I<line>. If I<line> is omitted, deletes
243 the breakpoint from the line about to be executed.
247 Delete all installed breakpoints.
249 =item a [line] command
251 Set an action to be done before the line is executed. If I<line> is
252 omitted, set an action on the line about to be executed.
253 The sequence of steps taken by the debugger is
255 1. check for a breakpoint at this line
256 2. print the line if necessary (tracing)
257 3. do any actions associated with that line
258 4. prompt user if at a breakpoint or in single-step
261 For example, this will print out $foo every time line
264 a 53 print "DB FOUND $foo\n"
268 Delete an action from the specified line. If I<line> is omitted, delete
269 the action on the line that is about to be executed.
273 Delete all installed actions.
277 Add a global watch-expression. We hope you know what one of these
278 is, because they're supposed to be obvious. B<WARNING>: It is far
279 too easy to destroy your watch expressions by accidentally omitting
284 Delete all watch-expressions.
286 =item O booloption ...
288 Set each listed Boolean option to the value C<1>.
290 =item O anyoption? ...
292 Print out the value of one or more options.
294 =item O option=value ...
296 Set the value of one or more options. If the value has internal
297 whitespace, it should be quoted. For example, you could set C<O
298 pager="less -MQeicsNfr"> to call B<less> with those specific options.
299 You may use either single or double quotes, but if you do, you must
300 escape any embedded instances of same sort of quote you began with,
301 as well as any escaping any escapes that immediately precede that
302 quote but which are not meant to escape the quote itself. In other
303 words, you follow single-quoting rules irrespective of the quote;
304 eg: C<O option='this isn\'t bad'> or C<O option="She said, \"Isn't
307 For historical reasons, the C<=value> is optional, but defaults to
308 1 only where it is safe to do so--that is, mostly for Boolean
309 options. It is always better to assign a specific value using C<=>.
310 The C<option> can be abbreviated, but for clarity probably should
311 not be. Several options can be set together. See L<"Options"> for
316 List out all pre-prompt Perl command actions.
320 Set an action (Perl command) to happen before every debugger prompt.
321 A multi-line command may be entered by backslashing the newlines.
322 B<WARNING> If C<command> is missing, all actions are wiped out!
326 Add an action (Perl command) to happen before every debugger prompt.
327 A multi-line command may be entered by backwhacking the newlines.
331 List out post-prompt Perl command actions.
335 Set an action (Perl command) to happen after the prompt when you've
336 just given a command to return to executing the script. A multi-line
337 command may be entered by backslashing the newlines (we bet you
338 couldn't've guessed this by now). B<WARNING> If C<command> is
339 missing, all actions are wiped out!
343 Adds an action (Perl command) to happen after the prompt when you've
344 just given a command to return to executing the script. A multi-line
345 command may be entered by slackbashing the newlines.
349 List out pre-prompt debugger commands.
353 Set an action (debugger command) to happen before every debugger prompt.
354 A multi-line command may be entered in the customary fashion.
355 B<WARNING> If C<command> is missing, all actions are wiped out!
357 Because this command is in some senses new, a warning is issued if
358 you appear to have accidentally entered a block instead. If that's
359 what you mean to do, write it as with C<;{ ... }> or even
364 Add an action (debugger command) to happen before every debugger prompt.
365 A multi-line command may be entered, if you can guess how: see above.
369 Redo a previous command (defaults to the previous command).
373 Redo number'th previous command.
377 Redo last command that started with pattern.
378 See C<O recallCommand>, too.
382 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
383 C<O shellBang>, also. Note that the user's current shell (well,
384 their C<$ENV{SHELL}> variable) will be used, which can interfere
385 with proper interpretation of exit status or signal and coredump
390 Display last n commands. Only commands longer than one character are
391 listed. If I<number> is omitted, list them all.
395 Quit. ("quit" doesn't work for this, unless you've made an alias)
396 This is the only supported way to exit the debugger, though typing
397 C<exit> twice might work.
399 Set the C<inhibit_exit> option to 0 if you want to be able to step
400 off the end the script. You may also need to set $finished to 0
401 if you want to step through global destruction.
405 Restart the debugger by C<exec()>ing a new session. We try to maintain
406 your history across this, but internal settings and command-line options
409 The following setting are currently preserved: history, breakpoints,
410 actions, debugger options, and the Perl command-line
411 options B<-w>, B<-I>, and B<-e>.
415 Run the debugger command, piping DB::OUT into your current pager.
419 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
421 =item = [alias value]
423 Define a command alias, like
427 or list current aliases.
431 Execute command as a Perl statement. A trailing semicolon will be
432 supplied. If the Perl statement would otherwise be confused for a
433 Perl debugger, use a leading semicolon, too.
437 List which methods may be called on the result of the evaluated
438 expression. The expression may evaluated to a reference to a
439 blessed object, or to a package name.
443 Despite its name, this calls your system's default documentation
444 viewer on the given page, or on the viewer itself if I<manpage> is
445 omitted. If that viewer is B<man>, the current C<Config> information
446 is used to invoke B<man> using the proper MANPATH or S<B<-M>
447 I<manpath>> option. Failed lookups of the form C<XXX> that match
448 known manpages of the form I<perlXXX> will be retried. This lets
449 you type C<man debug> or C<man op> from the debugger.
451 On systems traditionally bereft of a usable B<man> command, the
452 debugger invokes B<perldoc>. Occasionally this determination is
453 incorrect due to recalcitrant vendors or rather more felicitously,
454 to enterprising users. If you fall into either category, just
455 manually set the $DB::doccmd variable to whatever viewer to view
456 the Perl documentation on your system. This may be set in an rc
457 file, or through direct assignment. We're still waiting for a
458 working example of something along the lines of:
460 $DB::doccmd = 'netscape -remote http://something.here/';
464 =head2 Configurable Options
466 The debugger has numerous options settable using the C<O> command,
467 either interactively or from the environment or an rc file.
471 =item C<recallCommand>, C<ShellBang>
473 The characters used to recall command or spawn shell. By
474 default, both are set to C<!>, which is unfortunate.
478 Program to use for output of pager-piped commands (those beginning
479 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
480 Because the debugger uses your current terminal characteristics
481 for bold and underlining, if the chosen pager does not pass escape
482 sequences through unchanged, the output of some debugger commands
483 will not be readable when sent through the pager.
487 Run Tk while prompting (with ReadLine).
489 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
491 Level of verbosity. By default, the debugger prints backtraces
492 upon receiving any kind of warning (this is often annoying) and
493 fatal exceptions (this is often valuable). It will attempt to print
494 a message when uncaught INT, BUS, or SEGV signals arrive.
496 To disable this behaviour, set these values to 0. If C<dieLevel>
497 is 2, the debugger usurps your own exception handler and prints out
498 a trace of these, replacing your exceptions with its own. This may
499 be useful for some tracing purposes, but tends to hopelessly destroy
500 any program that takes its exception handling seriously.
504 Trace mode (similar to C<t> command, but can be put into
509 File or pipe to print line number info to. If it is a pipe (say,
510 C<|visual_perl_db>), then a short message is used. This is the
511 mechanism used to interact with a slave editor or visual debugger,
512 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
515 =item C<inhibit_exit>
517 If 0, allows I<stepping off> the end of the script.
521 Print return value after C<r> command if set (default).
525 Affects screen appearance of the command line (see L<Term::ReadLine>).
526 There is currently no way to disable these, which can render
527 some output illegible on some displays, or with some pagers.
528 This is considered a bug.
532 Affects the printing of messages upon entry and exit from subroutines. If
533 C<frame & 2> is false, messages are printed on entry only. (Printing
534 on exit might be useful if interspersed with other messages.)
536 If C<frame & 4>, arguments to functions are printed, plus context
537 and caller info. If C<frame & 8>, overloaded C<stringify> and
538 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
539 & 16>, the return value from the subroutine is printed.
541 The length at which the argument list is truncated is governed by the
546 Length to truncate the argument list when the C<frame> option's
551 The following options affect what happens with C<V>, C<X>, and C<x>
556 =item C<arrayDepth>, C<hashDepth>
558 Print only first N elements ('' for all).
560 =item C<compactDump>, C<veryCompact>
562 Change the style of array and hash output. If C<compactDump>, short array
563 may be printed on one line.
567 Whether to print contents of globs.
571 Dump arrays holding debugged files.
573 =item C<DumpPackages>
575 Dump symbol tables of packages.
579 Dump contents of "reused" addresses.
581 =item C<quote>, C<HighBit>, C<undefPrint>
583 Change the style of string dump. The default value for C<quote>
584 is C<auto>; one can enable double-quotish or single-quotish format
585 by setting it to C<"> or C<'>, respectively. By default, characters
586 with their high bit set are printed verbatim.
590 Rudimentary per-package memory usage dump. Calculates total
591 size of strings found in variables in the package. This does not
592 include lexicals in a module's file scope, or lost in closures.
596 During startup, options are initialized from C<$ENV{PERLDB_OPTS}>.
597 You may place the initialization options C<TTY>, C<noTTY>,
598 C<ReadLine>, and C<NonStop> there.
600 If your rc file contains:
602 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
604 then your script will run without human intervention, putting trace
605 information into the file I<db.out>. (If you interrupt it, you'd
606 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
612 The TTY to use for debugging I/O.
616 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
617 interrupted (or if control goes to the debugger via explicit setting of
618 $DB::signal or $DB::single from the Perl script), it connects to a TTY
619 specified in the C<TTY> option at startup, or to a tty found at
620 runtime using the C<Term::Rendezvous> module of your choice.
622 This module should implement a method named C<new> that returns an object
623 with two methods: C<IN> and C<OUT. These should return filehandles to use
624 for debugging input and output correspondingly. The C<new> method should
625 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
626 startup, or C<"/tmp/perldbtty$$"> otherwise. This file is not
627 inspected for proper ownership, so security hazards are theoretically
632 If false, readline support in the debugger is disabled in order
633 to debug applications that themselves use ReadLine.
637 If set, the debugger goes into non-interactive mode until interrupted, or
638 programmatically by setting $DB::signal or $DB::single.
642 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
644 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
646 That will run the script B<myprogram> without human intervention,
647 printing out the call tree with entry and exit points. Note that
648 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
649 options could be uniquely abbreviated by the first letter (modulo
650 the C<Dump*> options). It is nevertheless recommended that you
651 always spell them out in full for legibility and future compatibility.
653 Other examples include
655 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
657 which runs script non-interactively, printing info on each entry
658 into a subroutine and each executed line into the file named F<listing>.
659 (If you interrupt it, you would better reset C<LineInfo> to something
662 Other examples include (using standard shell syntax to show environment
665 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
668 which may be useful for debugging a program that uses C<Term::ReadLine>
669 itself. Do not forget to detach your shell from the TTY in the window that
670 corresponds to F</dev/ttyXX>, say, by issuing a command like
674 See L<perldebguts/"Debugger Internals"> for details.
676 =head2 Debugger input/output
682 The debugger prompt is something like
690 where that number is the command number, and which you'd use to
691 access with the built-in B<csh>-like history mechanism. For example,
692 C<!17> would repeat command number 17. The depth of the angle
693 brackets indicates the nesting depth of the debugger. You could
694 get more than one set of brackets, for example, if you'd already
695 at a breakpoint and then printed the result of a function call that
696 itself has a breakpoint, or you step into an expression via C<s/n/t
699 =item Multiline commands
701 If you want to enter a multi-line command, such as a subroutine
702 definition with several statements or a format, escape the newline
703 that would normally end the debugger command with a backslash.
707 cont: print "ok\n"; \
714 Note that this business of escaping a newline is specific to interactive
715 commands typed into the debugger.
717 =item Stack backtrace
719 Here's an example of what a stack backtrace via C<T> command might
722 $ = main::infested called from file `Ambulation.pm' line 10
723 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
724 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
726 The left-hand character up there indicates the context in which the
727 function was called, with C<$> and C<@> meaning scalar or list
728 contexts respectively, and C<.> meaning void context (which is
729 actually a sort of scalar context). The display above says
730 that you were in the function C<main::infested> when you ran the
731 stack dump, and that it was called in scalar context from line
732 10 of the file I<Ambulation.pm>, but without any arguments at all,
733 meaning it was called as C<&infested>. The next stack frame shows
734 that the function C<Ambulation::legs> was called in list context
735 from the I<camel_flea> file with four arguments. The last stack
736 frame shows that C<main::pests> was called in scalar context,
737 also from I<camel_flea>, but from line 4.
739 If you execute the C<T> command from inside an active C<use>
740 statement, the backtrace will contain both a C<require> frame and
743 =item Line Listing Format
745 This shows the sorts of output the C<l> command can produce:
749 102:b @isa{@i,$pack} = ()
750 103 if(exists $i{$prevpack} || exists $isa{$pack});
754 107==> if(exists $isa{$pack});
756 109:a if ($extra-- > 0) {
757 110: %isa = ($pack,1);
759 Breakable lines are marked with C<:>. Lines with breakpoints are
760 marked by C<b> and those with actions by C<a>. The line that's
761 about to be executed is marked by C<< ==> >>.
765 When the C<frame> option is set, the debugger would print entered (and
766 optionally exited) subroutines in different styles. See L<perldebguts>
767 for incredibly long examples of these.
771 =head2 Debugging compile-time statements
773 If you have compile-time executable statements (such as code within
774 BEGIN and CHECK blocks or C<use> statements), these will I<not> be
775 stopped by debugger, although C<require>s and INIT blocks will, and
776 compile-time statements can be traced with C<AutoTrace> option set
777 in C<PERLDB_OPTS>). From your own Perl code, however, you can
778 transfer control back to the debugger using the following statement,
779 which is harmless if the debugger is not running:
783 If you set C<$DB::single> to 2, it's equivalent to having
784 just typed the C<n> command, whereas a value of 1 means the C<s>
785 command. The C<$DB::trace> variable should be set to 1 to simulate
786 having typed the C<t> command.
788 Another way to debug compile-time code is to start the debugger, set a
789 breakpoint on the I<load> of some module:
791 DB<7> b load f:/perllib/lib/Carp.pm
792 Will stop on load of `f:/perllib/lib/Carp.pm'.
794 and then restart the debugger using the C<R> command (if possible). One can use C<b
795 compile subname> for the same purpose.
797 =head2 Debugger Customization
799 The debugger probably contains enough configuration hooks that you
800 won't ever have to modify it yourself. You may change the behaviour
801 of debugger from within the debugger using its C<O> command, from
802 the command line via the C<PERLDB_OPTS> environment variable, and
803 from customization files.
805 You can do some customization by setting up a F<.perldb> file, which
806 contains initialization code. For instance, you could make aliases
807 like these (the last one is one people expect to be there):
809 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
810 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
811 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
812 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
814 You can change options from F<.perldb> by using calls like this one;
816 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
818 The code is executed in the package C<DB>. Note that F<.perldb> is
819 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
820 subroutine C<afterinit>, that function is called after debugger
821 initialization ends. F<.perldb> may be contained in the current
822 directory, or in the home directory. Because this file is sourced
823 in by Perl and may contain arbitrary commands, for security reasons,
824 it must be owned by the superuser or the current user, and writable
825 by no one but its owner.
827 If you want to modify the debugger, copy F<perl5db.pl> from the
828 Perl library to another name and hack it to your heart's content.
829 You'll then want to set your C<PERL5DB> environment variable to say
832 BEGIN { require "myperl5db.pl" }
834 As a last resort, you could also use C<PERL5DB> to customize the debugger
835 by directly setting internal variables or calling debugger functions.
837 Note that any variables and functions that are not documented in
838 this document (or in L<perldebguts>) are considered for internal
839 use only, and as such are subject to change without notice.
841 =head2 Readline Support
843 As shipped, the only command-line history supplied is a simplistic one
844 that checks for leading exclamation points. However, if you install
845 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
846 have full editing capabilities much like GNU I<readline>(3) provides.
847 Look for these in the F<modules/by-module/Term> directory on CPAN.
848 These do not support normal B<vi> command-line editing, however.
850 A rudimentary command-line completion is also available.
851 Unfortunately, the names of lexical variables are not available for
854 =head2 Editor Support for Debugging
856 If you have the FSF's version of B<emacs> installed on your system,
857 it can interact with the Perl debugger to provide an integrated
858 software development environment reminiscent of its interactions
861 Perl comes with a start file for making B<emacs> act like a
862 syntax-directed editor that understands (some of) Perl's syntax.
863 Look in the I<emacs> directory of the Perl source distribution.
865 A similar setup by Tom Christiansen for interacting with any
866 vendor-shipped B<vi> and the X11 window system is also available.
867 This works similarly to the integrated multiwindow support that
868 B<emacs> provides, where the debugger drives the editor. At the
869 time of this writing, however, that tool's eventual location in the
870 Perl distribution was uncertain.
872 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
873 and windy version, for coloring of Perl keywords.
875 Note that only perl can truly parse Perl, so all such CASE tools
876 fall somewhat short of the mark, especially if you don't program
877 your Perl as a C programmer might.
879 =head2 The Perl Profiler
881 If you wish to supply an alternative debugger for Perl to run, just
882 invoke your script with a colon and a package argument given to the
883 B<-d> flag. The most popular alternative debuggers for Perl is the
884 Perl profiler. Devel::DProf is now included with the standard Perl
885 distribution. To profile your Perl program in the file F<mycode.pl>,
888 $ perl -d:DProf mycode.pl
890 When the script terminates the profiler will dump the profile
891 information to a file called F<tmon.out>. A tool like B<dprofpp>,
892 also supplied with the standard Perl distribution, can be used to
893 interpret the information in that profile.
895 =head1 Debugging regular expressions
897 C<use re 'debug'> enables you to see the gory details of how the
898 Perl regular expression engine works. In order to understand this
899 typically voluminous output, one must not only have some idea about
900 about how regular expression matching works in general, but also
901 know how Perl's regular expressions are internally compiled into
902 an automaton. These matters are explored in some detail in
903 L<perldebguts/"Debugging regular expressions">.
905 =head1 Debugging memory usage
907 Perl contains internal support for reporting its own memory usage,
908 but this is a fairly advanced concept that requires some understanding
909 of how memory allocation works.
910 See L<perldebguts/"Debugging Perl memory usage"> for the details.
914 You did try the B<-w> switch, didn't you?
927 You cannot get stack frame information or in any fashion debug functions
928 that were not compiled by Perl, such as those from C or C++ extensions.
930 If you alter your @_ arguments in a subroutine (such as with B<shift>
931 or B<pop>, the stack backtrace will not show the original values.