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 {$DB::OUT} expr> in the current package. In particular,
71 because this is just Perl's own C<print> function, this means that nested
72 data structures and objects are not dumped, unlike with the C<x> command.
74 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
75 where STDOUT may be redirected to.
79 Evaluates its expression in list context and dumps out the result
80 in a pretty-printed fashion. Nested data structures are printed out
81 recursively, unlike the real C<print> function in Perl.
82 See L<Dumpvalue> if you'd like to do this yourself.
84 The output format is governed by multiple options described under
85 L<"Configurable Options">.
89 Display all (or some) variables in package (defaulting to C<main>)
90 using a data pretty-printer (hashes show their keys and values so
91 you see what's what, control characters are made printable, etc.).
92 Make sure you don't put the type specifier (like C<$>) 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.
185 The search is case-insensitive by default.
189 Search backwards for pattern; final ? is optional.
190 The search is case-insensitive by default.
194 List all breakpoints and actions.
198 List subroutine names [not] matching the regex.
202 Toggle trace mode (see also the C<AutoTrace> option).
206 Trace through execution of C<expr>.
207 See L<perldebguts/"Frame Listing Output Examples"> for examples.
209 =item b [line] [condition]
211 Set a breakpoint before the given line. If I<line> is omitted, set a
212 breakpoint on the line about to be executed. If a condition
213 is specified, it's evaluated each time the statement is reached: a
214 breakpoint is taken only if the condition is true. Breakpoints may
215 only be set on lines that begin an executable statement. Conditions
219 b 237 ++$count237 < 11
222 =item b subname [condition]
224 Set a breakpoint before the first line of the named subroutine. I<subname> may
225 be a variable containing a code reference (in this case I<condition>
228 =item b postpone subname [condition]
230 Set a breakpoint at first line of subroutine after it is compiled.
232 =item b load filename
234 Set a breakpoint before the first executed line of the I<filename>,
235 which should be a full pathname found amongst the %INC values.
237 =item b compile subname
239 Sets a breakpoint before the first statement executed after the specified
240 subroutine is compiled.
244 Delete a breakpoint from the specified I<line>. If I<line> is omitted, deletes
245 the breakpoint from the line about to be executed.
249 Delete all installed breakpoints.
251 =item a [line] command
253 Set an action to be done before the line is executed. If I<line> is
254 omitted, set an action on the line about to be executed.
255 The sequence of steps taken by the debugger is
257 1. check for a breakpoint at this line
258 2. print the line if necessary (tracing)
259 3. do any actions associated with that line
260 4. prompt user if at a breakpoint or in single-step
263 For example, this will print out $foo every time line
266 a 53 print "DB FOUND $foo\n"
270 Delete an action from the specified line. If I<line> is omitted, delete
271 the action on the line that is about to be executed.
275 Delete all installed actions.
279 Add a global watch-expression. We hope you know what one of these
280 is, because they're supposed to be obvious. B<WARNING>: It is far
281 too easy to destroy your watch expressions by accidentally omitting
286 Delete all watch-expressions.
288 =item O booloption ...
290 Set each listed Boolean option to the value C<1>.
292 =item O anyoption? ...
294 Print out the value of one or more options.
296 =item O option=value ...
298 Set the value of one or more options. If the value has internal
299 whitespace, it should be quoted. For example, you could set C<O
300 pager="less -MQeicsNfr"> to call B<less> with those specific options.
301 You may use either single or double quotes, but if you do, you must
302 escape any embedded instances of same sort of quote you began with,
303 as well as any escaping any escapes that immediately precede that
304 quote but which are not meant to escape the quote itself. In other
305 words, you follow single-quoting rules irrespective of the quote;
306 eg: C<O option='this isn\'t bad'> or C<O option="She said, \"Isn't
309 For historical reasons, the C<=value> is optional, but defaults to
310 1 only where it is safe to do so--that is, mostly for Boolean
311 options. It is always better to assign a specific value using C<=>.
312 The C<option> can be abbreviated, but for clarity probably should
313 not be. Several options can be set together. See L<"Configurable Options">
318 List out all pre-prompt Perl command actions.
322 Set an action (Perl command) to happen before every debugger prompt.
323 A multi-line command may be entered by backslashing the newlines.
324 B<WARNING> If C<command> is missing, all actions are wiped out!
328 Add an action (Perl command) to happen before every debugger prompt.
329 A multi-line command may be entered by backwhacking the newlines.
333 List out post-prompt Perl command actions.
337 Set an action (Perl command) to happen after the prompt when you've
338 just given a command to return to executing the script. A multi-line
339 command may be entered by backslashing the newlines (we bet you
340 couldn't've guessed this by now). B<WARNING> If C<command> is
341 missing, all actions are wiped out!
345 Adds an action (Perl command) to happen after the prompt when you've
346 just given a command to return to executing the script. A multi-line
347 command may be entered by backslashing the newlines.
351 List out pre-prompt debugger commands.
355 Set an action (debugger command) to happen before every debugger prompt.
356 A multi-line command may be entered in the customary fashion.
357 B<WARNING> If C<command> is missing, all actions are wiped out!
359 Because this command is in some senses new, a warning is issued if
360 you appear to have accidentally entered a block instead. If that's
361 what you mean to do, write it as with C<;{ ... }> or even
366 Add an action (debugger command) to happen before every debugger prompt.
367 A multi-line command may be entered, if you can guess how: see above.
371 Redo a previous command (defaults to the previous command).
375 Redo number'th previous command.
379 Redo last command that started with pattern.
380 See C<O recallCommand>, too.
384 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
385 C<O shellBang>, also. Note that the user's current shell (well,
386 their C<$ENV{SHELL}> variable) will be used, which can interfere
387 with proper interpretation of exit status or signal and coredump
392 Read and execute debugger commands from I<file>. I<file> may itself contain
397 Display last n commands. Only commands longer than one character are
398 listed. If I<number> is omitted, list them all.
402 Quit. ("quit" doesn't work for this, unless you've made an alias)
403 This is the only supported way to exit the debugger, though typing
404 C<exit> twice might work.
406 Set the C<inhibit_exit> option to 0 if you want to be able to step
407 off the end the script. You may also need to set $finished to 0
408 if you want to step through global destruction.
412 Restart the debugger by C<exec()>ing a new session. We try to maintain
413 your history across this, but internal settings and command-line options
416 The following setting are currently preserved: history, breakpoints,
417 actions, debugger options, and the Perl command-line
418 options B<-w>, B<-I>, and B<-e>.
422 Run the debugger command, piping DB::OUT into your current pager.
426 Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
428 =item = [alias value]
430 Define a command alias, like
434 or list current aliases.
438 Execute command as a Perl statement. A trailing semicolon will be
439 supplied. If the Perl statement would otherwise be confused for a
440 Perl debugger, use a leading semicolon, too.
444 List which methods may be called on the result of the evaluated
445 expression. The expression may evaluated to a reference to a
446 blessed object, or to a package name.
450 Despite its name, this calls your system's default documentation
451 viewer on the given page, or on the viewer itself if I<manpage> is
452 omitted. If that viewer is B<man>, the current C<Config> information
453 is used to invoke B<man> using the proper MANPATH or S<B<-M>
454 I<manpath>> option. Failed lookups of the form C<XXX> that match
455 known manpages of the form I<perlXXX> will be retried. This lets
456 you type C<man debug> or C<man op> from the debugger.
458 On systems traditionally bereft of a usable B<man> command, the
459 debugger invokes B<perldoc>. Occasionally this determination is
460 incorrect due to recalcitrant vendors or rather more felicitously,
461 to enterprising users. If you fall into either category, just
462 manually set the $DB::doccmd variable to whatever viewer to view
463 the Perl documentation on your system. This may be set in an rc
464 file, or through direct assignment. We're still waiting for a
465 working example of something along the lines of:
467 $DB::doccmd = 'netscape -remote http://something.here/';
471 =head2 Configurable Options
473 The debugger has numerous options settable using the C<O> command,
474 either interactively or from the environment or an rc file.
475 (./.perldb or ~/.perldb under Unix.)
480 =item C<recallCommand>, C<ShellBang>
482 The characters used to recall command or spawn shell. By
483 default, both are set to C<!>, which is unfortunate.
487 Program to use for output of pager-piped commands (those beginning
488 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
489 Because the debugger uses your current terminal characteristics
490 for bold and underlining, if the chosen pager does not pass escape
491 sequences through unchanged, the output of some debugger commands
492 will not be readable when sent through the pager.
496 Run Tk while prompting (with ReadLine).
498 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
500 Level of verbosity. By default, the debugger leaves your exceptions
501 and warnings alone, because altering them can break correctly running
502 programs. It will attempt to print a message when uncaught INT, BUS, or
503 SEGV signals arrive. (But see the mention of signals in L<BUGS> below.)
505 To disable this default safe mode, set these values to something higher
506 than 0. At a level of 1, you get backtraces upon receiving any kind
507 of warning (this is often annoying) or exception (this is
508 often valuable). Unfortunately, the debugger cannot discern fatal
509 exceptions from non-fatal ones. If C<dieLevel> is even 1, then your
510 non-fatal exceptions are also traced and unceremoniously altered if they
511 came from C<eval'd> strings or from any kind of C<eval> within modules
512 you're attempting to load. If C<dieLevel> is 2, the debugger doesn't
513 care where they came from: It usurps your exception handler and prints
514 out a trace, then modifies all exceptions with its own embellishments.
515 This may perhaps be useful for some tracing purposes, but tends to hopelessly
516 destroy any program that takes its exception handling seriously.
520 Trace mode (similar to C<t> command, but can be put into
525 File or pipe to print line number info to. If it is a pipe (say,
526 C<|visual_perl_db>), then a short message is used. This is the
527 mechanism used to interact with a slave editor or visual debugger,
528 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
531 =item C<inhibit_exit>
533 If 0, allows I<stepping off> the end of the script.
537 Print return value after C<r> command if set (default).
541 Affects screen appearance of the command line (see L<Term::ReadLine>).
542 There is currently no way to disable these, which can render
543 some output illegible on some displays, or with some pagers.
544 This is considered a bug.
548 Affects the printing of messages upon entry and exit from subroutines. If
549 C<frame & 2> is false, messages are printed on entry only. (Printing
550 on exit might be useful if interspersed with other messages.)
552 If C<frame & 4>, arguments to functions are printed, plus context
553 and caller info. If C<frame & 8>, overloaded C<stringify> and
554 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
555 & 16>, the return value from the subroutine is printed.
557 The length at which the argument list is truncated is governed by the
562 Length to truncate the argument list when the C<frame> option's
567 Change the size of code list window (default is 10 lines).
571 The following options affect what happens with C<V>, C<X>, and C<x>
576 =item C<arrayDepth>, C<hashDepth>
578 Print only first N elements ('' for all).
580 =item C<compactDump>, C<veryCompact>
582 Change the style of array and hash output. If C<compactDump>, short array
583 may be printed on one line.
587 Whether to print contents of globs.
591 Dump arrays holding debugged files.
593 =item C<DumpPackages>
595 Dump symbol tables of packages.
599 Dump contents of "reused" addresses.
601 =item C<quote>, C<HighBit>, C<undefPrint>
603 Change the style of string dump. The default value for C<quote>
604 is C<auto>; one can enable double-quotish or single-quotish format
605 by setting it to C<"> or C<'>, respectively. By default, characters
606 with their high bit set are printed verbatim.
610 Rudimentary per-package memory usage dump. Calculates total
611 size of strings found in variables in the package. This does not
612 include lexicals in a module's file scope, or lost in closures.
616 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
617 environment variable and parses this as the remainder of a `O ...'
618 line as one might enter at the debugger prompt. You may place the
619 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
622 If your rc file contains:
624 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
626 then your script will run without human intervention, putting trace
627 information into the file I<db.out>. (If you interrupt it, you'd
628 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
634 The TTY to use for debugging I/O.
638 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
639 interrupted (or if control goes to the debugger via explicit setting of
640 $DB::signal or $DB::single from the Perl script), it connects to a TTY
641 specified in the C<TTY> option at startup, or to a tty found at
642 runtime using the C<Term::Rendezvous> module of your choice.
644 This module should implement a method named C<new> that returns an object
645 with two methods: C<IN> and C<OUT>. These should return filehandles to use
646 for debugging input and output correspondingly. The C<new> method should
647 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
648 startup, or C<"/tmp/perldbtty$$"> otherwise. This file is not
649 inspected for proper ownership, so security hazards are theoretically
654 If false, readline support in the debugger is disabled in order
655 to debug applications that themselves use ReadLine.
659 If set, the debugger goes into non-interactive mode until interrupted, or
660 programmatically by setting $DB::signal or $DB::single.
664 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
666 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
668 That will run the script B<myprogram> without human intervention,
669 printing out the call tree with entry and exit points. Note that
670 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
671 options could be uniquely abbreviated by the first letter (modulo
672 the C<Dump*> options). It is nevertheless recommended that you
673 always spell them out in full for legibility and future compatibility.
675 Other examples include
677 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
679 which runs script non-interactively, printing info on each entry
680 into a subroutine and each executed line into the file named F<listing>.
681 (If you interrupt it, you would better reset C<LineInfo> to something
684 Other examples include (using standard shell syntax to show environment
687 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
690 which may be useful for debugging a program that uses C<Term::ReadLine>
691 itself. Do not forget to detach your shell from the TTY in the window that
692 corresponds to F</dev/ttyXX>, say, by issuing a command like
696 See L<perldebguts/"Debugger Internals"> for details.
698 =head2 Debugger input/output
704 The debugger prompt is something like
712 where that number is the command number, and which you'd use to
713 access with the built-in B<csh>-like history mechanism. For example,
714 C<!17> would repeat command number 17. The depth of the angle
715 brackets indicates the nesting depth of the debugger. You could
716 get more than one set of brackets, for example, if you'd already
717 at a breakpoint and then printed the result of a function call that
718 itself has a breakpoint, or you step into an expression via C<s/n/t
721 =item Multiline commands
723 If you want to enter a multi-line command, such as a subroutine
724 definition with several statements or a format, escape the newline
725 that would normally end the debugger command with a backslash.
729 cont: print "ok\n"; \
736 Note that this business of escaping a newline is specific to interactive
737 commands typed into the debugger.
739 =item Stack backtrace
741 Here's an example of what a stack backtrace via C<T> command might
744 $ = main::infested called from file `Ambulation.pm' line 10
745 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
746 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
748 The left-hand character up there indicates the context in which the
749 function was called, with C<$> and C<@> meaning scalar or list
750 contexts respectively, and C<.> meaning void context (which is
751 actually a sort of scalar context). The display above says
752 that you were in the function C<main::infested> when you ran the
753 stack dump, and that it was called in scalar context from line
754 10 of the file I<Ambulation.pm>, but without any arguments at all,
755 meaning it was called as C<&infested>. The next stack frame shows
756 that the function C<Ambulation::legs> was called in list context
757 from the I<camel_flea> file with four arguments. The last stack
758 frame shows that C<main::pests> was called in scalar context,
759 also from I<camel_flea>, but from line 4.
761 If you execute the C<T> command from inside an active C<use>
762 statement, the backtrace will contain both a C<require> frame and
765 =item Line Listing Format
767 This shows the sorts of output the C<l> command can produce:
771 102:b @isa{@i,$pack} = ()
772 103 if(exists $i{$prevpack} || exists $isa{$pack});
776 107==> if(exists $isa{$pack});
778 109:a if ($extra-- > 0) {
779 110: %isa = ($pack,1);
781 Breakable lines are marked with C<:>. Lines with breakpoints are
782 marked by C<b> and those with actions by C<a>. The line that's
783 about to be executed is marked by C<< ==> >>.
785 Please be aware that code in debugger listings may not look the same
786 as your original source code. Line directives and external source
787 filters can alter the code before Perl sees it, causing code to move
788 from its original positions or take on entirely different forms.
792 When the C<frame> option is set, the debugger would print entered (and
793 optionally exited) subroutines in different styles. See L<perldebguts>
794 for incredibly long examples of these.
798 =head2 Debugging compile-time statements
800 If you have compile-time executable statements (such as code within
801 BEGIN and CHECK blocks or C<use> statements), these will I<not> be
802 stopped by debugger, although C<require>s and INIT blocks will, and
803 compile-time statements can be traced with C<AutoTrace> option set
804 in C<PERLDB_OPTS>). From your own Perl code, however, you can
805 transfer control back to the debugger using the following statement,
806 which is harmless if the debugger is not running:
810 If you set C<$DB::single> to 2, it's equivalent to having
811 just typed the C<n> command, whereas a value of 1 means the C<s>
812 command. The C<$DB::trace> variable should be set to 1 to simulate
813 having typed the C<t> command.
815 Another way to debug compile-time code is to start the debugger, set a
816 breakpoint on the I<load> of some module:
818 DB<7> b load f:/perllib/lib/Carp.pm
819 Will stop on load of `f:/perllib/lib/Carp.pm'.
821 and then restart the debugger using the C<R> command (if possible). One can use C<b
822 compile subname> for the same purpose.
824 =head2 Debugger Customization
826 The debugger probably contains enough configuration hooks that you
827 won't ever have to modify it yourself. You may change the behaviour
828 of debugger from within the debugger using its C<O> command, from
829 the command line via the C<PERLDB_OPTS> environment variable, and
830 from customization files.
832 You can do some customization by setting up a F<.perldb> file, which
833 contains initialization code. For instance, you could make aliases
834 like these (the last one is one people expect to be there):
836 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
837 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
838 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
839 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
841 You can change options from F<.perldb> by using calls like this one;
843 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
845 The code is executed in the package C<DB>. Note that F<.perldb> is
846 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
847 subroutine C<afterinit>, that function is called after debugger
848 initialization ends. F<.perldb> may be contained in the current
849 directory, or in the home directory. Because this file is sourced
850 in by Perl and may contain arbitrary commands, for security reasons,
851 it must be owned by the superuser or the current user, and writable
852 by no one but its owner.
854 If you want to modify the debugger, copy F<perl5db.pl> from the
855 Perl library to another name and hack it to your heart's content.
856 You'll then want to set your C<PERL5DB> environment variable to say
859 BEGIN { require "myperl5db.pl" }
861 As a last resort, you could also use C<PERL5DB> to customize the debugger
862 by directly setting internal variables or calling debugger functions.
864 Note that any variables and functions that are not documented in
865 this document (or in L<perldebguts>) are considered for internal
866 use only, and as such are subject to change without notice.
868 =head2 Readline Support
870 As shipped, the only command-line history supplied is a simplistic one
871 that checks for leading exclamation points. However, if you install
872 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
873 have full editing capabilities much like GNU I<readline>(3) provides.
874 Look for these in the F<modules/by-module/Term> directory on CPAN.
875 These do not support normal B<vi> command-line editing, however.
877 A rudimentary command-line completion is also available.
878 Unfortunately, the names of lexical variables are not available for
881 =head2 Editor Support for Debugging
883 If you have the FSF's version of B<emacs> installed on your system,
884 it can interact with the Perl debugger to provide an integrated
885 software development environment reminiscent of its interactions
888 Perl comes with a start file for making B<emacs> act like a
889 syntax-directed editor that understands (some of) Perl's syntax.
890 Look in the I<emacs> directory of the Perl source distribution.
892 A similar setup by Tom Christiansen for interacting with any
893 vendor-shipped B<vi> and the X11 window system is also available.
894 This works similarly to the integrated multiwindow support that
895 B<emacs> provides, where the debugger drives the editor. At the
896 time of this writing, however, that tool's eventual location in the
897 Perl distribution was uncertain.
899 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
900 and windy version, for coloring of Perl keywords.
902 Note that only perl can truly parse Perl, so all such CASE tools
903 fall somewhat short of the mark, especially if you don't program
904 your Perl as a C programmer might.
906 =head2 The Perl Profiler
908 If you wish to supply an alternative debugger for Perl to run, just
909 invoke your script with a colon and a package argument given to the
910 B<-d> flag. The most popular alternative debuggers for Perl is the
911 Perl profiler. Devel::DProf is now included with the standard Perl
912 distribution. To profile your Perl program in the file F<mycode.pl>,
915 $ perl -d:DProf mycode.pl
917 When the script terminates the profiler will dump the profile
918 information to a file called F<tmon.out>. A tool like B<dprofpp>,
919 also supplied with the standard Perl distribution, can be used to
920 interpret the information in that profile.
922 =head1 Debugging regular expressions
924 C<use re 'debug'> enables you to see the gory details of how the Perl
925 regular expression engine works. In order to understand this typically
926 voluminous output, one must not only have some idea about how regular
927 expression matching works in general, but also know how Perl's regular
928 expressions are internally compiled into an automaton. These matters
929 are explored in some detail in
930 L<perldebguts/"Debugging regular expressions">.
932 =head1 Debugging memory usage
934 Perl contains internal support for reporting its own memory usage,
935 but this is a fairly advanced concept that requires some understanding
936 of how memory allocation works.
937 See L<perldebguts/"Debugging Perl memory usage"> for the details.
941 You did try the B<-w> switch, didn't you?
954 You cannot get stack frame information or in any fashion debug functions
955 that were not compiled by Perl, such as those from C or C++ extensions.
957 If you alter your @_ arguments in a subroutine (such as with C<shift>
958 or C<pop>), the stack backtrace will not show the original values.
960 The debugger does not currently work in conjunction with the B<-W>
961 command-line switch, because it itself is not free of warnings.
963 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
964 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
965 handler, then you won't be able to CTRL-C your way back to the debugger,
966 because the debugger's own C<$SIG{INT}> handler doesn't understand that
967 it needs to raise an exception to longjmp(3) out of slow syscalls.