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 Display last n commands. Only commands longer than one character are
393 listed. If I<number> is omitted, list them all.
397 Quit. ("quit" doesn't work for this, unless you've made an alias)
398 This is the only supported way to exit the debugger, though typing
399 C<exit> twice might work.
401 Set the C<inhibit_exit> option to 0 if you want to be able to step
402 off the end the script. You may also need to set $finished to 0
403 if you want to step through global destruction.
407 Restart the debugger by C<exec()>ing a new session. We try to maintain
408 your history across this, but internal settings and command-line options
411 The following setting are currently preserved: history, breakpoints,
412 actions, debugger options, and the Perl command-line
413 options B<-w>, B<-I>, and B<-e>.
417 Run the debugger command, piping DB::OUT into your current pager.
421 Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
423 =item = [alias value]
425 Define a command alias, like
429 or list current aliases.
433 Execute command as a Perl statement. A trailing semicolon will be
434 supplied. If the Perl statement would otherwise be confused for a
435 Perl debugger, use a leading semicolon, too.
439 List which methods may be called on the result of the evaluated
440 expression. The expression may evaluated to a reference to a
441 blessed object, or to a package name.
445 Despite its name, this calls your system's default documentation
446 viewer on the given page, or on the viewer itself if I<manpage> is
447 omitted. If that viewer is B<man>, the current C<Config> information
448 is used to invoke B<man> using the proper MANPATH or S<B<-M>
449 I<manpath>> option. Failed lookups of the form C<XXX> that match
450 known manpages of the form I<perlXXX> will be retried. This lets
451 you type C<man debug> or C<man op> from the debugger.
453 On systems traditionally bereft of a usable B<man> command, the
454 debugger invokes B<perldoc>. Occasionally this determination is
455 incorrect due to recalcitrant vendors or rather more felicitously,
456 to enterprising users. If you fall into either category, just
457 manually set the $DB::doccmd variable to whatever viewer to view
458 the Perl documentation on your system. This may be set in an rc
459 file, or through direct assignment. We're still waiting for a
460 working example of something along the lines of:
462 $DB::doccmd = 'netscape -remote http://something.here/';
466 =head2 Configurable Options
468 The debugger has numerous options settable using the C<O> command,
469 either interactively or from the environment or an rc file.
470 (./.perldb or ~/.perldb under Unix.)
475 =item C<recallCommand>, C<ShellBang>
477 The characters used to recall command or spawn shell. By
478 default, both are set to C<!>, which is unfortunate.
482 Program to use for output of pager-piped commands (those beginning
483 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
484 Because the debugger uses your current terminal characteristics
485 for bold and underlining, if the chosen pager does not pass escape
486 sequences through unchanged, the output of some debugger commands
487 will not be readable when sent through the pager.
491 Run Tk while prompting (with ReadLine).
493 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
495 Level of verbosity. By default, the debugger leaves your exceptions
496 and warnings alone, because altering them can break correctly running
497 programs. It will attempt to print a message when uncaught INT, BUS, or
498 SEGV signals arrive. (But see the mention of signals in L<BUGS> below.)
500 To disable this default safe mode, set these values to something higher
501 than 0. At a level of 1, you get backtraces upon receiving any kind
502 of warning (this is often annoying) or exception (this is
503 often valuable). Unfortunately, the debugger cannot discern fatal
504 exceptions from non-fatal ones. If C<dieLevel> is even 1, then your
505 non-fatal exceptions are also traced and unceremoniously altered if they
506 came from C<eval'd> strings or from any kind of C<eval> within modules
507 you're attempting to load. If C<dieLevel> is 2, the debugger doesn't
508 care where they came from: It usurps your exception handler and prints
509 out a trace, then modifies all exceptions with its own embellishments.
510 This may perhaps be useful for some tracing purposes, but tends to hopelessly
511 destroy any program that takes its exception handling seriously.
515 Trace mode (similar to C<t> command, but can be put into
520 File or pipe to print line number info to. If it is a pipe (say,
521 C<|visual_perl_db>), then a short message is used. This is the
522 mechanism used to interact with a slave editor or visual debugger,
523 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
526 =item C<inhibit_exit>
528 If 0, allows I<stepping off> the end of the script.
532 Print return value after C<r> command if set (default).
536 Affects screen appearance of the command line (see L<Term::ReadLine>).
537 There is currently no way to disable these, which can render
538 some output illegible on some displays, or with some pagers.
539 This is considered a bug.
543 Affects the printing of messages upon entry and exit from subroutines. If
544 C<frame & 2> is false, messages are printed on entry only. (Printing
545 on exit might be useful if interspersed with other messages.)
547 If C<frame & 4>, arguments to functions are printed, plus context
548 and caller info. If C<frame & 8>, overloaded C<stringify> and
549 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
550 & 16>, the return value from the subroutine is printed.
552 The length at which the argument list is truncated is governed by the
557 Length to truncate the argument list when the C<frame> option's
562 Change the size of code list window (default is 10 lines).
566 The following options affect what happens with C<V>, C<X>, and C<x>
571 =item C<arrayDepth>, C<hashDepth>
573 Print only first N elements ('' for all).
575 =item C<compactDump>, C<veryCompact>
577 Change the style of array and hash output. If C<compactDump>, short array
578 may be printed on one line.
582 Whether to print contents of globs.
586 Dump arrays holding debugged files.
588 =item C<DumpPackages>
590 Dump symbol tables of packages.
594 Dump contents of "reused" addresses.
596 =item C<quote>, C<HighBit>, C<undefPrint>
598 Change the style of string dump. The default value for C<quote>
599 is C<auto>; one can enable double-quotish or single-quotish format
600 by setting it to C<"> or C<'>, respectively. By default, characters
601 with their high bit set are printed verbatim.
605 Rudimentary per-package memory usage dump. Calculates total
606 size of strings found in variables in the package. This does not
607 include lexicals in a module's file scope, or lost in closures.
611 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
612 environment variable and parses this as the remainder of a `O ...'
613 line as one might enter at the debugger prompt. You may place the
614 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
617 If your rc file contains:
619 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
621 then your script will run without human intervention, putting trace
622 information into the file I<db.out>. (If you interrupt it, you'd
623 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
629 The TTY to use for debugging I/O.
633 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
634 interrupted (or if control goes to the debugger via explicit setting of
635 $DB::signal or $DB::single from the Perl script), it connects to a TTY
636 specified in the C<TTY> option at startup, or to a tty found at
637 runtime using the C<Term::Rendezvous> module of your choice.
639 This module should implement a method named C<new> that returns an object
640 with two methods: C<IN> and C<OUT>. These should return filehandles to use
641 for debugging input and output correspondingly. The C<new> method should
642 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
643 startup, or C<"/tmp/perldbtty$$"> otherwise. This file is not
644 inspected for proper ownership, so security hazards are theoretically
649 If false, readline support in the debugger is disabled in order
650 to debug applications that themselves use ReadLine.
654 If set, the debugger goes into non-interactive mode until interrupted, or
655 programmatically by setting $DB::signal or $DB::single.
659 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
661 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
663 That will run the script B<myprogram> without human intervention,
664 printing out the call tree with entry and exit points. Note that
665 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
666 options could be uniquely abbreviated by the first letter (modulo
667 the C<Dump*> options). It is nevertheless recommended that you
668 always spell them out in full for legibility and future compatibility.
670 Other examples include
672 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
674 which runs script non-interactively, printing info on each entry
675 into a subroutine and each executed line into the file named F<listing>.
676 (If you interrupt it, you would better reset C<LineInfo> to something
679 Other examples include (using standard shell syntax to show environment
682 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
685 which may be useful for debugging a program that uses C<Term::ReadLine>
686 itself. Do not forget to detach your shell from the TTY in the window that
687 corresponds to F</dev/ttyXX>, say, by issuing a command like
691 See L<perldebguts/"Debugger Internals"> for details.
693 =head2 Debugger input/output
699 The debugger prompt is something like
707 where that number is the command number, and which you'd use to
708 access with the built-in B<csh>-like history mechanism. For example,
709 C<!17> would repeat command number 17. The depth of the angle
710 brackets indicates the nesting depth of the debugger. You could
711 get more than one set of brackets, for example, if you'd already
712 at a breakpoint and then printed the result of a function call that
713 itself has a breakpoint, or you step into an expression via C<s/n/t
716 =item Multiline commands
718 If you want to enter a multi-line command, such as a subroutine
719 definition with several statements or a format, escape the newline
720 that would normally end the debugger command with a backslash.
724 cont: print "ok\n"; \
731 Note that this business of escaping a newline is specific to interactive
732 commands typed into the debugger.
734 =item Stack backtrace
736 Here's an example of what a stack backtrace via C<T> command might
739 $ = main::infested called from file `Ambulation.pm' line 10
740 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
741 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
743 The left-hand character up there indicates the context in which the
744 function was called, with C<$> and C<@> meaning scalar or list
745 contexts respectively, and C<.> meaning void context (which is
746 actually a sort of scalar context). The display above says
747 that you were in the function C<main::infested> when you ran the
748 stack dump, and that it was called in scalar context from line
749 10 of the file I<Ambulation.pm>, but without any arguments at all,
750 meaning it was called as C<&infested>. The next stack frame shows
751 that the function C<Ambulation::legs> was called in list context
752 from the I<camel_flea> file with four arguments. The last stack
753 frame shows that C<main::pests> was called in scalar context,
754 also from I<camel_flea>, but from line 4.
756 If you execute the C<T> command from inside an active C<use>
757 statement, the backtrace will contain both a C<require> frame and
760 =item Line Listing Format
762 This shows the sorts of output the C<l> command can produce:
766 102:b @isa{@i,$pack} = ()
767 103 if(exists $i{$prevpack} || exists $isa{$pack});
771 107==> if(exists $isa{$pack});
773 109:a if ($extra-- > 0) {
774 110: %isa = ($pack,1);
776 Breakable lines are marked with C<:>. Lines with breakpoints are
777 marked by C<b> and those with actions by C<a>. The line that's
778 about to be executed is marked by C<< ==> >>.
780 Please be aware that code in debugger listings may not look the same
781 as your original source code. Line directives and external source
782 filters can alter the code before Perl sees it, causing code to move
783 from its original positions or take on entirely different forms.
787 When the C<frame> option is set, the debugger would print entered (and
788 optionally exited) subroutines in different styles. See L<perldebguts>
789 for incredibly long examples of these.
793 =head2 Debugging compile-time statements
795 If you have compile-time executable statements (such as code within
796 BEGIN and CHECK blocks or C<use> statements), these will I<not> be
797 stopped by debugger, although C<require>s and INIT blocks will, and
798 compile-time statements can be traced with C<AutoTrace> option set
799 in C<PERLDB_OPTS>). From your own Perl code, however, you can
800 transfer control back to the debugger using the following statement,
801 which is harmless if the debugger is not running:
805 If you set C<$DB::single> to 2, it's equivalent to having
806 just typed the C<n> command, whereas a value of 1 means the C<s>
807 command. The C<$DB::trace> variable should be set to 1 to simulate
808 having typed the C<t> command.
810 Another way to debug compile-time code is to start the debugger, set a
811 breakpoint on the I<load> of some module:
813 DB<7> b load f:/perllib/lib/Carp.pm
814 Will stop on load of `f:/perllib/lib/Carp.pm'.
816 and then restart the debugger using the C<R> command (if possible). One can use C<b
817 compile subname> for the same purpose.
819 =head2 Debugger Customization
821 The debugger probably contains enough configuration hooks that you
822 won't ever have to modify it yourself. You may change the behaviour
823 of debugger from within the debugger using its C<O> command, from
824 the command line via the C<PERLDB_OPTS> environment variable, and
825 from customization files.
827 You can do some customization by setting up a F<.perldb> file, which
828 contains initialization code. For instance, you could make aliases
829 like these (the last one is one people expect to be there):
831 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
832 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
833 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
834 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
836 You can change options from F<.perldb> by using calls like this one;
838 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
840 The code is executed in the package C<DB>. Note that F<.perldb> is
841 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
842 subroutine C<afterinit>, that function is called after debugger
843 initialization ends. F<.perldb> may be contained in the current
844 directory, or in the home directory. Because this file is sourced
845 in by Perl and may contain arbitrary commands, for security reasons,
846 it must be owned by the superuser or the current user, and writable
847 by no one but its owner.
849 If you want to modify the debugger, copy F<perl5db.pl> from the
850 Perl library to another name and hack it to your heart's content.
851 You'll then want to set your C<PERL5DB> environment variable to say
854 BEGIN { require "myperl5db.pl" }
856 As a last resort, you could also use C<PERL5DB> to customize the debugger
857 by directly setting internal variables or calling debugger functions.
859 Note that any variables and functions that are not documented in
860 this document (or in L<perldebguts>) are considered for internal
861 use only, and as such are subject to change without notice.
863 =head2 Readline Support
865 As shipped, the only command-line history supplied is a simplistic one
866 that checks for leading exclamation points. However, if you install
867 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
868 have full editing capabilities much like GNU I<readline>(3) provides.
869 Look for these in the F<modules/by-module/Term> directory on CPAN.
870 These do not support normal B<vi> command-line editing, however.
872 A rudimentary command-line completion is also available.
873 Unfortunately, the names of lexical variables are not available for
876 =head2 Editor Support for Debugging
878 If you have the FSF's version of B<emacs> installed on your system,
879 it can interact with the Perl debugger to provide an integrated
880 software development environment reminiscent of its interactions
883 Perl comes with a start file for making B<emacs> act like a
884 syntax-directed editor that understands (some of) Perl's syntax.
885 Look in the I<emacs> directory of the Perl source distribution.
887 A similar setup by Tom Christiansen for interacting with any
888 vendor-shipped B<vi> and the X11 window system is also available.
889 This works similarly to the integrated multiwindow support that
890 B<emacs> provides, where the debugger drives the editor. At the
891 time of this writing, however, that tool's eventual location in the
892 Perl distribution was uncertain.
894 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
895 and windy version, for coloring of Perl keywords.
897 Note that only perl can truly parse Perl, so all such CASE tools
898 fall somewhat short of the mark, especially if you don't program
899 your Perl as a C programmer might.
901 =head2 The Perl Profiler
903 If you wish to supply an alternative debugger for Perl to run, just
904 invoke your script with a colon and a package argument given to the
905 B<-d> flag. The most popular alternative debuggers for Perl is the
906 Perl profiler. Devel::DProf is now included with the standard Perl
907 distribution. To profile your Perl program in the file F<mycode.pl>,
910 $ perl -d:DProf mycode.pl
912 When the script terminates the profiler will dump the profile
913 information to a file called F<tmon.out>. A tool like B<dprofpp>,
914 also supplied with the standard Perl distribution, can be used to
915 interpret the information in that profile.
917 =head1 Debugging regular expressions
919 C<use re 'debug'> enables you to see the gory details of how the
920 Perl regular expression engine works. In order to understand this
921 typically voluminous output, one must not only have some idea about
922 about how regular expression matching works in general, but also
923 know how Perl's regular expressions are internally compiled into
924 an automaton. These matters are explored in some detail in
925 L<perldebguts/"Debugging regular expressions">.
927 =head1 Debugging memory usage
929 Perl contains internal support for reporting its own memory usage,
930 but this is a fairly advanced concept that requires some understanding
931 of how memory allocation works.
932 See L<perldebguts/"Debugging Perl memory usage"> for the details.
936 You did try the B<-w> switch, didn't you?
949 You cannot get stack frame information or in any fashion debug functions
950 that were not compiled by Perl, such as those from C or C++ extensions.
952 If you alter your @_ arguments in a subroutine (such as with C<shift>
953 or C<pop>, the stack backtrace will not show the original values.
955 The debugger does not currently work in conjunction with the B<-W>
956 command-line switch, because it itself is not free of warnings.
958 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
959 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
960 handler, then you won't be able to CTRL-C your way back to the debugger,
961 because the debugger's own C<$SIG{INT}> handler doesn't understand that
962 it needs to raise an exception to longjmp(3) out of slow syscalls.