3 perldebug - Perl debugging
7 First of all, have you tried using the B<-w> switch?
10 If you're new to the Perl debugger, you may prefer to read
11 L<perldebtut>, which is a tutorial introduction to the debugger .
13 =head1 The Perl Debugger
15 If you invoke Perl with the B<-d> switch, your script runs under the
16 Perl source debugger. This works like an interactive Perl
17 environment, prompting for debugger commands that let you examine
18 source code, set breakpoints, get stack backtraces, change the values of
19 variables, etc. This is so convenient that you often fire up
20 the debugger all by itself just to test out Perl constructs
21 interactively to see what they do. For example:
25 In Perl, the debugger is not a separate program the way it usually is in the
26 typical compiled environment. Instead, the B<-d> flag tells the compiler
27 to insert source information into the parse trees it's about to hand off
28 to the interpreter. That means your code must first compile correctly
29 for the debugger to work on it. Then when the interpreter starts up, it
30 preloads a special Perl library file containing the debugger.
32 The program will halt I<right before> the first run-time executable
33 statement (but see below regarding compile-time statements) and ask you
34 to enter a debugger command. Contrary to popular expectations, whenever
35 the debugger halts and shows you a line of code, it always displays the
36 line it's I<about> to execute, rather than the one it has just executed.
38 Any command not recognized by the debugger is directly executed
39 (C<eval>'d) as Perl code in the current package. (The debugger
40 uses the DB package for keeping its own state information.)
42 For any text entered at the debugger prompt, leading and trailing whitespace
43 is first stripped before further processing. If a debugger command
44 coincides with some function in your own program, merely precede the
45 function with something that doesn't look like a debugger command, such
46 as a leading C<;> or perhaps a C<+>, or by wrapping it with parentheses
49 =head2 Debugger Commands
51 The debugger understands the following commands:
57 Prints out a summary help message
61 Prints out a help message for the given debugger command.
65 The special argument of C<h h> produces the entire help page, which is quite long.
67 If the output of the C<h h> command (or any command, for that matter) scrolls
68 past your screen, precede the command with a leading pipe symbol so
69 that it's run through your pager, as in
73 You may change the pager which is used via C<o pager=...> command.
78 Same as C<print {$DB::OUT} expr> in the current package. In particular,
79 because this is just Perl's own C<print> function, this means that nested
80 data structures and objects are not dumped, unlike with the C<x> command.
82 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
83 where STDOUT may be redirected to.
85 =item x [maxdepth] expr
87 Evaluates its expression in list context and dumps out the result
88 in a pretty-printed fashion. Nested data structures are printed out
89 recursively, unlike the real C<print> function in Perl.
90 See L<Dumpvalue> if you'd like to do this yourself.
92 The output format is governed by multiple options described under
93 L<"Configurable Options">.
95 If the C<maxdepth> is included, it must be a numeral I<N>; the value is
96 dumped only I<N> levels deep, as if the C<dumpDepth> option had been
97 temporarily set to I<N>.
101 Display all (or some) variables in package (defaulting to C<main>)
102 using a data pretty-printer (hashes show their keys and values so
103 you see what's what, control characters are made printable, etc.).
104 Make sure you don't put the type specifier (like C<$>) there, just
105 the symbol names, like this:
109 Use C<~pattern> and C<!pattern> for positive and negative regexes.
111 This is similar to calling the C<x> command on each applicable var.
115 Same as C<V currentpackage [vars]>.
117 =item y [level [vars]]
119 Display all (or some) lexical variables (mnemonic: C<mY> variables)
120 in the current scope or I<level> scopes higher. You can limit the
121 variables that you see with I<vars> which works exactly as it does
122 for the C<V> and C<X> commands. Requires the C<PadWalker> module
123 version 0.08 or higher; will warn if this isn't installed. Output
124 is pretty-printed in the same style as for C<V> and the format is
125 controlled by the same options.
129 Produce a stack backtrace. See below for details on its output.
133 Single step. Executes until the beginning of another
134 statement, descending into subroutine calls. If an expression is
135 supplied that includes function calls, it too will be single-stepped.
139 Next. Executes over subroutine calls, until the beginning
140 of the next statement. If an expression is supplied that includes
141 function calls, those functions will be executed with stops before
146 Continue until the return from the current subroutine.
147 Dump the return value if the C<PrintRet> option is set (default).
151 Repeat last C<n> or C<s> command.
155 Continue, optionally inserting a one-time-only breakpoint
156 at the specified line or subroutine.
160 List next window of lines.
164 List C<incr+1> lines starting at C<min>.
168 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
176 List first window of lines from subroutine. I<subname> may
177 be a variable that contains a code reference.
181 List previous window of lines.
185 View a few lines of code around the current line.
189 Return the internal debugger pointer to the line last
190 executed, and print out that line.
194 Switch to viewing a different file or C<eval> statement. If I<filename>
195 is not a full pathname found in the values of %INC, it is considered
198 C<eval>ed strings (when accessible) are considered to be filenames:
199 C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
200 (in the order of execution). The bodies of the currently executed C<eval>
201 and of C<eval>ed strings that define subroutines are saved and thus
206 Search forwards for pattern (a Perl regex); final / is optional.
207 The search is case-insensitive by default.
211 Search backwards for pattern; final ? is optional.
212 The search is case-insensitive by default.
216 List (default all) actions, breakpoints and watch expressions
220 List subroutine names [not] matching the regex.
224 Toggle trace mode (see also the C<AutoTrace> option).
228 Trace through execution of C<expr>.
229 See L<perldebguts/"Frame Listing Output Examples"> for examples.
233 Sets breakpoint on current line
235 =item b [line] [condition]
237 Set a breakpoint before the given line. If a condition
238 is specified, it's evaluated each time the statement is reached: a
239 breakpoint is taken only if the condition is true. Breakpoints may
240 only be set on lines that begin an executable statement. Conditions
244 b 237 ++$count237 < 11
247 =item b subname [condition]
249 Set a breakpoint before the first line of the named subroutine. I<subname> may
250 be a variable containing a code reference (in this case I<condition>
253 =item b postpone subname [condition]
255 Set a breakpoint at first line of subroutine after it is compiled.
257 =item b load filename
259 Set a breakpoint before the first executed line of the I<filename>,
260 which should be a full pathname found amongst the %INC values.
262 =item b compile subname
264 Sets a breakpoint before the first statement executed after the specified
265 subroutine is compiled.
269 Delete a breakpoint from the specified I<line>.
273 Delete all installed breakpoints.
275 =item a [line] command
277 Set an action to be done before the line is executed. If I<line> is
278 omitted, set an action on the line about to be executed.
279 The sequence of steps taken by the debugger is
281 1. check for a breakpoint at this line
282 2. print the line if necessary (tracing)
283 3. do any actions associated with that line
284 4. prompt user if at a breakpoint or in single-step
287 For example, this will print out $foo every time line
290 a 53 print "DB FOUND $foo\n"
294 Delete an action from the specified line.
298 Delete all installed actions.
302 Add a global watch-expression. We hope you know what one of these
303 is, because they're supposed to be obvious.
307 Delete watch-expression
311 Delete all watch-expressions.
317 =item o booloption ...
319 Set each listed Boolean option to the value C<1>.
321 =item o anyoption? ...
323 Print out the value of one or more options.
325 =item o option=value ...
327 Set the value of one or more options. If the value has internal
328 whitespace, it should be quoted. For example, you could set C<o
329 pager="less -MQeicsNfr"> to call B<less> with those specific options.
330 You may use either single or double quotes, but if you do, you must
331 escape any embedded instances of same sort of quote you began with,
332 as well as any escaping any escapes that immediately precede that
333 quote but which are not meant to escape the quote itself. In other
334 words, you follow single-quoting rules irrespective of the quote;
335 eg: C<o option='this isn\'t bad'> or C<o option="She said, \"Isn't
338 For historical reasons, the C<=value> is optional, but defaults to
339 1 only where it is safe to do so--that is, mostly for Boolean
340 options. It is always better to assign a specific value using C<=>.
341 The C<option> can be abbreviated, but for clarity probably should
342 not be. Several options can be set together. See L<"Configurable Options">
347 List out all pre-prompt Perl command actions.
351 Set an action (Perl command) to happen before every debugger prompt.
352 A multi-line command may be entered by backslashing the newlines.
353 B<WARNING> If C<command> is missing, all actions are wiped out!
357 Add an action (Perl command) to happen before every debugger prompt.
358 A multi-line command may be entered by backwhacking the newlines.
362 List out post-prompt Perl command actions.
366 Set an action (Perl command) to happen after the prompt when you've
367 just given a command to return to executing the script. A multi-line
368 command may be entered by backslashing the newlines (we bet you
369 couldn't've guessed this by now). B<WARNING> If C<command> is
370 missing, all actions are wiped out!
374 Adds an action (Perl command) to happen after the prompt when you've
375 just given a command to return to executing the script. A multi-line
376 command may be entered by backslashing the newlines.
380 List out pre-prompt debugger commands.
384 Set an action (debugger command) to happen before every debugger prompt.
385 A multi-line command may be entered in the customary fashion.
386 B<WARNING> If C<command> is missing, all actions are wiped out!
388 Because this command is in some senses new, a warning is issued if
389 you appear to have accidentally entered a block instead. If that's
390 what you mean to do, write it as with C<;{ ... }> or even
395 Add an action (debugger command) to happen before every debugger prompt.
396 A multi-line command may be entered, if you can guess how: see above.
400 Redo a previous command (defaults to the previous command).
404 Redo number'th previous command.
408 Redo last command that started with pattern.
409 See C<o recallCommand>, too.
413 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
414 C<o shellBang>, also. Note that the user's current shell (well,
415 their C<$ENV{SHELL}> variable) will be used, which can interfere
416 with proper interpretation of exit status or signal and coredump
421 Read and execute debugger commands from I<file>. I<file> may itself contain
426 Display last n commands. Only commands longer than one character are
427 listed. If I<number> is omitted, list them all.
431 Quit. ("quit" doesn't work for this, unless you've made an alias)
432 This is the only supported way to exit the debugger, though typing
433 C<exit> twice might work.
435 Set the C<inhibit_exit> option to 0 if you want to be able to step
436 off the end the script. You may also need to set $finished to 0
437 if you want to step through global destruction.
441 Restart the debugger by C<exec()>ing a new session. We try to maintain
442 your history across this, but internal settings and command-line options
445 The following setting are currently preserved: history, breakpoints,
446 actions, debugger options, and the Perl command-line
447 options B<-w>, B<-I>, and B<-e>.
451 Run the debugger command, piping DB::OUT into your current pager.
455 Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
457 =item = [alias value]
459 Define a command alias, like
463 or list current aliases.
467 Execute command as a Perl statement. A trailing semicolon will be
468 supplied. If the Perl statement would otherwise be confused for a
469 Perl debugger, use a leading semicolon, too.
473 List which methods may be called on the result of the evaluated
474 expression. The expression may evaluated to a reference to a
475 blessed object, or to a package name.
479 Displays all loaded modules and their versions
484 Despite its name, this calls your system's default documentation
485 viewer on the given page, or on the viewer itself if I<manpage> is
486 omitted. If that viewer is B<man>, the current C<Config> information
487 is used to invoke B<man> using the proper MANPATH or S<B<-M>
488 I<manpath>> option. Failed lookups of the form C<XXX> that match
489 known manpages of the form I<perlXXX> will be retried. This lets
490 you type C<man debug> or C<man op> from the debugger.
492 On systems traditionally bereft of a usable B<man> command, the
493 debugger invokes B<perldoc>. Occasionally this determination is
494 incorrect due to recalcitrant vendors or rather more felicitously,
495 to enterprising users. If you fall into either category, just
496 manually set the $DB::doccmd variable to whatever viewer to view
497 the Perl documentation on your system. This may be set in an rc
498 file, or through direct assignment. We're still waiting for a
499 working example of something along the lines of:
501 $DB::doccmd = 'netscape -remote http://something.here/';
505 =head2 Configurable Options
507 The debugger has numerous options settable using the C<o> command,
508 either interactively or from the environment or an rc file.
509 (./.perldb or ~/.perldb under Unix.)
514 =item C<recallCommand>, C<ShellBang>
516 The characters used to recall command or spawn shell. By
517 default, both are set to C<!>, which is unfortunate.
521 Program to use for output of pager-piped commands (those beginning
522 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
523 Because the debugger uses your current terminal characteristics
524 for bold and underlining, if the chosen pager does not pass escape
525 sequences through unchanged, the output of some debugger commands
526 will not be readable when sent through the pager.
530 Run Tk while prompting (with ReadLine).
532 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
534 Level of verbosity. By default, the debugger leaves your exceptions
535 and warnings alone, because altering them can break correctly running
536 programs. It will attempt to print a message when uncaught INT, BUS, or
537 SEGV signals arrive. (But see the mention of signals in L<BUGS> below.)
539 To disable this default safe mode, set these values to something higher
540 than 0. At a level of 1, you get backtraces upon receiving any kind
541 of warning (this is often annoying) or exception (this is
542 often valuable). Unfortunately, the debugger cannot discern fatal
543 exceptions from non-fatal ones. If C<dieLevel> is even 1, then your
544 non-fatal exceptions are also traced and unceremoniously altered if they
545 came from C<eval'd> strings or from any kind of C<eval> within modules
546 you're attempting to load. If C<dieLevel> is 2, the debugger doesn't
547 care where they came from: It usurps your exception handler and prints
548 out a trace, then modifies all exceptions with its own embellishments.
549 This may perhaps be useful for some tracing purposes, but tends to hopelessly
550 destroy any program that takes its exception handling seriously.
554 Trace mode (similar to C<t> command, but can be put into
559 File or pipe to print line number info to. If it is a pipe (say,
560 C<|visual_perl_db>), then a short message is used. This is the
561 mechanism used to interact with a slave editor or visual debugger,
562 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
565 =item C<inhibit_exit>
567 If 0, allows I<stepping off> the end of the script.
571 Print return value after C<r> command if set (default).
575 Affects screen appearance of the command line (see L<Term::ReadLine>).
576 There is currently no way to disable these, which can render
577 some output illegible on some displays, or with some pagers.
578 This is considered a bug.
582 Affects the printing of messages upon entry and exit from subroutines. If
583 C<frame & 2> is false, messages are printed on entry only. (Printing
584 on exit might be useful if interspersed with other messages.)
586 If C<frame & 4>, arguments to functions are printed, plus context
587 and caller info. If C<frame & 8>, overloaded C<stringify> and
588 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
589 & 16>, the return value from the subroutine is printed.
591 The length at which the argument list is truncated is governed by the
596 Length to truncate the argument list when the C<frame> option's
601 Change the size of code list window (default is 10 lines).
605 The following options affect what happens with C<V>, C<X>, and C<x>
610 =item C<arrayDepth>, C<hashDepth>
612 Print only first N elements ('' for all).
616 Limit recursion depth to N levels when dumping structures.
617 Negative values are interpreted as infinity. Default: infinity.
619 =item C<compactDump>, C<veryCompact>
621 Change the style of array and hash output. If C<compactDump>, short array
622 may be printed on one line.
626 Whether to print contents of globs.
630 Dump arrays holding debugged files.
632 =item C<DumpPackages>
634 Dump symbol tables of packages.
638 Dump contents of "reused" addresses.
640 =item C<quote>, C<HighBit>, C<undefPrint>
642 Change the style of string dump. The default value for C<quote>
643 is C<auto>; one can enable double-quotish or single-quotish format
644 by setting it to C<"> or C<'>, respectively. By default, characters
645 with their high bit set are printed verbatim.
649 Rudimentary per-package memory usage dump. Calculates total
650 size of strings found in variables in the package. This does not
651 include lexicals in a module's file scope, or lost in closures.
655 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
656 environment variable and parses this as the remainder of a `O ...'
657 line as one might enter at the debugger prompt. You may place the
658 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
661 If your rc file contains:
663 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
665 then your script will run without human intervention, putting trace
666 information into the file I<db.out>. (If you interrupt it, you'd
667 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
673 The TTY to use for debugging I/O.
677 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
678 interrupted (or if control goes to the debugger via explicit setting of
679 $DB::signal or $DB::single from the Perl script), it connects to a TTY
680 specified in the C<TTY> option at startup, or to a tty found at
681 runtime using the C<Term::Rendezvous> module of your choice.
683 This module should implement a method named C<new> that returns an object
684 with two methods: C<IN> and C<OUT>. These should return filehandles to use
685 for debugging input and output correspondingly. The C<new> method should
686 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
687 startup, or C<"/tmp/perldbtty$$"> otherwise. This file is not
688 inspected for proper ownership, so security hazards are theoretically
693 If false, readline support in the debugger is disabled in order
694 to debug applications that themselves use ReadLine.
698 If set, the debugger goes into non-interactive mode until interrupted, or
699 programmatically by setting $DB::signal or $DB::single.
703 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
705 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
707 That will run the script B<myprogram> without human intervention,
708 printing out the call tree with entry and exit points. Note that
709 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
710 options could be uniquely abbreviated by the first letter (modulo
711 the C<Dump*> options). It is nevertheless recommended that you
712 always spell them out in full for legibility and future compatibility.
714 Other examples include
716 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
718 which runs script non-interactively, printing info on each entry
719 into a subroutine and each executed line into the file named F<listing>.
720 (If you interrupt it, you would better reset C<LineInfo> to something
723 Other examples include (using standard shell syntax to show environment
726 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
729 which may be useful for debugging a program that uses C<Term::ReadLine>
730 itself. Do not forget to detach your shell from the TTY in the window that
731 corresponds to F</dev/ttyXX>, say, by issuing a command like
735 See L<perldebguts/"Debugger Internals"> for details.
737 =head2 Debugger input/output
743 The debugger prompt is something like
751 where that number is the command number, and which you'd use to
752 access with the built-in B<csh>-like history mechanism. For example,
753 C<!17> would repeat command number 17. The depth of the angle
754 brackets indicates the nesting depth of the debugger. You could
755 get more than one set of brackets, for example, if you'd already
756 at a breakpoint and then printed the result of a function call that
757 itself has a breakpoint, or you step into an expression via C<s/n/t
760 =item Multiline commands
762 If you want to enter a multi-line command, such as a subroutine
763 definition with several statements or a format, escape the newline
764 that would normally end the debugger command with a backslash.
768 cont: print "ok\n"; \
775 Note that this business of escaping a newline is specific to interactive
776 commands typed into the debugger.
778 =item Stack backtrace
780 Here's an example of what a stack backtrace via C<T> command might
783 $ = main::infested called from file `Ambulation.pm' line 10
784 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
785 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
787 The left-hand character up there indicates the context in which the
788 function was called, with C<$> and C<@> meaning scalar or list
789 contexts respectively, and C<.> meaning void context (which is
790 actually a sort of scalar context). The display above says
791 that you were in the function C<main::infested> when you ran the
792 stack dump, and that it was called in scalar context from line
793 10 of the file I<Ambulation.pm>, but without any arguments at all,
794 meaning it was called as C<&infested>. The next stack frame shows
795 that the function C<Ambulation::legs> was called in list context
796 from the I<camel_flea> file with four arguments. The last stack
797 frame shows that C<main::pests> was called in scalar context,
798 also from I<camel_flea>, but from line 4.
800 If you execute the C<T> command from inside an active C<use>
801 statement, the backtrace will contain both a C<require> frame and
804 =item Line Listing Format
806 This shows the sorts of output the C<l> command can produce:
810 102:b @isa{@i,$pack} = ()
811 103 if(exists $i{$prevpack} || exists $isa{$pack});
815 107==> if(exists $isa{$pack});
817 109:a if ($extra-- > 0) {
818 110: %isa = ($pack,1);
820 Breakable lines are marked with C<:>. Lines with breakpoints are
821 marked by C<b> and those with actions by C<a>. The line that's
822 about to be executed is marked by C<< ==> >>.
824 Please be aware that code in debugger listings may not look the same
825 as your original source code. Line directives and external source
826 filters can alter the code before Perl sees it, causing code to move
827 from its original positions or take on entirely different forms.
831 When the C<frame> option is set, the debugger would print entered (and
832 optionally exited) subroutines in different styles. See L<perldebguts>
833 for incredibly long examples of these.
837 =head2 Debugging compile-time statements
839 If you have compile-time executable statements (such as code within
840 BEGIN and CHECK blocks or C<use> statements), these will I<not> be
841 stopped by debugger, although C<require>s and INIT blocks will, and
842 compile-time statements can be traced with C<AutoTrace> option set
843 in C<PERLDB_OPTS>). From your own Perl code, however, you can
844 transfer control back to the debugger using the following statement,
845 which is harmless if the debugger is not running:
849 If you set C<$DB::single> to 2, it's equivalent to having
850 just typed the C<n> command, whereas a value of 1 means the C<s>
851 command. The C<$DB::trace> variable should be set to 1 to simulate
852 having typed the C<t> command.
854 Another way to debug compile-time code is to start the debugger, set a
855 breakpoint on the I<load> of some module:
857 DB<7> b load f:/perllib/lib/Carp.pm
858 Will stop on load of `f:/perllib/lib/Carp.pm'.
860 and then restart the debugger using the C<R> command (if possible). One can use C<b
861 compile subname> for the same purpose.
863 =head2 Debugger Customization
865 The debugger probably contains enough configuration hooks that you
866 won't ever have to modify it yourself. You may change the behaviour
867 of debugger from within the debugger using its C<o> command, from
868 the command line via the C<PERLDB_OPTS> environment variable, and
869 from customization files.
871 You can do some customization by setting up a F<.perldb> file, which
872 contains initialization code. For instance, you could make aliases
873 like these (the last one is one people expect to be there):
875 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
876 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
877 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
878 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
880 You can change options from F<.perldb> by using calls like this one;
882 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
884 The code is executed in the package C<DB>. Note that F<.perldb> is
885 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
886 subroutine C<afterinit>, that function is called after debugger
887 initialization ends. F<.perldb> may be contained in the current
888 directory, or in the home directory. Because this file is sourced
889 in by Perl and may contain arbitrary commands, for security reasons,
890 it must be owned by the superuser or the current user, and writable
891 by no one but its owner.
893 If you want to modify the debugger, copy F<perl5db.pl> from the
894 Perl library to another name and hack it to your heart's content.
895 You'll then want to set your C<PERL5DB> environment variable to say
898 BEGIN { require "myperl5db.pl" }
900 As a last resort, you could also use C<PERL5DB> to customize the debugger
901 by directly setting internal variables or calling debugger functions.
903 Note that any variables and functions that are not documented in
904 this document (or in L<perldebguts>) are considered for internal
905 use only, and as such are subject to change without notice.
907 =head2 Readline Support
909 As shipped, the only command-line history supplied is a simplistic one
910 that checks for leading exclamation points. However, if you install
911 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
912 have full editing capabilities much like GNU I<readline>(3) provides.
913 Look for these in the F<modules/by-module/Term> directory on CPAN.
914 These do not support normal B<vi> command-line editing, however.
916 A rudimentary command-line completion is also available.
917 Unfortunately, the names of lexical variables are not available for
920 =head2 Editor Support for Debugging
922 If you have the FSF's version of B<emacs> installed on your system,
923 it can interact with the Perl debugger to provide an integrated
924 software development environment reminiscent of its interactions
927 Perl comes with a start file for making B<emacs> act like a
928 syntax-directed editor that understands (some of) Perl's syntax.
929 Look in the I<emacs> directory of the Perl source distribution.
931 A similar setup by Tom Christiansen for interacting with any
932 vendor-shipped B<vi> and the X11 window system is also available.
933 This works similarly to the integrated multiwindow support that
934 B<emacs> provides, where the debugger drives the editor. At the
935 time of this writing, however, that tool's eventual location in the
936 Perl distribution was uncertain.
938 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
939 and windy version, for coloring of Perl keywords.
941 Note that only perl can truly parse Perl, so all such CASE tools
942 fall somewhat short of the mark, especially if you don't program
943 your Perl as a C programmer might.
945 =head2 The Perl Profiler
947 If you wish to supply an alternative debugger for Perl to run, just
948 invoke your script with a colon and a package argument given to the
949 B<-d> flag. The most popular alternative debuggers for Perl is the
950 Perl profiler. Devel::DProf is now included with the standard Perl
951 distribution. To profile your Perl program in the file F<mycode.pl>,
954 $ perl -d:DProf mycode.pl
956 When the script terminates the profiler will dump the profile
957 information to a file called F<tmon.out>. A tool like B<dprofpp>,
958 also supplied with the standard Perl distribution, can be used to
959 interpret the information in that profile.
961 =head1 Debugging regular expressions
963 C<use re 'debug'> enables you to see the gory details of how the Perl
964 regular expression engine works. In order to understand this typically
965 voluminous output, one must not only have some idea about how regular
966 expression matching works in general, but also know how Perl's regular
967 expressions are internally compiled into an automaton. These matters
968 are explored in some detail in
969 L<perldebguts/"Debugging regular expressions">.
971 =head1 Debugging memory usage
973 Perl contains internal support for reporting its own memory usage,
974 but this is a fairly advanced concept that requires some understanding
975 of how memory allocation works.
976 See L<perldebguts/"Debugging Perl memory usage"> for the details.
980 You did try the B<-w> switch, didn't you?
994 You cannot get stack frame information or in any fashion debug functions
995 that were not compiled by Perl, such as those from C or C++ extensions.
997 If you alter your @_ arguments in a subroutine (such as with C<shift>
998 or C<pop>), the stack backtrace will not show the original values.
1000 The debugger does not currently work in conjunction with the B<-W>
1001 command-line switch, because it itself is not free of warnings.
1003 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
1004 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
1005 handler, then you won't be able to CTRL-C your way back to the debugger,
1006 because the debugger's own C<$SIG{INT}> handler doesn't understand that
1007 it needs to raise an exception to longjmp(3) out of slow syscalls.