4 perldebug - Perl debugging
8 First of all, have you tried using the B<-w> switch?
11 If you're new to the Perl debugger, you may prefer to read
12 L<perldebtut>, which is a tutorial introduction to the debugger.
14 =head1 The Perl Debugger
16 If you invoke Perl with the B<-d> switch, your script runs under the
17 Perl source debugger. This works like an interactive Perl
18 environment, prompting for debugger commands that let you examine
19 source code, set breakpoints, get stack backtraces, change the values of
20 variables, etc. This is so convenient that you often fire up
21 the debugger all by itself just to test out Perl constructs
22 interactively to see what they do. For example:
27 In Perl, the debugger is not a separate program the way it usually is in the
28 typical compiled environment. Instead, the B<-d> flag tells the compiler
29 to insert source information into the parse trees it's about to hand off
30 to the interpreter. That means your code must first compile correctly
31 for the debugger to work on it. Then when the interpreter starts up, it
32 preloads a special Perl library file containing the debugger.
34 The program will halt I<right before> the first run-time executable
35 statement (but see below regarding compile-time statements) and ask you
36 to enter a debugger command. Contrary to popular expectations, whenever
37 the debugger halts and shows you a line of code, it always displays the
38 line it's I<about> to execute, rather than the one it has just executed.
40 Any command not recognized by the debugger is directly executed
41 (C<eval>'d) as Perl code in the current package. (The debugger
42 uses the DB package for keeping its own state information.)
44 Note that the said C<eval> is bound by an implicit scope. As a
45 result any newly introduced lexical variable or any modified
46 capture buffer content is lost after the eval. The debugger is a
47 nice environment to learn Perl, but if you interactively experiment using
48 material which should be in the same scope, stuff it in one line.
50 For any text entered at the debugger prompt, leading and trailing whitespace
51 is first stripped before further processing. If a debugger command
52 coincides with some function in your own program, merely precede the
53 function with something that doesn't look like a debugger command, such
54 as a leading C<;> or perhaps a C<+>, or by wrapping it with parentheses
57 =head2 Calling the Debugger
59 There are several ways to call the debugger:
63 =item perl -d program_name
65 On the given program identified by C<program_name>.
69 Interactively supply an arbitrary C<expression> using C<-e>.
71 =item perl -d:ptkdb program_name
73 Debug a given program via the C<Devel::ptkdb> GUI.
75 =item perl -dt threaded_program_name
77 Debug a given program using threads (experimental).
81 =head2 Debugger Commands
83 The interactive debugger understands the following commands:
88 X<debugger command, h>
90 Prints out a summary help message
94 Prints out a help message for the given debugger command.
98 The special argument of C<h h> produces the entire help page, which is quite long.
100 If the output of the C<h h> command (or any command, for that matter) scrolls
101 past your screen, precede the command with a leading pipe symbol so
102 that it's run through your pager, as in
106 You may change the pager which is used via C<o pager=...> command.
109 X<debugger command, p>
111 Same as C<print {$DB::OUT} expr> in the current package. In particular,
112 because this is just Perl's own C<print> function, this means that nested
113 data structures and objects are not dumped, unlike with the C<x> command.
115 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
116 where STDOUT may be redirected to.
118 =item x [maxdepth] expr
119 X<debugger command, x>
121 Evaluates its expression in list context and dumps out the result in a
122 pretty-printed fashion. Nested data structures are printed out
123 recursively, unlike the real C<print> function in Perl. When dumping
124 hashes, you'll probably prefer 'x \%h' rather than 'x %h'.
125 See L<Dumpvalue> if you'd like to do this yourself.
127 The output format is governed by multiple options described under
128 L<"Configurable Options">.
130 If the C<maxdepth> is included, it must be a numeral I<N>; the value is
131 dumped only I<N> levels deep, as if the C<dumpDepth> option had been
132 temporarily set to I<N>.
135 X<debugger command, V>
137 Display all (or some) variables in package (defaulting to C<main>)
138 using a data pretty-printer (hashes show their keys and values so
139 you see what's what, control characters are made printable, etc.).
140 Make sure you don't put the type specifier (like C<$>) there, just
141 the symbol names, like this:
145 Use C<~pattern> and C<!pattern> for positive and negative regexes.
147 This is similar to calling the C<x> command on each applicable var.
150 X<debugger command, X>
152 Same as C<V currentpackage [vars]>.
154 =item y [level [vars]]
155 X<debugger command, y>
157 Display all (or some) lexical variables (mnemonic: C<mY> variables)
158 in the current scope or I<level> scopes higher. You can limit the
159 variables that you see with I<vars> which works exactly as it does
160 for the C<V> and C<X> commands. Requires the C<PadWalker> module
161 version 0.08 or higher; will warn if this isn't installed. Output
162 is pretty-printed in the same style as for C<V> and the format is
163 controlled by the same options.
166 X<debugger command, T> X<backtrace> X<stack, backtrace>
168 Produce a stack backtrace. See below for details on its output.
171 X<debugger command, s> X<step>
173 Single step. Executes until the beginning of another
174 statement, descending into subroutine calls. If an expression is
175 supplied that includes function calls, it too will be single-stepped.
178 X<debugger command, n>
180 Next. Executes over subroutine calls, until the beginning
181 of the next statement. If an expression is supplied that includes
182 function calls, those functions will be executed with stops before
186 X<debugger command, r>
188 Continue until the return from the current subroutine.
189 Dump the return value if the C<PrintRet> option is set (default).
193 Repeat last C<n> or C<s> command.
196 X<debugger command, c>
198 Continue, optionally inserting a one-time-only breakpoint
199 at the specified line or subroutine.
202 X<debugger command, l>
204 List next window of lines.
208 List C<incr+1> lines starting at C<min>.
212 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
220 List first window of lines from subroutine. I<subname> may
221 be a variable that contains a code reference.
224 X<debugger command, ->
226 List previous window of lines.
229 X<debugger command, v>
231 View a few lines of code around the current line.
234 X<debugger command, .>
236 Return the internal debugger pointer to the line last
237 executed, and print out that line.
240 X<debugger command, f>
242 Switch to viewing a different file or C<eval> statement. If I<filename>
243 is not a full pathname found in the values of %INC, it is considered
246 C<eval>ed strings (when accessible) are considered to be filenames:
247 C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
248 (in the order of execution). The bodies of the currently executed C<eval>
249 and of C<eval>ed strings that define subroutines are saved and thus
254 Search forwards for pattern (a Perl regex); final / is optional.
255 The search is case-insensitive by default.
259 Search backwards for pattern; final ? is optional.
260 The search is case-insensitive by default.
263 X<debugger command, L>
265 List (default all) actions, breakpoints and watch expressions
268 X<debugger command, S>
270 List subroutine names [not] matching the regex.
273 X<debugger command, t>
275 Toggle trace mode (see also the C<AutoTrace> option).
276 Optional argument is the maximum number of levels to trace below
277 the current one; anything deeper than that will be silent.
280 X<debugger command, t>
282 Trace through execution of C<expr>.
283 Optional first argument is the maximum number of levels to trace below
284 the current one; anything deeper than that will be silent.
285 See L<perldebguts/"Frame Listing Output Examples"> for examples.
289 X<debugger command, b>
291 Sets breakpoint on current line
293 =item b [line] [condition]
295 X<debugger command, b>
297 Set a breakpoint before the given line. If a condition
298 is specified, it's evaluated each time the statement is reached: a
299 breakpoint is taken only if the condition is true. Breakpoints may
300 only be set on lines that begin an executable statement. Conditions
304 b 237 ++$count237 < 11
307 If the line number is C<.>, sets a breakpoint on the current line:
311 =item b [file]:[line] [condition]
313 X<debugger command, b>
315 Set a breakpoint before the given line in a (possibly different) file. If a
316 condition is specified, it's evaluated each time the statement is reached: a
317 breakpoint is taken only if the condition is true. Breakpoints may only be set
318 on lines that begin an executable statement. Conditions don't use C<if>:
320 b lib/MyModule.pm:237 $x > 30
321 b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
323 =item b subname [condition]
325 X<debugger command, b>
327 Set a breakpoint before the first line of the named subroutine. I<subname> may
328 be a variable containing a code reference (in this case I<condition>
331 =item b postpone subname [condition]
333 X<debugger command, b>
335 Set a breakpoint at first line of subroutine after it is compiled.
337 =item b load filename
339 X<debugger command, b>
341 Set a breakpoint before the first executed line of the I<filename>,
342 which should be a full pathname found amongst the %INC values.
344 =item b compile subname
346 X<debugger command, b>
348 Sets a breakpoint before the first statement executed after the specified
349 subroutine is compiled.
353 X<debugger command, B>
355 Delete a breakpoint from the specified I<line>.
359 X<debugger command, B>
361 Delete all installed breakpoints.
363 =item disable [file]:[line]
365 X<debugger command, disable>
368 Disable the breakpoint so it won't stop the execution of the program.
369 Breakpoints are enabled by default and can be re-enabled using the C<enable>
374 X<debugger command, disable>
377 Disable the breakpoint so it won't stop the execution of the program.
378 Breakpoints are enabled by default and can be re-enabled using the C<enable>
381 This is done for a breakpoint in the current file.
383 =item enable [file]:[line]
385 X<debugger command, disable>
388 Enable the breakpoint so it will stop the execution of the program.
392 X<debugger command, disable>
395 Enable the breakpoint so it will stop the execution of the program.
397 This is done for a breakpoint in the current file.
399 =item a [line] command
400 X<debugger command, a>
402 Set an action to be done before the line is executed. If I<line> is
403 omitted, set an action on the line about to be executed.
404 The sequence of steps taken by the debugger is
406 1. check for a breakpoint at this line
407 2. print the line if necessary (tracing)
408 3. do any actions associated with that line
409 4. prompt user if at a breakpoint or in single-step
412 For example, this will print out $foo every time line
415 a 53 print "DB FOUND $foo\n"
418 X<debugger command, A>
420 Delete an action from the specified line.
423 X<debugger command, A>
425 Delete all installed actions.
428 X<debugger command, w>
430 Add a global watch-expression. Whenever a watched global changes the
431 debugger will stop and display the old and new values.
434 X<debugger command, W>
436 Delete watch-expression
439 X<debugger command, W>
441 Delete all watch-expressions.
444 X<debugger command, o>
448 =item o booloption ...
449 X<debugger command, o>
451 Set each listed Boolean option to the value C<1>.
453 =item o anyoption? ...
454 X<debugger command, o>
456 Print out the value of one or more options.
458 =item o option=value ...
459 X<debugger command, o>
461 Set the value of one or more options. If the value has internal
462 whitespace, it should be quoted. For example, you could set C<o
463 pager="less -MQeicsNfr"> to call B<less> with those specific options.
464 You may use either single or double quotes, but if you do, you must
465 escape any embedded instances of same sort of quote you began with,
466 as well as any escaping any escapes that immediately precede that
467 quote but which are not meant to escape the quote itself. In other
468 words, you follow single-quoting rules irrespective of the quote;
469 eg: C<o option='this isn\'t bad'> or C<o option="She said, \"Isn't
472 For historical reasons, the C<=value> is optional, but defaults to
473 1 only where it is safe to do so--that is, mostly for Boolean
474 options. It is always better to assign a specific value using C<=>.
475 The C<option> can be abbreviated, but for clarity probably should
476 not be. Several options can be set together. See L<"Configurable Options">
480 X<< debugger command, < >>
482 List out all pre-prompt Perl command actions.
485 X<< debugger command, < >>
487 Set an action (Perl command) to happen before every debugger prompt.
488 A multi-line command may be entered by backslashing the newlines.
491 X<< debugger command, < >>
493 Delete all pre-prompt Perl command actions.
496 X<< debugger command, << >>
498 Add an action (Perl command) to happen before every debugger prompt.
499 A multi-line command may be entered by backwhacking the newlines.
502 X<< debugger command, > >>
504 List out post-prompt Perl command actions.
507 X<< debugger command, > >>
509 Set an action (Perl command) to happen after the prompt when you've
510 just given a command to return to executing the script. A multi-line
511 command may be entered by backslashing the newlines (we bet you
512 couldn't have guessed this by now).
515 X<< debugger command, > >>
517 Delete all post-prompt Perl command actions.
520 X<<< debugger command, >> >>>
522 Adds an action (Perl command) to happen after the prompt when you've
523 just given a command to return to executing the script. A multi-line
524 command may be entered by backslashing the newlines.
527 X<debugger command, {>
529 List out pre-prompt debugger commands.
533 Set an action (debugger command) to happen before every debugger prompt.
534 A multi-line command may be entered in the customary fashion.
536 Because this command is in some senses new, a warning is issued if
537 you appear to have accidentally entered a block instead. If that's
538 what you mean to do, write it as with C<;{ ... }> or even
542 X<debugger command, {>
544 Delete all pre-prompt debugger commands.
547 X<debugger command, {{>
549 Add an action (debugger command) to happen before every debugger prompt.
550 A multi-line command may be entered, if you can guess how: see above.
553 X<debugger command, !>
555 Redo a previous command (defaults to the previous command).
558 X<debugger command, !>
560 Redo number'th previous command.
563 X<debugger command, !>
565 Redo last command that started with pattern.
566 See C<o recallCommand>, too.
569 X<debugger command, !!>
571 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
572 C<o shellBang>, also. Note that the user's current shell (well,
573 their C<$ENV{SHELL}> variable) will be used, which can interfere
574 with proper interpretation of exit status or signal and coredump
578 X<debugger command, source>
580 Read and execute debugger commands from I<file>.
581 I<file> may itself contain C<source> commands.
584 X<debugger command, H>
586 Display last n commands. Only commands longer than one character are
587 listed. If I<number> is omitted, list them all.
590 X<debugger command, q>
591 X<debugger command, ^D>
593 Quit. ("quit" doesn't work for this, unless you've made an alias)
594 This is the only supported way to exit the debugger, though typing
595 C<exit> twice might work.
597 Set the C<inhibit_exit> option to 0 if you want to be able to step
598 off the end the script. You may also need to set $finished to 0
599 if you want to step through global destruction.
602 X<debugger command, R>
604 Restart the debugger by C<exec()>ing a new session. We try to maintain
605 your history across this, but internal settings and command-line options
608 The following setting are currently preserved: history, breakpoints,
609 actions, debugger options, and the Perl command-line
610 options B<-w>, B<-I>, and B<-e>.
613 X<debugger command, |>
615 Run the debugger command, piping DB::OUT into your current pager.
618 X<debugger command, ||>
620 Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
622 =item = [alias value]
623 X<debugger command, =>
625 Define a command alias, like
629 or list current aliases.
633 Execute command as a Perl statement. A trailing semicolon will be
634 supplied. If the Perl statement would otherwise be confused for a
635 Perl debugger, use a leading semicolon, too.
638 X<debugger command, m>
640 List which methods may be called on the result of the evaluated
641 expression. The expression may evaluated to a reference to a
642 blessed object, or to a package name.
645 X<debugger command, M>
647 Display all loaded modules and their versions.
650 X<debugger command, man>
652 Despite its name, this calls your system's default documentation
653 viewer on the given page, or on the viewer itself if I<manpage> is
654 omitted. If that viewer is B<man>, the current C<Config> information
655 is used to invoke B<man> using the proper MANPATH or S<B<-M>
656 I<manpath>> option. Failed lookups of the form C<XXX> that match
657 known manpages of the form I<perlXXX> will be retried. This lets
658 you type C<man debug> or C<man op> from the debugger.
660 On systems traditionally bereft of a usable B<man> command, the
661 debugger invokes B<perldoc>. Occasionally this determination is
662 incorrect due to recalcitrant vendors or rather more felicitously,
663 to enterprising users. If you fall into either category, just
664 manually set the $DB::doccmd variable to whatever viewer to view
665 the Perl documentation on your system. This may be set in an rc
666 file, or through direct assignment. We're still waiting for a
667 working example of something along the lines of:
669 $DB::doccmd = 'netscape -remote http://something.here/';
673 =head2 Configurable Options
675 The debugger has numerous options settable using the C<o> command,
676 either interactively or from the environment or an rc file.
677 (./.perldb or ~/.perldb under Unix.)
682 =item C<recallCommand>, C<ShellBang>
683 X<debugger option, recallCommand>
684 X<debugger option, ShellBang>
686 The characters used to recall a command or spawn a shell. By
687 default, both are set to C<!>, which is unfortunate.
690 X<debugger option, pager>
692 Program to use for output of pager-piped commands (those beginning
693 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
694 Because the debugger uses your current terminal characteristics
695 for bold and underlining, if the chosen pager does not pass escape
696 sequences through unchanged, the output of some debugger commands
697 will not be readable when sent through the pager.
700 X<debugger option, tkRunning>
702 Run Tk while prompting (with ReadLine).
704 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
705 X<debugger option, signalLevel> X<debugger option, warnLevel>
706 X<debugger option, dieLevel>
708 Level of verbosity. By default, the debugger leaves your exceptions
709 and warnings alone, because altering them can break correctly running
710 programs. It will attempt to print a message when uncaught INT, BUS, or
711 SEGV signals arrive. (But see the mention of signals in L</BUGS> below.)
713 To disable this default safe mode, set these values to something higher
714 than 0. At a level of 1, you get backtraces upon receiving any kind
715 of warning (this is often annoying) or exception (this is
716 often valuable). Unfortunately, the debugger cannot discern fatal
717 exceptions from non-fatal ones. If C<dieLevel> is even 1, then your
718 non-fatal exceptions are also traced and unceremoniously altered if they
719 came from C<eval'ed> strings or from any kind of C<eval> within modules
720 you're attempting to load. If C<dieLevel> is 2, the debugger doesn't
721 care where they came from: It usurps your exception handler and prints
722 out a trace, then modifies all exceptions with its own embellishments.
723 This may perhaps be useful for some tracing purposes, but tends to hopelessly
724 destroy any program that takes its exception handling seriously.
727 X<debugger option, AutoTrace>
729 Trace mode (similar to C<t> command, but can be put into
733 X<debugger option, LineInfo>
735 File or pipe to print line number info to. If it is a pipe (say,
736 C<|visual_perl_db>), then a short message is used. This is the
737 mechanism used to interact with a slave editor or visual debugger,
738 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
741 =item C<inhibit_exit>
742 X<debugger option, inhibit_exit>
744 If 0, allows I<stepping off> the end of the script.
747 X<debugger option, PrintRet>
749 Print return value after C<r> command if set (default).
752 X<debugger option, ornaments>
754 Affects screen appearance of the command line (see L<Term::ReadLine>).
755 There is currently no way to disable these, which can render
756 some output illegible on some displays, or with some pagers.
757 This is considered a bug.
760 X<debugger option, frame>
762 Affects the printing of messages upon entry and exit from subroutines. If
763 C<frame & 2> is false, messages are printed on entry only. (Printing
764 on exit might be useful if interspersed with other messages.)
766 If C<frame & 4>, arguments to functions are printed, plus context
767 and caller info. If C<frame & 8>, overloaded C<stringify> and
768 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
769 & 16>, the return value from the subroutine is printed.
771 The length at which the argument list is truncated is governed by the
775 X<debugger option, maxTraceLen>
777 Length to truncate the argument list when the C<frame> option's
781 X<debugger option, windowSize>
783 Change the size of code list window (default is 10 lines).
787 The following options affect what happens with C<V>, C<X>, and C<x>
792 =item C<arrayDepth>, C<hashDepth>
793 X<debugger option, arrayDepth> X<debugger option, hashDepth>
795 Print only first N elements ('' for all).
798 X<debugger option, dumpDepth>
800 Limit recursion depth to N levels when dumping structures.
801 Negative values are interpreted as infinity. Default: infinity.
803 =item C<compactDump>, C<veryCompact>
804 X<debugger option, compactDump> X<debugger option, veryCompact>
806 Change the style of array and hash output. If C<compactDump>, short array
807 may be printed on one line.
810 X<debugger option, globPrint>
812 Whether to print contents of globs.
815 X<debugger option, DumpDBFiles>
817 Dump arrays holding debugged files.
819 =item C<DumpPackages>
820 X<debugger option, DumpPackages>
822 Dump symbol tables of packages.
825 X<debugger option, DumpReused>
827 Dump contents of "reused" addresses.
829 =item C<quote>, C<HighBit>, C<undefPrint>
830 X<debugger option, quote> X<debugger option, HighBit>
831 X<debugger option, undefPrint>
833 Change the style of string dump. The default value for C<quote>
834 is C<auto>; one can enable double-quotish or single-quotish format
835 by setting it to C<"> or C<'>, respectively. By default, characters
836 with their high bit set are printed verbatim.
839 X<debugger option, UsageOnly>
841 Rudimentary per-package memory usage dump. Calculates total
842 size of strings found in variables in the package. This does not
843 include lexicals in a module's file scope, or lost in closures.
846 X<debugger option, history, HistFile>
848 The path of the file from which the history (assuming a usable
849 Term::ReadLine backend) will be read on the debugger's startup, and to which
850 it will be saved on shutdown (for persistence across sessions). Similar in
851 concept to Bash's C<.bash_history> file.
854 X<debugger option, history, HistSize>
856 The count of the saved lines in the history (assuming C<HistFile> above).
860 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
861 environment variable and parses this as the remainder of a "O ..."
862 line as one might enter at the debugger prompt. You may place the
863 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
866 If your rc file contains:
868 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
870 then your script will run without human intervention, putting trace
871 information into the file I<db.out>. (If you interrupt it, you'd
872 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
877 X<debugger option, TTY>
879 The TTY to use for debugging I/O.
882 X<debugger option, noTTY>
884 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
885 interrupted (or if control goes to the debugger via explicit setting of
886 $DB::signal or $DB::single from the Perl script), it connects to a TTY
887 specified in the C<TTY> option at startup, or to a tty found at
888 runtime using the C<Term::Rendezvous> module of your choice.
890 This module should implement a method named C<new> that returns an object
891 with two methods: C<IN> and C<OUT>. These should return filehandles to use
892 for debugging input and output correspondingly. The C<new> method should
893 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
894 startup, or C<"$ENV{HOME}/.perldbtty$$"> otherwise. This file is not
895 inspected for proper ownership, so security hazards are theoretically
899 X<debugger option, ReadLine>
901 If false, readline support in the debugger is disabled in order
902 to debug applications that themselves use ReadLine.
905 X<debugger option, NonStop>
907 If set, the debugger goes into non-interactive mode until interrupted, or
908 programmatically by setting $DB::signal or $DB::single.
912 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
914 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
916 That will run the script B<myprogram> without human intervention,
917 printing out the call tree with entry and exit points. Note that
918 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
919 options could be uniquely abbreviated by the first letter (modulo
920 the C<Dump*> options). It is nevertheless recommended that you
921 always spell them out in full for legibility and future compatibility.
923 Other examples include
925 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
927 which runs script non-interactively, printing info on each entry
928 into a subroutine and each executed line into the file named F<listing>.
929 (If you interrupt it, you would better reset C<LineInfo> to something
932 Other examples include (using standard shell syntax to show environment
935 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
938 which may be useful for debugging a program that uses C<Term::ReadLine>
939 itself. Do not forget to detach your shell from the TTY in the window that
940 corresponds to F</dev/ttyXX>, say, by issuing a command like
944 See L<perldebguts/"Debugger Internals"> for details.
946 =head2 Debugger Input/Output
952 The debugger prompt is something like
960 where that number is the command number, and which you'd use to
961 access with the built-in B<csh>-like history mechanism. For example,
962 C<!17> would repeat command number 17. The depth of the angle
963 brackets indicates the nesting depth of the debugger. You could
964 get more than one set of brackets, for example, if you'd already
965 at a breakpoint and then printed the result of a function call that
966 itself has a breakpoint, or you step into an expression via C<s/n/t
969 =item Multiline commands
971 If you want to enter a multi-line command, such as a subroutine
972 definition with several statements or a format, escape the newline
973 that would normally end the debugger command with a backslash.
977 cont: print "ok\n"; \
984 Note that this business of escaping a newline is specific to interactive
985 commands typed into the debugger.
987 =item Stack backtrace
988 X<backtrace> X<stack, backtrace>
990 Here's an example of what a stack backtrace via C<T> command might
993 $ = main::infested called from file 'Ambulation.pm' line 10
994 @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea' line 7
995 $ = main::pests('bactrian', 4) called from file 'camel_flea' line 4
997 The left-hand character up there indicates the context in which the
998 function was called, with C<$> and C<@> meaning scalar or list
999 contexts respectively, and C<.> meaning void context (which is
1000 actually a sort of scalar context). The display above says
1001 that you were in the function C<main::infested> when you ran the
1002 stack dump, and that it was called in scalar context from line
1003 10 of the file I<Ambulation.pm>, but without any arguments at all,
1004 meaning it was called as C<&infested>. The next stack frame shows
1005 that the function C<Ambulation::legs> was called in list context
1006 from the I<camel_flea> file with four arguments. The last stack
1007 frame shows that C<main::pests> was called in scalar context,
1008 also from I<camel_flea>, but from line 4.
1010 If you execute the C<T> command from inside an active C<use>
1011 statement, the backtrace will contain both a C<require> frame and
1014 =item Line Listing Format
1016 This shows the sorts of output the C<l> command can produce:
1020 102:b @isa{@i,$pack} = ()
1021 103 if(exists $i{$prevpack} || exists $isa{$pack});
1025 107==> if(exists $isa{$pack});
1027 109:a if ($extra-- > 0) {
1028 110: %isa = ($pack,1);
1030 Breakable lines are marked with C<:>. Lines with breakpoints are
1031 marked by C<b> and those with actions by C<a>. The line that's
1032 about to be executed is marked by C<< ==> >>.
1034 Please be aware that code in debugger listings may not look the same
1035 as your original source code. Line directives and external source
1036 filters can alter the code before Perl sees it, causing code to move
1037 from its original positions or take on entirely different forms.
1041 When the C<frame> option is set, the debugger would print entered (and
1042 optionally exited) subroutines in different styles. See L<perldebguts>
1043 for incredibly long examples of these.
1047 =head2 Debugging Compile-Time Statements
1049 If you have compile-time executable statements (such as code within
1050 BEGIN, UNITCHECK and CHECK blocks or C<use> statements), these will
1051 I<not> be stopped by debugger, although C<require>s and INIT blocks
1052 will, and compile-time statements can be traced with the C<AutoTrace>
1053 option set in C<PERLDB_OPTS>). From your own Perl code, however, you
1054 can transfer control back to the debugger using the following
1055 statement, which is harmless if the debugger is not running:
1059 If you set C<$DB::single> to 2, it's equivalent to having
1060 just typed the C<n> command, whereas a value of 1 means the C<s>
1061 command. The C<$DB::trace> variable should be set to 1 to simulate
1062 having typed the C<t> command.
1064 Another way to debug compile-time code is to start the debugger, set a
1065 breakpoint on the I<load> of some module:
1067 DB<7> b load f:/perllib/lib/Carp.pm
1068 Will stop on load of 'f:/perllib/lib/Carp.pm'.
1070 and then restart the debugger using the C<R> command (if possible). One can use C<b
1071 compile subname> for the same purpose.
1073 =head2 Debugger Customization
1075 The debugger probably contains enough configuration hooks that you
1076 won't ever have to modify it yourself. You may change the behaviour
1077 of the debugger from within the debugger using its C<o> command, from
1078 the command line via the C<PERLDB_OPTS> environment variable, and
1079 from customization files.
1081 You can do some customization by setting up a F<.perldb> file, which
1082 contains initialization code. For instance, you could make aliases
1083 like these (the last one is one people expect to be there):
1085 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
1086 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
1087 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
1088 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
1090 You can change options from F<.perldb> by using calls like this one;
1092 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
1094 The code is executed in the package C<DB>. Note that F<.perldb> is
1095 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
1096 subroutine C<afterinit>, that function is called after debugger
1097 initialization ends. F<.perldb> may be contained in the current
1098 directory, or in the home directory. Because this file is sourced
1099 in by Perl and may contain arbitrary commands, for security reasons,
1100 it must be owned by the superuser or the current user, and writable
1101 by no one but its owner.
1103 You can mock TTY input to debugger by adding arbitrary commands to
1104 @DB::typeahead. For example, your F<.perldb> file might contain:
1106 sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
1108 Which would attempt to set breakpoints on lines 4 and 6 immediately
1109 after debugger initialization. Note that @DB::typeahead is not a supported
1110 interface and is subject to change in future releases.
1112 If you want to modify the debugger, copy F<perl5db.pl> from the
1113 Perl library to another name and hack it to your heart's content.
1114 You'll then want to set your C<PERL5DB> environment variable to say
1115 something like this:
1117 BEGIN { require "myperl5db.pl" }
1119 As a last resort, you could also use C<PERL5DB> to customize the debugger
1120 by directly setting internal variables or calling debugger functions.
1122 Note that any variables and functions that are not documented in
1123 this document (or in L<perldebguts>) are considered for internal
1124 use only, and as such are subject to change without notice.
1126 =head2 Readline Support / History in the Debugger
1128 As shipped, the only command-line history supplied is a simplistic one
1129 that checks for leading exclamation points. However, if you install
1130 the Term::ReadKey and Term::ReadLine modules from CPAN (such as
1131 Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will
1132 have full editing capabilities much like those GNU I<readline>(3) provides.
1133 Look for these in the F<modules/by-module/Term> directory on CPAN.
1134 These do not support normal B<vi> command-line editing, however.
1136 A rudimentary command-line completion is also available, including
1137 lexical variables in the current scope if the C<PadWalker> module
1140 Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B",
1141 "^[[D"", "^H", ... when using the arrow keys and/or the backspace key.
1143 =head2 Editor Support for Debugging
1145 If you have the GNU's version of B<emacs> installed on your system,
1146 it can interact with the Perl debugger to provide an integrated
1147 software development environment reminiscent of its interactions
1150 Recent versions of Emacs come with a
1151 start file for making B<emacs> act like a
1152 syntax-directed editor that understands (some of) Perl's syntax.
1155 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
1156 and windy version, for coloring of Perl keywords.
1158 Note that only perl can truly parse Perl, so all such CASE tools
1159 fall somewhat short of the mark, especially if you don't program
1160 your Perl as a C programmer might.
1162 =head2 The Perl Profiler
1163 X<profile> X<profiling> X<profiler>
1165 If you wish to supply an alternative debugger for Perl to run,
1166 invoke your script with a colon and a package argument given to the
1167 B<-d> flag. Perl's alternative debuggers include a Perl profiler,
1168 L<Devel::NYTProf>, which is available separately as a CPAN
1169 distribution. To profile your Perl program in the file F<mycode.pl>,
1172 $ perl -d:NYTProf mycode.pl
1174 When the script terminates the profiler will create a database of the
1175 profile information that you can turn into reports using the profiler's
1176 tools. See <perlperf> for details.
1178 =head1 Debugging Regular Expressions
1179 X<regular expression, debugging>
1180 X<regex, debugging> X<regexp, debugging>
1182 C<use re 'debug'> enables you to see the gory details of how the Perl
1183 regular expression engine works. In order to understand this typically
1184 voluminous output, one must not only have some idea about how regular
1185 expression matching works in general, but also know how Perl's regular
1186 expressions are internally compiled into an automaton. These matters
1187 are explored in some detail in
1188 L<perldebguts/"Debugging Regular Expressions">.
1190 =head1 Debugging Memory Usage
1193 Perl contains internal support for reporting its own memory usage,
1194 but this is a fairly advanced concept that requires some understanding
1195 of how memory allocation works.
1196 See L<perldebguts/"Debugging Perl Memory Usage"> for the details.
1200 You did try the B<-w> switch, didn't you?
1211 When debugging a script that uses #! and is thus normally found in
1212 $PATH, the -S option causes perl to search $PATH for it, so you don't
1213 have to type the path or C<which $scriptname>.
1219 You cannot get stack frame information or in any fashion debug functions
1220 that were not compiled by Perl, such as those from C or C++ extensions.
1222 If you alter your @_ arguments in a subroutine (such as with C<shift>
1223 or C<pop>), the stack backtrace will not show the original values.
1225 The debugger does not currently work in conjunction with the B<-W>
1226 command-line switch, because it itself is not free of warnings.
1228 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
1229 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
1230 handler, then you won't be able to CTRL-C your way back to the debugger,
1231 because the debugger's own C<$SIG{INT}> handler doesn't understand that
1232 it needs to raise an exception to longjmp(3) out of slow syscalls.