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).
278 X<debugger command, t>
280 Trace through execution of C<expr>.
281 See L<perldebguts/"Frame Listing Output Examples"> for examples.
285 X<debugger command, b>
287 Sets breakpoint on current line
289 =item b [line] [condition]
291 X<debugger command, b>
293 Set a breakpoint before the given line. If a condition
294 is specified, it's evaluated each time the statement is reached: a
295 breakpoint is taken only if the condition is true. Breakpoints may
296 only be set on lines that begin an executable statement. Conditions
300 b 237 ++$count237 < 11
303 =item b [file]:[line] [condition]
305 X<debugger command, b>
307 Set a breakpoint before the given line in a (possibly different) file. If a
308 condition is specified, it's evaluated each time the statement is reached: a
309 breakpoint is taken only if the condition is true. Breakpoints may only be set
310 on lines that begin an executable statement. Conditions don't use C<if>:
312 b lib/MyModule.pm:237 $x > 30
313 b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
315 =item b subname [condition]
317 X<debugger command, b>
319 Set a breakpoint before the first line of the named subroutine. I<subname> may
320 be a variable containing a code reference (in this case I<condition>
323 =item b postpone subname [condition]
325 X<debugger command, b>
327 Set a breakpoint at first line of subroutine after it is compiled.
329 =item b load filename
331 X<debugger command, b>
333 Set a breakpoint before the first executed line of the I<filename>,
334 which should be a full pathname found amongst the %INC values.
336 =item b compile subname
338 X<debugger command, b>
340 Sets a breakpoint before the first statement executed after the specified
341 subroutine is compiled.
345 X<debugger command, B>
347 Delete a breakpoint from the specified I<line>.
351 X<debugger command, B>
353 Delete all installed breakpoints.
355 =item a [line] command
356 X<debugger command, a>
358 Set an action to be done before the line is executed. If I<line> is
359 omitted, set an action on the line about to be executed.
360 The sequence of steps taken by the debugger is
362 1. check for a breakpoint at this line
363 2. print the line if necessary (tracing)
364 3. do any actions associated with that line
365 4. prompt user if at a breakpoint or in single-step
368 For example, this will print out $foo every time line
371 a 53 print "DB FOUND $foo\n"
374 X<debugger command, A>
376 Delete an action from the specified line.
379 X<debugger command, A>
381 Delete all installed actions.
384 X<debugger command, w>
386 Add a global watch-expression. Whenever a watched global changes the
387 debugger will stop and display the old and new values.
390 X<debugger command, W>
392 Delete watch-expression
395 X<debugger command, W>
397 Delete all watch-expressions.
400 X<debugger command, o>
404 =item o booloption ...
405 X<debugger command, o>
407 Set each listed Boolean option to the value C<1>.
409 =item o anyoption? ...
410 X<debugger command, o>
412 Print out the value of one or more options.
414 =item o option=value ...
415 X<debugger command, o>
417 Set the value of one or more options. If the value has internal
418 whitespace, it should be quoted. For example, you could set C<o
419 pager="less -MQeicsNfr"> to call B<less> with those specific options.
420 You may use either single or double quotes, but if you do, you must
421 escape any embedded instances of same sort of quote you began with,
422 as well as any escaping any escapes that immediately precede that
423 quote but which are not meant to escape the quote itself. In other
424 words, you follow single-quoting rules irrespective of the quote;
425 eg: C<o option='this isn\'t bad'> or C<o option="She said, \"Isn't
428 For historical reasons, the C<=value> is optional, but defaults to
429 1 only where it is safe to do so--that is, mostly for Boolean
430 options. It is always better to assign a specific value using C<=>.
431 The C<option> can be abbreviated, but for clarity probably should
432 not be. Several options can be set together. See L<"Configurable Options">
436 X<< debugger command, < >>
438 List out all pre-prompt Perl command actions.
441 X<< debugger command, < >>
443 Set an action (Perl command) to happen before every debugger prompt.
444 A multi-line command may be entered by backslashing the newlines.
447 X<< debugger command, < >>
449 Delete all pre-prompt Perl command actions.
452 X<< debugger command, << >>
454 Add an action (Perl command) to happen before every debugger prompt.
455 A multi-line command may be entered by backwhacking the newlines.
458 X<< debugger command, > >>
460 List out post-prompt Perl command actions.
463 X<< debugger command, > >>
465 Set an action (Perl command) to happen after the prompt when you've
466 just given a command to return to executing the script. A multi-line
467 command may be entered by backslashing the newlines (we bet you
468 couldn't have guessed this by now).
471 X<< debugger command, > >>
473 Delete all post-prompt Perl command actions.
476 X<<< debugger command, >> >>>
478 Adds an action (Perl command) to happen after the prompt when you've
479 just given a command to return to executing the script. A multi-line
480 command may be entered by backslashing the newlines.
483 X<debugger command, {>
485 List out pre-prompt debugger commands.
489 Set an action (debugger command) to happen before every debugger prompt.
490 A multi-line command may be entered in the customary fashion.
492 Because this command is in some senses new, a warning is issued if
493 you appear to have accidentally entered a block instead. If that's
494 what you mean to do, write it as with C<;{ ... }> or even
498 X<debugger command, {>
500 Delete all pre-prompt debugger commands.
503 X<debugger command, {{>
505 Add an action (debugger command) to happen before every debugger prompt.
506 A multi-line command may be entered, if you can guess how: see above.
509 X<debugger command, !>
511 Redo a previous command (defaults to the previous command).
514 X<debugger command, !>
516 Redo number'th previous command.
519 X<debugger command, !>
521 Redo last command that started with pattern.
522 See C<o recallCommand>, too.
525 X<debugger command, !!>
527 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
528 C<o shellBang>, also. Note that the user's current shell (well,
529 their C<$ENV{SHELL}> variable) will be used, which can interfere
530 with proper interpretation of exit status or signal and coredump
534 X<debugger command, source>
536 Read and execute debugger commands from I<file>.
537 I<file> may itself contain C<source> commands.
540 X<debugger command, H>
542 Display last n commands. Only commands longer than one character are
543 listed. If I<number> is omitted, list them all.
546 X<debugger command, q>
547 X<debugger command, ^D>
549 Quit. ("quit" doesn't work for this, unless you've made an alias)
550 This is the only supported way to exit the debugger, though typing
551 C<exit> twice might work.
553 Set the C<inhibit_exit> option to 0 if you want to be able to step
554 off the end the script. You may also need to set $finished to 0
555 if you want to step through global destruction.
558 X<debugger command, R>
560 Restart the debugger by C<exec()>ing a new session. We try to maintain
561 your history across this, but internal settings and command-line options
564 The following setting are currently preserved: history, breakpoints,
565 actions, debugger options, and the Perl command-line
566 options B<-w>, B<-I>, and B<-e>.
569 X<debugger command, |>
571 Run the debugger command, piping DB::OUT into your current pager.
574 X<debugger command, ||>
576 Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
578 =item = [alias value]
579 X<debugger command, =>
581 Define a command alias, like
585 or list current aliases.
589 Execute command as a Perl statement. A trailing semicolon will be
590 supplied. If the Perl statement would otherwise be confused for a
591 Perl debugger, use a leading semicolon, too.
594 X<debugger command, m>
596 List which methods may be called on the result of the evaluated
597 expression. The expression may evaluated to a reference to a
598 blessed object, or to a package name.
601 X<debugger command, M>
603 Display all loaded modules and their versions.
606 X<debugger command, man>
608 Despite its name, this calls your system's default documentation
609 viewer on the given page, or on the viewer itself if I<manpage> is
610 omitted. If that viewer is B<man>, the current C<Config> information
611 is used to invoke B<man> using the proper MANPATH or S<B<-M>
612 I<manpath>> option. Failed lookups of the form C<XXX> that match
613 known manpages of the form I<perlXXX> will be retried. This lets
614 you type C<man debug> or C<man op> from the debugger.
616 On systems traditionally bereft of a usable B<man> command, the
617 debugger invokes B<perldoc>. Occasionally this determination is
618 incorrect due to recalcitrant vendors or rather more felicitously,
619 to enterprising users. If you fall into either category, just
620 manually set the $DB::doccmd variable to whatever viewer to view
621 the Perl documentation on your system. This may be set in an rc
622 file, or through direct assignment. We're still waiting for a
623 working example of something along the lines of:
625 $DB::doccmd = 'netscape -remote http://something.here/';
629 =head2 Configurable Options
631 The debugger has numerous options settable using the C<o> command,
632 either interactively or from the environment or an rc file.
633 (./.perldb or ~/.perldb under Unix.)
638 =item C<recallCommand>, C<ShellBang>
639 X<debugger option, recallCommand>
640 X<debugger option, ShellBang>
642 The characters used to recall command or spawn shell. By
643 default, both are set to C<!>, which is unfortunate.
646 X<debugger option, pager>
648 Program to use for output of pager-piped commands (those beginning
649 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
650 Because the debugger uses your current terminal characteristics
651 for bold and underlining, if the chosen pager does not pass escape
652 sequences through unchanged, the output of some debugger commands
653 will not be readable when sent through the pager.
656 X<debugger option, tkRunning>
658 Run Tk while prompting (with ReadLine).
660 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
661 X<debugger option, signalLevel> X<debugger option, warnLevel>
662 X<debugger option, dieLevel>
664 Level of verbosity. By default, the debugger leaves your exceptions
665 and warnings alone, because altering them can break correctly running
666 programs. It will attempt to print a message when uncaught INT, BUS, or
667 SEGV signals arrive. (But see the mention of signals in L</BUGS> below.)
669 To disable this default safe mode, set these values to something higher
670 than 0. At a level of 1, you get backtraces upon receiving any kind
671 of warning (this is often annoying) or exception (this is
672 often valuable). Unfortunately, the debugger cannot discern fatal
673 exceptions from non-fatal ones. If C<dieLevel> is even 1, then your
674 non-fatal exceptions are also traced and unceremoniously altered if they
675 came from C<eval'ed> strings or from any kind of C<eval> within modules
676 you're attempting to load. If C<dieLevel> is 2, the debugger doesn't
677 care where they came from: It usurps your exception handler and prints
678 out a trace, then modifies all exceptions with its own embellishments.
679 This may perhaps be useful for some tracing purposes, but tends to hopelessly
680 destroy any program that takes its exception handling seriously.
683 X<debugger option, AutoTrace>
685 Trace mode (similar to C<t> command, but can be put into
689 X<debugger option, LineInfo>
691 File or pipe to print line number info to. If it is a pipe (say,
692 C<|visual_perl_db>), then a short message is used. This is the
693 mechanism used to interact with a slave editor or visual debugger,
694 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
697 =item C<inhibit_exit>
698 X<debugger option, inhibit_exit>
700 If 0, allows I<stepping off> the end of the script.
703 X<debugger option, PrintRet>
705 Print return value after C<r> command if set (default).
708 X<debugger option, ornaments>
710 Affects screen appearance of the command line (see L<Term::ReadLine>).
711 There is currently no way to disable these, which can render
712 some output illegible on some displays, or with some pagers.
713 This is considered a bug.
716 X<debugger option, frame>
718 Affects the printing of messages upon entry and exit from subroutines. If
719 C<frame & 2> is false, messages are printed on entry only. (Printing
720 on exit might be useful if interspersed with other messages.)
722 If C<frame & 4>, arguments to functions are printed, plus context
723 and caller info. If C<frame & 8>, overloaded C<stringify> and
724 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
725 & 16>, the return value from the subroutine is printed.
727 The length at which the argument list is truncated is governed by the
731 X<debugger option, maxTraceLen>
733 Length to truncate the argument list when the C<frame> option's
737 X<debugger option, windowSize>
739 Change the size of code list window (default is 10 lines).
743 The following options affect what happens with C<V>, C<X>, and C<x>
748 =item C<arrayDepth>, C<hashDepth>
749 X<debugger option, arrayDepth> X<debugger option, hashDepth>
751 Print only first N elements ('' for all).
754 X<debugger option, dumpDepth>
756 Limit recursion depth to N levels when dumping structures.
757 Negative values are interpreted as infinity. Default: infinity.
759 =item C<compactDump>, C<veryCompact>
760 X<debugger option, compactDump> X<debugger option, veryCompact>
762 Change the style of array and hash output. If C<compactDump>, short array
763 may be printed on one line.
766 X<debugger option, globPrint>
768 Whether to print contents of globs.
771 X<debugger option, DumpDBFiles>
773 Dump arrays holding debugged files.
775 =item C<DumpPackages>
776 X<debugger option, DumpPackages>
778 Dump symbol tables of packages.
781 X<debugger option, DumpReused>
783 Dump contents of "reused" addresses.
785 =item C<quote>, C<HighBit>, C<undefPrint>
786 X<debugger option, quote> X<debugger option, HighBit>
787 X<debugger option, undefPrint>
789 Change the style of string dump. The default value for C<quote>
790 is C<auto>; one can enable double-quotish or single-quotish format
791 by setting it to C<"> or C<'>, respectively. By default, characters
792 with their high bit set are printed verbatim.
795 X<debugger option, UsageOnly>
797 Rudimentary per-package memory usage dump. Calculates total
798 size of strings found in variables in the package. This does not
799 include lexicals in a module's file scope, or lost in closures.
803 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
804 environment variable and parses this as the remainder of a "O ..."
805 line as one might enter at the debugger prompt. You may place the
806 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
809 If your rc file contains:
811 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
813 then your script will run without human intervention, putting trace
814 information into the file I<db.out>. (If you interrupt it, you'd
815 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
820 X<debugger option, TTY>
822 The TTY to use for debugging I/O.
825 X<debugger option, noTTY>
827 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
828 interrupted (or if control goes to the debugger via explicit setting of
829 $DB::signal or $DB::single from the Perl script), it connects to a TTY
830 specified in the C<TTY> option at startup, or to a tty found at
831 runtime using the C<Term::Rendezvous> module of your choice.
833 This module should implement a method named C<new> that returns an object
834 with two methods: C<IN> and C<OUT>. These should return filehandles to use
835 for debugging input and output correspondingly. The C<new> method should
836 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
837 startup, or C<"$ENV{HOME}/.perldbtty$$"> otherwise. This file is not
838 inspected for proper ownership, so security hazards are theoretically
842 X<debugger option, ReadLine>
844 If false, readline support in the debugger is disabled in order
845 to debug applications that themselves use ReadLine.
848 X<debugger option, NonStop>
850 If set, the debugger goes into non-interactive mode until interrupted, or
851 programmatically by setting $DB::signal or $DB::single.
855 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
857 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
859 That will run the script B<myprogram> without human intervention,
860 printing out the call tree with entry and exit points. Note that
861 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
862 options could be uniquely abbreviated by the first letter (modulo
863 the C<Dump*> options). It is nevertheless recommended that you
864 always spell them out in full for legibility and future compatibility.
866 Other examples include
868 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
870 which runs script non-interactively, printing info on each entry
871 into a subroutine and each executed line into the file named F<listing>.
872 (If you interrupt it, you would better reset C<LineInfo> to something
875 Other examples include (using standard shell syntax to show environment
878 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
881 which may be useful for debugging a program that uses C<Term::ReadLine>
882 itself. Do not forget to detach your shell from the TTY in the window that
883 corresponds to F</dev/ttyXX>, say, by issuing a command like
887 See L<perldebguts/"Debugger Internals"> for details.
889 =head2 Debugger Input/Output
895 The debugger prompt is something like
903 where that number is the command number, and which you'd use to
904 access with the built-in B<csh>-like history mechanism. For example,
905 C<!17> would repeat command number 17. The depth of the angle
906 brackets indicates the nesting depth of the debugger. You could
907 get more than one set of brackets, for example, if you'd already
908 at a breakpoint and then printed the result of a function call that
909 itself has a breakpoint, or you step into an expression via C<s/n/t
912 =item Multiline commands
914 If you want to enter a multi-line command, such as a subroutine
915 definition with several statements or a format, escape the newline
916 that would normally end the debugger command with a backslash.
920 cont: print "ok\n"; \
927 Note that this business of escaping a newline is specific to interactive
928 commands typed into the debugger.
930 =item Stack backtrace
931 X<backtrace> X<stack, backtrace>
933 Here's an example of what a stack backtrace via C<T> command might
936 $ = main::infested called from file `Ambulation.pm' line 10
937 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
938 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
940 The left-hand character up there indicates the context in which the
941 function was called, with C<$> and C<@> meaning scalar or list
942 contexts respectively, and C<.> meaning void context (which is
943 actually a sort of scalar context). The display above says
944 that you were in the function C<main::infested> when you ran the
945 stack dump, and that it was called in scalar context from line
946 10 of the file I<Ambulation.pm>, but without any arguments at all,
947 meaning it was called as C<&infested>. The next stack frame shows
948 that the function C<Ambulation::legs> was called in list context
949 from the I<camel_flea> file with four arguments. The last stack
950 frame shows that C<main::pests> was called in scalar context,
951 also from I<camel_flea>, but from line 4.
953 If you execute the C<T> command from inside an active C<use>
954 statement, the backtrace will contain both a C<require> frame and
957 =item Line Listing Format
959 This shows the sorts of output the C<l> command can produce:
963 102:b @isa{@i,$pack} = ()
964 103 if(exists $i{$prevpack} || exists $isa{$pack});
968 107==> if(exists $isa{$pack});
970 109:a if ($extra-- > 0) {
971 110: %isa = ($pack,1);
973 Breakable lines are marked with C<:>. Lines with breakpoints are
974 marked by C<b> and those with actions by C<a>. The line that's
975 about to be executed is marked by C<< ==> >>.
977 Please be aware that code in debugger listings may not look the same
978 as your original source code. Line directives and external source
979 filters can alter the code before Perl sees it, causing code to move
980 from its original positions or take on entirely different forms.
984 When the C<frame> option is set, the debugger would print entered (and
985 optionally exited) subroutines in different styles. See L<perldebguts>
986 for incredibly long examples of these.
990 =head2 Debugging Compile-Time Statements
992 If you have compile-time executable statements (such as code within
993 BEGIN, UNITCHECK and CHECK blocks or C<use> statements), these will
994 I<not> be stopped by debugger, although C<require>s and INIT blocks
995 will, and compile-time statements can be traced with the C<AutoTrace>
996 option set in C<PERLDB_OPTS>). From your own Perl code, however, you
997 can transfer control back to the debugger using the following
998 statement, which is harmless if the debugger is not running:
1002 If you set C<$DB::single> to 2, it's equivalent to having
1003 just typed the C<n> command, whereas a value of 1 means the C<s>
1004 command. The C<$DB::trace> variable should be set to 1 to simulate
1005 having typed the C<t> command.
1007 Another way to debug compile-time code is to start the debugger, set a
1008 breakpoint on the I<load> of some module:
1010 DB<7> b load f:/perllib/lib/Carp.pm
1011 Will stop on load of `f:/perllib/lib/Carp.pm'.
1013 and then restart the debugger using the C<R> command (if possible). One can use C<b
1014 compile subname> for the same purpose.
1016 =head2 Debugger Customization
1018 The debugger probably contains enough configuration hooks that you
1019 won't ever have to modify it yourself. You may change the behaviour
1020 of the debugger from within the debugger using its C<o> command, from
1021 the command line via the C<PERLDB_OPTS> environment variable, and
1022 from customization files.
1024 You can do some customization by setting up a F<.perldb> file, which
1025 contains initialization code. For instance, you could make aliases
1026 like these (the last one is one people expect to be there):
1028 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
1029 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
1030 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
1031 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
1033 You can change options from F<.perldb> by using calls like this one;
1035 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
1037 The code is executed in the package C<DB>. Note that F<.perldb> is
1038 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
1039 subroutine C<afterinit>, that function is called after debugger
1040 initialization ends. F<.perldb> may be contained in the current
1041 directory, or in the home directory. Because this file is sourced
1042 in by Perl and may contain arbitrary commands, for security reasons,
1043 it must be owned by the superuser or the current user, and writable
1044 by no one but its owner.
1046 You can mock TTY input to debugger by adding arbitrary commands to
1047 @DB::typeahead. For example, your F<.perldb> file might contain:
1049 sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
1051 Which would attempt to set breakpoints on lines 4 and 6 immediately
1052 after debugger initialization. Note that @DB::typeahead is not a supported
1053 interface and is subject to change in future releases.
1055 If you want to modify the debugger, copy F<perl5db.pl> from the
1056 Perl library to another name and hack it to your heart's content.
1057 You'll then want to set your C<PERL5DB> environment variable to say
1058 something like this:
1060 BEGIN { require "myperl5db.pl" }
1062 As a last resort, you could also use C<PERL5DB> to customize the debugger
1063 by directly setting internal variables or calling debugger functions.
1065 Note that any variables and functions that are not documented in
1066 this document (or in L<perldebguts>) are considered for internal
1067 use only, and as such are subject to change without notice.
1069 =head2 Readline Support / History in the Debugger
1071 As shipped, the only command-line history supplied is a simplistic one
1072 that checks for leading exclamation points. However, if you install
1073 the Term::ReadKey and Term::ReadLine modules from CPAN (such as
1074 Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will
1075 have full editing capabilities much like those GNU I<readline>(3) provides.
1076 Look for these in the F<modules/by-module/Term> directory on CPAN.
1077 These do not support normal B<vi> command-line editing, however.
1079 A rudimentary command-line completion is also available, including
1080 lexical variables in the current scope if the C<PadWalker> module
1083 Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B",
1084 "^[[D"", "^H", ... when using the arrow keys and/or the backspace key.
1086 =head2 Editor Support for Debugging
1088 If you have the FSF's version of B<emacs> installed on your system,
1089 it can interact with the Perl debugger to provide an integrated
1090 software development environment reminiscent of its interactions
1093 Recent versions of Emacs come with a
1094 start file for making B<emacs> act like a
1095 syntax-directed editor that understands (some of) Perl's syntax.
1098 A similar setup by Tom Christiansen for interacting with any
1099 vendor-shipped B<vi> and the X11 window system is also available.
1100 This works similarly to the integrated multiwindow support that
1101 B<emacs> provides, where the debugger drives the editor. At the
1102 time of this writing, however, that tool's eventual location in the
1103 Perl distribution was uncertain.
1105 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
1106 and windy version, for coloring of Perl keywords.
1108 Note that only perl can truly parse Perl, so all such CASE tools
1109 fall somewhat short of the mark, especially if you don't program
1110 your Perl as a C programmer might.
1112 =head2 The Perl Profiler
1113 X<profile> X<profiling> X<profiler>
1115 If you wish to supply an alternative debugger for Perl to run,
1116 invoke your script with a colon and a package argument given to the
1117 B<-d> flag. Perl's alternative debuggers include a Perl profiler,
1118 L<Devel::NYTProf>, which is available separately as a CPAN
1119 distribution. To profile your Perl program in the file F<mycode.pl>,
1122 $ perl -d:NYTProf mycode.pl
1124 When the script terminates the profiler will create a database of the
1125 profile information that you can turn into reports using the profiler's
1126 tools. See <perlperf> for details.
1128 =head1 Debugging Regular Expressions
1129 X<regular expression, debugging>
1130 X<regex, debugging> X<regexp, debugging>
1132 C<use re 'debug'> enables you to see the gory details of how the Perl
1133 regular expression engine works. In order to understand this typically
1134 voluminous output, one must not only have some idea about how regular
1135 expression matching works in general, but also know how Perl's regular
1136 expressions are internally compiled into an automaton. These matters
1137 are explored in some detail in
1138 L<perldebguts/"Debugging Regular Expressions">.
1140 =head1 Debugging Memory Usage
1143 Perl contains internal support for reporting its own memory usage,
1144 but this is a fairly advanced concept that requires some understanding
1145 of how memory allocation works.
1146 See L<perldebguts/"Debugging Perl Memory Usage"> for the details.
1150 You did try the B<-w> switch, didn't you?
1161 When debugging a script that uses #! and is thus normally found in
1162 $PATH, the -S option causes perl to search $PATH for it, so you don't
1163 have to type the path or C<which $scriptname>.
1169 You cannot get stack frame information or in any fashion debug functions
1170 that were not compiled by Perl, such as those from C or C++ extensions.
1172 If you alter your @_ arguments in a subroutine (such as with C<shift>
1173 or C<pop>), the stack backtrace will not show the original values.
1175 The debugger does not currently work in conjunction with the B<-W>
1176 command-line switch, because it itself is not free of warnings.
1178 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
1179 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
1180 handler, then you won't be able to CTRL-C your way back to the debugger,
1181 because the debugger's own C<$SIG{INT}> handler doesn't understand that
1182 it needs to raise an exception to longjmp(3) out of slow syscalls.