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 $DB::doccmd = 'netscape -remote http://something.here/';
673 =head2 Configurable Options
675 The debugger has numerous options settable using the C<o> command,
676 either interactively or from the environment or an rc file.
677 (./.perldb or ~/.perldb under Unix.)
682 =item C<recallCommand>, C<ShellBang>
683 X<debugger option, recallCommand>
684 X<debugger option, ShellBang>
686 The characters used to recall command or spawn shell. By
687 default, both are set to C<!>, which is unfortunate.
690 X<debugger option, pager>
692 Program to use for output of pager-piped commands (those beginning
693 with a C<|> character.) By default, C<$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.
847 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
848 environment variable and parses this as the remainder of a "O ..."
849 line as one might enter at the debugger prompt. You may place the
850 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
853 If your rc file contains:
855 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
857 then your script will run without human intervention, putting trace
858 information into the file I<db.out>. (If you interrupt it, you'd
859 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
864 X<debugger option, TTY>
866 The TTY to use for debugging I/O.
869 X<debugger option, noTTY>
871 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
872 interrupted (or if control goes to the debugger via explicit setting of
873 $DB::signal or $DB::single from the Perl script), it connects to a TTY
874 specified in the C<TTY> option at startup, or to a tty found at
875 runtime using the C<Term::Rendezvous> module of your choice.
877 This module should implement a method named C<new> that returns an object
878 with two methods: C<IN> and C<OUT>. These should return filehandles to use
879 for debugging input and output correspondingly. The C<new> method should
880 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
881 startup, or C<"$ENV{HOME}/.perldbtty$$"> otherwise. This file is not
882 inspected for proper ownership, so security hazards are theoretically
886 X<debugger option, ReadLine>
888 If false, readline support in the debugger is disabled in order
889 to debug applications that themselves use ReadLine.
892 X<debugger option, NonStop>
894 If set, the debugger goes into non-interactive mode until interrupted, or
895 programmatically by setting $DB::signal or $DB::single.
899 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
901 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
903 That will run the script B<myprogram> without human intervention,
904 printing out the call tree with entry and exit points. Note that
905 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
906 options could be uniquely abbreviated by the first letter (modulo
907 the C<Dump*> options). It is nevertheless recommended that you
908 always spell them out in full for legibility and future compatibility.
910 Other examples include
912 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
914 which runs script non-interactively, printing info on each entry
915 into a subroutine and each executed line into the file named F<listing>.
916 (If you interrupt it, you would better reset C<LineInfo> to something
919 Other examples include (using standard shell syntax to show environment
922 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
925 which may be useful for debugging a program that uses C<Term::ReadLine>
926 itself. Do not forget to detach your shell from the TTY in the window that
927 corresponds to F</dev/ttyXX>, say, by issuing a command like
931 See L<perldebguts/"Debugger Internals"> for details.
933 =head2 Debugger Input/Output
939 The debugger prompt is something like
947 where that number is the command number, and which you'd use to
948 access with the built-in B<csh>-like history mechanism. For example,
949 C<!17> would repeat command number 17. The depth of the angle
950 brackets indicates the nesting depth of the debugger. You could
951 get more than one set of brackets, for example, if you'd already
952 at a breakpoint and then printed the result of a function call that
953 itself has a breakpoint, or you step into an expression via C<s/n/t
956 =item Multiline commands
958 If you want to enter a multi-line command, such as a subroutine
959 definition with several statements or a format, escape the newline
960 that would normally end the debugger command with a backslash.
964 cont: print "ok\n"; \
971 Note that this business of escaping a newline is specific to interactive
972 commands typed into the debugger.
974 =item Stack backtrace
975 X<backtrace> X<stack, backtrace>
977 Here's an example of what a stack backtrace via C<T> command might
980 $ = main::infested called from file 'Ambulation.pm' line 10
981 @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea' line 7
982 $ = main::pests('bactrian', 4) called from file 'camel_flea' line 4
984 The left-hand character up there indicates the context in which the
985 function was called, with C<$> and C<@> meaning scalar or list
986 contexts respectively, and C<.> meaning void context (which is
987 actually a sort of scalar context). The display above says
988 that you were in the function C<main::infested> when you ran the
989 stack dump, and that it was called in scalar context from line
990 10 of the file I<Ambulation.pm>, but without any arguments at all,
991 meaning it was called as C<&infested>. The next stack frame shows
992 that the function C<Ambulation::legs> was called in list context
993 from the I<camel_flea> file with four arguments. The last stack
994 frame shows that C<main::pests> was called in scalar context,
995 also from I<camel_flea>, but from line 4.
997 If you execute the C<T> command from inside an active C<use>
998 statement, the backtrace will contain both a C<require> frame and
1001 =item Line Listing Format
1003 This shows the sorts of output the C<l> command can produce:
1007 102:b @isa{@i,$pack} = ()
1008 103 if(exists $i{$prevpack} || exists $isa{$pack});
1012 107==> if(exists $isa{$pack});
1014 109:a if ($extra-- > 0) {
1015 110: %isa = ($pack,1);
1017 Breakable lines are marked with C<:>. Lines with breakpoints are
1018 marked by C<b> and those with actions by C<a>. The line that's
1019 about to be executed is marked by C<< ==> >>.
1021 Please be aware that code in debugger listings may not look the same
1022 as your original source code. Line directives and external source
1023 filters can alter the code before Perl sees it, causing code to move
1024 from its original positions or take on entirely different forms.
1028 When the C<frame> option is set, the debugger would print entered (and
1029 optionally exited) subroutines in different styles. See L<perldebguts>
1030 for incredibly long examples of these.
1034 =head2 Debugging Compile-Time Statements
1036 If you have compile-time executable statements (such as code within
1037 BEGIN, UNITCHECK and CHECK blocks or C<use> statements), these will
1038 I<not> be stopped by debugger, although C<require>s and INIT blocks
1039 will, and compile-time statements can be traced with the C<AutoTrace>
1040 option set in C<PERLDB_OPTS>). From your own Perl code, however, you
1041 can transfer control back to the debugger using the following
1042 statement, which is harmless if the debugger is not running:
1046 If you set C<$DB::single> to 2, it's equivalent to having
1047 just typed the C<n> command, whereas a value of 1 means the C<s>
1048 command. The C<$DB::trace> variable should be set to 1 to simulate
1049 having typed the C<t> command.
1051 Another way to debug compile-time code is to start the debugger, set a
1052 breakpoint on the I<load> of some module:
1054 DB<7> b load f:/perllib/lib/Carp.pm
1055 Will stop on load of 'f:/perllib/lib/Carp.pm'.
1057 and then restart the debugger using the C<R> command (if possible). One can use C<b
1058 compile subname> for the same purpose.
1060 =head2 Debugger Customization
1062 The debugger probably contains enough configuration hooks that you
1063 won't ever have to modify it yourself. You may change the behaviour
1064 of the debugger from within the debugger using its C<o> command, from
1065 the command line via the C<PERLDB_OPTS> environment variable, and
1066 from customization files.
1068 You can do some customization by setting up a F<.perldb> file, which
1069 contains initialization code. For instance, you could make aliases
1070 like these (the last one is one people expect to be there):
1072 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
1073 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
1074 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
1075 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
1077 You can change options from F<.perldb> by using calls like this one;
1079 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
1081 The code is executed in the package C<DB>. Note that F<.perldb> is
1082 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
1083 subroutine C<afterinit>, that function is called after debugger
1084 initialization ends. F<.perldb> may be contained in the current
1085 directory, or in the home directory. Because this file is sourced
1086 in by Perl and may contain arbitrary commands, for security reasons,
1087 it must be owned by the superuser or the current user, and writable
1088 by no one but its owner.
1090 You can mock TTY input to debugger by adding arbitrary commands to
1091 @DB::typeahead. For example, your F<.perldb> file might contain:
1093 sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
1095 Which would attempt to set breakpoints on lines 4 and 6 immediately
1096 after debugger initialization. Note that @DB::typeahead is not a supported
1097 interface and is subject to change in future releases.
1099 If you want to modify the debugger, copy F<perl5db.pl> from the
1100 Perl library to another name and hack it to your heart's content.
1101 You'll then want to set your C<PERL5DB> environment variable to say
1102 something like this:
1104 BEGIN { require "myperl5db.pl" }
1106 As a last resort, you could also use C<PERL5DB> to customize the debugger
1107 by directly setting internal variables or calling debugger functions.
1109 Note that any variables and functions that are not documented in
1110 this document (or in L<perldebguts>) are considered for internal
1111 use only, and as such are subject to change without notice.
1113 =head2 Readline Support / History in the Debugger
1115 As shipped, the only command-line history supplied is a simplistic one
1116 that checks for leading exclamation points. However, if you install
1117 the Term::ReadKey and Term::ReadLine modules from CPAN (such as
1118 Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will
1119 have full editing capabilities much like those GNU I<readline>(3) provides.
1120 Look for these in the F<modules/by-module/Term> directory on CPAN.
1121 These do not support normal B<vi> command-line editing, however.
1123 A rudimentary command-line completion is also available, including
1124 lexical variables in the current scope if the C<PadWalker> module
1127 Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B",
1128 "^[[D"", "^H", ... when using the arrow keys and/or the backspace key.
1130 =head2 Editor Support for Debugging
1132 If you have the FSF's version of B<emacs> installed on your system,
1133 it can interact with the Perl debugger to provide an integrated
1134 software development environment reminiscent of its interactions
1137 Recent versions of Emacs come with a
1138 start file for making B<emacs> act like a
1139 syntax-directed editor that understands (some of) Perl's syntax.
1142 A similar setup by Tom Christiansen for interacting with any
1143 vendor-shipped B<vi> and the X11 window system is also available.
1144 This works similarly to the integrated multiwindow support that
1145 B<emacs> provides, where the debugger drives the editor. At the
1146 time of this writing, however, that tool's eventual location in the
1147 Perl distribution was uncertain.
1149 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
1150 and windy version, for coloring of Perl keywords.
1152 Note that only perl can truly parse Perl, so all such CASE tools
1153 fall somewhat short of the mark, especially if you don't program
1154 your Perl as a C programmer might.
1156 =head2 The Perl Profiler
1157 X<profile> X<profiling> X<profiler>
1159 If you wish to supply an alternative debugger for Perl to run,
1160 invoke your script with a colon and a package argument given to the
1161 B<-d> flag. Perl's alternative debuggers include a Perl profiler,
1162 L<Devel::NYTProf>, which is available separately as a CPAN
1163 distribution. To profile your Perl program in the file F<mycode.pl>,
1166 $ perl -d:NYTProf mycode.pl
1168 When the script terminates the profiler will create a database of the
1169 profile information that you can turn into reports using the profiler's
1170 tools. See <perlperf> for details.
1172 =head1 Debugging Regular Expressions
1173 X<regular expression, debugging>
1174 X<regex, debugging> X<regexp, debugging>
1176 C<use re 'debug'> enables you to see the gory details of how the Perl
1177 regular expression engine works. In order to understand this typically
1178 voluminous output, one must not only have some idea about how regular
1179 expression matching works in general, but also know how Perl's regular
1180 expressions are internally compiled into an automaton. These matters
1181 are explored in some detail in
1182 L<perldebguts/"Debugging Regular Expressions">.
1184 =head1 Debugging Memory Usage
1187 Perl contains internal support for reporting its own memory usage,
1188 but this is a fairly advanced concept that requires some understanding
1189 of how memory allocation works.
1190 See L<perldebguts/"Debugging Perl Memory Usage"> for the details.
1194 You did try the B<-w> switch, didn't you?
1205 When debugging a script that uses #! and is thus normally found in
1206 $PATH, the -S option causes perl to search $PATH for it, so you don't
1207 have to type the path or C<which $scriptname>.
1213 You cannot get stack frame information or in any fashion debug functions
1214 that were not compiled by Perl, such as those from C or C++ extensions.
1216 If you alter your @_ arguments in a subroutine (such as with C<shift>
1217 or C<pop>), the stack backtrace will not show the original values.
1219 The debugger does not currently work in conjunction with the B<-W>
1220 command-line switch, because it itself is not free of warnings.
1222 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
1223 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
1224 handler, then you won't be able to CTRL-C your way back to the debugger,
1225 because the debugger's own C<$SIG{INT}> handler doesn't understand that
1226 it needs to raise an exception to longjmp(3) out of slow syscalls.