4 perldebug - Perl debugging
8 First of all, have you tried using L<C<use strict;>|strict> and
9 L<C<use warnings;>|warnings>?
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 If you're looking for the nitty gritty details of how the debugger is
15 I<implemented>, you may prefer to read L<perldebguts>.
17 For in-depth technical usage details, see L<perl5db.pl>, the documentation
18 of the debugger itself.
20 =head1 The Perl Debugger
22 If you invoke Perl with the B<-d> switch, your script runs under the
23 Perl source debugger. This works like an interactive Perl
24 environment, prompting for debugger commands that let you examine
25 source code, set breakpoints, get stack backtraces, change the values of
26 variables, etc. This is so convenient that you often fire up
27 the debugger all by itself just to test out Perl constructs
28 interactively to see what they do. For example:
33 In Perl, the debugger is not a separate program the way it usually is in the
34 typical compiled environment. Instead, the B<-d> flag tells the compiler
35 to insert source information into the parse trees it's about to hand off
36 to the interpreter. That means your code must first compile correctly
37 for the debugger to work on it. Then when the interpreter starts up, it
38 preloads a special Perl library file containing the debugger.
40 The program will halt I<right before> the first run-time executable
41 statement (but see below regarding compile-time statements) and ask you
42 to enter a debugger command. Contrary to popular expectations, whenever
43 the debugger halts and shows you a line of code, it always displays the
44 line it's I<about> to execute, rather than the one it has just executed.
46 Any command not recognized by the debugger is directly executed
47 (C<eval>'d) as Perl code in the current package. (The debugger
48 uses the DB package for keeping its own state information.)
50 Note that the said C<eval> is bound by an implicit scope. As a
51 result any newly introduced lexical variable or any modified
52 capture buffer content is lost after the eval. The debugger is a
53 nice environment to learn Perl, but if you interactively experiment using
54 material which should be in the same scope, stuff it in one line.
56 For any text entered at the debugger prompt, leading and trailing whitespace
57 is first stripped before further processing. If a debugger command
58 coincides with some function in your own program, merely precede the
59 function with something that doesn't look like a debugger command, such
60 as a leading C<;> or perhaps a C<+>, or by wrapping it with parentheses
63 =head2 Calling the Debugger
65 There are several ways to call the debugger:
69 =item perl -d program_name
71 On the given program identified by C<program_name>.
75 Interactively supply an arbitrary C<expression> using C<-e>.
77 =item perl -d:ptkdb program_name
79 Debug a given program via the C<Devel::ptkdb> GUI.
81 =item perl -dt threaded_program_name
83 Debug a given program using threads (experimental).
87 If Perl is called with the C<-d> switch, the variable C<$^P> will hold a true
88 value. This is useful if you need to know if your code is running under the
92 # running under the debugger
95 See L<perlvar/$^P> for more information on the variable.
97 =head2 Debugger Commands
99 The interactive debugger understands the following commands:
104 X<debugger command, h>
106 Prints out a summary help message
110 Prints out a help message for the given debugger command.
114 The special argument of C<h h> produces the entire help page, which is quite long.
116 If the output of the C<h h> command (or any command, for that matter) scrolls
117 past your screen, precede the command with a leading pipe symbol so
118 that it's run through your pager, as in
122 You may change the pager which is used via C<o pager=...> command.
125 X<debugger command, p>
127 Same as C<print {$DB::OUT} expr> in the current package. In particular,
128 because this is just Perl's own C<print> function, this means that nested
129 data structures and objects are not dumped, unlike with the C<x> command.
131 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
132 where STDOUT may be redirected to.
134 =item x [maxdepth] expr
135 X<debugger command, x>
137 Evaluates its expression in list context and dumps out the result in a
138 pretty-printed fashion. Nested data structures are printed out
139 recursively, unlike the real C<print> function in Perl. When dumping
140 hashes, you'll probably prefer 'x \%h' rather than 'x %h'.
141 See L<Dumpvalue> if you'd like to do this yourself.
143 The output format is governed by multiple options described under
144 L</"Configurable Options">.
146 If the C<maxdepth> is included, it must be a numeral I<N>; the value is
147 dumped only I<N> levels deep, as if the C<dumpDepth> option had been
148 temporarily set to I<N>.
151 X<debugger command, V>
153 Display all (or some) variables in package (defaulting to C<main>)
154 using a data pretty-printer (hashes show their keys and values so
155 you see what's what, control characters are made printable, etc.).
156 Make sure you don't put the type specifier (like C<$>) there, just
157 the symbol names, like this:
161 Use C<~pattern> and C<!pattern> for positive and negative regexes.
163 This is similar to calling the C<x> command on each applicable var.
166 X<debugger command, X>
168 Same as C<V currentpackage [vars]>.
170 =item y [level [vars]]
171 X<debugger command, y>
173 Display all (or some) lexical variables (mnemonic: C<mY> variables)
174 in the current scope or I<level> scopes higher. You can limit the
175 variables that you see with I<vars> which works exactly as it does
176 for the C<V> and C<X> commands. Requires the C<PadWalker> module
177 version 0.08 or higher; will warn if this isn't installed. Output
178 is pretty-printed in the same style as for C<V> and the format is
179 controlled by the same options.
182 X<debugger command, T> X<backtrace> X<stack, backtrace>
184 Produce a stack backtrace. See below for details on its output.
187 X<debugger command, s> X<step>
189 Single step. Executes until the beginning of another
190 statement, descending into subroutine calls. If an expression is
191 supplied that includes function calls, it too will be single-stepped.
194 X<debugger command, n>
196 Next. Executes over subroutine calls, until the beginning
197 of the next statement. If an expression is supplied that includes
198 function calls, those functions will be executed with stops before
202 X<debugger command, r>
204 Continue until the return from the current subroutine.
205 Dump the return value if the C<PrintRet> option is set (default).
209 Repeat last C<n> or C<s> command.
212 X<debugger command, c>
214 Continue, optionally inserting a one-time-only breakpoint
215 at the specified line or subroutine.
218 X<debugger command, l>
220 List next window of lines.
224 List C<incr+1> lines starting at C<min>.
228 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
236 List first window of lines from subroutine. I<subname> may
237 be a variable that contains a code reference.
240 X<debugger command, ->
242 List previous window of lines.
245 X<debugger command, v>
247 View a few lines of code around the current line.
250 X<debugger command, .>
252 Return the internal debugger pointer to the line last
253 executed, and print out that line.
256 X<debugger command, f>
258 Switch to viewing a different file or C<eval> statement. If I<filename>
259 is not a full pathname found in the values of %INC, it is considered
262 C<eval>ed strings (when accessible) are considered to be filenames:
263 C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
264 (in the order of execution). The bodies of the currently executed C<eval>
265 and of C<eval>ed strings that define subroutines are saved and thus
270 Search forwards for pattern (a Perl regex); final / is optional.
271 The search is case-insensitive by default.
275 Search backwards for pattern; final ? is optional.
276 The search is case-insensitive by default.
279 X<debugger command, L>
281 List (default all) actions, breakpoints and watch expressions
284 X<debugger command, S>
286 List subroutine names [not] matching the regex.
289 X<debugger command, t>
291 Toggle trace mode (see also the C<AutoTrace> option).
292 Optional argument is the maximum number of levels to trace below
293 the current one; anything deeper than that will be silent.
296 X<debugger command, t>
298 Trace through execution of C<expr>.
299 Optional first argument is the maximum number of levels to trace below
300 the current one; anything deeper than that will be silent.
301 See L<perldebguts/"Frame Listing Output Examples"> for examples.
305 X<debugger command, b>
307 Sets breakpoint on current line
309 =item b [line] [condition]
311 X<debugger command, b>
313 Set a breakpoint before the given line. If a condition
314 is specified, it's evaluated each time the statement is reached: a
315 breakpoint is taken only if the condition is true. Breakpoints may
316 only be set on lines that begin an executable statement. Conditions
320 b 237 ++$count237 < 11
323 If the line number is C<.>, sets a breakpoint on the current line:
327 =item b [file]:[line] [condition]
329 X<debugger command, b>
331 Set a breakpoint before the given line in a (possibly different) file. If a
332 condition is specified, it's evaluated each time the statement is reached: a
333 breakpoint is taken only if the condition is true. Breakpoints may only be set
334 on lines that begin an executable statement. Conditions don't use C<if>:
336 b lib/MyModule.pm:237 $x > 30
337 b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
339 =item b subname [condition]
341 X<debugger command, b>
343 Set a breakpoint before the first line of the named subroutine. I<subname> may
344 be a variable containing a code reference (in this case I<condition>
347 =item b postpone subname [condition]
349 X<debugger command, b>
351 Set a breakpoint at first line of subroutine after it is compiled.
353 =item b load filename
355 X<debugger command, b>
357 Set a breakpoint before the first executed line of the I<filename>,
358 which should be a full pathname found amongst the %INC values.
360 =item b compile subname
362 X<debugger command, b>
364 Sets a breakpoint before the first statement executed after the specified
365 subroutine is compiled.
369 X<debugger command, B>
371 Delete a breakpoint from the specified I<line>.
375 X<debugger command, B>
377 Delete all installed breakpoints.
379 =item disable [file]:[line]
381 X<debugger command, disable>
384 Disable the breakpoint so it won't stop the execution of the program.
385 Breakpoints are enabled by default and can be re-enabled using the C<enable>
390 X<debugger command, disable>
393 Disable the breakpoint so it won't stop the execution of the program.
394 Breakpoints are enabled by default and can be re-enabled using the C<enable>
397 This is done for a breakpoint in the current file.
399 =item enable [file]:[line]
401 X<debugger command, disable>
404 Enable the breakpoint so it will stop the execution of the program.
408 X<debugger command, disable>
411 Enable the breakpoint so it will stop the execution of the program.
413 This is done for a breakpoint in the current file.
415 =item a [line] command
416 X<debugger command, a>
418 Set an action to be done before the line is executed. If I<line> is
419 omitted, set an action on the line about to be executed.
420 The sequence of steps taken by the debugger is
422 1. check for a breakpoint at this line
423 2. print the line if necessary (tracing)
424 3. do any actions associated with that line
425 4. prompt user if at a breakpoint or in single-step
428 For example, this will print out $foo every time line
431 a 53 print "DB FOUND $foo\n"
434 X<debugger command, A>
436 Delete an action from the specified line.
439 X<debugger command, A>
441 Delete all installed actions.
444 X<debugger command, w>
446 Add a global watch-expression. Whenever a watched global changes the
447 debugger will stop and display the old and new values.
450 X<debugger command, W>
452 Delete watch-expression
455 X<debugger command, W>
457 Delete all watch-expressions.
460 X<debugger command, o>
464 =item o booloption ...
465 X<debugger command, o>
467 Set each listed Boolean option to the value C<1>.
469 =item o anyoption? ...
470 X<debugger command, o>
472 Print out the value of one or more options.
474 =item o option=value ...
475 X<debugger command, o>
477 Set the value of one or more options. If the value has internal
478 whitespace, it should be quoted. For example, you could set C<o
479 pager="less -MQeicsNfr"> to call B<less> with those specific options.
480 You may use either single or double quotes, but if you do, you must
481 escape any embedded instances of same sort of quote you began with,
482 as well as any escaping any escapes that immediately precede that
483 quote but which are not meant to escape the quote itself. In other
484 words, you follow single-quoting rules irrespective of the quote;
485 eg: C<o option='this isn\'t bad'> or C<o option="She said, \"Isn't
488 For historical reasons, the C<=value> is optional, but defaults to
489 1 only where it is safe to do so--that is, mostly for Boolean
490 options. It is always better to assign a specific value using C<=>.
491 The C<option> can be abbreviated, but for clarity probably should
492 not be. Several options can be set together. See L</"Configurable Options">
496 X<< debugger command, < >>
498 List out all pre-prompt Perl command actions.
501 X<< debugger command, < >>
503 Set an action (Perl command) to happen before every debugger prompt.
504 A multi-line command may be entered by backslashing the newlines.
507 X<< debugger command, < >>
509 Delete all pre-prompt Perl command actions.
512 X<< debugger command, << >>
514 Add an action (Perl command) to happen before every debugger prompt.
515 A multi-line command may be entered by backwhacking the newlines.
518 X<< debugger command, > >>
520 List out post-prompt Perl command actions.
523 X<< debugger command, > >>
525 Set an action (Perl command) to happen after the prompt when you've
526 just given a command to return to executing the script. A multi-line
527 command may be entered by backslashing the newlines (we bet you
528 couldn't have guessed this by now).
531 X<< debugger command, > >>
533 Delete all post-prompt Perl command actions.
536 X<<< debugger command, >> >>>
538 Adds an action (Perl command) to happen after the prompt when you've
539 just given a command to return to executing the script. A multi-line
540 command may be entered by backslashing the newlines.
543 X<debugger command, {>
545 List out pre-prompt debugger commands.
549 Set an action (debugger command) to happen before every debugger prompt.
550 A multi-line command may be entered in the customary fashion.
552 Because this command is in some senses new, a warning is issued if
553 you appear to have accidentally entered a block instead. If that's
554 what you mean to do, write it as with C<;{ ... }> or even
558 X<debugger command, {>
560 Delete all pre-prompt debugger commands.
563 X<debugger command, {{>
565 Add an action (debugger command) to happen before every debugger prompt.
566 A multi-line command may be entered, if you can guess how: see above.
569 X<debugger command, !>
571 Redo a previous command (defaults to the previous command).
574 X<debugger command, !>
576 Redo number'th previous command.
579 X<debugger command, !>
581 Redo last command that started with pattern.
582 See C<o recallCommand>, too.
585 X<debugger command, !!>
587 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
588 C<o shellBang>, also. Note that the user's current shell (well,
589 their C<$ENV{SHELL}> variable) will be used, which can interfere
590 with proper interpretation of exit status or signal and coredump
594 X<debugger command, source>
596 Read and execute debugger commands from I<file>.
597 I<file> may itself contain C<source> commands.
600 X<debugger command, H>
602 Display last n commands. Only commands longer than one character are
603 listed. If I<number> is omitted, list them all.
606 X<debugger command, q>
607 X<debugger command, ^D>
609 Quit. ("quit" doesn't work for this, unless you've made an alias)
610 This is the only supported way to exit the debugger, though typing
611 C<exit> twice might work.
613 Set the C<inhibit_exit> option to 0 if you want to be able to step
614 off the end the script. You may also need to set $finished to 0
615 if you want to step through global destruction.
618 X<debugger command, R>
620 Restart the debugger by C<exec()>ing a new session. We try to maintain
621 your history across this, but internal settings and command-line options
624 The following setting are currently preserved: history, breakpoints,
625 actions, debugger options, and the Perl command-line
626 options B<-w>, B<-I>, and B<-e>.
629 X<debugger command, |>
631 Run the debugger command, piping DB::OUT into your current pager.
634 X<debugger command, ||>
636 Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
638 =item = [alias value]
639 X<debugger command, =>
641 Define a command alias, like
645 or list current aliases.
649 Execute command as a Perl statement. A trailing semicolon will be
650 supplied. If the Perl statement would otherwise be confused for a
651 Perl debugger, use a leading semicolon, too.
654 X<debugger command, m>
656 List which methods may be called on the result of the evaluated
657 expression. The expression may evaluated to a reference to a
658 blessed object, or to a package name.
661 X<debugger command, M>
663 Display all loaded modules and their versions.
666 X<debugger command, man>
668 Despite its name, this calls your system's default documentation
669 viewer on the given page, or on the viewer itself if I<manpage> is
670 omitted. If that viewer is B<man>, the current C<Config> information
671 is used to invoke B<man> using the proper MANPATH or S<B<-M>
672 I<manpath>> option. Failed lookups of the form C<XXX> that match
673 known manpages of the form I<perlXXX> will be retried. This lets
674 you type C<man debug> or C<man op> from the debugger.
676 On systems traditionally bereft of a usable B<man> command, the
677 debugger invokes B<perldoc>. Occasionally this determination is
678 incorrect due to recalcitrant vendors or rather more felicitously,
679 to enterprising users. If you fall into either category, just
680 manually set the $DB::doccmd variable to whatever viewer to view
681 the Perl documentation on your system. This may be set in an rc
682 file, or through direct assignment. We're still waiting for a
683 working example of something along the lines of:
685 $DB::doccmd = 'netscape -remote http://something.here/';
689 =head2 Configurable Options
691 The debugger has numerous options settable using the C<o> command,
692 either interactively or from the environment or an rc file. The file
693 is named F<./.perldb> or F<~/.perldb> under Unix with F</dev/tty>,
694 F<perldb.ini> otherwise.
698 =item C<recallCommand>, C<ShellBang>
699 X<debugger option, recallCommand>
700 X<debugger option, ShellBang>
702 The characters used to recall a command or spawn a shell. By
703 default, both are set to C<!>, which is unfortunate.
706 X<debugger option, pager>
708 Program to use for output of pager-piped commands (those beginning
709 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
710 Because the debugger uses your current terminal characteristics
711 for bold and underlining, if the chosen pager does not pass escape
712 sequences through unchanged, the output of some debugger commands
713 will not be readable when sent through the pager.
716 X<debugger option, tkRunning>
718 Run Tk while prompting (with ReadLine).
720 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
721 X<debugger option, signalLevel> X<debugger option, warnLevel>
722 X<debugger option, dieLevel>
724 Level of verbosity. By default, the debugger leaves your exceptions
725 and warnings alone, because altering them can break correctly running
726 programs. It will attempt to print a message when uncaught INT, BUS, or
727 SEGV signals arrive. (But see the mention of signals in L</BUGS> below.)
729 To disable this default safe mode, set these values to something higher
730 than 0. At a level of 1, you get backtraces upon receiving any kind
731 of warning (this is often annoying) or exception (this is
732 often valuable). Unfortunately, the debugger cannot discern fatal
733 exceptions from non-fatal ones. If C<dieLevel> is even 1, then your
734 non-fatal exceptions are also traced and unceremoniously altered if they
735 came from C<eval'ed> strings or from any kind of C<eval> within modules
736 you're attempting to load. If C<dieLevel> is 2, the debugger doesn't
737 care where they came from: It usurps your exception handler and prints
738 out a trace, then modifies all exceptions with its own embellishments.
739 This may perhaps be useful for some tracing purposes, but tends to hopelessly
740 destroy any program that takes its exception handling seriously.
743 X<debugger option, AutoTrace>
745 Trace mode (similar to C<t> command, but can be put into
749 X<debugger option, LineInfo>
751 File or pipe to print line number info to. If it is a pipe (say,
752 C<|visual_perl_db>), then a short message is used. This is the
753 mechanism used to interact with a client editor or visual debugger,
754 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
757 =item C<inhibit_exit>
758 X<debugger option, inhibit_exit>
760 If 0, allows I<stepping off> the end of the script.
763 X<debugger option, PrintRet>
765 Print return value after C<r> command if set (default).
768 X<debugger option, ornaments>
770 Affects screen appearance of the command line (see L<Term::ReadLine>).
771 There is currently no way to disable these, which can render
772 some output illegible on some displays, or with some pagers.
773 This is considered a bug.
776 X<debugger option, frame>
778 Affects the printing of messages upon entry and exit from subroutines. If
779 C<frame & 2> is false, messages are printed on entry only. (Printing
780 on exit might be useful if interspersed with other messages.)
782 If C<frame & 4>, arguments to functions are printed, plus context
783 and caller info. If C<frame & 8>, overloaded C<stringify> and
784 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
785 & 16>, the return value from the subroutine is printed.
787 The length at which the argument list is truncated is governed by the
791 X<debugger option, maxTraceLen>
793 Length to truncate the argument list when the C<frame> option's
797 X<debugger option, windowSize>
799 Change the size of code list window (default is 10 lines).
803 The following options affect what happens with C<V>, C<X>, and C<x>
808 =item C<arrayDepth>, C<hashDepth>
809 X<debugger option, arrayDepth> X<debugger option, hashDepth>
811 Print only first N elements ('' for all).
814 X<debugger option, dumpDepth>
816 Limit recursion depth to N levels when dumping structures.
817 Negative values are interpreted as infinity. Default: infinity.
819 =item C<compactDump>, C<veryCompact>
820 X<debugger option, compactDump> X<debugger option, veryCompact>
822 Change the style of array and hash output. If C<compactDump>, short array
823 may be printed on one line.
826 X<debugger option, globPrint>
828 Whether to print contents of globs.
831 X<debugger option, DumpDBFiles>
833 Dump arrays holding debugged files.
835 =item C<DumpPackages>
836 X<debugger option, DumpPackages>
838 Dump symbol tables of packages.
841 X<debugger option, DumpReused>
843 Dump contents of "reused" addresses.
845 =item C<quote>, C<HighBit>, C<undefPrint>
846 X<debugger option, quote> X<debugger option, HighBit>
847 X<debugger option, undefPrint>
849 Change the style of string dump. The default value for C<quote>
850 is C<auto>; one can enable double-quotish or single-quotish format
851 by setting it to C<"> or C<'>, respectively. By default, characters
852 with their high bit set are printed verbatim.
855 X<debugger option, UsageOnly>
857 Rudimentary per-package memory usage dump. Calculates total
858 size of strings found in variables in the package. This does not
859 include lexicals in a module's file scope, or lost in closures.
862 X<debugger option, history, HistFile>
864 The path of the file from which the history (assuming a usable
865 Term::ReadLine backend) will be read on the debugger's startup, and to which
866 it will be saved on shutdown (for persistence across sessions). Similar in
867 concept to Bash's C<.bash_history> file.
870 X<debugger option, history, HistSize>
872 The count of the saved lines in the history (assuming C<HistFile> above).
876 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
877 environment variable and parses this as the remainder of a "O ..."
878 line as one might enter at the debugger prompt. You may place the
879 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
882 If your rc file contains:
884 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
886 then your script will run without human intervention, putting trace
887 information into the file I<db.out>. (If you interrupt it, you'd
888 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
893 X<debugger option, TTY>
895 The TTY to use for debugging I/O.
898 X<debugger option, noTTY>
900 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
901 interrupted (or if control goes to the debugger via explicit setting of
902 $DB::signal or $DB::single from the Perl script), it connects to a TTY
903 specified in the C<TTY> option at startup, or to a tty found at
904 runtime using the C<Term::Rendezvous> module of your choice.
906 This module should implement a method named C<new> that returns an object
907 with two methods: C<IN> and C<OUT>. These should return filehandles to use
908 for debugging input and output correspondingly. The C<new> method should
909 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
910 startup, or C<"$ENV{HOME}/.perldbtty$$"> otherwise. This file is not
911 inspected for proper ownership, so security hazards are theoretically
915 X<debugger option, ReadLine>
917 If false, readline support in the debugger is disabled in order
918 to debug applications that themselves use ReadLine.
921 X<debugger option, NonStop>
923 If set, the debugger goes into non-interactive mode until interrupted, or
924 programmatically by setting $DB::signal or $DB::single.
928 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
930 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
932 That will run the script B<myprogram> without human intervention,
933 printing out the call tree with entry and exit points. Note that
934 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
935 options could be uniquely abbreviated by the first letter (modulo
936 the C<Dump*> options). It is nevertheless recommended that you
937 always spell them out in full for legibility and future compatibility.
939 Other examples include
941 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
943 which runs script non-interactively, printing info on each entry
944 into a subroutine and each executed line into the file named F<listing>.
945 (If you interrupt it, you would better reset C<LineInfo> to something
948 Other examples include (using standard shell syntax to show environment
951 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
954 which may be useful for debugging a program that uses C<Term::ReadLine>
955 itself. Do not forget to detach your shell from the TTY in the window that
956 corresponds to F</dev/ttyXX>, say, by issuing a command like
960 See L<perldebguts/"Debugger Internals"> for details.
962 =head2 Debugger Input/Output
968 The debugger prompt is something like
976 where that number is the command number, and which you'd use to
977 access with the built-in B<csh>-like history mechanism. For example,
978 C<!17> would repeat command number 17. The depth of the angle
979 brackets indicates the nesting depth of the debugger. You could
980 get more than one set of brackets, for example, if you'd already
981 at a breakpoint and then printed the result of a function call that
982 itself has a breakpoint, or you step into an expression via C<s/n/t
985 =item Multiline commands
987 If you want to enter a multi-line command, such as a subroutine
988 definition with several statements or a format, escape the newline
989 that would normally end the debugger command with a backslash.
993 cont: print "ok\n"; \
1000 Note that this business of escaping a newline is specific to interactive
1001 commands typed into the debugger.
1003 =item Stack backtrace
1004 X<backtrace> X<stack, backtrace>
1006 Here's an example of what a stack backtrace via C<T> command might
1009 $ = main::infested called from file 'Ambulation.pm' line 10
1010 @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea'
1012 $ = main::pests('bactrian', 4) called from file 'camel_flea'
1015 The left-hand character up there indicates the context in which the
1016 function was called, with C<$> and C<@> meaning scalar or list
1017 contexts respectively, and C<.> meaning void context (which is
1018 actually a sort of scalar context). The display above says
1019 that you were in the function C<main::infested> when you ran the
1020 stack dump, and that it was called in scalar context from line
1021 10 of the file I<Ambulation.pm>, but without any arguments at all,
1022 meaning it was called as C<&infested>. The next stack frame shows
1023 that the function C<Ambulation::legs> was called in list context
1024 from the I<camel_flea> file with four arguments. The last stack
1025 frame shows that C<main::pests> was called in scalar context,
1026 also from I<camel_flea>, but from line 4.
1028 If you execute the C<T> command from inside an active C<use>
1029 statement, the backtrace will contain both a C<require> frame and
1032 =item Line Listing Format
1034 This shows the sorts of output the C<l> command can produce:
1038 102:b @isa{@i,$pack} = ()
1039 103 if(exists $i{$prevpack} || exists $isa{$pack});
1043 107==> if(exists $isa{$pack});
1045 109:a if ($extra-- > 0) {
1046 110: %isa = ($pack,1);
1048 Breakable lines are marked with C<:>. Lines with breakpoints are
1049 marked by C<b> and those with actions by C<a>. The line that's
1050 about to be executed is marked by C<< ==> >>.
1052 Please be aware that code in debugger listings may not look the same
1053 as your original source code. Line directives and external source
1054 filters can alter the code before Perl sees it, causing code to move
1055 from its original positions or take on entirely different forms.
1059 When the C<frame> option is set, the debugger would print entered (and
1060 optionally exited) subroutines in different styles. See L<perldebguts>
1061 for incredibly long examples of these.
1065 =head2 Debugging Compile-Time Statements
1067 If you have compile-time executable statements (such as code within
1068 BEGIN, UNITCHECK and CHECK blocks or C<use> statements), these will
1069 I<not> be stopped by debugger, although C<require>s and INIT blocks
1070 will, and compile-time statements can be traced with the C<AutoTrace>
1071 option set in C<PERLDB_OPTS>). From your own Perl code, however, you
1072 can transfer control back to the debugger using the following
1073 statement, which is harmless if the debugger is not running:
1077 If you set C<$DB::single> to 2, it's equivalent to having
1078 just typed the C<n> command, whereas a value of 1 means the C<s>
1079 command. The C<$DB::trace> variable should be set to 1 to simulate
1080 having typed the C<t> command.
1082 Another way to debug compile-time code is to start the debugger, set a
1083 breakpoint on the I<load> of some module:
1085 DB<7> b load f:/perllib/lib/Carp.pm
1086 Will stop on load of 'f:/perllib/lib/Carp.pm'.
1088 and then restart the debugger using the C<R> command (if possible). One can use C<b
1089 compile subname> for the same purpose.
1091 =head2 Debugger Customization
1093 The debugger probably contains enough configuration hooks that you
1094 won't ever have to modify it yourself. You may change the behaviour
1095 of the debugger from within the debugger using its C<o> command, from
1096 the command line via the C<PERLDB_OPTS> environment variable, and
1097 from customization files.
1099 You can do some customization by setting up a F<.perldb> file, which
1100 contains initialization code. For instance, you could make aliases
1101 like these (the last one is one people expect to be there):
1103 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
1104 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
1105 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
1106 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
1108 You can change options from F<.perldb> by using calls like this one;
1110 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
1112 The code is executed in the package C<DB>. Note that F<.perldb> is
1113 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
1114 subroutine C<afterinit>, that function is called after debugger
1115 initialization ends. F<.perldb> may be contained in the current
1116 directory, or in the home directory. Because this file is sourced
1117 in by Perl and may contain arbitrary commands, for security reasons,
1118 it must be owned by the superuser or the current user, and writable
1119 by no one but its owner.
1121 You can mock TTY input to debugger by adding arbitrary commands to
1122 @DB::typeahead. For example, your F<.perldb> file might contain:
1124 sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
1126 Which would attempt to set breakpoints on lines 4 and 6 immediately
1127 after debugger initialization. Note that @DB::typeahead is not a supported
1128 interface and is subject to change in future releases.
1130 If you want to modify the debugger, copy F<perl5db.pl> from the
1131 Perl library to another name and hack it to your heart's content.
1132 You'll then want to set your C<PERL5DB> environment variable to say
1133 something like this:
1135 BEGIN { require "myperl5db.pl" }
1137 As a last resort, you could also use C<PERL5DB> to customize the debugger
1138 by directly setting internal variables or calling debugger functions.
1140 Note that any variables and functions that are not documented in
1141 this document (or in L<perldebguts>) are considered for internal
1142 use only, and as such are subject to change without notice.
1144 =head2 Readline Support / History in the Debugger
1146 As shipped, the only command-line history supplied is a simplistic one
1147 that checks for leading exclamation points. However, if you install
1148 the Term::ReadKey and Term::ReadLine modules from CPAN (such as
1149 Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will
1150 have full editing capabilities much like those GNU I<readline>(3) provides.
1151 Look for these in the F<modules/by-module/Term> directory on CPAN.
1152 These do not support normal B<vi> command-line editing, however.
1154 A rudimentary command-line completion is also available, including
1155 lexical variables in the current scope if the C<PadWalker> module
1158 Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B",
1159 "^[[D"", "^H", ... when using the arrow keys and/or the backspace key.
1161 =head2 Editor Support for Debugging
1163 If you have the GNU's version of B<emacs> installed on your system,
1164 it can interact with the Perl debugger to provide an integrated
1165 software development environment reminiscent of its interactions
1168 Recent versions of Emacs come with a
1169 start file for making B<emacs> act like a
1170 syntax-directed editor that understands (some of) Perl's syntax.
1173 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
1174 and windy version, for coloring of Perl keywords.
1176 Note that only perl can truly parse Perl, so all such CASE tools
1177 fall somewhat short of the mark, especially if you don't program
1178 your Perl as a C programmer might.
1180 =head2 The Perl Profiler
1181 X<profile> X<profiling> X<profiler>
1183 If you wish to supply an alternative debugger for Perl to run,
1184 invoke your script with a colon and a package argument given to the
1185 B<-d> flag. Perl's alternative debuggers include a Perl profiler,
1186 L<Devel::NYTProf>, which is available separately as a CPAN
1187 distribution. To profile your Perl program in the file F<mycode.pl>,
1190 $ perl -d:NYTProf mycode.pl
1192 When the script terminates the profiler will create a database of the
1193 profile information that you can turn into reports using the profiler's
1194 tools. See <perlperf> for details.
1196 =head1 Debugging Regular Expressions
1197 X<regular expression, debugging>
1198 X<regex, debugging> X<regexp, debugging>
1200 C<use re 'debug'> enables you to see the gory details of how the Perl
1201 regular expression engine works. In order to understand this typically
1202 voluminous output, one must not only have some idea about how regular
1203 expression matching works in general, but also know how Perl's regular
1204 expressions are internally compiled into an automaton. These matters
1205 are explored in some detail in
1206 L<perldebguts/"Debugging Regular Expressions">.
1208 =head1 Debugging Memory Usage
1211 Perl contains internal support for reporting its own memory usage,
1212 but this is a fairly advanced concept that requires some understanding
1213 of how memory allocation works.
1214 See L<perldebguts/"Debugging Perl Memory Usage"> for the details.
1218 You do have C<use strict> and C<use warnings> enabled, don't you?
1230 When debugging a script that uses #! and is thus normally found in
1231 $PATH, the -S option causes perl to search $PATH for it, so you don't
1232 have to type the path or C<which $scriptname>.
1238 You cannot get stack frame information or in any fashion debug functions
1239 that were not compiled by Perl, such as those from C or C++ extensions.
1241 If you alter your @_ arguments in a subroutine (such as with C<shift>
1242 or C<pop>), the stack backtrace will not show the original values.
1244 The debugger does not currently work in conjunction with the B<-W>
1245 command-line switch, because it itself is not free of warnings.
1247 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
1248 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
1249 handler, then you won't be able to CTRL-C your way back to the debugger,
1250 because the debugger's own C<$SIG{INT}> handler doesn't understand that
1251 it needs to raise an exception to longjmp(3) out of slow syscalls.