4 perldebug - Perl debugging
8 First of all, have you tried using L<C<use strict;>|strict> and
9 L<C<use warnings;>|warnings>?
12 If you're new to the Perl debugger, you may prefer to read
13 L<perldebtut>, which is a tutorial introduction to the debugger.
15 =head1 The Perl Debugger
17 If you invoke Perl with the B<-d> switch, your script runs under the
18 Perl source debugger. This works like an interactive Perl
19 environment, prompting for debugger commands that let you examine
20 source code, set breakpoints, get stack backtraces, change the values of
21 variables, etc. This is so convenient that you often fire up
22 the debugger all by itself just to test out Perl constructs
23 interactively to see what they do. For example:
28 In Perl, the debugger is not a separate program the way it usually is in the
29 typical compiled environment. Instead, the B<-d> flag tells the compiler
30 to insert source information into the parse trees it's about to hand off
31 to the interpreter. That means your code must first compile correctly
32 for the debugger to work on it. Then when the interpreter starts up, it
33 preloads a special Perl library file containing the debugger.
35 The program will halt I<right before> the first run-time executable
36 statement (but see below regarding compile-time statements) and ask you
37 to enter a debugger command. Contrary to popular expectations, whenever
38 the debugger halts and shows you a line of code, it always displays the
39 line it's I<about> to execute, rather than the one it has just executed.
41 Any command not recognized by the debugger is directly executed
42 (C<eval>'d) as Perl code in the current package. (The debugger
43 uses the DB package for keeping its own state information.)
45 Note that the said C<eval> is bound by an implicit scope. As a
46 result any newly introduced lexical variable or any modified
47 capture buffer content is lost after the eval. The debugger is a
48 nice environment to learn Perl, but if you interactively experiment using
49 material which should be in the same scope, stuff it in one line.
51 For any text entered at the debugger prompt, leading and trailing whitespace
52 is first stripped before further processing. If a debugger command
53 coincides with some function in your own program, merely precede the
54 function with something that doesn't look like a debugger command, such
55 as a leading C<;> or perhaps a C<+>, or by wrapping it with parentheses
58 =head2 Calling the Debugger
60 There are several ways to call the debugger:
64 =item perl -d program_name
66 On the given program identified by C<program_name>.
70 Interactively supply an arbitrary C<expression> using C<-e>.
72 =item perl -d:ptkdb program_name
74 Debug a given program via the C<Devel::ptkdb> GUI.
76 =item perl -dt threaded_program_name
78 Debug a given program using threads (experimental).
82 =head2 Debugger Commands
84 The interactive debugger understands the following commands:
89 X<debugger command, h>
91 Prints out a summary help message
95 Prints out a help message for the given debugger command.
99 The special argument of C<h h> produces the entire help page, which is quite long.
101 If the output of the C<h h> command (or any command, for that matter) scrolls
102 past your screen, precede the command with a leading pipe symbol so
103 that it's run through your pager, as in
107 You may change the pager which is used via C<o pager=...> command.
110 X<debugger command, p>
112 Same as C<print {$DB::OUT} expr> in the current package. In particular,
113 because this is just Perl's own C<print> function, this means that nested
114 data structures and objects are not dumped, unlike with the C<x> command.
116 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
117 where STDOUT may be redirected to.
119 =item x [maxdepth] expr
120 X<debugger command, x>
122 Evaluates its expression in list context and dumps out the result in a
123 pretty-printed fashion. Nested data structures are printed out
124 recursively, unlike the real C<print> function in Perl. When dumping
125 hashes, you'll probably prefer 'x \%h' rather than 'x %h'.
126 See L<Dumpvalue> if you'd like to do this yourself.
128 The output format is governed by multiple options described under
129 L</"Configurable Options">.
131 If the C<maxdepth> is included, it must be a numeral I<N>; the value is
132 dumped only I<N> levels deep, as if the C<dumpDepth> option had been
133 temporarily set to I<N>.
136 X<debugger command, V>
138 Display all (or some) variables in package (defaulting to C<main>)
139 using a data pretty-printer (hashes show their keys and values so
140 you see what's what, control characters are made printable, etc.).
141 Make sure you don't put the type specifier (like C<$>) there, just
142 the symbol names, like this:
146 Use C<~pattern> and C<!pattern> for positive and negative regexes.
148 This is similar to calling the C<x> command on each applicable var.
151 X<debugger command, X>
153 Same as C<V currentpackage [vars]>.
155 =item y [level [vars]]
156 X<debugger command, y>
158 Display all (or some) lexical variables (mnemonic: C<mY> variables)
159 in the current scope or I<level> scopes higher. You can limit the
160 variables that you see with I<vars> which works exactly as it does
161 for the C<V> and C<X> commands. Requires the C<PadWalker> module
162 version 0.08 or higher; will warn if this isn't installed. Output
163 is pretty-printed in the same style as for C<V> and the format is
164 controlled by the same options.
167 X<debugger command, T> X<backtrace> X<stack, backtrace>
169 Produce a stack backtrace. See below for details on its output.
172 X<debugger command, s> X<step>
174 Single step. Executes until the beginning of another
175 statement, descending into subroutine calls. If an expression is
176 supplied that includes function calls, it too will be single-stepped.
179 X<debugger command, n>
181 Next. Executes over subroutine calls, until the beginning
182 of the next statement. If an expression is supplied that includes
183 function calls, those functions will be executed with stops before
187 X<debugger command, r>
189 Continue until the return from the current subroutine.
190 Dump the return value if the C<PrintRet> option is set (default).
194 Repeat last C<n> or C<s> command.
197 X<debugger command, c>
199 Continue, optionally inserting a one-time-only breakpoint
200 at the specified line or subroutine.
203 X<debugger command, l>
205 List next window of lines.
209 List C<incr+1> lines starting at C<min>.
213 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
221 List first window of lines from subroutine. I<subname> may
222 be a variable that contains a code reference.
225 X<debugger command, ->
227 List previous window of lines.
230 X<debugger command, v>
232 View a few lines of code around the current line.
235 X<debugger command, .>
237 Return the internal debugger pointer to the line last
238 executed, and print out that line.
241 X<debugger command, f>
243 Switch to viewing a different file or C<eval> statement. If I<filename>
244 is not a full pathname found in the values of %INC, it is considered
247 C<eval>ed strings (when accessible) are considered to be filenames:
248 C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
249 (in the order of execution). The bodies of the currently executed C<eval>
250 and of C<eval>ed strings that define subroutines are saved and thus
255 Search forwards for pattern (a Perl regex); final / is optional.
256 The search is case-insensitive by default.
260 Search backwards for pattern; final ? is optional.
261 The search is case-insensitive by default.
264 X<debugger command, L>
266 List (default all) actions, breakpoints and watch expressions
269 X<debugger command, S>
271 List subroutine names [not] matching the regex.
274 X<debugger command, t>
276 Toggle trace mode (see also the C<AutoTrace> option).
277 Optional argument is the maximum number of levels to trace below
278 the current one; anything deeper than that will be silent.
281 X<debugger command, t>
283 Trace through execution of C<expr>.
284 Optional first argument is the maximum number of levels to trace below
285 the current one; anything deeper than that will be silent.
286 See L<perldebguts/"Frame Listing Output Examples"> for examples.
290 X<debugger command, b>
292 Sets breakpoint on current line
294 =item b [line] [condition]
296 X<debugger command, b>
298 Set a breakpoint before the given line. If a condition
299 is specified, it's evaluated each time the statement is reached: a
300 breakpoint is taken only if the condition is true. Breakpoints may
301 only be set on lines that begin an executable statement. Conditions
305 b 237 ++$count237 < 11
308 If the line number is C<.>, sets a breakpoint on the current line:
312 =item b [file]:[line] [condition]
314 X<debugger command, b>
316 Set a breakpoint before the given line in a (possibly different) file. If a
317 condition is specified, it's evaluated each time the statement is reached: a
318 breakpoint is taken only if the condition is true. Breakpoints may only be set
319 on lines that begin an executable statement. Conditions don't use C<if>:
321 b lib/MyModule.pm:237 $x > 30
322 b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
324 =item b subname [condition]
326 X<debugger command, b>
328 Set a breakpoint before the first line of the named subroutine. I<subname> may
329 be a variable containing a code reference (in this case I<condition>
332 =item b postpone subname [condition]
334 X<debugger command, b>
336 Set a breakpoint at first line of subroutine after it is compiled.
338 =item b load filename
340 X<debugger command, b>
342 Set a breakpoint before the first executed line of the I<filename>,
343 which should be a full pathname found amongst the %INC values.
345 =item b compile subname
347 X<debugger command, b>
349 Sets a breakpoint before the first statement executed after the specified
350 subroutine is compiled.
354 X<debugger command, B>
356 Delete a breakpoint from the specified I<line>.
360 X<debugger command, B>
362 Delete all installed breakpoints.
364 =item disable [file]:[line]
366 X<debugger command, disable>
369 Disable the breakpoint so it won't stop the execution of the program.
370 Breakpoints are enabled by default and can be re-enabled using the C<enable>
375 X<debugger command, disable>
378 Disable the breakpoint so it won't stop the execution of the program.
379 Breakpoints are enabled by default and can be re-enabled using the C<enable>
382 This is done for a breakpoint in the current file.
384 =item enable [file]:[line]
386 X<debugger command, disable>
389 Enable the breakpoint so it will stop the execution of the program.
393 X<debugger command, disable>
396 Enable the breakpoint so it will stop the execution of the program.
398 This is done for a breakpoint in the current file.
400 =item a [line] command
401 X<debugger command, a>
403 Set an action to be done before the line is executed. If I<line> is
404 omitted, set an action on the line about to be executed.
405 The sequence of steps taken by the debugger is
407 1. check for a breakpoint at this line
408 2. print the line if necessary (tracing)
409 3. do any actions associated with that line
410 4. prompt user if at a breakpoint or in single-step
413 For example, this will print out $foo every time line
416 a 53 print "DB FOUND $foo\n"
419 X<debugger command, A>
421 Delete an action from the specified line.
424 X<debugger command, A>
426 Delete all installed actions.
429 X<debugger command, w>
431 Add a global watch-expression. Whenever a watched global changes the
432 debugger will stop and display the old and new values.
435 X<debugger command, W>
437 Delete watch-expression
440 X<debugger command, W>
442 Delete all watch-expressions.
445 X<debugger command, o>
449 =item o booloption ...
450 X<debugger command, o>
452 Set each listed Boolean option to the value C<1>.
454 =item o anyoption? ...
455 X<debugger command, o>
457 Print out the value of one or more options.
459 =item o option=value ...
460 X<debugger command, o>
462 Set the value of one or more options. If the value has internal
463 whitespace, it should be quoted. For example, you could set C<o
464 pager="less -MQeicsNfr"> to call B<less> with those specific options.
465 You may use either single or double quotes, but if you do, you must
466 escape any embedded instances of same sort of quote you began with,
467 as well as any escaping any escapes that immediately precede that
468 quote but which are not meant to escape the quote itself. In other
469 words, you follow single-quoting rules irrespective of the quote;
470 eg: C<o option='this isn\'t bad'> or C<o option="She said, \"Isn't
473 For historical reasons, the C<=value> is optional, but defaults to
474 1 only where it is safe to do so--that is, mostly for Boolean
475 options. It is always better to assign a specific value using C<=>.
476 The C<option> can be abbreviated, but for clarity probably should
477 not be. Several options can be set together. See L</"Configurable Options">
481 X<< debugger command, < >>
483 List out all pre-prompt Perl command actions.
486 X<< debugger command, < >>
488 Set an action (Perl command) to happen before every debugger prompt.
489 A multi-line command may be entered by backslashing the newlines.
492 X<< debugger command, < >>
494 Delete all pre-prompt Perl command actions.
497 X<< debugger command, << >>
499 Add an action (Perl command) to happen before every debugger prompt.
500 A multi-line command may be entered by backwhacking the newlines.
503 X<< debugger command, > >>
505 List out post-prompt Perl command actions.
508 X<< debugger command, > >>
510 Set an action (Perl command) to happen after the prompt when you've
511 just given a command to return to executing the script. A multi-line
512 command may be entered by backslashing the newlines (we bet you
513 couldn't have guessed this by now).
516 X<< debugger command, > >>
518 Delete all post-prompt Perl command actions.
521 X<<< debugger command, >> >>>
523 Adds an action (Perl command) to happen after the prompt when you've
524 just given a command to return to executing the script. A multi-line
525 command may be entered by backslashing the newlines.
528 X<debugger command, {>
530 List out pre-prompt debugger commands.
534 Set an action (debugger command) to happen before every debugger prompt.
535 A multi-line command may be entered in the customary fashion.
537 Because this command is in some senses new, a warning is issued if
538 you appear to have accidentally entered a block instead. If that's
539 what you mean to do, write it as with C<;{ ... }> or even
543 X<debugger command, {>
545 Delete all pre-prompt debugger commands.
548 X<debugger command, {{>
550 Add an action (debugger command) to happen before every debugger prompt.
551 A multi-line command may be entered, if you can guess how: see above.
554 X<debugger command, !>
556 Redo a previous command (defaults to the previous command).
559 X<debugger command, !>
561 Redo number'th previous command.
564 X<debugger command, !>
566 Redo last command that started with pattern.
567 See C<o recallCommand>, too.
570 X<debugger command, !!>
572 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
573 C<o shellBang>, also. Note that the user's current shell (well,
574 their C<$ENV{SHELL}> variable) will be used, which can interfere
575 with proper interpretation of exit status or signal and coredump
579 X<debugger command, source>
581 Read and execute debugger commands from I<file>.
582 I<file> may itself contain C<source> commands.
585 X<debugger command, H>
587 Display last n commands. Only commands longer than one character are
588 listed. If I<number> is omitted, list them all.
591 X<debugger command, q>
592 X<debugger command, ^D>
594 Quit. ("quit" doesn't work for this, unless you've made an alias)
595 This is the only supported way to exit the debugger, though typing
596 C<exit> twice might work.
598 Set the C<inhibit_exit> option to 0 if you want to be able to step
599 off the end the script. You may also need to set $finished to 0
600 if you want to step through global destruction.
603 X<debugger command, R>
605 Restart the debugger by C<exec()>ing a new session. We try to maintain
606 your history across this, but internal settings and command-line options
609 The following setting are currently preserved: history, breakpoints,
610 actions, debugger options, and the Perl command-line
611 options B<-w>, B<-I>, and B<-e>.
614 X<debugger command, |>
616 Run the debugger command, piping DB::OUT into your current pager.
619 X<debugger command, ||>
621 Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
623 =item = [alias value]
624 X<debugger command, =>
626 Define a command alias, like
630 or list current aliases.
634 Execute command as a Perl statement. A trailing semicolon will be
635 supplied. If the Perl statement would otherwise be confused for a
636 Perl debugger, use a leading semicolon, too.
639 X<debugger command, m>
641 List which methods may be called on the result of the evaluated
642 expression. The expression may evaluated to a reference to a
643 blessed object, or to a package name.
646 X<debugger command, M>
648 Display all loaded modules and their versions.
651 X<debugger command, man>
653 Despite its name, this calls your system's default documentation
654 viewer on the given page, or on the viewer itself if I<manpage> is
655 omitted. If that viewer is B<man>, the current C<Config> information
656 is used to invoke B<man> using the proper MANPATH or S<B<-M>
657 I<manpath>> option. Failed lookups of the form C<XXX> that match
658 known manpages of the form I<perlXXX> will be retried. This lets
659 you type C<man debug> or C<man op> from the debugger.
661 On systems traditionally bereft of a usable B<man> command, the
662 debugger invokes B<perldoc>. Occasionally this determination is
663 incorrect due to recalcitrant vendors or rather more felicitously,
664 to enterprising users. If you fall into either category, just
665 manually set the $DB::doccmd variable to whatever viewer to view
666 the Perl documentation on your system. This may be set in an rc
667 file, or through direct assignment. We're still waiting for a
668 working example of something along the lines of:
670 $DB::doccmd = 'netscape -remote http://something.here/';
674 =head2 Configurable Options
676 The debugger has numerous options settable using the C<o> command,
677 either interactively or from the environment or an rc file.
678 (./.perldb or ~/.perldb under Unix.)
683 =item C<recallCommand>, C<ShellBang>
684 X<debugger option, recallCommand>
685 X<debugger option, ShellBang>
687 The characters used to recall a command or spawn a shell. By
688 default, both are set to C<!>, which is unfortunate.
691 X<debugger option, pager>
693 Program to use for output of pager-piped commands (those beginning
694 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
695 Because the debugger uses your current terminal characteristics
696 for bold and underlining, if the chosen pager does not pass escape
697 sequences through unchanged, the output of some debugger commands
698 will not be readable when sent through the pager.
701 X<debugger option, tkRunning>
703 Run Tk while prompting (with ReadLine).
705 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
706 X<debugger option, signalLevel> X<debugger option, warnLevel>
707 X<debugger option, dieLevel>
709 Level of verbosity. By default, the debugger leaves your exceptions
710 and warnings alone, because altering them can break correctly running
711 programs. It will attempt to print a message when uncaught INT, BUS, or
712 SEGV signals arrive. (But see the mention of signals in L</BUGS> below.)
714 To disable this default safe mode, set these values to something higher
715 than 0. At a level of 1, you get backtraces upon receiving any kind
716 of warning (this is often annoying) or exception (this is
717 often valuable). Unfortunately, the debugger cannot discern fatal
718 exceptions from non-fatal ones. If C<dieLevel> is even 1, then your
719 non-fatal exceptions are also traced and unceremoniously altered if they
720 came from C<eval'ed> strings or from any kind of C<eval> within modules
721 you're attempting to load. If C<dieLevel> is 2, the debugger doesn't
722 care where they came from: It usurps your exception handler and prints
723 out a trace, then modifies all exceptions with its own embellishments.
724 This may perhaps be useful for some tracing purposes, but tends to hopelessly
725 destroy any program that takes its exception handling seriously.
728 X<debugger option, AutoTrace>
730 Trace mode (similar to C<t> command, but can be put into
734 X<debugger option, LineInfo>
736 File or pipe to print line number info to. If it is a pipe (say,
737 C<|visual_perl_db>), then a short message is used. This is the
738 mechanism used to interact with a slave editor or visual debugger,
739 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
742 =item C<inhibit_exit>
743 X<debugger option, inhibit_exit>
745 If 0, allows I<stepping off> the end of the script.
748 X<debugger option, PrintRet>
750 Print return value after C<r> command if set (default).
753 X<debugger option, ornaments>
755 Affects screen appearance of the command line (see L<Term::ReadLine>).
756 There is currently no way to disable these, which can render
757 some output illegible on some displays, or with some pagers.
758 This is considered a bug.
761 X<debugger option, frame>
763 Affects the printing of messages upon entry and exit from subroutines. If
764 C<frame & 2> is false, messages are printed on entry only. (Printing
765 on exit might be useful if interspersed with other messages.)
767 If C<frame & 4>, arguments to functions are printed, plus context
768 and caller info. If C<frame & 8>, overloaded C<stringify> and
769 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
770 & 16>, the return value from the subroutine is printed.
772 The length at which the argument list is truncated is governed by the
776 X<debugger option, maxTraceLen>
778 Length to truncate the argument list when the C<frame> option's
782 X<debugger option, windowSize>
784 Change the size of code list window (default is 10 lines).
788 The following options affect what happens with C<V>, C<X>, and C<x>
793 =item C<arrayDepth>, C<hashDepth>
794 X<debugger option, arrayDepth> X<debugger option, hashDepth>
796 Print only first N elements ('' for all).
799 X<debugger option, dumpDepth>
801 Limit recursion depth to N levels when dumping structures.
802 Negative values are interpreted as infinity. Default: infinity.
804 =item C<compactDump>, C<veryCompact>
805 X<debugger option, compactDump> X<debugger option, veryCompact>
807 Change the style of array and hash output. If C<compactDump>, short array
808 may be printed on one line.
811 X<debugger option, globPrint>
813 Whether to print contents of globs.
816 X<debugger option, DumpDBFiles>
818 Dump arrays holding debugged files.
820 =item C<DumpPackages>
821 X<debugger option, DumpPackages>
823 Dump symbol tables of packages.
826 X<debugger option, DumpReused>
828 Dump contents of "reused" addresses.
830 =item C<quote>, C<HighBit>, C<undefPrint>
831 X<debugger option, quote> X<debugger option, HighBit>
832 X<debugger option, undefPrint>
834 Change the style of string dump. The default value for C<quote>
835 is C<auto>; one can enable double-quotish or single-quotish format
836 by setting it to C<"> or C<'>, respectively. By default, characters
837 with their high bit set are printed verbatim.
840 X<debugger option, UsageOnly>
842 Rudimentary per-package memory usage dump. Calculates total
843 size of strings found in variables in the package. This does not
844 include lexicals in a module's file scope, or lost in closures.
847 X<debugger option, history, HistFile>
849 The path of the file from which the history (assuming a usable
850 Term::ReadLine backend) will be read on the debugger's startup, and to which
851 it will be saved on shutdown (for persistence across sessions). Similar in
852 concept to Bash's C<.bash_history> file.
855 X<debugger option, history, HistSize>
857 The count of the saved lines in the history (assuming C<HistFile> above).
861 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
862 environment variable and parses this as the remainder of a "O ..."
863 line as one might enter at the debugger prompt. You may place the
864 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
867 If your rc file contains:
869 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
871 then your script will run without human intervention, putting trace
872 information into the file I<db.out>. (If you interrupt it, you'd
873 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
878 X<debugger option, TTY>
880 The TTY to use for debugging I/O.
883 X<debugger option, noTTY>
885 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
886 interrupted (or if control goes to the debugger via explicit setting of
887 $DB::signal or $DB::single from the Perl script), it connects to a TTY
888 specified in the C<TTY> option at startup, or to a tty found at
889 runtime using the C<Term::Rendezvous> module of your choice.
891 This module should implement a method named C<new> that returns an object
892 with two methods: C<IN> and C<OUT>. These should return filehandles to use
893 for debugging input and output correspondingly. The C<new> method should
894 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
895 startup, or C<"$ENV{HOME}/.perldbtty$$"> otherwise. This file is not
896 inspected for proper ownership, so security hazards are theoretically
900 X<debugger option, ReadLine>
902 If false, readline support in the debugger is disabled in order
903 to debug applications that themselves use ReadLine.
906 X<debugger option, NonStop>
908 If set, the debugger goes into non-interactive mode until interrupted, or
909 programmatically by setting $DB::signal or $DB::single.
913 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
915 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
917 That will run the script B<myprogram> without human intervention,
918 printing out the call tree with entry and exit points. Note that
919 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
920 options could be uniquely abbreviated by the first letter (modulo
921 the C<Dump*> options). It is nevertheless recommended that you
922 always spell them out in full for legibility and future compatibility.
924 Other examples include
926 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
928 which runs script non-interactively, printing info on each entry
929 into a subroutine and each executed line into the file named F<listing>.
930 (If you interrupt it, you would better reset C<LineInfo> to something
933 Other examples include (using standard shell syntax to show environment
936 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
939 which may be useful for debugging a program that uses C<Term::ReadLine>
940 itself. Do not forget to detach your shell from the TTY in the window that
941 corresponds to F</dev/ttyXX>, say, by issuing a command like
945 See L<perldebguts/"Debugger Internals"> for details.
947 =head2 Debugger Input/Output
953 The debugger prompt is something like
961 where that number is the command number, and which you'd use to
962 access with the built-in B<csh>-like history mechanism. For example,
963 C<!17> would repeat command number 17. The depth of the angle
964 brackets indicates the nesting depth of the debugger. You could
965 get more than one set of brackets, for example, if you'd already
966 at a breakpoint and then printed the result of a function call that
967 itself has a breakpoint, or you step into an expression via C<s/n/t
970 =item Multiline commands
972 If you want to enter a multi-line command, such as a subroutine
973 definition with several statements or a format, escape the newline
974 that would normally end the debugger command with a backslash.
978 cont: print "ok\n"; \
985 Note that this business of escaping a newline is specific to interactive
986 commands typed into the debugger.
988 =item Stack backtrace
989 X<backtrace> X<stack, backtrace>
991 Here's an example of what a stack backtrace via C<T> command might
994 $ = main::infested called from file 'Ambulation.pm' line 10
995 @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea'
997 $ = main::pests('bactrian', 4) called from file 'camel_flea'
1000 The left-hand character up there indicates the context in which the
1001 function was called, with C<$> and C<@> meaning scalar or list
1002 contexts respectively, and C<.> meaning void context (which is
1003 actually a sort of scalar context). The display above says
1004 that you were in the function C<main::infested> when you ran the
1005 stack dump, and that it was called in scalar context from line
1006 10 of the file I<Ambulation.pm>, but without any arguments at all,
1007 meaning it was called as C<&infested>. The next stack frame shows
1008 that the function C<Ambulation::legs> was called in list context
1009 from the I<camel_flea> file with four arguments. The last stack
1010 frame shows that C<main::pests> was called in scalar context,
1011 also from I<camel_flea>, but from line 4.
1013 If you execute the C<T> command from inside an active C<use>
1014 statement, the backtrace will contain both a C<require> frame and
1017 =item Line Listing Format
1019 This shows the sorts of output the C<l> command can produce:
1023 102:b @isa{@i,$pack} = ()
1024 103 if(exists $i{$prevpack} || exists $isa{$pack});
1028 107==> if(exists $isa{$pack});
1030 109:a if ($extra-- > 0) {
1031 110: %isa = ($pack,1);
1033 Breakable lines are marked with C<:>. Lines with breakpoints are
1034 marked by C<b> and those with actions by C<a>. The line that's
1035 about to be executed is marked by C<< ==> >>.
1037 Please be aware that code in debugger listings may not look the same
1038 as your original source code. Line directives and external source
1039 filters can alter the code before Perl sees it, causing code to move
1040 from its original positions or take on entirely different forms.
1044 When the C<frame> option is set, the debugger would print entered (and
1045 optionally exited) subroutines in different styles. See L<perldebguts>
1046 for incredibly long examples of these.
1050 =head2 Debugging Compile-Time Statements
1052 If you have compile-time executable statements (such as code within
1053 BEGIN, UNITCHECK and CHECK blocks or C<use> statements), these will
1054 I<not> be stopped by debugger, although C<require>s and INIT blocks
1055 will, and compile-time statements can be traced with the C<AutoTrace>
1056 option set in C<PERLDB_OPTS>). From your own Perl code, however, you
1057 can transfer control back to the debugger using the following
1058 statement, which is harmless if the debugger is not running:
1062 If you set C<$DB::single> to 2, it's equivalent to having
1063 just typed the C<n> command, whereas a value of 1 means the C<s>
1064 command. The C<$DB::trace> variable should be set to 1 to simulate
1065 having typed the C<t> command.
1067 Another way to debug compile-time code is to start the debugger, set a
1068 breakpoint on the I<load> of some module:
1070 DB<7> b load f:/perllib/lib/Carp.pm
1071 Will stop on load of 'f:/perllib/lib/Carp.pm'.
1073 and then restart the debugger using the C<R> command (if possible). One can use C<b
1074 compile subname> for the same purpose.
1076 =head2 Debugger Customization
1078 The debugger probably contains enough configuration hooks that you
1079 won't ever have to modify it yourself. You may change the behaviour
1080 of the debugger from within the debugger using its C<o> command, from
1081 the command line via the C<PERLDB_OPTS> environment variable, and
1082 from customization files.
1084 You can do some customization by setting up a F<.perldb> file, which
1085 contains initialization code. For instance, you could make aliases
1086 like these (the last one is one people expect to be there):
1088 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
1089 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
1090 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
1091 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
1093 You can change options from F<.perldb> by using calls like this one;
1095 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
1097 The code is executed in the package C<DB>. Note that F<.perldb> is
1098 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
1099 subroutine C<afterinit>, that function is called after debugger
1100 initialization ends. F<.perldb> may be contained in the current
1101 directory, or in the home directory. Because this file is sourced
1102 in by Perl and may contain arbitrary commands, for security reasons,
1103 it must be owned by the superuser or the current user, and writable
1104 by no one but its owner.
1106 You can mock TTY input to debugger by adding arbitrary commands to
1107 @DB::typeahead. For example, your F<.perldb> file might contain:
1109 sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
1111 Which would attempt to set breakpoints on lines 4 and 6 immediately
1112 after debugger initialization. Note that @DB::typeahead is not a supported
1113 interface and is subject to change in future releases.
1115 If you want to modify the debugger, copy F<perl5db.pl> from the
1116 Perl library to another name and hack it to your heart's content.
1117 You'll then want to set your C<PERL5DB> environment variable to say
1118 something like this:
1120 BEGIN { require "myperl5db.pl" }
1122 As a last resort, you could also use C<PERL5DB> to customize the debugger
1123 by directly setting internal variables or calling debugger functions.
1125 Note that any variables and functions that are not documented in
1126 this document (or in L<perldebguts>) are considered for internal
1127 use only, and as such are subject to change without notice.
1129 =head2 Readline Support / History in the Debugger
1131 As shipped, the only command-line history supplied is a simplistic one
1132 that checks for leading exclamation points. However, if you install
1133 the Term::ReadKey and Term::ReadLine modules from CPAN (such as
1134 Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will
1135 have full editing capabilities much like those GNU I<readline>(3) provides.
1136 Look for these in the F<modules/by-module/Term> directory on CPAN.
1137 These do not support normal B<vi> command-line editing, however.
1139 A rudimentary command-line completion is also available, including
1140 lexical variables in the current scope if the C<PadWalker> module
1143 Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B",
1144 "^[[D"", "^H", ... when using the arrow keys and/or the backspace key.
1146 =head2 Editor Support for Debugging
1148 If you have the GNU's version of B<emacs> installed on your system,
1149 it can interact with the Perl debugger to provide an integrated
1150 software development environment reminiscent of its interactions
1153 Recent versions of Emacs come with a
1154 start file for making B<emacs> act like a
1155 syntax-directed editor that understands (some of) Perl's syntax.
1158 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
1159 and windy version, for coloring of Perl keywords.
1161 Note that only perl can truly parse Perl, so all such CASE tools
1162 fall somewhat short of the mark, especially if you don't program
1163 your Perl as a C programmer might.
1165 =head2 The Perl Profiler
1166 X<profile> X<profiling> X<profiler>
1168 If you wish to supply an alternative debugger for Perl to run,
1169 invoke your script with a colon and a package argument given to the
1170 B<-d> flag. Perl's alternative debuggers include a Perl profiler,
1171 L<Devel::NYTProf>, which is available separately as a CPAN
1172 distribution. To profile your Perl program in the file F<mycode.pl>,
1175 $ perl -d:NYTProf mycode.pl
1177 When the script terminates the profiler will create a database of the
1178 profile information that you can turn into reports using the profiler's
1179 tools. See <perlperf> for details.
1181 =head1 Debugging Regular Expressions
1182 X<regular expression, debugging>
1183 X<regex, debugging> X<regexp, debugging>
1185 C<use re 'debug'> enables you to see the gory details of how the Perl
1186 regular expression engine works. In order to understand this typically
1187 voluminous output, one must not only have some idea about how regular
1188 expression matching works in general, but also know how Perl's regular
1189 expressions are internally compiled into an automaton. These matters
1190 are explored in some detail in
1191 L<perldebguts/"Debugging Regular Expressions">.
1193 =head1 Debugging Memory Usage
1196 Perl contains internal support for reporting its own memory usage,
1197 but this is a fairly advanced concept that requires some understanding
1198 of how memory allocation works.
1199 See L<perldebguts/"Debugging Perl Memory Usage"> for the details.
1203 You do have C<use strict> and C<use warnings> enabled, don't you?
1214 When debugging a script that uses #! and is thus normally found in
1215 $PATH, the -S option causes perl to search $PATH for it, so you don't
1216 have to type the path or C<which $scriptname>.
1222 You cannot get stack frame information or in any fashion debug functions
1223 that were not compiled by Perl, such as those from C or C++ extensions.
1225 If you alter your @_ arguments in a subroutine (such as with C<shift>
1226 or C<pop>), the stack backtrace will not show the original values.
1228 The debugger does not currently work in conjunction with the B<-W>
1229 command-line switch, because it itself is not free of warnings.
1231 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
1232 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
1233 handler, then you won't be able to CTRL-C your way back to the debugger,
1234 because the debugger's own C<$SIG{INT}> handler doesn't understand that
1235 it needs to raise an exception to longjmp(3) out of slow syscalls.