3 perldebug - Perl debugging
7 First of all, have you tried using the B<-w> switch?
9 =head1 The Perl Debugger
11 "As soon as we started programming, we found to our
12 surprise that it wasn't as easy to get programs right
13 as we had thought. Debugging had to be discovered.
14 I can remember the exact instant when I realized that
15 a large part of my life from then on was going to be
16 spent in finding mistakes in my own programs."
17 --Maurice Wilkes, 1949
19 If you invoke Perl with the B<-d> switch, your script runs under the
20 Perl source debugger. This works like an interactive Perl
21 environment, prompting for debugger commands that let you examine
22 source code, set breakpoints, get stack backtraces, change the values of
23 variables, etc. This is so convenient that you often fire up
24 the debugger all by itself just to test out Perl constructs
25 interactively to see what they do. For example:
29 In Perl, the debugger is not a separate program as it usually is in the
30 typical compiled environment. Instead, the B<-d> flag tells the compiler
31 to insert source information into the parse trees it's about to hand off
32 to the interpreter. That means your code must first compile correctly
33 for the debugger to work on it. Then when the interpreter starts up, it
34 preloads a Perl library file containing the debugger itself.
36 The program will halt I<right before> the first run-time executable
37 statement (but see below regarding compile-time statements) and ask you
38 to enter a debugger command. Contrary to popular expectations, whenever
39 the debugger halts and shows you a line of code, it always displays the
40 line it's I<about> to execute, rather than the one it has just executed.
42 Any command not recognized by the debugger is directly executed
43 (C<eval>'d) as Perl code in the current package. (The debugger uses the
44 DB package for its own state information.)
46 Leading white space before a command would cause the debugger to think
47 it's I<NOT> a debugger command but for Perl, so be careful not to do
50 =head2 Debugger Commands
52 The debugger understands the following commands:
58 Prints out a help message.
60 If you supply another debugger command as an argument to the C<h> command,
61 it prints out the description for just that command. The special
62 argument of C<h h> produces a more compact help listing, designed to fit
63 together on one screen.
65 If the output the C<h> command (or any command, for that matter) scrolls
66 past your screen, either precede the command with a leading pipe symbol so
67 it's run through your pager, as in
71 You may change the pager which is used via C<O pager=...> command.
75 Same as C<print {$DB::OUT} expr> in the current package. In particular,
76 because this is just Perl's own B<print> function, this means that nested
77 data structures and objects are not dumped, unlike with the C<x> command.
79 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
80 where STDOUT may be redirected to.
84 Evaluates its expression in list context and dumps out the result
85 in a pretty-printed fashion. Nested data structures are printed out
86 recursively, unlike the C<print> function.
88 The details of printout are governed by multiple C<O>ptions.
92 Display all (or some) variables in package (defaulting to the C<main>
93 package) using a data pretty-printer (hashes show their keys and values so
94 you see what's what, control characters are made printable, etc.). Make
95 sure you don't put the type specifier (like C<$>) there, just the symbol
100 Use C<~pattern> and C<!pattern> for positive and negative regexps.
102 Nested data structures are printed out in a legible fashion, unlike
103 the C<print> function.
105 The details of printout are governed by multiple C<O>ptions.
109 Same as C<V currentpackage [vars]>.
113 Produce a stack backtrace. See below for details on its output.
117 Single step. Executes until it reaches the beginning of another
118 statement, descending into subroutine calls. If an expression is
119 supplied that includes function calls, it too will be single-stepped.
123 Next. Executes over subroutine calls, until it reaches the beginning
124 of the next statement. If an expression is supplied that includes
125 function calls, those functions will be executed with stops before
130 Repeat last C<n> or C<s> command.
134 Continue, optionally inserting a one-time-only breakpoint
135 at the specified line or subroutine.
139 List next window of lines.
143 List C<incr+1> lines starting at C<min>.
147 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
155 List first window of lines from subroutine.
159 List previous window of lines.
163 List window (a few lines) around the current line.
167 Return debugger pointer to the last-executed line and
172 Switch to viewing a different file or eval statement. If C<filename>
173 is not a full filename as found in values of %INC, it is considered as
178 Search forwards for pattern; final / is optional.
182 Search backwards for pattern; final ? is optional.
186 List all breakpoints and actions.
190 List subroutine names [not] matching pattern.
194 Toggle trace mode (see also C<AutoTrace> C<O>ption).
198 Trace through execution of expr. For example:
201 Stack dump during die enabled outside of evals.
203 Loading DB routines from perl5db.pl patch level 0.94
204 Emacs support available.
206 Enter h or `h h' for help.
213 DB<3> t print foo() * bar()
214 main::((eval 172):3): print foo() + bar();
215 main::foo((eval 168):2):
216 main::bar((eval 170):2):
219 or, with the C<O>ption C<frame=2> set,
223 DB<5> t print foo() * bar()
233 =item b [line] [condition]
235 Set a breakpoint. If line is omitted, sets a breakpoint on the line
236 that is about to be executed. If a condition is specified, it's
237 evaluated each time the statement is reached and a breakpoint is taken
238 only if the condition is true. Breakpoints may be set on only lines
239 that begin an executable statement. Conditions don't use B<if>:
242 b 237 ++$count237 < 11
245 =item b subname [condition]
247 Set a breakpoint at the first line of the named subroutine.
249 =item b postpone subname [condition]
251 Set breakpoint at first line of subroutine after it is compiled.
253 =item b load filename
255 Set breakpoint at the first executed line of the file. Filename should
256 be a full name as found in values of %INC.
258 =item b compile subname
260 Sets breakpoint at the first statement executed after the subroutine
265 Delete a breakpoint at the specified line. If line is omitted, deletes
266 the breakpoint on the line that is about to be executed.
270 Delete all installed breakpoints.
272 =item a [line] command
274 Set an action to be done before the line is executed.
275 The sequence of steps taken by the debugger is
277 1. check for a breakpoint at this line
278 2. print the line if necessary (tracing)
279 3. do any actions associated with that line
280 4. prompt user if at a breakpoint or in single-step
283 For example, this will print out C<$foo> every time line
286 a 53 print "DB FOUND $foo\n"
290 Delete all installed actions.
292 =item O [opt[=val]] [opt"val"] [opt?]...
294 Set or query values of options. val defaults to 1. opt can
295 be abbreviated. Several options can be listed.
299 =item C<recallCommand>, C<ShellBang>
301 The characters used to recall command or spawn shell. By
302 default, these are both set to C<!>.
306 Program to use for output of pager-piped commands (those
307 beginning with a C<|> character.) By default,
308 C<$ENV{PAGER}> will be used.
312 Run Tk while prompting (with ReadLine).
314 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
316 Level of verbosity. By default the debugger is in a sane verbose mode,
317 thus it will print backtraces on all the warnings and die-messages
318 which are going to be printed out, and will print a message when
319 interesting uncaught signals arrive.
321 To disable this behaviour, set these values to 0. If C<dieLevel> is 2,
322 then the messages which will be caught by surrounding C<eval> are also
327 Trace mode (similar to C<t> command, but can be put into
332 File or pipe to print line number info to. If it is a pipe (say,
333 C<|visual_perl_db>), then a short, "emacs like" message is used.
335 =item C<inhibit_exit>
337 If 0, allows I<stepping off> the end of the script.
341 affects printing of return value after C<r> command.
345 affects screen appearance of the command line (see L<Term::ReadLine>).
349 affects printing messages on entry and exit from subroutines. If
350 C<frame & 2> is false, messages are printed on entry only. (Printing
351 on exit may be useful if inter(di)spersed with other messages.)
353 If C<frame & 4>, arguments to functions are printed as well as the
354 context and caller info. If C<frame & 8>, overloaded C<stringify> and
355 C<tie>d C<FETCH> are enabled on the printed arguments. If C<frame &
356 16>, the return value from the subroutine is printed as well.
358 The length at which the argument list is truncated is governed by the
363 length at which the argument list is truncated when C<frame> option's
368 The following options affect what happens with C<V>, C<X>, and C<x>
373 =item C<arrayDepth>, C<hashDepth>
375 Print only first N elements ('' for all).
377 =item C<compactDump>, C<veryCompact>
379 Change style of array and hash dump. If C<compactDump>, short array
380 may be printed on one line.
384 Whether to print contents of globs.
388 Dump arrays holding debugged files.
390 =item C<DumpPackages>
392 Dump symbol tables of packages.
394 =item C<quote>, C<HighBit>, C<undefPrint>
396 Change style of string dump. Default value of C<quote> is C<auto>, one
397 can enable either double-quotish dump, or single-quotish by setting it
398 to C<"> or C<'>. By default, characters with high bit set are printed
403 I<very> rudimentally per-package memory usage dump. Calculates total
404 size of strings in variables in the package.
408 During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
409 You can put additional initialization options C<TTY>, C<noTTY>,
410 C<ReadLine>, and C<NonStop> there.
414 &parse_options("NonStop=1 LineInfo=db.out AutoTrace");
416 The script will run without human intervention, putting trace information
417 into the file I<db.out>. (If you interrupt it, you would better reset
418 C<LineInfo> to something "interactive"!)
424 The TTY to use for debugging I/O.
428 If set, goes in C<NonStop> mode, and would not connect to a TTY. If
429 interrupt (or if control goes to debugger via explicit setting of
430 $DB::signal or $DB::single from the Perl script), connects to a TTY
431 specified by the C<TTY> option at startup, or to a TTY found at
432 runtime using C<Term::Rendezvous> module of your choice.
434 This module should implement a method C<new> which returns an object
435 with two methods: C<IN> and C<OUT>, returning two filehandles to use
436 for debugging input and output correspondingly. Method C<new> may
437 inspect an argument which is a value of C<$ENV{PERLDB_NOTTY}> at
438 startup, or is C<"/tmp/perldbtty$$"> otherwise.
442 If false, readline support in debugger is disabled, so you can debug
443 ReadLine applications.
447 If set, debugger goes into noninteractive mode until interrupted, or
448 programmatically by setting $DB::signal or $DB::single.
452 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
454 $ PERLDB_OPTS="N f=2" perl -d myprogram
456 will run the script C<myprogram> without human intervention, printing
457 out the call tree with entry and exit points. Note that C<N f=2> is
458 equivalent to C<NonStop=1 frame=2>. Note also that at the moment when
459 this documentation was written all the options to the debugger could
460 be uniquely abbreviated by the first letter (with exception of
463 Other examples may include
465 $ PERLDB_OPTS="N f A L=listing" perl -d myprogram
467 - runs script noninteractively, printing info on each entry into a
468 subroutine and each executed line into the file F<listing>. (If you
469 interrupt it, you would better reset C<LineInfo> to something
473 $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
475 may be useful for debugging a program which uses C<Term::ReadLine>
476 itself. Do not forget detach shell from the TTY in the window which
477 corresponds to F</dev/ttyc>, say, by issuing a command like
481 See L<"Debugger Internals"> below for more details.
483 =item E<lt> [ command ]
485 Set an action (Perl command) to happen before every debugger prompt.
486 A multi-line command may be entered by backslashing the newlines. If
487 C<command> is missing, resets the list of actions.
489 =item E<lt>E<lt> command
491 Add an action (Perl command) to happen before every debugger prompt.
492 A multi-line command may be entered by backslashing the newlines.
496 Set an action (Perl command) to happen after the prompt when you've
497 just given a command to return to executing the script. A multi-line
498 command may be entered by backslashing the newlines. If C<command> is
499 missing, resets the list of actions.
501 =item E<gt>E<gt> command
503 Adds an action (Perl command) to happen after the prompt when you've
504 just given a command to return to executing the script. A multi-line
505 command may be entered by backslashing the newlines.
509 Set an action (debugger command) to happen before every debugger prompt.
510 A multi-line command may be entered by backslashing the newlines. If
511 C<command> is missing, resets the list of actions.
515 Add an action (debugger command) to happen before every debugger prompt.
516 A multi-line command may be entered by backslashing the newlines.
520 Redo a previous command (default previous command).
524 Redo number'th-to-last command.
528 Redo last command that started with pattern.
529 See C<O recallCommand>, too.
533 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT)
534 See C<O shellBang> too.
538 Display last n commands. Only commands longer than one character are
539 listed. If number is omitted, lists them all.
543 Quit. ("quit" doesn't work for this.) This is the only supported way
544 to exit the debugger, though typing C<exit> twice may do it too.
546 Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
547 off> the end the script. You may also need to set C<$finished> to 0 at
548 some moment if you want to step through global destruction.
552 Restart the debugger by B<exec>ing a new session. It tries to maintain
553 your history across this, but internal settings and command line options
556 Currently the following setting are preserved: history, breakpoints,
557 actions, debugger C<O>ptions, and the following command line
558 options: B<-w>, B<-I>, and B<-e>.
562 Run debugger command, piping DB::OUT to current pager.
566 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
567 Often used with commands that would otherwise produce long
572 =item = [alias value]
574 Define a command alias, like
578 or list current aliases.
582 Execute command as a Perl statement. A missing semicolon will be
587 The expression is evaluated, and the methods which may be applied to
588 the result are listed.
592 The methods which may be applied to objects in the C<package> are listed.
596 =head2 Debugger input/output
602 The debugger prompt is something like
610 where that number is the command number, which you'd use to access with
611 the builtin B<csh>-like history mechanism, e.g., C<!17> would repeat
612 command number 17. The number of angle brackets indicates the depth of
613 the debugger. You could get more than one set of brackets, for example, if
614 you'd already at a breakpoint and then printed out the result of a
615 function call that itself also has a breakpoint, or you step into an
616 expression via C<s/n/t expression> command.
618 =item Multiline commands
620 If you want to enter a multi-line command, such as a subroutine
621 definition with several statements, or a format, you may escape the
622 newline that would normally end the debugger command with a backslash.
626 cont: print "ok\n"; \
633 Note that this business of escaping a newline is specific to interactive
634 commands typed into the debugger.
636 =item Stack backtrace
638 Here's an example of what a stack backtrace via C<T> command might
641 $ = main::infested called from file `Ambulation.pm' line 10
642 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
643 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
645 The left-hand character up there tells whether the function was called
646 in a scalar or list context (we bet you can tell which is which). What
647 that says is that you were in the function C<main::infested> when you ran
648 the stack dump, and that it was called in a scalar context from line 10
649 of the file I<Ambulation.pm>, but without any arguments at all, meaning
650 it was called as C<&infested>. The next stack frame shows that the
651 function C<Ambulation::legs> was called in a list context from the
652 I<camel_flea> file with four arguments. The last stack frame shows that
653 C<main::pests> was called in a scalar context, also from I<camel_flea>,
656 Note that if you execute C<T> command from inside an active C<use>
657 statement, the backtrace will contain both C<L<perlfunc/require>>
658 frame and an C<L<perlfunc/eval EXPR>>) frame.
662 Listing given via different flavors of C<l> command looks like this:
666 102:b @isa{@i,$pack} = ()
667 103 if(exists $i{$prevpack} || exists $isa{$pack});
671 107==> if(exists $isa{$pack});
673 109:a if ($extra-- > 0) {
674 110: %isa = ($pack,1);
676 Note that the breakable lines are marked with C<:>, lines with
677 breakpoints are marked by C<b>, with actions by C<a>, and the
678 next executed line is marked by C<==E<gt>>.
682 When C<frame> option is set, debugger would print entered (and
683 optionally exited) subroutines in different styles.
685 What follows is the start of the listing of
687 env "PERLDB_OPTS=f=n N" perl -d -V
689 for different values of C<n>:
696 entering Config::BEGIN
697 Package lib/Exporter.pm.
699 Package lib/Config.pm.
700 entering Config::TIEHASH
701 entering Exporter::import
702 entering Exporter::export
703 entering Config::myconfig
704 entering Config::FETCH
705 entering Config::FETCH
706 entering Config::FETCH
707 entering Config::FETCH
712 entering Config::BEGIN
713 Package lib/Exporter.pm.
716 Package lib/Config.pm.
717 entering Config::TIEHASH
718 exited Config::TIEHASH
719 entering Exporter::import
720 entering Exporter::export
721 exited Exporter::export
722 exited Exporter::import
724 entering Config::myconfig
725 entering Config::FETCH
727 entering Config::FETCH
729 entering Config::FETCH
733 in $=main::BEGIN() from /dev/nul:0
734 in $=Config::BEGIN() from lib/Config.pm:2
735 Package lib/Exporter.pm.
737 Package lib/Config.pm.
738 in $=Config::TIEHASH('Config') from lib/Config.pm:644
739 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
740 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
741 in @=Config::myconfig() from /dev/nul:0
742 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
743 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
744 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
745 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
746 in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
747 in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
751 in $=main::BEGIN() from /dev/nul:0
752 in $=Config::BEGIN() from lib/Config.pm:2
753 Package lib/Exporter.pm.
755 out $=Config::BEGIN() from lib/Config.pm:0
756 Package lib/Config.pm.
757 in $=Config::TIEHASH('Config') from lib/Config.pm:644
758 out $=Config::TIEHASH('Config') from lib/Config.pm:644
759 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
760 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
761 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
762 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
763 out $=main::BEGIN() from /dev/nul:0
764 in @=Config::myconfig() from /dev/nul:0
765 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
766 out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
767 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
768 out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
769 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
770 out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
771 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
775 in $=main::BEGIN() from /dev/nul:0
776 in $=Config::BEGIN() from lib/Config.pm:2
777 Package lib/Exporter.pm.
779 out $=Config::BEGIN() from lib/Config.pm:0
780 Package lib/Config.pm.
781 in $=Config::TIEHASH('Config') from lib/Config.pm:644
782 out $=Config::TIEHASH('Config') from lib/Config.pm:644
783 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
784 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
785 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
786 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
787 out $=main::BEGIN() from /dev/nul:0
788 in @=Config::myconfig() from /dev/nul:0
789 in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
790 out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
791 in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
792 out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
796 in $=CODE(0x15eca4)() from /dev/null:0
797 in $=CODE(0x182528)() from lib/Config.pm:2
798 Package lib/Exporter.pm.
799 out $=CODE(0x182528)() from lib/Config.pm:0
800 scalar context return from CODE(0x182528): undef
801 Package lib/Config.pm.
802 in $=Config::TIEHASH('Config') from lib/Config.pm:628
803 out $=Config::TIEHASH('Config') from lib/Config.pm:628
804 scalar context return from Config::TIEHASH: empty hash
805 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
806 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
807 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
808 scalar context return from Exporter::export: ''
809 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
810 scalar context return from Exporter::import: ''
815 In all the cases indentation of lines shows the call tree, if bit 2 of
816 C<frame> is set, then a line is printed on exit from a subroutine as
817 well, if bit 4 is set, then the arguments are printed as well as the
818 caller info, if bit 8 is set, the arguments are printed even if they
819 are tied or references, if bit 16 is set, the return value is printed
822 When a package is compiled, a line like this
826 is printed with proper indentation.
830 =head2 Debugging compile-time statements
832 If you have any compile-time executable statements (code within a BEGIN
833 block or a C<use> statement), these will C<NOT> be stopped by debugger,
834 although C<require>s will (and compile-time statements can be traced
835 with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
836 code, however, you can
837 transfer control back to the debugger using the following statement,
838 which is harmless if the debugger is not running:
842 If you set C<$DB::single> to the value 2, it's equivalent to having
843 just typed the C<n> command, whereas a value of 1 means the C<s>
844 command. The C<$DB::trace> variable should be set to 1 to simulate
845 having typed the C<t> command.
847 Another way to debug compile-time code is to start debugger, set a
848 breakpoint on I<load> of some module thusly
850 DB<7> b load f:/perllib/lib/Carp.pm
851 Will stop on load of `f:/perllib/lib/Carp.pm'.
853 and restart debugger by C<R> command (if possible). One can use C<b
854 compile subname> for the same purpose.
856 =head2 Debugger Customization
858 Most probably you not want to modify the debugger, it contains enough
859 hooks to satisfy most needs. You may change the behaviour of debugger
860 from the debugger itself, using C<O>ptions, from the command line via
861 C<PERLDB_OPTS> environment variable, and from I<customization files>.
863 You can do some customization by setting up a F<.perldb> file which
864 contains initialization code. For instance, you could make aliases
865 like these (the last one is one people expect to be there):
867 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
868 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
869 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
870 $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
872 One changes options from F<.perldb> file via calls like this one;
874 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
876 (the code is executed in the package C<DB>). Note that F<.perldb> is
877 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
878 subroutine C<afterinit>, it is called after all the debugger
879 initialization ends. F<.perldb> may be contained in the current
880 directory, or in the C<LOGDIR>/C<HOME> directory.
882 If you want to modify the debugger, copy F<perl5db.pl> from the Perl
883 library to another name and modify it as necessary. You'll also want
884 to set your C<PERL5DB> environment variable to say something like this:
886 BEGIN { require "myperl5db.pl" }
888 As the last resort, one can use C<PERL5DB> to customize debugger by
889 directly setting internal variables or calling debugger functions.
891 =head2 Readline Support
893 As shipped, the only command line history supplied is a simplistic one
894 that checks for leading exclamation points. However, if you install
895 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
896 have full editing capabilities much like GNU I<readline>(3) provides.
897 Look for these in the F<modules/by-module/Term> directory on CPAN.
899 A rudimentary command line completion is also available.
900 Unfortunately, the names of lexical variables are not available for
903 =head2 Editor Support for Debugging
905 If you have GNU B<emacs> installed on your system, it can interact with
906 the Perl debugger to provide an integrated software development
907 environment reminiscent of its interactions with C debuggers.
909 Perl is also delivered with a start file for making B<emacs> act like a
910 syntax-directed editor that understands (some of) Perl's syntax. Look in
911 the I<emacs> directory of the Perl source distribution.
913 (Historically, a similar setup for interacting with B<vi> and the
914 X11 window system had also been available, but at the time of this
915 writing, no debugger support for B<vi> currently exists.)
917 =head2 The Perl Profiler
919 If you wish to supply an alternative debugger for Perl to run, just
920 invoke your script with a colon and a package argument given to the B<-d>
921 flag. One of the most popular alternative debuggers for Perl is
922 B<DProf>, the Perl profiler. As of this writing, B<DProf> is not
923 included with the standard Perl distribution, but it is expected to
924 be included soon, for certain values of "soon".
926 Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming
927 it's properly installed on your system, to profile your Perl program in
928 the file F<mycode.pl>, just type:
930 perl -d:DProf mycode.pl
932 When the script terminates the profiler will dump the profile information
933 to a file called F<tmon.out>. A tool like B<dprofpp> (also supplied with
934 the Devel::DProf package) can be used to interpret the information which is
937 =head2 Debugger support in perl
939 When you call the B<caller> function (see L<perlfunc/caller>) from the
940 package DB, Perl sets the array @DB::args to contain the arguments the
941 corresponding stack frame was called with.
943 If perl is run with B<-d> option, the following additional features
950 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
951 'perl5db.pl'}> if not present) before the first line of the
956 The array C<@{"_<$filename"}> is the line-by-line contents of
957 $filename for all the compiled files. Same for C<eval>ed strings which
958 contain subroutines, or which are currently executed. The C<$filename>
959 for C<eval>ed strings looks like C<(eval 34)>.
963 The hash C<%{"_<$filename"}> contains breakpoints and action (it is
964 keyed by line number), and individual entries are settable (as opposed
965 to the whole hash). Only true/false is important to Perl, though the
966 values used by F<perl5db.pl> have the form
967 C<"$break_condition\0$action">. Values are magical in numeric context:
968 they are zeros if the line is not breakable.
970 Same for evaluated strings which contain subroutines, or which are
971 currently executed. The C<$filename> for C<eval>ed strings looks like
976 The scalar C<${"_<$filename"}> contains C<"_<$filename">. Same for
977 evaluated strings which contain subroutines, or which are currently
978 executed. The C<$filename> for C<eval>ed strings looks like C<(eval
983 After each C<require>d file is compiled, but before it is executed,
984 C<DB::postponed(*{"_<$filename"})> is called (if subroutine
985 C<DB::postponed> exists). Here the $filename is the expanded name of
986 the C<require>d file (as found in values of C<%INC>).
990 After each subroutine C<subname> is compiled existence of
991 C<$DB::postponed{subname}> is checked. If this key exists,
992 C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
997 A hash C<%DB::sub> is maintained, with keys being subroutine names,
998 values having the form C<filename:startline-endline>. C<filename> has
999 the form C<(eval 31)> for subroutines defined inside C<eval>s.
1003 When execution of the application reaches a place that can have
1004 a breakpoint, a call to C<DB::DB()> is performed if any one of
1005 variables $DB::trace, $DB::single, or $DB::signal is true. (Note that
1006 these variables are not C<local>izable.) This feature is disabled when
1007 the control is inside C<DB::DB()> or functions called from it (unless
1008 C<$^D & (1E<lt>E<lt>30)>).
1012 When execution of the application reaches a subroutine call, a call
1013 to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
1014 the name of the called subroutine. (Unless the subroutine is compiled
1015 in the package C<DB>.)
1019 Note that no subroutine call is possible until C<&DB::sub> is defined
1020 (for subroutines outside of package C<DB>). (This restriction is
1023 (In fact, for the standard debugger the same is true if C<$DB::deep>
1024 (how many levels of recursion deep into the debugger you can go before
1025 a mandatory break) is not defined.)
1027 With the recent updates the minimal possible debugger consists of one
1032 which is quite handy as contents of C<PERL5DB> environment
1035 env "PERL5DB=sub DB::DB {}" perl -d your-script
1037 Another (a little bit more useful) minimal debugger can be created
1038 with the only line being
1040 sub DB::DB {print ++$i; scalar <STDIN>}
1042 This debugger would print the sequential number of encountered
1043 statement, and would wait for your C<CR> to continue.
1045 The following debugger is quite functional:
1050 sub sub {print ++$i, " $sub\n"; &$sub}
1053 It prints the sequential number of subroutine call and the name of the
1054 called subroutine. Note that C<&DB::sub> should be compiled into the
1057 =head2 Debugger Internals
1059 At the start, the debugger reads your rc file (F<./.perldb> or
1060 F<~/.perldb> under Unix), which can set important options. This file may
1061 define a subroutine C<&afterinit> to be executed after the debugger is
1064 After the rc file is read, the debugger reads environment variable
1065 PERLDB_OPTS and parses it as a rest of C<O ...> line in debugger prompt.
1067 It also maintains magical internal variables, such as C<@DB::dbline>,
1068 C<%DB::dbline>, which are aliases for C<@{"::_<current_file"}>
1069 C<%{"::_<current_file"}>. Here C<current_file> is the currently
1070 selected (with the debugger's C<f> command, or by flow of execution)
1073 Some functions are provided to simplify customization. See L<"Debugger
1074 Customization"> for description of C<DB::parse_options(string)>. The
1075 function C<DB::dump_trace(skip[, count])> skips the specified number
1076 of frames, and returns an array containing info about the caller
1077 frames (all if C<count> is missing). Each entry is a hash with keys
1078 C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
1079 eval), C<args> (C<undef> or a reference to an array), C<file>, and
1082 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
1083 formatted info about caller frames. The last two functions may be
1084 convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.
1086 =head2 Other resources
1088 You did try the B<-w> switch, didn't you?
1092 You cannot get the stack frame information or otherwise debug functions
1093 that were not compiled by Perl, such as C or C++ extensions.
1095 If you alter your @_ arguments in a subroutine (such as with B<shift>
1096 or B<pop>, the stack backtrace will not show the original values.