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."
18 I< --Maurice Wilkes, 1949>
20 If you invoke Perl with the B<-d> switch, your script runs under the
21 Perl source debugger. This works like an interactive Perl
22 environment, prompting for debugger commands that let you examine
23 source code, set breakpoints, get stack backtraces, change the values of
24 variables, etc. This is so convenient that you often fire up
25 the debugger all by itself just to test out Perl constructs
26 interactively to see what they do. For example:
30 In Perl, the debugger is not a separate program as it usually is in the
31 typical compiled environment. Instead, the B<-d> flag tells the compiler
32 to insert source information into the parse trees it's about to hand off
33 to the interpreter. That means your code must first compile correctly
34 for the debugger to work on it. Then when the interpreter starts up, it
35 preloads a Perl library file containing the debugger itself.
37 The program will halt I<right before> the first run-time executable
38 statement (but see below regarding compile-time statements) and ask you
39 to enter a debugger command. Contrary to popular expectations, whenever
40 the debugger halts and shows you a line of code, it always displays the
41 line it's I<about> to execute, rather than the one it has just executed.
43 Any command not recognized by the debugger is directly executed
44 (C<eval>'d) as Perl code in the current package. (The debugger uses the
45 DB package for its own state information.)
47 Leading white space before a command would cause the debugger to think
48 it's I<NOT> a debugger command but for Perl, so be careful not to do
51 =head2 Debugger Commands
53 The debugger understands the following commands:
59 Prints out a help message.
61 If you supply another debugger command as an argument to the C<h> command,
62 it prints out the description for just that command. The special
63 argument of C<h h> produces a more compact help listing, designed to fit
64 together on one screen.
66 If the output of the C<h> command (or any command, for that matter) scrolls
67 past your screen, either precede the command with a leading pipe symbol so
68 it's run through your pager, as in
72 You may change the pager which is used via C<O pager=...> command.
76 Same as C<print {$DB::OUT} expr> in the current package. In particular,
77 because this is just Perl's own B<print> function, this means that nested
78 data structures and objects are not dumped, unlike with the C<x> command.
80 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
81 where STDOUT may be redirected to.
85 Evaluates its expression in list context and dumps out the result
86 in a pretty-printed fashion. Nested data structures are printed out
87 recursively, unlike the C<print> function.
89 The details of printout are governed by multiple C<O>ptions.
93 Display all (or some) variables in package (defaulting to the C<main>
94 package) using a data pretty-printer (hashes show their keys and values so
95 you see what's what, control characters are made printable, etc.). Make
96 sure you don't put the type specifier (like C<$>) there, just the symbol
101 Use C<~pattern> and C<!pattern> for positive and negative regexps.
103 Nested data structures are printed out in a legible fashion, unlike
104 the C<print> function.
106 The details of printout are governed by multiple C<O>ptions.
110 Same as C<V currentpackage [vars]>.
114 Produce a stack backtrace. See below for details on its output.
118 Single step. Executes until it reaches the beginning of another
119 statement, descending into subroutine calls. If an expression is
120 supplied that includes function calls, it too will be single-stepped.
124 Next. Executes over subroutine calls, until it reaches the beginning
125 of the next statement. If an expression is supplied that includes
126 function calls, those functions will be executed with stops before
131 Repeat last C<n> or C<s> command.
135 Continue, optionally inserting a one-time-only breakpoint
136 at the specified line or subroutine.
140 List next window of lines.
144 List C<incr+1> lines starting at C<min>.
148 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
156 List first window of lines from subroutine.
160 List previous window of lines.
164 List window (a few lines) around the current line.
168 Return debugger pointer to the last-executed line and
173 Switch to viewing a different file or eval statement. If C<filename>
174 is not a full filename as found in values of %INC, it is considered as
179 Search forwards for pattern; final / is optional.
183 Search backwards for pattern; final ? is optional.
187 List all breakpoints and actions.
191 List subroutine names [not] matching pattern.
195 Toggle trace mode (see also C<AutoTrace> C<O>ption).
199 Trace through execution of expr. For example:
202 Stack dump during die enabled outside of evals.
204 Loading DB routines from perl5db.pl patch level 0.94
205 Emacs support available.
207 Enter h or `h h' for help.
214 DB<3> t print foo() * bar()
215 main::((eval 172):3): print foo() + bar();
216 main::foo((eval 168):2):
217 main::bar((eval 170):2):
220 or, with the C<O>ption C<frame=2> set,
224 DB<5> t print foo() * bar()
234 =item b [line] [condition]
236 Set a breakpoint. If line is omitted, sets a breakpoint on the line
237 that is about to be executed. If a condition is specified, it's
238 evaluated each time the statement is reached and a breakpoint is taken
239 only if the condition is true. Breakpoints may be set on only lines
240 that begin an executable statement. Conditions don't use B<if>:
243 b 237 ++$count237 < 11
246 =item b subname [condition]
248 Set a breakpoint at the first line of the named subroutine.
250 =item b postpone subname [condition]
252 Set breakpoint at first line of subroutine after it is compiled.
254 =item b load filename
256 Set breakpoint at the first executed line of the file. Filename should
257 be a full name as found in values of %INC.
259 =item b compile subname
261 Sets breakpoint at the first statement executed after the subroutine
266 Delete a breakpoint at the specified line. If line is omitted, deletes
267 the breakpoint on the line that is about to be executed.
271 Delete all installed breakpoints.
273 =item a [line] command
275 Set an action to be done before the line is executed.
276 The sequence of steps taken by the debugger is
278 1. check for a breakpoint at this line
279 2. print the line if necessary (tracing)
280 3. do any actions associated with that line
281 4. prompt user if at a breakpoint or in single-step
284 For example, this will print out $foo every time line
287 a 53 print "DB FOUND $foo\n"
291 Delete all installed actions.
295 Add a global watch-expression.
299 Delete all watch-expressions.
301 =item O [opt[=val]] [opt"val"] [opt?]...
303 Set or query values of options. val defaults to 1. opt can
304 be abbreviated. Several options can be listed.
308 =item C<recallCommand>, C<ShellBang>
310 The characters used to recall command or spawn shell. By
311 default, these are both set to C<!>.
315 Program to use for output of pager-piped commands (those
316 beginning with a C<|> character.) By default,
317 C<$ENV{PAGER}> will be used.
321 Run Tk while prompting (with ReadLine).
323 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
325 Level of verbosity. By default the debugger is in a sane verbose mode,
326 thus it will print backtraces on all the warnings and die-messages
327 which are going to be printed out, and will print a message when
328 interesting uncaught signals arrive.
330 To disable this behaviour, set these values to 0. If C<dieLevel> is 2,
331 then the messages which will be caught by surrounding C<eval> are also
336 Trace mode (similar to C<t> command, but can be put into
341 File or pipe to print line number info to. If it is a pipe (say,
342 C<|visual_perl_db>), then a short, "emacs like" message is used.
344 =item C<inhibit_exit>
346 If 0, allows I<stepping off> the end of the script.
350 affects printing of return value after C<r> command.
354 affects screen appearance of the command line (see L<Term::ReadLine>).
358 affects printing messages on entry and exit from subroutines. If
359 C<frame & 2> is false, messages are printed on entry only. (Printing
360 on exit may be useful if inter(di)spersed with other messages.)
362 If C<frame & 4>, arguments to functions are printed as well as the
363 context and caller info. If C<frame & 8>, overloaded C<stringify> and
364 C<tie>d C<FETCH> are enabled on the printed arguments. If C<frame &
365 16>, the return value from the subroutine is printed as well.
367 The length at which the argument list is truncated is governed by the
372 length at which the argument list is truncated when C<frame> option's
377 The following options affect what happens with C<V>, C<X>, and C<x>
382 =item C<arrayDepth>, C<hashDepth>
384 Print only first N elements ('' for all).
386 =item C<compactDump>, C<veryCompact>
388 Change style of array and hash dump. If C<compactDump>, short array
389 may be printed on one line.
393 Whether to print contents of globs.
397 Dump arrays holding debugged files.
399 =item C<DumpPackages>
401 Dump symbol tables of packages.
405 Dump contents of "reused" addresses.
407 =item C<quote>, C<HighBit>, C<undefPrint>
409 Change style of string dump. Default value of C<quote> is C<auto>, one
410 can enable either double-quotish dump, or single-quotish by setting it
411 to C<"> or C<'>. By default, characters with high bit set are printed
416 I<very> rudimentally per-package memory usage dump. Calculates total
417 size of strings in variables in the package.
421 During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
422 You can put additional initialization options C<TTY>, C<noTTY>,
423 C<ReadLine>, and C<NonStop> there.
427 &parse_options("NonStop=1 LineInfo=db.out AutoTrace");
429 The script will run without human intervention, putting trace information
430 into the file I<db.out>. (If you interrupt it, you would better reset
431 C<LineInfo> to something "interactive"!)
437 The TTY to use for debugging I/O.
441 If set, goes in C<NonStop> mode, and would not connect to a TTY. If
442 interrupt (or if control goes to debugger via explicit setting of
443 $DB::signal or $DB::single from the Perl script), connects to a TTY
444 specified by the C<TTY> option at startup, or to a TTY found at
445 runtime using C<Term::Rendezvous> module of your choice.
447 This module should implement a method C<new> which returns an object
448 with two methods: C<IN> and C<OUT>, returning two filehandles to use
449 for debugging input and output correspondingly. Method C<new> may
450 inspect an argument which is a value of C<$ENV{PERLDB_NOTTY}> at
451 startup, or is C<"/tmp/perldbtty$$"> otherwise.
455 If false, readline support in debugger is disabled, so you can debug
456 ReadLine applications.
460 If set, debugger goes into noninteractive mode until interrupted, or
461 programmatically by setting $DB::signal or $DB::single.
465 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
467 $ PERLDB_OPTS="N f=2" perl -d myprogram
469 will run the script C<myprogram> without human intervention, printing
470 out the call tree with entry and exit points. Note that C<N f=2> is
471 equivalent to C<NonStop=1 frame=2>. Note also that at the moment when
472 this documentation was written all the options to the debugger could
473 be uniquely abbreviated by the first letter (with exception of
476 Other examples may include
478 $ PERLDB_OPTS="N f A L=listing" perl -d myprogram
480 - runs script noninteractively, printing info on each entry into a
481 subroutine and each executed line into the file F<listing>. (If you
482 interrupt it, you would better reset C<LineInfo> to something
486 $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
488 may be useful for debugging a program which uses C<Term::ReadLine>
489 itself. Do not forget detach shell from the TTY in the window which
490 corresponds to F</dev/ttyc>, say, by issuing a command like
494 See L<"Debugger Internals"> below for more details.
496 =item E<lt> [ command ]
498 Set an action (Perl command) to happen before every debugger prompt.
499 A multi-line command may be entered by backslashing the newlines. If
500 C<command> is missing, resets the list of actions.
502 =item E<lt>E<lt> command
504 Add an action (Perl command) to happen before every debugger prompt.
505 A multi-line command may be entered by backslashing the newlines.
509 Set an action (Perl command) to happen after the prompt when you've
510 just given a command to return to executing the script. A multi-line
511 command may be entered by backslashing the newlines. If C<command> is
512 missing, resets the list of actions.
514 =item E<gt>E<gt> command
516 Adds an action (Perl command) to happen after the prompt when you've
517 just given a command to return to executing the script. A multi-line
518 command may be entered by backslashing the newlines.
522 Set an action (debugger command) to happen before every debugger prompt.
523 A multi-line command may be entered by backslashing the newlines. If
524 C<command> is missing, resets the list of actions.
528 Add an action (debugger command) to happen before every debugger prompt.
529 A multi-line command may be entered by backslashing the newlines.
533 Redo a previous command (default previous command).
537 Redo number'th-to-last command.
541 Redo last command that started with pattern.
542 See C<O recallCommand>, too.
546 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT)
547 See C<O shellBang> too.
551 Display last n commands. Only commands longer than one character are
552 listed. If number is omitted, lists them all.
556 Quit. ("quit" doesn't work for this.) This is the only supported way
557 to exit the debugger, though typing C<exit> twice may do it too.
559 Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
560 off> the end the script. You may also need to set C<$finished> to 0 at
561 some moment if you want to step through global destruction.
565 Restart the debugger by B<exec>ing a new session. It tries to maintain
566 your history across this, but internal settings and command line options
569 Currently the following setting are preserved: history, breakpoints,
570 actions, debugger C<O>ptions, and the following command line
571 options: B<-w>, B<-I>, and B<-e>.
575 Run debugger command, piping DB::OUT to current pager.
579 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
580 Often used with commands that would otherwise produce long
585 =item = [alias value]
587 Define a command alias, like
591 or list current aliases.
595 Execute command as a Perl statement. A missing semicolon will be
600 The expression is evaluated, and the methods which may be applied to
601 the result are listed.
605 The methods which may be applied to objects in the C<package> are listed.
609 =head2 Debugger input/output
615 The debugger prompt is something like
623 where that number is the command number, which you'd use to access with
624 the builtin B<csh>-like history mechanism, e.g., C<!17> would repeat
625 command number 17. The number of angle brackets indicates the depth of
626 the debugger. You could get more than one set of brackets, for example, if
627 you'd already at a breakpoint and then printed out the result of a
628 function call that itself also has a breakpoint, or you step into an
629 expression via C<s/n/t expression> command.
631 =item Multiline commands
633 If you want to enter a multi-line command, such as a subroutine
634 definition with several statements, or a format, you may escape the
635 newline that would normally end the debugger command with a backslash.
639 cont: print "ok\n"; \
646 Note that this business of escaping a newline is specific to interactive
647 commands typed into the debugger.
649 =item Stack backtrace
651 Here's an example of what a stack backtrace via C<T> command might
654 $ = main::infested called from file `Ambulation.pm' line 10
655 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
656 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
658 The left-hand character up there tells whether the function was called
659 in a scalar or list context (we bet you can tell which is which). What
660 that says is that you were in the function C<main::infested> when you ran
661 the stack dump, and that it was called in a scalar context from line 10
662 of the file I<Ambulation.pm>, but without any arguments at all, meaning
663 it was called as C<&infested>. The next stack frame shows that the
664 function C<Ambulation::legs> was called in a list context from the
665 I<camel_flea> file with four arguments. The last stack frame shows that
666 C<main::pests> was called in a scalar context, also from I<camel_flea>,
669 Note that if you execute C<T> command from inside an active C<use>
670 statement, the backtrace will contain both C<require>
671 frame and an C<eval>) frame.
675 Listing given via different flavors of C<l> command looks like this:
679 102:b @isa{@i,$pack} = ()
680 103 if(exists $i{$prevpack} || exists $isa{$pack});
684 107==> if(exists $isa{$pack});
686 109:a if ($extra-- > 0) {
687 110: %isa = ($pack,1);
689 Note that the breakable lines are marked with C<:>, lines with
690 breakpoints are marked by C<b>, with actions by C<a>, and the
691 next executed line is marked by C<==E<gt>>.
695 When C<frame> option is set, debugger would print entered (and
696 optionally exited) subroutines in different styles.
698 What follows is the start of the listing of
700 env "PERLDB_OPTS=f=n N" perl -d -V
702 for different values of C<n>:
709 entering Config::BEGIN
710 Package lib/Exporter.pm.
712 Package lib/Config.pm.
713 entering Config::TIEHASH
714 entering Exporter::import
715 entering Exporter::export
716 entering Config::myconfig
717 entering Config::FETCH
718 entering Config::FETCH
719 entering Config::FETCH
720 entering Config::FETCH
725 entering Config::BEGIN
726 Package lib/Exporter.pm.
729 Package lib/Config.pm.
730 entering Config::TIEHASH
731 exited Config::TIEHASH
732 entering Exporter::import
733 entering Exporter::export
734 exited Exporter::export
735 exited Exporter::import
737 entering Config::myconfig
738 entering Config::FETCH
740 entering Config::FETCH
742 entering Config::FETCH
746 in $=main::BEGIN() from /dev/nul:0
747 in $=Config::BEGIN() from lib/Config.pm:2
748 Package lib/Exporter.pm.
750 Package lib/Config.pm.
751 in $=Config::TIEHASH('Config') from lib/Config.pm:644
752 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
753 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
754 in @=Config::myconfig() from /dev/nul:0
755 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
756 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
757 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
758 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
759 in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
760 in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
764 in $=main::BEGIN() from /dev/nul:0
765 in $=Config::BEGIN() from lib/Config.pm:2
766 Package lib/Exporter.pm.
768 out $=Config::BEGIN() from lib/Config.pm:0
769 Package lib/Config.pm.
770 in $=Config::TIEHASH('Config') from lib/Config.pm:644
771 out $=Config::TIEHASH('Config') from lib/Config.pm:644
772 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
773 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
774 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
775 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
776 out $=main::BEGIN() from /dev/nul:0
777 in @=Config::myconfig() from /dev/nul:0
778 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
779 out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
780 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
781 out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
782 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
783 out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
784 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
788 in $=main::BEGIN() from /dev/nul:0
789 in $=Config::BEGIN() from lib/Config.pm:2
790 Package lib/Exporter.pm.
792 out $=Config::BEGIN() from lib/Config.pm:0
793 Package lib/Config.pm.
794 in $=Config::TIEHASH('Config') from lib/Config.pm:644
795 out $=Config::TIEHASH('Config') from lib/Config.pm:644
796 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
797 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
798 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
799 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
800 out $=main::BEGIN() from /dev/nul:0
801 in @=Config::myconfig() from /dev/nul:0
802 in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
803 out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
804 in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
805 out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
809 in $=CODE(0x15eca4)() from /dev/null:0
810 in $=CODE(0x182528)() from lib/Config.pm:2
811 Package lib/Exporter.pm.
812 out $=CODE(0x182528)() from lib/Config.pm:0
813 scalar context return from CODE(0x182528): undef
814 Package lib/Config.pm.
815 in $=Config::TIEHASH('Config') from lib/Config.pm:628
816 out $=Config::TIEHASH('Config') from lib/Config.pm:628
817 scalar context return from Config::TIEHASH: empty hash
818 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
819 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
820 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
821 scalar context return from Exporter::export: ''
822 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
823 scalar context return from Exporter::import: ''
828 In all the cases indentation of lines shows the call tree, if bit 2 of
829 C<frame> is set, then a line is printed on exit from a subroutine as
830 well, if bit 4 is set, then the arguments are printed as well as the
831 caller info, if bit 8 is set, the arguments are printed even if they
832 are tied or references, if bit 16 is set, the return value is printed
835 When a package is compiled, a line like this
839 is printed with proper indentation.
843 =head2 Debugging compile-time statements
845 If you have any compile-time executable statements (code within a BEGIN
846 block or a C<use> statement), these will C<NOT> be stopped by debugger,
847 although C<require>s will (and compile-time statements can be traced
848 with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
849 code, however, you can
850 transfer control back to the debugger using the following statement,
851 which is harmless if the debugger is not running:
855 If you set C<$DB::single> to the value 2, it's equivalent to having
856 just typed the C<n> command, whereas a value of 1 means the C<s>
857 command. The C<$DB::trace> variable should be set to 1 to simulate
858 having typed the C<t> command.
860 Another way to debug compile-time code is to start debugger, set a
861 breakpoint on I<load> of some module thusly
863 DB<7> b load f:/perllib/lib/Carp.pm
864 Will stop on load of `f:/perllib/lib/Carp.pm'.
866 and restart debugger by C<R> command (if possible). One can use C<b
867 compile subname> for the same purpose.
869 =head2 Debugger Customization
871 Most probably you do not want to modify the debugger, it contains enough
872 hooks to satisfy most needs. You may change the behaviour of debugger
873 from the debugger itself, using C<O>ptions, from the command line via
874 C<PERLDB_OPTS> environment variable, and from I<customization files>.
876 You can do some customization by setting up a F<.perldb> file which
877 contains initialization code. For instance, you could make aliases
878 like these (the last one is one people expect to be there):
880 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
881 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
882 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
883 $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
885 One changes options from F<.perldb> file via calls like this one;
887 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
889 (the code is executed in the package C<DB>). Note that F<.perldb> is
890 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
891 subroutine C<afterinit>, it is called after all the debugger
892 initialization ends. F<.perldb> may be contained in the current
893 directory, or in the C<LOGDIR>/C<HOME> directory.
895 If you want to modify the debugger, copy F<perl5db.pl> from the Perl
896 library to another name and modify it as necessary. You'll also want
897 to set your C<PERL5DB> environment variable to say something like this:
899 BEGIN { require "myperl5db.pl" }
901 As the last resort, one can use C<PERL5DB> to customize debugger by
902 directly setting internal variables or calling debugger functions.
904 =head2 Readline Support
906 As shipped, the only command line history supplied is a simplistic one
907 that checks for leading exclamation points. However, if you install
908 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
909 have full editing capabilities much like GNU I<readline>(3) provides.
910 Look for these in the F<modules/by-module/Term> directory on CPAN.
912 A rudimentary command line completion is also available.
913 Unfortunately, the names of lexical variables are not available for
916 =head2 Editor Support for Debugging
918 If you have GNU B<emacs> installed on your system, it can interact with
919 the Perl debugger to provide an integrated software development
920 environment reminiscent of its interactions with C debuggers.
922 Perl is also delivered with a start file for making B<emacs> act like a
923 syntax-directed editor that understands (some of) Perl's syntax. Look in
924 the I<emacs> directory of the Perl source distribution.
926 (Historically, a similar setup for interacting with B<vi> and the
927 X11 window system had also been available, but at the time of this
928 writing, no debugger support for B<vi> currently exists.)
930 =head2 The Perl Profiler
932 If you wish to supply an alternative debugger for Perl to run, just
933 invoke your script with a colon and a package argument given to the B<-d>
934 flag. One of the most popular alternative debuggers for Perl is
935 B<DProf>, the Perl profiler. As of this writing, B<DProf> is not
936 included with the standard Perl distribution, but it is expected to
937 be included soon, for certain values of "soon".
939 Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming
940 it's properly installed on your system, to profile your Perl program in
941 the file F<mycode.pl>, just type:
943 perl -d:DProf mycode.pl
945 When the script terminates the profiler will dump the profile information
946 to a file called F<tmon.out>. A tool like B<dprofpp> (also supplied with
947 the Devel::DProf package) can be used to interpret the information which is
950 =head2 Debugger support in perl
952 When you call the B<caller> function (see L<perlfunc/caller>) from the
953 package DB, Perl sets the array @DB::args to contain the arguments the
954 corresponding stack frame was called with.
956 If perl is run with B<-d> option, the following additional features
957 are enabled (cf. L<perlvar/$^P>):
963 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
964 'perl5db.pl'}> if not present) before the first line of the
969 The array C<@{"_E<lt>$filename"}> is the line-by-line contents of
970 $filename for all the compiled files. Same for C<eval>ed strings which
971 contain subroutines, or which are currently executed. The C<$filename>
972 for C<eval>ed strings looks like C<(eval 34)>.
976 The hash C<%{"_E<lt>$filename"}> contains breakpoints and action (it is
977 keyed by line number), and individual entries are settable (as opposed
978 to the whole hash). Only true/false is important to Perl, though the
979 values used by F<perl5db.pl> have the form
980 C<"$break_condition\0$action">. Values are magical in numeric context:
981 they are zeros if the line is not breakable.
983 Same for evaluated strings which contain subroutines, or which are
984 currently executed. The $filename for C<eval>ed strings looks like
989 The scalar C<${"_E<lt>$filename"}> contains C<"_E<lt>$filename">. Same for
990 evaluated strings which contain subroutines, or which are currently
991 executed. The $filename for C<eval>ed strings looks like C<(eval
996 After each C<require>d file is compiled, but before it is executed,
997 C<DB::postponed(*{"_E<lt>$filename"})> is called (if subroutine
998 C<DB::postponed> exists). Here the $filename is the expanded name of
999 the C<require>d file (as found in values of %INC).
1003 After each subroutine C<subname> is compiled existence of
1004 C<$DB::postponed{subname}> is checked. If this key exists,
1005 C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
1010 A hash C<%DB::sub> is maintained, with keys being subroutine names,
1011 values having the form C<filename:startline-endline>. C<filename> has
1012 the form C<(eval 31)> for subroutines defined inside C<eval>s.
1016 When execution of the application reaches a place that can have
1017 a breakpoint, a call to C<DB::DB()> is performed if any one of
1018 variables $DB::trace, $DB::single, or $DB::signal is true. (Note that
1019 these variables are not C<local>izable.) This feature is disabled when
1020 the control is inside C<DB::DB()> or functions called from it (unless
1021 C<$^D & (1E<lt>E<lt>30)>).
1025 When execution of the application reaches a subroutine call, a call
1026 to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
1027 the name of the called subroutine. (Unless the subroutine is compiled
1028 in the package C<DB>.)
1032 Note that if C<&DB::sub> needs some external data to be setup for it
1033 to work, no subroutine call is possible until this is done. For the
1034 standard debugger C<$DB::deep> (how many levels of recursion deep into
1035 the debugger you can go before a mandatory break) gives an example of
1038 The minimal working debugger consists of one line
1042 which is quite handy as contents of C<PERL5DB> environment
1045 env "PERL5DB=sub DB::DB {}" perl -d your-script
1047 Another (a little bit more useful) minimal debugger can be created
1048 with the only line being
1050 sub DB::DB {print ++$i; scalar <STDIN>}
1052 This debugger would print the sequential number of encountered
1053 statement, and would wait for your C<CR> to continue.
1055 The following debugger is quite functional:
1060 sub sub {print ++$i, " $sub\n"; &$sub}
1063 It prints the sequential number of subroutine call and the name of the
1064 called subroutine. Note that C<&DB::sub> should be compiled into the
1067 =head2 Debugger Internals
1069 At the start, the debugger reads your rc file (F<./.perldb> or
1070 F<~/.perldb> under Unix), which can set important options. This file may
1071 define a subroutine C<&afterinit> to be executed after the debugger is
1074 After the rc file is read, the debugger reads environment variable
1075 PERLDB_OPTS and parses it as a rest of C<O ...> line in debugger prompt.
1077 It also maintains magical internal variables, such as C<@DB::dbline>,
1078 C<%DB::dbline>, which are aliases for C<@{"::_<current_file"}>
1079 C<%{"::_<current_file"}>. Here C<current_file> is the currently
1080 selected (with the debugger's C<f> command, or by flow of execution)
1083 Some functions are provided to simplify customization. See L<"Debugger
1084 Customization"> for description of C<DB::parse_options(string)>. The
1085 function C<DB::dump_trace(skip[, count])> skips the specified number
1086 of frames, and returns a list containing info about the caller
1087 frames (all if C<count> is missing). Each entry is a hash with keys
1088 C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
1089 eval), C<args> (C<undef> or a reference to an array), C<file>, and
1092 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
1093 formatted info about caller frames. The last two functions may be
1094 convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.
1096 =head2 Other resources
1098 You did try the B<-w> switch, didn't you?
1102 You cannot get the stack frame information or otherwise debug functions
1103 that were not compiled by Perl, such as C or C++ extensions.
1105 If you alter your @_ arguments in a subroutine (such as with B<shift>
1106 or B<pop>, the stack backtrace will not show the original values.