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 $foo\n"
291 Delete all installed actions.
295 Add a global watch-expression.
299 Delete all watch-expressions.
303 Add a global watch-expression.
307 Delete all watch-expressions.
309 =item O [opt[=val]] [opt"val"] [opt?]...
311 Set or query values of options. val defaults to 1. opt can
312 be abbreviated. Several options can be listed.
316 =item C<recallCommand>, C<ShellBang>
318 The characters used to recall command or spawn shell. By
319 default, these are both set to C<!>.
323 Program to use for output of pager-piped commands (those
324 beginning with a C<|> character.) By default,
325 C<$ENV{PAGER}> will be used.
329 Run Tk while prompting (with ReadLine).
331 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
333 Level of verbosity. By default the debugger is in a sane verbose mode,
334 thus it will print backtraces on all the warnings and die-messages
335 which are going to be printed out, and will print a message when
336 interesting uncaught signals arrive.
338 To disable this behaviour, set these values to 0. If C<dieLevel> is 2,
339 then the messages which will be caught by surrounding C<eval> are also
344 Trace mode (similar to C<t> command, but can be put into
349 File or pipe to print line number info to. If it is a pipe (say,
350 C<|visual_perl_db>), then a short, "emacs like" message is used.
352 =item C<inhibit_exit>
354 If 0, allows I<stepping off> the end of the script.
358 affects printing of return value after C<r> command.
362 affects screen appearance of the command line (see L<Term::ReadLine>).
366 affects printing messages on entry and exit from subroutines. If
367 C<frame & 2> is false, messages are printed on entry only. (Printing
368 on exit may be useful if inter(di)spersed with other messages.)
370 If C<frame & 4>, arguments to functions are printed as well as the
371 context and caller info. If C<frame & 8>, overloaded C<stringify> and
372 C<tie>d C<FETCH> are enabled on the printed arguments. If C<frame &
373 16>, the return value from the subroutine is printed as well.
375 The length at which the argument list is truncated is governed by the
380 length at which the argument list is truncated when C<frame> option's
385 The following options affect what happens with C<V>, C<X>, and C<x>
390 =item C<arrayDepth>, C<hashDepth>
392 Print only first N elements ('' for all).
394 =item C<compactDump>, C<veryCompact>
396 Change style of array and hash dump. If C<compactDump>, short array
397 may be printed on one line.
401 Whether to print contents of globs.
405 Dump arrays holding debugged files.
407 =item C<DumpPackages>
409 Dump symbol tables of packages.
413 Dump contents of "reused" addresses.
415 =item C<quote>, C<HighBit>, C<undefPrint>
417 Change style of string dump. Default value of C<quote> is C<auto>, one
418 can enable either double-quotish dump, or single-quotish by setting it
419 to C<"> or C<'>. By default, characters with high bit set are printed
424 I<very> rudimentally per-package memory usage dump. Calculates total
425 size of strings in variables in the package.
429 During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
430 You can put additional initialization options C<TTY>, C<noTTY>,
431 C<ReadLine>, and C<NonStop> there.
435 &parse_options("NonStop=1 LineInfo=db.out AutoTrace");
437 The script will run without human intervention, putting trace information
438 into the file I<db.out>. (If you interrupt it, you would better reset
439 C<LineInfo> to something "interactive"!)
445 The TTY to use for debugging I/O.
449 If set, goes in C<NonStop> mode, and would not connect to a TTY. If
450 interrupt (or if control goes to debugger via explicit setting of
451 $DB::signal or $DB::single from the Perl script), connects to a TTY
452 specified by the C<TTY> option at startup, or to a TTY found at
453 runtime using C<Term::Rendezvous> module of your choice.
455 This module should implement a method C<new> which returns an object
456 with two methods: C<IN> and C<OUT>, returning two filehandles to use
457 for debugging input and output correspondingly. Method C<new> may
458 inspect an argument which is a value of C<$ENV{PERLDB_NOTTY}> at
459 startup, or is C<"/tmp/perldbtty$$"> otherwise.
463 If false, readline support in debugger is disabled, so you can debug
464 ReadLine applications.
468 If set, debugger goes into noninteractive mode until interrupted, or
469 programmatically by setting $DB::signal or $DB::single.
473 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
475 $ PERLDB_OPTS="N f=2" perl -d myprogram
477 will run the script C<myprogram> without human intervention, printing
478 out the call tree with entry and exit points. Note that C<N f=2> is
479 equivalent to C<NonStop=1 frame=2>. Note also that at the moment when
480 this documentation was written all the options to the debugger could
481 be uniquely abbreviated by the first letter (with exception of
484 Other examples may include
486 $ PERLDB_OPTS="N f A L=listing" perl -d myprogram
488 - runs script noninteractively, printing info on each entry into a
489 subroutine and each executed line into the file F<listing>. (If you
490 interrupt it, you would better reset C<LineInfo> to something
494 $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
496 may be useful for debugging a program which uses C<Term::ReadLine>
497 itself. Do not forget detach shell from the TTY in the window which
498 corresponds to F</dev/ttyc>, say, by issuing a command like
502 See L<"Debugger Internals"> below for more details.
504 =item E<lt> [ command ]
506 Set an action (Perl command) to happen before every debugger prompt.
507 A multi-line command may be entered by backslashing the newlines. If
508 C<command> is missing, resets the list of actions.
510 =item E<lt>E<lt> command
512 Add an action (Perl command) to happen before every debugger prompt.
513 A multi-line command may be entered by backslashing the newlines.
517 Set an action (Perl command) to happen after the prompt when you've
518 just given a command to return to executing the script. A multi-line
519 command may be entered by backslashing the newlines. If C<command> is
520 missing, resets the list of actions.
522 =item E<gt>E<gt> command
524 Adds an action (Perl command) to happen after the prompt when you've
525 just given a command to return to executing the script. A multi-line
526 command may be entered by backslashing the newlines.
530 Set an action (debugger command) to happen before every debugger prompt.
531 A multi-line command may be entered by backslashing the newlines. If
532 C<command> is missing, resets the list of actions.
536 Add an action (debugger command) to happen before every debugger prompt.
537 A multi-line command may be entered by backslashing the newlines.
541 Redo a previous command (default previous command).
545 Redo number'th-to-last command.
549 Redo last command that started with pattern.
550 See C<O recallCommand>, too.
554 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT)
555 See C<O shellBang> too.
559 Display last n commands. Only commands longer than one character are
560 listed. If number is omitted, lists them all.
564 Quit. ("quit" doesn't work for this.) This is the only supported way
565 to exit the debugger, though typing C<exit> twice may do it too.
567 Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
568 off> the end the script. You may also need to set C<$finished> to 0 at
569 some moment if you want to step through global destruction.
573 Restart the debugger by B<exec>ing a new session. It tries to maintain
574 your history across this, but internal settings and command line options
577 Currently the following setting are preserved: history, breakpoints,
578 actions, debugger C<O>ptions, and the following command line
579 options: B<-w>, B<-I>, and B<-e>.
583 Run debugger command, piping DB::OUT to current pager.
587 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
588 Often used with commands that would otherwise produce long
593 =item = [alias value]
595 Define a command alias, like
599 or list current aliases.
603 Execute command as a Perl statement. A missing semicolon will be
608 The expression is evaluated, and the methods which may be applied to
609 the result are listed.
613 The methods which may be applied to objects in the C<package> are listed.
617 =head2 Debugger input/output
623 The debugger prompt is something like
631 where that number is the command number, which you'd use to access with
632 the builtin B<csh>-like history mechanism, e.g., C<!17> would repeat
633 command number 17. The number of angle brackets indicates the depth of
634 the debugger. You could get more than one set of brackets, for example, if
635 you'd already at a breakpoint and then printed out the result of a
636 function call that itself also has a breakpoint, or you step into an
637 expression via C<s/n/t expression> command.
639 =item Multiline commands
641 If you want to enter a multi-line command, such as a subroutine
642 definition with several statements, or a format, you may escape the
643 newline that would normally end the debugger command with a backslash.
647 cont: print "ok\n"; \
654 Note that this business of escaping a newline is specific to interactive
655 commands typed into the debugger.
657 =item Stack backtrace
659 Here's an example of what a stack backtrace via C<T> command might
662 $ = main::infested called from file `Ambulation.pm' line 10
663 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
664 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
666 The left-hand character up there tells whether the function was called
667 in a scalar or list context (we bet you can tell which is which). What
668 that says is that you were in the function C<main::infested> when you ran
669 the stack dump, and that it was called in a scalar context from line 10
670 of the file I<Ambulation.pm>, but without any arguments at all, meaning
671 it was called as C<&infested>. The next stack frame shows that the
672 function C<Ambulation::legs> was called in a list context from the
673 I<camel_flea> file with four arguments. The last stack frame shows that
674 C<main::pests> was called in a scalar context, also from I<camel_flea>,
677 Note that if you execute C<T> command from inside an active C<use>
678 statement, the backtrace will contain both C<require>
679 frame and an C<eval>) frame.
683 Listing given via different flavors of C<l> command looks like this:
687 102:b @isa{@i,$pack} = ()
688 103 if(exists $i{$prevpack} || exists $isa{$pack});
692 107==> if(exists $isa{$pack});
694 109:a if ($extra-- > 0) {
695 110: %isa = ($pack,1);
697 Note that the breakable lines are marked with C<:>, lines with
698 breakpoints are marked by C<b>, with actions by C<a>, and the
699 next executed line is marked by C<==E<gt>>.
703 When C<frame> option is set, debugger would print entered (and
704 optionally exited) subroutines in different styles.
706 What follows is the start of the listing of
708 env "PERLDB_OPTS=f=n N" perl -d -V
710 for different values of C<n>:
717 entering Config::BEGIN
718 Package lib/Exporter.pm.
720 Package lib/Config.pm.
721 entering Config::TIEHASH
722 entering Exporter::import
723 entering Exporter::export
724 entering Config::myconfig
725 entering Config::FETCH
726 entering Config::FETCH
727 entering Config::FETCH
728 entering Config::FETCH
733 entering Config::BEGIN
734 Package lib/Exporter.pm.
737 Package lib/Config.pm.
738 entering Config::TIEHASH
739 exited Config::TIEHASH
740 entering Exporter::import
741 entering Exporter::export
742 exited Exporter::export
743 exited Exporter::import
745 entering Config::myconfig
746 entering Config::FETCH
748 entering Config::FETCH
750 entering Config::FETCH
754 in $=main::BEGIN() from /dev/nul:0
755 in $=Config::BEGIN() from lib/Config.pm:2
756 Package lib/Exporter.pm.
758 Package lib/Config.pm.
759 in $=Config::TIEHASH('Config') from lib/Config.pm:644
760 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
761 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
762 in @=Config::myconfig() from /dev/nul:0
763 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
764 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
765 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
766 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
767 in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
768 in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
772 in $=main::BEGIN() from /dev/nul:0
773 in $=Config::BEGIN() from lib/Config.pm:2
774 Package lib/Exporter.pm.
776 out $=Config::BEGIN() from lib/Config.pm:0
777 Package lib/Config.pm.
778 in $=Config::TIEHASH('Config') from lib/Config.pm:644
779 out $=Config::TIEHASH('Config') from lib/Config.pm:644
780 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
781 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
782 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
783 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
784 out $=main::BEGIN() from /dev/nul:0
785 in @=Config::myconfig() from /dev/nul:0
786 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
787 out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
788 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
789 out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
790 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
791 out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
792 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
796 in $=main::BEGIN() from /dev/nul:0
797 in $=Config::BEGIN() from lib/Config.pm:2
798 Package lib/Exporter.pm.
800 out $=Config::BEGIN() from lib/Config.pm:0
801 Package lib/Config.pm.
802 in $=Config::TIEHASH('Config') from lib/Config.pm:644
803 out $=Config::TIEHASH('Config') from lib/Config.pm:644
804 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
805 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
806 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
807 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
808 out $=main::BEGIN() from /dev/nul:0
809 in @=Config::myconfig() from /dev/nul:0
810 in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
811 out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
812 in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
813 out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
817 in $=CODE(0x15eca4)() from /dev/null:0
818 in $=CODE(0x182528)() from lib/Config.pm:2
819 Package lib/Exporter.pm.
820 out $=CODE(0x182528)() from lib/Config.pm:0
821 scalar context return from CODE(0x182528): undef
822 Package lib/Config.pm.
823 in $=Config::TIEHASH('Config') from lib/Config.pm:628
824 out $=Config::TIEHASH('Config') from lib/Config.pm:628
825 scalar context return from Config::TIEHASH: empty hash
826 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
827 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
828 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
829 scalar context return from Exporter::export: ''
830 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
831 scalar context return from Exporter::import: ''
836 In all the cases indentation of lines shows the call tree, if bit 2 of
837 C<frame> is set, then a line is printed on exit from a subroutine as
838 well, if bit 4 is set, then the arguments are printed as well as the
839 caller info, if bit 8 is set, the arguments are printed even if they
840 are tied or references, if bit 16 is set, the return value is printed
843 When a package is compiled, a line like this
847 is printed with proper indentation.
851 =head2 Debugging compile-time statements
853 If you have any compile-time executable statements (code within a BEGIN
854 block or a C<use> statement), these will C<NOT> be stopped by debugger,
855 although C<require>s will (and compile-time statements can be traced
856 with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
857 code, however, you can
858 transfer control back to the debugger using the following statement,
859 which is harmless if the debugger is not running:
863 If you set C<$DB::single> to the value 2, it's equivalent to having
864 just typed the C<n> command, whereas a value of 1 means the C<s>
865 command. The C<$DB::trace> variable should be set to 1 to simulate
866 having typed the C<t> command.
868 Another way to debug compile-time code is to start debugger, set a
869 breakpoint on I<load> of some module thusly
871 DB<7> b load f:/perllib/lib/Carp.pm
872 Will stop on load of `f:/perllib/lib/Carp.pm'.
874 and restart debugger by C<R> command (if possible). One can use C<b
875 compile subname> for the same purpose.
877 =head2 Debugger Customization
879 Most probably you do not want to modify the debugger, it contains enough
880 hooks to satisfy most needs. You may change the behaviour of debugger
881 from the debugger itself, using C<O>ptions, from the command line via
882 C<PERLDB_OPTS> environment variable, and from I<customization files>.
884 You can do some customization by setting up a F<.perldb> file which
885 contains initialization code. For instance, you could make aliases
886 like these (the last one is one people expect to be there):
888 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
889 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
890 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
891 $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
893 One changes options from F<.perldb> file via calls like this one;
895 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
897 (the code is executed in the package C<DB>). Note that F<.perldb> is
898 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
899 subroutine C<afterinit>, it is called after all the debugger
900 initialization ends. F<.perldb> may be contained in the current
901 directory, or in the C<LOGDIR>/C<HOME> directory.
903 If you want to modify the debugger, copy F<perl5db.pl> from the Perl
904 library to another name and modify it as necessary. You'll also want
905 to set your C<PERL5DB> environment variable to say something like this:
907 BEGIN { require "myperl5db.pl" }
909 As the last resort, one can use C<PERL5DB> to customize debugger by
910 directly setting internal variables or calling debugger functions.
912 =head2 Readline Support
914 As shipped, the only command line history supplied is a simplistic one
915 that checks for leading exclamation points. However, if you install
916 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
917 have full editing capabilities much like GNU I<readline>(3) provides.
918 Look for these in the F<modules/by-module/Term> directory on CPAN.
920 A rudimentary command line completion is also available.
921 Unfortunately, the names of lexical variables are not available for
924 =head2 Editor Support for Debugging
926 If you have GNU B<emacs> installed on your system, it can interact with
927 the Perl debugger to provide an integrated software development
928 environment reminiscent of its interactions with C debuggers.
930 Perl is also delivered with a start file for making B<emacs> act like a
931 syntax-directed editor that understands (some of) Perl's syntax. Look in
932 the I<emacs> directory of the Perl source distribution.
934 (Historically, a similar setup for interacting with B<vi> and the
935 X11 window system had also been available, but at the time of this
936 writing, no debugger support for B<vi> currently exists.)
938 =head2 The Perl Profiler
940 If you wish to supply an alternative debugger for Perl to run, just
941 invoke your script with a colon and a package argument given to the B<-d>
942 flag. One of the most popular alternative debuggers for Perl is
943 B<DProf>, the Perl profiler. As of this writing, B<DProf> is not
944 included with the standard Perl distribution, but it is expected to
945 be included soon, for certain values of "soon".
947 Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming
948 it's properly installed on your system, to profile your Perl program in
949 the file F<mycode.pl>, just type:
951 perl -d:DProf mycode.pl
953 When the script terminates the profiler will dump the profile information
954 to a file called F<tmon.out>. A tool like B<dprofpp> (also supplied with
955 the Devel::DProf package) can be used to interpret the information which is
958 =head2 Debugger support in perl
960 When you call the B<caller> function (see L<perlfunc/caller>) from the
961 package DB, Perl sets the array @DB::args to contain the arguments the
962 corresponding stack frame was called with.
964 If perl is run with B<-d> option, the following additional features
965 are enabled (cf. L<perlvar/$^P>):
971 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
972 'perl5db.pl'}> if not present) before the first line of the
977 The array C<@{"_E<lt>$filename"}> is the line-by-line contents of
978 $filename for all the compiled files. Same for C<eval>ed strings which
979 contain subroutines, or which are currently executed. The C<$filename>
980 for C<eval>ed strings looks like C<(eval 34)>.
984 The hash C<%{"_E<lt>$filename"}> contains breakpoints and action (it is
985 keyed by line number), and individual entries are settable (as opposed
986 to the whole hash). Only true/false is important to Perl, though the
987 values used by F<perl5db.pl> have the form
988 C<"$break_condition\0$action">. Values are magical in numeric context:
989 they are zeros if the line is not breakable.
991 Same for evaluated strings which contain subroutines, or which are
992 currently executed. The $filename for C<eval>ed strings looks like
997 The scalar C<${"_E<lt>$filename"}> contains C<"_E<lt>$filename">. Same for
998 evaluated strings which contain subroutines, or which are currently
999 executed. The $filename for C<eval>ed strings looks like C<(eval
1004 After each C<require>d file is compiled, but before it is executed,
1005 C<DB::postponed(*{"_E<lt>$filename"})> is called (if subroutine
1006 C<DB::postponed> exists). Here the $filename is the expanded name of
1007 the C<require>d file (as found in values of %INC).
1011 After each subroutine C<subname> is compiled existence of
1012 C<$DB::postponed{subname}> is checked. If this key exists,
1013 C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
1018 A hash C<%DB::sub> is maintained, with keys being subroutine names,
1019 values having the form C<filename:startline-endline>. C<filename> has
1020 the form C<(eval 31)> for subroutines defined inside C<eval>s.
1024 When execution of the application reaches a place that can have
1025 a breakpoint, a call to C<DB::DB()> is performed if any one of
1026 variables $DB::trace, $DB::single, or $DB::signal is true. (Note that
1027 these variables are not C<local>izable.) This feature is disabled when
1028 the control is inside C<DB::DB()> or functions called from it (unless
1029 C<$^D & (1E<lt>E<lt>30)>).
1033 When execution of the application reaches a subroutine call, a call
1034 to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
1035 the name of the called subroutine. (Unless the subroutine is compiled
1036 in the package C<DB>.)
1040 Note that if C<&DB::sub> needs some external data to be setup for it
1041 to work, no subroutine call is possible until this is done. For the
1042 standard debugger C<$DB::deep> (how many levels of recursion deep into
1043 the debugger you can go before a mandatory break) gives an example of
1046 The minimal working debugger consists of one line
1050 which is quite handy as contents of C<PERL5DB> environment
1053 env "PERL5DB=sub DB::DB {}" perl -d your-script
1055 Another (a little bit more useful) minimal debugger can be created
1056 with the only line being
1058 sub DB::DB {print ++$i; scalar <STDIN>}
1060 This debugger would print the sequential number of encountered
1061 statement, and would wait for your C<CR> to continue.
1063 The following debugger is quite functional:
1068 sub sub {print ++$i, " $sub\n"; &$sub}
1071 It prints the sequential number of subroutine call and the name of the
1072 called subroutine. Note that C<&DB::sub> should be compiled into the
1075 =head2 Debugger Internals
1077 At the start, the debugger reads your rc file (F<./.perldb> or
1078 F<~/.perldb> under Unix), which can set important options. This file may
1079 define a subroutine C<&afterinit> to be executed after the debugger is
1082 After the rc file is read, the debugger reads environment variable
1083 PERLDB_OPTS and parses it as a rest of C<O ...> line in debugger prompt.
1085 It also maintains magical internal variables, such as C<@DB::dbline>,
1086 C<%DB::dbline>, which are aliases for C<@{"::_<current_file"}>
1087 C<%{"::_<current_file"}>. Here C<current_file> is the currently
1088 selected (with the debugger's C<f> command, or by flow of execution)
1091 Some functions are provided to simplify customization. See L<"Debugger
1092 Customization"> for description of C<DB::parse_options(string)>. The
1093 function C<DB::dump_trace(skip[, count])> skips the specified number
1094 of frames, and returns an array containing info about the caller
1095 frames (all if C<count> is missing). Each entry is a hash with keys
1096 C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
1097 eval), C<args> (C<undef> or a reference to an array), C<file>, and
1100 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
1101 formatted info about caller frames. The last two functions may be
1102 convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.
1104 =head2 Other resources
1106 You did try the B<-w> switch, didn't you?
1110 You cannot get the stack frame information or otherwise debug functions
1111 that were not compiled by Perl, such as C or C++ extensions.
1113 If you alter your @_ arguments in a subroutine (such as with B<shift>
1114 or B<pop>, the stack backtrace will not show the original values.