3 perldebug - Perl debugging
7 First of all, have you tried using the B<-w> switch?
9 =head1 The Perl Debugger
11 If you invoke Perl with the B<-d> switch, your script runs under the
12 Perl source debugger. This works like an interactive Perl
13 environment, prompting for debugger commands that let you examine
14 source code, set breakpoints, get stack back-traces, change the values of
15 variables, etc. This is so convenient that you often fire up
16 the debugger all by itself just to test out Perl constructs
17 interactively to see what they do. For example:
21 In Perl, the debugger is not a separate program as it usually is in the
22 typical compiled environment. Instead, the B<-d> flag tells the compiler
23 to insert source information into the parse trees it's about to hand off
24 to the interpreter. That means your code must first compile correctly
25 for the debugger to work on it. Then when the interpreter starts up, it
26 pre-loads a Perl library file containing the debugger itself.
28 The program will halt I<right before> the first run-time executable
29 statement (but see below regarding compile-time statements) and ask you
30 to enter a debugger command. Contrary to popular expectations, whenever
31 the debugger halts and shows you a line of code, it always displays the
32 line it's I<about> to execute, rather than the one it has just executed.
34 Any command not recognized by the debugger is directly executed
35 (C<eval>'d) as Perl code in the current package. (The debugger uses the
36 DB package for its own state information.)
38 Leading white space before a command would cause the debugger to think
39 it's I<NOT> a debugger command but for Perl, so be careful not to do
42 =head2 Debugger Commands
44 The debugger understands the following commands:
50 Prints out a help message.
52 If you supply another debugger command as an argument to the C<h> command,
53 it prints out the description for just that command. The special
54 argument of C<h h> produces a more compact help listing, designed to fit
55 together on one screen.
57 If the output the C<h> command (or any command, for that matter) scrolls
58 past your screen, either precede the command with a leading pipe symbol so
59 it's run through your pager, as in
63 You may change the pager which is used via C<O pager=...> command.
67 Same as C<print {$DB::OUT} expr> in the current package. In particular,
68 because this is just Perl's own B<print> function, this means that nested
69 data structures and objects are not dumped, unlike with the C<x> command.
71 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
72 where STDOUT may be redirected to.
76 Evaluates its expression in list context and dumps out the result
77 in a pretty-printed fashion. Nested data structures are printed out
78 recursively, unlike the C<print> function.
80 The details of printout are governed by multiple C<O>ptions.
84 Display all (or some) variables in package (defaulting to the C<main>
85 package) using a data pretty-printer (hashes show their keys and values so
86 you see what's what, control characters are made printable, etc.). Make
87 sure you don't put the type specifier (like C<$>) there, just the symbol
92 Use C<~pattern> and C<!pattern> for positive and negative regexps.
94 Nested data structures are printed out in a legible fashion, unlike
95 the C<print> function.
97 The details of printout are governed by multiple C<O>ptions.
101 Same as C<V currentpackage [vars]>.
105 Produce a stack back-trace. See below for details on its output.
109 Single step. Executes until it reaches the beginning of another
110 statement, descending into subroutine calls. If an expression is
111 supplied that includes function calls, it too will be single-stepped.
115 Next. Executes over subroutine calls, until it reaches the beginning
116 of the next statement. If an expression is supplied that includes
117 function calls, those functions will be executed with stops before
122 Repeat last C<n> or C<s> command.
126 Continue, optionally inserting a one-time-only breakpoint
127 at the specified line or subroutine.
131 List next window of lines.
135 List C<incr+1> lines starting at C<min>.
139 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
147 List first window of lines from subroutine.
151 List previous window of lines.
155 List window (a few lines) around the current line.
159 Return debugger pointer to the last-executed line and
164 Switch to viewing a different file or eval statement. If C<filename>
165 is not a full filename as found in values of %INC, it is considered as
170 Search forwards for pattern; final / is optional.
174 Search backwards for pattern; final ? is optional.
178 List all breakpoints and actions.
182 List subroutine names [not] matching pattern.
186 Toggle trace mode (see also C<AutoTrace> C<O>ption).
190 Trace through execution of expr. For example:
193 Stack dump during die enabled outside of evals.
195 Loading DB routines from perl5db.pl patch level 0.94
196 Emacs support available.
198 Enter h or `h h' for help.
205 DB<3> t print foo() * bar()
206 main::((eval 172):3): print foo() + bar();
207 main::foo((eval 168):2):
208 main::bar((eval 170):2):
211 or, with the C<O>ption C<frame=2> set,
215 DB<5> t print foo() * bar()
225 =item b [line] [condition]
227 Set a breakpoint. If line is omitted, sets a breakpoint on the line
228 that is about to be executed. If a condition is specified, it's
229 evaluated each time the statement is reached and a breakpoint is taken
230 only if the condition is true. Breakpoints may be set on only lines
231 that begin an executable statement. Conditions don't use B<if>:
234 b 237 ++$count237 < 11
237 =item b subname [condition]
239 Set a breakpoint at the first line of the named subroutine.
241 =item b postpone subname [condition]
243 Set breakpoint at first line of subroutine after it is compiled.
245 =item b load filename
247 Set breakpoint at the first executed line of the file. Filename should
248 be a full name as found in values of %INC.
250 =item b compile subname
252 Sets breakpoint at the first statement executed after the subroutine
257 Delete a breakpoint at the specified line. If line is omitted, deletes
258 the breakpoint on the line that is about to be executed.
262 Delete all installed breakpoints.
264 =item a [line] command
266 Set an action to be done before the line is executed.
267 The sequence of steps taken by the debugger is
269 1. check for a breakpoint at this line
270 2. print the line if necessary (tracing)
271 3. do any actions associated with that line
272 4. prompt user if at a breakpoint or in single-step
275 For example, this will print out C<$foo> every time line
278 a 53 print "DB FOUND $foo\n"
282 Delete all installed actions.
284 =item O [opt[=val]] [opt"val"] [opt?]...
286 Set or query values of options. val defaults to 1. opt can
287 be abbreviated. Several options can be listed.
291 =item C<recallCommand>, C<ShellBang>
293 The characters used to recall command or spawn shell. By
294 default, these are both set to C<!>.
298 Program to use for output of pager-piped commands (those
299 beginning with a C<|> character.) By default,
300 C<$ENV{PAGER}> will be used.
304 Run Tk while prompting (with ReadLine).
306 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
308 Level of verbosity. By default the debugger is in a sane verbose mode,
309 thus it will print backtraces on all the warnings and die-messages
310 which are going to be printed out, and will print a message when
311 interesting uncaught signals arrive.
313 To disable this behaviour, set these values to 0. If C<dieLevel> is 2,
314 then the messages which will be caught by surrounding C<eval> are also
319 Trace mode (similar to C<t> command, but can be put into
324 File or pipe to print line number info to. If it is a pipe (say,
325 C<|visual_perl_db>), then a short, "emacs like" message is used.
327 =item C<inhibit_exit>
329 If 0, allows I<stepping off> the end of the script.
333 affects printing of return value after C<r> command.
337 affects printing messages on entry and exit from subroutines. If
338 C<frame & 2> is false, messages are printed on entry only. (Printing
339 on exit may be useful if inter(di)spersed with other messages.)
341 If C<frame & 4>, arguments to functions are printed as well as the
342 context and caller info. If C<frame & 8>, overloaded C<stringify> and
343 C<tie>d C<FETCH> are enabled on the printed arguments. The length at
344 which the argument list is truncated is governed by the next option:
348 length at which the argument list is truncated when C<frame> option's
353 The following options affect what happens with C<V>, C<X>, and C<x>
358 =item C<arrayDepth>, C<hashDepth>
360 Print only first N elements ('' for all).
362 =item C<compactDump>, C<veryCompact>
364 Change style of array and hash dump. If C<compactDump>, short array
365 may be printed on one line.
369 Whether to print contents of globs.
373 Dump arrays holding debugged files.
375 =item C<DumpPackages>
377 Dump symbol tables of packages.
379 =item C<quote>, C<HighBit>, C<undefPrint>
381 Change style of string dump. Default value of C<quote> is C<auto>, one
382 can enable either double-quotish dump, or single-quotish by setting it
383 to C<"> or C<'>. By default, characters with high bit set are printed
388 I<very> rudimentally per-package memory usage dump. Calculates total
389 size of strings in variables in the package.
393 During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
394 You can put additional initialization options C<TTY>, C<noTTY>,
395 C<ReadLine>, and C<NonStop> there.
399 &parse_options("NonStop=1 LineInfo=db.out AutoTrace");
401 The script will run without human intervention, putting trace information
402 into the file I<db.out>. (If you interrupt it, you would better reset
403 C<LineInfo> to something "interactive"!)
409 The TTY to use for debugging I/O.
413 If set, goes in C<NonStop> mode, and would not connect to a TTY. If
414 interrupt (or if control goes to debugger via explicit setting of
415 $DB::signal or $DB::single from the Perl script), connects to a TTY
416 specified by the C<TTY> option at startup, or to a TTY found at
417 runtime using C<Term::Rendezvous> module of your choice.
419 This module should implement a method C<new> which returns an object
420 with two methods: C<IN> and C<OUT>, returning two filehandles to use
421 for debugging input and output correspondingly. Method C<new> may
422 inspect an argument which is a value of C<$ENV{PERLDB_NOTTY}> at
423 startup, or is C<"/tmp/perldbtty$$"> otherwise.
427 If false, readline support in debugger is disabled, so you can debug
428 ReadLine applications.
432 If set, debugger goes into non-interactive mode until interrupted, or
433 programmatically by setting $DB::signal or $DB::single.
437 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
439 $ PERLDB_OPTS="N f=2" perl -d myprogram
441 will run the script C<myprogram> without human intervention, printing
442 out the call tree with entry and exit points. Note that C<N f=2> is
443 equivalent to C<NonStop=1 frame=2>. Note also that at the moment when
444 this documentation was written all the options to the debugger could
445 be uniquely abbreviated by the first letter (with exception of
448 Other examples may include
450 $ PERLDB_OPTS="N f A L=listing" perl -d myprogram
452 - runs script non-interactively, printing info on each entry into a
453 subroutine and each executed line into the file F<listing>. (If you
454 interrupt it, you would better reset C<LineInfo> to something
458 $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
460 may be useful for debugging a program which uses C<Term::ReadLine>
461 itself. Do not forget detach shell from the TTY in the window which
462 corresponds to F</dev/ttyc>, say, by issuing a command like
466 See L<"Debugger Internals"> below for more details.
470 =item E<lt> [ command ]
472 Set an action (Perl command) to happen before every debugger prompt.
473 A multi-line command may be entered by backslashing the newlines. If
474 C<command> is missing, resets the list of actions.
476 =item E<lt>E<lt> command
478 Add an action (Perl command) to happen before every debugger prompt.
479 A multi-line command may be entered by backslashing the newlines.
483 Set an action (Perl command) to happen after the prompt when you've
484 just given a command to return to executing the script. A multi-line
485 command may be entered by backslashing the newlines. If C<command> is
486 missing, resets the list of actions.
488 =item E<gt>E<gt> command
490 Adds an action (Perl command) to happen after the prompt when you've
491 just given a command to return to executing the script. A multi-line
492 command may be entered by backslashing the newlines.
496 Set an action (debugger command) to happen before every debugger prompt.
497 A multi-line command may be entered by backslashing the newlines. If
498 C<command> is missing, resets the list of actions.
502 Add an action (debugger command) to happen before every debugger prompt.
503 A multi-line command may be entered by backslashing the newlines.
507 Redo a previous command (default previous command).
511 Redo number'th-to-last command.
515 Redo last command that started with pattern.
516 See C<O recallCommand>, too.
520 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT)
521 See C<O shellBang> too.
525 Display last n commands. Only commands longer than one character are
526 listed. If number is omitted, lists them all.
530 Quit. ("quit" doesn't work for this.) This is the only supported way
531 to exit the debugger, though typing C<exit> twice may do it too.
533 Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
534 off> the end the script. You may also need to set C<$finished> to 0 at
535 some moment if you want to step through global destruction.
539 Restart the debugger by B<exec>ing a new session. It tries to maintain
540 your history across this, but internal settings and command line options
543 Currently the following setting are preserved: history, breakpoints,
544 actions, debugger C<O>ptions, and the following command-line
545 options: B<-w>, B<-I>, and B<-e>.
549 Run debugger command, piping DB::OUT to current pager.
553 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
554 Often used with commands that would otherwise produce long
559 =item = [alias value]
561 Define a command alias, like
565 or list current aliases.
569 Execute command as a Perl statement. A missing semicolon will be
574 The expression is evaluated, and the methods which may be applied to
575 the result are listed.
579 The methods which may be applied to objects in the C<package> are listed.
583 =head2 Debugger input/output
589 The debugger prompt is something like
597 where that number is the command number, which you'd use to access with
598 the built-in B<csh>-like history mechanism, e.g., C<!17> would repeat
599 command number 17. The number of angle brackets indicates the depth of
600 the debugger. You could get more than one set of brackets, for example, if
601 you'd already at a breakpoint and then printed out the result of a
602 function call that itself also has a breakpoint, or you step into an
603 expression via C<s/n/t expression> command.
605 =item Multi-line commands
607 If you want to enter a multi-line command, such as a subroutine
608 definition with several statements, or a format, you may escape the
609 newline that would normally end the debugger command with a backslash.
613 cont: print "ok\n"; \
620 Note that this business of escaping a newline is specific to interactive
621 commands typed into the debugger.
623 =item Stack backtrace
625 Here's an example of what a stack back-trace via C<T> command might
628 $ = main::infested called from file `Ambulation.pm' line 10
629 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
630 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
632 The left-hand character up there tells whether the function was called
633 in a scalar or list context (we bet you can tell which is which). What
634 that says is that you were in the function C<main::infested> when you ran
635 the stack dump, and that it was called in a scalar context from line 10
636 of the file I<Ambulation.pm>, but without any arguments at all, meaning
637 it was called as C<&infested>. The next stack frame shows that the
638 function C<Ambulation::legs> was called in a list context from the
639 I<camel_flea> file with four arguments. The last stack frame shows that
640 C<main::pests> was called in a scalar context, also from I<camel_flea>,
643 Note that if you execute C<T> command from inside an active C<use>
644 statement, the backtrace will contain both C<L<perlfunc/require>>
645 frame and an C<L<perlfunc/eval EXPR>>) frame.
649 Listing given via different flavors of C<l> command looks like this:
653 102:b @isa{@i,$pack} = ()
654 103 if(exists $i{$prevpack} || exists $isa{$pack});
658 107==> if(exists $isa{$pack});
660 109:a if ($extra-- > 0) {
661 110: %isa = ($pack,1);
663 Note that the breakable lines are marked with C<:>, lines with
664 breakpoints are marked by C<b>, with actions by C<a>, and the
665 next executed line is marked by C<==E<gt>>.
669 When C<frame> option is set, debugger would print entered (and
670 optionally exited) subroutines in different styles.
672 What follows is the start of the listing of
674 env "PERLDB_OPTS=f=1 N" perl -d -V
681 entering Config::BEGIN
682 Package lib/Exporter.pm.
684 Package lib/Config.pm.
685 entering Config::TIEHASH
686 entering Exporter::import
687 entering Exporter::export
688 entering Config::myconfig
689 entering Config::FETCH
690 entering Config::FETCH
691 entering Config::FETCH
692 entering Config::FETCH
697 entering Config::BEGIN
698 Package lib/Exporter.pm.
701 Package lib/Config.pm.
702 entering Config::TIEHASH
703 exited Config::TIEHASH
704 entering Exporter::import
705 entering Exporter::export
706 exited Exporter::export
707 exited Exporter::import
709 entering Config::myconfig
710 entering Config::FETCH
712 entering Config::FETCH
714 entering Config::FETCH
718 in $=main::BEGIN() from /dev/nul:0
719 in $=Config::BEGIN() from lib/Config.pm:2
720 Package lib/Exporter.pm.
722 Package lib/Config.pm.
723 in $=Config::TIEHASH('Config') from lib/Config.pm:644
724 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
725 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
726 in @=Config::myconfig() from /dev/nul:0
727 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
728 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
729 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
730 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
731 in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
732 in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
736 in $=main::BEGIN() from /dev/nul:0
737 in $=Config::BEGIN() from lib/Config.pm:2
738 Package lib/Exporter.pm.
740 out $=Config::BEGIN() from lib/Config.pm:0
741 Package lib/Config.pm.
742 in $=Config::TIEHASH('Config') from lib/Config.pm:644
743 out $=Config::TIEHASH('Config') from lib/Config.pm:644
744 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
745 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
746 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
747 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
748 out $=main::BEGIN() from /dev/nul:0
749 in @=Config::myconfig() from /dev/nul:0
750 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
751 out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
752 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
753 out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
754 in $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
755 out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
756 in $=Config::FETCH(ref(Config), 'SUBVERSION') from lib/Config.pm:574
760 in $=main::BEGIN() from /dev/nul:0
761 in $=Config::BEGIN() from lib/Config.pm:2
762 Package lib/Exporter.pm.
764 out $=Config::BEGIN() from lib/Config.pm:0
765 Package lib/Config.pm.
766 in $=Config::TIEHASH('Config') from lib/Config.pm:644
767 out $=Config::TIEHASH('Config') from lib/Config.pm:644
768 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
769 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
770 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
771 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/nul:0
772 out $=main::BEGIN() from /dev/nul:0
773 in @=Config::myconfig() from /dev/nul:0
774 in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
775 out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
776 in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
777 out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
781 In all the cases indentation of lines shows the call tree, if bit 2 of
782 C<frame> is set, then a line is printed on exit from a subroutine as
783 well, if bit 4 is set, then the arguments are printed as well as the
784 caller info, if bit 8 is set, the arguments are printed even if they
785 are tied or references.
787 When a package is compiled, a line like this
791 is printed with proper indentation.
795 =head2 Debugging compile-time statements
797 If you have any compile-time executable statements (code within a BEGIN
798 block or a C<use> statement), these will C<NOT> be stopped by debugger,
799 although C<require>s will (and compile-time statements can be traced
800 with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
801 code, however, you can
802 transfer control back to the debugger using the following statement,
803 which is harmless if the debugger is not running:
807 If you set C<$DB::single> to the value 2, it's equivalent to having
808 just typed the C<n> command, whereas a value of 1 means the C<s>
809 command. The C<$DB::trace> variable should be set to 1 to simulate
810 having typed the C<t> command.
812 Another way to debug compile-time code is to start debugger, set a
813 breakpoint on I<load> of some module thusly
815 DB<7> b load f:/perllib/lib/Carp.pm
816 Will stop on load of `f:/perllib/lib/Carp.pm'.
818 and restart debugger by C<R> command (if possible). One can use C<b
819 compile subname> for the same purpose.
821 =head2 Debugger Customization
823 Most probably you not want to modify the debugger, it contains enough
824 hooks to satisfy most needs. You may change the behaviour of debugger
825 from the debugger itself, using C<O>ptions, from the command line via
826 C<PERLDB_OPTS> environment variable, and from I<customization files>.
828 You can do some customization by setting up a F<.perldb> file which
829 contains initialization code. For instance, you could make aliases
830 like these (the last one is one people expect to be there):
832 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
833 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
834 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
835 $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
837 One changes options from F<.perldb> file via calls like this one;
839 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
841 (the code is executed in the package C<DB>). Note that F<.perldb> is
842 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
843 subroutine C<afterinit>, it is called after all the debugger
844 initialization ends. F<.perldb> may be contained in the current
845 directory, or in the C<LOGDIR>/C<HOME> directory.
847 If you want to modify the debugger, copy F<perl5db.pl> from the Perl
848 library to another name and modify it as necessary. You'll also want
849 to set your C<PERL5DB> environment variable to say something like this:
851 BEGIN { require "myperl5db.pl" }
853 As the last resort, one can use C<PERL5DB> to customize debugger by
854 directly setting internal variables or calling debugger functions.
856 =head2 Readline Support
858 As shipped, the only command line history supplied is a simplistic one
859 that checks for leading exclamation points. However, if you install
860 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
861 have full editing capabilities much like GNU I<readline>(3) provides.
862 Look for these in the F<modules/by-module/Term> directory on CPAN.
864 A rudimentary command-line completion is also available.
865 Unfortunately, the names of lexical variables are not available for
868 =head2 Editor Support for Debugging
870 If you have GNU B<emacs> installed on your system, it can interact with
871 the Perl debugger to provide an integrated software development
872 environment reminiscent of its interactions with C debuggers.
874 Perl is also delivered with a start file for making B<emacs> act like a
875 syntax-directed editor that understands (some of) Perl's syntax. Look in
876 the I<emacs> directory of the Perl source distribution.
878 (Historically, a similar setup for interacting with B<vi> and the
879 X11 window system had also been available, but at the time of this
880 writing, no debugger support for B<vi> currently exists.)
882 =head2 The Perl Profiler
884 If you wish to supply an alternative debugger for Perl to run, just
885 invoke your script with a colon and a package argument given to the B<-d>
886 flag. One of the most popular alternative debuggers for Perl is
887 B<DProf>, the Perl profiler. As of this writing, B<DProf> is not
888 included with the standard Perl distribution, but it is expected to
889 be included soon, for certain values of "soon".
891 Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming
892 it's properly installed on your system, to profile your Perl program in
893 the file F<mycode.pl>, just type:
895 perl -d:DProf mycode.pl
897 When the script terminates the profiler will dump the profile information
898 to a file called F<tmon.out>. A tool like B<dprofpp> (also supplied with
899 the Devel::DProf package) can be used to interpret the information which is
902 =head2 Debugger support in perl
904 When you call the B<caller> function (see L<perlfunc/caller>) from the
905 package DB, Perl sets the array @DB::args to contain the arguments the
906 corresponding stack frame was called with.
908 If perl is run with B<-d> option, the following additional features
915 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
916 'perl5db.pl'}> if not present) before the first line of the
921 The array C<@{"_<$filename"}> is the line-by-line contents of
922 $filename for all the compiled files. Same for C<eval>ed strings which
923 contain subroutines, or which are currently executed. The C<$filename>
924 for C<eval>ed strings looks like C<(eval 34)>.
928 The hash C<%{"_<$filename"}> contains breakpoints and action (it is
929 keyed by line number), and individual entries are settable (as opposed
930 to the whole hash). Only true/false is important to Perl, though the
931 values used by F<perl5db.pl> have the form
932 C<"$break_condition\0$action">. Values are magical in numeric context:
933 they are zeros if the line is not breakable.
935 Same for evaluated strings which contain subroutines, or which are
936 currently executed. The C<$filename> for C<eval>ed strings looks like
941 The scalar C<${"_<$filename"}> contains C<"_<$filename">. Same for
942 evaluated strings which contain subroutines, or which are currently
943 executed. The C<$filename> for C<eval>ed strings looks like C<(eval
948 After each C<require>d file is compiled, but before it is executed,
949 C<DB::postponed(*{"_<$filename"})> is called (if subroutine
950 C<DB::postponed> exists). Here the $filename is the expanded name of
951 the C<require>d file (as found in values of C<%INC>).
955 After each subroutine C<subname> is compiled existence of
956 C<$DB::postponed{subname}> is checked. If this key exists,
957 C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
962 A hash C<%DB::sub> is maintained, with keys being subroutine names,
963 values having the form C<filename:startline-endline>. C<filename> has
964 the form C<(eval 31)> for subroutines defined inside C<eval>s.
968 When execution of the application reaches a place that can have
969 a breakpoint, a call to C<DB::DB()> is performed if any one of
970 variables $DB::trace, $DB::single, or $DB::signal is true. (Note that
971 these variables are not C<local>izable.) This feature is disabled when
972 the control is inside C<DB::DB()> or functions called from it (unless
973 C<$^D & (1E<lt>E<lt>30)>).
977 When execution of the application reaches a subroutine call, a call
978 to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
979 the name of the called subroutine. (Unless the subroutine is compiled
980 in the package C<DB>.)
984 Note that no subroutine call is possible until C<&DB::sub> is defined
985 (for subroutines outside of package C<DB>). (This restriction is
988 (In fact, for the standard debugger the same is true if C<$DB::deep>
989 (how many levels of recursion deep into the debugger you can go before
990 a mandatory break) is not defined.)
992 With the recent updates the minimal possible debugger consists of one
997 which is quite handy as contents of C<PERL5DB> environment
1000 env "PERL5DB=sub DB::DB {}" perl -d your-script
1002 Another (a little bit more useful) minimal debugger can be created
1003 with the only line being
1005 sub DB::DB {print ++$i; scalar <STDIN>}
1007 This debugger would print the sequential number of encountered
1008 statement, and would wait for your C<CR> to continue.
1010 The following debugger is quite functional:
1015 sub sub {print ++$i, " $sub\n"; &$sub}
1018 It prints the sequential number of subroutine call and the name of the
1019 called subroutine. Note that C<&DB::sub> should be compiled into the
1022 =head2 Debugger Internals
1024 At the start, the debugger reads your rc file (F<./.perldb> or
1025 F<~/.perldb> under UNIX), which can set important options. This file may
1026 define a subroutine C<&afterinit> to be executed after the debugger is
1029 After the rc file is read, the debugger reads environment variable
1030 PERLDB_OPTS and parses it as a rest of C<O ...> line in debugger prompt.
1032 It also maintains magical internal variables, such as C<@DB::dbline>,
1033 C<%DB::dbline>, which are aliases for C<@{"::_<current_file"}>
1034 C<%{"::_<current_file"}>. Here C<current_file> is the currently
1035 selected (with the debugger's C<f> command, or by flow of execution)
1038 Some functions are provided to simplify customization. See L<"Debugger
1039 Customization"> for description of C<DB::parse_options(string)>. The
1040 function C<DB::dump_trace(skip[, count])> skips the specified number
1041 of frames, and returns an array containing info about the caller
1042 frames (all if C<count> is missing). Each entry is a hash with keys
1043 C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
1044 eval), C<args> (C<undef> or a reference to an array), C<file>, and
1047 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
1048 formatted info about caller frames. The last two functions may be
1049 convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.
1051 =head2 Other resources
1053 You did try the B<-w> switch, didn't you?
1057 You cannot get the stack frame information or otherwise debug functions
1058 that were not compiled by Perl, such as C or C++ extensions.
1060 If you alter your @_ arguments in a subroutine (such as with B<shift>
1061 or B<pop>, the stack back-trace will not show the original values.