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
65 Same as C<print {$DB::OUT} expr> in the current package. In particular,
66 because this is just Perl's own B<print> function, this means that nested
67 data structures and objects are not dumped, unlike with the C<x> command.
71 Evaluates its expression in list context and dumps out the result
72 in a pretty-printed fashion. Nested data structures are printed out
73 recursively, unlike the C<print> function.
75 The details of printout are governed by multiple C<O>ptions.
79 Display all (or some) variables in package (defaulting to the C<main>
80 package) using a data pretty-printer (hashes show their keys and values so
81 you see what's what, control characters are made printable, etc.). Make
82 sure you don't put the type specifier (like C<$>) there, just the symbol
87 Use C<~pattern> and C<!pattern> for positive and negative regexps.
89 Nested data structures are printed out in a legible fashion, unlike
90 the C<print> function.
92 The details of printout are governed by multiple C<O>ptions.
96 Same as C<V currentpackage [vars]>.
100 Produce a stack back-trace. See below for details on its output.
104 Single step. Executes until it reaches the beginning of another
105 statement, descending into subroutine calls. If an expression is
106 supplied that includes function calls, it too will be single-stepped.
110 Next. Executes over subroutine calls, until it reaches the beginning
111 of the next statement.
115 Repeat last C<n> or C<s> command.
119 Continue, optionally inserting a one-time-only breakpoint
120 at the specified line or subroutine.
124 List next window of lines.
128 List C<incr+1> lines starting at C<min>.
132 List lines C<min> through C<max>.
140 List first window of lines from subroutine.
144 List previous window of lines.
148 List window (a few lines) around the current line.
152 Return debugger pointer to the last-executed line and
157 Switch to viewing a different file.
161 Search forwards for pattern; final / is optional.
165 Search backwards for pattern; final ? is optional.
169 List all breakpoints and actions.
173 List subroutine names [not] matching pattern.
177 Toggle trace mode (see also C<AutoTrace> C<O>ption).
181 Trace through execution of expr. For example:
184 Stack dump during die enabled outside of evals.
186 Loading DB routines from perl5db.pl patch level 0.94
187 Emacs support available.
189 Enter h or `h h' for help.
196 DB<3> t print foo() * bar()
197 main::((eval 172):3): print foo() + bar();
198 main::foo((eval 168):2):
199 main::bar((eval 170):2):
202 or, with the C<O>ption C<frame=2> set,
206 DB<5> t print foo() * bar()
216 =item b [line] [condition]
218 Set a breakpoint. If line is omitted, sets a breakpoint on the line
219 that is about to be executed. If a condition is specified, it's
220 evaluated each time the statement is reached and a breakpoint is taken
221 only if the condition is true. Breakpoints may be set on only lines
222 that begin an executable statement. Conditions don't use B<if>:
225 b 237 ++$count237 < 11
228 =item b subname [condition]
230 Set a breakpoint at the first line of the named subroutine.
232 =item b postpone subname [condition]
234 Set breakpoint at first line of subroutine after it is compiled.
236 =item b load filename
238 Set breakpoint at the first executed line of the file.
242 Delete a breakpoint at the specified line. If line is omitted, deletes
243 the breakpoint on the line that is about to be executed.
247 Delete all installed breakpoints.
249 =item a [line] command
251 Set an action to be done before the line is executed.
252 The sequence of steps taken by the debugger is
254 1. check for a breakpoint at this line
255 2. print the line if necessary (tracing)
256 3. do any actions associated with that line
257 4. prompt user if at a breakpoint or in single-step
260 For example, this will print out C<$foo> every time line
263 a 53 print "DB FOUND $foo\n"
267 Delete all installed actions.
269 =item O [opt[=val]] [opt"val"] [opt?]...
271 Set or query values of options. val defaults to 1. opt can
272 be abbreviated. Several options can be listed.
276 =item recallCommand, ShellBang
278 The characters used to recall command or spawn shell. By
279 default, these are both set to C<!>.
283 Program to use for output of pager-piped commands (those
284 beginning with a C<|> character.) By default,
285 C<$ENV{PAGER}> will be used.
289 Run Tk while prompting (with ReadLine).
291 =item signalLevel, warnLevel, dieLevel
297 Where to print all the breakable points in the executed program
298 (similar to C<t> command, but can be put into C<PERLDB_OPTS>).
302 File or pipe to print line number info to. If it is a
303 pipe, then a short, "emacs like" message is used.
305 =item C<inhibit_exit>
307 If 0, allows I<stepping off> the end of the script.
311 affects printing of return value after C<r> command.
315 affects printing messages on entry and exit from subroutines. If
316 C<frame & 2> is false, messages are printed on entry only. (Printing
317 on exit may be useful if inter(di)spersed with other messages.)
319 If C<frame & 4>, arguments to functions are printed as well as the
320 context and caller info.
324 The following options affect what happens with C<V>, C<X>, and C<x>
329 =item arrayDepth, hashDepth
331 Print only first N elements ('' for all).
333 =item compactDump, veryCompact
335 Change style of array and hash dump.
339 Whether to print contents of globs.
343 Dump arrays holding debugged files.
347 Dump symbol tables of packages.
349 =item quote, HighBit, undefPrint
351 Change style of string dump.
355 During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
356 You can put additional initialization options C<TTY>, C<noTTY>,
357 C<ReadLine>, and C<NonStop> there.
361 &parse_options("NonStop=1 LineInfo=db.out AutoTrace");
363 The script will run without human intervention, putting trace information
364 into the file I<db.out>. (If you interrupt it, you would better reset
365 C<LineInfo> to something "interactive"!)
371 The TTY to use for debugging I/O.
375 If set, goes in C<NonStop> mode. On interrupt if TTY is not set uses the
376 value of C<noTTY> or "/tmp/perldbtty$$" to find TTY using
377 C<Term::Rendezvous>. Current variant is to have the name of TTY in this
382 If set, goes in C<NonStop> mode, and would not connect to a TTY. If
383 interrupt (or if control goes to debugger via explicit setting of
384 $DB::signal or $DB::single from the Perl script), connects to a TTY
385 specified by the C<TTY> option at startup, or to a TTY found at
386 runtime using C<Term::Rendezvous> module of your choice.
388 This module should implement a method C<new> which returns an object
389 with two methods: C<IN> and C<OUT>, returning two filehandles to use
390 for debugging input and output correspondingly. Method C<new> may
391 inspect an argument which is a value of C<$ENV{PERLDB_NOTTY}> at
392 startup, or is C<"/tmp/perldbtty$$"> otherwise.
396 If false, readline support in debugger is disabled, so you can debug
397 ReadLine applications.
401 If set, debugger goes into non-interactive mode until interrupted, or
402 programmatically by setting $DB::signal or $DB::single.
406 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
408 $ PERLDB_OPTS="N f=2" perl -d myprogram
410 will run the script C<myprogram> without human intervention, printing
411 out the call tree with entry and exit points. Note that C<N f=2> is
412 equivalent to C<NonStop=1 frame=2>. Note also that at the moment when
413 this documentation was written all the options to the debugger could
414 be uniquely abbreviated by the first letter (with exception of
417 Other examples may include
419 $ PERLDB_OPTS="N f A L=listing" perl -d myprogram
421 - runs script non-interactively, printing info on each entry into a
422 subroutine and each executed line into the file F<listing>. (If you
423 interrupt it, you would better reset C<LineInfo> to something
427 $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
429 may be useful for debugging a program which uses C<Term::ReadLine>
430 itself. Do not forget detach shell from the TTY in the window which
431 corresponds to F</dev/ttyc>, say, by issuing a command like
435 See L<"Debugger Internals"> below for more details.
439 =item E<lt> [ command ]
441 Set an action (Perl command) to happen before every debugger prompt.
442 A multi-line command may be entered by backslashing the newlines. If
443 C<command> is missing, resets the list of actions.
445 =item E<lt>E<lt> command
447 Add an action (Perl command) to happen before every debugger prompt.
448 A multi-line command may be entered by backslashing the newlines.
452 Set an action (Perl command) to happen after the prompt when you've
453 just given a command to return to executing the script. A multi-line
454 command may be entered by backslashing the newlines. If C<command> is
455 missing, resets the list of actions.
457 =item E<gt>E<gt> command
459 Adds an action (Perl command) to happen after the prompt when you've
460 just given a command to return to executing the script. A multi-line
461 command may be entered by backslashing the newlines.
465 Set an action (debugger command) to happen before every debugger prompt.
466 A multi-line command may be entered by backslashing the newlines. If
467 C<command> is missing, resets the list of actions.
471 Add an action (debugger command) to happen before every debugger prompt.
472 A multi-line command may be entered by backslashing the newlines.
476 Redo a previous command (default previous command).
480 Redo number'th-to-last command.
484 Redo last command that started with pattern.
485 See C<O recallCommand>, too.
489 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT)
490 See C<O shellBang> too.
494 Display last n commands. Only commands longer than one character are
495 listed. If number is omitted, lists them all.
499 Quit. ("quit" doesn't work for this.) This is the only supported way
500 to exit the debugger, though typing C<exit> twice may do it too.
502 Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
503 off> the end the script. You may also need to set C<$finished> to 0 at
504 some moment if you want to step through global destruction.
508 Restart the debugger by B<exec>ing a new session. It tries to maintain
509 your history across this, but internal settings and command line options
512 Currently the following setting are preserved: history, breakpoints,
513 actions, debugger C<O>ptions, and the following command-line
514 options: B<-w>, B<-I>, and B<-e>.
518 Run debugger command, piping DB::OUT to current pager.
522 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
523 Often used with commands that would otherwise produce long
528 =item = [alias value]
530 Define a command alias, or list current aliases.
534 Execute command as a Perl statement. A missing semicolon will be
539 Same as C<print DB::OUT expr>. The DB::OUT filehandle is opened to
540 /dev/tty, regardless of where STDOUT may be redirected to.
544 The debugger prompt is something like
552 where that number is the command number, which you'd use to access with
553 the built-in B<csh>-like history mechanism, e.g., C<!17> would repeat
554 command number 17. The number of angle brackets indicates the depth of
555 the debugger. You could get more than one set of brackets, for example, if
556 you'd already at a breakpoint and then printed out the result of a
557 function call that itself also has a breakpoint, or you step into an
558 expression via C<s/n/t expression> command.
560 If you want to enter a multi-line command, such as a subroutine
561 definition with several statements, you may escape the newline that would
562 normally end the debugger command with a backslash. Here's an example:
565 cont: print "ok\n"; \
572 Note that this business of escaping a newline is specific to interactive
573 commands typed into the debugger.
575 Here's an example of what a stack back-trace might look like:
577 $ = main::infested called from file `Ambulation.pm' line 10
578 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
579 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
581 The left-hand character up there tells whether the function was called
582 in a scalar or list context (we bet you can tell which is which). What
583 that says is that you were in the function C<main::infested> when you ran
584 the stack dump, and that it was called in a scalar context from line 10
585 of the file I<Ambulation.pm>, but without any arguments at all, meaning
586 it was called as C<&infested>. The next stack frame shows that the
587 function C<Ambulation::legs> was called in a list context from the
588 I<camel_flea> file with four arguments. The last stack frame shows that
589 C<main::pests> was called in a scalar context, also from I<camel_flea>,
592 If you have any compile-time executable statements (code within a BEGIN
593 block or a C<use> statement), these will C<NOT> be stopped by debugger,
594 although C<require>s will (and compile-time statements can be traced
595 with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
596 code, however, you can
597 transfer control back to the debugger using the following statement,
598 which is harmless if the debugger is not running:
602 If you set C<$DB::single> to the value 2, it's equivalent to having
603 just typed the C<n> command, whereas a value of 1 means the C<s>
604 command. The C<$DB::trace> variable should be set to 1 to simulate
605 having typed the C<t> command.
607 =head2 Debugger Customization
609 Most probably you not want to modify the debugger, it contains enough
610 hooks to satisfy most needs. You may change the behaviour of debugger
611 from the debugger itself, using C<O>ptions, from the command line via
612 C<PERLDB_OPTS> environment variable, and from I<customization files>.
614 You can do some customization by setting up a F<.perldb> file which
615 contains initialization code. For instance, you could make aliases
616 like these (the last one is one people expect to be there):
618 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
619 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
620 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
621 $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
623 One changes options from F<.perldb> file via calls like this one;
625 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
627 (the code is executed in the package C<DB>). Note that F<.perldb> is
628 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
629 subroutine C<afterinit>, it is called after all the debugger
630 initialization ends. F<.perldb> may be contained in the current
631 directory, or in the C<LOGDIR>/C<HOME> directory.
633 If you want to modify the debugger, copy F<perl5db.pl> from the Perl
634 library to another name and modify it as necessary. You'll also want
635 to set your C<PERL5DB> environment variable to say something like this:
637 BEGIN { require "myperl5db.pl" }
639 As the last resort, one can use C<PERL5DB> to customize debugger by
640 directly setting internal variables or calling debugger functions.
642 =head2 Readline Support
644 As shipped, the only command line history supplied is a simplistic one
645 that checks for leading exclamation points. However, if you install
646 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
647 have full editing capabilities much like GNU I<readline>(3) provides.
648 Look for these in the F<modules/by-module/Term> directory on CPAN.
650 =head2 Editor Support for Debugging
652 If you have GNU B<emacs> installed on your system, it can interact with
653 the Perl debugger to provide an integrated software development
654 environment reminiscent of its interactions with C debuggers.
656 Perl is also delivered with a start file for making B<emacs> act like a
657 syntax-directed editor that understands (some of) Perl's syntax. Look in
658 the I<emacs> directory of the Perl source distribution.
660 (Historically, a similar setup for interacting with B<vi> and the
661 X11 window system had also been available, but at the time of this
662 writing, no debugger support for B<vi> currently exists.)
664 =head2 The Perl Profiler
666 If you wish to supply an alternative debugger for Perl to run, just
667 invoke your script with a colon and a package argument given to the B<-d>
668 flag. One of the most popular alternative debuggers for Perl is
669 B<DProf>, the Perl profiler. As of this writing, B<DProf> is not
670 included with the standard Perl distribution, but it is expected to
671 be included soon, for certain values of "soon".
673 Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming
674 it's properly installed on your system, to profile your Perl program in
675 the file F<mycode.pl>, just type:
677 perl -d:DProf mycode.pl
679 When the script terminates the profiler will dump the profile information
680 to a file called F<tmon.out>. A tool like B<dprofpp> (also supplied with
681 the Devel::DProf package) can be used to interpret the information which is
684 =head2 Debugger support in perl
686 When you call the B<caller> function from package DB, Perl sets the
687 C<@DB::args> array to contain the arguments that stack frame was called
690 If perl is run with B<-d> option, the following additional features
697 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
698 'perl5db.pl'}> if not present) before the first line of the
703 The array C<@{"_<$filename"}> is the line-by-line contents of
704 $filename for all the compiled files. Same for C<eval>ed strings which
705 contain subroutines, or which are currently executed. The C<$filename>
706 for C<eval>ed strings looks like C<(eval 34)>.
710 The hash C<%{"_<$filename"}> contains breakpoints and action (it is
711 keyed by line number), and individual entries are settable (as opposed
712 to the whole hash). Only true/false is important to Perl, though the
713 values used by F<perl5db.pl> have the form
714 C<"$break_condition\0$action">. Values are magical in numeric context:
715 they are zeros if the line is not breakable.
717 Same for evaluated strings which contain subroutines, or which are
718 currently executed. The C<$filename> for C<eval>ed strings looks like
723 The scalar C<${"_<$filename"}> contains C<"_<$filename">. Same for
724 evaluated strings which contain subroutines, or which are currently
725 executed. The C<$filename> for C<eval>ed strings looks like C<(eval
730 After each C<require>d file is compiled, but before it is executed,
731 C<DB::postponed(*{"_<$filename"})> is called (if subroutine
732 C<DB::postponed> exists). Here the $filename is the expanded name of
733 the C<require>d file (as found in values of C<%INC>).
737 After each subroutine C<subname> is compiled existence of
738 C<$DB::postponed{subname}> is checked. If this key exists,
739 C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
744 A hash C<%DB::sub> is maintained, with keys being subroutine names,
745 values having the form C<filename:startline-endline>. C<filename> has
746 the form C<(eval 31)> for subroutines defined inside C<eval>s.
750 When execution of the application reaches a place that can have
751 a breakpoint, a call to C<DB::DB()> is performed if any one of
752 variables $DB::trace, $DB::single, or $DB::signal is true. (Note that
753 these variables are not C<local>izable.) This feature is disabled when
754 the control is inside C<DB::DB()> or functions called from it (unless
755 C<$^D & 1 E<lt>E<lt> 30>).
759 When execution of the application reaches a subroutine call, a call
760 to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
761 the name of the called subroutine. (Unless the subroutine is compiled
762 in the package C<DB>.)
766 Note that no subroutine call is possible until C<&DB::sub> is defined
767 (for subroutines outside of package C<DB>). (In fact, for the
768 standard debugger the same is true if C<$DB::deep> (how many levels of
769 recursion deep into the debugger you can go before a mandatory break)
772 =head2 Debugger Internals
774 At the start, the debugger reads your rc file (F<./.perldb> or
775 F<~/.perldb> under UNIX), which can set important options. This file may
776 define a subroutine C<&afterinit> to be executed after the debugger is
779 After the rc file is read, the debugger reads environment variable
780 PERLDB_OPTS and parses it as a rest of C<O ...> line in debugger prompt.
782 It also maintains magical internal variables, such as C<@DB::dbline>,
783 C<%DB::dbline>, which are aliases for C<@{"::_<current_file"}>
784 C<%{"::_<current_file"}>. Here C<current_file> is the currently
785 selected (with the debugger's C<f> command, or by flow of execution)
788 Some functions are provided to simplify customization. See L<"Debugger
789 Customization"> for description of C<DB::parse_options(string)>. The
790 function C<DB::dump_trace(skip[, count])> skips the specified number
791 of frames, and returns an array containing info about the caller
792 frames (all if C<count> is missing). Each entry is a hash with keys
793 C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
794 eval), C<args> (C<undef> or a reference to an array), C<file>, and
797 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
798 formatted info about caller frames. The last two functions may be
799 convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.
801 =head2 Other resources
803 You did try the B<-w> switch, didn't you?
807 You cannot get the stack frame information or otherwise debug functions
808 that were not compiled by Perl, such as C or C++ extensions.
810 If you alter your @_ arguments in a subroutine (such as with B<shift>
811 or B<pop>, the stack back-trace will not show the original values.