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 backtraces, 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 {$DB::OUT} expr> in the current package. In particular,
66 since this is just Perl's own B<print> function, this means that nested
67 data structures and objects are not dumped, unlike with the C<x> command.
71 Evals its expression in list context and dumps out the result
72 in a pretty-printed fashion. Nested data structures are printed out
73 recursively, unlike the C<print> function.
75 The details of printout are governed by multiple C<O>ptions.
79 Display all (or some) variables in package (defaulting to the C<main>
80 package) using a data pretty-printer (hashes show their keys and values so
81 you see what's what, control characters are made printable, etc.). Make
82 sure you don't put the type specifier (like C<$>) 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 backtrace. 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 only be set on lines
222 that begin an executable statement. Conditions don't use B<if>:
225 b 237 ++$count237 < 11
228 =item b subname [condition]
230 Set a breakpoint at the first line of the named subroutine.
232 =item b postpone subname [condition]
234 Set breakpoint at first line of subroutine after it is compiled.
236 =item b load filename
238 Set breakpoint at the first executed line of the file.
242 Delete a breakpoint at the specified line. If line is omitted, deletes
243 the breakpoint on the line that is about to be executed.
247 Delete all installed breakpoints.
249 =item a [line] command
251 Set an action to be done before the line is executed.
252 The sequence of steps taken by the debugger is
258 check for a breakpoint at this line
262 print the line if necessary (tracing)
266 do any actions associated with that line
270 prompt user if at a breakpoint or in single-step
278 For example, this will print out C<$foo> every time line
281 a 53 print "DB FOUND $foo\n"
285 Delete all installed actions.
287 =item O [opt[=val]] [opt"val"] [opt?]...
289 Set or query values of options. val defaults to 1. opt can
290 be abbreviated. Several options can be listed.
294 =item recallCommand, ShellBang
296 The characters used to recall command or spawn shell. By
297 default, these are both set to C<!>.
301 Program to use for output of pager-piped commands (those
302 beginning with a C<|> character.) By default,
303 C<$ENV{PAGER}> will be used.
307 Run Tk while prompting (with ReadLine).
309 =item signalLevel, warnLevel, dieLevel
315 Where to print all the breakable points in the executed program
316 (similar to C<t> command, but can be put into C<PERLDB_OPTS>).
320 File or pipe to print line number info to. If it is a
321 pipe, then a short, "emacs like" message is used.
323 =item C<inhibit_exit>
325 If 0, allows I<stepping off> the end of the script.
329 affects printing of return value after C<r> command.
333 affects printing messages on entry and exit from subroutines. If
334 C<frame & 2> is false, messages are printed on entry only. (Printing
335 on exit may be useful if interdispersed with other messages.)
337 If C<frame & 4>, arguments to functions are printed as well as the
338 context and caller info.
342 The following options affect what happens with C<V>, C<X>, and C<x>
347 =item arrayDepth, hashDepth
349 Print only first N elements ('' for all).
351 =item compactDump, veryCompact
353 Change style of array and hash dump.
357 Whether to print contents of globs.
361 Dump arrays holding debugged files.
365 Dump symbol tables of packages.
367 =item quote, HighBit, undefPrint
369 Change style of string dump.
373 During startup options are initialized from C<$ENV{PERLDB_OPTS}>.
374 You can put additional initialization options C<TTY>, C<noTTY>,
375 C<ReadLine>, and C<NonStop> there.
379 &parse_options("NonStop=1 LineInfo=db.out AutoTrace");
381 The script will run without human intervention, putting trace information
382 into the file I<db.out>. (If you interrupt it, you would better reset
383 C<LineInfo> to something "interactive"!)
389 The TTY to use for debugging I/O.
393 If set, goes in C<NonStop> mode. On interrupt if TTY is not set uses the
394 value of C<noTTY> or "/tmp/perldbtty$$" to find TTY using
395 C<Term::Rendezvous>. Current variant is to have the name of TTY in this
400 If set, goes in C<NonStop> mode, and would not connect to a TTY. If
401 interrupt (or if control goes to debugger via explicit setting of
402 $DB::signal or $DB::single from the Perl script), connects to a TTY
403 specified by the C<TTY> option at startup, or to a TTY found at
404 runtime using C<Term::Rendezvous> module of your choice.
406 This module should implement a method C<new> which returns an object
407 with two methods: C<IN> and C<OUT>, returning two filehandles to use
408 for debugging input and output correspondingly. Method C<new> may
409 inspect an argument which is a value of C<$ENV{PERLDB_NOTTY}> at
410 startup, or is C<"/tmp/perldbtty$$"> otherwise.
414 If false, readline support in debugger is disabled, so you can debug
415 ReadLine applications.
419 If set, debugger goes into non-interactive mode until interrupted, or
420 programmatically by setting $DB::signal or $DB::single.
424 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
426 $ PERLDB_OPTS="N f=2" perl -d myprogram
428 will run the script C<myprogram> without human intervention, printing
429 out the call tree with entry and exit points. Note that C<N f=2> is
430 equivalent to C<NonStop=1 frame=2>. Note also that at the moment when
431 this documentation was written all the options to the debugger could
432 be uniquely abbreviated by the first letter (with exception of
435 Other examples may include
437 $ PERLDB_OPTS="N f A L=listing" perl -d myprogram
439 - runs script non-interactively, printing info on each entry into a
440 subroutine and each executed line into the file F<listing>. (If you
441 interrupt it, you would better reset C<LineInfo> to something
445 $ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
447 may be useful for debugging a program which uses C<Term::ReadLine>
448 itself. Do not forget detach shell from the TTY in the window which
449 corresponds to F</dev/ttyc>, say, by issuing a command like
453 See L<"Debugger Internals"> below for more details.
455 =item E<lt> [ command ]
457 Set an action (Perl command) to happen before every debugger prompt.
458 A multiline command may be entered by backslashing the newlines. If
459 C<command> is missing, resets the list of actions.
461 =item E<lt>E<lt> command
463 Add an action (Perl command) to happen before every debugger prompt.
464 A multiline command may be entered by backslashing the newlines.
468 Set an action (Perl command) to happen after the prompt when you've
469 just given a command to return to executing the script. A multiline
470 command may be entered by backslashing the newlines. If C<command> is
471 missing, resets the list of actions.
473 =item E<gt>E<gt> command
475 Adds an action (Perl command) to happen after the prompt when you've
476 just given a command to return to executing the script. A multiline
477 command may be entered by backslashing the newlines.
481 Set an action (debugger command) to happen before every debugger prompt.
482 A multiline command may be entered by backslashing the newlines. If
483 C<command> is missing, resets the list of actions.
487 Add an action (debugger command) to happen before every debugger prompt.
488 A multiline command may be entered by backslashing the newlines.
492 Redo a previous command (default previous command).
496 Redo number'th-to-last command.
500 Redo last command that started with pattern.
501 See C<O recallCommand>, too.
505 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT)
506 See C<O shellBang> too.
510 Display last n commands. Only commands longer than one character are
511 listed. If number is omitted, lists them all.
515 Quit. ("quit" doesn't work for this.) This is the only supported way
516 to exit the debugger, though typing C<exit> twice may do it too.
518 Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
519 off> the end the script. You may also need to set C<$finished> to 0 at
520 some moment if you want to step through global destruction.
524 Restart the debugger by B<exec>ing a new session. It tries to maintain
525 your history across this, but internal settings and command line options
528 Currently the following setting are preserved: history, breakpoints
529 and actions, debugger C<O>ptions and the following command-line
530 options: B<-w>, B<-I>, B<-e>.
534 Run debugger command, piping DB::OUT to current pager.
538 Same as C<|dbcmd> but DB::OUT is temporarily B<select>ed as well.
539 Often used with commands that would otherwise produce long
544 =item = [alias value]
546 Define a command alias, or list current aliases.
550 Execute command as a Perl statement. A missing semicolon will be
555 Same as C<print DB::OUT expr>. The DB::OUT filehandle is opened to
556 /dev/tty, regardless of where STDOUT may be redirected to.
560 The debugger prompt is something like
568 where that number is the command number, which you'd use to access with
569 the built-in B<csh>-like history mechanism, e.g. C<!17> would repeat
570 command number 17. The number of angle brackets indicates the depth of
571 the debugger. You could get more than one set of brackets, for example, if
572 you'd already at a breakpoint and then printed out the result of a
573 function call that itself also has a breakpoint, or you step into an
574 expression via C<s/n/t expression> command.
576 If you want to enter a multi-line command, such as a subroutine
577 definition with several statements, you may escape the newline that would
578 normally end the debugger command with a backslash. Here's an example:
581 cont: print "ok\n"; \
588 Note that this business of escaping a newline is specific to interactive
589 commands typed into the debugger.
591 Here's an example of what a stack backtrace might look like:
593 $ = main::infested called from file `Ambulation.pm' line 10
594 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
595 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
597 The left-hand character up there tells whether the function was called
598 in a scalar or list context (we bet you can tell which is which). What
599 that says is that you were in the function C<main::infested> when you ran
600 the stack dump, and that it was called in a scalar context from line 10
601 of the file I<Ambulation.pm>, but without any arguments at all, meaning
602 it was called as C<&infested>. The next stack frame shows that the
603 function C<Ambulation::legs> was called in a list context from the
604 I<camel_flea> file with four arguments. The last stack frame shows that
605 C<main::pests> was called in a scalar context, also from I<camel_flea>,
608 If you have any compile-time executable statements (code within a BEGIN
609 block or a C<use> statement), these will C<NOT> be stopped by debugger,
610 although C<require>s will (and compile-time statements can be traced
611 with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
612 code, however, you can
613 transfer control back to the debugger using the following statement,
614 which is harmless if the debugger is not running:
618 If you set C<$DB::single> to the value 2, it's equivalent to having
619 just typed the C<n> command, whereas a value of 1 means the C<s>
620 command. The C<$DB::trace> variable should be set to 1 to simulate
621 having typed the C<t> command.
623 =head2 Debugger Customization
625 Most probably you not want to modify the debugger, it contains enough
626 hooks to satisfy most needs. You may change the behaviour of debugger
627 from the debugger itself, using C<O>ptions, from the command line via
628 C<PERLDB_OPTS> environment variable, and from I<customization files>.
630 You can do some customization by setting up a F<.perldb> file which
631 contains initialization code. For instance, you could make aliases
632 like these (the last one is one people expect to be there):
634 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
635 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
636 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
637 $DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
639 One changes options from F<.perldb> file via calls like this one;
641 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
643 (the code is executed in the package C<DB>). Note that F<.perldb> is
644 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
645 subroutine C<afterinit>, it is called after all the debugger
646 initialization ends. F<.perldb> may be contained in the current
647 directory, or in the C<LOGDIR>/C<HOME> directory.
649 If you want to modify the debugger, copy F<perl5db.pl> from the Perl
650 library to another name and modify it as necessary. You'll also want
651 to set your C<PERL5DB> environment variable to say something like this:
653 BEGIN { require "myperl5db.pl" }
655 As the last resort, one can use C<PERL5DB> to customize debugger by
656 directly setting internal variables or calling debugger functions.
658 =head2 Readline Support
660 As shipped, the only command line history supplied is a simplistic one
661 that checks for leading exclamation points. However, if you install
662 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
663 have full editing capabilities much like GNU I<readline>(3) provides.
664 Look for these in the F<modules/by-module/Term> directory on CPAN.
666 =head2 Editor Support for Debugging
668 If you have GNU B<emacs> installed on your system, it can interact with
669 the Perl debugger to provide an integrated software development
670 environment reminiscent of its interactions with C debuggers.
672 Perl is also delivered with a start file for making B<emacs> act like a
673 syntax-directed editor that understands (some of) Perl's syntax. Look in
674 the I<emacs> directory of the Perl source distribution.
676 (Historically, a similar setup for interacting with B<vi> and the
677 X11 window system had also been available, but at the time of this
678 writing, no debugger support for B<vi> currently exists.)
680 =head2 The Perl Profiler
682 If you wish to supply an alternative debugger for Perl to run, just
683 invoke your script with a colon and a package argument given to the B<-d>
684 flag. One of the most popular alternative debuggers for Perl is
685 B<DProf>, the Perl profiler. As of this writing, B<DProf> is not
686 included with the standard Perl distribution, but it is expected to
687 be included soon, for certain values of "soon".
689 Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assuming
690 it's properly installed on your system, to profile your Perl program in
691 the file F<mycode.pl>, just type:
693 perl -d:DProf mycode.pl
695 When the script terminates the profiler will dump the profile information
696 to a file called F<tmon.out>. A tool like B<dprofpp> (also supplied with
697 the Devel::DProf package) can be used to interpret the information which is
700 =head2 Debugger support in perl
702 When you call the B<caller> function from package DB, Perl sets the
703 C<@DB::args> array to contain the arguments that stack frame was called
706 If perl is run with B<-d> option, the following additional features
713 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
714 'perl5db.pl'}> if not present) before the first line of the
719 The array C<@{"_<$filename"}> is the line-by-line contents of
720 $filename for all the compiled files. Same for C<eval>ed strings which
721 contain subroutines, or which are currently executed. The C<$filename>
722 for C<eval>ed strings looks like C<(eval 34)>.
726 The hash C<%{"_<$filename"}> contains breakpoints and action (it is
727 keyed by line number), and individual entries are settable (as opposed
728 to the whole hash). Only true/false is important to Perl, though the
729 values used by F<perl5db.pl> have the form
730 C<"$break_condition\0$action">. Values are magical in numeric context:
731 they are zeros if the line is not breakable.
733 Same for evaluated strings which contain subroutines, or which are
734 currently executed. The C<$filename> for C<eval>ed strings looks like
739 The scalar C<${"_<$filename"}> contains C<"_<$filename">. Same for
740 evaluated strings which contain subroutines, or which are currently
741 executed. The C<$filename> for C<eval>ed strings looks like C<(eval
746 After each C<require>d file is compiled, but before it is executed,
747 C<DB::postponed(*{"_<$filename"})> is called (if subroutine
748 C<DB::postponed> exists). Here the $filename is the expanded name of
749 the C<require>d file (as found in values of C<%INC>).
753 After each subroutine C<subname> is compiled existence of
754 C<$DB::postponed{subname}> is checked. If this key exists,
755 C<DB::postponed(subname)> is called (if subroutine C<DB::postponed>
760 A hash C<%DB::sub> is maintained, with keys being subroutine names,
761 values having the form C<filename:startline-endline>. C<filename> has
762 the form C<(eval 31)> for subroutines defined inside C<eval>s.
766 When an exection of the application reaches a place that can have a
767 breakpoint, a call to C<DB::DB()> is performed if any one of
768 variables $DB::trace, $DB::single, $DB::signal is true. (Note that
769 these variables are not C<local>izable.) This feature is disabled when
770 the control is inside C<DB::DB()> or functions called from it (unless
771 C<$^D & 1 E<lt>E<lt> 30>).
775 When an exection of the application reaches a subroutine call, a call
776 to C<&DB::sub>(I<args>) is performed instead, with C<$DB::sub> being
777 the name of the called subroutine. (Unless the subroutine is compiled
778 in the package C<DB>.)
782 Note that no subroutine call is possible until C<&DB::sub> is defined
783 (for subroutines outside of package C<DB>). (In fact, for the
784 standard debugger the same is true if C<$DB::deep> (how many levels of
785 recursion deep into the debugger you can go before a mandatory break)
788 =head2 Debugger Internals
790 At the start, the debugger reads your rc file (F<./.perldb> or
791 F<~/.perldb> under UNIX), which can set important options. This file may
792 define a subroutine C<&afterinit> to be executed after the debugger is
795 After the rc file is read, the debugger reads environment variable
796 PERLDB_OPTS and parses it as a rest of C<O ...> line in debugger prompt.
798 It also maintains magical internal variables, such as C<@DB::dbline>,
799 C<%DB::dbline>, which are aliases for C<@{"::_<current_file"}>
800 C<%{"::_<current_file"}>. Here C<current_file> is the currently
801 selected (with the debugger's C<f> command, or by flow of execution)
804 Some functions are provided to simplify customization. See L<"Debugger
805 Customization"> for description of C<DB::parse_options(string)>. The
806 function C<DB::dump_trace(skip[, count])> skips the specified number
807 of frames, and returns an array containing info about the caller
808 frames (all if C<count> is missing). Each entry is a hash with keys
809 C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
810 eval), C<args> (C<undef> or a reference to an array), C<file> and
813 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
814 formatted info about caller frames. The last two functions may be
815 convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.
817 =head2 Other resources
819 You did try the B<-w> switch, didn't you?
823 You cannot get the stack frame information or otherwise debug functions
824 that were not compiled by Perl, such as C or C++ extensions.
826 If you alter your @_ arguments in a subroutine (such as with B<shift>
827 or B<pop>, the stack backtrace will not show the original values.
829 Some subroutines are called without creating a call frame. This may
830 confuse backtrace C<T> and output of C<fE<gt>=4>.