1PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)
2
6 perldebug - Perl debugging
7
9 First of all, have you tried using "use strict;" and "use warnings;"?
10 If you're new to the Perl debugger, you may prefer to read perldebtut,
12 which is a tutorial introduction to the debugger.
13 If you're looking for the nitty gritty details of how the debugger is
15 implemented, you may prefer to read perldebguts.
16
18 If you invoke Perl with the -d switch, your script runs under the Perl
19 source debugger. This works like an interactive Perl environment,
20 prompting for debugger commands that let you examine source code, set
21 breakpoints, get stack backtraces, change the values of variables, etc.
22 This is so convenient that you often fire up the debugger all by itself
23 just to test out Perl constructs interactively to see what they do.
24 For example:
25 $ perl -d -e 42
27 In Perl, the debugger is not a separate program the way it usually is
29 in the typical compiled environment. Instead, the -d flag tells the
30 compiler to insert source information into the parse trees it's about
31 to hand off to the interpreter. That means your code must first
32 compile correctly for the debugger to work on it. Then when the
33 interpreter starts up, it preloads a special Perl library file
34 containing the debugger.
35 The program will halt right before the first run-time executable
37 statement (but see below regarding compile-time statements) and ask you
38 to enter a debugger command. Contrary to popular expectations,
39 whenever the debugger halts and shows you a line of code, it always
40 displays the line it's about to execute, rather than the one it has
41 just executed.
42 Any command not recognized by the debugger is directly executed
44 ("eval"'d) as Perl code in the current package. (The debugger uses the
45 DB package for keeping its own state information.)
46 Note that the said "eval" is bound by an implicit scope. As a result
48 any newly introduced lexical variable or any modified capture buffer
49 content is lost after the eval. The debugger is a nice environment to
50 learn Perl, but if you interactively experiment using material which
51 should be in the same scope, stuff it in one line.
52 For any text entered at the debugger prompt, leading and trailing
54 whitespace is first stripped before further processing. If a debugger
55 command coincides with some function in your own program, merely
56 precede the function with something that doesn't look like a debugger
57 command, such as a leading ";" or perhaps a "+", or by wrapping it with
58 parentheses or braces.
59 Calling the Debugger
61 There are several ways to call the debugger:
62 perl -d program_name
64 On the given program identified by "program_name".
65 perl -d -e 0
67 Interactively supply an arbitrary "expression" using "-e".
68 perl -d:ptkdb program_name
70 Debug a given program via the "Devel::ptkdb" GUI.
71 perl -dt threaded_program_name
73 Debug a given program using threads (experimental).
74 Debugger Commands
76 The interactive debugger understands the following commands:
77 h Prints out a summary help message
79 h [command] Prints out a help message for the given debugger command.
81 h h The special argument of "h h" produces the entire help
83 page, which is quite long.
84 If the output of the "h h" command (or any command, for
86 that matter) scrolls past your screen, precede the command
87 with a leading pipe symbol so that it's run through your
88 pager, as in
89 DB> |h h
91 You may change the pager which is used via "o pager=..."
93 command.
94 p expr Same as "print {$DB::OUT} expr" in the current package. In
96 particular, because this is just Perl's own "print"
97 function, this means that nested data structures and
98 objects are not dumped, unlike with the "x" command.
99 The "DB::OUT" filehandle is opened to /dev/tty, regardless
101 of where STDOUT may be redirected to.
102 x [maxdepth] expr
104 Evaluates its expression in list context and dumps out the
105 result in a pretty-printed fashion. Nested data structures
106 are printed out recursively, unlike the real "print"
107 function in Perl. When dumping hashes, you'll probably
108 prefer 'x \%h' rather than 'x %h'. See Dumpvalue if you'd
109 like to do this yourself.
110 The output format is governed by multiple options described
112 under "Configurable Options".
113 If the "maxdepth" is included, it must be a numeral N; the
115 value is dumped only N levels deep, as if the "dumpDepth"
116 option had been temporarily set to N.
117 V [pkg [vars]]
119 Display all (or some) variables in package (defaulting to
120 "main") using a data pretty-printer (hashes show their keys
121 and values so you see what's what, control characters are
122 made printable, etc.). Make sure you don't put the type
123 specifier (like "$") there, just the symbol names, like
124 this:
125 V DB filename line
127 Use "~pattern" and "!pattern" for positive and negative
129 regexes.
130 This is similar to calling the "x" command on each
132 applicable var.
133 X [vars] Same as "V currentpackage [vars]".
135 y [level [vars]]
137 Display all (or some) lexical variables (mnemonic: "mY"
138 variables) in the current scope or level scopes higher.
139 You can limit the variables that you see with vars which
140 works exactly as it does for the "V" and "X" commands.
141 Requires the "PadWalker" module version 0.08 or higher;
142 will warn if this isn't installed. Output is pretty-
143 printed in the same style as for "V" and the format is
144 controlled by the same options.
145 T Produce a stack backtrace. See below for details on its
147 output.
148 s [expr] Single step. Executes until the beginning of another
150 statement, descending into subroutine calls. If an
151 expression is supplied that includes function calls, it too
152 will be single-stepped.
153 n [expr] Next. Executes over subroutine calls, until the beginning
155 of the next statement. If an expression is supplied that
156 includes function calls, those functions will be executed
157 with stops before each statement.
158 r Continue until the return from the current subroutine.
160 Dump the return value if the "PrintRet" option is set
161 (default).
162 <CR> Repeat last "n" or "s" command.
164 c [line|sub]
166 Continue, optionally inserting a one-time-only breakpoint
167 at the specified line or subroutine.
168 l List next window of lines.
170 l min+incr List "incr+1" lines starting at "min".
172 l min-max List lines "min" through "max". "l -" is synonymous to
174 "-".
175 l line List a single line.
177 l subname List first window of lines from subroutine. subname may be
179 a variable that contains a code reference.
180 - List previous window of lines.
182 v [line] View a few lines of code around the current line.
184 . Return the internal debugger pointer to the line last
186 executed, and print out that line.
187 f filename Switch to viewing a different file or "eval" statement. If
189 filename is not a full pathname found in the values of
190 %INC, it is considered a regex.
191 "eval"ed strings (when accessible) are considered to be
193 filenames: "f (eval 7)" and "f eval 7\b" access the body of
194 the 7th "eval"ed string (in the order of execution). The
195 bodies of the currently executed "eval" and of "eval"ed
196 strings that define subroutines are saved and thus
197 accessible.
198 /pattern/ Search forwards for pattern (a Perl regex); final / is
200 optional. The search is case-insensitive by default.
201 ?pattern? Search backwards for pattern; final ? is optional. The
203 search is case-insensitive by default.
204 L [abw] List (default all) actions, breakpoints and watch
206 expressions
207 S [[!]regex]
209 List subroutine names [not] matching the regex.
210 t [n] Toggle trace mode (see also the "AutoTrace" option).
212 Optional argument is the maximum number of levels to trace
213 below the current one; anything deeper than that will be
214 silent.
215 t [n] expr Trace through execution of "expr". Optional first argument
217 is the maximum number of levels to trace below the current
218 one; anything deeper than that will be silent. See "Frame
219 Listing Output Examples" in perldebguts for examples.
220 b Sets breakpoint on current line
222 b [line] [condition]
224 Set a breakpoint before the given line. If a condition is
225 specified, it's evaluated each time the statement is
226 reached: a breakpoint is taken only if the condition is
227 true. Breakpoints may only be set on lines that begin an
228 executable statement. Conditions don't use "if":
229 b 237 $x > 30
231 b 237 ++$count237 < 11
232 b 33 /pattern/i
233 If the line number is ".", sets a breakpoint on the current
235 line:
236 b . $n > 100
238 b [file]:[line] [condition]
240 Set a breakpoint before the given line in a (possibly
241 different) file. If a condition is specified, it's
242 evaluated each time the statement is reached: a breakpoint
243 is taken only if the condition is true. Breakpoints may
244 only be set on lines that begin an executable statement.
245 Conditions don't use "if":
246 b lib/MyModule.pm:237 $x > 30
248 b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
249 b subname [condition]
251 Set a breakpoint before the first line of the named
252 subroutine. subname may be a variable containing a code
253 reference (in this case condition is not supported).
254 b postpone subname [condition]
256 Set a breakpoint at first line of subroutine after it is
257 compiled.
258 b load filename
260 Set a breakpoint before the first executed line of the
261 filename, which should be a full pathname found amongst the
262 %INC values.
263 b compile subname
265 Sets a breakpoint before the first statement executed after
266 the specified subroutine is compiled.
267 B line Delete a breakpoint from the specified line.
269 B * Delete all installed breakpoints.
271 disable [file]:[line]
273 Disable the breakpoint so it won't stop the execution of
274 the program. Breakpoints are enabled by default and can be
275 re-enabled using the "enable" command.
276 disable [line]
278 Disable the breakpoint so it won't stop the execution of
279 the program. Breakpoints are enabled by default and can be
280 re-enabled using the "enable" command.
281 This is done for a breakpoint in the current file.
283 enable [file]:[line]
285 Enable the breakpoint so it will stop the execution of the
286 program.
287 enable [line]
289 Enable the breakpoint so it will stop the execution of the
290 program.
291 This is done for a breakpoint in the current file.
293 a [line] command
295 Set an action to be done before the line is executed. If
296 line is omitted, set an action on the line about to be
297 executed. The sequence of steps taken by the debugger is
298 1. check for a breakpoint at this line
300 2. print the line if necessary (tracing)
301 3. do any actions associated with that line
302 4. prompt user if at a breakpoint or in single-step
303 5. evaluate line
304 For example, this will print out $foo every time line 53 is
306 passed:
307 a 53 print "DB FOUND $foo\n"
309 A line Delete an action from the specified line.
311 A * Delete all installed actions.
313 w expr Add a global watch-expression. Whenever a watched global
315 changes the debugger will stop and display the old and new
316 values.
317 W expr Delete watch-expression
319 W * Delete all watch-expressions.
321 o Display all options.
323 o booloption ...
325 Set each listed Boolean option to the value 1.
326 o anyoption? ...
328 Print out the value of one or more options.
329 o option=value ...
331 Set the value of one or more options. If the value has
332 internal whitespace, it should be quoted. For example, you
333 could set "o pager="less -MQeicsNfr"" to call less with
334 those specific options. You may use either single or
335 double quotes, but if you do, you must escape any embedded
336 instances of same sort of quote you began with, as well as
337 any escaping any escapes that immediately precede that
338 quote but which are not meant to escape the quote itself.
339 In other words, you follow single-quoting rules
340 irrespective of the quote; eg: "o option='this isn\'t bad'"
341 or "o option="She said, \"Isn't it?\""".
342 For historical reasons, the "=value" is optional, but
344 defaults to 1 only where it is safe to do so--that is,
345 mostly for Boolean options. It is always better to assign
346 a specific value using "=". The "option" can be
347 abbreviated, but for clarity probably should not be.
348 Several options can be set together. See "Configurable
349 Options" for a list of these.
350 < ? List out all pre-prompt Perl command actions.
352 < [ command ]
354 Set an action (Perl command) to happen before every
355 debugger prompt. A multi-line command may be entered by
356 backslashing the newlines.
357 < * Delete all pre-prompt Perl command actions.
359 << command Add an action (Perl command) to happen before every
361 debugger prompt. A multi-line command may be entered by
362 backwhacking the newlines.
363 > ? List out post-prompt Perl command actions.
365 > command Set an action (Perl command) to happen after the prompt
367 when you've just given a command to return to executing the
368 script. A multi-line command may be entered by
369 backslashing the newlines (we bet you couldn't have guessed
370 this by now).
371 > * Delete all post-prompt Perl command actions.
373 >> command Adds an action (Perl command) to happen after the prompt
375 when you've just given a command to return to executing the
376 script. A multi-line command may be entered by
377 backslashing the newlines.
378 { ? List out pre-prompt debugger commands.
380 { [ command ]
382 Set an action (debugger command) to happen before every
383 debugger prompt. A multi-line command may be entered in
384 the customary fashion.
385 Because this command is in some senses new, a warning is
387 issued if you appear to have accidentally entered a block
388 instead. If that's what you mean to do, write it as with
389 ";{ ... }" or even "do { ... }".
390 { * Delete all pre-prompt debugger commands.
392 {{ command Add an action (debugger command) to happen before every
394 debugger prompt. A multi-line command may be entered, if
395 you can guess how: see above.
396 ! number Redo a previous command (defaults to the previous command).
398 ! -number Redo number'th previous command.
400 ! pattern Redo last command that started with pattern. See "o
402 recallCommand", too.
403 !! cmd Run cmd in a subprocess (reads from DB::IN, writes to
405 DB::OUT) See "o shellBang", also. Note that the user's
406 current shell (well, their $ENV{SHELL} variable) will be
407 used, which can interfere with proper interpretation of
408 exit status or signal and coredump information.
409 source file Read and execute debugger commands from file. file may
411 itself contain "source" commands.
412 H -number Display last n commands. Only commands longer than one
414 character are listed. If number is omitted, list them all.
415 q or ^D Quit. ("quit" doesn't work for this, unless you've made an
417 alias) This is the only supported way to exit the debugger,
418 though typing "exit" twice might work.
419 Set the "inhibit_exit" option to 0 if you want to be able
421 to step off the end the script. You may also need to set
422 $finished to 0 if you want to step through global
423 destruction.
424 R Restart the debugger by "exec()"ing a new session. We try
426 to maintain your history across this, but internal settings
427 and command-line options may be lost.
428 The following setting are currently preserved: history,
430 breakpoints, actions, debugger options, and the Perl
431 command-line options -w, -I, and -e.
432 |dbcmd Run the debugger command, piping DB::OUT into your current
434 pager.
435 ||dbcmd Same as "|dbcmd" but DB::OUT is temporarily "select"ed as
437 well.
438 = [alias value]
440 Define a command alias, like
441 = quit q
443 or list current aliases.
445 command Execute command as a Perl statement. A trailing semicolon
447 will be supplied. If the Perl statement would otherwise be
448 confused for a Perl debugger, use a leading semicolon, too.
449 m expr List which methods may be called on the result of the
451 evaluated expression. The expression may evaluated to a
452 reference to a blessed object, or to a package name.
453 M Display all loaded modules and their versions.
455 man [manpage]
457 Despite its name, this calls your system's default
458 documentation viewer on the given page, or on the viewer
459 itself if manpage is omitted. If that viewer is man, the
460 current "Config" information is used to invoke man using
461 the proper MANPATH or -M manpath option. Failed lookups of
462 the form "XXX" that match known manpages of the form
463 perlXXX will be retried. This lets you type "man debug" or
464 "man op" from the debugger.
465 On systems traditionally bereft of a usable man command,
467 the debugger invokes perldoc. Occasionally this
468 determination is incorrect due to recalcitrant vendors or
469 rather more felicitously, to enterprising users. If you
470 fall into either category, just manually set the
471 $DB::doccmd variable to whatever viewer to view the Perl
472 documentation on your system. This may be set in an rc
473 file, or through direct assignment. We're still waiting
474 for a working example of something along the lines of:
475 $DB::doccmd = 'netscape -remote http://something.here/';
477 Configurable Options
479 The debugger has numerous options settable using the "o" command,
480 either interactively or from the environment or an rc file. (./.perldb
481 or ~/.perldb under Unix.)
482 "recallCommand", "ShellBang"
484 The characters used to recall a command or spawn a shell.
485 By default, both are set to "!", which is unfortunate.
486 "pager" Program to use for output of pager-piped commands (those
488 beginning with a "|" character.) By default, $ENV{PAGER}
489 will be used. Because the debugger uses your current
490 terminal characteristics for bold and underlining, if the
491 chosen pager does not pass escape sequences through
492 unchanged, the output of some debugger commands will not be
493 readable when sent through the pager.
494 "tkRunning" Run Tk while prompting (with ReadLine).
496 "signalLevel", "warnLevel", "dieLevel"
498 Level of verbosity. By default, the debugger leaves your
499 exceptions and warnings alone, because altering them can
500 break correctly running programs. It will attempt to print
501 a message when uncaught INT, BUS, or SEGV signals arrive.
502 (But see the mention of signals in "BUGS" below.)
503 To disable this default safe mode, set these values to
505 something higher than 0. At a level of 1, you get
506 backtraces upon receiving any kind of warning (this is
507 often annoying) or exception (this is often valuable).
508 Unfortunately, the debugger cannot discern fatal exceptions
509 from non-fatal ones. If "dieLevel" is even 1, then your
510 non-fatal exceptions are also traced and unceremoniously
511 altered if they came from "eval'ed" strings or from any
512 kind of "eval" within modules you're attempting to load.
513 If "dieLevel" is 2, the debugger doesn't care where they
514 came from: It usurps your exception handler and prints out
515 a trace, then modifies all exceptions with its own
516 embellishments. This may perhaps be useful for some
517 tracing purposes, but tends to hopelessly destroy any
518 program that takes its exception handling seriously.
519 "AutoTrace" Trace mode (similar to "t" command, but can be put into
521 "PERLDB_OPTS").
522 "LineInfo" File or pipe to print line number info to. If it is a pipe
524 (say, "|visual_perl_db"), then a short message is used.
525 This is the mechanism used to interact with a slave editor
526 or visual debugger, such as the special "vi" or "emacs"
527 hooks, or the "ddd" graphical debugger.
528 "inhibit_exit"
530 If 0, allows stepping off the end of the script.
531 "PrintRet" Print return value after "r" command if set (default).
533 "ornaments" Affects screen appearance of the command line (see
535 Term::ReadLine). There is currently no way to disable
536 these, which can render some output illegible on some
537 displays, or with some pagers. This is considered a bug.
538 "frame" Affects the printing of messages upon entry and exit from
540 subroutines. If "frame & 2" is false, messages are printed
541 on entry only. (Printing on exit might be useful if
542 interspersed with other messages.)
543 If "frame & 4", arguments to functions are printed, plus
545 context and caller info. If "frame & 8", overloaded
546 "stringify" and "tie"d "FETCH" is enabled on the printed
547 arguments. If "frame & 16", the return value from the
548 subroutine is printed.
549 The length at which the argument list is truncated is
551 governed by the next option:
552 "maxTraceLen"
554 Length to truncate the argument list when the "frame"
555 option's bit 4 is set.
556 "windowSize"
558 Change the size of code list window (default is 10 lines).
559 The following options affect what happens with "V", "X", and "x"
561 commands:
562 "arrayDepth", "hashDepth"
564 Print only first N elements ('' for all).
565 "dumpDepth" Limit recursion depth to N levels when dumping structures.
567 Negative values are interpreted as infinity. Default:
568 infinity.
569 "compactDump", "veryCompact"
571 Change the style of array and hash output. If
572 "compactDump", short array may be printed on one line.
573 "globPrint" Whether to print contents of globs.
575 "DumpDBFiles"
577 Dump arrays holding debugged files.
578 "DumpPackages"
580 Dump symbol tables of packages.
581 "DumpReused"
583 Dump contents of "reused" addresses.
584 "quote", "HighBit", "undefPrint"
586 Change the style of string dump. The default value for
587 "quote" is "auto"; one can enable double-quotish or single-
588 quotish format by setting it to """ or "'", respectively.
589 By default, characters with their high bit set are printed
590 verbatim.
591 "UsageOnly" Rudimentary per-package memory usage dump. Calculates
593 total size of strings found in variables in the package.
594 This does not include lexicals in a module's file scope, or
595 lost in closures.
596 "HistFile" The path of the file from which the history (assuming a
598 usable Term::ReadLine backend) will be read on the
599 debugger's startup, and to which it will be saved on
600 shutdown (for persistence across sessions). Similar in
601 concept to Bash's ".bash_history" file.
602 "HistSize" The count of the saved lines in the history (assuming
604 "HistFile" above).
605 After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
607 environment variable and parses this as the remainder of a "O ..."
608 line as one might enter at the debugger prompt. You may place the
609 initialization options "TTY", "noTTY", "ReadLine", and "NonStop" there.
610 If your rc file contains:
612 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
614 then your script will run without human intervention, putting trace
616 information into the file db.out. (If you interrupt it, you'd better
617 reset "LineInfo" to /dev/tty if you expect to see anything.)
618 "TTY" The TTY to use for debugging I/O.
620 "noTTY" If set, the debugger goes into "NonStop" mode and will not
622 connect to a TTY. If interrupted (or if control goes to
623 the debugger via explicit setting of $DB::signal or
624 $DB::single from the Perl script), it connects to a TTY
625 specified in the "TTY" option at startup, or to a tty found
626 at runtime using the "Term::Rendezvous" module of your
627 choice.
628 This module should implement a method named "new" that
630 returns an object with two methods: "IN" and "OUT". These
631 should return filehandles to use for debugging input and
632 output correspondingly. The "new" method should inspect an
633 argument containing the value of $ENV{PERLDB_NOTTY} at
634 startup, or "$ENV{HOME}/.perldbtty$$" otherwise. This file
635 is not inspected for proper ownership, so security hazards
636 are theoretically possible.
637 "ReadLine" If false, readline support in the debugger is disabled in
639 order to debug applications that themselves use ReadLine.
640 "NonStop" If set, the debugger goes into non-interactive mode until
642 interrupted, or programmatically by setting $DB::signal or
643 $DB::single.
644 Here's an example of using the $ENV{PERLDB_OPTS} variable:
646 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
648 That will run the script myprogram without human intervention, printing
650 out the call tree with entry and exit points. Note that "NonStop=1
651 frame=2" is equivalent to "N f=2", and that originally, options could
652 be uniquely abbreviated by the first letter (modulo the "Dump*"
653 options). It is nevertheless recommended that you always spell them
654 out in full for legibility and future compatibility.
655 Other examples include
657 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
659 which runs script non-interactively, printing info on each entry into a
661 subroutine and each executed line into the file named listing. (If you
662 interrupt it, you would better reset "LineInfo" to something
663 "interactive"!)
664 Other examples include (using standard shell syntax to show environment
666 variable settings):
667 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
669 perl -d myprogram )
670 which may be useful for debugging a program that uses "Term::ReadLine"
672 itself. Do not forget to detach your shell from the TTY in the window
673 that corresponds to /dev/ttyXX, say, by issuing a command like
674 $ sleep 1000000
676 See "Debugger Internals" in perldebguts for details.
678 Debugger Input/Output
680 Prompt The debugger prompt is something like
681 DB<8>
683 or even
685 DB<<17>>
687 where that number is the command number, and which you'd use to
689 access with the built-in csh-like history mechanism. For
690 example, "!17" would repeat command number 17. The depth of
691 the angle brackets indicates the nesting depth of the debugger.
692 You could get more than one set of brackets, for example, if
693 you'd already at a breakpoint and then printed the result of a
694 function call that itself has a breakpoint, or you step into an
695 expression via "s/n/t expression" command.
696 Multiline commands
698 If you want to enter a multi-line command, such as a subroutine
699 definition with several statements or a format, escape the
700 newline that would normally end the debugger command with a
701 backslash. Here's an example:
702 DB<1> for (1..4) { \
704 cont: print "ok\n"; \
705 cont: }
706 ok
707 ok
708 ok
709 ok
710 Note that this business of escaping a newline is specific to
712 interactive commands typed into the debugger.
713 Stack backtrace
715 Here's an example of what a stack backtrace via "T" command
716 might look like:
717 $ = main::infested called from file 'Ambulation.pm' line 10
719 @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea'
720 line 7
721 $ = main::pests('bactrian', 4) called from file 'camel_flea'
722 line 4
723 The left-hand character up there indicates the context in which
725 the function was called, with "$" and "@" meaning scalar or
726 list contexts respectively, and "." meaning void context (which
727 is actually a sort of scalar context). The display above says
728 that you were in the function "main::infested" when you ran the
729 stack dump, and that it was called in scalar context from line
730 10 of the file Ambulation.pm, but without any arguments at all,
731 meaning it was called as &infested. The next stack frame shows
732 that the function "Ambulation::legs" was called in list context
733 from the camel_flea file with four arguments. The last stack
734 frame shows that "main::pests" was called in scalar context,
735 also from camel_flea, but from line 4.
736 If you execute the "T" command from inside an active "use"
738 statement, the backtrace will contain both a "require" frame
739 and an "eval" frame.
740 Line Listing Format
742 This shows the sorts of output the "l" command can produce:
743 DB<<13>> l
745 101: @i{@i} = ();
746 102:b @isa{@i,$pack} = ()
747 103 if(exists $i{$prevpack} || exists $isa{$pack});
748 104 }
749 105
750 106 next
751 107==> if(exists $isa{$pack});
752 108
753 109:a if ($extra-- > 0) {
754 110: %isa = ($pack,1);
755 Breakable lines are marked with ":". Lines with breakpoints
757 are marked by "b" and those with actions by "a". The line
758 that's about to be executed is marked by "==>".
759 Please be aware that code in debugger listings may not look the
761 same as your original source code. Line directives and
762 external source filters can alter the code before Perl sees it,
763 causing code to move from its original positions or take on
764 entirely different forms.
765 Frame listing
767 When the "frame" option is set, the debugger would print
768 entered (and optionally exited) subroutines in different
769 styles. See perldebguts for incredibly long examples of these.
770 Debugging Compile-Time Statements
772 If you have compile-time executable statements (such as code within
773 BEGIN, UNITCHECK and CHECK blocks or "use" statements), these will not
774 be stopped by debugger, although "require"s and INIT blocks will, and
775 compile-time statements can be traced with the "AutoTrace" option set
776 in "PERLDB_OPTS"). From your own Perl code, however, you can transfer
777 control back to the debugger using the following statement, which is
778 harmless if the debugger is not running:
779 $DB::single = 1;
781 If you set $DB::single to 2, it's equivalent to having just typed the
783 "n" command, whereas a value of 1 means the "s" command. The
784 $DB::trace variable should be set to 1 to simulate having typed the
785 "t" command.
786 Another way to debug compile-time code is to start the debugger, set a
788 breakpoint on the load of some module:
789 DB<7> b load f:/perllib/lib/Carp.pm
791 Will stop on load of 'f:/perllib/lib/Carp.pm'.
792 and then restart the debugger using the "R" command (if possible). One
794 can use "b compile subname" for the same purpose.
795 Debugger Customization
797 The debugger probably contains enough configuration hooks that you
798 won't ever have to modify it yourself. You may change the behaviour of
799 the debugger from within the debugger using its "o" command, from the
800 command line via the "PERLDB_OPTS" environment variable, and from
801 customization files.
802 You can do some customization by setting up a .perldb file, which
804 contains initialization code. For instance, you could make aliases
805 like these (the last one is one people expect to be there):
806 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
808 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
809 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
810 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
811 You can change options from .perldb by using calls like this one;
813 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
815 The code is executed in the package "DB". Note that .perldb is
817 processed before processing "PERLDB_OPTS". If .perldb defines the
818 subroutine "afterinit", that function is called after debugger
819 initialization ends. .perldb may be contained in the current
820 directory, or in the home directory. Because this file is sourced in
821 by Perl and may contain arbitrary commands, for security reasons, it
822 must be owned by the superuser or the current user, and writable by no
823 one but its owner.
824 You can mock TTY input to debugger by adding arbitrary commands to
826 @DB::typeahead. For example, your .perldb file might contain:
827 sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
829 Which would attempt to set breakpoints on lines 4 and 6 immediately
831 after debugger initialization. Note that @DB::typeahead is not a
832 supported interface and is subject to change in future releases.
833 If you want to modify the debugger, copy perl5db.pl from the Perl
835 library to another name and hack it to your heart's content. You'll
836 then want to set your "PERL5DB" environment variable to say something
837 like this:
838 BEGIN { require "myperl5db.pl" }
840 As a last resort, you could also use "PERL5DB" to customize the
842 debugger by directly setting internal variables or calling debugger
843 functions.
844 Note that any variables and functions that are not documented in this
846 document (or in perldebguts) are considered for internal use only, and
847 as such are subject to change without notice.
848 Readline Support / History in the Debugger
850 As shipped, the only command-line history supplied is a simplistic one
851 that checks for leading exclamation points. However, if you install
852 the Term::ReadKey and Term::ReadLine modules from CPAN (such as
853 Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full
854 editing capabilities much like those GNU readline(3) provides. Look
855 for these in the modules/by-module/Term directory on CPAN. These do
856 not support normal vi command-line editing, however.
857 A rudimentary command-line completion is also available, including
859 lexical variables in the current scope if the "PadWalker" module is
860 installed.
861 Without Readline support you may see the symbols "^[[A", "^[[C",
863 "^[[B", "^[[D"", "^H", ... when using the arrow keys and/or the
864 backspace key.
865 Editor Support for Debugging
867 If you have the GNU's version of emacs installed on your system, it can
868 interact with the Perl debugger to provide an integrated software
869 development environment reminiscent of its interactions with C
870 debuggers.
871 Recent versions of Emacs come with a start file for making emacs act
873 like a syntax-directed editor that understands (some of) Perl's syntax.
874 See perlfaq3.
875 Users of vi should also look into vim and gvim, the mousey and windy
877 version, for coloring of Perl keywords.
878 Note that only perl can truly parse Perl, so all such CASE tools fall
880 somewhat short of the mark, especially if you don't program your Perl
881 as a C programmer might.
882 The Perl Profiler
884 If you wish to supply an alternative debugger for Perl to run, invoke
885 your script with a colon and a package argument given to the -d flag.
886 Perl's alternative debuggers include a Perl profiler, Devel::NYTProf,
887 which is available separately as a CPAN distribution. To profile your
888 Perl program in the file mycode.pl, just type:
889 $ perl -d:NYTProf mycode.pl
891 When the script terminates the profiler will create a database of the
893 profile information that you can turn into reports using the profiler's
894 tools. See <perlperf> for details.
895
897 "use re 'debug'" enables you to see the gory details of how the Perl
898 regular expression engine works. In order to understand this typically
899 voluminous output, one must not only have some idea about how regular
900 expression matching works in general, but also know how Perl's regular
901 expressions are internally compiled into an automaton. These matters
902 are explored in some detail in "Debugging Regular Expressions" in
903 perldebguts.
904
906 Perl contains internal support for reporting its own memory usage, but
907 this is a fairly advanced concept that requires some understanding of
908 how memory allocation works. See "Debugging Perl Memory Usage" in
909 perldebguts for the details.
910
912 You do have "use strict" and "use warnings" enabled, don't you?
913 perldebtut, perldebguts, re, DB, Devel::NYTProf, Dumpvalue, and
915 perlrun.
916 When debugging a script that uses #! and is thus normally found in
918 $PATH, the -S option causes perl to search $PATH for it, so you don't
919 have to type the path or "which $scriptname".
920 $ perl -Sd foo.pl
922
924 You cannot get stack frame information or in any fashion debug
925 functions that were not compiled by Perl, such as those from C or C++
926 extensions.
927 If you alter your @_ arguments in a subroutine (such as with "shift" or
929 "pop"), the stack backtrace will not show the original values.
930 The debugger does not currently work in conjunction with the -W
932 command-line switch, because it itself is not free of warnings.
933 If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing
935 from your keyboard or a socket) and haven't set up your own $SIG{INT}
936 handler, then you won't be able to CTRL-C your way back to the
937 debugger, because the debugger's own $SIG{INT} handler doesn't
938 understand that it needs to raise an exception to longjmp(3) out of
939 slow syscalls.
940perl v5.30.2 2020-03-27 PERLDEBUG(1)