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
15 If you invoke Perl with the -d switch, your script runs under the Perl
16 source debugger. This works like an interactive Perl environment,
17 prompting for debugger commands that let you examine source code, set
18 breakpoints, get stack backtraces, change the values of variables, etc.
19 This is so convenient that you often fire up the debugger all by itself
20 just to test out Perl constructs interactively to see what they do.
21 For example:
22 $ perl -d -e 42
24 In Perl, the debugger is not a separate program the way it usually is
26 in the typical compiled environment. Instead, the -d flag tells the
27 compiler to insert source information into the parse trees it's about
28 to hand off to the interpreter. That means your code must first
29 compile correctly for the debugger to work on it. Then when the
30 interpreter starts up, it preloads a special Perl library file
31 containing the debugger.
32 The program will halt right before the first run-time executable
34 statement (but see below regarding compile-time statements) and ask you
35 to enter a debugger command. Contrary to popular expectations,
36 whenever the debugger halts and shows you a line of code, it always
37 displays the line it's about to execute, rather than the one it has
38 just executed.
39 Any command not recognized by the debugger is directly executed
41 ("eval"'d) as Perl code in the current package. (The debugger uses the
42 DB package for keeping its own state information.)
43 Note that the said "eval" is bound by an implicit scope. As a result
45 any newly introduced lexical variable or any modified capture buffer
46 content is lost after the eval. The debugger is a nice environment to
47 learn Perl, but if you interactively experiment using material which
48 should be in the same scope, stuff it in one line.
49 For any text entered at the debugger prompt, leading and trailing
51 whitespace is first stripped before further processing. If a debugger
52 command coincides with some function in your own program, merely
53 precede the function with something that doesn't look like a debugger
54 command, such as a leading ";" or perhaps a "+", or by wrapping it with
55 parentheses or braces.
56 Calling the Debugger
58 There are several ways to call the debugger:
59 perl -d program_name
61 On the given program identified by "program_name".
62 perl -d -e 0
64 Interactively supply an arbitrary "expression" using "-e".
65 perl -d:ptkdb program_name
67 Debug a given program via the "Devel::ptkdb" GUI.
68 perl -dt threaded_program_name
70 Debug a given program using threads (experimental).
71 Debugger Commands
73 The interactive debugger understands the following commands:
74 h Prints out a summary help message
76 h [command] Prints out a help message for the given debugger command.
78 h h The special argument of "h h" produces the entire help
80 page, which is quite long.
81 If the output of the "h h" command (or any command, for
83 that matter) scrolls past your screen, precede the command
84 with a leading pipe symbol so that it's run through your
85 pager, as in
86 DB> |h h
88 You may change the pager which is used via "o pager=..."
90 command.
91 p expr Same as "print {$DB::OUT} expr" in the current package. In
93 particular, because this is just Perl's own "print"
94 function, this means that nested data structures and
95 objects are not dumped, unlike with the "x" command.
96 The "DB::OUT" filehandle is opened to /dev/tty, regardless
98 of where STDOUT may be redirected to.
99 x [maxdepth] expr
101 Evaluates its expression in list context and dumps out the
102 result in a pretty-printed fashion. Nested data structures
103 are printed out recursively, unlike the real "print"
104 function in Perl. When dumping hashes, you'll probably
105 prefer 'x \%h' rather than 'x %h'. See Dumpvalue if you'd
106 like to do this yourself.
107 The output format is governed by multiple options described
109 under "Configurable Options".
110 If the "maxdepth" is included, it must be a numeral N; the
112 value is dumped only N levels deep, as if the "dumpDepth"
113 option had been temporarily set to N.
114 V [pkg [vars]]
116 Display all (or some) variables in package (defaulting to
117 "main") using a data pretty-printer (hashes show their keys
118 and values so you see what's what, control characters are
119 made printable, etc.). Make sure you don't put the type
120 specifier (like "$") there, just the symbol names, like
121 this:
122 V DB filename line
124 Use "~pattern" and "!pattern" for positive and negative
126 regexes.
127 This is similar to calling the "x" command on each
129 applicable var.
130 X [vars] Same as "V currentpackage [vars]".
132 y [level [vars]]
134 Display all (or some) lexical variables (mnemonic: "mY"
135 variables) in the current scope or level scopes higher.
136 You can limit the variables that you see with vars which
137 works exactly as it does for the "V" and "X" commands.
138 Requires the "PadWalker" module version 0.08 or higher;
139 will warn if this isn't installed. Output is pretty-
140 printed in the same style as for "V" and the format is
141 controlled by the same options.
142 T Produce a stack backtrace. See below for details on its
144 output.
145 s [expr] Single step. Executes until the beginning of another
147 statement, descending into subroutine calls. If an
148 expression is supplied that includes function calls, it too
149 will be single-stepped.
150 n [expr] Next. Executes over subroutine calls, until the beginning
152 of the next statement. If an expression is supplied that
153 includes function calls, those functions will be executed
154 with stops before each statement.
155 r Continue until the return from the current subroutine.
157 Dump the return value if the "PrintRet" option is set
158 (default).
159 <CR> Repeat last "n" or "s" command.
161 c [line|sub]
163 Continue, optionally inserting a one-time-only breakpoint
164 at the specified line or subroutine.
165 l List next window of lines.
167 l min+incr List "incr+1" lines starting at "min".
169 l min-max List lines "min" through "max". "l -" is synonymous to
171 "-".
172 l line List a single line.
174 l subname List first window of lines from subroutine. subname may be
176 a variable that contains a code reference.
177 - List previous window of lines.
179 v [line] View a few lines of code around the current line.
181 . Return the internal debugger pointer to the line last
183 executed, and print out that line.
184 f filename Switch to viewing a different file or "eval" statement. If
186 filename is not a full pathname found in the values of
187 %INC, it is considered a regex.
188 "eval"ed strings (when accessible) are considered to be
190 filenames: "f (eval 7)" and "f eval 7\b" access the body of
191 the 7th "eval"ed string (in the order of execution). The
192 bodies of the currently executed "eval" and of "eval"ed
193 strings that define subroutines are saved and thus
194 accessible.
195 /pattern/ Search forwards for pattern (a Perl regex); final / is
197 optional. The search is case-insensitive by default.
198 ?pattern? Search backwards for pattern; final ? is optional. The
200 search is case-insensitive by default.
201 L [abw] List (default all) actions, breakpoints and watch
203 expressions
204 S [[!]regex]
206 List subroutine names [not] matching the regex.
207 t [n] Toggle trace mode (see also the "AutoTrace" option).
209 Optional argument is the maximum number of levels to trace
210 below the current one; anything deeper than that will be
211 silent.
212 t [n] expr Trace through execution of "expr". Optional first argument
214 is the maximum number of levels to trace below the current
215 one; anything deeper than that will be silent. See "Frame
216 Listing Output Examples" in perldebguts for examples.
217 b Sets breakpoint on current line
219 b [line] [condition]
221 Set a breakpoint before the given line. If a condition is
222 specified, it's evaluated each time the statement is
223 reached: a breakpoint is taken only if the condition is
224 true. Breakpoints may only be set on lines that begin an
225 executable statement. Conditions don't use "if":
226 b 237 $x > 30
228 b 237 ++$count237 < 11
229 b 33 /pattern/i
230 If the line number is ".", sets a breakpoint on the current
232 line:
233 b . $n > 100
235 b [file]:[line] [condition]
237 Set a breakpoint before the given line in a (possibly
238 different) file. If a condition is specified, it's
239 evaluated each time the statement is reached: a breakpoint
240 is taken only if the condition is true. Breakpoints may
241 only be set on lines that begin an executable statement.
242 Conditions don't use "if":
243 b lib/MyModule.pm:237 $x > 30
245 b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
246 b subname [condition]
248 Set a breakpoint before the first line of the named
249 subroutine. subname may be a variable containing a code
250 reference (in this case condition is not supported).
251 b postpone subname [condition]
253 Set a breakpoint at first line of subroutine after it is
254 compiled.
255 b load filename
257 Set a breakpoint before the first executed line of the
258 filename, which should be a full pathname found amongst the
259 %INC values.
260 b compile subname
262 Sets a breakpoint before the first statement executed after
263 the specified subroutine is compiled.
264 B line Delete a breakpoint from the specified line.
266 B * Delete all installed breakpoints.
268 disable [file]:[line]
270 Disable the breakpoint so it won't stop the execution of
271 the program. Breakpoints are enabled by default and can be
272 re-enabled using the "enable" command.
273 disable [line]
275 Disable the breakpoint so it won't stop the execution of
276 the program. Breakpoints are enabled by default and can be
277 re-enabled using the "enable" command.
278 This is done for a breakpoint in the current file.
280 enable [file]:[line]
282 Enable the breakpoint so it will stop the execution of the
283 program.
284 enable [line]
286 Enable the breakpoint so it will stop the execution of the
287 program.
288 This is done for a breakpoint in the current file.
290 a [line] command
292 Set an action to be done before the line is executed. If
293 line is omitted, set an action on the line about to be
294 executed. The sequence of steps taken by the debugger is
295 1. check for a breakpoint at this line
297 2. print the line if necessary (tracing)
298 3. do any actions associated with that line
299 4. prompt user if at a breakpoint or in single-step
300 5. evaluate line
301 For example, this will print out $foo every time line 53 is
303 passed:
304 a 53 print "DB FOUND $foo\n"
306 A line Delete an action from the specified line.
308 A * Delete all installed actions.
310 w expr Add a global watch-expression. Whenever a watched global
312 changes the debugger will stop and display the old and new
313 values.
314 W expr Delete watch-expression
316 W * Delete all watch-expressions.
318 o Display all options.
320 o booloption ...
322 Set each listed Boolean option to the value 1.
323 o anyoption? ...
325 Print out the value of one or more options.
326 o option=value ...
328 Set the value of one or more options. If the value has
329 internal whitespace, it should be quoted. For example, you
330 could set "o pager="less -MQeicsNfr"" to call less with
331 those specific options. You may use either single or
332 double quotes, but if you do, you must escape any embedded
333 instances of same sort of quote you began with, as well as
334 any escaping any escapes that immediately precede that
335 quote but which are not meant to escape the quote itself.
336 In other words, you follow single-quoting rules
337 irrespective of the quote; eg: "o option='this isn\'t bad'"
338 or "o option="She said, \"Isn't it?\""".
339 For historical reasons, the "=value" is optional, but
341 defaults to 1 only where it is safe to do so--that is,
342 mostly for Boolean options. It is always better to assign
343 a specific value using "=". The "option" can be
344 abbreviated, but for clarity probably should not be.
345 Several options can be set together. See "Configurable
346 Options" for a list of these.
347 < ? List out all pre-prompt Perl command actions.
349 < [ command ]
351 Set an action (Perl command) to happen before every
352 debugger prompt. A multi-line command may be entered by
353 backslashing the newlines.
354 < * Delete all pre-prompt Perl command actions.
356 << command Add an action (Perl command) to happen before every
358 debugger prompt. A multi-line command may be entered by
359 backwhacking the newlines.
360 > ? List out post-prompt Perl command actions.
362 > command Set an action (Perl command) to happen after the prompt
364 when you've just given a command to return to executing the
365 script. A multi-line command may be entered by
366 backslashing the newlines (we bet you couldn't have guessed
367 this by now).
368 > * Delete all post-prompt Perl command actions.
370 >> command Adds an action (Perl command) to happen after the prompt
372 when you've just given a command to return to executing the
373 script. A multi-line command may be entered by
374 backslashing the newlines.
375 { ? List out pre-prompt debugger commands.
377 { [ command ]
379 Set an action (debugger command) to happen before every
380 debugger prompt. A multi-line command may be entered in
381 the customary fashion.
382 Because this command is in some senses new, a warning is
384 issued if you appear to have accidentally entered a block
385 instead. If that's what you mean to do, write it as with
386 ";{ ... }" or even "do { ... }".
387 { * Delete all pre-prompt debugger commands.
389 {{ command Add an action (debugger command) to happen before every
391 debugger prompt. A multi-line command may be entered, if
392 you can guess how: see above.
393 ! number Redo a previous command (defaults to the previous command).
395 ! -number Redo number'th previous command.
397 ! pattern Redo last command that started with pattern. See "o
399 recallCommand", too.
400 !! cmd Run cmd in a subprocess (reads from DB::IN, writes to
402 DB::OUT) See "o shellBang", also. Note that the user's
403 current shell (well, their $ENV{SHELL} variable) will be
404 used, which can interfere with proper interpretation of
405 exit status or signal and coredump information.
406 source file Read and execute debugger commands from file. file may
408 itself contain "source" commands.
409 H -number Display last n commands. Only commands longer than one
411 character are listed. If number is omitted, list them all.
412 q or ^D Quit. ("quit" doesn't work for this, unless you've made an
414 alias) This is the only supported way to exit the debugger,
415 though typing "exit" twice might work.
416 Set the "inhibit_exit" option to 0 if you want to be able
418 to step off the end the script. You may also need to set
419 $finished to 0 if you want to step through global
420 destruction.
421 R Restart the debugger by "exec()"ing a new session. We try
423 to maintain your history across this, but internal settings
424 and command-line options may be lost.
425 The following setting are currently preserved: history,
427 breakpoints, actions, debugger options, and the Perl
428 command-line options -w, -I, and -e.
429 |dbcmd Run the debugger command, piping DB::OUT into your current
431 pager.
432 ||dbcmd Same as "|dbcmd" but DB::OUT is temporarily "select"ed as
434 well.
435 = [alias value]
437 Define a command alias, like
438 = quit q
440 or list current aliases.
442 command Execute command as a Perl statement. A trailing semicolon
444 will be supplied. If the Perl statement would otherwise be
445 confused for a Perl debugger, use a leading semicolon, too.
446 m expr List which methods may be called on the result of the
448 evaluated expression. The expression may evaluated to a
449 reference to a blessed object, or to a package name.
450 M Display all loaded modules and their versions.
452 man [manpage]
454 Despite its name, this calls your system's default
455 documentation viewer on the given page, or on the viewer
456 itself if manpage is omitted. If that viewer is man, the
457 current "Config" information is used to invoke man using
458 the proper MANPATH or -M manpath option. Failed lookups of
459 the form "XXX" that match known manpages of the form
460 perlXXX will be retried. This lets you type "man debug" or
461 "man op" from the debugger.
462 On systems traditionally bereft of a usable man command,
464 the debugger invokes perldoc. Occasionally this
465 determination is incorrect due to recalcitrant vendors or
466 rather more felicitously, to enterprising users. If you
467 fall into either category, just manually set the
468 $DB::doccmd variable to whatever viewer to view the Perl
469 documentation on your system. This may be set in an rc
470 file, or through direct assignment. We're still waiting
471 for a working example of something along the lines of:
472 $DB::doccmd = 'netscape -remote http://something.here/';
474 Configurable Options
476 The debugger has numerous options settable using the "o" command,
477 either interactively or from the environment or an rc file. (./.perldb
478 or ~/.perldb under Unix.)
479 "recallCommand", "ShellBang"
481 The characters used to recall a command or spawn a shell.
482 By default, both are set to "!", which is unfortunate.
483 "pager" Program to use for output of pager-piped commands (those
485 beginning with a "|" character.) By default, $ENV{PAGER}
486 will be used. Because the debugger uses your current
487 terminal characteristics for bold and underlining, if the
488 chosen pager does not pass escape sequences through
489 unchanged, the output of some debugger commands will not be
490 readable when sent through the pager.
491 "tkRunning" Run Tk while prompting (with ReadLine).
493 "signalLevel", "warnLevel", "dieLevel"
495 Level of verbosity. By default, the debugger leaves your
496 exceptions and warnings alone, because altering them can
497 break correctly running programs. It will attempt to print
498 a message when uncaught INT, BUS, or SEGV signals arrive.
499 (But see the mention of signals in "BUGS" below.)
500 To disable this default safe mode, set these values to
502 something higher than 0. At a level of 1, you get
503 backtraces upon receiving any kind of warning (this is
504 often annoying) or exception (this is often valuable).
505 Unfortunately, the debugger cannot discern fatal exceptions
506 from non-fatal ones. If "dieLevel" is even 1, then your
507 non-fatal exceptions are also traced and unceremoniously
508 altered if they came from "eval'ed" strings or from any
509 kind of "eval" within modules you're attempting to load.
510 If "dieLevel" is 2, the debugger doesn't care where they
511 came from: It usurps your exception handler and prints out
512 a trace, then modifies all exceptions with its own
513 embellishments. This may perhaps be useful for some
514 tracing purposes, but tends to hopelessly destroy any
515 program that takes its exception handling seriously.
516 "AutoTrace" Trace mode (similar to "t" command, but can be put into
518 "PERLDB_OPTS").
519 "LineInfo" File or pipe to print line number info to. If it is a pipe
521 (say, "|visual_perl_db"), then a short message is used.
522 This is the mechanism used to interact with a slave editor
523 or visual debugger, such as the special "vi" or "emacs"
524 hooks, or the "ddd" graphical debugger.
525 "inhibit_exit"
527 If 0, allows stepping off the end of the script.
528 "PrintRet" Print return value after "r" command if set (default).
530 "ornaments" Affects screen appearance of the command line (see
532 Term::ReadLine). There is currently no way to disable
533 these, which can render some output illegible on some
534 displays, or with some pagers. This is considered a bug.
535 "frame" Affects the printing of messages upon entry and exit from
537 subroutines. If "frame & 2" is false, messages are printed
538 on entry only. (Printing on exit might be useful if
539 interspersed with other messages.)
540 If "frame & 4", arguments to functions are printed, plus
542 context and caller info. If "frame & 8", overloaded
543 "stringify" and "tie"d "FETCH" is enabled on the printed
544 arguments. If "frame & 16", the return value from the
545 subroutine is printed.
546 The length at which the argument list is truncated is
548 governed by the next option:
549 "maxTraceLen"
551 Length to truncate the argument list when the "frame"
552 option's bit 4 is set.
553 "windowSize"
555 Change the size of code list window (default is 10 lines).
556 The following options affect what happens with "V", "X", and "x"
558 commands:
559 "arrayDepth", "hashDepth"
561 Print only first N elements ('' for all).
562 "dumpDepth" Limit recursion depth to N levels when dumping structures.
564 Negative values are interpreted as infinity. Default:
565 infinity.
566 "compactDump", "veryCompact"
568 Change the style of array and hash output. If
569 "compactDump", short array may be printed on one line.
570 "globPrint" Whether to print contents of globs.
572 "DumpDBFiles"
574 Dump arrays holding debugged files.
575 "DumpPackages"
577 Dump symbol tables of packages.
578 "DumpReused"
580 Dump contents of "reused" addresses.
581 "quote", "HighBit", "undefPrint"
583 Change the style of string dump. The default value for
584 "quote" is "auto"; one can enable double-quotish or single-
585 quotish format by setting it to """ or "'", respectively.
586 By default, characters with their high bit set are printed
587 verbatim.
588 "UsageOnly" Rudimentary per-package memory usage dump. Calculates
590 total size of strings found in variables in the package.
591 This does not include lexicals in a module's file scope, or
592 lost in closures.
593 "HistFile" The path of the file from which the history (assuming a
595 usable Term::ReadLine backend) will be read on the
596 debugger's startup, and to which it will be saved on
597 shutdown (for persistence across sessions). Similar in
598 concept to Bash's ".bash_history" file.
599 "HistSize" The count of the saved lines in the history (assuming
601 "HistFile" above).
602 After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
604 environment variable and parses this as the remainder of a "O ..."
605 line as one might enter at the debugger prompt. You may place the
606 initialization options "TTY", "noTTY", "ReadLine", and "NonStop" there.
607 If your rc file contains:
609 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
611 then your script will run without human intervention, putting trace
613 information into the file db.out. (If you interrupt it, you'd better
614 reset "LineInfo" to /dev/tty if you expect to see anything.)
615 "TTY" The TTY to use for debugging I/O.
617 "noTTY" If set, the debugger goes into "NonStop" mode and will not
619 connect to a TTY. If interrupted (or if control goes to
620 the debugger via explicit setting of $DB::signal or
621 $DB::single from the Perl script), it connects to a TTY
622 specified in the "TTY" option at startup, or to a tty found
623 at runtime using the "Term::Rendezvous" module of your
624 choice.
625 This module should implement a method named "new" that
627 returns an object with two methods: "IN" and "OUT". These
628 should return filehandles to use for debugging input and
629 output correspondingly. The "new" method should inspect an
630 argument containing the value of $ENV{PERLDB_NOTTY} at
631 startup, or "$ENV{HOME}/.perldbtty$$" otherwise. This file
632 is not inspected for proper ownership, so security hazards
633 are theoretically possible.
634 "ReadLine" If false, readline support in the debugger is disabled in
636 order to debug applications that themselves use ReadLine.
637 "NonStop" If set, the debugger goes into non-interactive mode until
639 interrupted, or programmatically by setting $DB::signal or
640 $DB::single.
641 Here's an example of using the $ENV{PERLDB_OPTS} variable:
643 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
645 That will run the script myprogram without human intervention, printing
647 out the call tree with entry and exit points. Note that "NonStop=1
648 frame=2" is equivalent to "N f=2", and that originally, options could
649 be uniquely abbreviated by the first letter (modulo the "Dump*"
650 options). It is nevertheless recommended that you always spell them
651 out in full for legibility and future compatibility.
652 Other examples include
654 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
656 which runs script non-interactively, printing info on each entry into a
658 subroutine and each executed line into the file named listing. (If you
659 interrupt it, you would better reset "LineInfo" to something
660 "interactive"!)
661 Other examples include (using standard shell syntax to show environment
663 variable settings):
664 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
666 perl -d myprogram )
667 which may be useful for debugging a program that uses "Term::ReadLine"
669 itself. Do not forget to detach your shell from the TTY in the window
670 that corresponds to /dev/ttyXX, say, by issuing a command like
671 $ sleep 1000000
673 See "Debugger Internals" in perldebguts for details.
675 Debugger Input/Output
677 Prompt The debugger prompt is something like
678 DB<8>
680 or even
682 DB<<17>>
684 where that number is the command number, and which you'd use to
686 access with the built-in csh-like history mechanism. For
687 example, "!17" would repeat command number 17. The depth of
688 the angle brackets indicates the nesting depth of the debugger.
689 You could get more than one set of brackets, for example, if
690 you'd already at a breakpoint and then printed the result of a
691 function call that itself has a breakpoint, or you step into an
692 expression via "s/n/t expression" command.
693 Multiline commands
695 If you want to enter a multi-line command, such as a subroutine
696 definition with several statements or a format, escape the
697 newline that would normally end the debugger command with a
698 backslash. Here's an example:
699 DB<1> for (1..4) { \
701 cont: print "ok\n"; \
702 cont: }
703 ok
704 ok
705 ok
706 ok
707 Note that this business of escaping a newline is specific to
709 interactive commands typed into the debugger.
710 Stack backtrace
712 Here's an example of what a stack backtrace via "T" command
713 might look like:
714 $ = main::infested called from file 'Ambulation.pm' line 10
716 @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea'
717 line 7
718 $ = main::pests('bactrian', 4) called from file 'camel_flea'
719 line 4
720 The left-hand character up there indicates the context in which
722 the function was called, with "$" and "@" meaning scalar or
723 list contexts respectively, and "." meaning void context (which
724 is actually a sort of scalar context). The display above says
725 that you were in the function "main::infested" when you ran the
726 stack dump, and that it was called in scalar context from line
727 10 of the file Ambulation.pm, but without any arguments at all,
728 meaning it was called as &infested. The next stack frame shows
729 that the function "Ambulation::legs" was called in list context
730 from the camel_flea file with four arguments. The last stack
731 frame shows that "main::pests" was called in scalar context,
732 also from camel_flea, but from line 4.
733 If you execute the "T" command from inside an active "use"
735 statement, the backtrace will contain both a "require" frame
736 and an "eval" frame.
737 Line Listing Format
739 This shows the sorts of output the "l" command can produce:
740 DB<<13>> l
742 101: @i{@i} = ();
743 102:b @isa{@i,$pack} = ()
744 103 if(exists $i{$prevpack} || exists $isa{$pack});
745 104 }
746 105
747 106 next
748 107==> if(exists $isa{$pack});
749 108
750 109:a if ($extra-- > 0) {
751 110: %isa = ($pack,1);
752 Breakable lines are marked with ":". Lines with breakpoints
754 are marked by "b" and those with actions by "a". The line
755 that's about to be executed is marked by "==>".
756 Please be aware that code in debugger listings may not look the
758 same as your original source code. Line directives and
759 external source filters can alter the code before Perl sees it,
760 causing code to move from its original positions or take on
761 entirely different forms.
762 Frame listing
764 When the "frame" option is set, the debugger would print
765 entered (and optionally exited) subroutines in different
766 styles. See perldebguts for incredibly long examples of these.
767 Debugging Compile-Time Statements
769 If you have compile-time executable statements (such as code within
770 BEGIN, UNITCHECK and CHECK blocks or "use" statements), these will not
771 be stopped by debugger, although "require"s and INIT blocks will, and
772 compile-time statements can be traced with the "AutoTrace" option set
773 in "PERLDB_OPTS"). From your own Perl code, however, you can transfer
774 control back to the debugger using the following statement, which is
775 harmless if the debugger is not running:
776 $DB::single = 1;
778 If you set $DB::single to 2, it's equivalent to having just typed the
780 "n" command, whereas a value of 1 means the "s" command. The
781 $DB::trace variable should be set to 1 to simulate having typed the
782 "t" command.
783 Another way to debug compile-time code is to start the debugger, set a
785 breakpoint on the load of some module:
786 DB<7> b load f:/perllib/lib/Carp.pm
788 Will stop on load of 'f:/perllib/lib/Carp.pm'.
789 and then restart the debugger using the "R" command (if possible). One
791 can use "b compile subname" for the same purpose.
792 Debugger Customization
794 The debugger probably contains enough configuration hooks that you
795 won't ever have to modify it yourself. You may change the behaviour of
796 the debugger from within the debugger using its "o" command, from the
797 command line via the "PERLDB_OPTS" environment variable, and from
798 customization files.
799 You can do some customization by setting up a .perldb file, which
801 contains initialization code. For instance, you could make aliases
802 like these (the last one is one people expect to be there):
803 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
805 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
806 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
807 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
808 You can change options from .perldb by using calls like this one;
810 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
812 The code is executed in the package "DB". Note that .perldb is
814 processed before processing "PERLDB_OPTS". If .perldb defines the
815 subroutine "afterinit", that function is called after debugger
816 initialization ends. .perldb may be contained in the current
817 directory, or in the home directory. Because this file is sourced in
818 by Perl and may contain arbitrary commands, for security reasons, it
819 must be owned by the superuser or the current user, and writable by no
820 one but its owner.
821 You can mock TTY input to debugger by adding arbitrary commands to
823 @DB::typeahead. For example, your .perldb file might contain:
824 sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
826 Which would attempt to set breakpoints on lines 4 and 6 immediately
828 after debugger initialization. Note that @DB::typeahead is not a
829 supported interface and is subject to change in future releases.
830 If you want to modify the debugger, copy perl5db.pl from the Perl
832 library to another name and hack it to your heart's content. You'll
833 then want to set your "PERL5DB" environment variable to say something
834 like this:
835 BEGIN { require "myperl5db.pl" }
837 As a last resort, you could also use "PERL5DB" to customize the
839 debugger by directly setting internal variables or calling debugger
840 functions.
841 Note that any variables and functions that are not documented in this
843 document (or in perldebguts) are considered for internal use only, and
844 as such are subject to change without notice.
845 Readline Support / History in the Debugger
847 As shipped, the only command-line history supplied is a simplistic one
848 that checks for leading exclamation points. However, if you install
849 the Term::ReadKey and Term::ReadLine modules from CPAN (such as
850 Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full
851 editing capabilities much like those GNU readline(3) provides. Look
852 for these in the modules/by-module/Term directory on CPAN. These do
853 not support normal vi command-line editing, however.
854 A rudimentary command-line completion is also available, including
856 lexical variables in the current scope if the "PadWalker" module is
857 installed.
858 Without Readline support you may see the symbols "^[[A", "^[[C",
860 "^[[B", "^[[D"", "^H", ... when using the arrow keys and/or the
861 backspace key.
862 Editor Support for Debugging
864 If you have the GNU's version of emacs installed on your system, it can
865 interact with the Perl debugger to provide an integrated software
866 development environment reminiscent of its interactions with C
867 debuggers.
868 Recent versions of Emacs come with a start file for making emacs act
870 like a syntax-directed editor that understands (some of) Perl's syntax.
871 See perlfaq3.
872 Users of vi should also look into vim and gvim, the mousey and windy
874 version, for coloring of Perl keywords.
875 Note that only perl can truly parse Perl, so all such CASE tools fall
877 somewhat short of the mark, especially if you don't program your Perl
878 as a C programmer might.
879 The Perl Profiler
881 If you wish to supply an alternative debugger for Perl to run, invoke
882 your script with a colon and a package argument given to the -d flag.
883 Perl's alternative debuggers include a Perl profiler, Devel::NYTProf,
884 which is available separately as a CPAN distribution. To profile your
885 Perl program in the file mycode.pl, just type:
886 $ perl -d:NYTProf mycode.pl
888 When the script terminates the profiler will create a database of the
890 profile information that you can turn into reports using the profiler's
891 tools. See <perlperf> for details.
892
894 "use re 'debug'" enables you to see the gory details of how the Perl
895 regular expression engine works. In order to understand this typically
896 voluminous output, one must not only have some idea about how regular
897 expression matching works in general, but also know how Perl's regular
898 expressions are internally compiled into an automaton. These matters
899 are explored in some detail in "Debugging Regular Expressions" in
900 perldebguts.
901
903 Perl contains internal support for reporting its own memory usage, but
904 this is a fairly advanced concept that requires some understanding of
905 how memory allocation works. See "Debugging Perl Memory Usage" in
906 perldebguts for the details.
907
909 You do have "use strict" and "use warnings" enabled, don't you?
910 perldebtut, perldebguts, re, DB, Devel::NYTProf, Dumpvalue, and
912 perlrun.
913 When debugging a script that uses #! and is thus normally found in
915 $PATH, the -S option causes perl to search $PATH for it, so you don't
916 have to type the path or "which $scriptname".
917 $ perl -Sd foo.pl
919
921 You cannot get stack frame information or in any fashion debug
922 functions that were not compiled by Perl, such as those from C or C++
923 extensions.
924 If you alter your @_ arguments in a subroutine (such as with "shift" or
926 "pop"), the stack backtrace will not show the original values.
927 The debugger does not currently work in conjunction with the -W
929 command-line switch, because it itself is not free of warnings.
930 If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing
932 from your keyboard or a socket) and haven't set up your own $SIG{INT}
933 handler, then you won't be able to CTRL-C your way back to the
934 debugger, because the debugger's own $SIG{INT} handler doesn't
935 understand that it needs to raise an exception to longjmp(3) out of
936 slow syscalls.
937perl v5.26.3 2018-03-23 PERLDEBUG(1)