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 For in-depth technical usage details, see perl5db.pl, the documentation
18 of the debugger itself.
19
21 If you invoke Perl with the -d switch, your script runs under the Perl
22 source debugger. This works like an interactive Perl environment,
23 prompting for debugger commands that let you examine source code, set
24 breakpoints, get stack backtraces, change the values of variables, etc.
25 This is so convenient that you often fire up the debugger all by itself
26 just to test out Perl constructs interactively to see what they do.
27 For example:
28 $ perl -d -e 42
30 In Perl, the debugger is not a separate program the way it usually is
32 in the typical compiled environment. Instead, the -d flag tells the
33 compiler to insert source information into the parse trees it's about
34 to hand off to the interpreter. That means your code must first
35 compile correctly for the debugger to work on it. Then when the
36 interpreter starts up, it preloads a special Perl library file
37 containing the debugger.
38 The program will halt right before the first run-time executable
40 statement (but see below regarding compile-time statements) and ask you
41 to enter a debugger command. Contrary to popular expectations,
42 whenever the debugger halts and shows you a line of code, it always
43 displays the line it's about to execute, rather than the one it has
44 just executed.
45 Any command not recognized by the debugger is directly executed
47 ("eval"'d) as Perl code in the current package. (The debugger uses the
48 DB package for keeping its own state information.)
49 Note that the said "eval" is bound by an implicit scope. As a result
51 any newly introduced lexical variable or any modified capture buffer
52 content is lost after the eval. The debugger is a nice environment to
53 learn Perl, but if you interactively experiment using material which
54 should be in the same scope, stuff it in one line.
55 For any text entered at the debugger prompt, leading and trailing
57 whitespace is first stripped before further processing. If a debugger
58 command coincides with some function in your own program, merely
59 precede the function with something that doesn't look like a debugger
60 command, such as a leading ";" or perhaps a "+", or by wrapping it with
61 parentheses or braces.
62 Calling the Debugger
64 There are several ways to call the debugger:
65 perl -d program_name
67 On the given program identified by "program_name".
68 perl -d -e 0
70 Interactively supply an arbitrary "expression" using "-e".
71 perl -d:ptkdb program_name
73 Debug a given program via the "Devel::ptkdb" GUI.
74 perl -dt threaded_program_name
76 Debug a given program using threads (experimental).
77 Debugger Commands
79 The interactive debugger understands the following commands:
80 h Prints out a summary help message
82 h [command] Prints out a help message for the given debugger command.
84 h h The special argument of "h h" produces the entire help
86 page, which is quite long.
87 If the output of the "h h" command (or any command, for
89 that matter) scrolls past your screen, precede the command
90 with a leading pipe symbol so that it's run through your
91 pager, as in
92 DB> |h h
94 You may change the pager which is used via "o pager=..."
96 command.
97 p expr Same as "print {$DB::OUT} expr" in the current package. In
99 particular, because this is just Perl's own "print"
100 function, this means that nested data structures and
101 objects are not dumped, unlike with the "x" command.
102 The "DB::OUT" filehandle is opened to /dev/tty, regardless
104 of where STDOUT may be redirected to.
105 x [maxdepth] expr
107 Evaluates its expression in list context and dumps out the
108 result in a pretty-printed fashion. Nested data structures
109 are printed out recursively, unlike the real "print"
110 function in Perl. When dumping hashes, you'll probably
111 prefer 'x \%h' rather than 'x %h'. See Dumpvalue if you'd
112 like to do this yourself.
113 The output format is governed by multiple options described
115 under "Configurable Options".
116 If the "maxdepth" is included, it must be a numeral N; the
118 value is dumped only N levels deep, as if the "dumpDepth"
119 option had been temporarily set to N.
120 V [pkg [vars]]
122 Display all (or some) variables in package (defaulting to
123 "main") using a data pretty-printer (hashes show their keys
124 and values so you see what's what, control characters are
125 made printable, etc.). Make sure you don't put the type
126 specifier (like "$") there, just the symbol names, like
127 this:
128 V DB filename line
130 Use "~pattern" and "!pattern" for positive and negative
132 regexes.
133 This is similar to calling the "x" command on each
135 applicable var.
136 X [vars] Same as "V currentpackage [vars]".
138 y [level [vars]]
140 Display all (or some) lexical variables (mnemonic: "mY"
141 variables) in the current scope or level scopes higher.
142 You can limit the variables that you see with vars which
143 works exactly as it does for the "V" and "X" commands.
144 Requires the "PadWalker" module version 0.08 or higher;
145 will warn if this isn't installed. Output is pretty-
146 printed in the same style as for "V" and the format is
147 controlled by the same options.
148 T Produce a stack backtrace. See below for details on its
150 output.
151 s [expr] Single step. Executes until the beginning of another
153 statement, descending into subroutine calls. If an
154 expression is supplied that includes function calls, it too
155 will be single-stepped.
156 n [expr] Next. Executes over subroutine calls, until the beginning
158 of the next statement. If an expression is supplied that
159 includes function calls, those functions will be executed
160 with stops before each statement.
161 r Continue until the return from the current subroutine.
163 Dump the return value if the "PrintRet" option is set
164 (default).
165 <CR> Repeat last "n" or "s" command.
167 c [line|sub]
169 Continue, optionally inserting a one-time-only breakpoint
170 at the specified line or subroutine.
171 l List next window of lines.
173 l min+incr List "incr+1" lines starting at "min".
175 l min-max List lines "min" through "max". "l -" is synonymous to
177 "-".
178 l line List a single line.
180 l subname List first window of lines from subroutine. subname may be
182 a variable that contains a code reference.
183 - List previous window of lines.
185 v [line] View a few lines of code around the current line.
187 . Return the internal debugger pointer to the line last
189 executed, and print out that line.
190 f filename Switch to viewing a different file or "eval" statement. If
192 filename is not a full pathname found in the values of
193 %INC, it is considered a regex.
194 "eval"ed strings (when accessible) are considered to be
196 filenames: "f (eval 7)" and "f eval 7\b" access the body of
197 the 7th "eval"ed string (in the order of execution). The
198 bodies of the currently executed "eval" and of "eval"ed
199 strings that define subroutines are saved and thus
200 accessible.
201 /pattern/ Search forwards for pattern (a Perl regex); final / is
203 optional. The search is case-insensitive by default.
204 ?pattern? Search backwards for pattern; final ? is optional. The
206 search is case-insensitive by default.
207 L [abw] List (default all) actions, breakpoints and watch
209 expressions
210 S [[!]regex]
212 List subroutine names [not] matching the regex.
213 t [n] Toggle trace mode (see also the "AutoTrace" option).
215 Optional argument is the maximum number of levels to trace
216 below the current one; anything deeper than that will be
217 silent.
218 t [n] expr Trace through execution of "expr". Optional first argument
220 is the maximum number of levels to trace below the current
221 one; anything deeper than that will be silent. See "Frame
222 Listing Output Examples" in perldebguts for examples.
223 b Sets breakpoint on current line
225 b [line] [condition]
227 Set a breakpoint before the given line. If a condition is
228 specified, it's evaluated each time the statement is
229 reached: a breakpoint is taken only if the condition is
230 true. Breakpoints may only be set on lines that begin an
231 executable statement. Conditions don't use "if":
232 b 237 $x > 30
234 b 237 ++$count237 < 11
235 b 33 /pattern/i
236 If the line number is ".", sets a breakpoint on the current
238 line:
239 b . $n > 100
241 b [file]:[line] [condition]
243 Set a breakpoint before the given line in a (possibly
244 different) file. If a condition is specified, it's
245 evaluated each time the statement is reached: a breakpoint
246 is taken only if the condition is true. Breakpoints may
247 only be set on lines that begin an executable statement.
248 Conditions don't use "if":
249 b lib/MyModule.pm:237 $x > 30
251 b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
252 b subname [condition]
254 Set a breakpoint before the first line of the named
255 subroutine. subname may be a variable containing a code
256 reference (in this case condition is not supported).
257 b postpone subname [condition]
259 Set a breakpoint at first line of subroutine after it is
260 compiled.
261 b load filename
263 Set a breakpoint before the first executed line of the
264 filename, which should be a full pathname found amongst the
265 %INC values.
266 b compile subname
268 Sets a breakpoint before the first statement executed after
269 the specified subroutine is compiled.
270 B line Delete a breakpoint from the specified line.
272 B * Delete all installed breakpoints.
274 disable [file]:[line]
276 Disable the breakpoint so it won't stop the execution of
277 the program. Breakpoints are enabled by default and can be
278 re-enabled using the "enable" command.
279 disable [line]
281 Disable the breakpoint so it won't stop the execution of
282 the program. Breakpoints are enabled by default and can be
283 re-enabled using the "enable" command.
284 This is done for a breakpoint in the current file.
286 enable [file]:[line]
288 Enable the breakpoint so it will stop the execution of the
289 program.
290 enable [line]
292 Enable the breakpoint so it will stop the execution of the
293 program.
294 This is done for a breakpoint in the current file.
296 a [line] command
298 Set an action to be done before the line is executed. If
299 line is omitted, set an action on the line about to be
300 executed. The sequence of steps taken by the debugger is
301 1. check for a breakpoint at this line
303 2. print the line if necessary (tracing)
304 3. do any actions associated with that line
305 4. prompt user if at a breakpoint or in single-step
306 5. evaluate line
307 For example, this will print out $foo every time line 53 is
309 passed:
310 a 53 print "DB FOUND $foo\n"
312 A line Delete an action from the specified line.
314 A * Delete all installed actions.
316 w expr Add a global watch-expression. Whenever a watched global
318 changes the debugger will stop and display the old and new
319 values.
320 W expr Delete watch-expression
322 W * Delete all watch-expressions.
324 o Display all options.
326 o booloption ...
328 Set each listed Boolean option to the value 1.
329 o anyoption? ...
331 Print out the value of one or more options.
332 o option=value ...
334 Set the value of one or more options. If the value has
335 internal whitespace, it should be quoted. For example, you
336 could set "o pager="less -MQeicsNfr"" to call less with
337 those specific options. You may use either single or
338 double quotes, but if you do, you must escape any embedded
339 instances of same sort of quote you began with, as well as
340 any escaping any escapes that immediately precede that
341 quote but which are not meant to escape the quote itself.
342 In other words, you follow single-quoting rules
343 irrespective of the quote; eg: "o option='this isn\'t bad'"
344 or "o option="She said, \"Isn't it?\""".
345 For historical reasons, the "=value" is optional, but
347 defaults to 1 only where it is safe to do so--that is,
348 mostly for Boolean options. It is always better to assign
349 a specific value using "=". The "option" can be
350 abbreviated, but for clarity probably should not be.
351 Several options can be set together. See "Configurable
352 Options" for a list of these.
353 < ? List out all pre-prompt Perl command actions.
355 < [ command ]
357 Set an action (Perl command) to happen before every
358 debugger prompt. A multi-line command may be entered by
359 backslashing the newlines.
360 < * Delete all pre-prompt Perl command actions.
362 << command Add an action (Perl command) to happen before every
364 debugger prompt. A multi-line command may be entered by
365 backwhacking the newlines.
366 > ? List out post-prompt Perl command actions.
368 > command Set an action (Perl command) to happen after the prompt
370 when you've just given a command to return to executing the
371 script. A multi-line command may be entered by
372 backslashing the newlines (we bet you couldn't have guessed
373 this by now).
374 > * Delete all post-prompt Perl command actions.
376 >> command Adds an action (Perl command) to happen after the prompt
378 when you've just given a command to return to executing the
379 script. A multi-line command may be entered by
380 backslashing the newlines.
381 { ? List out pre-prompt debugger commands.
383 { [ command ]
385 Set an action (debugger command) to happen before every
386 debugger prompt. A multi-line command may be entered in
387 the customary fashion.
388 Because this command is in some senses new, a warning is
390 issued if you appear to have accidentally entered a block
391 instead. If that's what you mean to do, write it as with
392 ";{ ... }" or even "do { ... }".
393 { * Delete all pre-prompt debugger commands.
395 {{ command Add an action (debugger command) to happen before every
397 debugger prompt. A multi-line command may be entered, if
398 you can guess how: see above.
399 ! number Redo a previous command (defaults to the previous command).
401 ! -number Redo number'th previous command.
403 ! pattern Redo last command that started with pattern. See "o
405 recallCommand", too.
406 !! cmd Run cmd in a subprocess (reads from DB::IN, writes to
408 DB::OUT) See "o shellBang", also. Note that the user's
409 current shell (well, their $ENV{SHELL} variable) will be
410 used, which can interfere with proper interpretation of
411 exit status or signal and coredump information.
412 source file Read and execute debugger commands from file. file may
414 itself contain "source" commands.
415 H -number Display last n commands. Only commands longer than one
417 character are listed. If number is omitted, list them all.
418 q or ^D Quit. ("quit" doesn't work for this, unless you've made an
420 alias) This is the only supported way to exit the debugger,
421 though typing "exit" twice might work.
422 Set the "inhibit_exit" option to 0 if you want to be able
424 to step off the end the script. You may also need to set
425 $finished to 0 if you want to step through global
426 destruction.
427 R Restart the debugger by "exec()"ing a new session. We try
429 to maintain your history across this, but internal settings
430 and command-line options may be lost.
431 The following setting are currently preserved: history,
433 breakpoints, actions, debugger options, and the Perl
434 command-line options -w, -I, and -e.
435 |dbcmd Run the debugger command, piping DB::OUT into your current
437 pager.
438 ||dbcmd Same as "|dbcmd" but DB::OUT is temporarily "select"ed as
440 well.
441 = [alias value]
443 Define a command alias, like
444 = quit q
446 or list current aliases.
448 command Execute command as a Perl statement. A trailing semicolon
450 will be supplied. If the Perl statement would otherwise be
451 confused for a Perl debugger, use a leading semicolon, too.
452 m expr List which methods may be called on the result of the
454 evaluated expression. The expression may evaluated to a
455 reference to a blessed object, or to a package name.
456 M Display all loaded modules and their versions.
458 man [manpage]
460 Despite its name, this calls your system's default
461 documentation viewer on the given page, or on the viewer
462 itself if manpage is omitted. If that viewer is man, the
463 current "Config" information is used to invoke man using
464 the proper MANPATH or -M manpath option. Failed lookups of
465 the form "XXX" that match known manpages of the form
466 perlXXX will be retried. This lets you type "man debug" or
467 "man op" from the debugger.
468 On systems traditionally bereft of a usable man command,
470 the debugger invokes perldoc. Occasionally this
471 determination is incorrect due to recalcitrant vendors or
472 rather more felicitously, to enterprising users. If you
473 fall into either category, just manually set the
474 $DB::doccmd variable to whatever viewer to view the Perl
475 documentation on your system. This may be set in an rc
476 file, or through direct assignment. We're still waiting
477 for a working example of something along the lines of:
478 $DB::doccmd = 'netscape -remote http://something.here/';
480 Configurable Options
482 The debugger has numerous options settable using the "o" command,
483 either interactively or from the environment or an rc file. The file is
484 named ./.perldb or ~/.perldb under Unix with /dev/tty, perldb.ini
485 otherwise.
486 "recallCommand", "ShellBang"
488 The characters used to recall a command or spawn a shell.
489 By default, both are set to "!", which is unfortunate.
490 "pager" Program to use for output of pager-piped commands (those
492 beginning with a "|" character.) By default, $ENV{PAGER}
493 will be used. Because the debugger uses your current
494 terminal characteristics for bold and underlining, if the
495 chosen pager does not pass escape sequences through
496 unchanged, the output of some debugger commands will not be
497 readable when sent through the pager.
498 "tkRunning" Run Tk while prompting (with ReadLine).
500 "signalLevel", "warnLevel", "dieLevel"
502 Level of verbosity. By default, the debugger leaves your
503 exceptions and warnings alone, because altering them can
504 break correctly running programs. It will attempt to print
505 a message when uncaught INT, BUS, or SEGV signals arrive.
506 (But see the mention of signals in "BUGS" below.)
507 To disable this default safe mode, set these values to
509 something higher than 0. At a level of 1, you get
510 backtraces upon receiving any kind of warning (this is
511 often annoying) or exception (this is often valuable).
512 Unfortunately, the debugger cannot discern fatal exceptions
513 from non-fatal ones. If "dieLevel" is even 1, then your
514 non-fatal exceptions are also traced and unceremoniously
515 altered if they came from "eval'ed" strings or from any
516 kind of "eval" within modules you're attempting to load.
517 If "dieLevel" is 2, the debugger doesn't care where they
518 came from: It usurps your exception handler and prints out
519 a trace, then modifies all exceptions with its own
520 embellishments. This may perhaps be useful for some
521 tracing purposes, but tends to hopelessly destroy any
522 program that takes its exception handling seriously.
523 "AutoTrace" Trace mode (similar to "t" command, but can be put into
525 "PERLDB_OPTS").
526 "LineInfo" File or pipe to print line number info to. If it is a pipe
528 (say, "|visual_perl_db"), then a short message is used.
529 This is the mechanism used to interact with a slave editor
530 or visual debugger, such as the special "vi" or "emacs"
531 hooks, or the "ddd" graphical debugger.
532 "inhibit_exit"
534 If 0, allows stepping off the end of the script.
535 "PrintRet" Print return value after "r" command if set (default).
537 "ornaments" Affects screen appearance of the command line (see
539 Term::ReadLine). There is currently no way to disable
540 these, which can render some output illegible on some
541 displays, or with some pagers. This is considered a bug.
542 "frame" Affects the printing of messages upon entry and exit from
544 subroutines. If "frame & 2" is false, messages are printed
545 on entry only. (Printing on exit might be useful if
546 interspersed with other messages.)
547 If "frame & 4", arguments to functions are printed, plus
549 context and caller info. If "frame & 8", overloaded
550 "stringify" and "tie"d "FETCH" is enabled on the printed
551 arguments. If "frame & 16", the return value from the
552 subroutine is printed.
553 The length at which the argument list is truncated is
555 governed by the next option:
556 "maxTraceLen"
558 Length to truncate the argument list when the "frame"
559 option's bit 4 is set.
560 "windowSize"
562 Change the size of code list window (default is 10 lines).
563 The following options affect what happens with "V", "X", and "x"
565 commands:
566 "arrayDepth", "hashDepth"
568 Print only first N elements ('' for all).
569 "dumpDepth" Limit recursion depth to N levels when dumping structures.
571 Negative values are interpreted as infinity. Default:
572 infinity.
573 "compactDump", "veryCompact"
575 Change the style of array and hash output. If
576 "compactDump", short array may be printed on one line.
577 "globPrint" Whether to print contents of globs.
579 "DumpDBFiles"
581 Dump arrays holding debugged files.
582 "DumpPackages"
584 Dump symbol tables of packages.
585 "DumpReused"
587 Dump contents of "reused" addresses.
588 "quote", "HighBit", "undefPrint"
590 Change the style of string dump. The default value for
591 "quote" is "auto"; one can enable double-quotish or single-
592 quotish format by setting it to """ or "'", respectively.
593 By default, characters with their high bit set are printed
594 verbatim.
595 "UsageOnly" Rudimentary per-package memory usage dump. Calculates
597 total size of strings found in variables in the package.
598 This does not include lexicals in a module's file scope, or
599 lost in closures.
600 "HistFile" The path of the file from which the history (assuming a
602 usable Term::ReadLine backend) will be read on the
603 debugger's startup, and to which it will be saved on
604 shutdown (for persistence across sessions). Similar in
605 concept to Bash's ".bash_history" file.
606 "HistSize" The count of the saved lines in the history (assuming
608 "HistFile" above).
609 After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
611 environment variable and parses this as the remainder of a "O ..."
612 line as one might enter at the debugger prompt. You may place the
613 initialization options "TTY", "noTTY", "ReadLine", and "NonStop" there.
614 If your rc file contains:
616 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
618 then your script will run without human intervention, putting trace
620 information into the file db.out. (If you interrupt it, you'd better
621 reset "LineInfo" to /dev/tty if you expect to see anything.)
622 "TTY" The TTY to use for debugging I/O.
624 "noTTY" If set, the debugger goes into "NonStop" mode and will not
626 connect to a TTY. If interrupted (or if control goes to
627 the debugger via explicit setting of $DB::signal or
628 $DB::single from the Perl script), it connects to a TTY
629 specified in the "TTY" option at startup, or to a tty found
630 at runtime using the "Term::Rendezvous" module of your
631 choice.
632 This module should implement a method named "new" that
634 returns an object with two methods: "IN" and "OUT". These
635 should return filehandles to use for debugging input and
636 output correspondingly. The "new" method should inspect an
637 argument containing the value of $ENV{PERLDB_NOTTY} at
638 startup, or "$ENV{HOME}/.perldbtty$$" otherwise. This file
639 is not inspected for proper ownership, so security hazards
640 are theoretically possible.
641 "ReadLine" If false, readline support in the debugger is disabled in
643 order to debug applications that themselves use ReadLine.
644 "NonStop" If set, the debugger goes into non-interactive mode until
646 interrupted, or programmatically by setting $DB::signal or
647 $DB::single.
648 Here's an example of using the $ENV{PERLDB_OPTS} variable:
650 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
652 That will run the script myprogram without human intervention, printing
654 out the call tree with entry and exit points. Note that "NonStop=1
655 frame=2" is equivalent to "N f=2", and that originally, options could
656 be uniquely abbreviated by the first letter (modulo the "Dump*"
657 options). It is nevertheless recommended that you always spell them
658 out in full for legibility and future compatibility.
659 Other examples include
661 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
663 which runs script non-interactively, printing info on each entry into a
665 subroutine and each executed line into the file named listing. (If you
666 interrupt it, you would better reset "LineInfo" to something
667 "interactive"!)
668 Other examples include (using standard shell syntax to show environment
670 variable settings):
671 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
673 perl -d myprogram )
674 which may be useful for debugging a program that uses "Term::ReadLine"
676 itself. Do not forget to detach your shell from the TTY in the window
677 that corresponds to /dev/ttyXX, say, by issuing a command like
678 $ sleep 1000000
680 See "Debugger Internals" in perldebguts for details.
682 Debugger Input/Output
684 Prompt The debugger prompt is something like
685 DB<8>
687 or even
689 DB<<17>>
691 where that number is the command number, and which you'd use to
693 access with the built-in csh-like history mechanism. For
694 example, "!17" would repeat command number 17. The depth of
695 the angle brackets indicates the nesting depth of the debugger.
696 You could get more than one set of brackets, for example, if
697 you'd already at a breakpoint and then printed the result of a
698 function call that itself has a breakpoint, or you step into an
699 expression via "s/n/t expression" command.
700 Multiline commands
702 If you want to enter a multi-line command, such as a subroutine
703 definition with several statements or a format, escape the
704 newline that would normally end the debugger command with a
705 backslash. Here's an example:
706 DB<1> for (1..4) { \
708 cont: print "ok\n"; \
709 cont: }
710 ok
711 ok
712 ok
713 ok
714 Note that this business of escaping a newline is specific to
716 interactive commands typed into the debugger.
717 Stack backtrace
719 Here's an example of what a stack backtrace via "T" command
720 might look like:
721 $ = main::infested called from file 'Ambulation.pm' line 10
723 @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea'
724 line 7
725 $ = main::pests('bactrian', 4) called from file 'camel_flea'
726 line 4
727 The left-hand character up there indicates the context in which
729 the function was called, with "$" and "@" meaning scalar or
730 list contexts respectively, and "." meaning void context (which
731 is actually a sort of scalar context). The display above says
732 that you were in the function "main::infested" when you ran the
733 stack dump, and that it was called in scalar context from line
734 10 of the file Ambulation.pm, but without any arguments at all,
735 meaning it was called as &infested. The next stack frame shows
736 that the function "Ambulation::legs" was called in list context
737 from the camel_flea file with four arguments. The last stack
738 frame shows that "main::pests" was called in scalar context,
739 also from camel_flea, but from line 4.
740 If you execute the "T" command from inside an active "use"
742 statement, the backtrace will contain both a "require" frame
743 and an "eval" frame.
744 Line Listing Format
746 This shows the sorts of output the "l" command can produce:
747 DB<<13>> l
749 101: @i{@i} = ();
750 102:b @isa{@i,$pack} = ()
751 103 if(exists $i{$prevpack} || exists $isa{$pack});
752 104 }
753 105
754 106 next
755 107==> if(exists $isa{$pack});
756 108
757 109:a if ($extra-- > 0) {
758 110: %isa = ($pack,1);
759 Breakable lines are marked with ":". Lines with breakpoints
761 are marked by "b" and those with actions by "a". The line
762 that's about to be executed is marked by "==>".
763 Please be aware that code in debugger listings may not look the
765 same as your original source code. Line directives and
766 external source filters can alter the code before Perl sees it,
767 causing code to move from its original positions or take on
768 entirely different forms.
769 Frame listing
771 When the "frame" option is set, the debugger would print
772 entered (and optionally exited) subroutines in different
773 styles. See perldebguts for incredibly long examples of these.
774 Debugging Compile-Time Statements
776 If you have compile-time executable statements (such as code within
777 BEGIN, UNITCHECK and CHECK blocks or "use" statements), these will not
778 be stopped by debugger, although "require"s and INIT blocks will, and
779 compile-time statements can be traced with the "AutoTrace" option set
780 in "PERLDB_OPTS"). From your own Perl code, however, you can transfer
781 control back to the debugger using the following statement, which is
782 harmless if the debugger is not running:
783 $DB::single = 1;
785 If you set $DB::single to 2, it's equivalent to having just typed the
787 "n" command, whereas a value of 1 means the "s" command. The
788 $DB::trace variable should be set to 1 to simulate having typed the
789 "t" command.
790 Another way to debug compile-time code is to start the debugger, set a
792 breakpoint on the load of some module:
793 DB<7> b load f:/perllib/lib/Carp.pm
795 Will stop on load of 'f:/perllib/lib/Carp.pm'.
796 and then restart the debugger using the "R" command (if possible). One
798 can use "b compile subname" for the same purpose.
799 Debugger Customization
801 The debugger probably contains enough configuration hooks that you
802 won't ever have to modify it yourself. You may change the behaviour of
803 the debugger from within the debugger using its "o" command, from the
804 command line via the "PERLDB_OPTS" environment variable, and from
805 customization files.
806 You can do some customization by setting up a .perldb file, which
808 contains initialization code. For instance, you could make aliases
809 like these (the last one is one people expect to be there):
810 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
812 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
813 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
814 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
815 You can change options from .perldb by using calls like this one;
817 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
819 The code is executed in the package "DB". Note that .perldb is
821 processed before processing "PERLDB_OPTS". If .perldb defines the
822 subroutine "afterinit", that function is called after debugger
823 initialization ends. .perldb may be contained in the current
824 directory, or in the home directory. Because this file is sourced in
825 by Perl and may contain arbitrary commands, for security reasons, it
826 must be owned by the superuser or the current user, and writable by no
827 one but its owner.
828 You can mock TTY input to debugger by adding arbitrary commands to
830 @DB::typeahead. For example, your .perldb file might contain:
831 sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
833 Which would attempt to set breakpoints on lines 4 and 6 immediately
835 after debugger initialization. Note that @DB::typeahead is not a
836 supported interface and is subject to change in future releases.
837 If you want to modify the debugger, copy perl5db.pl from the Perl
839 library to another name and hack it to your heart's content. You'll
840 then want to set your "PERL5DB" environment variable to say something
841 like this:
842 BEGIN { require "myperl5db.pl" }
844 As a last resort, you could also use "PERL5DB" to customize the
846 debugger by directly setting internal variables or calling debugger
847 functions.
848 Note that any variables and functions that are not documented in this
850 document (or in perldebguts) are considered for internal use only, and
851 as such are subject to change without notice.
852 Readline Support / History in the Debugger
854 As shipped, the only command-line history supplied is a simplistic one
855 that checks for leading exclamation points. However, if you install
856 the Term::ReadKey and Term::ReadLine modules from CPAN (such as
857 Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full
858 editing capabilities much like those GNU readline(3) provides. Look
859 for these in the modules/by-module/Term directory on CPAN. These do
860 not support normal vi command-line editing, however.
861 A rudimentary command-line completion is also available, including
863 lexical variables in the current scope if the "PadWalker" module is
864 installed.
865 Without Readline support you may see the symbols "^[[A", "^[[C",
867 "^[[B", "^[[D"", "^H", ... when using the arrow keys and/or the
868 backspace key.
869 Editor Support for Debugging
871 If you have the GNU's version of emacs installed on your system, it can
872 interact with the Perl debugger to provide an integrated software
873 development environment reminiscent of its interactions with C
874 debuggers.
875 Recent versions of Emacs come with a start file for making emacs act
877 like a syntax-directed editor that understands (some of) Perl's syntax.
878 See perlfaq3.
879 Users of vi should also look into vim and gvim, the mousey and windy
881 version, for coloring of Perl keywords.
882 Note that only perl can truly parse Perl, so all such CASE tools fall
884 somewhat short of the mark, especially if you don't program your Perl
885 as a C programmer might.
886 The Perl Profiler
888 If you wish to supply an alternative debugger for Perl to run, invoke
889 your script with a colon and a package argument given to the -d flag.
890 Perl's alternative debuggers include a Perl profiler, Devel::NYTProf,
891 which is available separately as a CPAN distribution. To profile your
892 Perl program in the file mycode.pl, just type:
893 $ perl -d:NYTProf mycode.pl
895 When the script terminates the profiler will create a database of the
897 profile information that you can turn into reports using the profiler's
898 tools. See <perlperf> for details.
899
901 "use re 'debug'" enables you to see the gory details of how the Perl
902 regular expression engine works. In order to understand this typically
903 voluminous output, one must not only have some idea about how regular
904 expression matching works in general, but also know how Perl's regular
905 expressions are internally compiled into an automaton. These matters
906 are explored in some detail in "Debugging Regular Expressions" in
907 perldebguts.
908
910 Perl contains internal support for reporting its own memory usage, but
911 this is a fairly advanced concept that requires some understanding of
912 how memory allocation works. See "Debugging Perl Memory Usage" in
913 perldebguts for the details.
914
916 You do have "use strict" and "use warnings" enabled, don't you?
917 perldebtut, perldebguts, perl5db.pl, re, DB, Devel::NYTProf, Dumpvalue,
919 and perlrun.
920 When debugging a script that uses #! and is thus normally found in
922 $PATH, the -S option causes perl to search $PATH for it, so you don't
923 have to type the path or "which $scriptname".
924 $ perl -Sd foo.pl
926
928 You cannot get stack frame information or in any fashion debug
929 functions that were not compiled by Perl, such as those from C or C++
930 extensions.
931 If you alter your @_ arguments in a subroutine (such as with "shift" or
933 "pop"), the stack backtrace will not show the original values.
934 The debugger does not currently work in conjunction with the -W
936 command-line switch, because it itself is not free of warnings.
937 If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing
939 from your keyboard or a socket) and haven't set up your own $SIG{INT}
940 handler, then you won't be able to CTRL-C your way back to the
941 debugger, because the debugger's own $SIG{INT} handler doesn't
942 understand that it needs to raise an exception to longjmp(3) out of
943 slow syscalls.
944perl v5.34.1 2022-03-15 PERLDEBUG(1)