1PERLDEBUG(1)           Perl Programmers Reference Guide           PERLDEBUG(1)
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NAME

6       perldebug - Perl debugging
7

DESCRIPTION

9       First of all, have you tried using the -w switch?
10
11       If you're new to the Perl debugger, you may prefer to read perldebtut,
12       which is a tutorial introduction to the debugger .
13

The Perl Debugger

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
23           $ perl -d -e 42
24
25       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
33       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
40       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
44       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
50       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
57   Calling the debugger
58       There are several ways to call the debugger:
59
60       perl -d program_name
61           On the given program identified by "program_name".
62
63       perl -d -e 0
64           Interactively supply an arbitrary "expression" using "-e".
65
66       perl -d:Ptkdb program_name
67           Debug a given program via the "Devel::Ptkdb" GUI.
68
69       perl -dt threaded_program_name
70           Debug a given program using threads (experimental).
71
72   Debugger Commands
73       The interactive debugger understands the following commands:
74
75       h           Prints out a summary help message
76
77       h [command] Prints out a help message for the given debugger command.
78
79       h h         The special argument of "h h" produces the entire help
80                   page, which is quite long.
81
82                   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
87                       DB> |h h
88
89                   You may change the pager which is used via "o pager=..."
90                   command.
91
92       p expr      Same as "print {$DB::OUT} expr" in the current package.  In
93                   particular, because this is just Perl's own "print"
94                   function, this means that nested data structures and
95                   objects are not dumped, unlike with the "x" command.
96
97                   The "DB::OUT" filehandle is opened to /dev/tty, regardless
98                   of where STDOUT may be redirected to.
99
100       x [maxdepth] expr
101                   Evaluates its expression in list context and dumps out the
102                   result in a pretty-printed fashion.  Nested data structures
103                   are printed out recursively, unlike the real "print"
104                   function in Perl.  When dumping hashes, you'll probably
105                   prefer 'x \%h' rather than 'x %h'.  See Dumpvalue if you'd
106                   like to do this yourself.
107
108                   The output format is governed by multiple options described
109                   under "Configurable Options".
110
111                   If the "maxdepth" is included, it must be a numeral N; the
112                   value is dumped only N levels deep, as if the "dumpDepth"
113                   option had been temporarily set to N.
114
115       V [pkg [vars]]
116                   Display all (or some) variables in package (defaulting to
117                   "main") using a data pretty-printer (hashes show their keys
118                   and values so you see what's what, control characters are
119                   made printable, etc.).  Make sure you don't put the type
120                   specifier (like "$") there, just the symbol names, like
121                   this:
122
123                       V DB filename line
124
125                   Use "~pattern" and "!pattern" for positive and negative
126                   regexes.
127
128                   This is similar to calling the "x" command on each
129                   applicable var.
130
131       X [vars]    Same as "V currentpackage [vars]".
132
133       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
143       T           Produce a stack backtrace.  See below for details on its
144                   output.
145
146       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
151       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
156       r           Continue until the return from the current subroutine.
157                   Dump the return value if the "PrintRet" option is set
158                   (default).
159
160       <CR>        Repeat last "n" or "s" command.
161
162       c [line|sub]
163                   Continue, optionally inserting a one-time-only breakpoint
164                   at the specified line or subroutine.
165
166       l           List next window of lines.
167
168       l min+incr  List "incr+1" lines starting at "min".
169
170       l min-max   List lines "min" through "max".  "l -" is synonymous to
171                   "-".
172
173       l line      List a single line.
174
175       l subname   List first window of lines from subroutine.  subname may be
176                   a variable that contains a code reference.
177
178       -           List previous window of lines.
179
180       v [line]    View a few lines of code around the current line.
181
182       .           Return the internal debugger pointer to the line last
183                   executed, and print out that line.
184
185       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
189                   "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
196       /pattern/   Search forwards for pattern (a Perl regex); final / is
197                   optional.  The search is case-insensitive by default.
198
199       ?pattern?   Search backwards for pattern; final ? is optional.  The
200                   search is case-insensitive by default.
201
202       L [abw]     List (default all) actions, breakpoints and watch
203                   expressions
204
205       S [[!]regex]
206                   List subroutine names [not] matching the regex.
207
208       t           Toggle trace mode (see also the "AutoTrace" option).
209
210       t expr      Trace through execution of "expr".  See "Frame Listing
211                   Output Examples" in perldebguts for examples.
212
213       b           Sets breakpoint on current line
214
215       b [line] [condition]
216                   Set a breakpoint before the given line.  If a condition is
217                   specified, it's evaluated each time the statement is
218                   reached: a breakpoint is taken only if the condition is
219                   true.  Breakpoints may only be set on lines that begin an
220                   executable statement.  Conditions don't use "if":
221
222                       b 237 $x > 30
223                       b 237 ++$count237 < 11
224                       b 33 /pattern/i
225
226       b subname [condition]
227                   Set a breakpoint before the first line of the named
228                   subroutine.  subname may be a variable containing a code
229                   reference (in this case condition is not supported).
230
231       b postpone subname [condition]
232                   Set a breakpoint at first line of subroutine after it is
233                   compiled.
234
235       b load filename
236                   Set a breakpoint before the first executed line of the
237                   filename, which should be a full pathname found amongst the
238                   %INC values.
239
240       b compile subname
241                   Sets a breakpoint before the first statement executed after
242                   the specified subroutine is compiled.
243
244       B line      Delete a breakpoint from the specified line.
245
246       B *         Delete all installed breakpoints.
247
248       a [line] command
249                   Set an action to be done before the line is executed.  If
250                   line is omitted, set an action on the line about to be
251                   executed.  The sequence of steps taken by the debugger is
252
253                     1. check for a breakpoint at this line
254                     2. print the line if necessary (tracing)
255                     3. do any actions associated with that line
256                     4. prompt user if at a breakpoint or in single-step
257                     5. evaluate line
258
259                   For example, this will print out $foo every time line 53 is
260                   passed:
261
262                       a 53 print "DB FOUND $foo\n"
263
264       A line      Delete an action from the specified line.
265
266       A *         Delete all installed actions.
267
268       w expr      Add a global watch-expression. Whenever a watched global
269                   changes the debugger will stop and display the old and new
270                   values.
271
272       W expr      Delete watch-expression
273
274       W *         Delete all watch-expressions.
275
276       o           Display all options
277
278       o booloption ...
279                   Set each listed Boolean option to the value 1.
280
281       o anyoption? ...
282                   Print out the value of one or more options.
283
284       o option=value ...
285                   Set the value of one or more options.  If the value has
286                   internal whitespace, it should be quoted.  For example, you
287                   could set "o pager="less -MQeicsNfr"" to call less with
288                   those specific options.  You may use either single or
289                   double quotes, but if you do, you must escape any embedded
290                   instances of same sort of quote you began with, as well as
291                   any escaping any escapes that immediately precede that
292                   quote but which are not meant to escape the quote itself.
293                   In other words, you follow single-quoting rules
294                   irrespective of the quote; eg: "o option='this isn\'t bad'"
295                   or "o option="She said, \"Isn't it?\""".
296
297                   For historical reasons, the "=value" is optional, but
298                   defaults to 1 only where it is safe to do so--that is,
299                   mostly for Boolean options.  It is always better to assign
300                   a specific value using "=".  The "option" can be
301                   abbreviated, but for clarity probably should not be.
302                   Several options can be set together.  See "Configurable
303                   Options" for a list of these.
304
305       < ?         List out all pre-prompt Perl command actions.
306
307       < [ command ]
308                   Set an action (Perl command) to happen before every
309                   debugger prompt.  A multi-line command may be entered by
310                   backslashing the newlines.
311
312       < *         Delete all pre-prompt Perl command actions.
313
314       << command  Add an action (Perl command) to happen before every
315                   debugger prompt.  A multi-line command may be entered by
316                   backwhacking the newlines.
317
318       > ?         List out post-prompt Perl command actions.
319
320       > command   Set an action (Perl command) to happen after the prompt
321                   when you've just given a command to return to executing the
322                   script.  A multi-line command may be entered by
323                   backslashing the newlines (we bet you couldn't have guessed
324                   this by now).
325
326       > *         Delete all post-prompt Perl command actions.
327
328       >> command  Adds an action (Perl command) to happen after the prompt
329                   when you've just given a command to return to executing the
330                   script.  A multi-line command may be entered by
331                   backslashing the newlines.
332
333       { ?         List out pre-prompt debugger commands.
334
335       { [ command ]
336                   Set an action (debugger command) to happen before every
337                   debugger prompt.  A multi-line command may be entered in
338                   the customary fashion.
339
340                   Because this command is in some senses new, a warning is
341                   issued if you appear to have accidentally entered a block
342                   instead.  If that's what you mean to do, write it as with
343                   ";{ ... }" or even "do { ... }".
344
345       { *         Delete all pre-prompt debugger commands.
346
347       {{ command  Add an action (debugger command) to happen before every
348                   debugger prompt.  A multi-line command may be entered, if
349                   you can guess how: see above.
350
351       ! number    Redo a previous command (defaults to the previous command).
352
353       ! -number   Redo number'th previous command.
354
355       ! pattern   Redo last command that started with pattern.  See "o
356                   recallCommand", too.
357
358       !! cmd      Run cmd in a subprocess (reads from DB::IN, writes to
359                   DB::OUT) See "o shellBang", also.  Note that the user's
360                   current shell (well, their $ENV{SHELL} variable) will be
361                   used, which can interfere with proper interpretation of
362                   exit status or signal and coredump information.
363
364       source file Read and execute debugger commands from file.  file may
365                   itself contain "source" commands.
366
367       H -number   Display last n commands.  Only commands longer than one
368                   character are listed.  If number is omitted, list them all.
369
370       q or ^D     Quit.  ("quit" doesn't work for this, unless you've made an
371                   alias) This is the only supported way to exit the debugger,
372                   though typing "exit" twice might work.
373
374                   Set the "inhibit_exit" option to 0 if you want to be able
375                   to step off the end the script.  You may also need to set
376                   $finished to 0 if you want to step through global
377                   destruction.
378
379       R           Restart the debugger by "exec()"ing a new session.  We try
380                   to maintain your history across this, but internal settings
381                   and command-line options may be lost.
382
383                   The following setting are currently preserved: history,
384                   breakpoints, actions, debugger options, and the Perl
385                   command-line options -w, -I, and -e.
386
387       |dbcmd      Run the debugger command, piping DB::OUT into your current
388                   pager.
389
390       ||dbcmd     Same as "|dbcmd" but DB::OUT is temporarily "select"ed as
391                   well.
392
393       = [alias value]
394                   Define a command alias, like
395
396                       = quit q
397
398                   or list current aliases.
399
400       command     Execute command as a Perl statement.  A trailing semicolon
401                   will be supplied.  If the Perl statement would otherwise be
402                   confused for a Perl debugger, use a leading semicolon, too.
403
404       m expr      List which methods may be called on the result of the
405                   evaluated expression.  The expression may evaluated to a
406                   reference to a blessed object, or to a package name.
407
408       M           Displays all loaded modules and their versions
409
410       man [manpage]
411                   Despite its name, this calls your system's default
412                   documentation viewer on the given page, or on the viewer
413                   itself if manpage is omitted.  If that viewer is man, the
414                   current "Config" information is used to invoke man using
415                   the proper MANPATH or -M manpath option.  Failed lookups of
416                   the form "XXX" that match known manpages of the form
417                   perlXXX will be retried.  This lets you type "man debug" or
418                   "man op" from the debugger.
419
420                   On systems traditionally bereft of a usable man command,
421                   the debugger invokes perldoc.  Occasionally this
422                   determination is incorrect due to recalcitrant vendors or
423                   rather more felicitously, to enterprising users.  If you
424                   fall into either category, just manually set the
425                   $DB::doccmd variable to whatever viewer to view the Perl
426                   documentation on your system.  This may be set in an rc
427                   file, or through direct assignment.  We're still waiting
428                   for a working example of something along the lines of:
429
430                       $DB::doccmd = 'netscape -remote http://something.here/';
431
432   Configurable Options
433       The debugger has numerous options settable using the "o" command,
434       either interactively or from the environment or an rc file.  (./.perldb
435       or ~/.perldb under Unix.)
436
437       "recallCommand", "ShellBang"
438                   The characters used to recall command or spawn shell.  By
439                   default, both are set to "!", which is unfortunate.
440
441       "pager"     Program to use for output of pager-piped commands (those
442                   beginning with a "|" character.)  By default, $ENV{PAGER}
443                   will be used.  Because the debugger uses your current
444                   terminal characteristics for bold and underlining, if the
445                   chosen pager does not pass escape sequences through
446                   unchanged, the output of some debugger commands will not be
447                   readable when sent through the pager.
448
449       "tkRunning" Run Tk while prompting (with ReadLine).
450
451       "signalLevel", "warnLevel", "dieLevel"
452                   Level of verbosity.  By default, the debugger leaves your
453                   exceptions and warnings alone, because altering them can
454                   break correctly running programs.  It will attempt to print
455                   a message when uncaught INT, BUS, or SEGV signals arrive.
456                   (But see the mention of signals in BUGS below.)
457
458                   To disable this default safe mode, set these values to
459                   something higher than 0.  At a level of 1, you get
460                   backtraces upon receiving any kind of warning (this is
461                   often annoying) or exception (this is often valuable).
462                   Unfortunately, the debugger cannot discern fatal exceptions
463                   from non-fatal ones.  If "dieLevel" is even 1, then your
464                   non-fatal exceptions are also traced and unceremoniously
465                   altered if they came from "eval'ed" strings or from any
466                   kind of "eval" within modules you're attempting to load.
467                   If "dieLevel" is 2, the debugger doesn't care where they
468                   came from:  It usurps your exception handler and prints out
469                   a trace, then modifies all exceptions with its own
470                   embellishments.  This may perhaps be useful for some
471                   tracing purposes, but tends to hopelessly destroy any
472                   program that takes its exception handling seriously.
473
474       "AutoTrace" Trace mode (similar to "t" command, but can be put into
475                   "PERLDB_OPTS").
476
477       "LineInfo"  File or pipe to print line number info to.  If it is a pipe
478                   (say, "|visual_perl_db"), then a short message is used.
479                   This is the mechanism used to interact with a slave editor
480                   or visual debugger, such as the special "vi" or "emacs"
481                   hooks, or the "ddd" graphical debugger.
482
483       "inhibit_exit"
484                   If 0, allows stepping off the end of the script.
485
486       "PrintRet"  Print return value after "r" command if set (default).
487
488       "ornaments" Affects screen appearance of the command line (see
489                   Term::ReadLine).  There is currently no way to disable
490                   these, which can render some output illegible on some
491                   displays, or with some pagers.  This is considered a bug.
492
493       "frame"     Affects the printing of messages upon entry and exit from
494                   subroutines.  If "frame & 2" is false, messages are printed
495                   on entry only. (Printing on exit might be useful if
496                   interspersed with other messages.)
497
498                   If "frame & 4", arguments to functions are printed, plus
499                   context and caller info.  If "frame & 8", overloaded
500                   "stringify" and "tie"d "FETCH" is enabled on the printed
501                   arguments.  If "frame & 16", the return value from the
502                   subroutine is printed.
503
504                   The length at which the argument list is truncated is
505                   governed by the next option:
506
507       "maxTraceLen"
508                   Length to truncate the argument list when the "frame"
509                   option's bit 4 is set.
510
511       "windowSize"
512                   Change the size of code list window (default is 10 lines).
513
514       The following options affect what happens with "V", "X", and "x"
515       commands:
516
517       "arrayDepth", "hashDepth"
518                   Print only first N elements ('' for all).
519
520       "dumpDepth" Limit recursion depth to N levels when dumping structures.
521                   Negative values are interpreted as infinity.  Default:
522                   infinity.
523
524       "compactDump", "veryCompact"
525                   Change the style of array and hash output.  If
526                   "compactDump", short array may be printed on one line.
527
528       "globPrint" Whether to print contents of globs.
529
530       "DumpDBFiles"
531                   Dump arrays holding debugged files.
532
533       "DumpPackages"
534                   Dump symbol tables of packages.
535
536       "DumpReused"
537                   Dump contents of "reused" addresses.
538
539       "quote", "HighBit", "undefPrint"
540                   Change the style of string dump.  The default value for
541                   "quote" is "auto"; one can enable double-quotish or single-
542                   quotish format by setting it to """ or "'", respectively.
543                   By default, characters with their high bit set are printed
544                   verbatim.
545
546       "UsageOnly" Rudimentary per-package memory usage dump.  Calculates
547                   total size of strings found in variables in the package.
548                   This does not include lexicals in a module's file scope, or
549                   lost in closures.
550
551       After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
552       environment variable and parses this as the remainder of a "O ..."
553       line as one might enter at the debugger prompt.  You may place the
554       initialization options "TTY", "noTTY", "ReadLine", and "NonStop" there.
555
556       If your rc file contains:
557
558         parse_options("NonStop=1 LineInfo=db.out AutoTrace");
559
560       then your script will run without human intervention, putting trace
561       information into the file db.out.  (If you interrupt it, you'd better
562       reset "LineInfo" to /dev/tty if you expect to see anything.)
563
564       "TTY"       The TTY to use for debugging I/O.
565
566       "noTTY"     If set, the debugger goes into "NonStop" mode and will not
567                   connect to a TTY.  If interrupted (or if control goes to
568                   the debugger via explicit setting of $DB::signal or
569                   $DB::single from the Perl script), it connects to a TTY
570                   specified in the "TTY" option at startup, or to a tty found
571                   at runtime using the "Term::Rendezvous" module of your
572                   choice.
573
574                   This module should implement a method named "new" that
575                   returns an object with two methods: "IN" and "OUT".  These
576                   should return filehandles to use for debugging input and
577                   output correspondingly.  The "new" method should inspect an
578                   argument containing the value of $ENV{PERLDB_NOTTY} at
579                   startup, or "$ENV{HOME}/.perldbtty$$" otherwise.  This file
580                   is not inspected for proper ownership, so security hazards
581                   are theoretically possible.
582
583       "ReadLine"  If false, readline support in the debugger is disabled in
584                   order to debug applications that themselves use ReadLine.
585
586       "NonStop"   If set, the debugger goes into non-interactive mode until
587                   interrupted, or programmatically by setting $DB::signal or
588                   $DB::single.
589
590       Here's an example of using the $ENV{PERLDB_OPTS} variable:
591
592           $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
593
594       That will run the script myprogram without human intervention, printing
595       out the call tree with entry and exit points.  Note that "NonStop=1
596       frame=2" is equivalent to "N f=2", and that originally, options could
597       be uniquely abbreviated by the first letter (modulo the "Dump*"
598       options).  It is nevertheless recommended that you always spell them
599       out in full for legibility and future compatibility.
600
601       Other examples include
602
603           $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
604
605       which runs script non-interactively, printing info on each entry into a
606       subroutine and each executed line into the file named listing.  (If you
607       interrupt it, you would better reset "LineInfo" to something
608       "interactive"!)
609
610       Other examples include (using standard shell syntax to show environment
611       variable settings):
612
613         $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
614             perl -d myprogram )
615
616       which may be useful for debugging a program that uses "Term::ReadLine"
617       itself.  Do not forget to detach your shell from the TTY in the window
618       that corresponds to /dev/ttyXX, say, by issuing a command like
619
620         $ sleep 1000000
621
622       See "Debugger Internals" in perldebguts for details.
623
624   Debugger input/output
625       Prompt  The debugger prompt is something like
626
627                   DB<8>
628
629               or even
630
631                   DB<<17>>
632
633               where that number is the command number, and which you'd use to
634               access with the built-in csh-like history mechanism.  For
635               example, "!17" would repeat command number 17.  The depth of
636               the angle brackets indicates the nesting depth of the debugger.
637               You could get more than one set of brackets, for example, if
638               you'd already at a breakpoint and then printed the result of a
639               function call that itself has a breakpoint, or you step into an
640               expression via "s/n/t expression" command.
641
642       Multiline commands
643               If you want to enter a multi-line command, such as a subroutine
644               definition with several statements or a format, escape the
645               newline that would normally end the debugger command with a
646               backslash.  Here's an example:
647
648                     DB<1> for (1..4) {         \
649                     cont:     print "ok\n";   \
650                     cont: }
651                     ok
652                     ok
653                     ok
654                     ok
655
656               Note that this business of escaping a newline is specific to
657               interactive commands typed into the debugger.
658
659       Stack backtrace
660               Here's an example of what a stack backtrace via "T" command
661               might look like:
662
663                   $ = main::infested called from file `Ambulation.pm' line 10
664                   @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
665                   $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
666
667               The left-hand character up there indicates the context in which
668               the function was called, with "$" and "@" meaning scalar or
669               list contexts respectively, and "." meaning void context (which
670               is actually a sort of scalar context).  The display above says
671               that you were in the function "main::infested" when you ran the
672               stack dump, and that it was called in scalar context from line
673               10 of the file Ambulation.pm, but without any arguments at all,
674               meaning it was called as &infested.  The next stack frame shows
675               that the function "Ambulation::legs" was called in list context
676               from the camel_flea file with four arguments.  The last stack
677               frame shows that "main::pests" was called in scalar context,
678               also from camel_flea, but from line 4.
679
680               If you execute the "T" command from inside an active "use"
681               statement, the backtrace will contain both a "require" frame
682               and an "eval") frame.
683
684       Line Listing Format
685               This shows the sorts of output the "l" command can produce:
686
687                   DB<<13>> l
688                 101:                @i{@i} = ();
689                 102:b               @isa{@i,$pack} = ()
690                 103                     if(exists $i{$prevpack} || exists $isa{$pack});
691                 104             }
692                 105
693                 106             next
694                 107==>              if(exists $isa{$pack});
695                 108
696                 109:a           if ($extra-- > 0) {
697                 110:                %isa = ($pack,1);
698
699               Breakable lines are marked with ":".  Lines with breakpoints
700               are marked by "b" and those with actions by "a".  The line
701               that's about to be executed is marked by "==>".
702
703               Please be aware that code in debugger listings may not look the
704               same as your original source code.  Line directives and
705               external source filters can alter the code before Perl sees it,
706               causing code to move from its original positions or take on
707               entirely different forms.
708
709       Frame listing
710               When the "frame" option is set, the debugger would print
711               entered (and optionally exited) subroutines in different
712               styles.  See perldebguts for incredibly long examples of these.
713
714   Debugging compile-time statements
715       If you have compile-time executable statements (such as code within
716       BEGIN, UNITCHECK and CHECK blocks or "use" statements), these will not
717       be stopped by debugger, although "require"s and INIT blocks will, and
718       compile-time statements can be traced with "AutoTrace" option set in
719       "PERLDB_OPTS").  From your own Perl code, however, you can transfer
720       control back to the debugger using the following statement, which is
721       harmless if the debugger is not running:
722
723           $DB::single = 1;
724
725       If you set $DB::single to 2, it's equivalent to having just typed the
726       "n" command, whereas a value of 1 means the "s" command.  The
727       $DB::trace  variable should be set to 1 to simulate having typed the
728       "t" command.
729
730       Another way to debug compile-time code is to start the debugger, set a
731       breakpoint on the load of some module:
732
733           DB<7> b load f:/perllib/lib/Carp.pm
734         Will stop on load of `f:/perllib/lib/Carp.pm'.
735
736       and then restart the debugger using the "R" command (if possible).  One
737       can use "b compile subname" for the same purpose.
738
739   Debugger Customization
740       The debugger probably contains enough configuration hooks that you
741       won't ever have to modify it yourself.  You may change the behaviour of
742       debugger from within the debugger using its "o" command, from the
743       command line via the "PERLDB_OPTS" environment variable, and from
744       customization files.
745
746       You can do some customization by setting up a .perldb file, which
747       contains initialization code.  For instance, you could make aliases
748       like these (the last one is one people expect to be there):
749
750           $DB::alias{'len'}  = 's/^len(.*)/p length($1)/';
751           $DB::alias{'stop'} = 's/^stop (at|in)/b/';
752           $DB::alias{'ps'}   = 's/^ps\b/p scalar /';
753           $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
754
755       You can change options from .perldb by using calls like this one;
756
757           parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
758
759       The code is executed in the package "DB".  Note that .perldb is
760       processed before processing "PERLDB_OPTS".  If .perldb defines the
761       subroutine "afterinit", that function is called after debugger
762       initialization ends.  .perldb may be contained in the current
763       directory, or in the home directory.  Because this file is sourced in
764       by Perl and may contain arbitrary commands, for security reasons, it
765       must be owned by the superuser or the current user, and writable by no
766       one but its owner.
767
768       You can mock TTY input to debugger by adding arbitrary commands to
769       @DB::typeahead. For example, your .perldb file might contain:
770
771           sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
772
773       Which would attempt to set breakpoints on lines 4 and 6 immediately
774       after debugger initialization. Note that @DB::typeahead is not a
775       supported interface and is subject to change in future releases.
776
777       If you want to modify the debugger, copy perl5db.pl from the Perl
778       library to another name and hack it to your heart's content.  You'll
779       then want to set your "PERL5DB" environment variable to say something
780       like this:
781
782           BEGIN { require "myperl5db.pl" }
783
784       As a last resort, you could also use "PERL5DB" to customize the
785       debugger by directly setting internal variables or calling debugger
786       functions.
787
788       Note that any variables and functions that are not documented in this
789       document (or in perldebguts) are considered for internal use only, and
790       as such are subject to change without notice.
791
792   Readline Support / History in the debugger
793       As shipped, the only command-line history supplied is a simplistic one
794       that checks for leading exclamation points.  However, if you install
795       the Term::ReadKey and Term::ReadLine modules from CPAN (such as
796       Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full
797       editing capabilities much like GNU readline(3) provides.  Look for
798       these in the modules/by-module/Term directory on CPAN.  These do not
799       support normal vi command-line editing, however.
800
801       A rudimentary command-line completion is also available.
802       Unfortunately, the names of lexical variables are not available for
803       completion.
804
805       Without Readline support you may see the symbols "^[[A", "^[[C",
806       "^[[B", "^[[D"", "^H", ... when using the arrow keys and/or the
807       backspace key.
808
809   Editor Support for Debugging
810       If you have the FSF's version of emacs installed on your system, it can
811       interact with the Perl debugger to provide an integrated software
812       development environment reminiscent of its interactions with C
813       debuggers.
814
815       Perl comes with a start file for making emacs act like a syntax-
816       directed editor that understands (some of) Perl's syntax.  Look in the
817       emacs directory of the Perl source distribution.
818
819       A similar setup by Tom Christiansen for interacting with any vendor-
820       shipped vi and the X11 window system is also available.  This works
821       similarly to the integrated multiwindow support that emacs provides,
822       where the debugger drives the editor.  At the time of this writing,
823       however, that tool's eventual location in the Perl distribution was
824       uncertain.
825
826       Users of vi should also look into vim and gvim, the mousey and windy
827       version, for coloring of Perl keywords.
828
829       Note that only perl can truly parse Perl, so all such CASE tools fall
830       somewhat short of the mark, especially if you don't program your Perl
831       as a C programmer might.
832
833   The Perl Profiler
834       If you wish to supply an alternative debugger for Perl to run, just
835       invoke your script with a colon and a package argument given to the -d
836       flag.  The most popular alternative debuggers for Perl is the Perl
837       profiler.  Devel::DProf is included with the standard Perl
838       distribution.  To profile your Perl program in the file mycode.pl, just
839       type:
840
841           $ perl -d:DProf mycode.pl
842
843       When the script terminates the profiler will dump the profile
844       information to a file called tmon.out.  A tool like dprofpp, also
845       supplied with the standard Perl distribution, can be used to interpret
846       the information in that profile.  More powerful profilers, such as
847       "Devel::NYTProf" are available from the CPAN.
848

Debugging regular expressions

850       "use re 'debug'" enables you to see the gory details of how the Perl
851       regular expression engine works. In order to understand this typically
852       voluminous output, one must not only have some idea about how regular
853       expression matching works in general, but also know how Perl's regular
854       expressions are internally compiled into an automaton. These matters
855       are explored in some detail in "Debugging regular expressions" in
856       perldebguts.
857

Debugging memory usage

859       Perl contains internal support for reporting its own memory usage, but
860       this is a fairly advanced concept that requires some understanding of
861       how memory allocation works.  See "Debugging Perl memory usage" in
862       perldebguts for the details.
863

SEE ALSO

865       You did try the -w switch, didn't you?
866
867       perldebtut, perldebguts, re, DB, Devel::DProf, dprofpp, Dumpvalue, and
868       perlrun.
869
870       When debugging a script that uses #! and is thus normally found in
871       $PATH, the -S option causes perl to search $PATH for it, so you don't
872       have to type the path or "which $scriptname".
873
874         $ perl -Sd foo.pl
875

BUGS

877       You cannot get stack frame information or in any fashion debug
878       functions that were not compiled by Perl, such as those from C or C++
879       extensions.
880
881       If you alter your @_ arguments in a subroutine (such as with "shift" or
882       "pop"), the stack backtrace will not show the original values.
883
884       The debugger does not currently work in conjunction with the -W
885       command-line switch, because it itself is not free of warnings.
886
887       If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing
888       from your keyboard or a socket) and haven't set up your own $SIG{INT}
889       handler, then you won't be able to CTRL-C your way back to the
890       debugger, because the debugger's own $SIG{INT} handler doesn't
891       understand that it needs to raise an exception to longjmp(3) out of
892       slow syscalls.
893
894
895
896perl v5.10.1                      2009-06-26                      PERLDEBUG(1)
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