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 "use strict;" and "use warnings;"?
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
14       If you're looking for the nitty gritty details of how the debugger is
15       implemented, you may prefer to read perldebguts.
16
17       For in-depth technical usage details, see perl5db.pl, the documentation
18       of the debugger itself.
19

The Perl Debugger

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
29           $ perl -d -e 42
30
31       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
39       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
46       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
50       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
56       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
63   Calling the Debugger
64       There are several ways to call the debugger:
65
66       perl -d program_name
67           On the given program identified by "program_name".
68
69       perl -d -e 0
70           Interactively supply an arbitrary "expression" using "-e".
71
72       perl -d:ptkdb program_name
73           Debug a given program via the "Devel::ptkdb" GUI.
74
75       perl -dt threaded_program_name
76           Debug a given program using threads (experimental).
77
78       If Perl is called with the "-d" switch, the variable $^P will hold a
79       true value. This is useful if you need to know if your code is running
80       under the debugger:
81
82           if ( $^P ) {
83               # running under the debugger
84           }
85
86       See "$^P" in perlvar for more information on the variable.
87
88   Debugger Commands
89       The interactive debugger understands the following commands:
90
91       h           Prints out a summary help message
92
93       h [command] Prints out a help message for the given debugger command.
94
95       h h         The special argument of "h h" produces the entire help
96                   page, which is quite long.
97
98                   If the output of the "h h" command (or any command, for
99                   that matter) scrolls past your screen, precede the command
100                   with a leading pipe symbol so that it's run through your
101                   pager, as in
102
103                       DB> |h h
104
105                   You may change the pager which is used via "o pager=..."
106                   command.
107
108       p expr      Same as "print {$DB::OUT} expr" in the current package.  In
109                   particular, because this is just Perl's own "print"
110                   function, this means that nested data structures and
111                   objects are not dumped, unlike with the "x" command.
112
113                   The "DB::OUT" filehandle is opened to /dev/tty, regardless
114                   of where STDOUT may be redirected to.
115
116       x [maxdepth] expr
117                   Evaluates its expression in list context and dumps out the
118                   result in a pretty-printed fashion.  Nested data structures
119                   are printed out recursively, unlike the real "print"
120                   function in Perl.  When dumping hashes, you'll probably
121                   prefer 'x \%h' rather than 'x %h'.  See Dumpvalue if you'd
122                   like to do this yourself.
123
124                   The output format is governed by multiple options described
125                   under "Configurable Options".
126
127                   If the "maxdepth" is included, it must be a numeral N; the
128                   value is dumped only N levels deep, as if the "dumpDepth"
129                   option had been temporarily set to N.
130
131       V [pkg [vars]]
132                   Display all (or some) variables in package (defaulting to
133                   "main") using a data pretty-printer (hashes show their keys
134                   and values so you see what's what, control characters are
135                   made printable, etc.).  Make sure you don't put the type
136                   specifier (like "$") there, just the symbol names, like
137                   this:
138
139                       V DB filename line
140
141                   Use "~pattern" and "!pattern" for positive and negative
142                   regexes.
143
144                   This is similar to calling the "x" command on each
145                   applicable var.
146
147       X [vars]    Same as "V currentpackage [vars]".
148
149       y [level [vars]]
150                   Display all (or some) lexical variables (mnemonic: "mY"
151                   variables) in the current scope or level scopes higher.
152                   You can limit the variables that you see with vars which
153                   works exactly as it does for the "V" and "X" commands.
154                   Requires the "PadWalker" module version 0.08 or higher;
155                   will warn if this isn't installed.  Output is pretty-
156                   printed in the same style as for "V" and the format is
157                   controlled by the same options.
158
159       T           Produce a stack backtrace.  See below for details on its
160                   output.
161
162       s [expr]    Single step.  Executes until the beginning of another
163                   statement, descending into subroutine calls.  If an
164                   expression is supplied that includes function calls, it too
165                   will be single-stepped.
166
167       n [expr]    Next.  Executes over subroutine calls, until the beginning
168                   of the next statement.  If an expression is supplied that
169                   includes function calls, those functions will be executed
170                   with stops before each statement.
171
172       r           Continue until the return from the current subroutine.
173                   Dump the return value if the "PrintRet" option is set
174                   (default).
175
176       <CR>        Repeat last "n" or "s" command.
177
178       c [line|sub]
179                   Continue, optionally inserting a one-time-only breakpoint
180                   at the specified line or subroutine.
181
182       l           List next window of lines.
183
184       l min+incr  List "incr+1" lines starting at "min".
185
186       l min-max   List lines "min" through "max".  "l -" is synonymous to
187                   "-".
188
189       l line      List a single line.
190
191       l subname   List first window of lines from subroutine.  subname may be
192                   a variable that contains a code reference.
193
194       -           List previous window of lines.
195
196       v [line]    View a few lines of code around the current line.
197
198       .           Return the internal debugger pointer to the line last
199                   executed, and print out that line.
200
201       f filename  Switch to viewing a different file or "eval" statement.  If
202                   filename is not a full pathname found in the values of
203                   %INC, it is considered a regex.
204
205                   "eval"ed strings (when accessible) are considered to be
206                   filenames: "f (eval 7)" and "f eval 7\b" access the body of
207                   the 7th "eval"ed string (in the order of execution).  The
208                   bodies of the currently executed "eval" and of "eval"ed
209                   strings that define subroutines are saved and thus
210                   accessible.
211
212       /pattern/   Search forwards for pattern (a Perl regex); final / is
213                   optional.  The search is case-insensitive by default.
214
215       ?pattern?   Search backwards for pattern; final ? is optional.  The
216                   search is case-insensitive by default.
217
218       L [abw]     List (default all) actions, breakpoints and watch
219                   expressions
220
221       S [[!]regex]
222                   List subroutine names [not] matching the regex.
223
224       t [n]       Toggle trace mode (see also the "AutoTrace" option).
225                   Optional argument is the maximum number of levels to trace
226                   below the current one; anything deeper than that will be
227                   silent.
228
229       t [n] expr  Trace through execution of "expr".  Optional first argument
230                   is the maximum number of levels to trace below the current
231                   one; anything deeper than that will be silent.  See "Frame
232                   Listing Output Examples" in perldebguts for examples.
233
234       b           Sets breakpoint on current line
235
236       b [line] [condition]
237                   Set a breakpoint before the given line.  If a condition is
238                   specified, it's evaluated each time the statement is
239                   reached: a breakpoint is taken only if the condition is
240                   true.  Breakpoints may only be set on lines that begin an
241                   executable statement.  Conditions don't use "if":
242
243                       b 237 $x > 30
244                       b 237 ++$count237 < 11
245                       b 33 /pattern/i
246
247                   If the line number is ".", sets a breakpoint on the current
248                   line:
249
250                       b . $n > 100
251
252       b [file]:[line] [condition]
253                   Set a breakpoint before the given line in a (possibly
254                   different) file.  If a condition is specified, it's
255                   evaluated each time the statement is reached: a breakpoint
256                   is taken only if the condition is true.  Breakpoints may
257                   only be set on lines that begin an executable statement.
258                   Conditions don't use "if":
259
260                       b lib/MyModule.pm:237 $x > 30
261                       b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
262
263       b subname [condition]
264                   Set a breakpoint before the first line of the named
265                   subroutine.  subname may be a variable containing a code
266                   reference (in this case condition is not supported).
267
268       b postpone subname [condition]
269                   Set a breakpoint at first line of subroutine after it is
270                   compiled.
271
272       b load filename
273                   Set a breakpoint before the first executed line of the
274                   filename, which should be a full pathname found amongst the
275                   %INC values.
276
277       b compile subname
278                   Sets a breakpoint before the first statement executed after
279                   the specified subroutine is compiled.
280
281       B line      Delete a breakpoint from the specified line.
282
283       B *         Delete all installed breakpoints.
284
285       disable [file]:[line]
286                   Disable the breakpoint so it won't stop the execution of
287                   the program.  Breakpoints are enabled by default and can be
288                   re-enabled using the "enable" command.
289
290       disable [line]
291                   Disable the breakpoint so it won't stop the execution of
292                   the program.  Breakpoints are enabled by default and can be
293                   re-enabled using the "enable" command.
294
295                   This is done for a breakpoint in the current file.
296
297       enable [file]:[line]
298                   Enable the breakpoint so it will stop the execution of the
299                   program.
300
301       enable [line]
302                   Enable the breakpoint so it will stop the execution of the
303                   program.
304
305                   This is done for a breakpoint in the current file.
306
307       a [line] command
308                   Set an action to be done before the line is executed.  If
309                   line is omitted, set an action on the line about to be
310                   executed.  The sequence of steps taken by the debugger is
311
312                     1. check for a breakpoint at this line
313                     2. print the line if necessary (tracing)
314                     3. do any actions associated with that line
315                     4. prompt user if at a breakpoint or in single-step
316                     5. evaluate line
317
318                   For example, this will print out $foo every time line 53 is
319                   passed:
320
321                       a 53 print "DB FOUND $foo\n"
322
323       A line      Delete an action from the specified line.
324
325       A *         Delete all installed actions.
326
327       w expr      Add a global watch-expression. Whenever a watched global
328                   changes the debugger will stop and display the old and new
329                   values.
330
331       W expr      Delete watch-expression
332
333       W *         Delete all watch-expressions.
334
335       o           Display all options.
336
337       o booloption ...
338                   Set each listed Boolean option to the value 1.
339
340       o anyoption? ...
341                   Print out the value of one or more options.
342
343       o option=value ...
344                   Set the value of one or more options.  If the value has
345                   internal whitespace, it should be quoted.  For example, you
346                   could set "o pager="less -MQeicsNfr"" to call less with
347                   those specific options.  You may use either single or
348                   double quotes, but if you do, you must escape any embedded
349                   instances of same sort of quote you began with, as well as
350                   any escaping any escapes that immediately precede that
351                   quote but which are not meant to escape the quote itself.
352                   In other words, you follow single-quoting rules
353                   irrespective of the quote; eg: "o option='this isn\'t bad'"
354                   or "o option="She said, \"Isn't it?\""".
355
356                   For historical reasons, the "=value" is optional, but
357                   defaults to 1 only where it is safe to do so--that is,
358                   mostly for Boolean options.  It is always better to assign
359                   a specific value using "=".  The "option" can be
360                   abbreviated, but for clarity probably should not be.
361                   Several options can be set together.  See "Configurable
362                   Options" for a list of these.
363
364       < ?         List out all pre-prompt Perl command actions.
365
366       < [ command ]
367                   Set an action (Perl command) to happen before every
368                   debugger prompt.  A multi-line command may be entered by
369                   backslashing the newlines.
370
371       < *         Delete all pre-prompt Perl command actions.
372
373       << command  Add an action (Perl command) to happen before every
374                   debugger prompt.  A multi-line command may be entered by
375                   backwhacking the newlines.
376
377       > ?         List out post-prompt Perl command actions.
378
379       > command   Set an action (Perl command) to happen after the prompt
380                   when you've just given a command to return to executing the
381                   script.  A multi-line command may be entered by
382                   backslashing the newlines (we bet you couldn't have guessed
383                   this by now).
384
385       > *         Delete all post-prompt Perl command actions.
386
387       >> command  Adds an action (Perl command) to happen after the prompt
388                   when you've just given a command to return to executing the
389                   script.  A multi-line command may be entered by
390                   backslashing the newlines.
391
392       { ?         List out pre-prompt debugger commands.
393
394       { [ command ]
395                   Set an action (debugger command) to happen before every
396                   debugger prompt.  A multi-line command may be entered in
397                   the customary fashion.
398
399                   Because this command is in some senses new, a warning is
400                   issued if you appear to have accidentally entered a block
401                   instead.  If that's what you mean to do, write it as with
402                   ";{ ... }" or even "do { ... }".
403
404       { *         Delete all pre-prompt debugger commands.
405
406       {{ command  Add an action (debugger command) to happen before every
407                   debugger prompt.  A multi-line command may be entered, if
408                   you can guess how: see above.
409
410       ! number    Redo a previous command (defaults to the previous command).
411
412       ! -number   Redo number'th previous command.
413
414       ! pattern   Redo last command that started with pattern.  See "o
415                   recallCommand", too.
416
417       !! cmd      Run cmd in a subprocess (reads from DB::IN, writes to
418                   DB::OUT) See "o shellBang", also.  Note that the user's
419                   current shell (well, their $ENV{SHELL} variable) will be
420                   used, which can interfere with proper interpretation of
421                   exit status or signal and coredump information.
422
423       source file Read and execute debugger commands from file.  file may
424                   itself contain "source" commands.
425
426       H -number   Display last n commands.  Only commands longer than one
427                   character are listed.  If number is omitted, list them all.
428
429       q or ^D     Quit.  ("quit" doesn't work for this, unless you've made an
430                   alias) This is the only supported way to exit the debugger,
431                   though typing "exit" twice might work.
432
433                   Set the "inhibit_exit" option to 0 if you want to be able
434                   to step off the end the script.  You may also need to set
435                   $finished to 0 if you want to step through global
436                   destruction.
437
438       R           Restart the debugger by "exec()"ing a new session.  We try
439                   to maintain your history across this, but internal settings
440                   and command-line options may be lost.
441
442                   The following setting are currently preserved: history,
443                   breakpoints, actions, debugger options, and the Perl
444                   command-line options -w, -I, and -e.
445
446       |dbcmd      Run the debugger command, piping DB::OUT into your current
447                   pager.
448
449       ||dbcmd     Same as "|dbcmd" but DB::OUT is temporarily "select"ed as
450                   well.
451
452       = [alias value]
453                   Define a command alias, like
454
455                       = quit q
456
457                   or list current aliases.
458
459       command     Execute command as a Perl statement.  A trailing semicolon
460                   will be supplied.  If the Perl statement would otherwise be
461                   confused for a Perl debugger, use a leading semicolon, too.
462
463       m expr      List which methods may be called on the result of the
464                   evaluated expression.  The expression may evaluated to a
465                   reference to a blessed object, or to a package name.
466
467       M           Display all loaded modules and their versions.
468
469       man [manpage]
470                   Despite its name, this calls your system's default
471                   documentation viewer on the given page, or on the viewer
472                   itself if manpage is omitted.  If that viewer is man, the
473                   current "Config" information is used to invoke man using
474                   the proper MANPATH or -M manpath option.  Failed lookups of
475                   the form "XXX" that match known manpages of the form
476                   perlXXX will be retried.  This lets you type "man debug" or
477                   "man op" from the debugger.
478
479                   On systems traditionally bereft of a usable man command,
480                   the debugger invokes perldoc.  Occasionally this
481                   determination is incorrect due to recalcitrant vendors or
482                   rather more felicitously, to enterprising users.  If you
483                   fall into either category, just manually set the
484                   $DB::doccmd variable to whatever viewer to view the Perl
485                   documentation on your system.  This may be set in an rc
486                   file, or through direct assignment.  We're still waiting
487                   for a working example of something along the lines of:
488
489                       $DB::doccmd = 'netscape -remote http://something.here/';
490
491   Configurable Options
492       The debugger has numerous options settable using the "o" command,
493       either interactively or from the environment or an rc file. The file is
494       named ./.perldb or ~/.perldb under Unix with /dev/tty, perldb.ini
495       otherwise.
496
497       "recallCommand", "ShellBang"
498                   The characters used to recall a command or spawn a shell.
499                   By default, both are set to "!", which is unfortunate.
500
501       "pager"     Program to use for output of pager-piped commands (those
502                   beginning with a "|" character.)  By default, $ENV{PAGER}
503                   will be used.  Because the debugger uses your current
504                   terminal characteristics for bold and underlining, if the
505                   chosen pager does not pass escape sequences through
506                   unchanged, the output of some debugger commands will not be
507                   readable when sent through the pager.
508
509       "tkRunning" Run Tk while prompting (with ReadLine).
510
511       "signalLevel", "warnLevel", "dieLevel"
512                   Level of verbosity.  By default, the debugger leaves your
513                   exceptions and warnings alone, because altering them can
514                   break correctly running programs.  It will attempt to print
515                   a message when uncaught INT, BUS, or SEGV signals arrive.
516                   (But see the mention of signals in "BUGS" below.)
517
518                   To disable this default safe mode, set these values to
519                   something higher than 0.  At a level of 1, you get
520                   backtraces upon receiving any kind of warning (this is
521                   often annoying) or exception (this is often valuable).
522                   Unfortunately, the debugger cannot discern fatal exceptions
523                   from non-fatal ones.  If "dieLevel" is even 1, then your
524                   non-fatal exceptions are also traced and unceremoniously
525                   altered if they came from "eval'ed" strings or from any
526                   kind of "eval" within modules you're attempting to load.
527                   If "dieLevel" is 2, the debugger doesn't care where they
528                   came from:  It usurps your exception handler and prints out
529                   a trace, then modifies all exceptions with its own
530                   embellishments.  This may perhaps be useful for some
531                   tracing purposes, but tends to hopelessly destroy any
532                   program that takes its exception handling seriously.
533
534       "AutoTrace" Trace mode (similar to "t" command, but can be put into
535                   "PERLDB_OPTS").
536
537       "LineInfo"  File or pipe to print line number info to.  If it is a pipe
538                   (say, "|visual_perl_db"), then a short message is used.
539                   This is the mechanism used to interact with a client editor
540                   or visual debugger, such as the special "vi" or "emacs"
541                   hooks, or the "ddd" graphical debugger.
542
543       "inhibit_exit"
544                   If 0, allows stepping off the end of the script.
545
546       "PrintRet"  Print return value after "r" command if set (default).
547
548       "ornaments" Affects screen appearance of the command line (see
549                   Term::ReadLine).  There is currently no way to disable
550                   these, which can render some output illegible on some
551                   displays, or with some pagers.  This is considered a bug.
552
553       "frame"     Affects the printing of messages upon entry and exit from
554                   subroutines.  If "frame & 2" is false, messages are printed
555                   on entry only. (Printing on exit might be useful if
556                   interspersed with other messages.)
557
558                   If "frame & 4", arguments to functions are printed, plus
559                   context and caller info.  If "frame & 8", overloaded
560                   "stringify" and "tie"d "FETCH" is enabled on the printed
561                   arguments.  If "frame & 16", the return value from the
562                   subroutine is printed.
563
564                   The length at which the argument list is truncated is
565                   governed by the next option:
566
567       "maxTraceLen"
568                   Length to truncate the argument list when the "frame"
569                   option's bit 4 is set.
570
571       "windowSize"
572                   Change the size of code list window (default is 10 lines).
573
574       The following options affect what happens with "V", "X", and "x"
575       commands:
576
577       "arrayDepth", "hashDepth"
578                   Print only first N elements ('' for all).
579
580       "dumpDepth" Limit recursion depth to N levels when dumping structures.
581                   Negative values are interpreted as infinity.  Default:
582                   infinity.
583
584       "compactDump", "veryCompact"
585                   Change the style of array and hash output.  If
586                   "compactDump", short array may be printed on one line.
587
588       "globPrint" Whether to print contents of globs.
589
590       "DumpDBFiles"
591                   Dump arrays holding debugged files.
592
593       "DumpPackages"
594                   Dump symbol tables of packages.
595
596       "DumpReused"
597                   Dump contents of "reused" addresses.
598
599       "quote", "HighBit", "undefPrint"
600                   Change the style of string dump.  The default value for
601                   "quote" is "auto"; one can enable double-quotish or single-
602                   quotish format by setting it to """ or "'", respectively.
603                   By default, characters with their high bit set are printed
604                   verbatim.
605
606       "UsageOnly" Rudimentary per-package memory usage dump.  Calculates
607                   total size of strings found in variables in the package.
608                   This does not include lexicals in a module's file scope, or
609                   lost in closures.
610
611       "HistFile"  The path of the file from which the history (assuming a
612                   usable Term::ReadLine backend) will be read on the
613                   debugger's startup, and to which it will be saved on
614                   shutdown (for persistence across sessions). Similar in
615                   concept to Bash's ".bash_history" file.
616
617       "HistSize"  The count of the saved lines in the history (assuming
618                   "HistFile" above).
619
620       After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
621       environment variable and parses this as the remainder of a "O ..."
622       line as one might enter at the debugger prompt.  You may place the
623       initialization options "TTY", "noTTY", "ReadLine", and "NonStop" there.
624
625       If your rc file contains:
626
627         parse_options("NonStop=1 LineInfo=db.out AutoTrace");
628
629       then your script will run without human intervention, putting trace
630       information into the file db.out.  (If you interrupt it, you'd better
631       reset "LineInfo" to /dev/tty if you expect to see anything.)
632
633       "TTY"       The TTY to use for debugging I/O.
634
635       "noTTY"     If set, the debugger goes into "NonStop" mode and will not
636                   connect to a TTY.  If interrupted (or if control goes to
637                   the debugger via explicit setting of $DB::signal or
638                   $DB::single from the Perl script), it connects to a TTY
639                   specified in the "TTY" option at startup, or to a tty found
640                   at runtime using the "Term::Rendezvous" module of your
641                   choice.
642
643                   This module should implement a method named "new" that
644                   returns an object with two methods: "IN" and "OUT".  These
645                   should return filehandles to use for debugging input and
646                   output correspondingly.  The "new" method should inspect an
647                   argument containing the value of $ENV{PERLDB_NOTTY} at
648                   startup, or "$ENV{HOME}/.perldbtty$$" otherwise.  This file
649                   is not inspected for proper ownership, so security hazards
650                   are theoretically possible.
651
652       "ReadLine"  If false, readline support in the debugger is disabled in
653                   order to debug applications that themselves use ReadLine.
654
655       "NonStop"   If set, the debugger goes into non-interactive mode until
656                   interrupted, or programmatically by setting $DB::signal or
657                   $DB::single.
658
659       Here's an example of using the $ENV{PERLDB_OPTS} variable:
660
661           $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
662
663       That will run the script myprogram without human intervention, printing
664       out the call tree with entry and exit points.  Note that "NonStop=1
665       frame=2" is equivalent to "N f=2", and that originally, options could
666       be uniquely abbreviated by the first letter (modulo the "Dump*"
667       options).  It is nevertheless recommended that you always spell them
668       out in full for legibility and future compatibility.
669
670       Other examples include
671
672           $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
673
674       which runs script non-interactively, printing info on each entry into a
675       subroutine and each executed line into the file named listing.  (If you
676       interrupt it, you would better reset "LineInfo" to something
677       "interactive"!)
678
679       Other examples include (using standard shell syntax to show environment
680       variable settings):
681
682         $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
683             perl -d myprogram )
684
685       which may be useful for debugging a program that uses "Term::ReadLine"
686       itself.  Do not forget to detach your shell from the TTY in the window
687       that corresponds to /dev/ttyXX, say, by issuing a command like
688
689         $ sleep 1000000
690
691       See "Debugger Internals" in perldebguts for details.
692
693   Debugger Input/Output
694       Prompt  The debugger prompt is something like
695
696                   DB<8>
697
698               or even
699
700                   DB<<17>>
701
702               where that number is the command number, and which you'd use to
703               access with the built-in csh-like history mechanism.  For
704               example, "!17" would repeat command number 17.  The depth of
705               the angle brackets indicates the nesting depth of the debugger.
706               You could get more than one set of brackets, for example, if
707               you'd already at a breakpoint and then printed the result of a
708               function call that itself has a breakpoint, or you step into an
709               expression via "s/n/t expression" command.
710
711       Multiline commands
712               If you want to enter a multi-line command, such as a subroutine
713               definition with several statements or a format, escape the
714               newline that would normally end the debugger command with a
715               backslash.  Here's an example:
716
717                     DB<1> for (1..4) {         \
718                     cont:     print "ok\n";   \
719                     cont: }
720                     ok
721                     ok
722                     ok
723                     ok
724
725               Note that this business of escaping a newline is specific to
726               interactive commands typed into the debugger.
727
728       Stack backtrace
729               Here's an example of what a stack backtrace via "T" command
730               might look like:
731
732                $ = main::infested called from file 'Ambulation.pm' line 10
733                @ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea'
734                                                                         line 7
735                $ = main::pests('bactrian', 4) called from file 'camel_flea'
736                                                                         line 4
737
738               The left-hand character up there indicates the context in which
739               the function was called, with "$" and "@" meaning scalar or
740               list contexts respectively, and "." meaning void context (which
741               is actually a sort of scalar context).  The display above says
742               that you were in the function "main::infested" when you ran the
743               stack dump, and that it was called in scalar context from line
744               10 of the file Ambulation.pm, but without any arguments at all,
745               meaning it was called as &infested.  The next stack frame shows
746               that the function "Ambulation::legs" was called in list context
747               from the camel_flea file with four arguments.  The last stack
748               frame shows that "main::pests" was called in scalar context,
749               also from camel_flea, but from line 4.
750
751               If you execute the "T" command from inside an active "use"
752               statement, the backtrace will contain both a "require" frame
753               and an "eval" frame.
754
755       Line Listing Format
756               This shows the sorts of output the "l" command can produce:
757
758                  DB<<13>> l
759                101:        @i{@i} = ();
760                102:b       @isa{@i,$pack} = ()
761                103             if(exists $i{$prevpack} || exists $isa{$pack});
762                104     }
763                105
764                106     next
765                107==>      if(exists $isa{$pack});
766                108
767                109:a   if ($extra-- > 0) {
768                110:        %isa = ($pack,1);
769
770               Breakable lines are marked with ":".  Lines with breakpoints
771               are marked by "b" and those with actions by "a".  The line
772               that's about to be executed is marked by "==>".
773
774               Please be aware that code in debugger listings may not look the
775               same as your original source code.  Line directives and
776               external source filters can alter the code before Perl sees it,
777               causing code to move from its original positions or take on
778               entirely different forms.
779
780       Frame listing
781               When the "frame" option is set, the debugger would print
782               entered (and optionally exited) subroutines in different
783               styles.  See perldebguts for incredibly long examples of these.
784
785   Debugging Compile-Time Statements
786       If you have compile-time executable statements (such as code within
787       BEGIN, UNITCHECK and CHECK blocks or "use" statements), these will not
788       be stopped by debugger, although "require"s and INIT blocks will, and
789       compile-time statements can be traced with the "AutoTrace" option set
790       in "PERLDB_OPTS").  From your own Perl code, however, you can transfer
791       control back to the debugger using the following statement, which is
792       harmless if the debugger is not running:
793
794           $DB::single = 1;
795
796       If you set $DB::single to 2, it's equivalent to having just typed the
797       "n" command, whereas a value of 1 means the "s" command.  The
798       $DB::trace  variable should be set to 1 to simulate having typed the
799       "t" command.
800
801       Another way to debug compile-time code is to start the debugger, set a
802       breakpoint on the load of some module:
803
804           DB<7> b load f:/perllib/lib/Carp.pm
805         Will stop on load of 'f:/perllib/lib/Carp.pm'.
806
807       and then restart the debugger using the "R" command (if possible).  One
808       can use "b compile subname" for the same purpose.
809
810   Debugger Customization
811       The debugger probably contains enough configuration hooks that you
812       won't ever have to modify it yourself.  You may change the behaviour of
813       the debugger from within the debugger using its "o" command, from the
814       command line via the "PERLDB_OPTS" environment variable, and from
815       customization files.
816
817       You can do some customization by setting up a .perldb file, which
818       contains initialization code.  For instance, you could make aliases
819       like these (the last one is one people expect to be there):
820
821           $DB::alias{'len'}  = 's/^len(.*)/p length($1)/';
822           $DB::alias{'stop'} = 's/^stop (at|in)/b/';
823           $DB::alias{'ps'}   = 's/^ps\b/p scalar /';
824           $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
825
826       You can change options from .perldb by using calls like this one;
827
828           parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
829
830       The code is executed in the package "DB".  Note that .perldb is
831       processed before processing "PERLDB_OPTS".  If .perldb defines the
832       subroutine "afterinit", that function is called after debugger
833       initialization ends.  .perldb may be contained in the current
834       directory, or in the home directory.  Because this file is sourced in
835       by Perl and may contain arbitrary commands, for security reasons, it
836       must be owned by the superuser or the current user, and writable by no
837       one but its owner.
838
839       You can mock TTY input to debugger by adding arbitrary commands to
840       @DB::typeahead. For example, your .perldb file might contain:
841
842           sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
843
844       Which would attempt to set breakpoints on lines 4 and 6 immediately
845       after debugger initialization. Note that @DB::typeahead is not a
846       supported interface and is subject to change in future releases.
847
848       If you want to modify the debugger, copy perl5db.pl from the Perl
849       library to another name and hack it to your heart's content.  You'll
850       then want to set your "PERL5DB" environment variable to say something
851       like this:
852
853           BEGIN { require "myperl5db.pl" }
854
855       As a last resort, you could also use "PERL5DB" to customize the
856       debugger by directly setting internal variables or calling debugger
857       functions.
858
859       Note that any variables and functions that are not documented in this
860       document (or in perldebguts) are considered for internal use only, and
861       as such are subject to change without notice.
862
863   Readline Support / History in the Debugger
864       As shipped, the only command-line history supplied is a simplistic one
865       that checks for leading exclamation points.  However, if you install
866       the Term::ReadKey and Term::ReadLine modules from CPAN (such as
867       Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full
868       editing capabilities much like those GNU readline(3) provides.  Look
869       for these in the modules/by-module/Term directory on CPAN.  These do
870       not support normal vi command-line editing, however.
871
872       A rudimentary command-line completion is also available, including
873       lexical variables in the current scope if the "PadWalker" module is
874       installed.
875
876       Without Readline support you may see the symbols "^[[A", "^[[C",
877       "^[[B", "^[[D"", "^H", ... when using the arrow keys and/or the
878       backspace key.
879
880   Editor Support for Debugging
881       If you have the GNU's version of emacs installed on your system, it can
882       interact with the Perl debugger to provide an integrated software
883       development environment reminiscent of its interactions with C
884       debuggers.
885
886       Recent versions of Emacs come with a start file for making emacs act
887       like a syntax-directed editor that understands (some of) Perl's syntax.
888       See perlfaq3.
889
890       Users of vi should also look into vim and gvim, the mousey and windy
891       version, for coloring of Perl keywords.
892
893       Note that only perl can truly parse Perl, so all such CASE tools fall
894       somewhat short of the mark, especially if you don't program your Perl
895       as a C programmer might.
896
897   The Perl Profiler
898       If you wish to supply an alternative debugger for Perl to run, invoke
899       your script with a colon and a package argument given to the -d flag.
900       Perl's alternative debuggers include a Perl profiler, Devel::NYTProf,
901       which is available separately as a CPAN distribution.  To profile your
902       Perl program in the file mycode.pl, just type:
903
904           $ perl -d:NYTProf mycode.pl
905
906       When the script terminates the profiler will create a database of the
907       profile information that you can turn into reports using the profiler's
908       tools. See <perlperf> for details.
909

Debugging Regular Expressions

911       "use re 'debug'" enables you to see the gory details of how the Perl
912       regular expression engine works. In order to understand this typically
913       voluminous output, one must not only have some idea about how regular
914       expression matching works in general, but also know how Perl's regular
915       expressions are internally compiled into an automaton. These matters
916       are explored in some detail in "Debugging Regular Expressions" in
917       perldebguts.
918

Debugging Memory Usage

920       Perl contains internal support for reporting its own memory usage, but
921       this is a fairly advanced concept that requires some understanding of
922       how memory allocation works.  See "Debugging Perl Memory Usage" in
923       perldebguts for the details.
924

SEE ALSO

926       You do have "use strict" and "use warnings" enabled, don't you?
927
928       perldebtut, perldebguts, perl5db.pl, re, DB, Devel::NYTProf, Dumpvalue,
929       and perlrun.
930
931       When debugging a script that uses #! and is thus normally found in
932       $PATH, the -S option causes perl to search $PATH for it, so you don't
933       have to type the path or "which $scriptname".
934
935         $ perl -Sd foo.pl
936

BUGS

938       You cannot get stack frame information or in any fashion debug
939       functions that were not compiled by Perl, such as those from C or C++
940       extensions.
941
942       If you alter your @_ arguments in a subroutine (such as with "shift" or
943       "pop"), the stack backtrace will not show the original values.
944
945       The debugger does not currently work in conjunction with the -W
946       command-line switch, because it itself is not free of warnings.
947
948       If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing
949       from your keyboard or a socket) and haven't set up your own $SIG{INT}
950       handler, then you won't be able to CTRL-C your way back to the
951       debugger, because the debugger's own $SIG{INT} handler doesn't
952       understand that it needs to raise an exception to longjmp(3) out of
953       slow syscalls.
954
955
956
957perl v5.36.0                      2022-08-30                      PERLDEBUG(1)
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