3 perldebug - Perl debugging
7 First of all, have you tried using the B<-w> switch?
10 If you're new to the Perl debugger, you may prefer to read
11 L<perldebtut>, which is a tutorial introduction to the debugger .
13 =head1 The Perl Debugger
15 If you invoke Perl with the B<-d> switch, your script runs under the
16 Perl source debugger. This works like an interactive Perl
17 environment, prompting for debugger commands that let you examine
18 source code, set breakpoints, get stack backtraces, change the values of
19 variables, etc. This is so convenient that you often fire up
20 the debugger all by itself just to test out Perl constructs
21 interactively to see what they do. For example:
25 In Perl, the debugger is not a separate program the way it usually is in the
26 typical compiled environment. Instead, the B<-d> flag tells the compiler
27 to insert source information into the parse trees it's about to hand off
28 to the interpreter. That means your code must first compile correctly
29 for the debugger to work on it. Then when the interpreter starts up, it
30 preloads a special Perl library file containing the debugger.
32 The program will halt I<right before> the first run-time executable
33 statement (but see below regarding compile-time statements) and ask you
34 to enter a debugger command. Contrary to popular expectations, whenever
35 the debugger halts and shows you a line of code, it always displays the
36 line it's I<about> to execute, rather than the one it has just executed.
38 Any command not recognized by the debugger is directly executed
39 (C<eval>'d) as Perl code in the current package. (The debugger
40 uses the DB package for keeping its own state information.)
42 For any text entered at the debugger prompt, leading and trailing whitespace
43 is first stripped before further processing. If a debugger command
44 coincides with some function in your own program, merely precede the
45 function with something that doesn't look like a debugger command, such
46 as a leading C<;> or perhaps a C<+>, or by wrapping it with parentheses
49 =head2 Debugger Commands
51 The debugger understands the following commands:
57 Prints out a summary help message
61 Prints out a help message for the given debugger command.
65 The special argument of C<h h> produces the entire help page, which is quite long.
67 If the output of the C<h h> command (or any command, for that matter) scrolls
68 past your screen, precede the command with a leading pipe symbol so
69 that it's run through your pager, as in
73 You may change the pager which is used via C<o pager=...> command.
78 Same as C<print {$DB::OUT} expr> in the current package. In particular,
79 because this is just Perl's own C<print> function, this means that nested
80 data structures and objects are not dumped, unlike with the C<x> command.
82 The C<DB::OUT> filehandle is opened to F</dev/tty>, regardless of
83 where STDOUT may be redirected to.
85 =item x [maxdepth] expr
87 Evaluates its expression in list context and dumps out the result in a
88 pretty-printed fashion. Nested data structures are printed out
89 recursively, unlike the real C<print> function in Perl. When dumping
90 hashes, you'll probably prefer 'x \%h' rather than 'x %h'.
91 See L<Dumpvalue> if you'd like to do this yourself.
93 The output format is governed by multiple options described under
94 L<"Configurable Options">.
96 If the C<maxdepth> is included, it must be a numeral I<N>; the value is
97 dumped only I<N> levels deep, as if the C<dumpDepth> option had been
98 temporarily set to I<N>.
102 Display all (or some) variables in package (defaulting to C<main>)
103 using a data pretty-printer (hashes show their keys and values so
104 you see what's what, control characters are made printable, etc.).
105 Make sure you don't put the type specifier (like C<$>) there, just
106 the symbol names, like this:
110 Use C<~pattern> and C<!pattern> for positive and negative regexes.
112 This is similar to calling the C<x> command on each applicable var.
116 Same as C<V currentpackage [vars]>.
118 =item y [level [vars]]
120 Display all (or some) lexical variables (mnemonic: C<mY> variables)
121 in the current scope or I<level> scopes higher. You can limit the
122 variables that you see with I<vars> which works exactly as it does
123 for the C<V> and C<X> commands. Requires the C<PadWalker> module
124 version 0.08 or higher; will warn if this isn't installed. Output
125 is pretty-printed in the same style as for C<V> and the format is
126 controlled by the same options.
130 Produce a stack backtrace. See below for details on its output.
134 Single step. Executes until the beginning of another
135 statement, descending into subroutine calls. If an expression is
136 supplied that includes function calls, it too will be single-stepped.
140 Next. Executes over subroutine calls, until the beginning
141 of the next statement. If an expression is supplied that includes
142 function calls, those functions will be executed with stops before
147 Continue until the return from the current subroutine.
148 Dump the return value if the C<PrintRet> option is set (default).
152 Repeat last C<n> or C<s> command.
156 Continue, optionally inserting a one-time-only breakpoint
157 at the specified line or subroutine.
161 List next window of lines.
165 List C<incr+1> lines starting at C<min>.
169 List lines C<min> through C<max>. C<l -> is synonymous to C<->.
177 List first window of lines from subroutine. I<subname> may
178 be a variable that contains a code reference.
182 List previous window of lines.
186 View a few lines of code around the current line.
190 Return the internal debugger pointer to the line last
191 executed, and print out that line.
195 Switch to viewing a different file or C<eval> statement. If I<filename>
196 is not a full pathname found in the values of %INC, it is considered
199 C<eval>ed strings (when accessible) are considered to be filenames:
200 C<f (eval 7)> and C<f eval 7\b> access the body of the 7th C<eval>ed string
201 (in the order of execution). The bodies of the currently executed C<eval>
202 and of C<eval>ed strings that define subroutines are saved and thus
207 Search forwards for pattern (a Perl regex); final / is optional.
208 The search is case-insensitive by default.
212 Search backwards for pattern; final ? is optional.
213 The search is case-insensitive by default.
217 List (default all) actions, breakpoints and watch expressions
221 List subroutine names [not] matching the regex.
225 Toggle trace mode (see also the C<AutoTrace> option).
229 Trace through execution of C<expr>.
230 See L<perldebguts/"Frame Listing Output Examples"> for examples.
234 Sets breakpoint on current line
236 =item b [line] [condition]
238 Set a breakpoint before the given line. If a condition
239 is specified, it's evaluated each time the statement is reached: a
240 breakpoint is taken only if the condition is true. Breakpoints may
241 only be set on lines that begin an executable statement. Conditions
245 b 237 ++$count237 < 11
248 =item b subname [condition]
250 Set a breakpoint before the first line of the named subroutine. I<subname> may
251 be a variable containing a code reference (in this case I<condition>
254 =item b postpone subname [condition]
256 Set a breakpoint at first line of subroutine after it is compiled.
258 =item b load filename
260 Set a breakpoint before the first executed line of the I<filename>,
261 which should be a full pathname found amongst the %INC values.
263 =item b compile subname
265 Sets a breakpoint before the first statement executed after the specified
266 subroutine is compiled.
270 Delete a breakpoint from the specified I<line>.
274 Delete all installed breakpoints.
276 =item a [line] command
278 Set an action to be done before the line is executed. If I<line> is
279 omitted, set an action on the line about to be executed.
280 The sequence of steps taken by the debugger is
282 1. check for a breakpoint at this line
283 2. print the line if necessary (tracing)
284 3. do any actions associated with that line
285 4. prompt user if at a breakpoint or in single-step
288 For example, this will print out $foo every time line
291 a 53 print "DB FOUND $foo\n"
295 Delete an action from the specified line.
299 Delete all installed actions.
303 Add a global watch-expression. We hope you know what one of these
304 is, because they're supposed to be obvious.
308 Delete watch-expression
312 Delete all watch-expressions.
318 =item o booloption ...
320 Set each listed Boolean option to the value C<1>.
322 =item o anyoption? ...
324 Print out the value of one or more options.
326 =item o option=value ...
328 Set the value of one or more options. If the value has internal
329 whitespace, it should be quoted. For example, you could set C<o
330 pager="less -MQeicsNfr"> to call B<less> with those specific options.
331 You may use either single or double quotes, but if you do, you must
332 escape any embedded instances of same sort of quote you began with,
333 as well as any escaping any escapes that immediately precede that
334 quote but which are not meant to escape the quote itself. In other
335 words, you follow single-quoting rules irrespective of the quote;
336 eg: C<o option='this isn\'t bad'> or C<o option="She said, \"Isn't
339 For historical reasons, the C<=value> is optional, but defaults to
340 1 only where it is safe to do so--that is, mostly for Boolean
341 options. It is always better to assign a specific value using C<=>.
342 The C<option> can be abbreviated, but for clarity probably should
343 not be. Several options can be set together. See L<"Configurable Options">
348 List out all pre-prompt Perl command actions.
352 Set an action (Perl command) to happen before every debugger prompt.
353 A multi-line command may be entered by backslashing the newlines.
354 B<WARNING> If C<command> is missing, all actions are wiped out!
358 Add an action (Perl command) to happen before every debugger prompt.
359 A multi-line command may be entered by backwhacking the newlines.
363 List out post-prompt Perl command actions.
367 Set an action (Perl command) to happen after the prompt when you've
368 just given a command to return to executing the script. A multi-line
369 command may be entered by backslashing the newlines (we bet you
370 couldn't've guessed this by now). B<WARNING> If C<command> is
371 missing, all actions are wiped out!
375 Adds an action (Perl command) to happen after the prompt when you've
376 just given a command to return to executing the script. A multi-line
377 command may be entered by backslashing the newlines.
381 List out pre-prompt debugger commands.
385 Set an action (debugger command) to happen before every debugger prompt.
386 A multi-line command may be entered in the customary fashion.
387 B<WARNING> If C<command> is missing, all actions are wiped out!
389 Because this command is in some senses new, a warning is issued if
390 you appear to have accidentally entered a block instead. If that's
391 what you mean to do, write it as with C<;{ ... }> or even
396 Add an action (debugger command) to happen before every debugger prompt.
397 A multi-line command may be entered, if you can guess how: see above.
401 Redo a previous command (defaults to the previous command).
405 Redo number'th previous command.
409 Redo last command that started with pattern.
410 See C<o recallCommand>, too.
414 Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See
415 C<o shellBang>, also. Note that the user's current shell (well,
416 their C<$ENV{SHELL}> variable) will be used, which can interfere
417 with proper interpretation of exit status or signal and coredump
422 Read and execute debugger commands from I<file>.
423 I<file> may itself contain C<source> commands.
427 Display last n commands. Only commands longer than one character are
428 listed. If I<number> is omitted, list them all.
432 Quit. ("quit" doesn't work for this, unless you've made an alias)
433 This is the only supported way to exit the debugger, though typing
434 C<exit> twice might work.
436 Set the C<inhibit_exit> option to 0 if you want to be able to step
437 off the end the script. You may also need to set $finished to 0
438 if you want to step through global destruction.
442 Restart the debugger by C<exec()>ing a new session. We try to maintain
443 your history across this, but internal settings and command-line options
446 The following setting are currently preserved: history, breakpoints,
447 actions, debugger options, and the Perl command-line
448 options B<-w>, B<-I>, and B<-e>.
452 Run the debugger command, piping DB::OUT into your current pager.
456 Same as C<|dbcmd> but DB::OUT is temporarily C<select>ed as well.
458 =item = [alias value]
460 Define a command alias, like
464 or list current aliases.
468 Execute command as a Perl statement. A trailing semicolon will be
469 supplied. If the Perl statement would otherwise be confused for a
470 Perl debugger, use a leading semicolon, too.
474 List which methods may be called on the result of the evaluated
475 expression. The expression may evaluated to a reference to a
476 blessed object, or to a package name.
480 Displays all loaded modules and their versions
485 Despite its name, this calls your system's default documentation
486 viewer on the given page, or on the viewer itself if I<manpage> is
487 omitted. If that viewer is B<man>, the current C<Config> information
488 is used to invoke B<man> using the proper MANPATH or S<B<-M>
489 I<manpath>> option. Failed lookups of the form C<XXX> that match
490 known manpages of the form I<perlXXX> will be retried. This lets
491 you type C<man debug> or C<man op> from the debugger.
493 On systems traditionally bereft of a usable B<man> command, the
494 debugger invokes B<perldoc>. Occasionally this determination is
495 incorrect due to recalcitrant vendors or rather more felicitously,
496 to enterprising users. If you fall into either category, just
497 manually set the $DB::doccmd variable to whatever viewer to view
498 the Perl documentation on your system. This may be set in an rc
499 file, or through direct assignment. We're still waiting for a
500 working example of something along the lines of:
502 $DB::doccmd = 'netscape -remote http://something.here/';
506 =head2 Configurable Options
508 The debugger has numerous options settable using the C<o> command,
509 either interactively or from the environment or an rc file.
510 (./.perldb or ~/.perldb under Unix.)
515 =item C<recallCommand>, C<ShellBang>
517 The characters used to recall command or spawn shell. By
518 default, both are set to C<!>, which is unfortunate.
522 Program to use for output of pager-piped commands (those beginning
523 with a C<|> character.) By default, C<$ENV{PAGER}> will be used.
524 Because the debugger uses your current terminal characteristics
525 for bold and underlining, if the chosen pager does not pass escape
526 sequences through unchanged, the output of some debugger commands
527 will not be readable when sent through the pager.
531 Run Tk while prompting (with ReadLine).
533 =item C<signalLevel>, C<warnLevel>, C<dieLevel>
535 Level of verbosity. By default, the debugger leaves your exceptions
536 and warnings alone, because altering them can break correctly running
537 programs. It will attempt to print a message when uncaught INT, BUS, or
538 SEGV signals arrive. (But see the mention of signals in L<BUGS> below.)
540 To disable this default safe mode, set these values to something higher
541 than 0. At a level of 1, you get backtraces upon receiving any kind
542 of warning (this is often annoying) or exception (this is
543 often valuable). Unfortunately, the debugger cannot discern fatal
544 exceptions from non-fatal ones. If C<dieLevel> is even 1, then your
545 non-fatal exceptions are also traced and unceremoniously altered if they
546 came from C<eval'd> strings or from any kind of C<eval> within modules
547 you're attempting to load. If C<dieLevel> is 2, the debugger doesn't
548 care where they came from: It usurps your exception handler and prints
549 out a trace, then modifies all exceptions with its own embellishments.
550 This may perhaps be useful for some tracing purposes, but tends to hopelessly
551 destroy any program that takes its exception handling seriously.
555 Trace mode (similar to C<t> command, but can be put into
560 File or pipe to print line number info to. If it is a pipe (say,
561 C<|visual_perl_db>), then a short message is used. This is the
562 mechanism used to interact with a slave editor or visual debugger,
563 such as the special C<vi> or C<emacs> hooks, or the C<ddd> graphical
566 =item C<inhibit_exit>
568 If 0, allows I<stepping off> the end of the script.
572 Print return value after C<r> command if set (default).
576 Affects screen appearance of the command line (see L<Term::ReadLine>).
577 There is currently no way to disable these, which can render
578 some output illegible on some displays, or with some pagers.
579 This is considered a bug.
583 Affects the printing of messages upon entry and exit from subroutines. If
584 C<frame & 2> is false, messages are printed on entry only. (Printing
585 on exit might be useful if interspersed with other messages.)
587 If C<frame & 4>, arguments to functions are printed, plus context
588 and caller info. If C<frame & 8>, overloaded C<stringify> and
589 C<tie>d C<FETCH> is enabled on the printed arguments. If C<frame
590 & 16>, the return value from the subroutine is printed.
592 The length at which the argument list is truncated is governed by the
597 Length to truncate the argument list when the C<frame> option's
602 Change the size of code list window (default is 10 lines).
606 The following options affect what happens with C<V>, C<X>, and C<x>
611 =item C<arrayDepth>, C<hashDepth>
613 Print only first N elements ('' for all).
617 Limit recursion depth to N levels when dumping structures.
618 Negative values are interpreted as infinity. Default: infinity.
620 =item C<compactDump>, C<veryCompact>
622 Change the style of array and hash output. If C<compactDump>, short array
623 may be printed on one line.
627 Whether to print contents of globs.
631 Dump arrays holding debugged files.
633 =item C<DumpPackages>
635 Dump symbol tables of packages.
639 Dump contents of "reused" addresses.
641 =item C<quote>, C<HighBit>, C<undefPrint>
643 Change the style of string dump. The default value for C<quote>
644 is C<auto>; one can enable double-quotish or single-quotish format
645 by setting it to C<"> or C<'>, respectively. By default, characters
646 with their high bit set are printed verbatim.
650 Rudimentary per-package memory usage dump. Calculates total
651 size of strings found in variables in the package. This does not
652 include lexicals in a module's file scope, or lost in closures.
656 After the rc file is read, the debugger reads the C<$ENV{PERLDB_OPTS}>
657 environment variable and parses this as the remainder of a `O ...'
658 line as one might enter at the debugger prompt. You may place the
659 initialization options C<TTY>, C<noTTY>, C<ReadLine>, and C<NonStop>
662 If your rc file contains:
664 parse_options("NonStop=1 LineInfo=db.out AutoTrace");
666 then your script will run without human intervention, putting trace
667 information into the file I<db.out>. (If you interrupt it, you'd
668 better reset C<LineInfo> to F</dev/tty> if you expect to see anything.)
674 The TTY to use for debugging I/O.
678 If set, the debugger goes into C<NonStop> mode and will not connect to a TTY. If
679 interrupted (or if control goes to the debugger via explicit setting of
680 $DB::signal or $DB::single from the Perl script), it connects to a TTY
681 specified in the C<TTY> option at startup, or to a tty found at
682 runtime using the C<Term::Rendezvous> module of your choice.
684 This module should implement a method named C<new> that returns an object
685 with two methods: C<IN> and C<OUT>. These should return filehandles to use
686 for debugging input and output correspondingly. The C<new> method should
687 inspect an argument containing the value of C<$ENV{PERLDB_NOTTY}> at
688 startup, or C<"/tmp/perldbtty$$"> otherwise. This file is not
689 inspected for proper ownership, so security hazards are theoretically
694 If false, readline support in the debugger is disabled in order
695 to debug applications that themselves use ReadLine.
699 If set, the debugger goes into non-interactive mode until interrupted, or
700 programmatically by setting $DB::signal or $DB::single.
704 Here's an example of using the C<$ENV{PERLDB_OPTS}> variable:
706 $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram
708 That will run the script B<myprogram> without human intervention,
709 printing out the call tree with entry and exit points. Note that
710 C<NonStop=1 frame=2> is equivalent to C<N f=2>, and that originally,
711 options could be uniquely abbreviated by the first letter (modulo
712 the C<Dump*> options). It is nevertheless recommended that you
713 always spell them out in full for legibility and future compatibility.
715 Other examples include
717 $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
719 which runs script non-interactively, printing info on each entry
720 into a subroutine and each executed line into the file named F<listing>.
721 (If you interrupt it, you would better reset C<LineInfo> to something
724 Other examples include (using standard shell syntax to show environment
727 $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
730 which may be useful for debugging a program that uses C<Term::ReadLine>
731 itself. Do not forget to detach your shell from the TTY in the window that
732 corresponds to F</dev/ttyXX>, say, by issuing a command like
736 See L<perldebguts/"Debugger Internals"> for details.
738 =head2 Debugger input/output
744 The debugger prompt is something like
752 where that number is the command number, and which you'd use to
753 access with the built-in B<csh>-like history mechanism. For example,
754 C<!17> would repeat command number 17. The depth of the angle
755 brackets indicates the nesting depth of the debugger. You could
756 get more than one set of brackets, for example, if you'd already
757 at a breakpoint and then printed the result of a function call that
758 itself has a breakpoint, or you step into an expression via C<s/n/t
761 =item Multiline commands
763 If you want to enter a multi-line command, such as a subroutine
764 definition with several statements or a format, escape the newline
765 that would normally end the debugger command with a backslash.
769 cont: print "ok\n"; \
776 Note that this business of escaping a newline is specific to interactive
777 commands typed into the debugger.
779 =item Stack backtrace
781 Here's an example of what a stack backtrace via C<T> command might
784 $ = main::infested called from file `Ambulation.pm' line 10
785 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
786 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4
788 The left-hand character up there indicates the context in which the
789 function was called, with C<$> and C<@> meaning scalar or list
790 contexts respectively, and C<.> meaning void context (which is
791 actually a sort of scalar context). The display above says
792 that you were in the function C<main::infested> when you ran the
793 stack dump, and that it was called in scalar context from line
794 10 of the file I<Ambulation.pm>, but without any arguments at all,
795 meaning it was called as C<&infested>. The next stack frame shows
796 that the function C<Ambulation::legs> was called in list context
797 from the I<camel_flea> file with four arguments. The last stack
798 frame shows that C<main::pests> was called in scalar context,
799 also from I<camel_flea>, but from line 4.
801 If you execute the C<T> command from inside an active C<use>
802 statement, the backtrace will contain both a C<require> frame and
805 =item Line Listing Format
807 This shows the sorts of output the C<l> command can produce:
811 102:b @isa{@i,$pack} = ()
812 103 if(exists $i{$prevpack} || exists $isa{$pack});
816 107==> if(exists $isa{$pack});
818 109:a if ($extra-- > 0) {
819 110: %isa = ($pack,1);
821 Breakable lines are marked with C<:>. Lines with breakpoints are
822 marked by C<b> and those with actions by C<a>. The line that's
823 about to be executed is marked by C<< ==> >>.
825 Please be aware that code in debugger listings may not look the same
826 as your original source code. Line directives and external source
827 filters can alter the code before Perl sees it, causing code to move
828 from its original positions or take on entirely different forms.
832 When the C<frame> option is set, the debugger would print entered (and
833 optionally exited) subroutines in different styles. See L<perldebguts>
834 for incredibly long examples of these.
838 =head2 Debugging compile-time statements
840 If you have compile-time executable statements (such as code within
841 BEGIN and CHECK blocks or C<use> statements), these will I<not> be
842 stopped by debugger, although C<require>s and INIT blocks will, and
843 compile-time statements can be traced with C<AutoTrace> option set
844 in C<PERLDB_OPTS>). From your own Perl code, however, you can
845 transfer control back to the debugger using the following statement,
846 which is harmless if the debugger is not running:
850 If you set C<$DB::single> to 2, it's equivalent to having
851 just typed the C<n> command, whereas a value of 1 means the C<s>
852 command. The C<$DB::trace> variable should be set to 1 to simulate
853 having typed the C<t> command.
855 Another way to debug compile-time code is to start the debugger, set a
856 breakpoint on the I<load> of some module:
858 DB<7> b load f:/perllib/lib/Carp.pm
859 Will stop on load of `f:/perllib/lib/Carp.pm'.
861 and then restart the debugger using the C<R> command (if possible). One can use C<b
862 compile subname> for the same purpose.
864 =head2 Debugger Customization
866 The debugger probably contains enough configuration hooks that you
867 won't ever have to modify it yourself. You may change the behaviour
868 of debugger from within the debugger using its C<o> command, from
869 the command line via the C<PERLDB_OPTS> environment variable, and
870 from customization files.
872 You can do some customization by setting up a F<.perldb> file, which
873 contains initialization code. For instance, you could make aliases
874 like these (the last one is one people expect to be there):
876 $DB::alias{'len'} = 's/^len(.*)/p length($1)/';
877 $DB::alias{'stop'} = 's/^stop (at|in)/b/';
878 $DB::alias{'ps'} = 's/^ps\b/p scalar /';
879 $DB::alias{'quit'} = 's/^quit(\s*)/exit/';
881 You can change options from F<.perldb> by using calls like this one;
883 parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
885 The code is executed in the package C<DB>. Note that F<.perldb> is
886 processed before processing C<PERLDB_OPTS>. If F<.perldb> defines the
887 subroutine C<afterinit>, that function is called after debugger
888 initialization ends. F<.perldb> may be contained in the current
889 directory, or in the home directory. Because this file is sourced
890 in by Perl and may contain arbitrary commands, for security reasons,
891 it must be owned by the superuser or the current user, and writable
892 by no one but its owner.
894 If you want to modify the debugger, copy F<perl5db.pl> from the
895 Perl library to another name and hack it to your heart's content.
896 You'll then want to set your C<PERL5DB> environment variable to say
899 BEGIN { require "myperl5db.pl" }
901 As a last resort, you could also use C<PERL5DB> to customize the debugger
902 by directly setting internal variables or calling debugger functions.
904 Note that any variables and functions that are not documented in
905 this document (or in L<perldebguts>) are considered for internal
906 use only, and as such are subject to change without notice.
908 =head2 Readline Support
910 As shipped, the only command-line history supplied is a simplistic one
911 that checks for leading exclamation points. However, if you install
912 the Term::ReadKey and Term::ReadLine modules from CPAN, you will
913 have full editing capabilities much like GNU I<readline>(3) provides.
914 Look for these in the F<modules/by-module/Term> directory on CPAN.
915 These do not support normal B<vi> command-line editing, however.
917 A rudimentary command-line completion is also available.
918 Unfortunately, the names of lexical variables are not available for
921 =head2 Editor Support for Debugging
923 If you have the FSF's version of B<emacs> installed on your system,
924 it can interact with the Perl debugger to provide an integrated
925 software development environment reminiscent of its interactions
928 Perl comes with a start file for making B<emacs> act like a
929 syntax-directed editor that understands (some of) Perl's syntax.
930 Look in the I<emacs> directory of the Perl source distribution.
932 A similar setup by Tom Christiansen for interacting with any
933 vendor-shipped B<vi> and the X11 window system is also available.
934 This works similarly to the integrated multiwindow support that
935 B<emacs> provides, where the debugger drives the editor. At the
936 time of this writing, however, that tool's eventual location in the
937 Perl distribution was uncertain.
939 Users of B<vi> should also look into B<vim> and B<gvim>, the mousey
940 and windy version, for coloring of Perl keywords.
942 Note that only perl can truly parse Perl, so all such CASE tools
943 fall somewhat short of the mark, especially if you don't program
944 your Perl as a C programmer might.
946 =head2 The Perl Profiler
948 If you wish to supply an alternative debugger for Perl to run, just
949 invoke your script with a colon and a package argument given to the
950 B<-d> flag. The most popular alternative debuggers for Perl is the
951 Perl profiler. Devel::DProf is now included with the standard Perl
952 distribution. To profile your Perl program in the file F<mycode.pl>,
955 $ perl -d:DProf mycode.pl
957 When the script terminates the profiler will dump the profile
958 information to a file called F<tmon.out>. A tool like B<dprofpp>,
959 also supplied with the standard Perl distribution, can be used to
960 interpret the information in that profile.
962 =head1 Debugging regular expressions
964 C<use re 'debug'> enables you to see the gory details of how the Perl
965 regular expression engine works. In order to understand this typically
966 voluminous output, one must not only have some idea about how regular
967 expression matching works in general, but also know how Perl's regular
968 expressions are internally compiled into an automaton. These matters
969 are explored in some detail in
970 L<perldebguts/"Debugging regular expressions">.
972 =head1 Debugging memory usage
974 Perl contains internal support for reporting its own memory usage,
975 but this is a fairly advanced concept that requires some understanding
976 of how memory allocation works.
977 See L<perldebguts/"Debugging Perl memory usage"> for the details.
981 You did try the B<-w> switch, didn't you?
995 You cannot get stack frame information or in any fashion debug functions
996 that were not compiled by Perl, such as those from C or C++ extensions.
998 If you alter your @_ arguments in a subroutine (such as with C<shift>
999 or C<pop>), the stack backtrace will not show the original values.
1001 The debugger does not currently work in conjunction with the B<-W>
1002 command-line switch, because it itself is not free of warnings.
1004 If you're in a slow syscall (like C<wait>ing, C<accept>ing, or C<read>ing
1005 from your keyboard or a socket) and haven't set up your own C<$SIG{INT}>
1006 handler, then you won't be able to CTRL-C your way back to the debugger,
1007 because the debugger's own C<$SIG{INT}> handler doesn't understand that
1008 it needs to raise an exception to longjmp(3) out of slow syscalls.