MirBSD manpage: perldebug(1)

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


     perldebug - Perl debugging


     First of all, have you tried using the -w switch?

     If you're new to the Perl debugger, you may prefer to read
     perldebtut, which is a tutorial introduction to the debugger

The Perl Debugger

     If you invoke Perl with the -d switch, your script runs
     under the Perl source debugger.  This works like an interac-
     tive Perl environment, prompting for debugger commands that
     let you examine source code, set breakpoints, get stack
     backtraces, change the values of variables, etc.  This is so
     convenient that you often fire up the debugger all by itself
     just to test out Perl constructs interactively to see what
     they do.  For example:

         $ perl -d -e 42

     In Perl, the debugger is not a separate program the way it
     usually is in the typical compiled environment.  Instead,
     the -d flag tells the compiler to insert source information
     into the parse trees it's about to hand off to the inter-
     preter.  That means your code must first compile correctly
     for the debugger to work on it.  Then when the interpreter
     starts up, it preloads a special Perl library file contain-
     ing the debugger.

     The program will halt right before the first run-time exe-
     cutable statement (but see below regarding compile-time
     statements) and ask you to enter a debugger command.  Con-
     trary to popular expectations, whenever the debugger halts
     and shows you a line of code, it always displays the line
     it's about to execute, rather than the one it has just exe-

     Any command not recognized by the debugger is directly exe-
     cuted ("eval"'d) as Perl code in the current package.  (The
     debugger uses the DB package for keeping its own state

     Note that the said "eval" is bound by an implicit scope. As
     a result any newly introduced lexical variable or any modi-
     fied capture buffer content is lost after the eval. The
     debugger is a nice environment to learn Perl, but if you
     interactively experiment using material which should be in
     the same scope, stuff it in one line.

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     For any text entered at the debugger prompt, leading and
     trailing whitespace is first stripped before further pro-
     cessing.  If a debugger command coincides with some function
     in your own program, merely precede the function with some-
     thing that doesn't look like a debugger command, such as a
     leading ";" or perhaps a "+", or by wrapping it with
     parentheses or braces.

     Debugger Commands

     The debugger understands the following commands:

     h           Prints out a summary help message

     h [command] Prints out a help message for the given debugger

     h h         The special argument of "h h" produces the
                 entire help page, which is quite long.

                 If the output of the "h h" command (or any com-
                 mand, for that matter) scrolls past your screen,
                 precede the command with a leading pipe symbol
                 so that it's run through your pager, as in

                     DB> |h h

                 You may change the pager which is used via "o
                 pager=..." command.

     p expr      Same as "print {$DB::OUT} expr" in the current
                 package.  In particular, because this is just
                 Perl's own "print" function, this means that
                 nested data structures and objects are not
                 dumped, unlike with the "x" command.

                 The "DB::OUT" filehandle is opened to /dev/tty,
                 regardless of where STDOUT may be redirected to.

     x [maxdepth] expr
                 Evaluates its expression in list context and
                 dumps out the result in a pretty-printed
                 fashion.  Nested data structures are printed out
                 recursively, unlike the real "print" function in
                 Perl.  When dumping hashes, you'll probably
                 prefer 'x \%h' rather than 'x %h'. See Dumpvalue
                 if you'd like to do this yourself.

                 The output format is governed by multiple
                 options described under "Configurable Options".

                 If the "maxdepth" is included, it must be a

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                 numeral N; the value is dumped only N levels
                 deep, as if the "dumpDepth" option had been tem-
                 porarily set to N.

     V [pkg [vars]]
                 Display all (or some) variables in package
                 (defaulting to "main") using a data pretty-
                 printer (hashes show their keys and values so
                 you see what's what, control characters are made
                 printable, etc.). Make sure you don't put the
                 type specifier (like "$") there, just the symbol
                 names, like this:

                     V DB filename line

                 Use "~pattern" and "!pattern" for positive and
                 negative regexes.

                 This is similar to calling the "x" command on
                 each applicable var.

     X [vars]    Same as "V currentpackage [vars]".

     y [level [vars]]
                 Display all (or some) lexical variables
                 (mnemonic: "mY" variables) in the current scope
                 or level scopes higher.  You can limit the vari-
                 ables that you see with vars which works exactly
                 as it does for the "V" and "X" commands.
                 Requires the "PadWalker" module version 0.08 or
                 higher; will warn if this isn't installed.  Out-
                 put is pretty-printed in the same style as for
                 "V" and the format is controlled by the same

     T           Produce a stack backtrace.  See below for
                 details on its output.

     s [expr]    Single step.  Executes until the beginning of
                 another statement, descending into subroutine
                 calls.  If an expression is supplied that
                 includes function calls, it too will be

     n [expr]    Next.  Executes over subroutine calls, until the
                 beginning of the next statement.  If an expres-
                 sion is supplied that includes function calls,
                 those functions will be executed with stops
                 before each statement.

     r           Continue until the return from the current sub-
                 routine. Dump the return value if the "PrintRet"

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                 option is set (default).

     <CR>        Repeat last "n" or "s" command.

     c [line|sub]
                 Continue, optionally inserting a one-time-only
                 breakpoint at the specified line or subroutine.

     l           List next window of lines.

     l min+incr  List "incr+1" lines starting at "min".

     l min-max   List lines "min" through "max".  "l -" is
                 synonymous to "-".

     l line      List a single line.

     l subname   List first window of lines from subroutine.
                 subname may be a variable that contains a code

     -           List previous window of lines.

     v [line]    View a few lines of code around the current

     .           Return the internal debugger pointer to the line
                 last executed, and print out that line.

     f filename  Switch to viewing a different file or "eval"
                 statement.  If filename is not a full pathname
                 found in the values of %INC, it is considered a

                 "eval"ed strings (when accessible) are con-
                 sidered to be filenames: "f (eval 7)" and "f
                 eval 7\b" access the body of the 7th "eval"ed
                 string (in the order of execution).  The bodies
                 of the currently executed "eval" and of "eval"ed
                 strings that define subroutines are saved and
                 thus accessible.

     /pattern/   Search forwards for pattern (a Perl regex);
                 final / is optional. The search is case-
                 insensitive by default.

     ?pattern?   Search backwards for pattern; final ? is
                 optional. The search is case-insensitive by

     L [abw]     List (default all) actions, breakpoints and
                 watch expressions

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     S [[!]regex]
                 List subroutine names [not] matching the regex.

     t           Toggle trace mode (see also the "AutoTrace"

     t expr      Trace through execution of "expr". See "Frame
                 Listing Output Examples" in perldebguts for

     b           Sets breakpoint on current line

     b [line] [condition]
                 Set a breakpoint before the given line.  If a
                 condition is specified, it's evaluated each time
                 the statement is reached: a breakpoint is taken
                 only if the condition is true.  Breakpoints may
                 only be set on lines that begin an executable
                 statement.  Conditions don't use "if":

                     b 237 $x > 30
                     b 237 ++$count237 < 11
                     b 33 /pattern/i

     b subname [condition]
                 Set a breakpoint before the first line of the
                 named subroutine.  subname may be a variable
                 containing a code reference (in this case condi-
                 tion is not supported).

     b postpone subname [condition]
                 Set a breakpoint at first line of subroutine
                 after it is compiled.

     b load filename
                 Set a breakpoint before the first executed line
                 of the filename, which should be a full pathname
                 found amongst the %INC values.

     b compile subname
                 Sets a breakpoint before the first statement
                 executed after the specified subroutine is com-

     B line      Delete a breakpoint from the specified line.

     B *         Delete all installed breakpoints.

     a [line] command
                 Set an action to be done before the line is exe-
                 cuted.  If line is omitted, set an action on the
                 line about to be executed. The sequence of steps

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                 taken by the debugger is

                   1. check for a breakpoint at this line
                   2. print the line if necessary (tracing)
                   3. do any actions associated with that line
                   4. prompt user if at a breakpoint or in single-step
                   5. evaluate line

                 For example, this will print out $foo every time
                 line 53 is passed:

                     a 53 print "DB FOUND $foo\n"

     A line      Delete an action from the specified line.

     A *         Delete all installed actions.

     w expr      Add a global watch-expression.  We hope you know
                 what one of these is, because they're supposed
                 to be obvious.

     W expr      Delete watch-expression

     W *         Delete all watch-expressions.

     o           Display all options

     o booloption ...
                 Set each listed Boolean option to the value 1.

     o anyoption? ...
                 Print out the value of one or more options.

     o option=value ...
                 Set the value of one or more options.  If the
                 value has internal whitespace, it should be
                 quoted.  For example, you could set "o
                 pager="less -MQeicsNfr"" to call less with those
                 specific options. You may use either single or
                 double quotes, but if you do, you must escape
                 any embedded instances of same sort of quote you
                 began with, as well as any escaping any escapes
                 that immediately precede that quote but which
                 are not meant to escape the quote itself.  In
                 other words, you follow single-quoting rules
                 irrespective of the quote; eg: "o option='this
                 isn\'t bad'" or "o option="She said, \"Isn't

                 For historical reasons, the "=value" is
                 optional, but defaults to 1 only where it is
                 safe to do so--that is, mostly for Boolean

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                 options.  It is always better to assign a
                 specific value using "=". The "option" can be
                 abbreviated, but for clarity probably should not
                 be.  Several options can be set together.  See
                 "Configurable Options" for a list of these.

     < ?         List out all pre-prompt Perl command actions.

     < [ command ]
                 Set an action (Perl command) to happen before
                 every debugger prompt. A multi-line command may
                 be entered by backslashing the newlines.

     < *         Delete all pre-prompt Perl command actions.

     << command  Add an action (Perl command) to happen before
                 every debugger prompt. A multi-line command may
                 be entered by backwhacking the newlines.

     > ?         List out post-prompt Perl command actions.

     > command   Set an action (Perl command) to happen after the
                 prompt when you've just given a command to
                 return to executing the script.  A multi-line
                 command may be entered by backslashing the new-
                 lines (we bet you couldn't've guessed this by

     > *         Delete all post-prompt Perl command actions.

     >> command  Adds an action (Perl command) to happen after
                 the prompt when you've just given a command to
                 return to executing the script.  A multi-line
                 command may be entered by backslashing the new-

     { ?         List out pre-prompt debugger commands.

     { [ command ]
                 Set an action (debugger command) to happen
                 before every debugger prompt. A multi-line com-
                 mand may be entered in the customary fashion.

                 Because this command is in some senses new, a
                 warning is issued if you appear to have acciden-
                 tally entered a block instead.  If that's what
                 you mean to do, write it as with ";{ ... }" or
                 even "do { ... }".

     { *         Delete all pre-prompt debugger commands.

     {{ command  Add an action (debugger command) to happen

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                 before every debugger prompt. A multi-line com-
                 mand may be entered, if you can guess how: see

     ! number    Redo a previous command (defaults to the previ-
                 ous command).

     ! -number   Redo number'th previous command.

     ! pattern   Redo last command that started with pattern. See
                 "o recallCommand", too.

     !! cmd      Run cmd in a subprocess (reads from DB::IN,
                 writes to DB::OUT) See "o shellBang", also.
                 Note that the user's current shell (well, their
                 $ENV{SHELL} variable) will be used, which can
                 interfere with proper interpretation of exit
                 status or signal and coredump information.

     source file Read and execute debugger commands from file.
                 file may itself contain "source" commands.

     H -number   Display last n commands.  Only commands longer
                 than one character are listed.  If number is
                 omitted, list them all.

     q or ^D     Quit.  ("quit" doesn't work for this, unless
                 you've made an alias) This is the only supported
                 way to exit the debugger, though typing "exit"
                 twice might work.

                 Set the "inhibit_exit" option to 0 if you want
                 to be able to step off the end the script.  You
                 may also need to set $finished to 0 if you want
                 to step through global destruction.

     R           Restart the debugger by "exec()"ing a new ses-
                 sion.  We try to maintain your history across
                 this, but internal settings and command-line
                 options may be lost.

                 The following setting are currently preserved:
                 history, breakpoints, actions, debugger options,
                 and the Perl command-line options -w, -I, and

     |dbcmd      Run the debugger command, piping DB::OUT into
                 your current pager.

     ||dbcmd     Same as "|dbcmd" but DB::OUT is temporarily
                 "select"ed as well.

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     = [alias value]
                 Define a command alias, like

                     = quit q

                 or list current aliases.

     command     Execute command as a Perl statement.  A trailing
                 semicolon will be supplied.  If the Perl state-
                 ment would otherwise be confused for a Perl
                 debugger, use a leading semicolon, too.

     m expr      List which methods may be called on the result
                 of the evaluated expression.  The expression may
                 evaluated to a reference to a blessed object, or
                 to a package name.

     M           Displays all loaded modules and their versions

     man [manpage]
                 Despite its name, this calls your system's
                 default documentation viewer on the given page,
                 or on the viewer itself if manpage is omitted.
                 If that viewer is man, the current "Config"
                 information is used to invoke man using the
                 proper MANPATH or -M manpath option.  Failed
                 lookups of the form "XXX" that match known man-
                 pages of the form perlXXX will be retried.  This
                 lets you type "man debug" or "man op" from the

                 On systems traditionally bereft of a usable man
                 command, the debugger invokes perldoc.  Occa-
                 sionally this determination is incorrect due to
                 recalcitrant vendors or rather more felici-
                 tously, to enterprising users.  If you fall into
                 either category, just manually set the
                 $DB::doccmd variable to whatever viewer to view
                 the Perl documentation on your system.  This may
                 be set in an rc file, or through direct assign-
                 ment.  We're still waiting for a working example
                 of something along the lines of:

                     $DB::doccmd = 'netscape -remote http://something.here/';

     Configurable Options

     The debugger has numerous options settable using the "o"
     command, either interactively or from the environment or an
     rc file. (./.perldb or ~/.perldb under Unix.)

     "recallCommand", "ShellBang"

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                 The characters used to recall command or spawn
                 shell.  By default, both are set to "!", which
                 is unfortunate.

     "pager"     Program to use for output of pager-piped com-
                 mands (those beginning with a "|" character.)
                 By default, $ENV{PAGER} will be used. Because
                 the debugger uses your current terminal charac-
                 teristics for bold and underlining, if the
                 chosen pager does not pass escape sequences
                 through unchanged, the output of some debugger
                 commands will not be readable when sent through
                 the pager.

     "tkRunning" Run Tk while prompting (with ReadLine).

     "signalLevel", "warnLevel", "dieLevel"
                 Level of verbosity.  By default, the debugger
                 leaves your exceptions and warnings alone,
                 because altering them can break correctly run-
                 ning programs.  It will attempt to print a mes-
                 sage when uncaught INT, BUS, or SEGV signals
                 arrive.  (But see the mention of signals in BUGS

                 To disable this default safe mode, set these
                 values to something higher than 0.  At a level
                 of 1, you get backtraces upon receiving any kind
                 of warning (this is often annoying) or exception
                 (this is often valuable).  Unfortunately, the
                 debugger cannot discern fatal exceptions from
                 non-fatal ones.  If "dieLevel" is even 1, then
                 your non-fatal exceptions are also traced and
                 unceremoniously altered if they came from
                 "eval'd" strings or from any kind of "eval"
                 within modules you're attempting to load.  If
                 "dieLevel" is 2, the debugger doesn't care where
                 they came from:  It usurps your exception
                 handler and prints out a trace, then modifies
                 all exceptions with its own embellishments. This
                 may perhaps be useful for some tracing purposes,
                 but tends to hopelessly destroy any program that
                 takes its exception handling seriously.

     "AutoTrace" Trace mode (similar to "t" command, but can be
                 put into "PERLDB_OPTS").

     "LineInfo"  File or pipe to print line number info to.  If
                 it is a pipe (say, "|visual_perl_db"), then a
                 short message is used.  This is the mechanism
                 used to interact with a slave editor or visual
                 debugger, such as the special "vi" or "emacs"

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                 hooks, or the "ddd" graphical debugger.

                 If 0, allows stepping off the end of the script.

     "PrintRet"  Print return value after "r" command if set

     "ornaments" Affects screen appearance of the command line
                 (see Term::ReadLine). There is currently no way
                 to disable these, which can render some output
                 illegible on some displays, or with some pagers.
                 This is considered a bug.

     "frame"     Affects the printing of messages upon entry and
                 exit from subroutines.  If "frame & 2" is false,
                 messages are printed on entry only. (Printing on
                 exit might be useful if interspersed with other

                 If "frame & 4", arguments to functions are
                 printed, plus context and caller info.  If
                 "frame & 8", overloaded "stringify" and "tie"d
                 "FETCH" is enabled on the printed arguments.  If
                 "frame & 16", the return value from the subrou-
                 tine is printed.

                 The length at which the argument list is trun-
                 cated is governed by the next option:

                 Length to truncate the argument list when the
                 "frame" option's bit 4 is set.

                 Change the size of code list window (default is
                 10 lines).

     The following options affect what happens with "V", "X", and
     "x" commands:

     "arrayDepth", "hashDepth"
                 Print only first N elements ('' for all).

     "dumpDepth" Limit recursion depth to N levels when dumping
                 structures. Negative values are interpreted as
                 infinity.  Default: infinity.

     "compactDump", "veryCompact"
                 Change the style of array and hash output.  If
                 "compactDump", short array may be printed on one

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     "globPrint" Whether to print contents of globs.

                 Dump arrays holding debugged files.

                 Dump symbol tables of packages.

                 Dump contents of "reused" addresses.

     "quote", "HighBit", "undefPrint"
                 Change the style of string dump.  The default
                 value for "quote" is "auto"; one can enable
                 double-quotish or single-quotish format by set-
                 ting it to """ or "'", respectively.  By
                 default, characters with their high bit set are
                 printed verbatim.

     "UsageOnly" Rudimentary per-package memory usage dump.  Cal-
                 culates total size of strings found in variables
                 in the package.  This does not include lexicals
                 in a module's file scope, or lost in closures.

     After the rc file is read, the debugger reads the
     $ENV{PERLDB_OPTS} environment variable and parses this as
     the remainder of a "O ..." line as one might enter at the
     debugger prompt.  You may place the initialization options
     "TTY", "noTTY", "ReadLine", and "NonStop" there.

     If your rc file contains:

       parse_options("NonStop=1 LineInfo=db.out AutoTrace");

     then your script will run without human intervention, put-
     ting trace information into the file db.out.  (If you inter-
     rupt it, you'd better reset "LineInfo" to /dev/tty if you
     expect to see anything.)

     "TTY"       The TTY to use for debugging I/O.

     "noTTY"     If set, the debugger goes into "NonStop" mode
                 and will not connect to a TTY.  If interrupted
                 (or if control goes to the debugger via explicit
                 setting of $DB::signal or $DB::single from the
                 Perl script), it connects to a TTY specified in
                 the "TTY" option at startup, or to a tty found
                 at runtime using the "Term::Rendezvous" module
                 of your choice.

                 This module should implement a method named
                 "new" that returns an object with two methods:

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                 "IN" and "OUT".  These should return filehandles
                 to use for debugging input and output
                 correspondingly.  The "new" method should
                 inspect an argument containing the value of
                 $ENV{PERLDB_NOTTY} at startup, or
                 "$ENV{HOME}/.perldbtty$$" otherwise.  This file
                 is not inspected for proper ownership, so secu-
                 rity hazards are theoretically possible.

     "ReadLine"  If false, readline support in the debugger is
                 disabled in order to debug applications that
                 themselves use ReadLine.

     "NonStop"   If set, the debugger goes into non-interactive
                 mode until interrupted, or programmatically by
                 setting $DB::signal or $DB::single.

     Here's an example of using the $ENV{PERLDB_OPTS} variable:

         $ PERLDB_OPTS="NonStop frame=2" perl -d myprogram

     That will run the script myprogram without human interven-
     tion, printing out the call tree with entry and exit points.
     Note that "NonStop=1 frame=2" is equivalent to "N f=2", and
     that originally, options could be uniquely abbreviated by
     the first letter (modulo the "Dump*" options).  It is
     nevertheless recommended that you always spell them out in
     full for legibility and future compatibility.

     Other examples include

         $ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram

     which runs script non-interactively, printing info on each
     entry into a subroutine and each executed line into the file
     named listing. (If you interrupt it, you would better reset
     "LineInfo" to something "interactive"!)

     Other examples include (using standard shell syntax to show
     environment variable settings):

       $ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
           perl -d myprogram )

     which may be useful for debugging a program that uses
     "Term::ReadLine" itself.  Do not forget to detach your shell
     from the TTY in the window that corresponds to /dev/ttyXX,
     say, by issuing a command like

       $ sleep 1000000

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     See "Debugger Internals" in perldebguts for details.

     Debugger input/output

     Prompt  The debugger prompt is something like


             or even


             where that number is the command number, and which
             you'd use to access with the built-in csh-like his-
             tory mechanism.  For example, "!17" would repeat
             command number 17.  The depth of the angle brackets
             indicates the nesting depth of the debugger.  You
             could get more than one set of brackets, for exam-
             ple, if you'd already at a breakpoint and then
             printed the result of a function call that itself
             has a breakpoint, or you step into an expression via
             "s/n/t expression" command.

     Multiline commands
             If you want to enter a multi-line command, such as a
             subroutine definition with several statements or a
             format, escape the newline that would normally end
             the debugger command with a backslash. Here's an

                   DB<1> for (1..4) {         \
                   cont:     print "ok\n";   \
                   cont: }

             Note that this business of escaping a newline is
             specific to interactive commands typed into the

     Stack backtrace
             Here's an example of what a stack backtrace via "T"
             command might look like:

                 $ = main::infested called from file `Ambulation.pm' line 10
                 @ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
                 $ = main::pests('bactrian', 4) called from file `camel_flea' line 4

             The left-hand character up there indicates the con-
             text in which the function was called, with "$" and

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             "@" meaning scalar or list contexts respectively,
             and "." meaning void context (which is actually a
             sort of scalar context).  The display above says
             that you were in the function "main::infested" when
             you ran the stack dump, and that it was called in
             scalar context from line 10 of the file
             Ambulation.pm, but without any arguments at all,
             meaning it was called as &infested.  The next stack
             frame shows that the function "Ambulation::legs" was
             called in list context from the camel_flea file with
             four arguments.  The last stack frame shows that
             "main::pests" was called in scalar context, also
             from camel_flea, but from line 4.

             If you execute the "T" command from inside an active
             "use" statement, the backtrace will contain both a
             "require" frame and an "eval") frame.

     Line Listing Format
             This shows the sorts of output the "l" command can

                 DB<<13>> l
               101:                @i{@i} = ();
               102:b               @isa{@i,$pack} = ()
               103                     if(exists $i{$prevpack} || exists $isa{$pack});
               104             }
               106             next
               107==>              if(exists $isa{$pack});
               109:a           if ($extra-- > 0) {
               110:                %isa = ($pack,1);

             Breakable lines are marked with ":".  Lines with
             breakpoints are marked by "b" and those with actions
             by "a".  The line that's about to be executed is
             marked by "==>".

             Please be aware that code in debugger listings may
             not look the same as your original source code.
             Line directives and external source filters can
             alter the code before Perl sees it, causing code to
             move from its original positions or take on entirely
             different forms.

     Frame listing
             When the "frame" option is set, the debugger would
             print entered (and optionally exited) subroutines in
             different styles.  See perldebguts for incredibly
             long examples of these.

perl v5.8.8                2006-06-30                          15

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

     Debugging compile-time statements

     If you have compile-time executable statements (such as code
     within BEGIN and CHECK blocks or "use" statements), these
     will not be stopped by debugger, although "require"s and
     INIT blocks will, and compile-time statements can be traced
     with "AutoTrace" option set in "PERLDB_OPTS").  From your
     own Perl code, however, you can transfer control back to the
     debugger using the following statement, which is harmless if
     the debugger is not running:

         $DB::single = 1;

     If you set $DB::single to 2, it's equivalent to having just
     typed the "n" command, whereas a value of 1 means the "s"
     command.  The $DB::trace  variable should be set to 1 to
     simulate having typed the "t" command.

     Another way to debug compile-time code is to start the
     debugger, set a breakpoint on the load of some module:

         DB<7> b load f:/perllib/lib/Carp.pm
       Will stop on load of `f:/perllib/lib/Carp.pm'.

     and then restart the debugger using the "R" command (if pos-
     sible).  One can use "b compile subname" for the same pur-

     Debugger Customization

     The debugger probably contains enough configuration hooks
     that you won't ever have to modify it yourself.  You may
     change the behaviour of debugger from within the debugger
     using its "o" command, from the command line via the
     "PERLDB_OPTS" environment variable, and from customization

     You can do some customization by setting up a .perldb file,
     which contains initialization code.  For instance, you could
     make aliases like these (the last one is one people expect
     to be there):

         $DB::alias{'len'}  = 's/^len(.*)/p length($1)/';
         $DB::alias{'stop'} = 's/^stop (at|in)/b/';
         $DB::alias{'ps'}   = 's/^ps\b/p scalar /';
         $DB::alias{'quit'} = 's/^quit(\s*)/exit/';

     You can change options from .perldb by using calls like this

         parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");

perl v5.8.8                2006-06-30                          16

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

     The code is executed in the package "DB".  Note that .perldb
     is processed before processing "PERLDB_OPTS".  If .perldb
     defines the subroutine "afterinit", that function is called
     after debugger initialization ends.  .perldb may be con-
     tained in the current directory, or in the home directory.
     Because this file is sourced in by Perl and may contain
     arbitrary commands, for security reasons, it must be owned
     by the superuser or the current user, and writable by no one
     but its owner.

     You can mock TTY input to debugger by adding arbitrary com-
     mands to @DB::typeahead. For example, your .perldb file
     might contain:

         sub afterinit { push @DB::typeahead, "b 4", "b 6"; }

     Which would attempt to set breakpoints on lines 4 and 6
     immediately after debugger initialization. Note that
     @DB::typeahead is not a supported interface and is subject
     to change in future releases.

     If you want to modify the debugger, copy perl5db.pl from the
     Perl library to another name and hack it to your heart's
     content. You'll then want to set your "PERL5DB" environment
     variable to say something like this:

         BEGIN { require "myperl5db.pl" }

     As a last resort, you could also use "PERL5DB" to customize
     the debugger by directly setting internal variables or cal-
     ling debugger functions.

     Note that any variables and functions that are not docu-
     mented in this document (or in perldebguts) are considered
     for internal use only, and as such are subject to change
     without notice.

     Readline Support

     As shipped, the only command-line history supplied is a
     simplistic one that checks for leading exclamation points.
     However, if you install the Term::ReadKey and Term::ReadLine
     modules from CPAN, you will have full editing capabilities
     much like GNU readline(3) provides. Look for these in the
     modules/by-module/Term directory on CPAN. These do not sup-
     port normal vi command-line editing, however.

     A rudimentary command-line completion is also available.
     Unfortunately, the names of lexical variables are not avail-
     able for completion.

perl v5.8.8                2006-06-30                          17

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

     Editor Support for Debugging

     If you have the FSF's version of emacs installed on your
     system, it can interact with the Perl debugger to provide an
     integrated software development environment reminiscent of
     its interactions with C debuggers.

     Perl comes with a start file for making emacs act like a
     syntax-directed editor that understands (some of) Perl's
     syntax. Look in the emacs directory of the Perl source dis-

     A similar setup by Tom Christiansen for interacting with any
     vendor-shipped vi and the X11 window system is also avail-
     able. This works similarly to the integrated multiwindow
     support that emacs provides, where the debugger drives the
     editor.  At the time of this writing, however, that tool's
     eventual location in the Perl distribution was uncertain.

     Users of vi should also look into vim and gvim, the mousey
     and windy version, for coloring of Perl keywords.

     Note that only perl can truly parse Perl, so all such CASE
     tools fall somewhat short of the mark, especially if you
     don't program your Perl as a C programmer might.

     The Perl Profiler

     If you wish to supply an alternative debugger for Perl to
     run, just invoke your script with a colon and a package
     argument given to the -d flag.  The most popular alternative
     debuggers for Perl is the Perl profiler.  Devel::DProf is
     now included with the standard Perl distribution.  To pro-
     file your Perl program in the file mycode.pl, just type:

         $ perl -d:DProf mycode.pl

     When the script terminates the profiler will dump the pro-
     file information to a file called tmon.out.  A tool like
     dprofpp, also supplied with the standard Perl distribution,
     can be used to interpret the information in that profile.

Debugging regular expressions

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

perl v5.8.8                2006-06-30                          18

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

Debugging memory usage

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


     You did try the -w switch, didn't you?

     perldebtut, perldebguts, re, DB, Devel::DProf, dprofpp,
     Dumpvalue, and perlrun.

     When debugging a script that uses #! and is thus normally
     found in $PATH, the -S option causes perl to search $PATH
     for it, so you don't have to type the path or "which

       $ perl -Sd foo.pl


     You cannot get stack frame information or in any fashion
     debug functions that were not compiled by Perl, such as
     those from C or C++ extensions.

     If you alter your @_ arguments in a subroutine (such as with
     "shift" or "pop"), the stack backtrace will not show the
     original values.

     The debugger does not currently work in conjunction with the
     -W command-line switch, because it itself is not free of

     If you're in a slow syscall (like "wait"ing, "accept"ing, or
     "read"ing from your keyboard or a socket) and haven't set up
     your own $SIG{INT} handler, then you won't be able to CTRL-C
     your way back to the debugger, because the debugger's own
     $SIG{INT} handler doesn't understand that it needs to raise
     an exception to longjmp(3) out of slow syscalls.

perl v5.8.8                2006-06-30                          19

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