


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


NAME
       perldebug - Perl debugging

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

The Perl Debugger
       If you invoke Perl with the -d switch, your script runs
       under the Perl source debugger.  This works like an inter-
       active 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 interac-
       tively 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 informa-
       tion into the parse trees it's about to hand off to the
       interpreter.  That means your code must first compile cor-
       rectly for the debugger to work on it.  Then when the
       interpreter starts up, it preloads a special Perl library
       file containing 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
       executed.

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

       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 func-
       tion in your own program, merely precede the function with
       something 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 [command] Prints out a help message.




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                   If you supply another debugger command as an
                   argument to the "h" command, it prints out the
                   description for just that command.  The spe-
                   cial argument of "h h" produces a more compact
                   help listing, designed to fit together on one
                   screen.

                   If the output of the "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

                   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 expr      Evaluates its expression in list context and
                   dumps out the result in a pretty-printed fash-
                   ion.  Nested data structures are printed out
                   recursively, unlike the real "print" function
                   in Perl.  See the Dumpvalue manpage if you'd
                   like to do this yourself.

                   The output format is governed by multiple
                   options described under the section on "Con-
                   figurable Options".

       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



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                   each applicable var.

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

       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 sin-
                   gle-stepped.

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

       r           Continue until the return from the current
                   subroutine.  Dump the return value if the
                   "PrintRet" 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 subrou-
                   tine.

       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 syn-
                   onymous 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
                   reference.

       -           List previous window of lines.

       w [line]    List window (a few lines) around the current
                   line.

       .           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



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                   a regex.

                   "eval"ed strings (when accessible) are consid-
                   ered 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 bod-
                   ies 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.

       ?pattern?   Search backwards for pattern; final ? is
                   optional.

       L           List all breakpoints and actions.

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

       t           Toggle trace mode (see also the "AutoTrace"
                   option).

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

       b [line] [condition]
                   Set a breakpoint before the given line.  If
                   line is omitted, set a breakpoint on the line
                   about to be executed.  If a condition is spec-
                   ified, 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 state-
                   ment.  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 con-
                   dition is not supported).

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




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       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
                   compiled.

       d [line]    Delete a breakpoint from the specified line.
                   If line is omitted, deletes the breakpoint
                   from the line about to be executed.

       D           Delete all installed breakpoints.

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

       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.  WARNING: It is far
                   too easy to destroy your watch expressions by
                   accidentally omitting the expr.

       W           Delete all watch-expressions.

       O booloption ...
                   Set each listed Boolean option to the value
                   "1".

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



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       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 it?\""".

                   For historical reasons, the "=value" is
                   optional, but defaults to 1 only where it is
                   safe to do so--that is, mostly for Boolean
                   options.  It is always better to assign a spe-
                   cific value using "=".  The "option" can be
                   abbreviated, but for clarity probably should
                   not be.  Several options can be set together.
                   See the section on "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.
                   WARNING If "command" is missing, all actions
                   are wiped out!

       << 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
                   newlines (we bet you couldn't've guessed this
                   by now).  WARNING If "command" is missing, all
                   actions are wiped out!

       >> 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
                   newlines.



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       { ?         List out pre-prompt debugger commands.

       { [ command ]
                   Set an action (debugger command) to happen
                   before every debugger prompt.  A multi-line
                   command may be entered in the customary fash-
                   ion.  WARNING If "command" is missing, all
                   actions are wiped out!

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

       {{ command  Add an action (debugger command) to happen
                   before every debugger prompt.  A multi-line
                   command may be entered, if you can guess how:
                   see above.

       ! number    Redo a previous command (defaults to the pre-
                   vious 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 informa-
                   tion.

       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 sup-
                   ported 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.



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

       |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.

       = [alias value]
                   Define a command alias, like

                       = quit q

                   or list current aliases.

       command     Execute command as a Perl statement.  A trail-
                   ing semicolon will be supplied.  If the Perl
                   statement 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.

       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 manpages of the form perlXXX will be
                   retried.  This lets you type "man debug" or
                   "man op" from the debugger.

                   On systems traditionally bereft of a usable
                   man command, the debugger invokes perldoc.
                   Occasionally this determination is incorrect
                   due to recalcitrant vendors or rather more
                   felicitously, 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 assignment.  We're still waiting for a
                   working example of something along the lines
                   of:

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



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       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""
                   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 termi-
                   nal characteristics 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
                   message when uncaught INT, BUS, or SEGV sig-
                   nals arrive.  (But see the mention of signals
                   in the BUGS manpage below.)

                   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).  Unfortu-
                   nately, 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 attempt-
                   ing 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 hope-
                   lessly destroy any program that takes its
                   exception handling seriously.





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       ""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"
                   hooks, or the "ddd" graphical debugger.

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

       ""PrintRet""
                   Print return value after "r" command if set
                   (default).

       ""ornaments""
                   Affects screen appearance of the command line
                   (see the Term::ReadLine manpage).  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 inter-
                   spersed with other messages.)

                   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
                   subroutine is printed.

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

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

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

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




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

       ""globPrint""
                   Whether to print contents of globs.

       ""DumpDBFiles""
                   Dump arrays holding debugged files.

       ""DumpPackages""
                   Dump symbol tables of packages.

       ""DumpReused""
                   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
                   setting it to """ or "'", respectively.  By
                   default, characters with their high bit set
                   are printed verbatim.

       ""UsageOnly""
                   Rudimentary per-package memory usage dump.
                   Calculates 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,
       putting trace information into the file db.out.  (If you
       interrupt 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



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                   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:
                   "IN" and "OUT".  These should return filehan-
                   dles 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
                   ""/tmp/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}" vari-
       able:

           $ 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 compati-
       bility.

       Other examples include

           $ PERLDB_OPTS="NonStop 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 bet-
       ter 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



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       "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

       See the Debugger Internals entry in the perldebguts man-
       page for details.

       Debugger input/output


       Prompt  The debugger prompt is something like

                   DB<8>

               or even

                   DB<<17>>

               where that number is the command number, and which
               you'd use to access with the built-in csh-like
               history mechanism.  For example, "!17" would
               repeat command number 17.  The depth of the angle
               brackets indicates the nesting depth of the debug-
               ger.  You could get more than one set of brackets,
               for example, 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 example:

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

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

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




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                   $ = 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
               context in which the function was called, with "$"
               and "@" meaning scalar or list contexts respec-
               tively, 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 argu-
               ments 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 argu-
               ments.  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
               produce:

                   DB<<13>> l
                 101:                @i{@i} = ();
                 102:b               @isa{@i,$pack} = ()
                 103                     if(exists $i{$prevpack} || exists $isa{$pack});
                 104             }
                 105
                 106             next
                 107==>              if(exists $isa{$pack});
                 108
                 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 exe-
               cuted 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.





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

       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
       possible).  One can use "b compile subname" for the same
       purpose.

       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
       files.

       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



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       this one;

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

       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 contained 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.

       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 cus-
       tomize the debugger by directly setting internal variables
       or calling debugger functions.

       Note that any variables and functions that are not docu-
       mented in this document (or in the perldebguts manpage)
       are considered for internal use only, and as such are sub-
       ject 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::Read-
       Line modules from CPAN, you will have full editing capa-
       bilities much like GNU readline(3) provides.  Look for
       these in the modules/by-module/Term directory on CPAN.
       These do not support normal vi command-line editing, how-
       ever.

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

       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



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       syntax.  Look in the emacs directory of the Perl source
       distribution.

       A similar setup by Tom Christiansen for interacting with
       any vendor-shipped vi and the X11 window system is also
       available.  This works similarly to the integrated multi-
       window 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 alterna-
       tive debuggers for Perl is the Perl profiler.
       Devel::DProf is now included with the standard Perl dis-
       tribution.  To profile 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 distribu-
       tion, 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
       understand this typically voluminous output, one must not
       only have some idea about about how regular expression
       matching works in general, but also know how Perl's regu-
       lar expressions are internally compiled into an automaton.
       These matters are explored in some detail in the Debugging
       regular expressions entry in the perldebguts manpage.

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





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

       the perldebguts manpage, the re manpage, the DB manpage,
       the Devel::Dprof manpage, the dprofpp manpage, the Dump-
       value manpage, and the perlrun manpage.

BUGS
       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 warnings.

       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.






























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