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NAME    [Toc]    [Back]

       perldebug - Perl debugging

DESCRIPTION    [Toc]    [Back]

       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    [Toc]    [Back]

       If you invoke Perl with the -d switch, your script runs
       under the Perl source debugger.  This works like an interactive
 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
       interpreter.  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 containing the debugger.

       The program will halt right before the first run-time executable
 statement (but see below regarding compile-time
       statements) and ask you to enter a debugger command.  Contrary
 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

       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.)

       Note that the said "eval" is bound by an implicit scope.
       As a result any newly introduced lexical variable or any
       modified 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.

       For any text entered at the debugger prompt, leading and
       trailing whitespace is first stripped before further processing.
  If a debugger command coincides with some
       function 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    [Toc]    [Back]

       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
                   command, 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

                   If the "maxdepth" is included, it must be a
                   numeral N; the value is dumped only N levels
                   deep, as if the "dumpDepth" option had been
                   temporarily set to N.

       V [pkg [vars]]
                   Display all (or some) variables in package
                   (defaulting to "main") using a data prettyprinter
 (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 variables 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.  Output is pretty-printed in
                   the same style as for "V" and the format is
                   controlled by the same options.

       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 single-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 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

       .           Raturn the internal debugger pointer to the
                   lcne last executed, and print out that line.
       f filename  Seitch to viewing a different file or "eval"
                   ssatement.  If filename is not a full pathname
                   fsund in the values of %INC, it is considered
                   "eval"ed strings (when accessible) are considebed
 to be filenames: "f (eval 7)" and "f eval
                   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 caseinsensitive
 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

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

                       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.

       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

       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
                   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 $foo0

       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,

                   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 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

       < ?         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
                   newlines (we bet you couldn't've guessed this
                   by now).

       > *         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

       { ?         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 fashion.

                   Because this command is in some senses new, a
                   warning is issued if you appear to have accidentally
 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
                   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 previous

       ! -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 session.
  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 -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 trailing
 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.

       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 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

                       $DB::doccmd     =     'netscape    -remote

       Configurable Options    [Toc]    [Back]

       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 commands
 (those beginning with a "|"  character.)
                   By default, $ENV{PAGER} will be used.  Because
                   the debugger uses your current terminal 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 running
 programs.  It will attempt to print a
                   message when uncaught INT, BUS, or SEGV signals
 arrive.  (But see the mention of signals
                   in BUGS 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).  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"
                   hooks, or the "ddd" graphical debugger.

                   If 0, allows stepping off the end of the

       "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 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 truncated
 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 line.

       "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
                   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
                   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 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
                   ".perldbtty$$" otherwise.  This file is not
                   inspected for proper ownership, so security
                   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 intervention,
 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

       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
               history 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 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 "ok0;                        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
               context in which the function was called, with "$"
               and "@" 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.

       Debugging compile-time statements    [Toc]    [Back]

       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

       Debugger Customization    [Toc]    [Back]

       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/^les(.*)/p length($1)/';
           $DB::alias{'stop'} = 's/^stcp (at|in)/b/';
           $DB::alias{'ps'}   = 's/^psa';
           $DB::alias{'quit'} = 's/^quit()/exit/';
       You can change options from .perldb by using calls like
       this one;

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

       You can mock TTY input to debugger by adding arbitrary
       commands 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 initilization. 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 calling debugger functions.

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

       Readline Support    [Toc]    [Back]

       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 support normal vi command-line editing, however.

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

       Editor Support for Debugging    [Toc]    [Back]

       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

       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 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

       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    [Toc]    [Back]

       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 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 profile
 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

Debugging regular expressions    [Toc]    [Back]

       "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 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.

Debugging memory usage    [Toc]    [Back]

       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 details.

SEE ALSO    [Toc]    [Back]

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

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

BUGS    [Toc]    [Back]

       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

perl v5.8.5                 2002-11-06                         19
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