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NAME
       perldebug - Perl debugging

DESCRIPTION
       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
       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 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.)

       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



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       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.

   Calling the debugger
       There are several ways to call the debugger:

       perl -d program_name
           On the given program identified by "program_name".

       perl -d -e 0
           Interactively supply an arbitrary "expression" using
           "-e".

       perl -d:Ptkdb program_name
           Debug a given program via the "Devel::Ptkdb" GUI.

       perl -dt threaded_program_name
           Debug a given program using threads (experimental).

   Debugger Commands
       The interactive debugger understands the following
       commands:

       h           Prints out a summary help message

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

       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.



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       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
                   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 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 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



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                   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
                   reference.

       -           List previous window of lines.

       v [line]    View a few lines of code 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
                   a regex.

                   "eval"ed strings (when accessible) are
                   considered 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



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                   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
                   default.

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

       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 "Frame
                   Listing Output Examples" in perldebguts for
                   examples.

       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
                   condition 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.




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       b compile subname
                   Sets a breakpoint before the first statement
                   executed after the specified subroutine is
                   compiled.

       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 $foo\n"

       A line      Delete an action from the specified line.

       A *         Delete all installed actions.

       w expr      Add a global watch-expression. Whenever a
                   watched global changes the debugger will stop
                   and display the old and new values.

       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



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                   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
                   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
                   newlines (we bet you couldn't have 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
                   newlines.

       { ?         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



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                   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 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
                   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:



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                   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
                   of:




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                       $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"
                   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'ed" 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



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                   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 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:

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

       "windowSize"
                   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



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                   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.

       "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
                   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
                   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



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       /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

                   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
               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 "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:

                   $ = 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




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               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
               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
               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.



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


   Debugging compile-time statements
       If you have compile-time executable statements (such as
       code within BEGIN, UNITCHECK 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
       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



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


       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 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 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 / History in the debugger
       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 (such as
       Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) 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.

       Without Readline support you may see the symbols "^[[A",
       "^[[C", "^[[B", "^[[D"", "^H", ... when using the arrow
       keys and/or the backspace key.





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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
       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
       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 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 profile.  More powerful profilers, such as
       "Devel::NYTProf" are available from the CPAN.

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 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.




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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 details.

SEE ALSO
       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
       $scriptname".

         $ perl -Sd foo.pl

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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