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perldebug



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 debug­
       ger .


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.)

       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 pro­
       cessing.  If a debugger command coincides with some func­
       tion in your own program, merely precede the function with
       h [command] Prints out a help message for the given debug­
                   ger 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.

       x [maxdepth] 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.  When dumping hashes, you'll probably
                   prefer 'x \%h' rather than 'x %h'.  See Dump­
                   value 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
                   (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 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.

       v [line]    View a few lines of code around the current
                   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.  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.  Break­
                   points 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 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 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.  We hope you
                   know what one of these is, because they're
                   supposed to be obvious.

       W expr      Delete watch-expression

       W *         Delete all watch-expressions.

       o           Display all options

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

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

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

                   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
                   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 fash­
                   ion.

                   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 { ... }".

       { *         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 pre­
                   vious command).

       ! -number   Redo number'th previous command.

       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.

                   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.

       M           Displays all loaded modules and their versions

       man [manpage]
                   Despite its name, this calls your system's
                   default documentation viewer on the given
                   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/';

       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 terminal char­
                   acteristics 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 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).  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
                   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 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.

       "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
                   as infinity.  Default: infinity.

       "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
                   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

       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 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 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
       "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

                   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
                     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

               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 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);

               When the "frame" option is set, the debugger would
               print entered (and optionally exited) subroutines
               in different styles.  See perldebguts for incredi­
               bly 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 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 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 perldebguts) are considered
       for internal use only, and as such are subject to change
       without notice.

       Readline Support

       As shipped, the only command-line history supplied is a
       simplistic one that checks for leading exclamation points.
       However, if you install the Term::ReadKey and Term::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
       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 how regular expression matching
       works in general, but also know how Perl's regular expres­
       sions are internally compiled into an automaton. These
       matters are explored in some detail in "Debugging regular
       expressions" in perldebguts.


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 "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.


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

perl v5.8.1                 2003-09-02               PERLDEBUG(1)

An undefined database error occurred. SELECT distinct pages.pagepath,pages.pageid FROM pages, page2command WHERE pages.pageid = page2command.pageid AND commandid =


  
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