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



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 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.

       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 prettyprinter
                   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 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 subroutines
                   are saved and thus accessible.

       /pattern/   Search forwards for pattern (a Perl regex); final / is optional.  The search is case-insensitive caseinsensitive
                   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 [n]       Toggle trace mode (see also the "AutoTrace" option).  Optional argument is the maximum
                   number of levels to trace below the current one; anything deeper than that will be
                   silent.

       t [n] expr  Trace through execution of "expr".  Optional first argument is the maximum number of
                   levels to trace below the current one; anything deeper than that will be silent.  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

                   If the line number is ".", sets a breakpoint on the current line:

                       b . $n > 100

       b [file]:[line] [condition]
                   Set a breakpoint before the given line in a (possibly different) file.  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 lib/MyModule.pm:237 $x > 30
                       b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11

       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.

       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.

       disable [file]:[line]
                   Disable the breakpoint so it won't stop the execution of the program.  Breakpoints are
                   enabled by default and can be re-enabled using the "enable" command.

       disable [line]
                   Disable the breakpoint so it won't stop the execution of the program.  Breakpoints are
                   enabled by default and can be re-enabled using the "enable" command.

                   This is done for a breakpoint in the current file.

       enable [file]:[line]
                   Enable the breakpoint so it will stop the execution of the program.

       enable [line]
                   Enable the breakpoint so it will stop the execution of the program.

                   This is done for a breakpoint in the current file.

       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 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 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: 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           Display 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:

                       $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 nonfatal
                   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 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 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 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 /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

               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.

   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 the "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 the 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 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 those 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, including lexical variables in the current
       scope if the "PadWalker" module is installed.

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

   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.

       Recent versions of Emacs come with a start file for making emacs act like a syntax-directed editor
       that understands (some of) Perl's syntax.  See perlfaq3.

       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, invoke your script with a colon and a
       package argument given to the -d flag.  Perl's alternative debuggers include a Perl profiler,
       Devel::NYTProf, which is available separately as a CPAN distribution.  To profile your Perl program
       in the file mycode.pl, just type:

           $ perl -d:NYTProf mycode.pl

       When the script terminates the profiler will create a database of the profile information that you
       can turn into reports using the profiler's tools. See <perlperf> for details.

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.

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::NYTProf, 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.



perl v5.16.2                                     2012-10-25                                     PERLDEBUG(1)

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