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NAME
       perldebguts - Guts of Perl debugging

DESCRIPTION
       This is not the perldebug(1) manpage, which tells you how
       to use the debugger.  This manpage describes low-level
       details concerning the debugger's internals, which range
       from difficult to impossible to understand for anyone who
       isn't incredibly intimate with Perl's guts.  Caveat
       lector.

Debugger Internals
       Perl has special debugging hooks at compile-time and run-
       time used to create debugging environments.  These hooks
       are not to be confused with the perl -Dxxx command
       described in perlrun, which is usable only if a special
       Perl is built per the instructions in the INSTALL podpage
       in the Perl source tree.

       For example, whenever you call Perl's built-in "caller"
       function from the package "DB", the arguments that the
       corresponding stack frame was called with are copied to
       the @DB::args array.  These mechanisms are enabled by
       calling Perl with the -d switch.  Specifically, the
       following additional features are enabled (cf. "$^P" in
       perlvar):

       o   Perl inserts the contents of $ENV{PERL5DB} (or "BEGIN
           {require 'perl5db.pl'}" if not present) before the
           first line of your program.

       o   Each array "@{"_<$filename"}" holds the lines of
           $filename for a file compiled by Perl.  The same is
           also true for "eval"ed strings that contain
           subroutines, or which are currently being executed.
           The $filename for "eval"ed strings looks like "(eval
           34)".  Code assertions in regexes look like "(re_eval
           19)".

           Values in this array are magical in numeric context:
           they compare equal to zero only if the line is not
           breakable.

       o   Each hash "%{"_<$filename"}" contains breakpoints and
           actions keyed by line number.  Individual entries (as
           opposed to the whole hash) are settable.  Perl only
           cares about Boolean true here, although the values
           used by perl5db.pl have the form
           "$break_condition\0$action".

           The same holds for evaluated strings that contain
           subroutines, or which are currently being executed.
           The $filename for "eval"ed strings looks like "(eval
           34)" or  "(re_eval 19)".



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       o   Each scalar "${"_<$filename"}" contains "_<$filename".
           This is also the case for evaluated strings that
           contain subroutines, or which are currently being
           executed.  The $filename for "eval"ed strings looks
           like "(eval 34)" or "(re_eval 19)".

       o   After each "require"d file is compiled, but before it
           is executed, "DB::postponed(*{"_<$filename"})" is
           called if the subroutine "DB::postponed" exists.
           Here, the $filename is the expanded name of the
           "require"d file, as found in the values of %INC.

       o   After each subroutine "subname" is compiled, the
           existence of $DB::postponed{subname} is checked.  If
           this key exists, "DB::postponed(subname)" is called if
           the "DB::postponed" subroutine also exists.

       o   A hash %DB::sub is maintained, whose keys are
           subroutine names and whose values have the form
           "filename:startline-endline".  "filename" has the form
           "(eval 34)" for subroutines defined inside "eval"s, or
           "(re_eval 19)" for those within regex code assertions.

       o   When the execution of your program reaches a point
           that can hold a breakpoint, the "DB::DB()" subroutine
           is called if any of the variables $DB::trace,
           $DB::single, or $DB::signal is true.  These variables
           are not "local"izable.  This feature is disabled when
           executing inside "DB::DB()", including functions
           called from it unless "$^D & (1<<30)" is true.

       o   When execution of the program reaches a subroutine
           call, a call to &DB::sub(args) is made instead, with
           $DB::sub holding the name of the called subroutine.
           (This doesn't happen if the subroutine was compiled in
           the "DB" package.)

       Note that if &DB::sub needs external data for it to work,
       no subroutine call is possible without it. As an example,
       the standard debugger's &DB::sub depends on the $DB::deep
       variable (it defines how many levels of recursion deep
       into the debugger you can go before a mandatory break).
       If $DB::deep is not defined, subroutine calls are not
       possible, even though &DB::sub exists.

   Writing Your Own Debugger
       Environment Variables

       The "PERL5DB" environment variable can be used to define a
       debugger.  For example, the minimal "working" debugger (it
       actually doesn't do anything) consists of one line:

         sub DB::DB {}




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       It can easily be defined like this:

         $ PERL5DB="sub DB::DB {}" perl -d your-script

       Another brief debugger, slightly more useful, can be
       created with only the line:

         sub DB::DB {print ++$i; scalar <STDIN>}

       This debugger prints a number which increments for each
       statement encountered and waits for you to hit a newline
       before continuing to the next statement.

       The following debugger is actually useful:

         {
           package DB;
           sub DB  {}
           sub sub {print ++$i, " $sub\n"; &$sub}
         }

       It prints the sequence number of each subroutine call and
       the name of the called subroutine.  Note that &DB::sub is
       being compiled into the package "DB" through the use of
       the "package" directive.

       When it starts, the debugger reads your rc file (./.perldb
       or ~/.perldb under Unix), which can set important options.
       (A subroutine (&afterinit) can be defined here as well; it
       is executed after the debugger completes its own
       initialization.)

       After the rc file is read, the debugger reads the
       PERLDB_OPTS environment variable and uses it to set
       debugger options. The contents of this variable are
       treated as if they were the argument of an "o ..."
       debugger command (q.v. in "Options" in perldebug).

       Debugger internal variables

       In addition to the file and subroutine-related variables
       mentioned above, the debugger also maintains various
       magical internal variables.

       o   @DB::dbline is an alias for "@{"::_<current_file"}",
           which holds the lines of the currently-selected file
           (compiled by Perl), either explicitly chosen with the
           debugger's "f" command, or implicitly by flow of
           execution.

           Values in this array are magical in numeric context:
           they compare equal to zero only if the line is not
           breakable.




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       o   %DB::dbline, is an alias for "%{"::_<current_file"}",
           which contains breakpoints and actions keyed by line
           number in the currently-selected file, either
           explicitly chosen with the debugger's "f" command, or
           implicitly by flow of execution.

           As previously noted, individual entries (as opposed to
           the whole hash) are settable.  Perl only cares about
           Boolean true here, although the values used by
           perl5db.pl have the form "$break_condition\0$action".

       Debugger customization functions

       Some functions are provided to simplify customization.

       o   See "Configurable Options" in perldebug for a
           description of options parsed by
           "DB::parse_options(string)".

       o   "DB::dump_trace(skip[,count])" skips the specified
           number of frames and returns a list containing
           information about the calling frames (all of them, if
           "count" is missing).  Each entry is reference to a
           hash with keys "context" (either ".", "$", or "@"),
           "sub" (subroutine name, or info about "eval"), "args"
           ("undef" or a reference to an array), "file", and
           "line".

       o   "DB::print_trace(FH, skip[, count[, short]])" prints
           formatted info about caller frames.  The last two
           functions may be convenient as arguments to "<", "<<"
           commands.

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

Frame Listing Output Examples
       The "frame" option can be used to control the output of
       frame information.  For example, contrast this expression
       trace:

        $ perl -de 42
        Stack dump during die enabled outside of evals.

        Loading DB routines from perl5db.pl patch level 0.94
        Emacs support available.

        Enter h or `h h' for help.

        main::(-e:1):   0
          DB<1> sub foo { 14 }




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          DB<2> sub bar { 3 }

          DB<3> t print foo() * bar()
        main::((eval 172):3):   print foo() + bar();
        main::foo((eval 168):2):
        main::bar((eval 170):2):
        42

       with this one, once the "o"ption "frame=2" has been set:

          DB<4> o f=2
                       frame = '2'
          DB<5> t print foo() * bar()
        3:      foo() * bar()
        entering main::foo
         2:     sub foo { 14 };
        exited main::foo
        entering main::bar
         2:     sub bar { 3 };
        exited main::bar
        42

       By way of demonstration, we present below a laborious
       listing resulting from setting your "PERLDB_OPTS"
       environment variable to the value "f=n N", and running
       perl -d -V from the command line.  Examples use various
       values of "n" are shown to give you a feel for the
       difference between settings.  Long those it may be, this
       is not a complete listing, but only excerpts.

       1.
             entering main::BEGIN
              entering Config::BEGIN
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              Package lib/Config.pm.
              entering Config::TIEHASH
              entering Exporter::import
               entering Exporter::export
             entering Config::myconfig
              entering Config::FETCH
              entering Config::FETCH
              entering Config::FETCH
              entering Config::FETCH

       2.











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             entering main::BEGIN
              entering Config::BEGIN
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              exited Config::BEGIN
              Package lib/Config.pm.
              entering Config::TIEHASH
              exited Config::TIEHASH
              entering Exporter::import
               entering Exporter::export
               exited Exporter::export
              exited Exporter::import
             exited main::BEGIN
             entering Config::myconfig
              entering Config::FETCH
              exited Config::FETCH
              entering Config::FETCH
              exited Config::FETCH
              entering Config::FETCH

       3.
             in  $=main::BEGIN() from /dev/null:0
              in  $=Config::BEGIN() from lib/Config.pm:2
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              Package lib/Config.pm.
              in  $=Config::TIEHASH('Config') from lib/Config.pm:644
              in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
               in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
             in  @=Config::myconfig() from /dev/null:0
              in  $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574

       4.
             in  $=main::BEGIN() from /dev/null:0
              in  $=Config::BEGIN() from lib/Config.pm:2
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              out $=Config::BEGIN() from lib/Config.pm:0
              Package lib/Config.pm.
              in  $=Config::TIEHASH('Config') from lib/Config.pm:644
              out $=Config::TIEHASH('Config') from lib/Config.pm:644
              in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
               in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
               out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
              out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
             out $=main::BEGIN() from /dev/null:0
             in  @=Config::myconfig() from /dev/null:0
              in  $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
              out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574



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              in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
              out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
              out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
              in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574

       5.
             in  $=main::BEGIN() from /dev/null:0
              in  $=Config::BEGIN() from lib/Config.pm:2
               Package lib/Exporter.pm.
               Package lib/Carp.pm.
              out $=Config::BEGIN() from lib/Config.pm:0
              Package lib/Config.pm.
              in  $=Config::TIEHASH('Config') from lib/Config.pm:644
              out $=Config::TIEHASH('Config') from lib/Config.pm:644
              in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
               in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
               out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
              out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
             out $=main::BEGIN() from /dev/null:0
             in  @=Config::myconfig() from /dev/null:0
              in  $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
              out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
              in  $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
              out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574

       6.
             in  $=CODE(0x15eca4)() from /dev/null:0
              in  $=CODE(0x182528)() from lib/Config.pm:2
               Package lib/Exporter.pm.
              out $=CODE(0x182528)() from lib/Config.pm:0
              scalar context return from CODE(0x182528): undef
              Package lib/Config.pm.
              in  $=Config::TIEHASH('Config') from lib/Config.pm:628
              out $=Config::TIEHASH('Config') from lib/Config.pm:628
              scalar context return from Config::TIEHASH:   empty hash
              in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
               in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
               out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
               scalar context return from Exporter::export: ''
              out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
              scalar context return from Exporter::import: ''

       In all cases shown above, the line indentation shows the
       call tree.  If bit 2 of "frame" is set, a line is printed
       on exit from a subroutine as well.  If bit 4 is set, the
       arguments are printed along with the caller info.  If bit
       8 is set, the arguments are printed even if they are tied
       or references.  If bit 16 is set, the return value is
       printed, too.

       When a package is compiled, a line like this

           Package lib/Carp.pm.



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       is printed with proper indentation.

Debugging regular expressions
       There are two ways to enable debugging output for regular
       expressions.

       If your perl is compiled with "-DDEBUGGING", you may use
       the -Dr flag on the command line.

       Otherwise, one can "use re 'debug'", which has effects at
       compile time and run time.  It is not lexically scoped.

   Compile-time output
       The debugging output at compile time looks like this:

         Compiling REx `[bc]d(ef*g)+h[ij]k$'
         size 45 Got 364 bytes for offset annotations.
         first at 1
         rarest char g at 0
         rarest char d at 0
            1: ANYOF[bc](12)
           12: EXACT <d>(14)
           14: CURLYX[0] {1,32767}(28)
           16:   OPEN1(18)
           18:     EXACT <e>(20)
           20:     STAR(23)
           21:       EXACT <f>(0)
           23:     EXACT <g>(25)
           25:   CLOSE1(27)
           27:   WHILEM[1/1](0)
           28: NOTHING(29)
           29: EXACT <h>(31)
           31: ANYOF[ij](42)
           42: EXACT <k>(44)
           44: EOL(45)
           45: END(0)
         anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
               stclass `ANYOF[bc]' minlen 7
         Offsets: [45]
               1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
               0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
               11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
               0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]
         Omitting $` $& $' support.

       The first line shows the pre-compiled form of the regex.
       The second shows the size of the compiled form (in
       arbitrary units, usually 4-byte words) and the total
       number of bytes allocated for the offset/length table,
       usually 4+"size"*8.  The next line shows the label id of
       the first node that does a match.

       The




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         anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
               stclass `ANYOF[bc]' minlen 7

       line (split into two lines above) contains optimizer
       information.  In the example shown, the optimizer found
       that the match should contain a substring "de" at offset
       1, plus substring "gh" at some offset between 3 and
       infinity.  Moreover, when checking for these substrings
       (to abandon impossible matches quickly), Perl will check
       for the substring "gh" before checking for the substring
       "de".  The optimizer may also use the knowledge that the
       match starts (at the "first" id) with a character class,
       and no string shorter than 7 characters can possibly
       match.

       The fields of interest which may appear in this line are

       "anchored" STRING "at" POS
       "floating" STRING "at" POS1..POS2
           See above.

       "matching floating/anchored"
           Which substring to check first.

       "minlen"
           The minimal length of the match.

       "stclass" TYPE
           Type of first matching node.

       "noscan"
           Don't scan for the found substrings.

       "isall"
           Means that the optimizer information is all that the
           regular expression contains, and thus one does not
           need to enter the regex engine at all.

       "GPOS"
           Set if the pattern contains "\G".

       "plus"
           Set if the pattern starts with a repeated char (as in
           "x+y").

       "implicit"
           Set if the pattern starts with ".*".

       "with eval"
           Set if the pattern contain eval-groups, such as "(?{
           code })" and "(??{ code })".

       "anchored(TYPE)"
           If the pattern may match only at a handful of places,



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           (with "TYPE" being "BOL", "MBOL", or "GPOS".  See the
           table below.

       If a substring is known to match at end-of-line only, it
       may be followed by "$", as in "floating `k'$".

       The optimizer-specific information is used to avoid
       entering (a slow) regex engine on strings that will not
       definitely match.  If the "isall" flag is set, a call to
       the regex engine may be avoided even when the optimizer
       found an appropriate place for the match.

       Above the optimizer section is the list of nodes of the
       compiled form of the regex.  Each line has format

       "   "id: TYPE OPTIONAL-INFO (next-id)

   Types of nodes
       Here are the possible types, with short descriptions:

           # TYPE arg-description [num-args] [longjump-len] DESCRIPTION

           # Exit points
           END         no      End of program.
           SUCCEED     no      Return from a subroutine, basically.

           # Anchors:
           BOL         no      Match "" at beginning of line.
           MBOL        no      Same, assuming multiline.
           SBOL        no      Same, assuming singleline.
           EOS         no      Match "" at end of string.
           EOL         no      Match "" at end of line.
           MEOL        no      Same, assuming multiline.
           SEOL        no      Same, assuming singleline.
           BOUND       no      Match "" at any word boundary
           BOUNDL      no      Match "" at any word boundary
           NBOUND      no      Match "" at any word non-boundary
           NBOUNDL     no      Match "" at any word non-boundary
           GPOS        no      Matches where last m//g left off.

           # [Special] alternatives
           ANY         no      Match any one character (except newline).
           SANY        no      Match any one character.
           ANYOF       sv      Match character in (or not in) this class.
           ALNUM       no      Match any alphanumeric character
           ALNUML      no      Match any alphanumeric char in locale
           NALNUM      no      Match any non-alphanumeric character
           NALNUML     no      Match any non-alphanumeric char in locale
           SPACE       no      Match any whitespace character
           SPACEL      no      Match any whitespace char in locale
           NSPACE      no      Match any non-whitespace character
           NSPACEL     no      Match any non-whitespace char in locale
           DIGIT       no      Match any numeric character
           NDIGIT      no      Match any non-numeric character



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           # BRANCH    The set of branches constituting a single choice are hooked
           #           together with their "next" pointers, since precedence prevents
           #           anything being concatenated to any individual branch.  The
           #           "next" pointer of the last BRANCH in a choice points to the
           #           thing following the whole choice.  This is also where the
           #           final "next" pointer of each individual branch points; each
           #           branch starts with the operand node of a BRANCH node.
           #
           BRANCH      node    Match this alternative, or the next...

           # BACK      Normal "next" pointers all implicitly point forward; BACK
           #           exists to make loop structures possible.
           # not used
           BACK        no      Match "", "next" ptr points backward.

           # Literals
           EXACT       sv      Match this string (preceded by length).
           EXACTF      sv      Match this string, folded (prec. by length).
           EXACTFL     sv      Match this string, folded in locale (w/len).

           # Do nothing
           NOTHING     no      Match empty string.
           # A variant of above which delimits a group, thus stops optimizations
           TAIL        no      Match empty string. Can jump here from outside.

           # STAR,PLUS '?', and complex '*' and '+', are implemented as circular
           #           BRANCH structures using BACK.  Simple cases (one character
           #           per match) are implemented with STAR and PLUS for speed
           #           and to minimize recursive plunges.
           #
           STAR        node    Match this (simple) thing 0 or more times.
           PLUS        node    Match this (simple) thing 1 or more times.

           CURLY       sv 2    Match this simple thing {n,m} times.
           CURLYN      no 2    Match next-after-this simple thing
           #                   {n,m} times, set parens.
           CURLYM      no 2    Match this medium-complex thing {n,m} times.
           CURLYX      sv 2    Match this complex thing {n,m} times.

           # This terminator creates a loop structure for CURLYX
           WHILEM      no      Do curly processing and see if rest matches.

           # OPEN,CLOSE,GROUPP ...are numbered at compile time.
           OPEN        num 1   Mark this point in input as start of #n.
           CLOSE       num 1   Analogous to OPEN.

           REF         num 1   Match some already matched string
           REFF        num 1   Match already matched string, folded
           REFFL       num 1   Match already matched string, folded in loc.

           # grouping assertions
           IFMATCH     off 1 2 Succeeds if the following matches.
           UNLESSM     off 1 2 Fails if the following matches.



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           SUSPEND     off 1 1 "Independent" sub-regex.
           IFTHEN      off 1 1 Switch, should be preceded by switcher .
           GROUPP      num 1   Whether the group matched.

           # Support for long regex
           LONGJMP     off 1 1 Jump far away.
           BRANCHJ     off 1 1 BRANCH with long offset.

           # The heavy worker
           EVAL        evl 1   Execute some Perl code.

           # Modifiers
           MINMOD      no      Next operator is not greedy.
           LOGICAL     no      Next opcode should set the flag only.

           # This is not used yet
           RENUM       off 1 1 Group with independently numbered parens.

           # This is not really a node, but an optimized away piece of a "long" node.
           # To simplify debugging output, we mark it as if it were a node
           OPTIMIZED   off     Placeholder for dump.

       Following the optimizer information is a dump of the
       offset/length table, here split across several lines:

         Offsets: [45]
               1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
               0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
               11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
               0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]

       The first line here indicates that the offset/length table
       contains 45 entries.  Each entry is a pair of integers,
       denoted by "offset[length]".  Entries are numbered
       starting with 1, so entry #1 here is "1[4]" and entry #12
       is "5[1]".  "1[4]" indicates that the node labeled "1:"
       (the "1: ANYOF[bc]") begins at character position 1 in the
       pre-compiled form of the regex, and has a length of 4
       characters.  "5[1]" in position 12 indicates that the node
       labeled "12:" (the "12: EXACT <d>") begins at character
       position 5 in the pre-compiled form of the regex, and has
       a length of 1 character.  "12[1]" in position 14 indicates
       that the node labeled "14:" (the "14: CURLYX[0]
       {1,32767}") begins at character position 12 in the pre-
       compiled form of the regex, and has a length of 1
       character---that is, it corresponds to the "+" symbol in
       the precompiled regex.

       "0[0]" items indicate that there is no corresponding node.

   Run-time output
       First of all, when doing a match, one may get no run-time
       output even if debugging is enabled.  This means that the
       regex engine was never entered and that all of the job was



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       therefore done by the optimizer.

       If the regex engine was entered, the output may look like
       this:

         Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
           Setting an EVAL scope, savestack=3
            2 <ab> <cdefg__gh_>    |  1: ANYOF
            3 <abc> <defg__gh_>    | 11: EXACT <d>
            4 <abcd> <efg__gh_>    | 13: CURLYX {1,32767}
            4 <abcd> <efg__gh_>    | 26:   WHILEM
                                       0 out of 1..32767  cc=effff31c
            4 <abcd> <efg__gh_>    | 15:     OPEN1
            4 <abcd> <efg__gh_>    | 17:     EXACT <e>
            5 <abcde> <fg__gh_>    | 19:     STAR
                                    EXACT <f> can match 1 times out of 32767...
           Setting an EVAL scope, savestack=3
            6 <bcdef> <g__gh__>    | 22:       EXACT <g>
            7 <bcdefg> <__gh__>    | 24:       CLOSE1
            7 <bcdefg> <__gh__>    | 26:       WHILEM
                                           1 out of 1..32767  cc=effff31c
           Setting an EVAL scope, savestack=12
            7 <bcdefg> <__gh__>    | 15:         OPEN1
            7 <bcdefg> <__gh__>    | 17:         EXACT <e>
              restoring \1 to 4(4)..7
                                           failed, try continuation...
            7 <bcdefg> <__gh__>    | 27:         NOTHING
            7 <bcdefg> <__gh__>    | 28:         EXACT <h>
                                           failed...
                                       failed...

       The most significant information in the output is about
       the particular node of the compiled regex that is
       currently being tested against the target string.  The
       format of these lines is

       "    "STRING-OFFSET <PRE-STRING> <POST-STRING>   |ID:
       TYPE

       The TYPE info is indented with respect to the backtracking
       level.  Other incidental information appears interspersed
       within.

Debugging Perl memory usage
       Perl is a profligate wastrel when it comes to memory use.
       There is a saying that to estimate memory usage of Perl,
       assume a reasonable algorithm for memory allocation,
       multiply that estimate by 10, and while you still may miss
       the mark, at least you won't be quite so astonished.  This
       is not absolutely true, but may provide a good grasp of
       what happens.

       Assume that an integer cannot take less than 20 bytes of
       memory, a float cannot take less than 24 bytes, a string



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       cannot take less than 32 bytes (all these examples assume
       32-bit architectures, the result are quite a bit worse on
       64-bit architectures).  If a variable is accessed in two
       of three different ways (which require an integer, a
       float, or a string), the memory footprint may increase yet
       another 20 bytes.  A sloppy malloc(3) implementation can
       inflate these numbers dramatically.

       On the opposite end of the scale, a declaration like

         sub foo;

       may take up to 500 bytes of memory, depending on which
       release of Perl you're running.

       Anecdotal estimates of source-to-compiled code bloat
       suggest an eightfold increase.  This means that the
       compiled form of reasonable (normally commented, properly
       indented etc.) code will take about eight times more space
       in memory than the code took on disk.

       The -DL command-line switch is obsolete since circa Perl
       5.6.0 (it was available only if Perl was built with
       "-DDEBUGGING").  The switch was used to track Perl's
       memory allocations and possible memory leaks.  These days
       the use of malloc debugging tools like Purify or valgrind
       is suggested instead.  See also "PERL_MEM_LOG" in
       perlhack.

       One way to find out how much memory is being used by Perl
       data structures is to install the Devel::Size module from
       CPAN: it gives you the minimum number of bytes required to
       store a particular data structure.  Please be mindful of
       the difference between the size() and total_size().

       If Perl has been compiled using Perl's malloc you can
       analyze Perl memory usage by setting the
       $ENV{PERL_DEBUG_MSTATS}.

   Using $ENV{PERL_DEBUG_MSTATS}
       If your perl is using Perl's malloc() and was compiled
       with the necessary switches (this is the default), then it
       will print memory usage statistics after compiling your
       code when "$ENV{PERL_DEBUG_MSTATS} > 1", and before
       termination of the program when "$ENV{PERL_DEBUG_MSTATS}
       >= 1".  The report format is similar to the following
       example:










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         $ PERL_DEBUG_MSTATS=2 perl -e "require Carp"
         Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
            14216 free:   130   117    28     7     9   0   2     2   1 0 0
                       437    61    36     0     5
            60924 used:   125   137   161    55     7   8   6    16   2 0 1
                        74   109   304    84    20
         Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
         Memory allocation statistics after execution:   (buckets 4(4)..8188(8192)
            30888 free:   245    78    85    13     6   2   1     3   2 0 1
                       315   162    39    42    11
           175816 used:   265   176  1112   111    26  22  11    27   2 1 1
                       196   178  1066   798    39
         Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.

       It is possible to ask for such a statistic at arbitrary
       points in your execution using the mstat() function out of
       the standard Devel::Peek module.

       Here is some explanation of that format:

       "buckets SMALLEST(APPROX)..GREATEST(APPROX)"
           Perl's malloc() uses bucketed allocations.  Every
           request is rounded up to the closest bucket size
           available, and a bucket is taken from the pool of
           buckets of that size.

           The line above describes the limits of buckets
           currently in use.  Each bucket has two sizes: memory
           footprint and the maximal size of user data that can
           fit into this bucket.  Suppose in the above example
           that the smallest bucket were size 4.  The biggest
           bucket would have usable size 8188, and the memory
           footprint would be 8192.

           In a Perl built for debugging, some buckets may have
           negative usable size.  This means that these buckets
           cannot (and will not) be used.  For larger buckets,
           the memory footprint may be one page greater than a
           power of 2.  If so, case the corresponding power of
           two is printed in the "APPROX" field above.

       Free/Used
           The 1 or 2 rows of numbers following that correspond
           to the number of buckets of each size between
           "SMALLEST" and "GREATEST".  In the first row, the
           sizes (memory footprints) of buckets are powers of
           two--or possibly one page greater.  In the second row,
           if present, the memory footprints of the buckets are
           between the memory footprints of two buckets "above".

           For example, suppose under the previous example, the
           memory footprints were





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                free:    8     16    32    64    128  256 512 1024 2048 4096 8192
                      4     12    24    48    80

           With non-"DEBUGGING" perl, the buckets starting from
           128 have a 4-byte overhead, and thus an 8192-long
           bucket may take up to 8188-byte allocations.

       "Total sbrk(): SBRKed/SBRKs:CONTINUOUS"
           The first two fields give the total amount of memory
           perl sbrk(2)ed (ess-broken? :-) and number of sbrk(2)s
           used.  The third number is what perl thinks about
           continuity of returned chunks.  So long as this number
           is positive, malloc() will assume that it is probable
           that sbrk(2) will provide continuous memory.

           Memory allocated by external libraries is not counted.

       "pad: 0"
           The amount of sbrk(2)ed memory needed to keep buckets
           aligned.

       "heads: 2192"
           Although memory overhead of bigger buckets is kept
           inside the bucket, for smaller buckets, it is kept in
           separate areas.  This field gives the total size of
           these areas.

       "chain: 0"
           malloc() may want to subdivide a bigger bucket into
           smaller buckets.  If only a part of the deceased
           bucket is left unsubdivided, the rest is kept as an
           element of a linked list.  This field gives the total
           size of these chunks.

       "tail: 6144"
           To minimize the number of sbrk(2)s, malloc() asks for
           more memory.  This field gives the size of the yet
           unused part, which is sbrk(2)ed, but never touched.

SEE ALSO
       perldebug, perlguts, perlrun re, and Devel::DProf.
















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