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PERLDEBGUTS(1)			 Perl Programmers Reference Guide		   PERLDEBGUTS(1)

NAME
       perldebguts - Guts of Perl debugging

DESCRIPTION
       This is not the perldebug(1) manpage, which tells you how to use the debugger.  This man-
       page 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_condi-
	   tion\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)".

       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::post-
	   poned(*{"_<$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::post-
	   poned" 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 sub-
	   routines defined inside "eval"s, or "(re_eval 19)" for those within regex code asser-
	   tions.

       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 vari-
       able (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 {}

       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 subrou-
       tine.  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 men-
       tioned above, the debugger also maintains various magical internal variables.

       o   @DB::dbline is an alias for "@{"::_<current_file"}", which holds the lines of the cur-
	   rently-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.

       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 }

	  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
	     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
	      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 argu-
       ments 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.

       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

	 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 sub-
       string "gh" at some offset between 3 and infinity.  Moreover, when checking for these sub-
       strings (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, (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

	   # 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.
	   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 indi-
       cates 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
       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 com-
       piled 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 infor-
       mation appears interspersed within.

Debugging Perl memory usage
       Perl is a profligate wastrel when it comes to memory use.  There is a saying that to esti-
       mate 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 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 alloca-
       tions 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:

	 $ 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 big-
	   gest 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 foot-
	   prints) 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 foot-
	   prints of two buckets "above".

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

		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.

perl v5.8.9				    2007-11-17				   PERLDEBGUTS(1)
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