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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 manpage describes low-level details
     concerning the debugger's internals, which range from diffi-
     cult 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 addi-
     tional features are enabled (cf. "$^P" in perlvar):

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

     +	 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.

     +	 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 sub-
	 routines, or which are currently being executed.  The
	 $filename for "eval"ed strings looks like "(eval 34)" or
	 "(re_eval 19)".

     +	 Each scalar "${"_<$filename"}" contains "_<$filename".

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	 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)".

     +	 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.

     +	 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.

     +	 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.

     +	 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.

     +	 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" pack-
	 age.)

     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:

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       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 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 initializa-
     tion.)

     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.

     +	 @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 execu-
	 tion.

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

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	 breakable.

     +	 %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.

     +	 See "Options" in perldebug for description of options
	 parsed by "DB::parse_options(string)" parses debugger
	 options; see "Options" in pperldebug for a description
	 of options recognized.

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

     +	 "DB::print_trace(FH, skip[, count[, short]])" prints
	 formatted info about caller frames.  The last two func-
	 tions may be convenient as arguments to "<", "<<" com-
	 mands.

     Note that any variables and functions that are not docu-
     mented 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.

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      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 list-
     ing 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 set-
     tings.  Long those it may be, this is not a complete list-
     ing, 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

     4
	   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

     6

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	   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

     14
	   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

     30

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	   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:

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       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 infor-
     mation.  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.	 More-
     over, when checking for these substrings (to abandon impos-
     sible 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.

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     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.

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     Above the optimizer section is the list of nodes of the com-
     piled 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.

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	 # 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 charac-
     ters. "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.

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     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 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, multi-
     ply that estimate by 10, and while you still may miss the

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     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 "-DDE-
     BUGGING"). The switch was used to track Perl's memory allo-
     cations and possible memory leaks.	 These days the use of
     malloc debugging tools like Purify or valgrind is suggested
     instead.

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

       $ 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 avail-
	 able, 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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PERLDEBGUTS(1)	Perl Programmers Reference Guide   PERLDEBGUTS(1)

	      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 con-
	 tinuity 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 v5.8.8		   2006-06-30			       17

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