perldebguts


DESCRIPTION
       This is not perldebug, 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)".

       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
           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 {}

       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.)
           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.

       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<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 using
       various values of "n" are shown to give you a feel for the difference
       between settings.  Long though 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  $=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.

       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.

       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.  Since Perl 5.9.5, this pragma is 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
       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, 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.
        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 using native charset
                           semantics for non-utf8
        BOUNDL     no      Match "" at any locale word boundary
        BOUNDU     no      Match "" at any word boundary using Unicode semantics
        BOUNDA     no      Match "" at any word boundary using ASCII semantics
        NBOUND     no      Match "" at any word non-boundary using native charset
                           semantics for non-utf8
        NBOUNDL    no      Match "" at any locale word non-boundary
        NBOUNDU    no      Match "" at any word non-boundary using Unicode semantics
        NBOUNDA    no      Match "" at any word non-boundary using ASCII semantics
        GPOS       no      Matches where last m//g left off.

        # [Special] alternatives:

        REG_ANY    no      Match any one character (except newline).
        SANY       no      Match any one character.
        CANY       no      Match any one byte.
        ANYOF      sv      Match character in (or not in) this class, single char
                           match only
        ANYOFV     sv      Match character in (or not in) this class, can
                           match-multiple chars
        ALNUM      no      Match any alphanumeric character using native charset
                           semantics for non-utf8
        ALNUML     no      Match any alphanumeric char in locale
        ALNUMU     no      Match any alphanumeric char using Unicode semantics
        ALNUMA     no      Match [A-Za-z_0-9]
        NALNUM     no      Match any non-alphanumeric character using native charset
                           semantics for non-utf8
        NALNUML    no      Match any non-alphanumeric char in locale
        NALNUMU    no      Match any non-alphanumeric char using Unicode semantics
        NALNUMA    no      Match [^A-Za-z_0-9]
        SPACE      no      Match any whitespace character using native charset
                           semantics for non-utf8
        SPACEL     no      Match any whitespace char in locale
        SPACEU     no      Match any whitespace char using Unicode semantics
        SPACEA     no      Match [ \t\n\f\r]
        NSPACE     no      Match any non-whitespace character using native charset
                           semantics for non-utf8
        NSPACEL    no      Match any non-whitespace char in locale
        NSPACEU    no      Match any non-whitespace char using Unicode semantics
        NSPACEA    no      Match [^ \t\n\f\r]
        DIGIT      no      Match any numeric character using native charset semantics
        #               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 pointer

        # 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      str     Match this string (preceded by length).
        EXACTF     str     Match this string, folded, native charset semantics for
                           non-utf8 (prec. by length).
        EXACTFL    str     Match this string, folded in locale (w/len).
        EXACTFU    str     Match this string, folded, Unicode semantics for non-utf8
                           (prec. by length).
        EXACTFA    str     Match this string, folded, Unicode semantics for non-utf8,
                           but no ASCII-range character matches outside ASCII (prec.
                           by length),.

        # Do nothing types

        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.

        # Loops

        # 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    Capture next-after-this simple thing
        CURLYM     no 2    Capture 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.

        # Buffer related


        # Named references.  Code in regcomp.c assumes that these all are after the
        # numbered references
        NREF       no-sv 1 Match some already matched string
        NREFF      no-sv 1 Match already matched string, folded using native charset
                           semantics for non-utf8
        NREFFL     no-sv 1 Match already matched string, folded in loc.
        NREFFU     num   1 Match already matched string, folded using unicode
                           semantics for non-utf8
        NREFFA     num   1 Match already matched string, folded using unicode
                           semantics for non-utf8, no mixing ASCII, non-ASCII

        IFMATCH    off 1 2 Succeeds if the following matches.
        UNLESSM    off 1 2 Fails if the following matches.
        SUSPEND    off 1 1 "Independent" sub-RE.
        IFTHEN     off 1 1 Switch, should be preceded by switcher.
        GROUPP     num 1   Whether the group matched.

        # Support for long RE

        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.

        # Trie Related

        # Behave the same as A|LIST|OF|WORDS would. The '..C' variants have
        # inline charclass data (ascii only), the 'C' store it in the structure.
        # NOTE: the relative order of the TRIE-like regops  is significant

        TRIE       trie 1    Match many EXACT(F[ALU]?)? at once. flags==type
        TRIEC      charclass Same as TRIE, but with embedded charclass data

        # For start classes, contains an added fail table.
        AHOCORASICK trie 1   Aho Corasick stclass. flags==type
        AHOCORASICKC charclass Same as AHOCORASICK, but with embedded charclass data

        # Regex Subroutines
        GOSUB      num/ofs 2L recurse to paren arg1 at (signed) ofs arg2
        GOSTART    no         recurse to start of pattern

        # Special conditionals
        NGROUPP    no-sv 1   Whether the group matched.
        MARKPOINT  no-sv 1   Push the current location for rollback by cut.
        SKIP       no-sv 1   On failure skip forward (to the mark) before retrying
        COMMIT     no-sv 1   Pattern fails outright if backtracking through this
        CUTGROUP   no-sv 1   On failure go to the next alternation in the group

        # Control what to keep in $&.
        KEEPS      no        $& begins here.

        # New charclass like patterns
        LNBREAK    none      generic newline pattern
        VERTWS     none      vertical whitespace         (Perl 6)
        NVERTWS    none      not vertical whitespace     (Perl 6)
        HORIZWS    none      horizontal whitespace       (Perl 6)
        NHORIZWS   none      not horizontal whitespace   (Perl 6)

        FOLDCHAR   codepoint 1 codepoint with tricky case folding properties.

        # SPECIAL  REGOPS

        # 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.

        # Special opcode with the property that no opcode in a compiled program
        # will ever be of this type. Thus it can be used as a flag value that
        # no other opcode has been seen. END is used similarly, in that an END
        # node cant be optimized. So END implies "unoptimizable" and PSEUDO mean
        # "not seen anything to optimize yet".
        PSEUDO     off       Pseudo opcode for internal use.

       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.
            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 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.

       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 $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 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, the corresponding power of two
           is printed in the "APPROX" field above.

                      4     12    24    48    80

           With a 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 v5.14.2                      2011-09-19                    PERLDEBGUTS(1)
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