MirOS Manual: perldebguts(1)


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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       $ 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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              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.

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