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perldebguts ()
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    NAME

         perldebguts - Guts of Perl debugging
    
    
    

    DESCRIPTION

         This is not the perldebug(1) manpage, which tells you how to
         use the debugger.  This manpage describes low-level details
         ranging between difficult and impossible for anyone who
         isn't incredibly intimate with Perl's guts to understand.
         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
         the perlrun manpage, which are usable only if a special Perl
         built per the instructions 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
         the @DB::args array.  The general mechanisms is enabled by
         calling Perl with the -d switch, the following additional
         features are enabled (cf. the section on "$^P" in the
         perlvar manpage):
    
         o   Perl inserts the contents of `$ENV{PERL5DB}' (or `BEGIN
             {require 'perl5db.pl'}' if not present) before the first
             line of your program.
    
         o   The array `@{"_<$filename"}' holds the lines of
             $filename for all files compiled by Perl.  The same 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)'.
    
         o   The 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"'.  Values in this hash are
             magical in numeric context: they are zeros if the line
             is not breakable.
    
             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   The scalar `${"_<$filename"}' contains `"_<$filename"'.
             This is also the case for evaluated strings that contain
             subroutines, or which are currently being executed.  The
             $filename for `eval'ed strings looks like `(eval 34)' or
             `(re_eval 19)'.
    
         o   After each `require'd file is compiled, but before it is
             executed, `DB::postponed(*{"_<$filename"})' is called if
             the subroutine `DB::postponed' exists.  Here, the
             $filename is the expanded name of the `require'd file,
             as found in the values of %INC.
    
         o   After each subroutine `subname' is compiled, the
             existence of `$DB::postponed{subname}' is checked.  If
             this key exists, `DB::postponed(subname)' is called if
             the `DB::postponed' subroutine also exists.
    
         o   A hash `%DB::sub' is maintained, whose keys are
             subroutine names and whose values have the form
             `filename:startline-endline'.  `filename' has the form
             `(eval 34)' for subroutines defined inside `eval's, or
             `(re_eval 19)' for those within regex code assertions.
    
         o   When the execution of your program reaches a point that
             can hold a breakpoint, the `DB::DB()' subroutine is
             called any of the variables $DB::trace, $DB::single, or
             $DB::signal is true.  These variables are not
             `local'izable.  This feature is disabled when executing
             inside `DB::DB()', including functions called from it
             unless `$^D & (1<<30)' is true.
    
         o   When execution of the program reaches a subroutine call,
             a call to `&DB::sub'(args) is made instead, with
             `$DB::sub' holding the name of the called subroutine.
             This doesn't happen if the subroutine was compiled in
             the `DB' package.)
    
         Note that if `&DB::sub' needs external data for it to work,
         no subroutine call is possible until this is done.  For the
         standard debugger, the  `$DB::deep' variable (how many
         levels of recursion deep into the debugger you can go before
         a mandatory break) gives an example of such a dependency.
    
         Writing Your Own Debugger
    
         The minimal working debugger consists of one line
    
           sub DB::DB {}
    
         which is quite handy as contents of `PERL5DB' environment
         variable:
    
           $ PERL5DB="sub DB::DB {}" perl -d your-script
    
         Another brief debugger, slightly more useful, could be
         created with only the line:
    
           sub DB::DB {print ++$i; scalar <STDIN>}
    
         This debugger would print the sequential number of
         encountered statement, and would wait for you to hit a
         newline before continuing.
    
         The following debugger is quite functional:
    
           {
             package DB;
             sub DB  {}
             sub sub {print ++$i, " $sub\n"; &$sub}
           }
    
         It prints the sequential number of subroutine call and the
         name of the called subroutine.  Note that `&DB::sub' should
         be compiled into the package `DB'.
    
         At the start, the debugger reads your rc file (./.perldb or
         ~/.perldb under Unix), which can set important options.
         This file may define a subroutine `&afterinit' to be
         executed after the debugger is initialized.
    
         After the rc file is read, the debugger reads the
         PERLDB_OPTS environment variable and parses this as the
         remainder of a `O ...'  line as one might enter at the
         debugger prompt.
    
         The debugger also maintains magical internal variables, such
         as `@DB::dbline', `%DB::dbline', which are aliases for
         `@{"::_<current_file"}' `%{"::_<current_file"}'.  Here
         `current_file' is the currently selected file, either
         explicitly chosen with the debugger's `f' command, or
         implicitly by flow of execution.
    
         Some functions are provided to simplify customization.  See
         the Options entry in the perldebug manpage for description
         of options parsed by `DB::parse_options(string)'.  The
         function `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 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'.
    
    
         The function `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 the perldebug manpage)
         are considered for internal use only, and as such are
         subject to change without notice.
    
    
    

    Frame Listing Output Examples

         The `frame' option can be used to control the output of
         frame information.  For example, contrast this expression
         trace:
    
          $ perl -de 42
          Stack dump during die enabled outside of evals.
    
          Loading DB routines from perl5db.pl patch level 0.94
          Emacs support available.
    
          Enter h or `h h' for help.
    
          main::(-e:1):   0
            DB<1> sub foo { 14 }
    
            DB<2> sub bar { 3 }
    
            DB<3> t print foo() * bar()
          main::((eval 172):3):   print foo() + bar();
          main::foo((eval 168):2):
          main::bar((eval 170):2):
          42
    
         with this one, once the `O'ption `frame=2' has been set:
    
            DB<4> O f=2
                         frame = '2'
            DB<5> t print foo() * bar()
          3:      foo() * bar()
          entering main::foo
           2:     sub foo { 14 };
          exited main::foo
          entering main::bar
           2:     sub bar { 3 };
          exited main::bar
          42
    
         By way of demonstration, we present below a laborious
         listing resulting from setting your `PERLDB_OPTS'
         environment variable to the value `f=n N', and running perl
         -d -V from the command line.  Examples use various values of
         `n' are shown to give you a feel for the difference between
         settings.  Long those it may be, this is not a complete
         listing, but only excerpts.
    
         1
               entering main::BEGIN
                entering Config::BEGIN
                 Package lib/Exporter.pm.
                 Package lib/Carp.pm.
                Package lib/Config.pm.
                entering Config::TIEHASH
                entering Exporter::import
                 entering Exporter::export
               entering Config::myconfig
                entering Config::FETCH
                entering Config::FETCH
                entering Config::FETCH
                entering Config::FETCH
    
    
         2
               entering main::BEGIN
                entering Config::BEGIN
                 Package lib/Exporter.pm.
                 Package lib/Carp.pm.
                exited Config::BEGIN
                Package lib/Config.pm.
                entering Config::TIEHASH
                exited Config::TIEHASH
                entering Exporter::import
                 entering Exporter::export
                 exited Exporter::export
                exited Exporter::import
               exited main::BEGIN
               entering Config::myconfig
                entering Config::FETCH
                exited Config::FETCH
                entering Config::FETCH
                exited Config::FETCH
                entering Config::FETCH
    
    
         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
               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
               in  $=CODE(0x15eca4)() from /dev/null:0
                in  $=CODE(0x182528)() from lib/Config.pm:2
                 Package lib/Exporter.pm.
                out $=CODE(0x182528)() from lib/Config.pm:0
                scalar context return from CODE(0x182528): undef
                Package lib/Config.pm.
                in  $=Config::TIEHASH('Config') from lib/Config.pm:628
                out $=Config::TIEHASH('Config') from lib/Config.pm:628
                scalar context return from Config::TIEHASH:   empty hash
                in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
                 in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
                 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
                 scalar context return from Exporter::export: ''
                out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
                scalar context return from Exporter::import: ''
    
    
         In all cases shown above, the line indentation shows the
         call tree.  If bit 2 of `frame' is set, a line is printed on
         exit from a subroutine as well.  If bit 4 is set, the
         arguments are printed along with the caller info.  If bit 8
         is set, the arguments are printed even if they are tied or
         references.  If bit 16 is set, the return value is printed,
         too.
    
         When a package is compiled, a line like this
    
             Package lib/Carp.pm.
    
         is printed with proper indentation.
    
    
    

    Debugging regular expressions

         There are two ways to enable debugging output for regular
         expressions.
    
         If your perl is compiled with `-DDEBUGGING', you may use the
         -Dr flag on the command line.
    
         Otherwise, one can `use re 'debug'', which has effects at
         compile time and run time.  It is not lexically scoped.
    
         Compile-time output
    
         The debugging output at compile time looks like this:
    
           compiling RE `[bc]d(ef*g)+h[ij]k$'
           size 43 first at 1
              1: ANYOF(11)
             11: EXACT <d>(13)
             13: CURLYX {1,32767}(27)
             15:   OPEN1(17)
             17:     EXACT <e>(19)
             19:     STAR(22)
             20:       EXACT <f>(0)
             22:     EXACT <g>(24)
             24:   CLOSE1(26)
             26:   WHILEM(0)
             27: NOTHING(28)
             28: EXACT <h>(30)
             30: ANYOF(40)
             40: EXACT <k>(42)
             42: EOL(43)
             43: END(0)
           anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
                                             stclass `ANYOF' minlen 7
    
         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 label id of the first
         node that does a match.
    
         The last line (split into two lines above) contains
         optimizer information.  In the example shown, the optimizer
         found that the match should contain a substring `de' at
         offset 1, plus substring `gh' at some offset between 3 and
         infinity.  Moreover, when checking for these substrings (to
         abandon impossible matches quickly), Perl will check for the
         substring `gh' before checking for the substring `de'.  The
         optimizer may also use the knowledge that the match starts
         (at the `first' id) with a character class, and the match
         cannot be shorter than 7 chars.
    
    
         The fields of interest which may appear in the last 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 info 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 info is used to avoid entering (a
         slow) regex engine on strings that will not definitely
         match.  If `isall' flag is set, a call to the regex engine
         may be avoided even when the optimizer found an appropriate
         place for the match.
    
         The rest of the output contains the list of nodes of the
         compiled form of the regex.  Each line has format
    
         `   'id: TYPE OPTIONAL-INFO (next-id)
    
         Types of nodes
    
         Here are the possible types, with short descriptions:
    
             # TYPE arg-description [num-args] [longjump-len] DESCRIPTION
    
             # Exit points
             END         no      End of program.
             SUCCEED     no      Return from a subroutine, basically.
    
             # Anchors:
             BOL         no      Match "" at beginning of line.
             MBOL        no      Same, assuming multiline.
             SBOL        no      Same, assuming singleline.
             EOS         no      Match "" at end of string.
             EOL         no      Match "" at end of line.
             MEOL        no      Same, assuming multiline.
             SEOL        no      Same, assuming singleline.
             BOUND       no      Match "" at any word boundary
             BOUNDL      no      Match "" at any word boundary
             NBOUND      no      Match "" at any word non-boundary
             NBOUNDL     no      Match "" at any word non-boundary
             GPOS        no      Matches where last m//g left off.
    
             # [Special] alternatives
             ANY         no      Match any one character (except newline).
             SANY        no      Match any one character.
             ANYOF       sv      Match character in (or not in) this class.
             ALNUM       no      Match any alphanumeric character
             ALNUML      no      Match any alphanumeric char in locale
             NALNUM      no      Match any non-alphanumeric character
             NALNUML     no      Match any non-alphanumeric char in locale
             SPACE       no      Match any whitespace character
             SPACEL      no      Match any whitespace char in locale
             NSPACE      no      Match any non-whitespace character
             NSPACEL     no      Match any non-whitespace char in locale
             DIGIT       no      Match any numeric character
             NDIGIT      no      Match any non-numeric character
    
    
    
             # BRANCH    The set of branches constituting a single choice are hooked
             #           together with their "next" pointers, since precedence prevents
             #           anything being concatenated to any individual branch.  The
             #           "next" pointer of the last BRANCH in a choice points to the
             #           thing following the whole choice.  This is also where the
             #           final "next" pointer of each individual branch points; each
             #           branch starts with the operand node of a BRANCH node.
             #
             BRANCH      node    Match this alternative, or the next...
    
             # BACK      Normal "next" pointers all implicitly point forward; BACK
             #           exists to make loop structures possible.
             # not used
             BACK        no      Match "", "next" ptr points backward.
    
             # Literals
             EXACT       sv      Match this string (preceded by length).
             EXACTF      sv      Match this string, folded (prec. by length).
             EXACTFL     sv      Match this string, folded in locale (w/len).
    
             # Do nothing
             NOTHING     no      Match empty string.
             # A variant of above which delimits a group, thus stops optimizations
             TAIL        no      Match empty string. Can jump here from outside.
    
             # STAR,PLUS '?', and complex '*' and '+', are implemented as circular
             #           BRANCH structures using BACK.  Simple cases (one character
             #           per match) are implemented with STAR and PLUS for speed
             #           and to minimize recursive plunges.
             #
             STAR        node    Match this (simple) thing 0 or more times.
             PLUS        node    Match this (simple) thing 1 or more times.
    
             CURLY       sv 2    Match this simple thing {n,m} times.
             CURLYN      no 2    Match next-after-this simple thing
             #                   {n,m} times, set parens.
             CURLYM      no 2    Match this medium-complex thing {n,m} times.
             CURLYX      sv 2    Match this complex thing {n,m} times.
    
             # This terminator creates a loop structure for CURLYX
             WHILEM      no      Do curly processing and see if rest matches.
    
             # OPEN,CLOSE,GROUPP ...are numbered at compile time.
             OPEN        num 1   Mark this point in input as start of #n.
             CLOSE       num 1   Analogous to OPEN.
    
             REF         num 1   Match some already matched string
             REFF        num 1   Match already matched string, folded
             REFFL       num 1   Match already matched string, folded in loc.
    
    
    
             # grouping assertions
             IFMATCH     off 1 2 Succeeds if the following matches.
             UNLESSM     off 1 2 Fails if the following matches.
             SUSPEND     off 1 1 "Independent" sub-regex.
             IFTHEN      off 1 1 Switch, should be preceded by switcher .
             GROUPP      num 1   Whether the group matched.
    
             # Support for long regex
             LONGJMP     off 1 1 Jump far away.
             BRANCHJ     off 1 1 BRANCH with long offset.
    
             # The heavy worker
             EVAL        evl 1   Execute some Perl code.
    
             # Modifiers
             MINMOD      no      Next operator is not greedy.
             LOGICAL     no      Next opcode should set the flag only.
    
             # This is not used yet
             RENUM       off 1 1 Group with independently numbered parens.
    
             # This is not really a node, but an optimized away piece of a "long" node.
             # To simplify debugging output, we mark it as if it were a node
             OPTIMIZED   off     Placeholder for dump.
    
    
         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,
         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 prvide 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 make 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.
    
         There are two Perl-specific ways to analyze memory usage:
         $ENV{PERL_DEBUG_MSTATS} and -DL command-line switch.  The
         first is available only if Perl is compiled with Perl's
         malloc(); the second only if Perl was built with
         `-DDEBUGGING'.  See the instructions for how to do this in
         the INSTALL podpage at the top level of the Perl source
         tree.
    
         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 hwen
         `$ENV{PERL_DEBUG_MSTATS} > 1', and before termination of the
         program when `$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 mstats() function out of
         the standard Devel::Peek module.
    
         Here is some explanation of that format:
    
         `buckets SMALLEST(APPROX)..GREATEST(APPROX)'
             Perl's malloc() uses bucketed allocations.  Every
             request is rounded up to the closest bucket size
             available, and a bucket is taken from the pool of
             buckets of that size.
    
             The line above describes the limits of buckets currently
             in use.  Each bucket has two sizes: memory footprint and
             the maximal size of user data that can fit into this
             bucket.  Suppose in the above example that the smallest
             bucket were size 4.  The biggest bucket would have
             usable size 8188, and the memory footprint would be
             8192.
    
             In a Perl built for debugging, some buckets may have
             negative usable size.  This means that these buckets
             cannot (and will not) be used.  For larger buckets, the
             memory footprint may be one page greater than a power of
             2.  If so, case the corresponding power of two is
             printed in the `APPROX' field above.
    
         Free/Used
             The 1 or 2 rows of numbers following that correspond to
             the number of buckets of each size between `SMALLEST'
             and `GREATEST'.  In the first row, the sizes (memory
             footprints) of buckets are powers of two--or possibly
             one page greater.  In the second row, if present, the
             memory footprints of the buckets are between the memory
             footprints of two buckets "above".
    
             For example, suppose under the pervious example, the
             memory footprints were
    
                  free:    8     16    32    64    128  256 512 1024 2048 4096 8192
                        4     12    24    48    80
    
             With non-`DEBUGGING' perl, the buckets starting from
             `128' have a 4-byte overhead, and thus a 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.
    
         Example of using -DL switch
    
         Below we show how to analyse memory usage by
    
           do 'lib/auto/POSIX/autosplit.ix';
    
         The file in question contains a header and 146 lines similar
         to
    
           sub getcwd;
    
         WARNING: The discussion below supposes 32-bit architecture.
         In newer releases of Perl, memory usage of the constructs
         discussed here is greatly improved, but the story discussed
         below is a real-life story.  This story is mercilessly
         terse, and assumes rather more than cursory knowledge of
         Perl internals.  Type space to continue, `q' to quit.
         (Actually, you just want to skip to the next section.)
    
         Here is the itemized list of Perl allocations performed
         during parsing of this file:
    
    
    
          !!! "after" at test.pl line 3.
             Id  subtot   4   8  12  16  20  24  28  32  36  40  48  56  64  72  80 80+
           0 02   13752   .   .   .   . 294   .   .   .   .   .   .   .   .   .   .   4
           0 54    5545   .   .   8 124  16   .   .   .   1   1   .   .   .   .   .   3
           5 05      32   .   .   .   .   .   .   .   1   .   .   .   .   .   .   .   .
           6 02    7152   .   .   .   .   .   .   .   .   .   . 149   .   .   .   .   .
           7 02    3600   .   .   .   .   . 150   .   .   .   .   .   .   .   .   .   .
           7 03      64   .  -1   .   1   .   .   2   .   .   .   .   .   .   .   .   .
           7 04    7056   .   .   .   .   .   .   .   .   .   .   .   .   .   .   .   7
           7 17   38404   .   .   .   .   .   .   .   1   .   . 442 149   .   . 147   .
           9 03    2078  17 249  32   .   .   .   .   2   .   .   .   .   .   .   .   .
    
         To see this list, insert two `warn('!...')' statements
         around the call:
    
           warn('!');
           do 'lib/auto/POSIX/autosplit.ix';
           warn('!!! "after"');
    
         and run it with PErl's -DL option.  The first warn() will
         print memory allocation info before parsing the file and
         will memorize the statistics at this point (we ignore what
         it prints).  The second warn() prints increments with
         respect to these memorized data.  This is the printout shown
         above.
    
         Different Ids on the left correspond to different subsystems
         of the perl interpreter.  They are just the first argument
         given to the perl memory allocation API named New().  To
         find what `9 03' means, just grep the perl source for `903'.
         You'll find it in util.c, function savepvn().  (I know, you
         wonder why we told you to grep and then gave away the
         answer.  That's because grepping the source is good for the
         soul.)  This function is used to store a copy of an existing
         chunk of memory.  Using a C debugger, one can see that the
         function was called either directly from gv_init() or via
         sv_magic(), and that gv_init() is called from
         gv_fetchpv()--which was itself called from newSUB().  Please
         stop to catch your breath now.
    
         NOTE: To reach this point in the debugger and skip the calls
         to savepvn() during the compilation of the main program, you
         should set a C breakpoint in Perl_warn(), continue until
         this point is reached, and then set a C breakpoint in
         Perl_savepvn().  Note that you may need to skip a handful of
         Perl_savepvn() calls that do not correspond to mass
         production of CVs (there are more `903' allocations than 146
         similar lines of lib/auto/POSIX/autosplit.ix).  Note also
         that `Perl_' prefixes are added by macroization code in perl
         header files to avoid conflicts with external libraries.
    
    
         Anyway, we see that `903' ids correspond to creation of
         globs, twice per glob - for glob name, and glob
         stringification magic.
    
         Here are explanations for other Ids above:
    
         `717'
             CReates bigger `XPV*' structures.  In the case above, it
             creates 3 `AV's per subroutine, one for a list of
             lexical variable names, one for a scratchpad (which
             contains lexical variables and `targets'), and one for
             the array of scratchpads needed for recursion.
    
             It also creates a `GV' and a `CV' per subroutine, all
             called from start_subparse().
    
         `002'
             Creates a C array corresponding to the `AV' of
             scratchpads and the scratchpad itself.  The first fake
             entry of this scratchpad is created though the
             subroutine itself is not defined yet.
    
             It also creates C arrays to keep data for the stash.
             This is one HV, but it grows; thus, there are 4 big
             allocations: the big chunks are not freed, but are kept
             as additional arenas for `SV' allocations.
    
         `054'
             Creates a `HEK' for the name of the glob for the
             subroutine.  This name is a key in a stash.
    
             Big allocations with this Id correspond to allocations
             of new arenas to keep `HE'.
    
         `602'
             Creates a `GP' for the glob for the subroutine.
    
         `702'
             Creates the `MAGIC' for the glob for the subroutine.
    
         `704'
             Creates arenas which keep SVs.
    
         -DL details
    
         If Perl is run with -DL option, then warn()s that start with
         `!'  behave specially.  They print a list of categories of
         memory allocations, and statistics of allocations of
         different sizes for these categories.
    
         If warn() string starts with
    
         `!!!'
             print changed categories only, print the differences in
             counts of allocations.
    
         `!!'
             print grown categories only; print the absolute values
             of counts, and totals.
    
         `!' print nonempty categories, print the absolute values of
             counts and totals.
    
         Limitations of -DL statistics
    
         If an extension or external library does not use the Perl
         API to allocate memory, such allocations are not counted.
    
    
    

    SEE ALSO

         the perldebug manpage, the perlguts manpage, the perlrun
         manpage the re manpage, and the Devel::Dprof manpage.
    
    
    
    


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