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perldebguts



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 difficult to impossible to understand for anyone who
       isn't incredibly intimate with Perl's guts.  Caveat lec­
       tor.


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 fol­
       lowing additional features are enabled (cf. "$^P" in perl­
       var):

       ·   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 subrou­
           tines, 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_condi­
           tion\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".
           "DB::postponed" subroutine also exists.

       ·   A hash %DB::sub is maintained, whose keys are subrou­
           tine names and whose values have the form "file­
           name: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" 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 pos­
       sible, 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 cre­
       ated 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 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 initial­
       ization.)

       After the rc file is read, the debugger reads the
       PERLDB_OPTS environment variable and uses it to set debug­
       ger 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 exe­
           cution.

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

       ·   %DB::dbline, is an alias for "%{"::_<current_file"}",
           which contains breakpoints and actions keyed by line
           number in the currently-selected file, either explic­
           itly 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 infor­

       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" environ­
       ment 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 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

              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: ''


Debugging regular expressions

       There are two ways to enable debugging output for regular
       expressions.

       If your perl is compiled with "-DDEBUGGING", you may use
       the -Dr flag on the command line.

       Otherwise, one can "use re 'debug'", which has effects at
       compile time and run time.  It is not lexically scoped.

       Compile-time output

       The debugging output at compile time looks like this:

         Compiling REx `[bc]d(ef*g)+h[ij]k$'
         size 45 Got 364 bytes for offset annotations.
         first at 1
         rarest char g at 0
         rarest char d at 0
            1: ANYOF[bc](12)
           12: EXACT <d>(14)
           14: CURLYX[0] {1,32767}(28)
           16:   OPEN1(18)
           18:     EXACT <e>(20)
           20:     STAR(23)
           21:       EXACT <f>(0)
           23:     EXACT <g>(25)
           25:   CLOSE1(27)
           27:   WHILEM[1/1](0)
           28: NOTHING(29)
           29: EXACT <h>(31)
           31: ANYOF[ij](42)
           42: EXACT <k>(44)
           44: EOL(45)
           45: END(0)
         anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
               stclass `ANYOF[bc]' minlen 7
         Offsets: [45]
               1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
               0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
               11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
               0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]
         Omitting $` $& $' support.

       The first line shows the pre-compiled form of the regex.
       The second shows the size of the compiled form (in arbi­
       trary 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

       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 enter­
       ing (a slow) regex engine on strings that will not defi­
       nitely 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.

           # 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).

           CURLY       sv 2    Match this simple thing {n,m} times.
           CURLYN      no 2    Match next-after-this simple thing
           #                   {n,m} times, set parens.
           CURLYM      no 2    Match this medium-complex thing {n,m} times.
           CURLYX      sv 2    Match this complex thing {n,m} times.

           # This terminator creates a loop structure for CURLYX
           WHILEM      no      Do curly processing and see if rest matches.

           # OPEN,CLOSE,GROUPP ...are numbered at compile time.
           OPEN        num 1   Mark this point in input as start of #n.
           CLOSE       num 1   Analogous to OPEN.

           REF         num 1   Match some already matched string
           REFF        num 1   Match already matched string, folded
           REFFL       num 1   Match already matched string, folded in loc.

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

           # Support for long regex
           LONGJMP     off 1 1 Jump far away.
           BRANCHJ     off 1 1 BRANCH with long offset.

           # The heavy worker
           EVAL        evl 1   Execute some Perl code.

           # Modifiers
           MINMOD      no      Next operator is not greedy.
           LOGICAL     no      Next opcode should set the flag only.

           # This is not used yet
           RENUM       off 1 1 Group with independently numbered parens.

           # This is not really a node, but an optimized away piece of a "long" node.
           # To simplify debugging output, we mark it as if it were a node
           OPTIMIZED   off     Placeholder for dump.

       Following the optimizer information is a dump of the off­
       set/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]

       ter---that is, it corresponds to the "+" symbol in the
       precompiled regex.

       "0[0]" items indicate that there is no corresponding node.

       Run-time output

       First of all, when doing a match, one may get no run-time
       output even if debugging is enabled.  This means that the
       regex engine was never entered and that all of the job was
       therefore done by the optimizer.

       If the regex engine was entered, the output may look like
       this:

         Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
           Setting an EVAL scope, savestack=3
            2 <ab> <cdefg__gh_>    |  1: ANYOF
            3 <abc> <defg__gh_>    | 11: EXACT <d>
            4 <abcd> <efg__gh_>    | 13: CURLYX {1,32767}
            4 <abcd> <efg__gh_>    | 26:   WHILEM
                                       0 out of 1..32767  cc=effff31c
            4 <abcd> <efg__gh_>    | 15:     OPEN1
            4 <abcd> <efg__gh_>    | 17:     EXACT <e>
            5 <abcde> <fg__gh_>    | 19:     STAR
                                    EXACT <f> can match 1 times out of 32767...
           Setting an EVAL scope, savestack=3
            6 <bcdef> <g__gh__>    | 22:       EXACT <g>
            7 <bcdefg> <__gh__>    | 24:       CLOSE1
            7 <bcdefg> <__gh__>    | 26:       WHILEM
                                           1 out of 1..32767  cc=effff31c
           Setting an EVAL scope, savestack=12
            7 <bcdefg> <__gh__>    | 15:         OPEN1
            7 <bcdefg> <__gh__>    | 17:         EXACT <e>
              restoring \1 to 4(4)..7
                                           failed, try continuation...
            7 <bcdefg> <__gh__>    | 27:         NOTHING
            7 <bcdefg> <__gh__>    | 28:         EXACT <h>
                                           failed...
                                       failed...

       The most significant information in the output is about
       the particular node of the compiled regex that is cur­
       rently 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.
       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 sug­
       gest 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 mem­
       ory 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
       allocations and possible memory leaks.  These days the use
       of malloc debugging tools like Purify or valgrind is sug­
       gested 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 ana­
       lyze 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 "$ENV{PERL_DEBUG_MSTATS} > 1", and before termi­
       nation of the program when "$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.
           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 cur­
           rently 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 "SMALL­
           EST" 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 pre­
           sent, 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

                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.

       "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

       (Note that -DL is obsolete since circa 5.6.0, and even
       before that Perl needed to be compiled with -DDEBUGGING.)

       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 simi­
       lar to

         sub getcwd;

       WARNING: The discussion below supposes 32-bit architec­
       ture.  In newer releases of Perl, memory usage of the con­
       structs discussed here is greatly improved, but the story
       discussed below is a real-life story.  This story is mer­
       cilessly 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
       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 pro­
       gram, you should set a C breakpoint in Perl_warn(), con­
       tinue until this point is reached, and then set a C break­
       point 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 stringifi­
       cation 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 subrou­
           tine 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 sub­
           routine.  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.
           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

       perldebug, perlguts, perlrun re, and Devel::DProf.

perl v5.8.1                 2003-09-02             PERLDEBGUTS(1)

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