perldebguts - Guts of Perl debugging

- NAME
- DESCRIPTION
- Debugger Internals

- Writing Your Own Debugger

- Frame Listing Output Examples
- Debugging regular expressions

- Compile-time output
- Types of nodes
- Run-time output

- Debugging Perl memory usage

- Using `$ENV{PERL_DEBUG_MSTATS}`
- Example of using -DL switch
- -DL details
- Limitations of -DL statistics

- SEE ALSO

# 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 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. The general mechanisms is enabled by calling Perl with the -d switch, the following additional features are enabled (cf. $^P in the perlvar manpage):

- 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 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 subroutines, 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"`. 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 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 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 Options 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 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

3.       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

4.       in  $=main::BEGIN() from /dev/null:0
          in  $=Config::BEGIN() from lib/Config.pm:2
           Package lib/Exporter.pm.
           Package lib/Carp.pm.
          out $=Config::BEGIN() from lib/Config.pm:0
          Package lib/Config.pm.
          in  $=Config::TIEHASH('Config') from lib/Config.pm:644
          out $=Config::TIEHASH('Config') from lib/Config.pm:644
          in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
           in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
           out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
          out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
         out $=main::BEGIN() from /dev/null:0
         in  @=Config::myconfig() from /dev/null:0
          in  $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
          out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
          in  $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
          out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
          in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
          out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
          in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574

5.       in  $=main::BEGIN() from /dev/null:0
          in  $=Config::BEGIN() from lib/Config.pm:2
           Package lib/Exporter.pm.
           Package lib/Carp.pm.
          out $=Config::BEGIN() from lib/Config.pm:0
          Package lib/Config.pm.
          in  $=Config::TIEHASH('Config') from lib/Config.pm:644
          out $=Config::TIEHASH('Config') from lib/Config.pm:644
          in  $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
           in  $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
           out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
          out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
         out $=main::BEGIN() from /dev/null:0
         in  @=Config::myconfig() from /dev/null:0
          in  $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
          out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
          in  $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
          out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574

6.       in  $=CODE(0x15eca4)() from /dev/null:0
          in  $=CODE(0x182528)() from lib/Config.pm:2
           Package lib/Exporter.pm.
          out $=CODE(0x182528)() from lib/Config.pm:0
          scalar context return from CODE(0x182528): undef
          Package lib/Config.pm.
          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 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.

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 when `$ENV{PERL_DEBUG_MSTATS} > 1`, and before termination of the program when `< $ENV{PERL_DEBUG_MSTATS}` = 1 >>. The report format is similar to the following example:

      $ PERL_DEBUG_MSTATS=2 perl -e "require Carp"
      Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
         14216 free:   130   117    28     7     9   0   2     2   1 0 0
                    437    61    36     0     5
         60924 used:   125   137   161    55     7   8   6    16   2 0 1
                     74   109   304    84    20
      Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
      Memory allocation statistics after execution:   (buckets 4(4)..8188(8192)
         30888 free:   245    78    85    13     6   2   1     3   2 0 1
                    315   162    39    42    11
        175816 used:   265   176  1112   111    26  22  11    27   2 1 1
                    196   178  1066   798    39
      Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.

It is possible to ask for such a statistic at arbitrary points in your execution using the `mstat()` function out of the standard Devel::Peek module.

Here is some explanation of that format:

`buckets SMALLEST(APPROX)..GREATEST(APPROX)`
  Perl's `malloc()` uses bucketed allocations. Every request is rounded up to the closest bucket size available, and a bucket is taken from the pool of buckets of that size.

  The line above describes the limits of buckets currently in use. Each bucket has two sizes: memory footprint and the maximal size of user data that can fit into this bucket. Suppose in the above example that the smallest bucket were size 4. The biggest bucket would have usable size 8188, and the memory footprint would be 8192.

  In a Perl built for debugging, some buckets may have negative usable size. This means that these buckets cannot (and will not) be used. For larger buckets, the memory footprint may be one page greater than a power of 2. If so, 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

           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:

1. 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().

2. 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.

3. 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`.

4. Creates a `GP` for the glob for the subroutine.

5. Creates the `MAGIC` for the glob for the subroutine.

6. 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.

perldebguts - Guts of Perl debugging