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perldebguts - Guts of Perl debugging


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 perlrun, 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. "$^P" in perlvar):

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 perldebug 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 perldebug) are considered for internal use only, and as such are subject to change without notice.

Frame Listing Output Examples

The frame option can be used to control the output of frame information. For example, contrast this expression trace:

$ perl -de 42
Stack dump during die enabled outside of evals.

Loading DB routines from 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):

with this one, once the Option 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

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/
      Package lib/
     Package lib/
     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/
      Package lib/
     exited Config::BEGIN
     Package lib/
     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/
      Package lib/
      Package lib/
     Package lib/
     in  $=Config::TIEHASH('Config') from lib/
     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/
     in  $=Config::FETCH(ref(Config), 'baserev') from lib/
     in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/
     in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/
     in  $=Config::FETCH(ref(Config), 'osname') from lib/
     in  $=Config::FETCH(ref(Config), 'osvers') from lib/
  4. in  $=main::BEGIN() from /dev/null:0
     in  $=Config::BEGIN() from lib/
      Package lib/
      Package lib/
     out $=Config::BEGIN() from lib/
     Package lib/
     in  $=Config::TIEHASH('Config') from lib/
     out $=Config::TIEHASH('Config') from lib/
     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/
     out $=Config::FETCH(ref(Config), 'package') from lib/
     in  $=Config::FETCH(ref(Config), 'baserev') from lib/
     out $=Config::FETCH(ref(Config), 'baserev') from lib/
     in  $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/
     out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/
     in  $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/
  5. in  $=main::BEGIN() from /dev/null:0
     in  $=Config::BEGIN() from lib/
      Package lib/
      Package lib/
     out $=Config::BEGIN() from lib/
     Package lib/
     in  $=Config::TIEHASH('Config') from lib/
     out $=Config::TIEHASH('Config') from lib/
     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/
     out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/
     in  $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/
     out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/
  6. in  $=CODE(0x15eca4)() from /dev/null:0
     in  $=CODE(0x182528)() from lib/
      Package lib/
     out $=CODE(0x182528)() from lib/
     scalar context return from CODE(0x182528): undef
     Package lib/
     in  $=Config::TIEHASH('Config') from lib/
     out $=Config::TIEHASH('Config') from lib/
     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/
      out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
      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/

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.


The minimal length of the match.

stclass TYPE

Type of first matching node.


Don't scan for the found substrings.


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.


Set if the pattern contains \G.


Set if the pattern starts with a repeated char (as in x+y).


Set if the pattern starts with .*.

with eval

Set if the pattern contain eval-groups, such as (?{ code }) and (??{ code }).


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>

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


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.


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:


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.


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.


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 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:

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:


CReates bigger XPV* structures. In the case above, it creates 3 AVs 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().


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.


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.


Creates a GP for the glob for the subroutine.


Creates the MAGIC for the glob for the subroutine.


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.


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

4 POD Errors

The following errors were encountered while parsing the POD:

Around line 255:

You have '=item 4' instead of the expected '=item 3'

Around line 273:

You have '=item 6' instead of the expected '=item 4'

Around line 297:

You have '=item 14' instead of the expected '=item 5'

Around line 318:

You have '=item 30' instead of the expected '=item 6'