Perl 5 version 8.2 documentation



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 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 lector.

Debugger Internals

Perl has special debugging hooks at compile-time and run-time used to create debugging environments. These hooks are not to be confused with the perl -Dxxx command described in perlrun, which is usable only if a special Perl is built per the instructions in the INSTALL podpage in the Perl source tree.

For example, whenever you call Perl's built-in caller function from the package DB , the arguments that the corresponding stack frame was called with are copied to the @DB::args array. These mechanisms are enabled by calling Perl with the -d switch. Specifically, the following additional features are enabled (cf. $^P in perlvar):

  • Perl inserts the contents of $ENV{PERL5DB} (or BEGIN {require ''} 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 evaled strings that contain subroutines, or which are currently being executed. The $filename for evaled 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 %{"_<$filename"} contains breakpoints and actions keyed by line number. Individual entries (as opposed to the whole hash) are settable. Perl only cares about Boolean true here, although the values used by have the form "$break_condition\0$action" .

    The same holds for evaluated strings that contain subroutines, or which are currently being executed. The $filename for evaled 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 evaled strings looks like (eval 34) or (re_eval 19) .

  • After each required 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 required 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 evals, 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 localizable. 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 possible, even though &DB::sub exists.

Writing Your Own Debugger

Environment Variables

The PERL5DB environment variable can be used to define a debugger. For example, the minimal "working" debugger (it actually doesn't do anything) consists of one line:

  1. sub DB::DB {}

It can easily be defined like this:

  1. $ PERL5DB="sub DB::DB {}" perl -d your-script

Another brief debugger, slightly more useful, can be created with only the line:

  1. sub DB::DB {print ++$i; scalar <STDIN>}

This debugger prints a number which increments for each statement encountered and waits for you to hit a newline before continuing to the next statement.

The following debugger is actually useful:

  1. {
  2. package DB;
  3. sub DB {}
  4. sub sub {print ++$i, " $sub\n"; &$sub}
  5. }

It prints the sequence number of each subroutine call and the name of the called subroutine. Note that &DB::sub is being compiled into the package DB through the use of the package directive.

When it starts, the debugger reads your rc file (./.perldb or ~/.perldb under Unix), which can set important options. (A subroutine (&afterinit ) can be defined here as well; it is executed after the debugger completes its own initialization.)

After the rc file is read, the debugger reads the PERLDB_OPTS environment variable and uses it to set debugger options. The contents of this variable are treated as if they were the argument of an o ... debugger command (q.v. in Options in perldebug).

Debugger internal variables In addition to the file and subroutine-related variables mentioned above, the debugger also maintains various magical internal variables.

  • @DB::dbline is an alias for @{"::_<current_file"} , which holds the lines of the currently-selected file (compiled by Perl), either explicitly chosen with the debugger's f command, or implicitly by flow of execution.

    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 explicitly chosen with the debugger's f command, or implicitly by flow of execution.

    As previously noted, individual entries (as opposed to the whole hash) are settable. Perl only cares about Boolean true here, although the values used by 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 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 .

  • 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:

  1. $ perl -de 42
  2. Stack dump during die enabled outside of evals.
  3. Loading DB routines from patch level 0.94
  4. Emacs support available.
  5. Enter h or `h h' for help.
  6. main::(-e:1): 0
  7. DB<1> sub foo { 14 }
  8. DB<2> sub bar { 3 }
  9. DB<3> t print foo() * bar()
  10. main::((eval 172):3): print foo() + bar();
  11. main::foo((eval 168):2):
  12. main::bar((eval 170):2):
  13. 42

with this one, once the o ption frame=2 has been set:

  1. DB<4> o f=2
  2. frame = '2'
  3. DB<5> t print foo() * bar()
  4. 3: foo() * bar()
  5. entering main::foo
  6. 2: sub foo { 14 };
  7. exited main::foo
  8. entering main::bar
  9. 2: sub bar { 3 };
  10. exited main::bar
  11. 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
    2. entering Config::BEGIN
    3. Package lib/
    4. Package lib/
    5. Package lib/
    6. entering Config::TIEHASH
    7. entering Exporter::import
    8. entering Exporter::export
    9. entering Config::myconfig
    10. entering Config::FETCH
    11. entering Config::FETCH
    12. entering Config::FETCH
    13. entering Config::FETCH
    1. entering main::BEGIN
    2. entering Config::BEGIN
    3. Package lib/
    4. Package lib/
    5. exited Config::BEGIN
    6. Package lib/
    7. entering Config::TIEHASH
    8. exited Config::TIEHASH
    9. entering Exporter::import
    10. entering Exporter::export
    11. exited Exporter::export
    12. exited Exporter::import
    13. exited main::BEGIN
    14. entering Config::myconfig
    15. entering Config::FETCH
    16. exited Config::FETCH
    17. entering Config::FETCH
    18. exited Config::FETCH
    19. entering Config::FETCH
    1. in $=main::BEGIN() from /dev/null:0
    2. in $=Config::BEGIN() from lib/
    3. Package lib/
    4. Package lib/
    5. Package lib/
    6. in $=Config::TIEHASH('Config') from lib/
    7. in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    8. in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
    9. in @=Config::myconfig() from /dev/null:0
    10. in $=Config::FETCH(ref(Config), 'package') from lib/
    11. in $=Config::FETCH(ref(Config), 'baserev') from lib/
    12. in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/
    13. in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/
    14. in $=Config::FETCH(ref(Config), 'osname') from lib/
    15. in $=Config::FETCH(ref(Config), 'osvers') from lib/
    1. in $=main::BEGIN() from /dev/null:0
    2. in $=Config::BEGIN() from lib/
    3. Package lib/
    4. Package lib/
    5. out $=Config::BEGIN() from lib/
    6. Package lib/
    7. in $=Config::TIEHASH('Config') from lib/
    8. out $=Config::TIEHASH('Config') from lib/
    9. in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    10. in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
    11. out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
    12. out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    13. out $=main::BEGIN() from /dev/null:0
    14. in @=Config::myconfig() from /dev/null:0
    15. in $=Config::FETCH(ref(Config), 'package') from lib/
    16. out $=Config::FETCH(ref(Config), 'package') from lib/
    17. in $=Config::FETCH(ref(Config), 'baserev') from lib/
    18. out $=Config::FETCH(ref(Config), 'baserev') from lib/
    19. in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/
    20. out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/
    21. in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/
    1. in $=main::BEGIN() from /dev/null:0
    2. in $=Config::BEGIN() from lib/
    3. Package lib/
    4. Package lib/
    5. out $=Config::BEGIN() from lib/
    6. Package lib/
    7. in $=Config::TIEHASH('Config') from lib/
    8. out $=Config::TIEHASH('Config') from lib/
    9. in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    10. in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
    11. out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
    12. out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    13. out $=main::BEGIN() from /dev/null:0
    14. in @=Config::myconfig() from /dev/null:0
    15. in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/
    16. out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/
    17. in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/
    18. out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/
    1. in $=CODE(0x15eca4)() from /dev/null:0
    2. in $=CODE(0x182528)() from lib/
    3. Package lib/
    4. out $=CODE(0x182528)() from lib/
    5. scalar context return from CODE(0x182528): undef
    6. Package lib/
    7. in $=Config::TIEHASH('Config') from lib/
    8. out $=Config::TIEHASH('Config') from lib/
    9. scalar context return from Config::TIEHASH: empty hash
    10. in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    11. in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
    12. out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
    13. scalar context return from Exporter::export: ''
    14. out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
    15. 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

    1. 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:

    1. Compiling REx `[bc]d(ef*g)+h[ij]k$'
    2. size 45 Got 364 bytes for offset annotations.
    3. first at 1
    4. rarest char g at 0
    5. rarest char d at 0
    6. 1: ANYOF[bc](12)
    7. 12: EXACT <d>(14)
    8. 14: CURLYX[0] {1,32767}(28)
    9. 16: OPEN1(18)
    10. 18: EXACT <e>(20)
    11. 20: STAR(23)
    12. 21: EXACT <f>(0)
    13. 23: EXACT <g>(25)
    14. 25: CLOSE1(27)
    15. 27: WHILEM[1/1](0)
    16. 28: NOTHING(29)
    17. 29: EXACT <h>(31)
    18. 31: ANYOF[ij](42)
    19. 42: EXACT <k>(44)
    20. 44: EOL(45)
    21. 45: END(0)
    22. anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
    23. stclass `ANYOF[bc]' minlen 7
    24. Offsets: [45]
    25. 1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
    26. 0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
    27. 11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
    28. 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]
    29. Omitting $` $& $' support.

    The first line shows the pre-compiled form of the regex. The second shows the size of the compiled form (in arbitrary units, usually 4-byte words) and the total number of bytes allocated for the offset/length table, usually 4+size *8. The next line shows the label id of the first node that does a match.


    1. anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
    2. stclass `ANYOF[bc]' minlen 7

    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 no string shorter than 7 characters can possibly match.

    The fields of interest which may appear in this line are

    • anchored STRING at POS
    • floating STRING at POS1..POS2

      See above.

    • matching floating/anchored

      Which substring to check first.

    • minlen

      The minimal length of the match.

    • stclass TYPE

      Type of first matching node.

    • noscan

      Don't scan for the found substrings.

    • isall

      Means that the optimizer information is all that the regular expression contains, and thus one does not need to enter the regex engine at all.

    • GPOS

      Set if the pattern contains \G .

    • plus

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

    • implicit

      Set if the pattern starts with .* .

    • with eval

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

    • anchored(TYPE)

      If the pattern may match only at a handful of places, (with TYPE being BOL , MBOL , or GPOS . See the table below.

    If a substring is known to match at end-of-line only, it may be followed by $ , as in floating `k'$ .

    The optimizer-specific information is used to avoid entering (a slow) regex engine on strings that will not definitely match. If the isall flag is set, a call to the regex engine may be avoided even when the optimizer found an appropriate place for the match.

    Above the optimizer section is 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:

    1. # TYPE arg-description [num-args] [longjump-len] DESCRIPTION
    2. # Exit points
    3. END no End of program.
    4. SUCCEED no Return from a subroutine, basically.
    5. # Anchors:
    6. BOL no Match "" at beginning of line.
    7. MBOL no Same, assuming multiline.
    8. SBOL no Same, assuming singleline.
    9. EOS no Match "" at end of string.
    10. EOL no Match "" at end of line.
    11. MEOL no Same, assuming multiline.
    12. SEOL no Same, assuming singleline.
    13. BOUND no Match "" at any word boundary
    14. BOUNDL no Match "" at any word boundary
    15. NBOUND no Match "" at any word non-boundary
    16. NBOUNDL no Match "" at any word non-boundary
    17. GPOS no Matches where last m//g left off.
    18. # [Special] alternatives
    19. ANY no Match any one character (except newline).
    20. SANY no Match any one character.
    21. ANYOF sv Match character in (or not in) this class.
    22. ALNUM no Match any alphanumeric character
    23. ALNUML no Match any alphanumeric char in locale
    24. NALNUM no Match any non-alphanumeric character
    25. NALNUML no Match any non-alphanumeric char in locale
    26. SPACE no Match any whitespace character
    27. SPACEL no Match any whitespace char in locale
    28. NSPACE no Match any non-whitespace character
    29. NSPACEL no Match any non-whitespace char in locale
    30. DIGIT no Match any numeric character
    31. NDIGIT no Match any non-numeric character
    32. # BRANCH The set of branches constituting a single choice are hooked
    33. # together with their "next" pointers, since precedence prevents
    34. # anything being concatenated to any individual branch. The
    35. # "next" pointer of the last BRANCH in a choice points to the
    36. # thing following the whole choice. This is also where the
    37. # final "next" pointer of each individual branch points; each
    38. # branch starts with the operand node of a BRANCH node.
    39. #
    40. BRANCH node Match this alternative, or the next...
    41. # BACK Normal "next" pointers all implicitly point forward; BACK
    42. # exists to make loop structures possible.
    43. # not used
    44. BACK no Match "", "next" ptr points backward.
    45. # Literals
    46. EXACT sv Match this string (preceded by length).
    47. EXACTF sv Match this string, folded (prec. by length).
    48. EXACTFL sv Match this string, folded in locale (w/len).
    49. # Do nothing
    50. NOTHING no Match empty string.
    51. # A variant of above which delimits a group, thus stops optimizations
    52. TAIL no Match empty string. Can jump here from outside.
    53. # STAR,PLUS '?', and complex '*' and '+', are implemented as circular
    54. # BRANCH structures using BACK. Simple cases (one character
    55. # per match) are implemented with STAR and PLUS for speed
    56. # and to minimize recursive plunges.
    57. #
    58. STAR node Match this (simple) thing 0 or more times.
    59. PLUS node Match this (simple) thing 1 or more times.
    60. CURLY sv 2 Match this simple thing {n,m} times.
    61. CURLYN no 2 Match next-after-this simple thing
    62. # {n,m} times, set parens.
    63. CURLYM no 2 Match this medium-complex thing {n,m} times.
    64. CURLYX sv 2 Match this complex thing {n,m} times.
    65. # This terminator creates a loop structure for CURLYX
    66. WHILEM no Do curly processing and see if rest matches.
    67. # OPEN,CLOSE,GROUPP ...are numbered at compile time.
    68. OPEN num 1 Mark this point in input as start of #n.
    69. CLOSE num 1 Analogous to OPEN.
    70. REF num 1 Match some already matched string
    71. REFF num 1 Match already matched string, folded
    72. REFFL num 1 Match already matched string, folded in loc.
    73. # grouping assertions
    74. IFMATCH off 1 2 Succeeds if the following matches.
    75. UNLESSM off 1 2 Fails if the following matches.
    76. SUSPEND off 1 1 "Independent" sub-regex.
    77. IFTHEN off 1 1 Switch, should be preceded by switcher .
    78. GROUPP num 1 Whether the group matched.
    79. # Support for long regex
    80. LONGJMP off 1 1 Jump far away.
    81. BRANCHJ off 1 1 BRANCH with long offset.
    82. # The heavy worker
    83. EVAL evl 1 Execute some Perl code.
    84. # Modifiers
    85. MINMOD no Next operator is not greedy.
    86. LOGICAL no Next opcode should set the flag only.
    87. # This is not used yet
    88. RENUM off 1 1 Group with independently numbered parens.
    89. # This is not really a node, but an optimized away piece of a "long" node.
    90. # To simplify debugging output, we mark it as if it were a node
    91. OPTIMIZED off Placeholder for dump.

    Following the optimizer information is a dump of the offset/length table, here split across several lines:

    1. Offsets: [45]
    2. 1[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 5[1]
    3. 0[0] 12[1] 0[0] 6[1] 0[0] 7[1] 0[0] 9[1] 8[1] 0[0] 10[1] 0[0]
    4. 11[1] 0[0] 12[0] 12[0] 13[1] 0[0] 14[4] 0[0] 0[0] 0[0] 0[0]
    5. 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 18[1] 0[0] 19[1] 20[0]

    The first line here indicates that the offset/length table contains 45 entries. Each entry is a pair of integers, denoted by offset[length] . Entries are numbered starting with 1, so entry #1 here is 1[4] and entry #12 is 5[1] . 1[4] indicates that the node labeled 1: (the 1: ANYOF[bc] ) begins at character position 1 in the pre-compiled form of the regex, and has a length of 4 characters. 5[1] in position 12 indicates that the node labeled 12: (the 12: EXACT <d> ) begins at character position 5 in the pre-compiled form of the regex, and has a length of 1 character. 12[1] in position 14 indicates that the node labeled 14: (the 14: CURLYX[0] {1,32767} ) begins at character position 12 in the pre-compiled form of the regex, and has a length of 1 character---that is, it corresponds to the + symbol in the precompiled regex.

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

    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:

    1. Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
    2. Setting an EVAL scope, savestack=3
    3. 2 <ab> <cdefg__gh_> | 1: ANYOF
    4. 3 <abc> <defg__gh_> | 11: EXACT <d>
    5. 4 <abcd> <efg__gh_> | 13: CURLYX {1,32767}
    6. 4 <abcd> <efg__gh_> | 26: WHILEM
    7. 0 out of 1..32767 cc=effff31c
    8. 4 <abcd> <efg__gh_> | 15: OPEN1
    9. 4 <abcd> <efg__gh_> | 17: EXACT <e>
    10. 5 <abcde> <fg__gh_> | 19: STAR
    11. EXACT <f> can match 1 times out of 32767...
    12. Setting an EVAL scope, savestack=3
    13. 6 <bcdef> <g__gh__> | 22: EXACT <g>
    14. 7 <bcdefg> <__gh__> | 24: CLOSE1
    15. 7 <bcdefg> <__gh__> | 26: WHILEM
    16. 1 out of 1..32767 cc=effff31c
    17. Setting an EVAL scope, savestack=12
    18. 7 <bcdefg> <__gh__> | 15: OPEN1
    19. 7 <bcdefg> <__gh__> | 17: EXACT <e>
    20. restoring \1 to 4(4)..7
    21. failed, try continuation...
    22. 7 <bcdefg> <__gh__> | 27: NOTHING
    23. 7 <bcdefg> <__gh__> | 28: EXACT <h>
    24. failed...
    25. 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


    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

    1. sub foo;

    may take up to 500 bytes of memory, depending on which release of Perl you're running.

    Anecdotal estimates of source-to-compiled code bloat suggest an eightfold increase. This means that the compiled form of reasonable (normally commented, properly indented etc.) code will take about eight times more space in memory than the code took on disk.

    The -DL command-line switch is obsolete since circa Perl 5.6.0 (it was available only if Perl was built with -DDEBUGGING ). 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 suggested instead.

    One way to find out how much memory is being used by Perl data structures is to install the Devel::Size module from CPAN: it gives you the minimum number of bytes required to store a particular data structure. Please be mindful of the difference between the size() and total_size().

    If Perl has been compiled using Perl's malloc you can analyze Perl memory usage by setting the $ENV{PERL_DEBUG_MSTATS}.


    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:

    1. $ PERL_DEBUG_MSTATS=2 perl -e "require Carp"
    2. Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
    3. 14216 free: 130 117 28 7 9 0 2 2 1 0 0
    4. 437 61 36 0 5
    5. 60924 used: 125 137 161 55 7 8 6 16 2 0 1
    6. 74 109 304 84 20
    7. Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
    8. Memory allocation statistics after execution: (buckets 4(4)..8188(8192)
    9. 30888 free: 245 78 85 13 6 2 1 3 2 0 1
    10. 315 162 39 42 11
    11. 175816 used: 265 176 1112 111 26 22 11 27 2 1 1
    12. 196 178 1066 798 39
    13. 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:


      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

      1. free: 8 16 32 64 128 256 512 1024 2048 4096 8192
      2. 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 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

    (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

    1. do 'lib/auto/POSIX/autosplit.ix';

    The file in question contains a header and 146 lines similar to

    1. 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:

    1. !!! "after" at line 3.
    2. Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+
    3. 0 02 13752 . . . . 294 . . . . . . . . . . 4
    4. 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3
    5. 5 05 32 . . . . . . . 1 . . . . . . . .
    6. 6 02 7152 . . . . . . . . . . 149 . . . . .
    7. 7 02 3600 . . . . . 150 . . . . . . . . . .
    8. 7 03 64 . -1 . 1 . . 2 . . . . . . . . .
    9. 7 04 7056 . . . . . . . . . . . . . . . 7
    10. 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 .
    11. 9 03 2078 17 249 32 . . . . 2 . . . . . . . .

    To see this list, insert two warn('!...') statements around the call:

    1. warn('!');
    2. do 'lib/auto/POSIX/autosplit.ix';
    3. warn('!!! "after"');

    and run it with Perl's -DL option. The first warn() will print memory allocation info before parsing the file and will memorize the statistics at this point (we ignore what it prints). The second warn() prints increments with respect to these memorized data. This is the printout shown above.

    Different Ids on the left correspond to different subsystems of the perl interpreter. They are just the first argument given to the perl memory allocation API named New(). To find what 9 03 means, just grep the perl source for 903 . You'll find it in util.c, function savepvn(). (I know, you wonder why we told you to grep and then gave away the answer. That's because grepping the source is good for the soul.) This function is used to store a copy of an existing chunk of memory. Using a C debugger, one can see that the function was called either directly from gv_init() or via sv_magic(), and that gv_init() is called from gv_fetchpv()--which was itself called from newSUB(). Please stop to catch your breath now.

    NOTE: To reach this point in the debugger and skip the calls to savepvn() during the compilation of the main program, you should set a C breakpoint in Perl_warn(), continue until this point is reached, and then set a C breakpoint in Perl_savepvn(). Note that you may need to skip a handful of Perl_savepvn() calls that do not correspond to mass production of CVs (there are more 903 allocations than 146 similar lines of lib/auto/POSIX/autosplit.ix). Note also that Perl_ prefixes are added by macroization code in perl header files to avoid conflicts with external libraries.

    Anyway, we see that 903 ids correspond to creation of globs, twice per glob - for glob name, and glob stringification magic.

    Here are explanations for other Ids above:

    • 717

      Creates bigger XPV* structures. In the case above, it creates 3 AV s per subroutine, one for a list of lexical variable names, one for a scratchpad (which contains lexical variables and targets ), and one for the array of scratchpads needed for recursion.

      It also creates a GV and a CV per subroutine, all called from start_subparse().

    • 002

      Creates a C array corresponding to the AV of scratchpads and the scratchpad itself. The first fake entry of this scratchpad is created though the subroutine itself is not defined yet.

      It also creates C arrays to keep data for the stash. This is one HV, but it grows; thus, there are 4 big allocations: the big chunks are not freed, but are kept as additional arenas for SV allocations.

    • 054

      Creates a HEK for the name of the glob for the subroutine. This name is a key in a stash.

      Big allocations with this Id correspond to allocations of new arenas to keep HE .

    • 602

      Creates a GP for the glob for the subroutine.

    • 702

      Creates the MAGIC for the glob for the subroutine.

    • 704

      Creates arenas which keep SVs.

    -DL details

    If Perl is run with -DL option, then warn()s that start with `!' behave specially. They print a list of categories of memory allocations, and statistics of allocations of different sizes for these categories.

    If warn() string starts with

    • !!!

      print changed categories only, print the differences in counts of allocations.

    • !!

      print grown categories only; print the absolute values of counts, and totals.

    • !

      print nonempty categories, print the absolute values of counts and totals.

    Limitations of -DL statistics

    If an extension or external library does not use the Perl API to allocate memory, such allocations are not counted.


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