pod::perldebguts(3p) | Perl Programmers Reference Guide | pod::perldebguts(3p)

# NAME

perldebguts - Guts of Perl debugging

# DESCRIPTION

This is not perldebug, 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 file 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 $ENV{PERL5DB} (or "BEGIN {require 'perl5db.pl'}" if not present) before the first line of your program.
- Each array "@{"_<$filename"}" holds the lines of $filename for a file compiled by Perl. The same is also true for "eval"ed strings that contain subroutines, or which are currently being executed. The $filename for "eval"ed strings looks like "(eval 34)".

  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)".

- Each scalar "${"_<$filename"}" contains $filename. This is also the case for evaluated strings that contain subroutines, or which are currently being executed. The $filename for "eval"ed strings looks like "(eval 34)".
- After each "require"d file is compiled, but before it is executed, DB::postponed(*{"_<$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.
- When the execution of your program reaches a point that can hold a breakpoint, the DB::DB() subroutine is called if any of the variables $DB::trace, $DB::single, or $DB::signal is true. These variables are not "local"izable. This feature is disabled when executing inside DB::DB(), including functions called from it unless "$^D & (1<<30)" is true.
- When execution of the program reaches a subroutine call, a call to &DB::sub(args) is made instead, with $DB::sub set to identify the called subroutine. (This doesn't happen if the calling subroutine was compiled in the "DB" package.) $DB::sub normally holds the name of the called subroutine, if it has a name by which it can be looked up. Failing that, $DB::sub will hold a reference to the called subroutine. Either way, the &DB::sub subroutine can use $DB::sub as a reference by which to call the called subroutine, which it will normally want to do.

  If the call is to an lvalue subroutine, and &DB::lsub is defined &DB::lsub(args) is called instead, otherwise falling back to &DB::sub(args).

- When execution of the program uses "goto" to enter a non-XS subroutine and the 0x80 bit is set in $^P, a call to &DB::goto is made, with $DB::sub set to identify the subroutine being entered. The call to &DB::goto does not replace the "goto"; the requested subroutine will still be entered once &DB::goto has returned. $DB::sub normally holds the name of the subroutine being entered, if it has one. Failing that, $DB::sub will hold a reference to the subroutine being entered. Unlike when &DB::sub is called, it is not guaranteed that $DB::sub can be used as a reference to operate on the subroutine being entered.

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:

      sub DB::DB {}

It can easily be defined like this:

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

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

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

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

The following debugger is actually useful:

      {
        package DB;
        sub DB  {}
        sub sub {print ++$i, " $sub\n"; &$sub}
      }

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 "Configurable 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 perl5db.pl have the form "$break_condition\0$action".

Debugger Customization Functions

Some functions are provided to simplify customization.

- See "Configurable Options" in perldebug for a description of options parsed by DB::parse_options(string).
- "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:

     $ 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 using various values of "n" are shown to give you a feel for the difference between settings. Long though 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, and "-Drv" for more verbose information.

Otherwise, one can "use re 'debug'", which has effects at both compile time and run time. Since Perl 5.9.5, this pragma is lexically scoped.

## Compile-time Output

The debugging output at compile time looks like this:

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

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

The

      anchored 'de' at 1 floating 'gh' at 3..2147483647 (checking floating)
            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 "SBOL", "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 current possible types, with short descriptions:

     # TYPE arg-description [regnode-struct-suffix] [longjump-len] DESCRIPTION
     # Exit points
     END              no         End of program.
     SUCCEED          no         Return from a subroutine, basically.
     # Line Start Anchors:
     SBOL             no         Match "" at beginning of line: /^/, /\A/
     MBOL             no         Same, assuming multiline: /^/m
     # Line End Anchors:
     SEOL             no         Match "" at end of line: /$/
     MEOL             no         Same, assuming multiline: /$/m
     EOS              no         Match "" at end of string: /\z/
     # Match Start Anchors:
     GPOS             no         Matches where last m//g left off.
     # Word Boundary Opcodes:
     BOUND            no         Like BOUNDA for non-utf8, otherwise like
                                 BOUNDU
     BOUNDL           no         Like BOUND/BOUNDU, but \w and \W are
                                 defined by current locale
     BOUNDU           no         Match "" at any boundary of a given type
                                 using /u rules.
     BOUNDA           no         Match "" at any boundary between \w\W or
                                 \W\w, where \w is [_a-zA-Z0-9]
     NBOUND           no         Like NBOUNDA for non-utf8, otherwise like
                                 BOUNDU
     NBOUNDL          no         Like NBOUND/NBOUNDU, but \w and \W are
                                 defined by current locale
     NBOUNDU          no         Match "" at any non-boundary of a given
                                 type using using /u rules.
     NBOUNDA          no         Match "" betweeen any \w\w or \W\W, where
                                 \w is [_a-zA-Z0-9]
     # [Special] alternatives:
     REG_ANY          no         Match any one character (except newline).
     SANY             no         Match any one character.
     ANYOF            sv         Match character in (or not in) this class,
                      charclass  single char match only
     ANYOFD           sv         Like ANYOF, but /d is in effect
                      charclass
     ANYOFL           sv         Like ANYOF, but /l is in effect
                      charclass
     ANYOFPOSIXL      sv         Like ANYOFL, but matches [[:posix:]]
                      charclass_ classes
                      posixl
     ANYOFH           sv 1       Like ANYOF, but only has "High" matches,
                                 none in the bitmap; the flags field
                                 contains the lowest matchable UTF-8 start
                                 byte
     ANYOFHb          sv 1       Like ANYOFH, but all matches share the same
                                 UTF-8 start byte, given in the flags field
     ANYOFHr          sv 1       Like ANYOFH, but the flags field contains
                                 packed bounds for all matchable UTF-8 start
                                 bytes.
     ANYOFHs          sv:str 1   Like ANYOFHb, but has a string field that
                                 gives the leading matchable UTF-8 bytes;
                                 flags field is len
     ANYOFR           packed 1   Matches any character in the range given by
                                 its packed args: upper 12 bits is the max
                                 delta from the base lower 20; the flags
                                 field contains the lowest matchable UTF-8
                                 start byte
     ANYOFRb          packed 1   Like ANYOFR, but all matches share the same
                                 UTF-8 start byte, given in the flags field
     ANYOFHbbm        none bbm   Like ANYOFHb, but only for 2-byte UTF-8
                                 characters; uses a bitmap to match the
                                 continuation byte
     ANYOFM           byte 1     Like ANYOF, but matches an invariant byte
                                 as determined by the mask and arg
     NANYOFM          byte 1     complement of ANYOFM
     # POSIX Character Classes:
     POSIXD           none       Some [[:class:]] under /d; the FLAGS field
                                 gives which one
     POSIXL           none       Some [[:class:]] under /l; the FLAGS field
                                 gives which one
     POSIXU           none       Some [[:class:]] under /u; the FLAGS field
                                 gives which one
     POSIXA           none       Some [[:class:]] under /a; the FLAGS field
                                 gives which one
     NPOSIXD          none       complement of POSIXD, [[:^class:]]
     NPOSIXL          none       complement of POSIXL, [[:^class:]]
     NPOSIXU          none       complement of POSIXU, [[:^class:]]
     NPOSIXA          none       complement of POSIXA, [[:^class:]]
     CLUMP            no         Match any extended grapheme cluster
                                 sequence
     # Alternation
     # 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 1     Match this alternative, or the next...
     # Literals
     EXACT            str        Match this string (flags field is the
                                 length).
     # In a long string node, the U32 argument is the length, and is
     # immediately followed by the string.
     LEXACT           len:str 1  Match this long string (preceded by length;
                                 flags unused).
     EXACTL           str        Like EXACT, but /l is in effect (used so
                                 locale-related warnings can be checked for)
     EXACTF           str        Like EXACT, but match using /id rules;
                                 (string not UTF-8, ASCII folded; non-ASCII
                                 not)
     EXACTFL          str        Like EXACT, but match using /il rules;
                                 (string not likely to be folded)
     EXACTFU          str        Like EXACT, but match using /iu rules;
                                 (string folded)
     EXACTFAA         str        Like EXACT, but match using /iaa rules;
                                 (string folded except MICRO in non-UTF8
                                 patterns; doesn't contain SHARP S unless
                                 UTF-8; folded length <= unfolded)
     EXACTFAA_NO_TRIE str        Like EXACTFAA, (string not UTF-8, folded
                                 except: MICRO, SHARP S; folded length <=
                                 unfolded, not currently trie-able)
     EXACTFUP         str        Like EXACT, but match using /iu rules;
                                 (string not UTF-8, folded except MICRO:
                                 hence Problematic)
     EXACTFLU8        str        Like EXACTFU, but use /il, UTF-8, (string
                                 is folded, and everything in it is above
                                 255
     EXACT_REQ8       str        Like EXACT, but only UTF-8 encoded targets
                                 can match
     LEXACT_REQ8      len:str 1  Like LEXACT, but only UTF-8 encoded targets
                                 can match
     EXACTFU_REQ8     str        Like EXACTFU, but only UTF-8 encoded
                                 targets can match
     EXACTFU_S_EDGE   str        /di rules, but nothing in it precludes /ui,
                                 except begins and/or ends with [Ss];
                                 (string not UTF-8; compile-time only)
     # New charclass like patterns
     LNBREAK          none       generic newline pattern
     # Trie Related
     # Behave the same as A|LIST|OF|WORDS would. The '..C' variants
     # have inline charclass data (ascii only), the 'C' store it in the
     # structure.
     TRIE             trie 1     Match many EXACT(F[ALU]?)? at once.
                                 flags==type
     TRIEC            trie       Same as TRIE, but with embedded charclass
                      charclass  data
     AHOCORASICK      trie 1     Aho Corasick stclass. flags==type
     AHOCORASICKC     trie       Same as AHOCORASICK, but with embedded
                      charclass  charclass data
     # Do nothing types
     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.
     # Loops
     # STAR,PLUS    '?', and complex '*' and '+', are implemented as
     #               circular BRANCH structures.  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:
                                 /A{0,}B/ where A is width 1 char
     PLUS             node       Match this (simple) thing 1 or more times:
                                 /A{1,}B/ where A is width 1 char
     CURLY            sv 3       Match this (simple) thing {n,m} times:
                                 /A{m,n}B/ where A is width 1 char
     CURLYN           no 3       Capture next-after-this simple thing:
                                 /(A){m,n}B/ where A is width 1 char
     CURLYM           no 3       Capture this medium-complex thing {n,m}
                                 times: /(A){m,n}B/ where A is fixed-length
     CURLYX           sv 3       Match/Capture this complex thing {n,m}
                                 times.
     # This terminator creates a loop structure for CURLYX
     WHILEM           no         Do curly processing and see if rest
                                 matches.
     # Buffer related
     # OPEN,CLOSE,GROUPP     ...are numbered at compile time.
     OPEN             num 1      Mark this point in input as start of #n.
     CLOSE            num 1      Close corresponding OPEN of #n.
     SROPEN           none       Same as OPEN, but for script run
     SRCLOSE          none       Close preceding SROPEN
     REF              num 2      Match some already matched string
     REFF             num 2      Match already matched string, using /di
                                 rules.
     REFFL            num 2      Match already matched string, using /li
                                 rules.
     REFFU            num 2      Match already matched string, usng /ui.
     REFFA            num 2      Match already matched string, using /aai
                                 rules.
     # Named references.  Code in regcomp.c assumes that these all are after
     # the numbered references
     REFN             no-sv 2    Match some already matched string
     REFFN            no-sv 2    Match already matched string, using /di
                                 rules.
     REFFLN           no-sv 2    Match already matched string, using /li
                                 rules.
     REFFUN           num 2      Match already matched string, using /ui
                                 rules.
     REFFAN           num 2      Match already matched string, using /aai
                                 rules.
     # Support for long RE
     LONGJMP          off 1 1    Jump far away.
     BRANCHJ          off 2 1    BRANCH with long offset.
     # Special Case Regops
     IFMATCH          off 1 1    Succeeds if the following matches; non-zero
                                 flags "f", next_off "o" means lookbehind
                                 assertion starting "f..(f-o)" characters
                                 before current
     UNLESSM          off 1 1    Fails if the following matches; non-zero
                                 flags "f", next_off "o" means lookbehind
                                 assertion starting "f..(f-o)" characters
                                 before current
     SUSPEND          off 1 1    "Independent" sub-RE.
     IFTHEN           off 1 1    Switch, should be preceded by switcher.
     GROUPP           num 1      Whether the group matched.
     # The heavy worker
     EVAL             evl/flags  Execute some Perl code.
                      2
     # 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.
     # Regex Subroutines
     GOSUB            num/ofs 2  recurse to paren arg1 at (signed) ofs arg2
     # Special conditionals
     GROUPPN          no-sv 1    Whether the group matched.
     INSUBP           num 1      Whether we are in a specific recurse.
     DEFINEP          none 1     Never execute directly.
     # Backtracking Verbs
     ENDLIKE          none       Used only for the type field of verbs
     OPFAIL           no-sv 1    Same as (?!), but with verb arg
     ACCEPT           no-sv/num  Accepts the current matched string, with
                      2          verbar
     # Verbs With Arguments
     VERB             no-sv 1    Used only for the type field of verbs
     PRUNE            no-sv 1    Pattern fails at this startpoint if no-
                                 backtracking through this
     MARKPOINT        no-sv 1    Push the current location for rollback by
                                 cut.
     SKIP             no-sv 1    On failure skip forward (to the mark)
                                 before retrying
     COMMIT           no-sv 1    Pattern fails outright if backtracking
                                 through this
     CUTGROUP         no-sv 1    On failure go to the next alternation in
                                 the group
     # Control what to keep in $&.
     KEEPS            no         $& begins here.
     # Validate that lookbehind IFMATCH and UNLESSM end at the right place
     LOOKBEHIND_END   no         Return from lookbehind (IFMATCH/UNLESSM)
                                 and validate position
     # SPECIAL  REGOPS
     # 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.
     # Special opcode with the property that no opcode in a compiled program
     # will ever be of this type. Thus it can be used as a flag value that
     # no other opcode has been seen. END is used similarly, in that an END
     # node cant be optimized. So END implies "unoptimizable" and PSEUDO
     # mean "not seen anything to optimize yet".
     PSEUDO           off        Pseudo opcode for internal use.
     REGEX_SET        depth p    Regex set, temporary node used in pre-
                                 optimization compilation

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

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

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:

      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.

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. See also "PERL_MEM_LOG" in perlhacktips.

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 $ENV{PERL_DEBUG_MSTATS}.

## Using $ENV{PERL_DEBUG_MSTATS}

If your perl is using Perl's malloc() and was compiled with the necessary switches (this is the default), then it will print memory usage statistics after compiling your code when "$ENV{PERL_DEBUG_MSTATS} > 1", and before termination of the program when "$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, 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 a 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.

# SEE ALSO

perldebug, perl5db.pl, perlguts, perlrun, re, and Devel::DProf.

2024-05-14 | perl v5.40.1