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download PDF document: perldebguts.1.pdf

PERLDEBGUTS(1) Perl Programmers Reference Guide PERLDEBGUTS(1)

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 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):
o Perl inserts the contents of $ENV{PERL5DB} (or "BEGIN {require 'perl5db.pl'}" if not present) before the first line of your program.
o Each array "@{"_<$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 $filename for "eval"ed strings looks like "(eval 34)".
Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable.
o Each hash "%{"_<$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 $filename for "eval"ed strings looks like "(eval 34)".
o Each scalar "${"_<$filename"}" contains $filename. This is also the case for evaluated strings that contain subroutines, or which are currently being executed. The $filename for "eval"ed strings looks like "(eval 34)".
o After each "require"d file is compiled, but before it is executed, "DB::postponed(*{"_<$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.
o After each subroutine "subname" is compiled, the existence of $DB::postponed{subname} is checked. If this key exists,
o 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.
o 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).
o 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 ++$i; scalar <STDIN>}
This debugger prints a number which increments for each statement sub DB {} sub sub {print ++$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.
o @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.
o %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.
o See "Configurable Options" in perldebug for a description of options parsed by "DB::parse_options(string)".
o "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".
o "DB::print_trace(FH, skip[, count[, short]])" prints formatted info about caller frames. The last two functions may be convenient as arguments to "<", "<<" commands.

$ 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 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 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) 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 $` $& $' support.
The first line shows the pre-compiled form of the regex. The second shows the size of the compiled form (in arbitrary units, usually 4-byte words) and the total number of bytes allocated for the offset/length table, usually 4+"size"*8. The next line shows the label id of the first node that does a match.
The
anchored 'de' at 1 floating 'gh' at 3..2147483647 (checking floating) stclass 'ANYOF[bc]' minlen 7
line (split into two lines above) contains optimizer information. In the example shown, the optimizer found that the match should contain a substring "de" at offset 1, plus substring "gh" at some offset between 3 and infinity. Moreover, when checking for these substrings (to abandon impossible matches quickly), Perl will check for the substring "gh" before checking for the substring "de". The optimizer may also use the knowledge that the match starts (at the "first" id) with a character class, and no string shorter than 7 characters can possibly match.
The fields of interest which may appear in this line are
"anchored" STRING "at" POS "floating" STRING "at" POS1..POS2 See above.
"matching floating/anchored" Which substring to check first.
"minlen" The minimal length of the match.
"stclass" TYPE Type of first matching node.
"noscan" Don't scan for the found substrings.
"isall" Means that the optimizer information is all that the regular expression contains, and thus one does not need to enter the regex engine at all.
"GPOS" Set if the pattern contains "\G".
"plus" Set if the pattern starts with a repeated char (as in "x+y").
"anchored(TYPE)" If the pattern may match only at a handful of places, with "TYPE" being "SBOL", "MBOL", or "GPOS". See the table below.
If a substring is known to match at end-of-line only, it may be followed by "$", 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: 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 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
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 Match this alternative, or the next...
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. # 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. PLUS node Match this (simple) thing 1 or more times.
CURLY sv 2 Match this simple thing {n,m} times. CURLYN no 2 Capture next-after-this simple thing CURLYM no 2 Capture 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.
# 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 1 Match some already matched string REFF num 1 Match already matched string, using /di rules. REFFL num 1 Match already matched string, using /li rules. REFFU num 1 Match already matched string, usng /ui. REFFA num 1 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 1 Match some already matched string REFFN no-sv 1 Match already matched string, using /di rules. REFFLN no-sv 1 Match already matched string, using /li rules. REFFUN num 1 Match already matched string, using /ui rules. REFFAN num 1 Match already matched string, using /aai rules.
# Support for long RE LONGJMP off 1 1 Jump far away. BRANCHJ off 1 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
2L
# 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 2L 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 2L 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 $& begins here.
# SPECIAL REGOPS
# This is not really a node, but an optimized away piece of a "long" # node. To simplify debugging output, we mark it as if it were a node OPTIMIZED off Placeholder for dump.
# Special opcode with the property that no opcode in a compiled program # will ever be of this type. Thus it can be used as a flag value that # no other opcode has been seen. END is used similarly, in that an END # node cant be optimized. So END implies "unoptimizable" and PSEUDO # mean "not seen anything to optimize yet". PSEUDO off Pseudo opcode for internal use.
REGEX_SET depth p Regex set, temporary node used in pre- optimization compilation
Following the optimizer information is a dump of the offset/length table, here split across several lines:
entries. Each entry is a pair of integers, denoted by "offset[length]". Entries are numbered starting with 1, so entry #1 here is "1[4]" and entry #12 is "5[1]". "1[4]" indicates that the node labeled "1:" (the "1: ANYOF[bc]") begins at character position 1 in the pre-compiled form of the regex, and has a length of 4 characters. "5[1]" in position 12 indicates that the node labeled "12:" (the "12: EXACT <d>") begins at character position 5 in the pre-compiled form of the regex, and has a length of 1 character. "12[1]" in position 14 indicates that the node labeled "14:" (the "14: CURLYX[0] {1,32767}") begins at character position 12 in the pre-compiled form of the regex, and has a length of 1 character---that is, it corresponds to the "+" symbol in the precompiled regex.
"0[0]" items indicate that there is no corresponding node.
Run-time Output First of all, when doing a match, one may get no run-time output even if debugging is enabled. This means that the regex engine was never entered and that all of the job was therefore done by the optimizer.
If the regex engine was entered, the output may look like this:
Matching '[bc]d(ef*g)+h[ij]k$' 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 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.
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.