perldebguts - Guts of Perl debugging
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.
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 $ENV{PERL5DB}
(or BEGIN {require
'perl5db.pl'}
if not present) before the first line of your program.
Each array @{"_<$filename"}
holds the lines of $filename for a
file compiled by Perl. The same is also true for evaled strings
that contain subroutines, or which are currently being executed.
The $filename for evaled strings looks like (eval 34)
.
Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable.
Each hash %{"_<$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 evaled 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 $filename for evaled
strings looks like (eval 34)
.
After each required file is compiled, but before it is executed,
DB::postponed(*{"_<$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.
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.
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:
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:
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).
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"
.
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.
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.
- 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
- 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
- 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
- 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
- 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
- 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 $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
- scalar context return from Exporter::import: ''
In all cases shown above, the line indentation shows the call tree.
If bit 2 of frame
is set, a line is printed on exit from a
subroutine as well. If bit 4 is set, the arguments are printed
along with the caller info. If bit 8 is set, the arguments are
printed even if they are tied or references. If bit 16 is set, the
return value is printed, too.
When a package is compiled, a line like this
- Package lib/Carp.pm.
is printed with proper indentation.
There are two ways to enable debugging output for regular expressions.
If your perl is compiled with -DDEBUGGING
, you may use the
-Dr flag on the command line.
Otherwise, one can use re 'debug'
, which has effects at
compile time and run time. Since Perl 5.9.5, this pragma is lexically
scoped.
The debugging output at compile time looks like this:
- Compiling REx '[bc]d(ef*g)+h[ij]k$'
- 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 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)
Here are the possible types, with short descriptions:
- # TYPE arg-description [num-args] [longjump-len] DESCRIPTION
- # Exit points
- END no End of program.
- SUCCEED no Return from a subroutine, basically.
- # Anchors:
- BOL no Match "" at beginning of line.
- MBOL no Same, assuming multiline.
- SBOL no Same, assuming singleline.
- EOS no Match "" at end of string.
- EOL no Match "" at end of line.
- MEOL no Same, assuming multiline.
- SEOL no Same, assuming singleline.
- BOUND no Match "" at any word boundary using
- native charset semantics for non-utf8
- BOUNDL no Match "" at any locale word boundary
- BOUNDU no Match "" at any word boundary using
- Unicode semantics
- BOUNDA no Match "" at any word boundary using ASCII
- semantics
- NBOUND no Match "" at any word non-boundary using
- native charset semantics for non-utf8
- NBOUNDL no Match "" at any locale word non-boundary
- NBOUNDU no Match "" at any word non-boundary using
- Unicode semantics
- NBOUNDA no Match "" at any word non-boundary using
- ASCII semantics
- GPOS no Matches where last m//g left off.
- # [Special] alternatives:
- REG_ANY no Match any one character (except newline).
- SANY no Match any one character.
- CANY no Match any one byte.
- ANYOF sv Match character in (or not in) this
- class, single char match only
- ANYOF_WARN_SUPER sv Match character in (or not in) this
- class, warn (if enabled) upon matching a
- char above Unicode max;
- ANYOF_SYNTHETIC sv Synthetic start class
- 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...
- # Back pointer
- # BACK Normal "next" pointers all implicitly point forward;
- # BACK exists to make loop structures possible.
- # not used
- BACK no Match "", "next" ptr points backward.
- # Literals
- EXACT str Match this string (preceded by length).
- EXACTF str Match this non-UTF-8 string (not
- guaranteed to be folded) using /id rules
- (w/len).
- EXACTFL str Match this string (not guaranteed to be
- folded) using /il rules (w/len).
- EXACTFU str Match this string (folded iff in UTF-8,
- length in folding doesn't change if not
- in UTF-8) using /iu rules (w/len).
- EXACTFA str Match this string (not guaranteed to be
- folded) using /iaa rules (w/len).
- EXACTFU_SS str Match this string (folded iff in UTF-8,
- length in folding may change even if not
- in UTF-8) using /iu rules (w/len).
- EXACTFU_TRICKYFOLD str Match this folded UTF-8 string using /iu
- rules
- # 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 using BACK. Simple cases
- # (one character per match) are implemented with STAR
- # and PLUS for speed and to minimize recursive plunges.
- #
- STAR node Match this (simple) thing 0 or more
- times.
- PLUS node Match this (simple) thing 1 or more
- times.
- CURLY sv 2 Match this simple thing {n,m} times.
- CURLYN no 2 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 Analogous to OPEN.
- REF num 1 Match some already matched string
- REFF num 1 Match already matched string, folded
- using native charset semantics for non-
- utf8
- REFFL num 1 Match already matched string, folded in
- loc.
- REFFU num 1 Match already matched string, folded
- using unicode semantics for non-utf8
- REFFA num 1 Match already matched string, folded
- using unicode semantics for non-utf8, no
- mixing ASCII, non-ASCII
- # Named references. Code in regcomp.c assumes that these all are after
- # the numbered references
- NREF no-sv 1 Match some already matched string
- NREFF no-sv 1 Match already matched string, folded
- using native charset semantics for non-
- utf8
- NREFFL no-sv 1 Match already matched string, folded in
- loc.
- NREFFU num 1 Match already matched string, folded
- using unicode semantics for non-utf8
- NREFFA num 1 Match already matched string, folded
- using unicode semantics for non-utf8, no
- mixing ASCII, non-ASCII
- IFMATCH off 1 2 Succeeds if the following matches.
- UNLESSM off 1 2 Fails if the following matches.
- SUSPEND off 1 1 "Independent" sub-RE.
- IFTHEN off 1 1 Switch, should be preceded by switcher.
- GROUPP num 1 Whether the group matched.
- # Support for long RE
- LONGJMP off 1 1 Jump far away.
- BRANCHJ off 1 1 BRANCH with long offset.
- # The heavy worker
- EVAL evl 1 Execute some Perl code.
- # Modifiers
- MINMOD no Next operator is not greedy.
- LOGICAL no Next opcode should set the flag only.
- # This is not used yet
- RENUM off 1 1 Group with independently numbered parens.
- # 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
- # Regex Subroutines
- GOSUB num/ofs 2L recurse to paren arg1 at (signed) ofs
- arg2
- GOSTART no recurse to start of pattern
- # Special conditionals
- NGROUPP 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 none Same as (?!)
- ACCEPT parno 1 Accepts the current matched string.
- # 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.
- # New charclass like patterns
- LNBREAK none generic newline pattern
- # 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.
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.
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.
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}.
$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.
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.
perldebug, perlguts, perlrun re, and Devel::DProf.