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):
Perl inserts the contents of
$ENV{PERL5DB}
(orBEGIN {require 'perl5db.pl'}
if not present) before the first line of your program.Each array
@{"_<$filename"}
holds the lines of $filename for a file compiled by Perl. The same is also true foreval
ed strings that contain subroutines, or which are currently being executed. The $filename foreval
ed strings looks like(eval 34)
. Code assertions in regexes look like(re_eval 19)
.Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable.
Each hash
%{"_<$filename"}
contains breakpoints and actions keyed by line number. Individual entries (as opposed to the whole hash) are settable. Perl only cares about Boolean true here, although the values used by 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)
or(re_eval 19)
.Each scalar
${"_<$filename"}
contains"_<$filename"
. This is also the case for evaluated strings that contain subroutines, or which are currently being executed. The $filename foreval
ed strings looks like(eval 34)
or(re_eval 19)
.After each
require
d file is compiled, but before it is executed,DB::postponed(*{"_<$filename"})
is called if the subroutineDB::postponed
exists. Here, the $filename is the expanded name of therequire
d file, as found in the values of %INC.After each subroutine
subname
is compiled, the existence of$DB::postponed{subname}
is checked. If this key exists,DB::postponed(subname)
is called if theDB::postponed
subroutine also exists.A hash
%DB::sub
is maintained, whose keys are subroutine names and whose values have the formfilename:startline-endline
.filename
has the form(eval 34)
for subroutines defined insideeval
s, or(re_eval 19)
for those within regex code assertions.When the execution of your program reaches a point that can hold a breakpoint, the
DB::DB()
subroutine is called if any of the variables$DB::trace
,$DB::single
, or$DB::signal
is true. These variables are notlocal
izable. This feature is disabled when executing insideDB::DB()
, including functions called from it unless$^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 theDB
package.)
Note that if &DB::sub
needs external data for it to work, no subroutine call is possible without it. As an example, the standard debugger's &DB::sub
depends on the $DB::deep
variable (it defines how many levels of recursion deep into the debugger you can go before a mandatory break). If $DB::deep
is not defined, subroutine calls are not possible, even though &DB::sub
exists.
Writing Your Own Debugger
Environment Variables
The PERL5DB
environment variable can be used to define a debugger. For example, the minimal "working" debugger (it actually doesn't do anything) consists of one line:
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 {
++
$i
;
scalar
<STDIN>}
This debugger prints a number which increments for each statement encountered and waits for you to hit a newline before continuing to the next statement.
The following debugger is actually useful:
It prints the sequence number of each subroutine call and the name of the called subroutine. Note that &DB::sub
is being compiled into the package DB
through the use of the package
directive.
When it starts, the debugger reads your rc file (./.perldb or ~/.perldb under Unix), which can set important options. (A subroutine (&afterinit
) can be defined here as well; it is executed after the debugger completes its own initialization.)
After the rc file is read, the debugger reads the PERLDB_OPTS environment variable and uses it to set debugger options. The contents of this variable are treated as if they were the argument of an o ...
debugger command (q.v. in "Configurable Options" in perldebug).
Debugger Internal Variables
In addition to the file and subroutine-related variables mentioned above, the debugger also maintains various magical internal variables.
@DB::dbline
is an alias for@{"::_<current_file"}
, which holds the lines of the currently-selected file (compiled by Perl), either explicitly chosen with the debugger'sf
command, or implicitly by flow of execution.Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable.
%DB::dbline
is an alias for%{"::_<current_file"}
, which contains breakpoints and actions keyed by line number in the currently-selected file, either explicitly chosen with the debugger'sf
command, or implicitly by flow of execution.As previously noted, individual entries (as opposed to the whole hash) are settable. Perl only cares about Boolean true here, although the values used by perl5db.pl have the form
"$break_condition\0$action"
.
Debugger Customization Functions
Some functions are provided to simplify customization.
See "Configurable Options" in perldebug for a description of options parsed by
DB::parse_options(string)
.DB::dump_trace(skip[,count])
skips the specified number of frames and returns a list containing information about the calling frames (all of them, ifcount
is missing). Each entry is reference to a hash with keyscontext
(either.
,$
, or@
),sub
(subroutine name, or info abouteval
),args
(undef
or a reference to an array),file
, andline
.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
foo() * bar()
main::((
eval
172):3):
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
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 $=Exporter::
import
(
'Config'
,
'myconfig'
,
'config_vars'
) from /dev/null:0
scalar
context
return
from Exporter::
import
:
''
In all cases shown above, the line indentation shows the call tree. If bit 2 of frame
is set, a line is printed on exit from a subroutine as well. If bit 4 is set, the arguments are printed along with the caller info. If bit 8 is set, the arguments are printed even if they are tied or references. If bit 16 is set, the return value is printed, too.
When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
Debugging Regular Expressions
There are two ways to enable debugging output for regular expressions.
If your perl is compiled with -DDEBUGGING
, you may use the -Dr flag on the command line.
Otherwise, one can use re 'debug'
, which has effects at compile time and run time. Since Perl 5.9.5, this pragma is lexically scoped.
Compile-time Output
The debugging output at compile time looks like this:
Compiling REx
'[bc]d(ef*g)+h[ij]k$'
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
STRINGat
POSfloating
STRINGat
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
beingBOL
,MBOL
, orGPOS
. See the table below.
If a substring is known to match at end-of-line only, it may be followed by $
, as in floating 'k'$
.
The optimizer-specific information is used to avoid entering (a slow) regex engine on strings that will not definitely match. If the isall
flag is set, a call to the regex engine may be avoided even when the optimizer found an appropriate place for the match.
Above the optimizer section is the list of nodes of the compiled form of the regex. Each line has format
id: TYPE OPTIONAL-INFO (next-id)
Types of Nodes
Here are the possible types, with short descriptions:
# 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
ANYOFV sv Match character in (or not in) this class, can
match-multiple chars
ALNUM
no
Match any alphanumeric character using native charset
semantics
for
non-utf8
ALNUML
no
Match any alphanumeric char in locale
ALNUMU
no
Match any alphanumeric char using Unicode semantics
ALNUMA
no
Match [A-Za-z_0-9]
NALNUM
no
Match any non-alphanumeric character using native charset
semantics
for
non-utf8
NALNUML
no
Match any non-alphanumeric char in locale
NALNUMU
no
Match any non-alphanumeric char using Unicode semantics
NALNUMA
no
Match [^A-Za-z_0-9]
SPACE
no
Match any whitespace character using native charset
semantics
for
non-utf8
SPACEL
no
Match any whitespace char in locale
SPACEU
no
Match any whitespace char using Unicode semantics
SPACEA
no
Match [ \t\n\f\r]
NSPACE
no
Match any non-whitespace character using native charset
semantics
for
non-utf8
NSPACEL
no
Match any non-whitespace char in locale
NSPACEU
no
Match any non-whitespace char using Unicode semantics
NSPACEA
no
Match [^ \t\n\f\r]
DIGIT
no
Match any numeric character using native charset semantics
for
non-utf8
DIGITL
no
Match any numeric character in locale
DIGITA
no
Match [0-9]
NDIGIT
no
Match any non-numeric character using native charset
i semantics
for
non-utf8
NDIGITL
no
Match any non-numeric character in locale
NDIGITA
no
Match [^0-9]
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 string, folded, native charset semantics
for
non-utf8 (prec. by
length
).
EXACTFL str Match this string, folded in locale (w/len).
EXACTFU str Match this string, folded, Unicode semantics
for
non-utf8
(prec. by
length
).
EXACTFA str Match this string, folded, Unicode semantics
for
non-utf8,
but
no
ASCII-range character matches outside ASCII (prec.
by
length
),.
# 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.
# NOTE: the relative order of the TRIE-like regops is significant
TRIE trie 1 Match many EXACT(F[ALU]?)? at once. flags==type
TRIEC charclass Same as TRIE, but
with
embedded charclass data
# For start classes, contains an added fail table.
AHOCORASICK trie 1 Aho Corasick stclass. flags==type
AHOCORASICKC charclass Same as AHOCORASICK, but
with
embedded 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
VERTWS none vertical whitespace (Perl 6)
NVERTWS none not vertical whitespace (Perl 6)
HORIZWS none horizontal whitespace (Perl 6)
NHORIZWS none not horizontal whitespace (Perl 6)
FOLDCHAR codepoint 1 codepoint
with
tricky case folding properties.
# 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.
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
andGREATEST
. 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 from128
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, perlguts, perlrun re, and Devel::DProf.