perldebguts
(1)
Name
perldebguts - Guts of Perl debugging
Synopsis
Please see following description for synopsis
Description
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 $filename for
"eval"ed strings looks like "(eval 34)". Code
assertions in regexes look like "(re_eval 19)".
Values in this array are magical in numeric context:
they compare equal to zero only if the line is not
breakable.
o Each hash "%{"_<$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
$filename for "eval"ed strings looks like "(eval 34)" or
"(re_eval 19)".
o Each scalar "${"_<$filename"}" contains "_<$filename".
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This is also the case for evaluated strings that contain
subroutines, or which are currently being executed. The
$filename for "eval"ed strings looks like "(eval 34)" or
"(re_eval 19)".
o After each "require"d file is compiled, but before it is
executed, "DB::postponed(*{"_<$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, "DB::postponed(subname)" is called if
the "DB::postponed" subroutine also exists.
o A hash %DB::sub is maintained, whose keys are subroutine
names and whose values have the form
"filename:startline-endline". "filename" has the form
"(eval 34)" for subroutines defined inside "eval"s, or
"(re_eval 19)" for those within regex code assertions.
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
holding the name of the called subroutine. (This doesn't
happen if the subroutine was compiled in the "DB"
package.)
Note that if &DB::sub needs external data for it to work, no
subroutine call is possible without it. As an example, the
standard debugger's &DB::sub depends on the $DB::deep
variable (it defines how many levels of recursion deep into
the debugger you can go before a mandatory break). If
$DB::deep is not defined, subroutine calls are not possible,
even though &DB::sub exists.
Writing Your Own Debugger
Environment Variables
The "PERL5DB" environment variable can be used to define a
debugger. For example, the minimal "working" debugger (it
actually doesn't do anything) consists of one line:
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sub DB::DB {}
It can easily be defined like this:
$ PERL5DB="sub DB::DB {}" perl -d your-script
Another brief debugger, slightly more useful, can be created
with only the line:
sub DB::DB {print ++$i; scalar <STDIN>}
This debugger prints a number which increments for each
statement encountered and waits for you to hit a newline
before continuing to the next statement.
The following debugger is actually useful:
{
package DB;
sub DB {}
sub sub {print ++$i, " $sub\n"; &$sub}
}
It prints the sequence number of each subroutine call and
the name of the called subroutine. Note that &DB::sub is
being compiled into the package "DB" through the use of the
"package" directive.
When it starts, the debugger reads your rc file (./.perldb
or ~/.perldb under Unix), which can set important options.
(A subroutine (&afterinit) can be defined here as well; it
is executed after the debugger completes its own
initialization.)
After the rc file is read, the debugger reads the
PERLDB_OPTS environment variable and uses it to set debugger
options. The contents of this variable are treated as if
they were the argument of an "o ..." debugger command (q.v.
in "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.
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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.
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.
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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 use various values of
"n" are shown to give you a feel for the difference between
settings. Long those it may be, this is not a complete
listing, but only excerpts.
1.
entering main::BEGIN
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.
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entering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
exited Config::BEGIN
Package lib/Config.pm.
entering Config::TIEHASH
exited Config::TIEHASH
entering Exporter::import
entering Exporter::export
exited Exporter::export
exited Exporter::import
exited main::BEGIN
entering Config::myconfig
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
3.
in $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
in @=Config::myconfig() from /dev/null:0
in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
4.
in $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
out $=Config::BEGIN() from lib/Config.pm:0
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
out $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
out $=main::BEGIN() from /dev/null:0
in @=Config::myconfig() from /dev/null:0
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in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
5.
in $=main::BEGIN() from /dev/null:0
in $=Config::BEGIN() from lib/Config.pm:2
Package lib/Exporter.pm.
Package lib/Carp.pm.
out $=Config::BEGIN() from lib/Config.pm:0
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:644
out $=Config::TIEHASH('Config') from lib/Config.pm:644
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
out $=main::BEGIN() from /dev/null:0
in @=Config::myconfig() from /dev/null:0
in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
6.
in $=CODE(0x15eca4)() from /dev/null:0
in $=CODE(0x182528)() from lib/Config.pm:2
Package lib/Exporter.pm.
out $=CODE(0x182528)() from lib/Config.pm:0
scalar context return from CODE(0x182528): undef
Package lib/Config.pm.
in $=Config::TIEHASH('Config') from lib/Config.pm:628
out $=Config::TIEHASH('Config') from lib/Config.pm:628
scalar context return from Config::TIEHASH: empty hash
in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
scalar context return from Exporter::export: ''
out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
scalar context return from Exporter::import: ''
In all cases shown above, the line indentation shows the
call tree. If bit 2 of "frame" is set, a line is printed on
exit from a subroutine as well. If bit 4 is set, the
arguments are printed along with the caller info. If bit 8
is set, the arguments are printed even if they are tied or
references. If bit 16 is set, the return value is printed,
too.
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When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
Debugging regular expressions
There are two ways to enable debugging output for regular
expressions.
If your perl is compiled with "-DDEBUGGING", you may use the
-Dr flag on the command line.
Otherwise, one can "use re 'debug'", which has effects at
compile time and run time. Since Perl 5.9.5, this pragma is
lexically scoped.
Compile-time output
The debugging output at compile time looks like this:
Compiling REx `[bc]d(ef*g)+h[ij]k$'
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
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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"
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Set if the pattern starts with ".*".
"with eval"
Set if the pattern contain eval-groups, such as "(?{
code })" and "(??{ code })".
"anchored(TYPE)"
If the pattern may match only at a handful of places,
(with "TYPE" being "BOL", "MBOL", or "GPOS". See the
table below.
If a substring is known to match at end-of-line only, it may
be followed by "$", as in "floating `k'$".
The optimizer-specific information is used to avoid entering
(a slow) regex engine on strings that will not definitely
match. If the "isall" flag is set, a call to the regex
engine may be avoided even when the optimizer found an
appropriate place for the match.
Above the optimizer section is the list of nodes of the
compiled form of the regex. Each line has format
" "id: TYPE OPTIONAL-INFO (next-id)
Types of nodes
Here are the possible types, with short descriptions:
# 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
BOUNDL no Match "" at any word boundary
NBOUND no Match "" at any word non-boundary
NBOUNDL no Match "" at any word non-boundary
GPOS no Matches where last m//g left off.
# [Special] alternatives
ANY no Match any one character (except newline).
SANY no Match any one character.
ANYOF sv Match character in (or not in) this class.
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ALNUM no Match any alphanumeric character
ALNUML no Match any alphanumeric char in locale
NALNUM no Match any non-alphanumeric character
NALNUML no Match any non-alphanumeric char in locale
SPACE no Match any whitespace character
SPACEL no Match any whitespace char in locale
NSPACE no Match any non-whitespace character
NSPACEL no Match any non-whitespace char in locale
DIGIT no Match any numeric character
NDIGIT no Match any non-numeric character
# 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 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 sv Match this string (preceded by length).
EXACTF sv Match this string, folded (prec. by length).
EXACTFL sv Match this string, folded in locale (w/len).
# Do nothing
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.
# 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 Match next-after-this simple thing
# {n,m} times, set parens.
CURLYM no 2 Match 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.
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# 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
REFFL num 1 Match already matched string, folded in loc.
# grouping assertions
IFMATCH off 1 2 Succeeds if the following matches.
UNLESSM off 1 2 Fails if the following matches.
SUSPEND off 1 1 "Independent" sub-regex.
IFTHEN off 1 1 Switch, should be preceded by switcher .
GROUPP num 1 Whether the group matched.
# Support for long regex
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.
# 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.
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
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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
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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 perlhack.
One way to find out how much memory is being used by Perl
data structures is to install the Devel::Size module from
CPAN: it gives you the minimum number of bytes required to
store a particular data structure. Please be mindful of the
difference between the size() and total_size().
If Perl has been compiled using Perl's malloc you can
analyze Perl memory usage by setting the
$ENV{PERL_DEBUG_MSTATS}.
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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, case the corresponding power of two is
printed in the "APPROX" field above.
Free/Used
The 1 or 2 rows of numbers following that correspond to
the number of buckets of each size between "SMALLEST"
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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 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.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
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+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
perldebug, perlguts, perlrun re, and Devel::DProf.
NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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