- NAME
- DESCRIPTION
- Debugger Internals
- Frame Listing Output Examples
- Debugging regular expressions
- Debugging Perl memory usage
- SEE ALSO
NAME
perldebguts - Guts of Perl debugging
DESCRIPTION
This is not the perldebug(1) manpage, 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 forevaled strings that contain subroutines, or which are currently being executed. The $filename forevaled strings looks like(eval 34). Code assertions in regexes look like(re_eval 19).Values in this array are magical in numeric context: they compare equal to zero only if the line is not breakable.
-
Each hash
%{"_<$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
evaled 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 $filename forevaled strings looks like(eval 34)or(re_eval 19). -
After each
required file is compiled, but before it is executed,DB::postponed(*{"_<$filename"})is called if the subroutineDB::postponedexists. Here, the $filename is the expanded name of therequired file, as found in the values of %INC. -
After each subroutine
subnameis compiled, the existence of$DB::postponed{subname}is checked. If this key exists,DB::postponed(subname)is called if theDB::postponedsubroutine also exists. -
A hash
%DB::subis maintained, whose keys are subroutine names and whose values have the formfilename:startline-endline.filenamehas the form(eval 34)for subroutines defined insideevals, 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$DB::signalis true. These variables are notlocalizable. This feature is disabled when executing insideDB::DB(), including functions called from it unless$^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::subholding the name of the called subroutine. (This doesn't happen if the subroutine was compiled in theDBpackage.)
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-scriptAnother 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.
-
@DB::dblineis an alias for@{"::_<current_file"}, which holds the lines of the currently-selected file (compiled by Perl), either explicitly chosen with the debugger'sfcommand, 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'sfcommand, 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, ifcountis missing). Each entry is reference to a hash with keyscontext(either.,$, or@),sub(subroutine name, or info abouteval),args(undefor 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 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
42By 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.
-
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 $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0 scalar context return from Exporter::import: ''
In all cases shown above, the line indentation shows the call tree.
If bit 2 of frame
is set, a line is printed on exit from a
subroutine as well. If bit 4 is set, the arguments are printed
along with the caller info. If bit 8 is set, the arguments are
printed even if they are tied or references. If bit 16 is set, the
return value is printed, too.
When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
Debugging regular expressions
There are two ways to enable debugging output for regular expressions.
If your perl is compiled with -DDEBUGGING
, you may use the
-Dr flag on the command line.
Otherwise, one can use re 'debug'
, which has effects at
compile time and run time. It is not lexically scoped.
Compile-time output
The debugging output at compile time looks like this:
Compiling REx `[bc]d(ef*g)+h[ij]k$'
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 7line (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
anchoredSTRINGatPOSfloatingSTRINGatPOS1..POS2See above.
matching floating/anchoredWhich substring to check first.
minlenThe minimal length of the match.
stclassTYPEType of first matching node.
noscanDon't scan for the found substrings.
isallMeans that the optimizer information is all that the regular expression contains, and thus one does not need to enter the regex engine at all.
GPOSSet if the pattern contains
\G.plusSet if the pattern starts with a repeated char (as in
x+y).implicitSet if the pattern starts with
.*.with evalSet 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
TYPEbeingBOL,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
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.
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.
# 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 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 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}.
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
APPROXfield above.- Free/Used
The 1 or 2 rows of numbers following that correspond to the number of buckets of each size between
SMALLESTandGREATEST. 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-
DEBUGGINGperl, the buckets starting from128have a 4-byte overhead, and thus an 8192-long bucket may take up to 8188-byte allocations. Total sbrk(): SBRKed/SBRKs:CONTINUOUSThe 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: 0The amount of sbrk(2)ed memory needed to keep buckets aligned.
heads: 2192Although 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: 0malloc() 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: 6144To 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.
