PERLDEBGUTS(1) Perl Programmers Reference Guide PERLDEBGUTS(1)
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 impos-sible impossible
sible 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 mecha-nisms mechanisms
nisms are enabled by calling Perl with the -d switch. Specifically, the following additional fea-tures features
tures are enabled (cf. "$^P" in perlvar):
Perl inserts the contents of $ENV{PERL5DB} (or "BEGIN {require 'perl5db.pl'}" if not present)
before the first line of your program.
Each array "@{"_<$filename"}" holds the lines of $filename for a file compiled by Perl. The same
is also true for "eval"ed strings that contain subroutines, or which are currently being exe-
cuted. The $filename for "eval"ed strings looks like "(eval 34)". Code assertions in regexes
look like "(re_eval 19)".
Values in this array are magical in numeric context: they compare equal to zero only if the line
is not breakable.
Each hash "%{"_<$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 exe-
cuted. 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 $filename for
"eval"ed strings looks like "(eval 34)" or "(re_eval 19)".
After each "require"d file is compiled, but before it is executed, "DB::postponed(*{"_<$file-
name"})" 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.
After each subroutine "subname" is compiled, the existence of $DB::postponed{subname} is checked.
If this key exists, "DB::postponed(subname)" is called if the "DB::postponed" subroutine also
exists.
A hash %DB::sub is maintained, whose keys are subroutine names and whose values have the form
"filename:startline-endline". "filename" has the form "(eval 34)" for subroutines defined inside
"eval"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 $DB::signal is true.
These variables are not "local"izable. This feature is disabled when executing inside
"DB::DB()", including functions called from it unless "$^D & (1<<30)" is true.
When execution of the program reaches a subroutine call, a call to &DB::sub(args) is made
instead, with $DB::sub holding the name of the called subroutine. (This doesn't happen if the
subroutine was compiled in the "DB" package.)
Note that if &DB::sub needs external data for it to work, no subroutine call is possible without it.
As an example, the standard debugger's &DB::sub depends on the $DB::deep variable (it defines how
many levels of recursion deep into the debugger you can go before a mandatory break). If $DB::deep
is not defined, subroutine calls are not possible, even though &DB::sub exists.
Writing Your Own Debugger
Environment Variables
The "PERL5DB" environment variable can be used to define a debugger. For example, the minimal "work-
ing" debugger (it actually doesn't do anything) consists of one line:
sub DB::DB {}
It can easily be defined like this:
$ PERL5DB="sub DB::DB {}" perl -d your-script
Another brief debugger, slightly more useful, can be created with only the line:
sub DB::DB {print ++$i; scalar <STDIN>}
This debugger prints a number which increments for each statement encountered and waits for you to
hit a newline before continuing to the next statement.
The following debugger is actually useful:
{
package DB;
sub DB {}
sub sub {print ++$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::dbline is an alias for "@{"::_<current_file"}", which holds the lines of the currently-
selected file (compiled by Perl), either explicitly chosen with the debugger's "f" command, or
implicitly by flow of execution.
Values in this array are magical in numeric context: they compare equal to zero only if the line
is not breakable.
%DB::dbline, is an alias for "%{"::_<current_file"}", which contains breakpoints and actions
keyed by line number in the currently-selected file, either explicitly chosen with the debugger's
"f" command, or implicitly by flow of execution.
As previously noted, individual entries (as opposed to the whole hash) are settable. Perl only
cares about Boolean true here, although the values used by perl5db.pl have the form "$break_con-
dition\0$action".
Debugger customization functions
Some functions are provided to simplify customization.
See "Options" in perldebug for description of options parsed by "DB::parse_options(string)"
parses debugger options; see "Options" in pperldebug for a description of options recognized.
"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 refer-ence reference
ence to a hash with keys "context" (either ".", "$", or "@"), "sub" (subroutine name, or info
about "eval"), "args" ("undef" or a reference to an array), "file", and "line".
"DB::print_trace(FH, skip[, count[, short]])" prints formatted info about caller frames. The
last two functions may be convenient as arguments to "<", "<<" commands.
Note that any variables and functions that are not documented in this manpages (or in perldebug) are
considered for internal use only, and as such are subject to change without notice.
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
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
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
4
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
6
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
14
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
30
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 $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
scalar context return from Exporter::import: ''
In all cases shown above, the line indentation shows the call tree. If bit 2 of "frame" is set, a
line is printed on exit from a subroutine as well. If bit 4 is set, the arguments are printed along
with the caller info. If bit 8 is set, the arguments are printed even if they are tied or refer-ences. references.
ences. If bit 16 is set, the return value is printed, too.
When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
Debugging regular expressions
There are two ways to enable debugging output for regular expressions.
If your perl is compiled with "-DDEBUGGING", you may use the -Dr flag on the command line.
Otherwise, one can "use re 'debug'", which has effects at compile time and run time. It is not lexi-cally lexically
cally scoped.
Compile-time output
The debugging output at compile time looks like this:
Compiling REx `[bc]d(ef*g)+h[ij]k$'
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 $` $& $' support.
The first line shows the pre-compiled form of the regex. The second shows the size of the compiled
form (in arbitrary units, usually 4-byte words) and the total number of bytes allocated for the off-set/length offset/length
set/length table, usually 4+"size"*8. The next line shows the label id of the first node that does a
match.
The
anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
stclass `ANYOF[bc]' minlen 7
line (split into two lines above) contains optimizer information. In the example shown, the opti-mizer optimizer
mizer found that the match should contain a substring "de" at offset 1, plus substring "gh" at some
offset between 3 and infinity. Moreover, when checking for these substrings (to abandon impossible
matches quickly), Perl will check for the substring "gh" before checking for the substring "de". The
optimizer may also use the knowledge that the match starts (at the "first" id) with a character
class, and no string shorter than 7 characters can possibly match.
The fields of interest which may appear in this line are
"anchored" STRING "at" POS
"floating" STRING "at" POS1..POS2
See above.
"matching floating/anchored"
Which substring to check first.
"minlen"
The minimal length of the match.
"stclass" TYPE
Type of first matching node.
"noscan"
Don't scan for the found substrings.
"isall"
Means that the optimizer information is all that the regular expression contains, and thus one
does not need to enter the regex engine at all.
"GPOS"
Set if the pattern contains "\G".
"plus"
Set if the pattern starts with a repeated char (as in "x+y").
"implicit"
Set if the pattern starts with ".*".
"with eval"
Set if the pattern contain eval-groups, such as "(?{ code })" and "(??{ code })".
"anchored(TYPE)"
If the pattern may match only at a handful of places, (with "TYPE" being "BOL", "MBOL", or
"GPOS". See the table below.
If a substring is known to match at end-of-line only, it may be followed by "$", as in "floating
`k'$".
The optimizer-specific information is used to avoid entering (a slow) regex engine on strings that
will not definitely match. If the "isall" flag is set, a call to the regex engine may be avoided
even when the optimizer found an appropriate place for the match.
Above the optimizer section is the list of nodes of the compiled form of the regex. Each line has
format
" "id: TYPE OPTIONAL-INFO (next-id)
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 charac-ters. characters.
ters. "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 char-acter character
acter 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 opti-mizer. optimizer.
mizer.
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 mem-ory memory
ory leaks. These days the use of malloc debugging tools like Purify or valgrind is suggested
instead.
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: mem-ory memory
ory 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" 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 foot-prints footprints
prints 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 pro-vide provide
vide 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.
perl v5.8.8 2006-01-07 PERLDEBGUTS(1)
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