3 perldebguts - Guts of Perl debugging
7 This is not the perldebug(1) manpage, which tells you how to use
8 the debugger. This manpage describes low-level details ranging
9 between difficult and impossible for anyone who isn't incredibly
10 intimate with Perl's guts to understand. Caveat lector.
12 =head1 Debugger Internals
14 Perl has special debugging hooks at compile-time and run-time used
15 to create debugging environments. These hooks are not to be confused
16 with the I<perl -Dxxx> command described in L<perlrun>, which is
17 usable only if a special Perl is built per the instructions in the
18 F<INSTALL> podpage in the Perl source tree.
20 For example, whenever you call Perl's built-in C<caller> function
21 from the package DB, the arguments that the corresponding stack
22 frame was called with are copied to the @DB::args array. The
23 general mechanisms is enabled by calling Perl with the B<-d> switch, the
24 following additional features are enabled (cf. L<perlvar/$^P>):
30 Perl inserts the contents of C<$ENV{PERL5DB}> (or C<BEGIN {require
31 'perl5db.pl'}> if not present) before the first line of your program.
35 The array C<@{"_<$filename"}> holds the lines of $filename for all
36 files compiled by Perl. The same for C<eval>ed strings that contain
37 subroutines, or which are currently being executed. The $filename
38 for C<eval>ed strings looks like C<(eval 34)>. Code assertions
39 in regexes look like C<(re_eval 19)>.
43 The hash C<%{"_<$filename"}> contains breakpoints and actions keyed
44 by line number. Individual entries (as opposed to the whole hash)
45 are settable. Perl only cares about Boolean true here, although
46 the values used by F<perl5db.pl> have the form
47 C<"$break_condition\0$action">. Values in this hash are magical
48 in numeric context: they are zeros if the line is not breakable.
50 The same holds for evaluated strings that contain subroutines, or
51 which are currently being executed. The $filename for C<eval>ed strings
52 looks like C<(eval 34)> or C<(re_eval 19)>.
56 The scalar C<${"_<$filename"}> contains C<"_<$filename">. This is
57 also the case for evaluated strings that contain subroutines, or
58 which are currently being executed. The $filename for C<eval>ed
59 strings looks like C<(eval 34)> or C<(re_eval 19)>.
63 After each C<require>d file is compiled, but before it is executed,
64 C<DB::postponed(*{"_<$filename"})> is called if the subroutine
65 C<DB::postponed> exists. Here, the $filename is the expanded name of
66 the C<require>d file, as found in the values of %INC.
70 After each subroutine C<subname> is compiled, the existence of
71 C<$DB::postponed{subname}> is checked. If this key exists,
72 C<DB::postponed(subname)> is called if the C<DB::postponed> subroutine
77 A hash C<%DB::sub> is maintained, whose keys are subroutine names
78 and whose values have the form C<filename:startline-endline>.
79 C<filename> has the form C<(eval 34)> for subroutines defined inside
80 C<eval>s, or C<(re_eval 19)> for those within regex code assertions.
84 When the execution of your program reaches a point that can hold a
85 breakpoint, the C<DB::DB()> subroutine is called any of the variables
86 $DB::trace, $DB::single, or $DB::signal is true. These variables
87 are not C<local>izable. This feature is disabled when executing
88 inside C<DB::DB()>, including functions called from it
89 unless C<< $^D & (1<<30) >> is true.
93 When execution of the program reaches a subroutine call, a call to
94 C<&DB::sub>(I<args>) is made instead, with C<$DB::sub> holding the
95 name of the called subroutine. This doesn't happen if the subroutine
96 was compiled in the C<DB> package.)
100 Note that if C<&DB::sub> needs external data for it to work, no
101 subroutine call is possible until this is done. For the standard
102 debugger, the C<$DB::deep> variable (how many levels of recursion
103 deep into the debugger you can go before a mandatory break) gives
104 an example of such a dependency.
106 =head2 Writing Your Own Debugger
108 The minimal working debugger consists of one line
112 which is quite handy as contents of C<PERL5DB> environment
115 $ PERL5DB="sub DB::DB {}" perl -d your-script
117 Another brief debugger, slightly more useful, could be created
120 sub DB::DB {print ++$i; scalar <STDIN>}
122 This debugger would print the sequential number of encountered
123 statement, and would wait for you to hit a newline before continuing.
125 The following debugger is quite functional:
130 sub sub {print ++$i, " $sub\n"; &$sub}
133 It prints the sequential number of subroutine call and the name of the
134 called subroutine. Note that C<&DB::sub> should be compiled into the
137 At the start, the debugger reads your rc file (F<./.perldb> or
138 F<~/.perldb> under Unix), which can set important options. This file may
139 define a subroutine C<&afterinit> to be executed after the debugger is
142 After the rc file is read, the debugger reads the PERLDB_OPTS
143 environment variable and parses this as the remainder of a C<O ...>
144 line as one might enter at the debugger prompt.
146 The debugger also maintains magical internal variables, such as
147 C<@DB::dbline>, C<%DB::dbline>, which are aliases for
148 C<@{"::_<current_file"}> C<%{"::_<current_file"}>. Here C<current_file>
149 is the currently selected file, either explicitly chosen with the
150 debugger's C<f> command, or implicitly by flow of execution.
152 Some functions are provided to simplify customization. See
153 L<perldebug/"Options"> for description of options parsed by
154 C<DB::parse_options(string)>. The function C<DB::dump_trace(skip[,
155 count])> skips the specified number of frames and returns a list
156 containing information about the calling frames (all of them, if
157 C<count> is missing). Each entry is reference to a hash with
158 keys C<context> (either C<.>, C<$>, or C<@>), C<sub> (subroutine
159 name, or info about C<eval>), C<args> (C<undef> or a reference to
160 an array), C<file>, and C<line>.
162 The function C<DB::print_trace(FH, skip[, count[, short]])> prints
163 formatted info about caller frames. The last two functions may be
164 convenient as arguments to C<< < >>, C<< << >> commands.
166 Note that any variables and functions that are not documented in
167 this manpages (or in L<perldebug>) are considered for internal
168 use only, and as such are subject to change without notice.
170 =head1 Frame Listing Output Examples
172 The C<frame> option can be used to control the output of frame
173 information. For example, contrast this expression trace:
176 Stack dump during die enabled outside of evals.
178 Loading DB routines from perl5db.pl patch level 0.94
179 Emacs support available.
181 Enter h or `h h' for help.
188 DB<3> t print foo() * bar()
189 main::((eval 172):3): print foo() + bar();
190 main::foo((eval 168):2):
191 main::bar((eval 170):2):
194 with this one, once the C<O>ption C<frame=2> has been set:
198 DB<5> t print foo() * bar()
208 By way of demonstration, we present below a laborious listing
209 resulting from setting your C<PERLDB_OPTS> environment variable to
210 the value C<f=n N>, and running I<perl -d -V> from the command line.
211 Examples use various values of C<n> are shown to give you a feel
212 for the difference between settings. Long those it may be, this
213 is not a complete listing, but only excerpts.
220 entering Config::BEGIN
221 Package lib/Exporter.pm.
223 Package lib/Config.pm.
224 entering Config::TIEHASH
225 entering Exporter::import
226 entering Exporter::export
227 entering Config::myconfig
228 entering Config::FETCH
229 entering Config::FETCH
230 entering Config::FETCH
231 entering Config::FETCH
236 entering Config::BEGIN
237 Package lib/Exporter.pm.
240 Package lib/Config.pm.
241 entering Config::TIEHASH
242 exited Config::TIEHASH
243 entering Exporter::import
244 entering Exporter::export
245 exited Exporter::export
246 exited Exporter::import
248 entering Config::myconfig
249 entering Config::FETCH
251 entering Config::FETCH
253 entering Config::FETCH
257 in $=main::BEGIN() from /dev/null:0
258 in $=Config::BEGIN() from lib/Config.pm:2
259 Package lib/Exporter.pm.
261 Package lib/Config.pm.
262 in $=Config::TIEHASH('Config') from lib/Config.pm:644
263 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
264 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
265 in @=Config::myconfig() from /dev/null:0
266 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
267 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
268 in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
269 in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
270 in $=Config::FETCH(ref(Config), 'osname') from lib/Config.pm:574
271 in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
275 in $=main::BEGIN() from /dev/null:0
276 in $=Config::BEGIN() from lib/Config.pm:2
277 Package lib/Exporter.pm.
279 out $=Config::BEGIN() from lib/Config.pm:0
280 Package lib/Config.pm.
281 in $=Config::TIEHASH('Config') from lib/Config.pm:644
282 out $=Config::TIEHASH('Config') from lib/Config.pm:644
283 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
284 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
285 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/
286 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
287 out $=main::BEGIN() from /dev/null:0
288 in @=Config::myconfig() from /dev/null:0
289 in $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
290 out $=Config::FETCH(ref(Config), 'package') from lib/Config.pm:574
291 in $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
292 out $=Config::FETCH(ref(Config), 'baserev') from lib/Config.pm:574
293 in $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
294 out $=Config::FETCH(ref(Config), 'PERL_VERSION') from lib/Config.pm:574
295 in $=Config::FETCH(ref(Config), 'PERL_SUBVERSION') from lib/Config.pm:574
299 in $=main::BEGIN() from /dev/null:0
300 in $=Config::BEGIN() from lib/Config.pm:2
301 Package lib/Exporter.pm.
303 out $=Config::BEGIN() from lib/Config.pm:0
304 Package lib/Config.pm.
305 in $=Config::TIEHASH('Config') from lib/Config.pm:644
306 out $=Config::TIEHASH('Config') from lib/Config.pm:644
307 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
308 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
309 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/E
310 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
311 out $=main::BEGIN() from /dev/null:0
312 in @=Config::myconfig() from /dev/null:0
313 in $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
314 out $=Config::FETCH('Config=HASH(0x1aa444)', 'package') from lib/Config.pm:574
315 in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
316 out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
320 in $=CODE(0x15eca4)() from /dev/null:0
321 in $=CODE(0x182528)() from lib/Config.pm:2
322 Package lib/Exporter.pm.
323 out $=CODE(0x182528)() from lib/Config.pm:0
324 scalar context return from CODE(0x182528): undef
325 Package lib/Config.pm.
326 in $=Config::TIEHASH('Config') from lib/Config.pm:628
327 out $=Config::TIEHASH('Config') from lib/Config.pm:628
328 scalar context return from Config::TIEHASH: empty hash
329 in $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
330 in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
331 out $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from lib/Exporter.pm:171
332 scalar context return from Exporter::export: ''
333 out $=Exporter::import('Config', 'myconfig', 'config_vars') from /dev/null:0
334 scalar context return from Exporter::import: ''
338 In all cases shown above, the line indentation shows the call tree.
339 If bit 2 of C<frame> is set, a line is printed on exit from a
340 subroutine as well. If bit 4 is set, the arguments are printed
341 along with the caller info. If bit 8 is set, the arguments are
342 printed even if they are tied or references. If bit 16 is set, the
343 return value is printed, too.
345 When a package is compiled, a line like this
349 is printed with proper indentation.
351 =head1 Debugging regular expressions
353 There are two ways to enable debugging output for regular expressions.
355 If your perl is compiled with C<-DDEBUGGING>, you may use the
356 B<-Dr> flag on the command line.
358 Otherwise, one can C<use re 'debug'>, which has effects at
359 compile time and run time. It is not lexically scoped.
361 =head2 Compile-time output
363 The debugging output at compile time looks like this:
365 compiling RE `[bc]d(ef*g)+h[ij]k$'
369 13: CURLYX {1,32767}(27)
383 anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
384 stclass `ANYOF' minlen 7
386 The first line shows the pre-compiled form of the regex. The second
387 shows the size of the compiled form (in arbitrary units, usually
388 4-byte words) and the label I<id> of the first node that does a
391 The last line (split into two lines above) contains optimizer
392 information. In the example shown, the optimizer found that the match
393 should contain a substring C<de> at offset 1, plus substring C<gh>
394 at some offset between 3 and infinity. Moreover, when checking for
395 these substrings (to abandon impossible matches quickly), Perl will check
396 for the substring C<gh> before checking for the substring C<de>. The
397 optimizer may also use the knowledge that the match starts (at the
398 C<first> I<id>) with a character class, and the match cannot be
399 shorter than 7 chars.
401 The fields of interest which may appear in the last line are
405 =item C<anchored> I<STRING> C<at> I<POS>
407 =item C<floating> I<STRING> C<at> I<POS1..POS2>
411 =item C<matching floating/anchored>
413 Which substring to check first.
417 The minimal length of the match.
419 =item C<stclass> I<TYPE>
421 Type of first matching node.
425 Don't scan for the found substrings.
429 Means that the optimizer info is all that the regular
430 expression contains, and thus one does not need to enter the regex engine at
435 Set if the pattern contains C<\G>.
439 Set if the pattern starts with a repeated char (as in C<x+y>).
443 Set if the pattern starts with C<.*>.
447 Set if the pattern contain eval-groups, such as C<(?{ code })> and
450 =item C<anchored(TYPE)>
452 If the pattern may match only at a handful of places, (with C<TYPE>
453 being C<BOL>, C<MBOL>, or C<GPOS>. See the table below.
457 If a substring is known to match at end-of-line only, it may be
458 followed by C<$>, as in C<floating `k'$>.
460 The optimizer-specific info is used to avoid entering (a slow) regex
461 engine on strings that will not definitely match. If C<isall> flag
462 is set, a call to the regex engine may be avoided even when the optimizer
463 found an appropriate place for the match.
465 The rest of the output contains the list of I<nodes> of the compiled
466 form of the regex. Each line has format
468 C< >I<id>: I<TYPE> I<OPTIONAL-INFO> (I<next-id>)
470 =head2 Types of nodes
472 Here are the possible types, with short descriptions:
474 # TYPE arg-description [num-args] [longjump-len] DESCRIPTION
477 END no End of program.
478 SUCCEED no Return from a subroutine, basically.
481 BOL no Match "" at beginning of line.
482 MBOL no Same, assuming multiline.
483 SBOL no Same, assuming singleline.
484 EOS no Match "" at end of string.
485 EOL no Match "" at end of line.
486 MEOL no Same, assuming multiline.
487 SEOL no Same, assuming singleline.
488 BOUND no Match "" at any word boundary
489 BOUNDL no Match "" at any word boundary
490 NBOUND no Match "" at any word non-boundary
491 NBOUNDL no Match "" at any word non-boundary
492 GPOS no Matches where last m//g left off.
494 # [Special] alternatives
495 ANY no Match any one character (except newline).
496 SANY no Match any one character.
497 ANYOF sv Match character in (or not in) this class.
498 ALNUM no Match any alphanumeric character
499 ALNUML no Match any alphanumeric char in locale
500 NALNUM no Match any non-alphanumeric character
501 NALNUML no Match any non-alphanumeric char in locale
502 SPACE no Match any whitespace character
503 SPACEL no Match any whitespace char in locale
504 NSPACE no Match any non-whitespace character
505 NSPACEL no Match any non-whitespace char in locale
506 DIGIT no Match any numeric character
507 NDIGIT no Match any non-numeric character
509 # BRANCH The set of branches constituting a single choice are hooked
510 # together with their "next" pointers, since precedence prevents
511 # anything being concatenated to any individual branch. The
512 # "next" pointer of the last BRANCH in a choice points to the
513 # thing following the whole choice. This is also where the
514 # final "next" pointer of each individual branch points; each
515 # branch starts with the operand node of a BRANCH node.
517 BRANCH node Match this alternative, or the next...
519 # BACK Normal "next" pointers all implicitly point forward; BACK
520 # exists to make loop structures possible.
522 BACK no Match "", "next" ptr points backward.
525 EXACT sv Match this string (preceded by length).
526 EXACTF sv Match this string, folded (prec. by length).
527 EXACTFL sv Match this string, folded in locale (w/len).
530 NOTHING no Match empty string.
531 # A variant of above which delimits a group, thus stops optimizations
532 TAIL no Match empty string. Can jump here from outside.
534 # STAR,PLUS '?', and complex '*' and '+', are implemented as circular
535 # BRANCH structures using BACK. Simple cases (one character
536 # per match) are implemented with STAR and PLUS for speed
537 # and to minimize recursive plunges.
539 STAR node Match this (simple) thing 0 or more times.
540 PLUS node Match this (simple) thing 1 or more times.
542 CURLY sv 2 Match this simple thing {n,m} times.
543 CURLYN no 2 Match next-after-this simple thing
544 # {n,m} times, set parens.
545 CURLYM no 2 Match this medium-complex thing {n,m} times.
546 CURLYX sv 2 Match this complex thing {n,m} times.
548 # This terminator creates a loop structure for CURLYX
549 WHILEM no Do curly processing and see if rest matches.
551 # OPEN,CLOSE,GROUPP ...are numbered at compile time.
552 OPEN num 1 Mark this point in input as start of #n.
553 CLOSE num 1 Analogous to OPEN.
555 REF num 1 Match some already matched string
556 REFF num 1 Match already matched string, folded
557 REFFL num 1 Match already matched string, folded in loc.
559 # grouping assertions
560 IFMATCH off 1 2 Succeeds if the following matches.
561 UNLESSM off 1 2 Fails if the following matches.
562 SUSPEND off 1 1 "Independent" sub-regex.
563 IFTHEN off 1 1 Switch, should be preceded by switcher .
564 GROUPP num 1 Whether the group matched.
566 # Support for long regex
567 LONGJMP off 1 1 Jump far away.
568 BRANCHJ off 1 1 BRANCH with long offset.
571 EVAL evl 1 Execute some Perl code.
574 MINMOD no Next operator is not greedy.
575 LOGICAL no Next opcode should set the flag only.
577 # This is not used yet
578 RENUM off 1 1 Group with independently numbered parens.
580 # This is not really a node, but an optimized away piece of a "long" node.
581 # To simplify debugging output, we mark it as if it were a node
582 OPTIMIZED off Placeholder for dump.
584 =head2 Run-time output
586 First of all, when doing a match, one may get no run-time output even
587 if debugging is enabled. This means that the regex engine was never
588 entered and that all of the job was therefore done by the optimizer.
590 If the regex engine was entered, the output may look like this:
592 Matching `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'
593 Setting an EVAL scope, savestack=3
594 2 <ab> <cdefg__gh_> | 1: ANYOF
595 3 <abc> <defg__gh_> | 11: EXACT <d>
596 4 <abcd> <efg__gh_> | 13: CURLYX {1,32767}
597 4 <abcd> <efg__gh_> | 26: WHILEM
598 0 out of 1..32767 cc=effff31c
599 4 <abcd> <efg__gh_> | 15: OPEN1
600 4 <abcd> <efg__gh_> | 17: EXACT <e>
601 5 <abcde> <fg__gh_> | 19: STAR
602 EXACT <f> can match 1 times out of 32767...
603 Setting an EVAL scope, savestack=3
604 6 <bcdef> <g__gh__> | 22: EXACT <g>
605 7 <bcdefg> <__gh__> | 24: CLOSE1
606 7 <bcdefg> <__gh__> | 26: WHILEM
607 1 out of 1..32767 cc=effff31c
608 Setting an EVAL scope, savestack=12
609 7 <bcdefg> <__gh__> | 15: OPEN1
610 7 <bcdefg> <__gh__> | 17: EXACT <e>
611 restoring \1 to 4(4)..7
612 failed, try continuation...
613 7 <bcdefg> <__gh__> | 27: NOTHING
614 7 <bcdefg> <__gh__> | 28: EXACT <h>
618 The most significant information in the output is about the particular I<node>
619 of the compiled regex that is currently being tested against the target string.
620 The format of these lines is
622 C< >I<STRING-OFFSET> <I<PRE-STRING>> <I<POST-STRING>> |I<ID>: I<TYPE>
624 The I<TYPE> info is indented with respect to the backtracking level.
625 Other incidental information appears interspersed within.
627 =head1 Debugging Perl memory usage
629 Perl is a profligate wastrel when it comes to memory use. There
630 is a saying that to estimate memory usage of Perl, assume a reasonable
631 algorithm for memory allocation, multiply that estimate by 10, and
632 while you still may miss the mark, at least you won't be quite so
633 astonished. This is not absolutely true, but may provide a good
634 grasp of what happens.
636 Assume that an integer cannot take less than 20 bytes of memory, a
637 float cannot take less than 24 bytes, a string cannot take less
638 than 32 bytes (all these examples assume 32-bit architectures, the
639 result are quite a bit worse on 64-bit architectures). If a variable
640 is accessed in two of three different ways (which require an integer,
641 a float, or a string), the memory footprint may increase yet another
642 20 bytes. A sloppy malloc(3) implementation can make inflate these
643 numbers dramatically.
645 On the opposite end of the scale, a declaration like
649 may take up to 500 bytes of memory, depending on which release of Perl
652 Anecdotal estimates of source-to-compiled code bloat suggest an
653 eightfold increase. This means that the compiled form of reasonable
654 (normally commented, properly indented etc.) code will take
655 about eight times more space in memory than the code took
658 There are two Perl-specific ways to analyze memory usage:
659 $ENV{PERL_DEBUG_MSTATS} and B<-DL> command-line switch. The first
660 is available only if Perl is compiled with Perl's malloc(); the
661 second only if Perl was built with C<-DDEBUGGING>. See the
662 instructions for how to do this in the F<INSTALL> podpage at
663 the top level of the Perl source tree.
665 =head2 Using C<$ENV{PERL_DEBUG_MSTATS}>
667 If your perl is using Perl's malloc() and was compiled with the
668 necessary switches (this is the default), then it will print memory
669 usage statistics after compiling your code when C<< $ENV{PERL_DEBUG_MSTATS}
670 > 1 >>, and before termination of the program when C<<
671 $ENV{PERL_DEBUG_MSTATS} >= 1 >>. The report format is similar to
672 the following example:
674 $ PERL_DEBUG_MSTATS=2 perl -e "require Carp"
675 Memory allocation statistics after compilation: (buckets 4(4)..8188(8192)
676 14216 free: 130 117 28 7 9 0 2 2 1 0 0
678 60924 used: 125 137 161 55 7 8 6 16 2 0 1
680 Total sbrk(): 77824/21:119. Odd ends: pad+heads+chain+tail: 0+636+0+2048.
681 Memory allocation statistics after execution: (buckets 4(4)..8188(8192)
682 30888 free: 245 78 85 13 6 2 1 3 2 0 1
684 175816 used: 265 176 1112 111 26 22 11 27 2 1 1
686 Total sbrk(): 215040/47:145. Odd ends: pad+heads+chain+tail: 0+2192+0+6144.
688 It is possible to ask for such a statistic at arbitrary points in
689 your execution using the mstats() function out of the standard
692 Here is some explanation of that format:
696 =item C<buckets SMALLEST(APPROX)..GREATEST(APPROX)>
698 Perl's malloc() uses bucketed allocations. Every request is rounded
699 up to the closest bucket size available, and a bucket is taken from
700 the pool of buckets of that size.
702 The line above describes the limits of buckets currently in use.
703 Each bucket has two sizes: memory footprint and the maximal size
704 of user data that can fit into this bucket. Suppose in the above
705 example that the smallest bucket were size 4. The biggest bucket
706 would have usable size 8188, and the memory footprint would be 8192.
708 In a Perl built for debugging, some buckets may have negative usable
709 size. This means that these buckets cannot (and will not) be used.
710 For larger buckets, the memory footprint may be one page greater
711 than a power of 2. If so, case the corresponding power of two is
712 printed in the C<APPROX> field above.
716 The 1 or 2 rows of numbers following that correspond to the number
717 of buckets of each size between C<SMALLEST> and C<GREATEST>. In
718 the first row, the sizes (memory footprints) of buckets are powers
719 of two--or possibly one page greater. In the second row, if present,
720 the memory footprints of the buckets are between the memory footprints
721 of two buckets "above".
723 For example, suppose under the previous example, the memory footprints
726 free: 8 16 32 64 128 256 512 1024 2048 4096 8192
729 With non-C<DEBUGGING> perl, the buckets starting from C<128> have
730 a 4-byte overhead, and thus a 8192-long bucket may take up to
731 8188-byte allocations.
733 =item C<Total sbrk(): SBRKed/SBRKs:CONTINUOUS>
735 The first two fields give the total amount of memory perl sbrk(2)ed
736 (ess-broken? :-) and number of sbrk(2)s used. The third number is
737 what perl thinks about continuity of returned chunks. So long as
738 this number is positive, malloc() will assume that it is probable
739 that sbrk(2) will provide continuous memory.
741 Memory allocated by external libraries is not counted.
745 The amount of sbrk(2)ed memory needed to keep buckets aligned.
749 Although memory overhead of bigger buckets is kept inside the bucket, for
750 smaller buckets, it is kept in separate areas. This field gives the
751 total size of these areas.
755 malloc() may want to subdivide a bigger bucket into smaller buckets.
756 If only a part of the deceased bucket is left unsubdivided, the rest
757 is kept as an element of a linked list. This field gives the total
758 size of these chunks.
762 To minimize the number of sbrk(2)s, malloc() asks for more memory. This
763 field gives the size of the yet unused part, which is sbrk(2)ed, but
768 =head2 Example of using B<-DL> switch
770 Below we show how to analyse memory usage by
772 do 'lib/auto/POSIX/autosplit.ix';
774 The file in question contains a header and 146 lines similar to
778 B<WARNING>: The discussion below supposes 32-bit architecture. In
779 newer releases of Perl, memory usage of the constructs discussed
780 here is greatly improved, but the story discussed below is a real-life
781 story. This story is mercilessly terse, and assumes rather more than cursory
782 knowledge of Perl internals. Type space to continue, `q' to quit.
783 (Actually, you just want to skip to the next section.)
785 Here is the itemized list of Perl allocations performed during parsing
788 !!! "after" at test.pl line 3.
789 Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+
790 0 02 13752 . . . . 294 . . . . . . . . . . 4
791 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3
792 5 05 32 . . . . . . . 1 . . . . . . . .
793 6 02 7152 . . . . . . . . . . 149 . . . . .
794 7 02 3600 . . . . . 150 . . . . . . . . . .
795 7 03 64 . -1 . 1 . . 2 . . . . . . . . .
796 7 04 7056 . . . . . . . . . . . . . . . 7
797 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 .
798 9 03 2078 17 249 32 . . . . 2 . . . . . . . .
801 To see this list, insert two C<warn('!...')> statements around the call:
804 do 'lib/auto/POSIX/autosplit.ix';
807 and run it with Perl's B<-DL> option. The first warn() will print
808 memory allocation info before parsing the file and will memorize
809 the statistics at this point (we ignore what it prints). The second
810 warn() prints increments with respect to these memorized data. This
811 is the printout shown above.
813 Different I<Id>s on the left correspond to different subsystems of
814 the perl interpreter. They are just the first argument given to
815 the perl memory allocation API named New(). To find what C<9 03>
816 means, just B<grep> the perl source for C<903>. You'll find it in
817 F<util.c>, function savepvn(). (I know, you wonder why we told you
818 to B<grep> and then gave away the answer. That's because grepping
819 the source is good for the soul.) This function is used to store
820 a copy of an existing chunk of memory. Using a C debugger, one can
821 see that the function was called either directly from gv_init() or
822 via sv_magic(), and that gv_init() is called from gv_fetchpv()--which
823 was itself called from newSUB(). Please stop to catch your breath now.
825 B<NOTE>: To reach this point in the debugger and skip the calls to
826 savepvn() during the compilation of the main program, you should
828 in Perl_warn(), continue until this point is reached, and I<then> set
829 a C breakpoint in Perl_savepvn(). Note that you may need to skip a
830 handful of Perl_savepvn() calls that do not correspond to mass production
831 of CVs (there are more C<903> allocations than 146 similar lines of
832 F<lib/auto/POSIX/autosplit.ix>). Note also that C<Perl_> prefixes are
833 added by macroization code in perl header files to avoid conflicts
834 with external libraries.
836 Anyway, we see that C<903> ids correspond to creation of globs, twice
837 per glob - for glob name, and glob stringification magic.
839 Here are explanations for other I<Id>s above:
845 Creates bigger C<XPV*> structures. In the case above, it
846 creates 3 C<AV>s per subroutine, one for a list of lexical variable
847 names, one for a scratchpad (which contains lexical variables and
848 C<targets>), and one for the array of scratchpads needed for
851 It also creates a C<GV> and a C<CV> per subroutine, all called from
856 Creates a C array corresponding to the C<AV> of scratchpads and the
857 scratchpad itself. The first fake entry of this scratchpad is
858 created though the subroutine itself is not defined yet.
860 It also creates C arrays to keep data for the stash. This is one HV,
861 but it grows; thus, there are 4 big allocations: the big chunks are not
862 freed, but are kept as additional arenas for C<SV> allocations.
866 Creates a C<HEK> for the name of the glob for the subroutine. This
867 name is a key in a I<stash>.
869 Big allocations with this I<Id> correspond to allocations of new
870 arenas to keep C<HE>.
874 Creates a C<GP> for the glob for the subroutine.
878 Creates the C<MAGIC> for the glob for the subroutine.
882 Creates I<arenas> which keep SVs.
886 =head2 B<-DL> details
888 If Perl is run with B<-DL> option, then warn()s that start with `!'
889 behave specially. They print a list of I<categories> of memory
890 allocations, and statistics of allocations of different sizes for
893 If warn() string starts with
899 print changed categories only, print the differences in counts of allocations.
903 print grown categories only; print the absolute values of counts, and totals.
907 print nonempty categories, print the absolute values of counts and totals.
911 =head2 Limitations of B<-DL> statistics
913 If an extension or external library does not use the Perl API to
914 allocate memory, such allocations are not counted.