=head1 The Perl Debugger
+"As soon as we started programming, we found to our
+surprise that it wasn't as easy to get programs right
+as we had thought. Debugging had to be discovered.
+I can remember the exact instant when I realized that
+a large part of my life from then on was going to be
+spent in finding mistakes in my own programs."
+
+I< --Maurice Wilkes, 1949>
+
If you invoke Perl with the B<-d> switch, your script runs under the
Perl source debugger. This works like an interactive Perl
environment, prompting for debugger commands that let you examine
source code, set breakpoints, get stack backtraces, change the values of
variables, etc. This is so convenient that you often fire up
-the debugger all by itself just to test out Perl constructs
+the debugger all by itself just to test out Perl constructs
interactively to see what they do. For example:
perl -d -e 42
to insert source information into the parse trees it's about to hand off
to the interpreter. That means your code must first compile correctly
for the debugger to work on it. Then when the interpreter starts up, it
-pre-loads a Perl library file containing the debugger itself.
+preloads a Perl library file containing the debugger itself.
The program will halt I<right before> the first run-time executable
statement (but see below regarding compile-time statements) and ask you
=item h [command]
-Prints out a help message.
+Prints out a help message.
If you supply another debugger command as an argument to the C<h> command,
it prints out the description for just that command. The special
argument of C<h h> produces a more compact help listing, designed to fit
together on one screen.
-If the output the C<h> command (or any command, for that matter) scrolls
+If the output of the C<h> command (or any command, for that matter) scrolls
past your screen, either precede the command with a leading pipe symbol so
it's run through your pager, as in
=item x expr
-Evaluates its expression in list context and dumps out the result
+Evaluates its expression in list context and dumps out the result
in a pretty-printed fashion. Nested data structures are printed out
recursively, unlike the C<print> function.
4. prompt user if at a breakpoint or in single-step
5. evaluate line
-For example, this will print out C<$foo> every time line
+For example, this will print out $foo every time line
53 is passed:
a 53 print "DB FOUND $foo\n"
Delete all installed actions.
+=item W [expr]
+
+Add a global watch-expression.
+
+=item W
+
+Delete all watch-expressions.
+
=item O [opt[=val]] [opt"val"] [opt?]...
Set or query values of options. val defaults to 1. opt can
Level of verbosity. By default the debugger is in a sane verbose mode,
thus it will print backtraces on all the warnings and die-messages
which are going to be printed out, and will print a message when
-interesting uncaught signals arrive.
+interesting uncaught signals arrive.
To disable this behaviour, set these values to 0. If C<dieLevel> is 2,
then the messages which will be caught by surrounding C<eval> are also
If 0, allows I<stepping off> the end of the script.
-=item C<PrintRet>
+=item C<PrintRet>
affects printing of return value after C<r> command.
-=item C<frame>
+=item C<ornaments>
+
+affects screen appearance of the command line (see L<Term::ReadLine>).
+
+=item C<frame>
affects printing messages on entry and exit from subroutines. If
C<frame & 2> is false, messages are printed on entry only. (Printing
If C<frame & 4>, arguments to functions are printed as well as the
context and caller info. If C<frame & 8>, overloaded C<stringify> and
-C<tie>d C<FETCH> are enabled on the printed arguments. The length at
-which the argument list is truncated is governed by the next option:
+C<tie>d C<FETCH> are enabled on the printed arguments. If C<frame &
+16>, the return value from the subroutine is printed as well.
+
+The length at which the argument list is truncated is governed by the
+next option:
=item C<maxTraceLen>
Dump symbol tables of packages.
+=item C<DumpReused>
+
+Dump contents of "reused" addresses.
+
=item C<quote>, C<HighBit>, C<undefPrint>
Change style of string dump. Default value of C<quote> is C<auto>, one
to C<"> or C<'>. By default, characters with high bit set are printed
I<as is>.
-=item C<UsageOnly>
+=item C<UsageOnly>
I<very> rudimentally per-package memory usage dump. Calculates total
size of strings in variables in the package.
=item C<NonStop>
-If set, debugger goes into non-interactive mode until interrupted, or
+If set, debugger goes into noninteractive mode until interrupted, or
programmatically by setting $DB::signal or $DB::single.
=back
$ PERLDB_OPTS="N f A L=listing" perl -d myprogram
-- runs script non-interactively, printing info on each entry into a
+- runs script noninteractively, printing info on each entry into a
subroutine and each executed line into the file F<listing>. (If you
interrupt it, you would better reset C<LineInfo> to something
"interactive"!)
to exit the debugger, though typing C<exit> twice may do it too.
Set an C<O>ption C<inhibit_exit> to 0 if you want to be able to I<step
-off> the end the script. You may also need to set C<$finished> to 0 at
+off> the end the script. You may also need to set $finished to 0 at
some moment if you want to step through global destruction.
=item R
may be lost.
Currently the following setting are preserved: history, breakpoints,
-actions, debugger C<O>ptions, and the following command-line
+actions, debugger C<O>ptions, and the following command line
options: B<-w>, B<-I>, and B<-e>.
=item |dbcmd
DB<<17>>
where that number is the command number, which you'd use to access with
-the built-in B<csh>-like history mechanism, e.g., C<!17> would repeat
+the builtin B<csh>-like history mechanism, e.g., C<!17> would repeat
command number 17. The number of angle brackets indicates the depth of
the debugger. You could get more than one set of brackets, for example, if
you'd already at a breakpoint and then printed out the result of a
function call that itself also has a breakpoint, or you step into an
expression via C<s/n/t expression> command.
-=item Multi-line commands
+=item Multiline commands
If you want to enter a multi-line command, such as a subroutine
definition with several statements, or a format, you may escape the
but from line 4.
Note that if you execute C<T> command from inside an active C<use>
-statement, the backtrace will contain both C<L<perlfunc/require>>
-frame and an C<L<perlfunc/eval EXPR>>) frame.
+statement, the backtrace will contain both C<require>
+frame and an C<eval>) frame.
=item Listing
When C<frame> option is set, debugger would print entered (and
optionally exited) subroutines in different styles.
-What follows is the start of the listing of
+What follows is the start of the listing of
+
+ env "PERLDB_OPTS=f=n N" perl -d -V
- env "PERLDB_OPTS=f=1 N" perl -d -V
+for different values of C<n>:
=over 4
in @=Config::myconfig() from /dev/nul: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), 'PATCHLEVEL') from lib/Config.pm:574
- in $=Config::FETCH(ref(Config), 'SUBVERSION') 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
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), 'PATCHLEVEL') from lib/Config.pm:574
- out $=Config::FETCH(ref(Config), 'PATCHLEVEL') from lib/Config.pm:574
- in $=Config::FETCH(ref(Config), 'SUBVERSION') 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
=item 14
in $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
out $=Config::FETCH('Config=HASH(0x1aa444)', 'baserev') from lib/Config.pm:574
+=item 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: ''
+
+
=back
In all the cases indentation of lines shows the call tree, if bit 2 of
C<frame> is set, then a line is printed on exit from a subroutine as
well, if bit 4 is set, then the arguments are printed as well as the
caller info, if bit 8 is set, the arguments are printed even if they
-are tied or references.
+are tied or references, if bit 16 is set, the return value is printed
+as well.
When a package is compiled, a line like this
If you have any compile-time executable statements (code within a BEGIN
block or a C<use> statement), these will C<NOT> be stopped by debugger,
although C<require>s will (and compile-time statements can be traced
-with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
+with C<AutoTrace> option set in C<PERLDB_OPTS>). From your own Perl
code, however, you can
transfer control back to the debugger using the following statement,
which is harmless if the debugger is not running:
=head2 Debugger Customization
-Most probably you not want to modify the debugger, it contains enough
+Most probably you do not want to modify the debugger, it contains enough
hooks to satisfy most needs. You may change the behaviour of debugger
from the debugger itself, using C<O>ptions, from the command line via
C<PERLDB_OPTS> environment variable, and from I<customization files>.
have full editing capabilities much like GNU I<readline>(3) provides.
Look for these in the F<modules/by-module/Term> directory on CPAN.
-A rudimentary command-line completion is also available.
+A rudimentary command line completion is also available.
Unfortunately, the names of lexical variables are not available for
completion.
When you call the B<caller> function (see L<perlfunc/caller>) from the
package DB, Perl sets the array @DB::args to contain the arguments the
-corresponding stack frame was called with.
+corresponding stack frame was called with.
If perl is run with B<-d> option, the following additional features
-are enabled:
+are enabled (cf. L<perlvar/$^P>):
=over
=item *
-The array C<@{"_<$filename"}> is the line-by-line contents of
+The array C<@{"_E<lt>$filename"}> is the line-by-line contents of
$filename for all the compiled files. Same for C<eval>ed strings which
-contain subroutines, or which are currently executed. The C<$filename>
+contain subroutines, or which are currently executed. The $filename
for C<eval>ed strings looks like C<(eval 34)>.
=item *
-The hash C<%{"_<$filename"}> contains breakpoints and action (it is
+The hash C<%{"_E<lt>$filename"}> contains breakpoints and action (it is
keyed by line number), and individual entries are settable (as opposed
to the whole hash). Only true/false is important to Perl, though the
values used by F<perl5db.pl> have the form
they are zeros if the line is not breakable.
Same for evaluated strings which contain subroutines, or which are
-currently executed. The C<$filename> for C<eval>ed strings looks like
+currently executed. The $filename for C<eval>ed strings looks like
C<(eval 34)>.
=item *
-The scalar C<${"_<$filename"}> contains C<"_<$filename">. Same for
+The scalar C<${"_E<lt>$filename"}> contains C<"_E<lt>$filename">. Same for
evaluated strings which contain subroutines, or which are currently
-executed. The C<$filename> for C<eval>ed strings looks like C<(eval
+executed. The $filename for C<eval>ed strings looks like C<(eval
34)>.
=item *
After each C<require>d file is compiled, but before it is executed,
-C<DB::postponed(*{"_<$filename"})> is called (if subroutine
+C<DB::postponed(*{"_E<lt>$filename"})> is called (if subroutine
C<DB::postponed> exists). Here the $filename is the expanded name of
-the C<require>d file (as found in values of C<%INC>).
+the C<require>d file (as found in values of %INC).
=item *
=back
-Note that no subroutine call is possible until C<&DB::sub> is defined
-(for subroutines outside of package C<DB>). (This restriction is
-recently lifted.)
+Note that if C<&DB::sub> needs some external data to be setup for it
+to work, no subroutine call is possible until this is done. For the
+standard debugger C<$DB::deep> (how many levels of recursion deep into
+the debugger you can go before a mandatory break) gives an example of
+such a dependency.
-(In fact, for the standard debugger the same is true if C<$DB::deep>
-(how many levels of recursion deep into the debugger you can go before
-a mandatory break) is not defined.)
-
-With the recent updates the minimal possible debugger consists of one
-line
+The minimal working debugger consists of one line
sub DB::DB {}
The following debugger is quite functional:
- {
- package DB;
- sub DB {}
+ {
+ package DB;
+ sub DB {}
sub sub {print ++$i, " $sub\n"; &$sub}
}
=head2 Debugger Internals
At the start, the debugger reads your rc file (F<./.perldb> or
-F<~/.perldb> under UNIX), which can set important options. This file may
+F<~/.perldb> under Unix), which can set important options. This file may
define a subroutine C<&afterinit> to be executed after the debugger is
initialized.
Some functions are provided to simplify customization. See L<"Debugger
Customization"> for description of C<DB::parse_options(string)>. The
function C<DB::dump_trace(skip[, count])> skips the specified number
-of frames, and returns an array containing info about the caller
+of frames, and returns a list containing info about the caller
frames (all if C<count> is missing). Each entry is a hash with keys
C<context> (C<$> or C<@>), C<sub> (subroutine name, or info about
eval), C<args> (C<undef> or a reference to an array), C<file>, and
C<line>.
-The function C<DB::print_trace(FH, skip[, count[, short]])> prints
+The function C<DB::print_trace(FH, skip[, count[, short]])> prints
formatted info about caller frames. The last two functions may be
convenient as arguments to C<E<lt>>, C<E<lt>E<lt>> commands.
You did try the B<-w> switch, didn't you?
-=head1 BUGS
+=head2 BUGS
You cannot get the stack frame information or otherwise debug functions
that were not compiled by Perl, such as C or C++ extensions.
If you alter your @_ arguments in a subroutine (such as with B<shift>
or B<pop>, the stack backtrace will not show the original values.
+
+=head1 Debugging Perl memory usage
+
+Perl is I<very> frivolous with memory. There is a saying that to
+estimate memory usage of Perl, assume a reasonable algorithm of
+allocation, and multiply your estimates by 10. This is not absolutely
+true, but may give you a good grasp of what happens.
+
+Say, an integer cannot take less than 20 bytes of memory, a float
+cannot take less than 24 bytes, a string cannot take less than 32
+bytes (all these examples assume 32-bit architectures, the result are
+much worse on 64-bit architectures). If a variable is accessed in two
+of three different ways (which require an integer, a float, or a
+string), the memory footprint may increase by another 20 bytes. A
+sloppy malloc() implementation will make these numbers yet more.
+
+On the opposite end of the scale, a declaration like
+
+ sub foo;
+
+may take (on some versions of perl) up to 500 bytes of memory.
+
+Off-the-cuff anecdotal estimates of a code bloat give a factor around
+8. This means that the compiled form of reasonable (commented
+indented etc.) code will take approximately 8 times more than the
+disk space the code takes.
+
+There are two Perl-specific ways to analyze the memory usage:
+$ENV{PERL_DEBUG_MSTATS} and B<-DL> switch. First one is available
+only if perl is compiled with Perl's malloc(), the second one only if
+Perl compiled with C<-DDEBUGGING> (as with giving C<-D optimise=-g>
+option to F<Configure>).
+
+=head2 Using C<$ENV{PERL_DEBUG_MSTATS}>
+
+If your perl is using Perl's malloc(), and compiled with correct
+switches (this is the default), then it will print memory usage
+statistics after compiling your code (if C<$ENV{PERL_DEBUG_MSTATS}> >
+1), and before termination of the script (if
+C<$ENV{PERL_DEBUG_MSTATS}> >= 1). The report format is similar to one
+in the following example:
+
+ env 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 moment by
+using Devel::Peek::mstats() (module Devel::Peek is available on CPAN).
+
+Here is the explanation of different parts of the format:
+
+=over
+
+=item C<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 of these size is
+taken from the pool of the buckets of this size.
+
+The above line describes limits of buckets currently in use. Each
+bucket has two sizes: memory footprint, and the maximal size of user
+data which may be put into this bucket. Say, in the above example the
+smallest bucket is both sizes 4. The biggest bucket has usable size
+8188, and the memory footprint 8192.
+
+With debugging Perl some buckets may have negative usable size. This
+means that these buckets cannot (and will not) be used. For greater
+buckets the memory footprint may be one page greater than a power of
+2. In such a case the corresponding power of two is printed instead
+in the C<APPROX> field above.
+
+=item Free/Used
+
+The following 1 or 2 rows of numbers correspond to the number of
+buckets of each size between C<SMALLEST> and C<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 memory footprints of two
+buckets "above".
+
+Say, with the above example the memory footprints are (with current
+algorithm)
+
+ free: 8 16 32 64 128 256 512 1024 2048 4096 8192
+ 4 12 24 48 80
+
+With non-C<DEBUGGING> perl the buckets starting from C<128>-long ones
+have 4-byte overhead, thus 8192-long bucket may take up to
+8188-byte-long allocations.
+
+=item C<Total sbrk(): SBRKed/SBRKs:CONTINUOUS>
+
+The first two fields give the total amount of memory perl sbrk()ed,
+and number of sbrk()s used. The third number is what perl thinks
+about continuity of returned chunks. As far as this number is
+positive, malloc() will assume that it is probable that sbrk() will
+provide continuous memory.
+
+The amounts sbrk()ed by external libraries is not counted.
+
+=item C<pad: 0>
+
+The amount of sbrk()ed memory needed to keep buckets aligned.
+
+=item C<heads: 2192>
+
+While 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.
+
+=item C<chain: 0>
+
+malloc() may want to subdivide a bigger bucket into smaller buckets.
+If only a part of the deceased-bucket is left non-subdivided, the rest
+is kept as an element of a linked list. This field gives the total
+size of these chunks.
+
+=item C<tail: 6144>
+
+To minimize amount of sbrk()s malloc() asks for more memory. This
+field gives the size of the yet-unused part, which is sbrk()ed, but
+never touched.
+
+=back
+
+=head2 Example of using B<-DL> switch
+
+Below we show how to analyse memory usage by
+
+ do 'lib/auto/POSIX/autosplit.ix';
+
+The file in question contains a header and 146 lines similar to
+
+ sub getcwd ;
+
+B<Note:> I<the discussion below supposes 32-bit architecture. In the
+newer versions of perl the memory usage of the constructs discussed
+here is much improved, but the story discussed below is a real-life
+story. This story is very terse, and assumes more than cursory
+knowledge of Perl internals.>
+
+Here is the itemized list of Perl allocations performed during parsing
+of this file:
+
+ !!! "after" at test.pl line 3.
+ Id subtot 4 8 12 16 20 24 28 32 36 40 48 56 64 72 80 80+
+ 0 02 13752 . . . . 294 . . . . . . . . . . 4
+ 0 54 5545 . . 8 124 16 . . . 1 1 . . . . . 3
+ 5 05 32 . . . . . . . 1 . . . . . . . .
+ 6 02 7152 . . . . . . . . . . 149 . . . . .
+ 7 02 3600 . . . . . 150 . . . . . . . . . .
+ 7 03 64 . -1 . 1 . . 2 . . . . . . . . .
+ 7 04 7056 . . . . . . . . . . . . . . . 7
+ 7 17 38404 . . . . . . . 1 . . 442 149 . . 147 .
+ 9 03 2078 17 249 32 . . . . 2 . . . . . . . .
+
+
+To see this list insert two C<warn('!...')> statements around the call:
+
+ warn('!');
+ do 'lib/auto/POSIX/autosplit.ix';
+ warn('!!! "after"');
+
+and run it with B<-DL> option. The first warn() will print memory
+allocation info before the parsing of the file, and will memorize the
+statistics at this point (we ignore what it prints). The second warn()
+will print increments w.r.t. this memorized statistics. This is the
+above printout.
+
+Different I<Id>s on the left correspond to different subsystems of
+perl interpreter, they are just first argument given to perl memory
+allocation API New(). To find what C<9 03> means C<grep> the perl
+source for C<903>. You will see that it is F<util.c>, function
+savepvn(). This function is used to store a copy of existing chunk of
+memory. Using C debugger, one can see that it is called either
+directly from gv_init(), or via sv_magic(), and gv_init() is called
+from gv_fetchpv() - which is called from newSUB().
+
+B<Note:> to reach this place in debugger and skip all the calls to
+savepvn during the compilation of the main script, set a C breakpoint
+in Perl_warn(), C<continue> this point is reached, I<then> set
+breakpoint in Perl_savepvn(). Note that you may need to skip a
+handful of Perl_savepvn() which do not correspond to mass production
+of CVs (there are more C<903> allocations than 146 similar lines of
+F<lib/auto/POSIX/autosplit.ix>). Note also that C<Perl_> prefixes are
+added by macroization code in perl header files to avoid conflicts
+with external libraries.
+
+Anyway, we see that C<903> ids correspond to creation of globs, twice
+per glob - for glob name, and glob stringification magic.
+
+Here are explanations for other I<Id>s above:
+
+=over
+
+=item C<717>
+
+is for creation of bigger C<XPV*> structures. In the above case it
+creates 3 C<AV> per subroutine, one for a list of lexical variable
+names, one for a scratchpad (which contains lexical variables and
+C<targets>), and one for the array of scratchpads needed for
+recursion.
+
+It also creates a C<GV> and a C<CV> per subroutine (all called from
+start_subparse()).
+
+=item C<002>
+
+Creates C array corresponding to the C<AV> of scratchpads, and the
+scratchpad itself (the first fake entry of this scratchpad is created
+though the subroutine itself is not defined yet).
+
+It also creates C arrays to keep data for the stash (this is one HV,
+but it grows, thus there are 4 big allocations: the big chunks are not
+freed, but are kept as additional arenas for C<SV> allocations).
+
+=item C<054>
+
+creates a C<HEK> for the name of the glob for the subroutine (this
+name is a key in a I<stash>).
+
+Big allocations with this I<Id> correspond to allocations of new
+arenas to keep C<HE>.
+
+=item C<602>
+
+creates a C<GP> for the glob for the subroutine.
+
+=item C<702>
+
+creates the C<MAGIC> for the glob for the subroutine.
+
+=item C<704>
+
+creates I<arenas> which keep SVs.
+
+=back
+
+=head2 B<-DL> details
+
+If Perl is run with B<-DL> option, then warn()s which start with `!'
+behave specially. They print a list of I<categories> of memory
+allocations, and statistics of allocations of different sizes for
+these categories.
+
+If warn() string starts with
+
+=over
+
+=item C<!!!>
+
+print changed categories only, print the differences in counts of allocations;
+
+=item C<!!>
+
+print grown categories only; print the absolute values of counts, and totals;
+
+=item C<!>
+
+print nonempty categories, print the absolute values of counts and totals.
+
+=back
+
+=head2 Limitations of B<-DL> statistic
+
+If an extension or an external library does not use Perl API to
+allocate memory, these allocations are not counted.
+
+=head1 Debugging regular expressions
+
+There are two ways to enable debugging output for regular expressions.
+
+If your perl is compiled with C<-DDEBUGGING>, you may use the
+B<-Dr> flag on the command line.
+
+Otherwise, one can C<use re 'debug'>, which has effects both at
+compile time, and at run time (and is I<not> lexically scoped).
+
+=head2 Compile-time output
+
+The debugging output for the compile time looks like this:
+
+ compiling RE `[bc]d(ef*g)+h[ij]k$'
+ size 43 first at 1
+ 1: ANYOF(11)
+ 11: EXACT <d>(13)
+ 13: CURLYX {1,32767}(27)
+ 15: OPEN1(17)
+ 17: EXACT <e>(19)
+ 19: STAR(22)
+ 20: EXACT <f>(0)
+ 22: EXACT <g>(24)
+ 24: CLOSE1(26)
+ 26: WHILEM(0)
+ 27: NOTHING(28)
+ 28: EXACT <h>(30)
+ 30: ANYOF(40)
+ 40: EXACT <k>(42)
+ 42: EOL(43)
+ 43: END(0)
+ anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
+ stclass `ANYOF' minlen 7
+
+The first line shows the pre-compiled form of the regexp, and the
+second shows the size of the compiled form (in arbitrary units,
+usually 4-byte words) and the label I<id> of the first node which
+does a match.
+
+The last line (split into two lines in the above) contains the optimizer
+info. In the example shown, the optimizer found that the match
+should contain a substring C<de> at the offset 1, and substring C<gh>
+at some offset between 3 and infinity. Moreover, when checking for
+these substrings (to abandon impossible matches quickly) it will check
+for the substring C<gh> before checking for the substring C<de>. The
+optimizer may also use the knowledge that the match starts (at the
+C<first> I<id>) with a character class, and the match cannot be
+shorter than 7 chars.
+
+The fields of interest which may appear in the last line are
+
+=over
+
+=item C<anchored> I<STRING> C<at> I<POS>
+
+=item C<floating> I<STRING> C<at> I<POS1..POS2>
+
+see above;
+
+=item C<matching floating/anchored>
+
+which substring to check first;
+
+=item C<minlen>
+
+the minimal length of the match;
+
+=item C<stclass> I<TYPE>
+
+The type of the first matching node.
+
+=item C<noscan>
+
+which advises to not scan for the found substrings;
+
+=item C<isall>
+
+which says that the optimizer info is in fact all that the regular
+expression contains (thus one does not need to enter the RE engine at
+all);
+
+=item C<GPOS>
+
+if the pattern contains C<\G>;
+
+=item C<plus>
+
+if the pattern starts with a repeated char (as in C<x+y>);
+
+=item C<implicit>
+
+if the pattern starts with C<.*>;
+
+=item C<with eval>
+
+if the pattern contain eval-groups (see L<perlre/(?{ code })>);
+
+=item C<anchored(TYPE)>
+
+if the pattern may
+match only at a handful of places (with C<TYPE> being
+C<BOL>, C<MBOL>, or C<GPOS>, see the table below).
+
+=back
+
+If a substring is known to match at end-of-line only, it may be
+followed by C<$>, as in C<floating `k'$>.
+
+The optimizer-specific info is used to avoid entering (a slow) RE
+engine on strings which will definitely not match. If C<isall> flag
+is set, a call to the RE engine may be avoided even when optimizer
+found an appropriate place for the match.
+
+The rest of the output contains the list of I<nodes> of the compiled
+form of the RE. Each line has format
+
+C< >I<id>: I<TYPE> I<OPTIONAL-INFO> (I<next-id>)
+
+=head2 Types of nodes
+
+Here is the list of 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 parenths.
+ 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-RE.
+ IFTHEN off 1 1 Switch, should be preceeded by switcher .
+ GROUPP num 1 Whether the group matched.
+
+ # Support for long RE
+ LONGJMP off 1 1 Jump far away.
+ BRANCHJ off 1 1 BRANCH with long offset.
+
+ # The heavy worker
+ EVAL evl 1 Execute some Perl code.
+
+ # Modifiers
+ MINMOD no Next operator is not greedy.
+ LOGICAL no Next opcode should set the flag only.
+
+ # This is not used yet
+ RENUM off 1 1 Group with independently numbered parens.
+
+ # 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.
+
+=head2 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 RE engine was never
+entered, all of the job was done by the optimizer.
+
+If RE 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 I<node>
+of the compiled RE which is currently being tested against the target string.
+The format of these lines is
+
+C< >I<STRING-OFFSET> <I<PRE-STRING>> <I<POST-STRING>> |I<ID>: I<TYPE>
+
+The I<TYPE> info is indented with respect to the backtracking level.
+Other incidental information appears interspersed within.
+
+=cut
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