PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)NAMEperldebug - Perl debugging
DESCRIPTION
First of all, have you tried using the -w switch?
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."
--Maurice Wilkes, 1949
If you invoke Perl with the -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 interactively to see what they do. For
example:
perl -d -e 42
In Perl, the debugger is not a separate program as it
usually is in the typical compiled environment. Instead,
the -d flag tells the compiler 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 preloads a Perl library file
containing the debugger itself.
The program will halt right before the first run-time
executable statement (but see below regarding compile-time
statements) and ask you to enter a debugger command.
Contrary to popular expectations, whenever the debugger
halts and shows you a line of code, it always displays the
line it's about to execute, rather than the one it has
just executed.
Any command not recognized by the debugger is directly
executed (eval'd) as Perl code in the current package.
(The debugger uses the DB package for its own state
information.)
Leading white space before a command would cause the
debugger to think it's NOT a debugger command but for
Perl, so be careful not to do that.
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Debugger Commands
The debugger understands the following commands:
h [command] Prints out a help message.
If you supply another debugger command as an
argument to the h command, it prints out the
description for just that command. The
special argument of h h produces a more
compact help listing, designed to fit together
on one screen.
If the output of the 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
DB> |h
You may change the pager which is used via O
pager=... command.
p expr Same as print {$DB::OUT} expr in the current
package. In particular, because this is just
Perl's own print function, this means that
nested data structures and objects are not
dumped, unlike with the x command.
The DB::OUT filehandle is opened to /dev/tty,
regardless of where STDOUT may be redirected
to.
x expr 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 print function.
The details of printout are governed by
multiple Options.
V [pkg [vars]]
Display all (or some) variables in package
(defaulting to the main package) using a data
pretty-printer (hashes show their keys and
values so you see what's what, control
characters are made printable, etc.). Make
sure you don't put the type specifier (like $)
there, just the symbol names, like this:
V DB filename line
Use ~pattern and !pattern for positive and
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negative regexps.
Nested data structures are printed out in a
legible fashion, unlike the print function.
The details of printout are governed by
multiple Options.
X [vars] Same as V currentpackage [vars].
T Produce a stack backtrace. See below for
details on its output.
s [expr] Single step. Executes until it reaches the
beginning of another statement, descending
into subroutine calls. If an expression is
supplied that includes function calls, it too
will be single-stepped.
n [expr] Next. Executes over subroutine calls, until
it reaches the beginning of the next
statement. If an expression is supplied that
includes function calls, those functions will
be executed with stops before each statement.
<CR> Repeat last n or s command.
c [line|sub]
Continue, optionally inserting a one-time-only
breakpoint at the specified line or
subroutine.
l List next window of lines.
l min+incr List incr+1 lines starting at min.
l min-max List lines min through max. l - is synonymous
to -.
l line List a single line.
l subname List first window of lines from subroutine.
- List previous window of lines.
w [line] List window (a few lines) around the current
line.
. Return debugger pointer to the last-executed
line and print it out.
f filename Switch to viewing a different file or eval
statement. If filename is not a full filename
as found in values of %INC, it is considered
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as a regexp.
/pattern/ Search forwards for pattern; final / is
optional.
?pattern? Search backwards for pattern; final ? is
optional.
L List all breakpoints and actions.
S [[!]pattern]
List subroutine names [not] matching pattern.
t Toggle trace mode (see also AutoTrace Option).
t expr Trace through execution of expr. For example:
$ 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
or, with the Option frame=2 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
b [line] [condition]
Set a breakpoint. If line is omitted, sets a
breakpoint on the line that is about to be
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executed. If a condition is specified, it's
evaluated each time the statement is reached
and a breakpoint is taken only if the
condition is true. Breakpoints may be set on
only lines that begin an executable statement.
Conditions don't use if:
b 237 $x > 30
b 237 ++$count237 < 11
b 33 /pattern/i
b subname [condition]
Set a breakpoint at the first line of the
named subroutine.
b postpone subname [condition]
Set breakpoint at first line of subroutine
after it is compiled.
b load filename
Set breakpoint at the first executed line of
the file. Filename should be a full name as
found in values of %INC.
b compile subname
Sets breakpoint at the first statement
executed after the subroutine is compiled.
d [line] Delete a breakpoint at the specified line. If
line is omitted, deletes the breakpoint on the
line that is about to be executed.
D Delete all installed breakpoints.
a [line] command
Set an action to be done before the line is
executed. The sequence of steps taken by the
debugger is
1. check for a breakpoint at this line
2. print the line if necessary (tracing)
3. do any actions associated with that line
4. prompt user if at a breakpoint or in single-step
5. evaluate line
For example, this will print out $foo every
time line 53 is passed:
a 53 print "DB FOUND $foo\n"
A Delete all installed actions.
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W [expr] Add a global watch-expression.
W Delete all watch-expressions.
O [opt[=val]] [op'val' [opt?]...
Set or query values of options. val defaults
to 1. opt can be abbreviated. Several
options can be listed.
recallCommand, ShellBang
The characters used to recall
command or spawn shell. By
default, these are both set to !.
pager Program to use for output of
pager-piped commands (those
beginning with a | character.) By
default, $ENV{PAGER} will be used.
tkRunning Run Tk while prompting (with
ReadLine).
signalLevel, warnLevel, dieLevel
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.
To disable this behaviour, set
these values to 0. If dieLevel is
2, then the messages which will be
caught by surrounding eval are
also printed.
AutoTrace Trace mode (similar to t command,
but can be put into PERLDB_OPTS).
LineInfo File or pipe to print line number
info to. If it is a pipe (say,
|visual_perl_db), then a short,
"emacs like" message is used.
inhibit_exit If 0, allows stepping off the end
of the script.
PrintRet affects printing of return value
after r command.
ornaments affects screen appearance of the
command line (see the
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Term::ReadLine manpage).
frame affects printing messages on entry
and exit from subroutines. If
frame & 2 is false, messages are
printed on entry only. (Printing
on exit may be useful if
inter(di)spersed with other
messages.)
If frame & 4, arguments to
functions are printed as well as
the context and caller info. If
frame & 8, overloaded stringify
and tied FETCH are enabled on the
printed arguments. If 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:
maxTraceLen length at which the argument list
is truncated when frame option's
bit 4 is set.
The following options affect what
happens with V, X, and x commands:
arrayDepth, hashDepth Print only first N elements (''
for all).
compactDump, veryCompact
Change style of array and hash
dump. If compactDump, short array
may be printed on one line.
globPrint Whether to print contents of
globs.
DumpDBFiles Dump arrays holding debugged
files.
DumpPackages Dump symbol tables of packages.
DumpReused Dump contents of "reused"
addresses.
quote, HighBit, undefPrint
Change style of string dump.
Default value of quote is auto,
one can enable either double-
quotish dump, or single-quotish by
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setting it to " or '. By default,
characters with high bit set are
printed as is.
UsageOnly very rudimentally per-package
memory usage dump. Calculates
total size of strings in variables
in the package.
During startup options are
initialized from
$ENV{PERLDB_OPTS}. You can put
additional initialization options
TTY, noTTY, ReadLine, and NonStop
there.
Example rc file:
&parse_options("NonStop=1 LineInfo=db.out AutoTrace");
The script will run without human
intervention, putting trace
information into the file db.out.
(If you interrupt it, you would
better reset LineInfo to something
"interactive"!)
TTY The TTY to use for debugging I/O.
noTTY If set, goes in NonStop mode, and
would not connect to a TTY. If
interrupt (or if control goes to
debugger via explicit setting of
$DB::signal or $DB::single from
the Perl script), connects to a
TTY specified by the TTY option at
startup, or to a TTY found at
runtime using Term::Rendezvous
module of your choice.
This module should implement a
method new which returns an object
with two methods: IN and OUT,
returning two filehandles to use
for debugging input and output
correspondingly. Method new may
inspect an argument which is a
value of $ENV{PERLDB_NOTTY} at
startup, or is "/tmp/perldbtty$$"
otherwise.
ReadLine If false, readline support in
debugger is disabled, so you can
debug ReadLine applications.
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NonStop If set, debugger goes into
noninteractive mode until
interrupted, or programmatically
by setting $DB::signal or
$DB::single.
Here's an example of using the
$ENV{PERLDB_OPTS} variable:
$ PERLDB_OPTS="N f=2" perl -d myprogram
will run the script myprogram
without human intervention,
printing out the call tree with
entry and exit points. Note that
N f=2 is equivalent to NonStop=1
frame=2. Note also that at the
moment when this documentation was
written all the options to the
debugger could be uniquely
abbreviated by the first letter
(with exception of Dump* options).
Other examples may include
$ PERLDB_OPTS="N f A L=listing" perl -d myprogram
- runs script noninteractively,
printing info on each entry into a
subroutine and each executed line
into the file listing. (If you
interrupt it, you would better
reset LineInfo to something
"interactive"!)
$ env "PERLDB_OPTS=R=0 TTY=/dev/ttyc" perl -d myprogram
may be useful for debugging a
program which uses Term::ReadLine
itself. Do not forget detach
shell from the TTY in the window
which corresponds to /dev/ttyc,
say, by issuing a command like
$ sleep 1000000
See the section on Debugger
Internals below for more details.
< [ command ]
Set an action (Perl command) to happen before
every debugger prompt. A multi-line command
may be entered by backslashing the newlines.
If command is missing, resets the list of
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actions.
<< command Add an action (Perl command) to happen before
every debugger prompt. A multi-line command
may be entered by backslashing the newlines.
> command Set an action (Perl command) to happen after
the prompt when you've just given a command to
return to executing the script. A multi-line
command may be entered by backslashing the
newlines. If command is missing, resets the
list of actions.
>> command Adds an action (Perl command) to happen after
the prompt when you've just given a command to
return to executing the script. A multi-line
command may be entered by backslashing the
newlines.
{ [ command ]
Set an action (debugger command) to happen
before every debugger prompt. A multi-line
command may be entered by backslashing the
newlines. If command is missing, resets the
list of actions.
{{ command Add an action (debugger command) to happen
before every debugger prompt. A multi-line
command may be entered by backslashing the
newlines.
! number Redo a previous command (default previous
command).
! -number Redo number'th-to-last command.
! pattern Redo last command that started with pattern.
See O recallCommand, too.
!! cmd Run cmd in a subprocess (reads from DB::IN,
writes to DB::OUT) See O shellBang too.
H -number Display last n commands. Only commands longer
than one character are listed. If number is
omitted, lists them all.
q or ^D Quit. ("quit" doesn't work for this.) This
is the only supported way to exit the
debugger, though typing exit twice may do it
too.
Set an Option inhibit_exit to 0 if you want to
be able to step off the end the script. You
may also need to set $finished to 0 at some
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moment if you want to step through global
destruction.
R Restart the debugger by execing a new session.
It tries to maintain your history across this,
but internal settings and command line options
may be lost.
Currently the following setting are preserved:
history, breakpoints, actions, debugger
Options, and the following command line
options: -w, -I, and -e.
|dbcmd Run debugger command, piping DB::OUT to
current pager.
||dbcmd Same as |dbcmd but DB::OUT is temporarily
selected as well. Often used with commands
that would otherwise produce long output, such
as
|V main
= [alias value]
Define a command alias, like
= quit q
or list current aliases.
command Execute command as a Perl statement. A
missing semicolon will be supplied.
m expr The expression is evaluated, and the methods
which may be applied to the result are listed.
m package The methods which may be applied to objects in
the package are listed.
Debugger input/output
Prompt The debugger prompt is something like
DB<8>
or even
DB<<17>>
where that number is the command number, which
you'd use to access with the builtin csh-like
history mechanism, e.g., !17 would repeat command
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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
s/n/t expression command.
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 newline that would
normally end the debugger command with a
backslash. Here's an example:
DB<1> for (1..4) { \
cont: print "ok\n"; \
cont: }
ok
ok
ok
ok
Note that this business of escaping a newline is
specific to interactive commands typed into the
debugger.
Stack backtrace
Here's an example of what a stack backtrace via T
command might look like:
$ = main::infested called from file `Ambulation.pm' line 10
@ = Ambulation::legs(1, 2, 3, 4) called from file `camel_flea' line 7
$ = main::pests('bactrian', 4) called from file `camel_flea' line 4
The left-hand character up there tells whether the
function was called in a scalar or list context
(we bet you can tell which is which). What that
says is that you were in the function
main::infested when you ran the stack dump, and
that it was called in a scalar context from line
10 of the file Ambulation.pm, but without any
arguments at all, meaning it was called as
&infested. The next stack frame shows that the
function Ambulation::legs was called in a list
context from the camel_flea file with four
arguments. The last stack frame shows that
main::pests was called in a scalar context, also
from camel_flea, but from line 4.
Note that if you execute T command from inside an
active use statement, the backtrace will contain
both require frame and an eval) frame.
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Listing Listing given via different flavors of l command
looks like this:
DB<<13>> l
101: @i{@i} = ();
102:b @isa{@i,$pack} = ()
103 if(exists $i{$prevpack} || exists $isa{$pack});
104 }
105
106 next
107==> if(exists $isa{$pack});
108
109:a if ($extra-- > 0) {
110: %isa = ($pack,1);
Note that the breakable lines are marked with :,
lines with breakpoints are marked by b, with
actions by a, and the next executed line is marked
by ==>.
Frame listing
When frame option is set, debugger would print
entered (and optionally exited) subroutines in
different styles.
What follows is the start of the listing of
env "PERLDB_OPTS=f=n N" perl -d -V
for different values of n:
1
entering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
Package lib/Config.pm.
entering Config::TIEHASH
entering Exporter::import
entering Exporter::export
entering Config::myconfig
entering Config::FETCH
entering Config::FETCH
entering Config::FETCH
entering Config::FETCH
2
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entering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
exited Config::BEGIN
Package lib/Config.pm.
entering Config::TIEHASH
exited Config::TIEHASH
entering Exporter::import
entering Exporter::export
exited Exporter::export
exited Exporter::import
exited main::BEGIN
entering Config::myconfig
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
4
in $=main::BEGIN() from /dev/nul: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/nul:0
in $=Exporter::export('Config', 'main', 'myconfig', 'config_vars') from li
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), 'osname') from lib/Config.pm:574
in $=Config::FETCH(ref(Config), 'osvers') from lib/Config.pm:574
6
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in $=main::BEGIN() from /dev/nul: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/nul: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/nul:0
out $=main::BEGIN() from /dev/nul:0
in @=Config::myconfig() from /dev/nul: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), '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
14
in $=main::BEGIN() from /dev/nul: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/nul: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/nul:0
out $=main::BEGIN() from /dev/nul:0
in @=Config::myconfig() from /dev/nul: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
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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 the cases indentation of lines shows
the call tree, if bit 2 of 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, if bit 16 is set, the
return value is printed as well.
When a package is compiled, a line like this
Package lib/Carp.pm.
is printed with proper indentation.
Debugging compile-time statements
If you have any compile-time executable statements (code
within a BEGIN block or a use statement), these will NOT
be stopped by debugger, although requires will (and
compile-time statements can be traced with AutoTrace
option set in 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:
$DB::single = 1;
If you set $DB::single to the value 2, it's equivalent to
having just typed the n command, whereas a value of 1
means the s command. The $DB::trace variable should be
set to 1 to simulate having typed the t command.
Another way to debug compile-time code is to start
debugger, set a breakpoint on load of some module thusly
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DB<7> b load f:/perllib/lib/Carp.pm
Will stop on load of `f:/perllib/lib/Carp.pm'.
and restart debugger by R command (if possible). One can
use b compile subname for the same purpose.
Debugger Customization
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 Options, from the command line via PERLDB_OPTS
environment variable, and from customization files.
You can do some customization by setting up a .perldb file
which contains initialization code. For instance, you
could make aliases like these (the last one is one people
expect to be there):
$DB::alias{'len'} = 's/^len(.*)/p length($1)/';
$DB::alias{'stop'} = 's/^stop (at|in)/b/';
$DB::alias{'ps'} = 's/^ps\b/p scalar /';
$DB::alias{'quit'} = 's/^quit(\s*)/exit\$/';
One changes options from .perldb file via calls like this
one;
parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");
(the code is executed in the package DB). Note that
.perldb is processed before processing PERLDB_OPTS. If
.perldb defines the subroutine afterinit, it is called
after all the debugger initialization ends. .perldb may
be contained in the current directory, or in the
LOGDIR/HOME directory.
If you want to modify the debugger, copy perl5db.pl from
the Perl library to another name and modify it as
necessary. You'll also want to set your PERL5DB
environment variable to say something like this:
BEGIN { require "myperl5db.pl" }
As the last resort, one can use PERL5DB to customize
debugger by directly setting internal variables or calling
debugger functions.
Readline Support
As shipped, the only command line history supplied is a
simplistic one that checks for leading exclamation points.
However, if you install the Term::ReadKey and
Term::ReadLine modules from CPAN, you will have full
editing capabilities much like GNU readline(3) provides.
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Look for these in the modules/by-module/Term directory on
CPAN.
A rudimentary command line completion is also available.
Unfortunately, the names of lexical variables are not
available for completion.
Editor Support for Debugging
If you have GNU emacs installed on your system, it can
interact with the Perl debugger to provide an integrated
software development environment reminiscent of its
interactions with C debuggers.
Perl is also delivered with a start file for making emacs
act like a syntax-directed editor that understands (some
of) Perl's syntax. Look in the emacs directory of the
Perl source distribution.
(Historically, a similar setup for interacting with vi and
the X11 window system had also been available, but at the
time of this writing, no debugger support for vi currently
exists.)
The Perl Profiler
If you wish to supply an alternative debugger for Perl to
run, just invoke your script with a colon and a package
argument given to the -d flag. One of the most popular
alternative debuggers for Perl is DProf, the Perl
profiler. As of this writing, DProf is not included with
the standard Perl distribution, but it is expected to be
included soon, for certain values of "soon".
Meanwhile, you can fetch the Devel::Dprof module from
CPAN. Assuming it's properly installed on your system, to
profile your Perl program in the file mycode.pl, just
type:
perl -d:DProf mycode.pl
When the script terminates the profiler will dump the
profile information to a file called tmon.out. A tool
like dprofpp (also supplied with the Devel::DProf package)
can be used to interpret the information which is in that
profile.
Debugger support in perl
When you call the caller function (see the caller entry in
the perlfunc manpage) from the package DB, Perl sets the
array @DB::args to contain the arguments the corresponding
stack frame was called with.
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If perl is run with -d option, the following additional
features are enabled (cf. the section on $^P in the
perlvar manpage):
o Perl inserts the contents of $ENV{PERL5DB} (or BEGIN
{require 'perl5db.pl'} if not present) before the
first line of the application.
o The array @{"_<$filename"} is the line-by-line
contents of $filename for all the compiled files.
Same for evaled strings which contain subroutines, or
which are currently executed. The $filename for
evaled strings looks like (eval 34).
o The hash %{"_<$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 perl5db.pl have the form
"$break_condition\0$action". Values are magical in
numeric context: they are zeros if the line is not
breakable.
Same for evaluated strings which contain subroutines,
or which are currently executed. The $filename for
evaled strings looks like (eval 34).
o The scalar ${"_<$filename"} contains "_<$filename".
Same for evaluated strings which contain subroutines,
or which are currently executed. The $filename for
evaled strings looks like (eval 34).
o After each required file is compiled, but before it
is executed, DB::postponed(*{"_<$filename"}) is
called (if subroutine DB::postponed exists). Here
the $filename is the expanded name of the required
file (as found in values of %INC).
o After each subroutine subname is compiled existence
of $DB::postponed{subname} is checked. If this key
exists, DB::postponed(subname) is called (if
subroutine DB::postponed exists).
o A hash %DB::sub is maintained, with keys being
subroutine names, values having the form
filename:startline-endline. filename has the form
(eval 31) for subroutines defined inside evals.
o When execution of the application reaches a place
that can have a breakpoint, a call to DB::DB() is
performed if any one of variables $DB::trace,
$DB::single, or $DB::signal is true. (Note that these
variables are not localizable.) This feature is
disabled when the control is inside DB::DB() or
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functions called from it (unless $^D & (1<<30)).
o When execution of the application reaches a
subroutine call, a call to &DB::sub(args) is
performed instead, with $DB::sub being the name of
the called subroutine. (Unless the subroutine is
compiled in the package DB.)
Note that if &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 $DB::deep (how many
levels of recursion deep into the debugger you can go
before a mandatory break) gives an example of such a
dependency.
The minimal working debugger consists of one line
sub DB::DB {}
which is quite handy as contents of PERL5DB environment
variable:
env "PERL5DB=sub DB::DB {}" perl -d your-script
Another (a little bit more useful) minimal debugger can be
created with the only line being
sub DB::DB {print ++$i; scalar <STDIN>}
This debugger would print the sequential number of
encountered statement, and would wait for your CR to
continue.
The following debugger is quite functional:
{
package DB;
sub DB {}
sub sub {print ++$i, " $sub\n"; &$sub}
}
It prints the sequential number of subroutine call and the
name of the called subroutine. Note that &DB::sub should
be compiled into the package DB.
Debugger Internals
At the start, the debugger reads your rc file (./.perldb
or ~/.perldb under Unix), which can set important options.
This file may define a subroutine &afterinit to be
executed after the debugger is initialized.
After the rc file is read, the debugger reads environment
variable PERLDB_OPTS and parses it as a rest of O ... line
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in debugger prompt.
It also maintains magical internal variables, such as
@DB::dbline, %DB::dbline, which are aliases for
@{"::_<current_file"} %{"::_<current_file"}. Here
current_file is the currently selected (with the
debugger's f command, or by flow of execution) file.
Some functions are provided to simplify customization.
See the section on Debugger Customization for description
of DB::parse_options(string). The function
DB::dump_trace(skip[, count]) skips the specified number
of frames, and returns a list containing info about the
caller frames (all if count is missing). Each entry is a
hash with keys context ($ or @), sub (subroutine name, or
info about eval), args (undef or a reference to an array),
file, and line.
The function DB::print_trace(FH, skip[, count[, short]])
prints formatted info about caller frames. The last two
functions may be convenient as arguments to <, <<
commands.
Other resources
You did try the -w switch, didn't you?
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 shift or pop, the stack backtrace will not show the
original values.
Debugging Perl memory usage
Perl is 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.
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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 -DL switch. First one
is available only if perl is compiled with Perl's
malloc(), the second one only if Perl compiled with
-DDEBUGGING (as with giving -D optimise=-g option to
Configure).
Using $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
$ENV{PERL_DEBUG_MSTATS} > 1), and before termination of
the script (if $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:
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.
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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 APPROX field above.
Free/Used
The following 1 or 2 rows of numbers 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 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-DEBUGGING perl the buckets starting from
128-long ones have 4-byte overhead, thus 8192-long
bucket may take up to 8188-byte-long allocations.
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.
pad: 0
The amount of sbrk()ed memory needed to keep buckets
aligned.
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.
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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.
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.
Example of using -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 ;
Note: 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 warn('!...') statements around
the call:
warn('!');
do 'lib/auto/POSIX/autosplit.ix';
warn('!!! "after"');
and run it with -DL option. The first warn() will print
memory allocation info before the parsing of the file, and
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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 Ids 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 9 03 means grep the perl source for 903. You
will see that it is 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().
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(), continue this
point is reached, 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 903 allocations than 146 similar lines of
lib/auto/POSIX/autosplit.ix). Note also that Perl_
prefixes are added by macroization code in perl header
files to avoid conflicts with external libraries.
Anyway, we see that 903 ids correspond to creation of
globs, twice per glob - for glob name, and glob
stringification magic.
Here are explanations for other Ids above:
717 is for creation of bigger XPV* structures. In the
above case it creates 3 AV per subroutine, one for a
list of lexical variable names, one for a scratchpad
(which contains lexical variables and targets), and
one for the array of scratchpads needed for
recursion.
It also creates a GV and a CV per subroutine (all
called from start_subparse()).
002 Creates C array corresponding to the 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 SV allocations).
054 creates a HEK for the name of the glob for the
subroutine (this name is a key in a stash).
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Big allocations with this Id correspond to
allocations of new arenas to keep HE.
602 creates a GP for the glob for the subroutine.
702 creates the MAGIC for the glob for the subroutine.
704 creates arenas which keep SVs.
-DL details
If Perl is run with -DL option, then warn()s which start
with `!' behave specially. They print a list of
categories of memory allocations, and statistics of
allocations of different sizes for these categories.
If warn() string starts with
!!! print changed categories only, print the differences
in counts of allocations;
!! print grown categories only; print the absolute
values of counts, and totals;
! print nonempty categories, print the absolute values
of counts and totals.
Limitations of -DL statistic
If an extension or an external library does not use Perl
API to allocate memory, these allocations are not counted.
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 both
at compile time, and at run time (and is not lexically
scoped).
Compile-time output
The debugging output for the compile time looks like this:
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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 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 de at the
offset 1, and substring 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 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 the
match cannot be shorter than 7 chars.
The fields of interest which may appear in the last 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
The type of the first matching node.
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noscan
which advises to not scan for the found substrings;
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);
GPOS if the pattern contains \G;
plus if the pattern starts with a repeated char (as in
x+y);
implicit
if the pattern starts with .*;
with eval
if the pattern contain eval-groups (see the section
on (?{ code }) in the perlre manpage);
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 info is used to avoid entering (a
slow) RE engine on strings which will definitely not
match. If 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 nodes of the
compiled form of the RE. Each line has format
id: TYPE OPTIONAL-INFO (next-id)
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.
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# 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.
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# 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.
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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 node of the compiled RE which 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.
16/Sep/1999 perl 5.005, patch 03 31
PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)16/Sep/1999 perl 5.005, patch 03 32
