PERLDEBUG(1) PERLDEBUG(1)
NAME
perldebug - 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.
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
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PERLDEBUG(1) PERLDEBUG(1)
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 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 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.
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PERLDEBUG(1) PERLDEBUG(1)
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 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.
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PERLDEBUG(1) PERLDEBUG(1)
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 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.
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PERLDEBUG(1) PERLDEBUG(1)
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.
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
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PERLDEBUG(1) PERLDEBUG(1)
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 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.
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PERLDEBUG(1) PERLDEBUG(1)
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 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.
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:
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PERLDEBUG(1) PERLDEBUG(1)
$ 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
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.
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PERLDEBUG(1) PERLDEBUG(1)
>> 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 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
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PERLDEBUG(1) PERLDEBUG(1)
= [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 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.
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PERLDEBUG(1) PERLDEBUG(1)
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 the require entry in
the perlfunc manpage frame and an the section on eval EXPR in the
perlfunc manpage) frame.
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:
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PERLDEBUG(1) PERLDEBUG(1)
1
entering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
Package lib/Config.pm.
entering Config::TIEHASH
entering Exporter::import
entering Exporter::export
entering Config::myconfig
entering Config::FETCH
entering Config::FETCH
entering Config::FETCH
entering Config::FETCH
2
entering main::BEGIN
entering Config::BEGIN
Package lib/Exporter.pm.
Package lib/Carp.pm.
exited Config::BEGIN
Package lib/Config.pm.
entering Config::TIEHASH
exited Config::TIEHASH
entering Exporter::import
entering Exporter::export
exited Exporter::export
exited Exporter::import
exited main::BEGIN
entering Config::myconfig
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
exited Config::FETCH
entering Config::FETCH
4
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PERLDEBUG(1) PERLDEBUG(1)
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
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
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PERLDEBUG(1) PERLDEBUG(1)
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
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.
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PERLDEBUG(1) PERLDEBUG(1)
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
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 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.
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PERLDEBUG(1) PERLDEBUG(1)
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. 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
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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.
If perl is run with -d option, the following additional features are
enabled (cf. the section on $^P in the perlvar manpage):
⊕ Perl inserts the contents of $ENV{PERL5DB} (or BEGIN {require
'perl5db.pl'} if not present) before the first line of the
application.
⊕ The array C<@{"_<$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 $filename
for evaled strings looks like (eval 34).
⊕ The hash C<%{"_<$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 $filename for evaled strings looks like
(eval 34).
⊕ The scalar C<${"_<$filename"}> contains C<"_<$filename">. Same for
evaluated strings which contain subroutines, or which are currently
executed. The $filename for evaled strings looks like (eval 34).
⊕ After each required file is compiled, but before it is executed,
C<DB::postponed(*{"_<$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).
⊕ 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).
⊕ 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.
⊕ 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
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variables are not localizable.) This feature is disabled when the
control is inside DB::DB() or functions called from it (unless $^D &
(1<<30)).
⊕ 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 in debugger prompt.
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It also maintains magical internal variables, such as @DB::dbline,
%DB::dbline, which are aliases for C<@{"::_<current_file"}>
C<%{"::_<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 an array 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.
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