PERLDEBUG(1) Perl Programmers Reference Guide PERLDEBUG(1)
NAME
perldebug - Perl debugging
DESCRIPTION
First of all, have you tried using "use strict;" and "use warnings;"?
If you're new to the Perl debugger, you may prefer to read perldebtut,
which is a tutorial introduction to the debugger.
If you're looking for the nitty gritty details of how the debugger is
implemented, you may prefer to read perldebguts.
For in-depth technical usage details, see perl5db.pl, the documentation
of the debugger itself.
The Perl Debugger
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 the way 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 special Perl library file containing the
debugger.
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 keeping its own state information.)
Note that the said "eval" is bound by an implicit scope. As a result any
newly introduced lexical variable or any modified capture buffer content
is lost after the eval. The debugger is a nice environment to learn
Perl, but if you interactively experiment using material which should be
in the same scope, stuff it in one line.
For any text entered at the debugger prompt, leading and trailing
whitespace is first stripped before further processing. If a debugger
command coincides with some function in your own program, merely precede
the function with something that doesn't look like a debugger command,
such as a leading ";" or perhaps a "+", or by wrapping it with
parentheses or braces.
Calling the Debugger
There are several ways to call the debugger:
perl -d program_name
On the given program identified by "program_name".
perl -d -e 0
Interactively supply an arbitrary "expression" using "-e".
perl -d:ptkdb program_name
Debug a given program via the Devel::ptkdb GUI.
perl -dt threaded_program_name
Debug a given program using threads (experimental).
If Perl is called with the "-d" switch, the variable $^P will hold a
true value. This is useful if you need to know if your code is running
under the debugger:
if ( $^P ) {
# running under the debugger
}
See "$^P" in perlvar for more information on the variable.
Debugger Commands
The interactive debugger understands the following commands:
h Prints out a summary help message
h [command] Prints out a help message for the given debugger command.
h h The special argument of "h h" produces the entire help page,
which is quite long.
If the output of the "h h" command (or any command, for that
matter) scrolls past your screen, precede the command with a
leading pipe symbol so that it's run through your pager, as
in
DB> |h h
You may change the pager which is used via "o pager=..."
command.
p expr Same as "print {$DB::OUT} expr" in the current package. In
particular, because this is just Perl's own "print"
function, this means that nested data structures and objects
are not dumped, unlike with the "x" command.
The "DB::OUT" filehandle is opened to /dev/tty, regardless
of where STDOUT may be redirected to.
x [maxdepth] expr
Evaluates its expression in list context and dumps out the
result in a pretty-printed fashion. Nested data structures
are printed out recursively, unlike the real "print"
function in Perl. When dumping hashes, you'll probably
prefer 'x \%h' rather than 'x %h'. See Dumpvalue if you'd
like to do this yourself.
The output format is governed by multiple options described
under "Configurable Options".
If the "maxdepth" is included, it must be a numeral N; the
value is dumped only N levels deep, as if the "dumpDepth"
option had been temporarily set to N.
V [pkg [vars]]
Display all (or some) variables in package (defaulting to
"main") using a data pretty-printer (hashes show their keys
and values so you see what's what, control characters are
made printable, etc.). Make sure you don't put the type
specifier (like "$") there, just the symbol names, like
this:
V DB filename line
Use "~pattern" and "!pattern" for positive and negative
regexes.
This is similar to calling the "x" command on each
applicable var.
X [vars] Same as "V currentpackage [vars]".
y [level [vars]]
Display all (or some) lexical variables (mnemonic: "mY"
variables) in the current scope or level scopes higher. You
can limit the variables that you see with vars which works
exactly as it does for the "V" and "X" commands. Requires
the PadWalker module version 0.08 or higher; will warn if
this isn't installed. Output is pretty-printed in the same
style as for "V" and the format is controlled by the same
options.
T Produce a stack backtrace. See below for details on its
output.
s [expr] Single step. Executes until 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 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.
r Continue until the return from the current subroutine. Dump
the return value if the "PrintRet" option is set (default).
<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. subname may be
a variable that contains a code reference.
- List previous window of lines.
v [line] View a few lines of code around the current line.
. Return the internal debugger pointer to the line last
executed, and print out that line.
f filename Switch to viewing a different file or "eval" statement. If
filename is not a full pathname found in the values of %INC,
it is considered a regex.
"eval"ed strings (when accessible) are considered to be
filenames: "f (eval 7)" and "f eval 7\b" access the body of
the 7th "eval"ed string (in the order of execution). The
bodies of the currently executed "eval" and of "eval"ed
strings that define subroutines are saved and thus
accessible.
/pattern/ Search forwards for pattern (a Perl regex); final / is
optional. The search is case-insensitive by default.
?pattern? Search backwards for pattern; final ? is optional. The
search is case-insensitive by default.
L [abw] List (default all) actions, breakpoints and watch
expressions
S [[!]regex]
List subroutine names [not] matching the regex.
t [n] Toggle trace mode (see also the "AutoTrace" option).
Optional argument is the maximum number of levels to trace
below the current one; anything deeper than that will be
silent.
t [n] expr Trace through execution of "expr". Optional first argument
is the maximum number of levels to trace below the current
one; anything deeper than that will be silent. See "Frame
Listing Output Examples" in perldebguts for examples.
b Sets breakpoint on current line
b [line] [condition]
Set a breakpoint before the given line. If a condition is
specified, it's evaluated each time the statement is
reached: a breakpoint is taken only if the condition is
true. Breakpoints may only be set on lines that begin an
executable statement. Conditions don't use "if":
b 237 $x > 30
b 237 ++$count237 < 11
b 33 /pattern/i
If the line number is ".", sets a breakpoint on the current
line:
b . $n > 100
b [file]:[line] [condition]
Set a breakpoint before the given line in a (possibly
different) file. If a condition is specified, it's
evaluated each time the statement is reached: a breakpoint
is taken only if the condition is true. Breakpoints may
only be set on lines that begin an executable statement.
Conditions don't use "if":
b lib/MyModule.pm:237 $x > 30
b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11
b subname [condition]
Set a breakpoint before the first line of the named
subroutine. subname may be a variable containing a code
reference (in this case condition is not supported).
b postpone subname [condition]
Set a breakpoint at first line of subroutine after it is
compiled.
b load filename
Set a breakpoint before the first executed line of the
filename, which should be a full pathname found amongst the
%INC values.
b compile subname
Sets a breakpoint before the first statement executed after
the specified subroutine is compiled.
B line Delete a breakpoint from the specified line.
B * Delete all installed breakpoints.
disable [file]:[line]
Disable the breakpoint so it won't stop the execution of the
program. Breakpoints are enabled by default and can be re-
enabled using the "enable" command.
disable [line]
Disable the breakpoint so it won't stop the execution of the
program. Breakpoints are enabled by default and can be re-
enabled using the "enable" command.
This is done for a breakpoint in the current file.
enable [file]:[line]
Enable the breakpoint so it will stop the execution of the
program.
enable [line]
Enable the breakpoint so it will stop the execution of the
program.
This is done for a breakpoint in the current file.
a [line] command
Set an action to be done before the line is executed. If
line is omitted, set an action on the line about to be
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 line Delete an action from the specified line.
A * Delete all installed actions.
w expr Add a global watch-expression. Whenever a watched global
changes the debugger will stop and display the old and new
values.
W expr Delete watch-expression
W * Delete all watch-expressions.
o Display all options.
o booloption ...
Set each listed Boolean option to the value 1.
o anyoption? ...
Print out the value of one or more options.
o option=value ...
Set the value of one or more options. If the value has
internal whitespace, it should be quoted. For example, you
could set "o pager="less -MQeicsNfr"" to call less with
those specific options. You may use either single or double
quotes, but if you do, you must escape any embedded
instances of same sort of quote you began with, as well as
any escaping any escapes that immediately precede that quote
but which are not meant to escape the quote itself. In
other words, you follow single-quoting rules irrespective of
the quote; eg: "o option='this isn\'t bad'" or "o
option="She said, \"Isn't it?\""".
For historical reasons, the "=value" is optional, but
defaults to 1 only where it is safe to do so--that is,
mostly for Boolean options. It is always better to assign a
specific value using "=". The "option" can be abbreviated,
but for clarity probably should not be. Several options can
be set together. See "Configurable Options" for a list of
these.
< ? List out all pre-prompt Perl command actions.
< [ command ]
Set an action (Perl command) to happen before every debugger
prompt. A multi-line command may be entered by backslashing
the newlines.
< * Delete all pre-prompt Perl command actions.
<< command Add an action (Perl command) to happen before every debugger
prompt. A multi-line command may be entered by backwhacking
the newlines.
> ? List out post-prompt Perl command actions.
> 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 (we bet you couldn't have guessed this by now).
> * Delete all post-prompt Perl command 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.
{ ? List out pre-prompt debugger commands.
{ [ command ]
Set an action (debugger command) to happen before every
debugger prompt. A multi-line command may be entered in the
customary fashion.
Because this command is in some senses new, a warning is
issued if you appear to have accidentally entered a block
instead. If that's what you mean to do, write it as with
";{ ... }" or even "do { ... }".
{ * Delete all pre-prompt debugger commands.
{{ command Add an action (debugger command) to happen before every
debugger prompt. A multi-line command may be entered, if
you can guess how: see above.
! number Redo a previous command (defaults to the previous command).
! -number Redo number'th previous 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", also. Note that the user's
current shell (well, their $ENV{SHELL} variable) will be
used, which can interfere with proper interpretation of exit
status or signal and coredump information.
source file Read and execute debugger commands from file. file may
itself contain "source" commands.
H -number Display last n commands. Only commands longer than one
character are listed. If number is omitted, list them all.
q or ^D Quit. ("quit" doesn't work for this, unless you've made an
alias) This is the only supported way to exit the debugger,
though typing "exit" twice might work.
Set the "inhibit_exit" option to 0 if you want to be able to
step off the end the script. You may also need to set
$finished to 0 if you want to step through global
destruction.
R Restart the debugger by exec()ing a new session. We try to
maintain your history across this, but internal settings and
command-line options may be lost.
The following setting are currently preserved: history,
breakpoints, actions, debugger options, and the Perl
command-line options -w, -I, and -e.
|dbcmd Run the debugger command, piping DB::OUT into your current
pager.
||dbcmd Same as "|dbcmd" but DB::OUT is temporarily "select"ed as
well.
= [alias value]
Define a command alias, like
= quit q
or list current aliases.
command Execute command as a Perl statement. A trailing semicolon
will be supplied. If the Perl statement would otherwise be
confused for a Perl debugger, use a leading semicolon, too.
m expr List which methods may be called on the result of the
evaluated expression. The expression may evaluated to a
reference to a blessed object, or to a package name.
M Display all loaded modules and their versions.
man [manpage]
Despite its name, this calls your system's default
documentation viewer on the given page, or on the viewer
itself if manpage is omitted. If that viewer is man, the
current "Config" information is used to invoke man using the
proper MANPATH or -M manpath option. Failed lookups of the
form "XXX" that match known manpages of the form perlXXX
will be retried. This lets you type "man debug" or "man op"
from the debugger.
On systems traditionally bereft of a usable man command, the
debugger invokes perldoc. Occasionally this determination
is incorrect due to recalcitrant vendors or rather more
felicitously, to enterprising users. If you fall into
either category, just manually set the $DB::doccmd variable
to whatever viewer to view the Perl documentation on your
system. This may be set in an rc file, or through direct
assignment. We're still waiting for a working example of
something along the lines of:
$DB::doccmd = 'netscape -remote http://something.here/';
Configurable Options
The debugger has numerous options settable using the "o" command, either
interactively or from the environment or an rc file. The file is named
./.perldb or ~/.perldb under Unix with /dev/tty, perldb.ini otherwise.
"recallCommand", "ShellBang"
The characters used to recall a command or spawn a shell.
By default, both are set to "!", which is unfortunate.
"pager" Program to use for output of pager-piped commands (those
beginning with a "|" character.) By default, $ENV{PAGER}
will be used. Because the debugger uses your current
terminal characteristics for bold and underlining, if the
chosen pager does not pass escape sequences through
unchanged, the output of some debugger commands will not be
readable when sent through the pager.
"tkRunning" Run Tk while prompting (with ReadLine).
"signalLevel", "warnLevel", "dieLevel"
Level of verbosity. By default, the debugger leaves your
exceptions and warnings alone, because altering them can
break correctly running programs. It will attempt to print
a message when uncaught INT, BUS, or SEGV signals arrive.
(But see the mention of signals in "BUGS" below.)
To disable this default safe mode, set these values to
something higher than 0. At a level of 1, you get
backtraces upon receiving any kind of warning (this is often
annoying) or exception (this is often valuable).
Unfortunately, the debugger cannot discern fatal exceptions
from non-fatal ones. If "dieLevel" is even 1, then your
non-fatal exceptions are also traced and unceremoniously
altered if they came from "eval'ed" strings or from any kind
of "eval" within modules you're attempting to load. If
"dieLevel" is 2, the debugger doesn't care where they came
from: It usurps your exception handler and prints out a
trace, then modifies all exceptions with its own
embellishments. This may perhaps be useful for some tracing
purposes, but tends to hopelessly destroy any program that
takes its exception handling seriously.
"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 message is used.
This is the mechanism used to interact with a client editor
or visual debugger, such as the special "vi" or "emacs"
hooks, or the "ddd" graphical debugger.
"inhibit_exit"
If 0, allows stepping off the end of the script.
"PrintRet" Print return value after "r" command if set (default).
"ornaments" Affects screen appearance of the command line (see
Term::ReadLine). There is currently no way to disable
these, which can render some output illegible on some
displays, or with some pagers. This is considered a bug.
"frame" Affects the printing of messages upon entry and exit from
subroutines. If "frame & 2" is false, messages are printed
on entry only. (Printing on exit might be useful if
interspersed with other messages.)
If "frame & 4", arguments to functions are printed, plus
context and caller info. If "frame & 8", overloaded
"stringify" and "tie"d "FETCH" is enabled on the printed
arguments. If "frame & 16", the return value from the
subroutine is printed.
The length at which the argument list is truncated is
governed by the next option:
"maxTraceLen"
Length to truncate the argument list when the "frame"
option's bit 4 is set.
"windowSize"
Change the size of code list window (default is 10 lines).
The following options affect what happens with "V", "X", and "x"
commands:
"arrayDepth", "hashDepth"
Print only first N elements ('' for all).
"dumpDepth" Limit recursion depth to N levels when dumping structures.
Negative values are interpreted as infinity. Default:
infinity.
"compactDump", "veryCompact"
Change the style of array and hash output. 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 the style of string dump. The default value for
"quote" is "auto"; one can enable double-quotish or single-
quotish format by setting it to """ or "'", respectively.
By default, characters with their high bit set are printed
verbatim.
"UsageOnly" Rudimentary per-package memory usage dump. Calculates total
size of strings found in variables in the package. This
does not include lexicals in a module's file scope, or lost
in closures.
"HistFile" The path of the file from which the history (assuming a
usable Term::ReadLine backend) will be read on the
debugger's startup, and to which it will be saved on
shutdown (for persistence across sessions). Similar in
concept to Bash's ".bash_history" file.
"HistSize" The count of the saved lines in the history (assuming
"HistFile" above).
After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS}
environment variable and parses this as the remainder of a "O ..." line
as one might enter at the debugger prompt. You may place the
initialization options "TTY", "noTTY", "ReadLine", and "NonStop" there.
If your rc file contains:
parse_options("NonStop=1 LineInfo=db.out AutoTrace");
then your script will run without human intervention, putting trace
information into the file db.out. (If you interrupt it, you'd better
reset "LineInfo" to /dev/tty if you expect to see anything.)
"TTY" The TTY to use for debugging I/O.
"noTTY" If set, the debugger goes into "NonStop" mode and will not
connect to a TTY. If interrupted (or if control goes to the
debugger via explicit setting of $DB::signal or $DB::single
from the Perl script), it connects to a TTY specified in the
"TTY" option at startup, or to a tty found at runtime using
the "Term::Rendezvous" module of your choice.
This module should implement a method named "new" that
returns an object with two methods: "IN" and "OUT". These
should return filehandles to use for debugging input and
output correspondingly. The "new" method should inspect an
argument containing the value of $ENV{PERLDB_NOTTY} at
startup, or "$ENV{HOME}/.perldbtty$$" otherwise. This file
is not inspected for proper ownership, so security hazards
are theoretically possible.
"ReadLine" If false, readline support in the debugger is disabled in
order to debug applications that themselves use ReadLine.
"NonStop" If set, the debugger goes into non-interactive 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="NonStop frame=2" perl -d myprogram
That will run the script myprogram without human intervention, printing
out the call tree with entry and exit points. Note that "NonStop=1
frame=2" is equivalent to "N f=2", and that originally, options could be
uniquely abbreviated by the first letter (modulo the "Dump*" options).
It is nevertheless recommended that you always spell them out in full
for legibility and future compatibility.
Other examples include
$ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram
which runs script non-interactively, printing info on each entry into a
subroutine and each executed line into the file named listing. (If you
interrupt it, you would better reset "LineInfo" to something
"interactive"!)
Other examples include (using standard shell syntax to show environment
variable settings):
$ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
perl -d myprogram )
which may be useful for debugging a program that uses Term::ReadLine
itself. Do not forget to detach your shell from the TTY in the window
that corresponds to /dev/ttyXX, say, by issuing a command like
$ sleep 1000000
See "Debugger Internals" in perldebguts for details.
Debugger Input/Output
Prompt The debugger prompt is something like
DB<8>
or even
DB<<17>>
where that number is the command number, and which you'd use to
access with the built-in csh-like history mechanism. For
example, "!17" would repeat command number 17. The depth of the
angle brackets indicates the nesting 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 the result of a
function call that itself 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, 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 indicates the context in which
the function was called, with "$" and "@" meaning scalar or list
contexts respectively, and "." meaning void context (which is
actually a sort of scalar context). The display above says that
you were in the function "main::infested" when you ran the stack
dump, and that it was called in scalar context from line 10 of
the file Ambulation.pm, but without any arguments at all,
meaning it was called as &infested. The next stack frame shows
that the function "Ambulation::legs" was called in list context
from the camel_flea file with four arguments. The last stack
frame shows that "main::pests" was called in scalar context,
also from camel_flea, but from line 4.
If you execute the "T" command from inside an active "use"
statement, the backtrace will contain both a "require" frame and
an "eval" frame.
Line Listing Format
This shows the sorts of output the "l" command can produce:
DB<<13>> l
101: @i{@i} = ();
102:b @isa{@i,$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);
Breakable lines are marked with ":". Lines with breakpoints are
marked by "b" and those with actions by "a". The line that's
about to be executed is marked by "==>".
Please be aware that code in debugger listings may not look the
same as your original source code. Line directives and external
source filters can alter the code before Perl sees it, causing
code to move from its original positions or take on entirely
different forms.
Frame listing
When the "frame" option is set, the debugger would print entered
(and optionally exited) subroutines in different styles. See
perldebguts for incredibly long examples of these.
Debugging Compile-Time Statements
If you have compile-time executable statements (such as code within
BEGIN, UNITCHECK and CHECK blocks or "use" statements), these will not
be stopped by debugger, although "require"s and INIT blocks will, and
compile-time statements can be traced with the "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 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 the debugger, set a
breakpoint on the load of some module:
DB<7> b load f:/perllib/lib/Carp.pm
Will stop on load of 'f:/perllib/lib/Carp.pm'.
and then restart the debugger using the "R" command (if possible). One
can use "b compile subname" for the same purpose.
Debugger Customization
The debugger probably contains enough configuration hooks that you won't
ever have to modify it yourself. You may change the behaviour of the
debugger from within the debugger using its "o" command, from the
command line via the "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/';
You can change options from .perldb by using 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", that function is called after debugger
initialization ends. .perldb may be contained in the current directory,
or in the home directory. Because this file is sourced in by Perl and
may contain arbitrary commands, for security reasons, it must be owned
by the superuser or the current user, and writable by no one but its
owner.
You can mock TTY input to debugger by adding arbitrary commands to
@DB::typeahead. For example, your .perldb file might contain:
sub afterinit { push @DB::typeahead, "b 4", "b 6"; }
Which would attempt to set breakpoints on lines 4 and 6 immediately
after debugger initialization. Note that @DB::typeahead is not a
supported interface and is subject to change in future releases.
If you want to modify the debugger, copy perl5db.pl from the Perl
library to another name and hack it to your heart's content. You'll
then want to set your "PERL5DB" environment variable to say something
like this:
BEGIN { require "myperl5db.pl" }
As a last resort, you could also use "PERL5DB" to customize the debugger
by directly setting internal variables or calling debugger functions.
Note that any variables and functions that are not documented in this
document (or in perldebguts) are considered for internal use only, and
as such are subject to change without notice.
Readline Support / History in the Debugger
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 one of the "Term::ReadLine::*" modules from CPAN (such
as Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have full
editing capabilities much like those GNU readline(3) provides. Look for
these in the modules/by-module/Term directory on CPAN. These do not
support normal vi command-line editing, however.
A rudimentary command-line completion is also available, including
lexical variables in the current scope if the PadWalker module is
installed.
Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B",
"^[[D"", "^H", ... when using the arrow keys and/or the backspace key.
Editor Support for Debugging
If you have the GNU's version of 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.
Recent versions of Emacs come with a start file for making emacs act
like a syntax-directed editor that understands (some of) Perl's syntax.
See perlfaq3.
Users of vi should also look into vim and gvim, the mousey and windy
version, for coloring of Perl keywords.
Note that only perl can truly parse Perl, so all such CASE tools fall
somewhat short of the mark, especially if you don't program your Perl as
a C programmer might.
The Perl Profiler
If you wish to supply an alternative debugger for Perl to run, invoke
your script with a colon and a package argument given to the -d flag.
Perl's alternative debuggers include a Perl profiler, Devel::NYTProf,
which is available separately as a CPAN distribution. To profile your
Perl program in the file mycode.pl, just type:
$ perl -d:NYTProf mycode.pl
When the script terminates the profiler will create a database of the
profile information that you can turn into reports using the profiler's
tools. See <perlperf> for details.
Debugging Regular Expressions
"use re 'debug'" enables you to see the gory details of how the Perl
regular expression engine works. In order to understand this typically
voluminous output, one must not only have some idea about how regular
expression matching works in general, but also know how Perl's regular
expressions are internally compiled into an automaton. These matters are
explored in some detail in "Debugging Regular Expressions" in
perldebguts.
Debugging Memory Usage
Perl contains internal support for reporting its own memory usage, but
this is a fairly advanced concept that requires some understanding of
how memory allocation works. See "Debugging Perl Memory Usage" in
perldebguts for the details.
SEE ALSO
You do have "use strict" and "use warnings" enabled, don't you?
perldebtut, perldebguts, perl5db.pl, re, DB, Devel::NYTProf, Dumpvalue,
and perlrun.
When debugging a script that uses #! and is thus normally found in
$PATH, the -S option causes perl to search $PATH for it, so you don't
have to type the path or "which $scriptname".
$ perl -Sd foo.pl
BUGS
You cannot get stack frame information or in any fashion debug functions
that were not compiled by Perl, such as those from 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.
The debugger does not currently work in conjunction with the -W command-
line switch, because it itself is not free of warnings.
If you're in a slow syscall (like "wait"ing, "accept"ing, or "read"ing
from your keyboard or a socket) and haven't set up your own $SIG{INT}
handler, then you won't be able to CTRL-C your way back to the debugger,
because the debugger's own $SIG{INT} handler doesn't understand that it
needs to raise an exception to longjmp(3) out of slow syscalls.
perl v5.40.1 2026-08-30 PERLDEBUG(1)
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