MAN page from Old RedHat 5.X perl-5.004-4.i386.rpm

PERLDEBUG

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index

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 oursurprise that it wasn't as easy to get programs rightas we had thought. Debugging had to be discovered.I can remember the exact instant when I realized thata large part of my life from then on was going to bespent in finding mistakes in my own programs.''

--Maurice Wilkes, 1949

If you invoke Perl with the -d switch, your script runs under thePerl source debugger. This works like an interactive Perlenvironment, prompting for debugger commands that let you examinesource code, set breakpoints, get stack backtraces, change the values ofvariables, etc. This is so convenient that you often fire upthe debugger all by itself just to test out Perl constructsinteractively 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 thetypical compiled environment. Instead, the -d flag tells the compilerto insert source information into the parse trees it's about to hand offto the interpreter. That means your code must first compile correctlyfor the debugger to work on it. Then when the interpreter starts up, itpreloads a Perl library file containing the debugger itself.

The program will halt right before the first run-time executablestatement (but see below regarding compile-time statements) and ask youto enter a debugger command. Contrary to popular expectations, wheneverthe debugger halts and shows you a line of code, it always displays theline 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 theDB package for its own state information.)

Leading white space before a command would cause the debugger to thinkit's NOT a debugger command but for Perl, so be careful not to dothat.

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 specialargument of h h produces a more compact help listing, designed to fittogether on one screen.

If the output the h command (or any command, for that matter) scrollspast your screen, either precede the command with a leading pipe symbol soit'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 nesteddata structures and objects are not dumped, unlike with the x command.

The DB::OUT filehandle is opened to /dev/tty, regardless ofwhere STDOUT may be redirected to.

x expr

Evaluates its expression in list context and dumps out the resultin a pretty-printed fashion. Nested data structures are printed outrecursively, 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 mainpackage) using a data pretty-printer (hashes show their keys and values soyou see what's what, control characters are made printable, etc.). Makesure you don't put the type specifier (like $) there, just the symbolnames, 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, unlikethe 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 anotherstatement, descending into subroutine calls. If an expression issupplied that includes function calls, it too will be single-stepped.

n [expr]

Next. Executes over subroutine calls, until it reaches the beginningof the next statement. If an expression is supplied that includesfunction calls, those functions will be executed with stops beforeeach statement.

<CR>

Repeat last n or s command.

c [line|sub]

Continue, optionally inserting a one-time-only breakpointat 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 andprint it out.

f filename

Switch to viewing a different file or eval statement. If filenameis not a full filename as found in values of %INC, it is considered asa 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 linethat is about to be executed. If a condition is specified, it'sevaluated each time the statement is reached and a breakpoint is takenonly if the condition is true. Breakpoints may be set on only linesthat 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 shouldbe a full name as found in values of %INC.

b compile subname

Sets breakpoint at the first statement executed after the subroutineis compiled.

d [line]

Delete a breakpoint at the specified line. If line is omitted, deletesthe 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 line53 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 canbe abbreviated. Several options can be listed.

recallCommand, ShellBang

The characters used to recall command or spawn shell. Bydefault, these are both set to !.

pager

Program to use for output of pager-piped commands (thosebeginning 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-messageswhich are going to be printed out, and will print a message wheninteresting 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 alsoprinted.

AutoTrace

Trace mode (similar to t command, but can be put intoPERLDB_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. Ifframe & 2 is false, messages are printed on entry only. (Printingon exit may be useful if inter(di)spersed with other messages.)

If frame & 4, arguments to functions are printed as well as thecontext and caller info. If frame & 8, overloaded stringify andtied 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 thenext option:

maxTraceLen

length at which the argument list is truncated when frame option'sbit 4 is set.

The following options affect what happens with V, X, and xcommands:

arrayDepth, hashDepth

Print only first N elements ('' for all).

compactDump, veryCompact

Change style of array and hash dump. If compactDump, short arraymay be printed on one line.

globPrint

Whether to print contents of globs.

DumpDBFiles

Dump arrays holding debugged files.

DumpPackages

Dump symbol tables of packages.

quote, HighBit, undefPrint

Change style of string dump. Default value of quote is auto, onecan enable either double-quotish dump, or single-quotish by setting itto " or '. By default, characters with high bit set are printedas is.

UsageOnly

very rudimentally per-package memory usage dump. Calculates totalsize 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 informationinto the file db.out. (If you interrupt it, you would better resetLineInfo 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. Ifinterrupt (or if control goes to debugger via explicit setting of$DB::signal or $DB::single from the Perl script), connects to a TTYspecified by the TTY option at startup, or to a TTY found atruntime using Term::Rendezvous module of your choice.

This module should implement a method new which returns an objectwith two methods: IN and OUT, returning two filehandles to usefor debugging input and output correspondingly. Method new mayinspect an argument which is a value of $ENV{PERLDB_NOTTY} atstartup, or is "/tmp/perldbtty$$" otherwise.

ReadLine

If false, readline support in debugger is disabled, so you can debugReadLine applications.

NonStop

If set, debugger goes into noninteractive mode until interrupted, orprogrammatically 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, printingout the call tree with entry and exit points. Note that N f=2 isequivalent to NonStop=1 frame=2. Note also that at the moment whenthis documentation was written all the options to the debugger couldbe uniquely abbreviated by the first letter (with exception ofDump* options).

Other examples may include

  $ PERLDB_OPTS="N f A L=listing" perl -d myprogram

- runs script noninteractively, printing info on each entry into asubroutine and each executed line into the file listing. (If youinterrupt 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::ReadLineitself. Do not forget detach shell from the TTY in the window whichcorresponds 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. Ifcommand 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'vejust given a command to return to executing the script. A multi-linecommand may be entered by backslashing the newlines. If command ismissing, resets the list of actions.

>> command

Adds an action (Perl command) to happen after the prompt when you'vejust given a command to return to executing the script. A multi-linecommand 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. Ifcommand 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 arelisted. If number is omitted, lists them all.

q or ^D

Quit. ("quit'' doesn't work for this.) This is the only supported wayto 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 stepoff the end the script. You may also need to set $finished to 0 atsome moment if you want to step through global destruction.

R

Restart the debugger by execing a new session. It tries to maintainyour history across this, but internal settings and command line optionsmay be lost.

Currently the following setting are preserved: history, breakpoints,actions, debugger Options, and the following command lineoptions: -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 longoutput, 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 besupplied.

m expr

The expression is evaluated, and the methods which may be applied tothe 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 withthe builtin csh-like history mechanism, e.g., !17 would repeatcommand number 17. The number of angle brackets indicates the depth ofthe debugger. You could get more than one set of brackets, for example, ifyou'd already at a breakpoint and then printed out the result of afunction call that itself also has a breakpoint, or you step into anexpression via s/n/t expression command.

Multiline commands

If you want to enter a multi-line command, such as a subroutinedefinition with several statements, or a format, you may escape thenewline 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 interactivecommands typed into the debugger.

Stack backtrace

Here's an example of what a stack backtrace via T command mightlook 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 calledin a scalar or list context (we bet you can tell which is which). Whatthat says is that you were in the function main::infested when you ranthe stack dump, and that it was called in a scalar context from line 10of the file Ambulation.pm, but without any arguments at all, meaningit was called as &infested. The next stack frame shows that thefunction Ambulation::legs was called in a list context from thecamel_flea file with four arguments. The last stack frame shows thatmain::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 usestatement, the backtrace will contain both the require entry in the perlfunc manpageframe 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 withbreakpoints are marked by b, with actions by a, and thenext executed line is marked by ==>.

Frame listing

When frame option is set, debugger would print entered (andoptionally 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

  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

  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

  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 offrame is set, then a line is printed on exit from a subroutine aswell, if bit 4 is set, then the arguments are printed as well as thecaller info, if bit 8 is set, the arguments are printed even if theyare tied or references, if bit 16 is set, the return value is printedas 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 BEGINblock or a use statement), these will NOT be stopped by debugger,although requires will (and compile-time statements can be tracedwith AutoTrace option set in PERLDB_OPTS). From your own Perlcode, however, you cantransfer 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 havingjust typed the n command, whereas a value of 1 means the scommand. The $DB::trace variable should be set to 1 to simulatehaving typed the t command.

Another way to debug compile-time code is to start debugger, set abreakpoint 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 bcompile subname for the same purpose.

Debugger Customization

Most probably you not want to modify the debugger, it contains enoughhooks to satisfy most needs. You may change the behaviour of debuggerfrom the debugger itself, using Options, from the command line viaPERLDB_OPTS environment variable, and from customization files.

You can do some customization by setting up a .perldb file whichcontains initialization code. For instance, you could make aliaseslike 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 isprocessed before processing PERLDB_OPTS. If .perldb defines thesubroutine afterinit, it is called after all the debuggerinitialization ends. .perldb may be contained in the currentdirectory, or in the LOGDIR/HOME directory.

If you want to modify the debugger, copy perl5db.pl from the Perllibrary to another name and modify it as necessary. You'll also wantto 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 bydirectly setting internal variables or calling debugger functions.

Readline Support

As shipped, the only command line history supplied is a simplistic onethat checks for leading exclamation points. However, if you installthe Term::ReadKey and Term::ReadLine modules from CPAN, you willhave 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 forcompletion.

Editor Support for Debugging

If you have GNU emacs installed on your system, it can interact withthe Perl debugger to provide an integrated software developmentenvironment reminiscent of its interactions with C debuggers.

Perl is also delivered with a start file for making emacs act like asyntax-directed editor that understands (some of) Perl's syntax. Look inthe emacs directory of the Perl source distribution.

(Historically, a similar setup for interacting with vi and theX11 window system had also been available, but at the time of thiswriting, no debugger support for vi currently exists.)

The Perl Profiler

If you wish to supply an alternative debugger for Perl to run, justinvoke your script with a colon and a package argument given to the -dflag. One of the most popular alternative debuggers for Perl isDProf, the Perl profiler. As of this writing, DProf is notincluded with the standard Perl distribution, but it is expected tobe included soon, for certain values of ``soon''.

Meanwhile, you can fetch the Devel::Dprof module from CPAN. Assumingit's properly installed on your system, to profile your Perl program inthe file mycode.pl, just type:

    perl -d:DProf mycode.pl

When the script terminates the profiler will dump the profile informationto a file called tmon.out. A tool like dprofpp (also supplied withthe Devel::DProf package) can be used to interpret the information which isin that profile.

Debugger support in perl

When you call the caller function (see the caller entry in the perlfunc manpage) from thepackage DB, Perl sets the array @DB::args to contain the arguments thecorresponding stack frame was called with.

If perl is run with -d option, the following additional featuresare enabled (cf. the section on $^P in the perlvar manpage):

*: Perl inserts the contents of $ENV{PERL5DB} (or BEGIN {require'perl5db.pl'} if not present) before the first line of theapplication.
*: The array C<@{"_<$filename"}> is the line-by-line contents of$filename for all the compiled files. Same for evaled strings whichcontain subroutines, or which are currently executed. The $filenamefor evaled strings looks like (eval 34).
*: The hash C<%{"_<$filename"}> contains breakpoints and action (it iskeyed by line number), and individual entries are settable (as opposedto the whole hash). Only true/false is important to Perl, though thevalues 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 arecurrently executed. The $filename for evaled strings looks like(eval 34).
*: The scalar C<${"_<$filename"}> contains C<"_<$filename">. Same forevaluated strings which contain subroutines, or which are currentlyexecuted. The $filename for evaled strings looks like (eval34).
*: After each required file is compiled, but before it is executed,C<DB::postponed(*{"_<$filename"})> is called (if subroutineDB::postponed exists). Here the $filename is the expanded name ofthe required file (as found in values of %INC).
*: After each subroutine subname is compiled existence of$DB::postponed{subname} is checked. If this key exists,DB::postponed(subname) is called (if subroutine DB::postponedexists).
*: A hash %DB::sub is maintained, with keys being subroutine names,values having the form filename:startline-endline. filename hasthe form (eval 31) for subroutines defined inside evals.
*: When execution of the application reaches a place that can havea breakpoint, a call to DB::DB() is performed if any one ofvariables $DB::trace, $DB::single, or $DB::signal is true. (Note thatthese variables are not localizable.) This feature is disabled whenthe control is inside DB::DB() or functions called from it (unless$^D & (1<<30)).
*: When execution of the application reaches a subroutine call, a callto &DB::sub(args) is performed instead, with $DB::sub beingthe name of the called subroutine. (Unless the subroutine is compiledin the package DB.)

Note that if &DB::sub needs some external data to be setup for itto work, no subroutine call is possible until this is done. For thestandard debugger $DB::deep (how many levels of recursion deep intothe debugger you can go before a mandatory break) gives an example ofsuch a dependency.

The minimal working debugger consists of one line

  sub DB::DB {}

which is quite handy as contents of PERL5DB environmentvariable:

  env "PERL5DB=sub DB::DB {}" perl -d your-script

Another (a little bit more useful) minimal debugger can be createdwith the only line being

  sub DB::DB {print ++$i; scalar <STDIN>}

This debugger would print the sequential number of encounteredstatement, 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 thecalled subroutine. Note that &DB::sub should be compiled into thepackage DB.

Debugger Internals

At the start, the debugger reads your rc file (./.perldb or~/.perldb under Unix), which can set important options. This file maydefine a subroutine &afterinit to be executed after the debugger isinitialized.

After the rc file is read, the debugger reads environment variablePERLDB_OPTS and parses it as a rest of O ... line in debugger prompt.

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 currentlyselected (with the debugger's f command, or by flow of execution)file.

Some functions are provided to simplify customization. See the section on DebuggerCustomization for description of DB::parse_options(string). Thefunction DB::dump_trace(skip[, count]) skips the specified numberof frames, and returns an array containing info about the callerframes (all if count is missing). Each entry is a hash with keyscontext ($ or @), sub (subroutine name, or info abouteval), args (undef or a reference to an array), file, andline.

The function DB::print_trace(FH, skip[, count[, short]]) printsformatted info about caller frames. The last two functions may beconvenient 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 functionsthat were not compiled by Perl, such as C or C++ extensions.

If you alter your @_ arguments in a subroutine (such as with shiftor pop, the stack backtrace will not show the original values.

This document was created byman2html,using the manual pages.