perldebug - Perl debugging


First of all, have you tried using the -w switch?

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 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 pre-loads 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 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

p expr
Same as print DB::OUT expr in the current package. In particular, since this is just Perl's own print function, this means that nested data structures and objects are not dumped, unlike with the x command.

x expr
Evals 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.

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.

X [vars]
Same as V currentpackage [vars] .

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.

Next. Executes over subroutine calls, until it reaches the beginning of the next statement.

Repeat last n or s command.

c [line]
Continue, optionally inserting a one-time-only breakpoint at the specified line.

List next window of lines.

l min+incr
List incr+1 lines starting at min.

l min-max
List lines min through max.

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.

Search forwards for pattern; final / is optional.

Search backwards for pattern; final ? is optional.

List all breakpoints and actions for the current file.

S [[!]pattern]
List subroutine names [not] matching pattern.

Toggle trace mode.

t expr
Trace through execution of expr. For example:

$ perl -de 42 Stack dump during die enabled outside of evals. Loading DB routines from 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 DB<4> q

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 only be set on lines that begin an executable statement. Conditions don't use if:

b 237 $x > 30 b 33 /pattern/i

b subname [condition]
Set a breakpoint at the first line of the named subroutine.

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.

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"

  • Delete all installed actions.

  • 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 ! .

    Program to use for output of pager-piped commands (those beginning with a | character.) By default, $ENV{PAGER} will be used.

    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.

    Whether to print contents of globs.

    Dump arrays holding debugged files.

    Dump symbol tables of packages.

    quote, HighBit, undefPrint
    Change style of string dump.

    Run Tk while prompting (with ReadLine).

    signalLevel, warnLevel. dieLevel
    Level of verbosity.

    The option PrintRet affects printing of return value after rcommand, The option frame affects printing messages on entry and exit from subroutines. If frame is 1, messages are printed on entry only; if it's set to more than that, they'll will be printed on exit as well, which may be useful if interdispersed with other messages.

    During startup options are initialized from $ENV{PERLDB_OPTS}. You can put additional initialization options TTY , noTTY , ReadLine , and NonStop there. Here's an example of using the $ENV{PERLDB_OPTS} variable:

    $ PERLDB_OPTS="N f=2" perl -d myprogram

    will run the script myprogram without human intervention, printing out the call tree with entry and exit points. Note that N f=2 is equivalent to NonStop=1 frame=2 . Note also that at the moment when this documentation was written all the options to the debugger could be uniquely abbreviated by the first letter.

    See ``Debugger Internals'' below for more details.

    < command
    Set an action to happen before every debugger prompt. A multiline command may be entered by backslashing the newlines.

    > command
    Set an action to happen after the prompt when you've just given a command to return to executing the script. A multiline 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.)

    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.

    Run debugger command, piping DB::OUT to current pager.

    Same as |dbcmd but DB::OUT is temporarily selected as well. Often used with commands that would otherwise produce long output, such as

    |V main

    = [alias value]
    Define a command alias, or list current aliases.

    Execute command as a Perl statement. A missing semicolon will be supplied.

    p expr
    Same as print DB::OUT expr. The DB::OUT filehandle is opened to /dev/tty, regardless of where STDOUT may be redirected to.

  • The debugger prompt is something like D8

    or even


    where that number is the command number, which you'd use to access with the built-in 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.

    If you want to enter a multi-line command, such as a subroutine definition with several statements, 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.

    Here's an example of what a stack backtrace might look like:

    $ = main::infested called from file `' 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, 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.

    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. 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.

    Debugger Customization

    If you want to modify the debugger, copy 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 "" }

    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\$/';

    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.

    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, just type:

    perl -d:DProf

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

    Debugger Internals

    When you call the caller function from package DB, Perl sets the @DB::args array to contain the arguments that stack frame was called with. It also maintains other magical internal variables, such as @DB::dbline, an array of the source code lines for the currently selected (with the debugger's f command) file. Perl effectively inserts a call to the function DB::DB(linenum) in front of every place that can have a breakpoint. Instead of a subroutine call it calls DB::sub setting $DB::sub being the called subroutine. It also inserts a BEGIN {require ''} before the first line.

    Note that no subroutine call is possible until &DB::sub is defined (for subroutines defined outside this file). In fact, the same is true if $DB::deep (how many levels of recursion deep into the debugger you are) is not defined.

    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.

    The following options can only be specified at startup. To set them in your rc file, call &parse_options(``optionName=new_value'').

    The TTY to use for debugging I/O.

    If set, goes in NonStop mode. On interrupt if TTY is not set uses the value of noTTY or ``/tmp/perldbtty$$'' to find TTY using Term::Rendezvous. Current variant is to have the name of TTY in this file.

    If false, dummy ReadLine is used, so you can debug ReadLine applications.

    If true, no I/O is performed until an interrupt.

    File or pipe to print line number info to. If it is a pipe, then a short, ``emacs like'' message is used.

    Example rc file:

    &parse_options("NonStop=1 LineInfo=db.out"); sub afterinit { $trace = 1; }

    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''!)

    Other resources

    You did try the -w switch, didn't you?


    If your program exit()s or die()s, so too does the debugger.

    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.