Debugging Perl Code with DDD 423

Debugging Perl Code with DDD

If you’ve written many Perl programs, you’re probably familiar with Perl’s source-level debugger. To run a program under the control of the Perl debugger, use the -d option, like this:

$ perl -d program.pl

After a bit of clicking and whirring, the debugger appears, waiting for commands. If you type h and press Enter, you’re greeted with a helpful page of intimidation, as shown in Listing 55-1.

Don’t panic; there’s an easier way to debug Perl. Even if you’re a hardened Perl veteran (and you have the mental scars to prove it), you’ll love DDD, the data display debugger. DDD is a graphical front end for a number of different debuggers (GDB, DBX, the Perl debugger, and even the Python debugger).

If the DDD RPM package isn’t included on your distribution media, you can download a copy of the RPM package from www.gnu.org/ software/ddd.

Examining the main window

DDD divides the main window into three panes, stacked vertically, as shown in Figure 55-1:

The top pane, named the data window, displays variable values. (The data window is empty at first, but we show you how to put stuff there in a moment.)

The middle pane, named the source window, shows the code that you’re executing. A big green arrow points to the line of code that’s about to execute.

The bottom pane, the Perl console, is where you type in debugger commands.

Installing and starting DDD

Source window

DDD is included on most distribution media, so get-

Data window

ting it up and running is simple. To install and start the DDD debugger, follow these steps:

1. Open a terminal window and give yourself superuser privileges with the su - command.

2. Insert and mount the distribution media.

3. Move to the directory containing the RPM packages and enter the following command:

# rpm -Uhv ddd-version.rpm

4. To run a Perl program under the control of the

DDD debugger, use this command: $ ddd --perl /usr/bin/xql.pl

DDD opens, displaying a Tip of the Day and an

information window in front of the main window.

Perl console

• Figure 55-1: The DDD main window.

424 Technique 55: Using the DDD Graphical Debugger with Perl

LISTING 55-1: PERL DEBUGGER COMMANDS

List/search source		lines:		Control script execution:
l [ln|sub]	List	source	code	T	Stack trace
- or .	List	previous/current line		s [expr]	Single step [in expr]
v [line]	View	around	line	n [expr]	Next, steps over subs
f filename	View	source	in file	<CR/Enter>	Repeat last n or s
/pattern/ ?patt?		Search forw/backw		r	Return from subroutine
M	Show	module	versions	c [ln|sub]	Continue until position
Debugger controls:				L	List break/watch/actions
o [...]	Set debugger options			t [expr]	Toggle trace [trace expr]
<[<]|{[{]|>[>] [cmd] Do			pre/post-prompt b [ln|event|sub] [cnd] Set breakpoint
! [N|pat]	Redo	a previous command		B ln|*	Delete a/all breakpoints
H [-num]	Display last num commands			a [ln] cmd	Do cmd before line
= [a val]	Define/list		an alias	A ln|*	Delete a/all actions
h [db_cmd]	Get help on		command	w expr	Add a watch expression
h h	Complete help page			W expr|*	Delete a/all watch exprs
|[|]db_cmd	Send	output	to pager	![!] syscmd	Run cmd in a subprocess
q or ^D	Quit			R	Attempt a restart

Data Examination:			expr	Execute perl		code, also see: s,n,t expr
x|m expr		Evals expr in list context,				dumps the result or lists methods.
p	expr	Print expression			(uses script’s current package).
S	[[!]pat]	List	subroutine names [not]			matching pattern
V	[Pk [Vars]]	List	Variables in Package.			Vars can be ~pattern or !pattern.
X	[Vars]	Same	as “V current_package			[Vars]”.
y	[n [Vars]]	List	lexicals in		higher scope <n>. Vars same as V.
For	more help, type h		cmd_letter,		or run man	perldebug for all docs.

Reviewing and stepping through source code

If displaying your code was all that DDD could do, it would be marginally useful, but not much of a timesaver. But find a Perl program that you want to dance through, and we’ll show a few of DDD’s more useful features.

In the following example, we use a script that comes with the kdeaddons RPM package, but you can use DDD to debug any Perl program.

First, fire up the debugger:

$ cd /usr/share/apps/knewsticker/scripts $ ddd --perl bbc.pl

• Figure 55-2: bbc.pl, as viewed from DDD.

The DDD debugger window opens, displaying bbc.pl (see Figure 55-2).