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Alternative Debuggers - Perl

In this second part of a two-part series on debugging Perl, we will go beyond the author's favorite debugger and look at a number of alternative Perl debuggers. This article is excerpted from chapter four of the book Mastering Perl, written by Brian D Foy (O'Reilly; ISBN: 0596527241). Copyright © 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

  1. Perl Debuggers in Detail
  2. perl5db.pl
  3. Alternative Debuggers
  4. Devel::ebug
  5. Other Debuggers
  6. Summary
By: O'Reilly Media
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July 31, 2008

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Besides the standard perl5db.pl, there are several other sorts of debuggers that I can use, and there are several code analysis tools which use the debugging infrastructure. There’s a long list of Devel:: modules on CPAN, and one of them probably suits your needs.

Using a Different Debugger with -D

I can use an alternative debugger by giving the -d switch an argument. In this case, I want to run my program under the Devel::DProf module. The -d switch implies the Devel::, so I leave that off. I’ll cover profilers in depth in Chapter 5.

  $ perl -d:DProf program.pl

If I write my own debugging module, I can pass arguments to the module just like I can with the -M switch. I add the arguments as a comma-separated list after the module name and an equal sign. In this example, I load the Devel::MyDebugger with the arguments foo and bar:

  $ perl -d:MyDebugger=foo,bar

As normal Perl code, this is the same as loading Devel::MyDebugger with use.

  use Devel::MyDebugger qw( foo bar );


I can use a Tk-based debugger that provides a graphical interface to the same features I have from perl5db.pl. The Devel::ptkdb module does not come with Perl, so I have to install it myself.* I start ptkdb by specifying it as the debugger I want to use with the -d switch:

  $ perl -d:ptkdb program.pl

Figure 4-1.  The Devel::ptkdb provides a graphical debugger using Tk

It starts by creating an application window. In the left pane, I see the program lines around the current line, along with their line numbers (Figure 4-1). Buttons along the code pane allow me to search through the code. In the right pane, I have tabs to examine expressions, subroutines, and the list of current breakpoints.

The “Subs” tab gives me a hierarchal list of package names and the subroutines defined in them (Figure 4-2). These are all of the loaded modules, and I can immediately display the code for any of those functions by selecting the one I want to see. I can select one either by double-clicking or navigating with the arrow keys and hitting <RETURN> when I get to the one I want. It doesn’t change the state of my program, and I can use the “Subs” tab to decide to step into a subroutine to watch its execution, or step over it and continue with the execution of the program.

The “Exprs” tab is especially useful. It has two text entries at the top. “Quick Expr” allows me to enter a Perl expression, which it then replaces with its result, and affects the state of the program if my quick expression sets or changes variables. This is the equivalent of trying a one-off expression in the terminal debugger. That’s nice, but the “Enter Expr” is even better. I enter a Perl expression and it adds it to the list of expressions in the pane below the tabs (Figure 4-3). As I run my code, these expressions update their results based on the current state of the program. I can add the variables I want to track, for instance, and watch their values update.

I start with a simple program where I want to add two numbers. It’s not something that I need to debug (I hope), but I can use it to show the expressions tab doing its thing. At the start of the program, I’m at the start of the program and nothing has run yet. I single-step over the first line of code and can see the values for $m and $n, which I had previously entered as expressions. I could enter much more complex expressions, too, and ptkdb will update them as I move through the code.

Figure 4-2.  In the Subs tab, I can see the subroutine in any loaded package

Figure 4-3.  I can track variable values in the Exprs tab 

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