The Perl Debugger

The Perl debugger, a part of the core Perl distribution, is a useful tool to master, allowing close interactive examination of executing Perl code.
Stack Traces

Now that we've got the recursive call debugged, let's play with the calling stack a bit. Giving the command T will display the current calling stack. The calling stack is a list of the subroutines which have been called between the current point in execution and the beginning of execution. In other words, if the main portion of the code executes subroutine “a”, which in turn executes subroutine “b”, which calls “c”, then pressing “T” while in the middle of subroutine “c” outputs a list going from “c” all the way back to “main”.

Start up the program and enter the following commands (omit the second one if you have fixed the bug we discovered in the last section):

b 34 ( $_ =~ /file2$/)
a 34 $_ = "$dir/$_"

These commands set a breakpoint that will only stop execution if the value of the variable $_ ends with the string file2. Effectively, this code will halt execution at arbitrary points in the program. Press T and you'll get this:

@ = main::searchdir('./dir1.0/file2') called from file '../' line
@ = main::searchdir(.) called from file '../' line 10

Enter c, then T again:

@ = main::searchdir('./dir1.0/dir1.1/file2') called from file
`../' line 45
@ = main::searchdir(undef) called from file '../' line 45
@ = main::searchdir(.) called from file '../' line 10

Do it once more:

@ = main::searchdir('./dir2.0/file2') called from file '../' line
@ = main::searchdir(.) called from file '../' line 10

You can go on, if you so desire, but I think we have enough data from the arbitrary stack dumps we've taken.

We see here which subroutines were called, the debugger's best guess of which arguments were passed to the subroutine and which line of which file the subroutine was called from. Since the lines begin with @ = , we know that searchdir will return a list. If it were going to return a scalar value, we'd see $ =. For hashes (also known as associative arrays), we would see % =.

I say “best guess of what arguments were passed” because in Perl, the arguments to subroutines are placed into the @_ magic list. However, manipulating @_ (or $_) in the body of the subroutine is allowed and even encouraged. When a T is entered, the stack trace is printed out, and the current value of @_ is printed as the arguments to the subroutine. So when @_ is changed, the trace doesn't reflect what was actually passed as arguments to the subroutine.


Well, by now you must be thinking, “Gosh, this Perl debugger is so keen that with it I can end world hunger, learn to play the piano and increase my productivity by 300%!” Well, this is the right attitude. You are now displaying the third programmer's virtue, hubris. However, there are some warnings.

Race Conditions

Race conditions are the scourge of the programmer. Race conditions are bugs that occur only under certain circumstances. These circumstances usually involve the time at which certain events correlate with other events. Using the debugger to debug these situations is not always possible, because the act of using the debugger may change the timing of the events in the program. This can cause a symptom to occur without the debugger, but while using the debugger, the symptom may disappear. The bug isn't gone, it just isn't being “tickled”.

There really isn't any stock method to get rid of race conditions. Usually, an intense analysis of the algorithms is necessary. Finite-state diagrams may also be useful, if you have the patience for it.

Process Management, IPC

When writing code that involves more than one process (for example, if your code uses a “fork” system call or its equivalent), using the debugger becomes very difficult. This is because when the fork occurs, you are left with two (or more) processes, all running under the debugger. But since the debugger is interactive, you have to interact with every process. The result is that you have to individually deal with each process, controlling each execution. All the processes will want to read debug commands from the controlling terminal, but only one at a time will be able to do so. The other(s) will block, waiting for the first to complete. When it does, another process will complete. Incidentally, we can't know for sure which process will be first. This is an example of the above mentioned race condition.