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.

There is one more variation of the list code command, l. It is the ability to list the code of a subroutine, by typing l sub, where sub is the subroutine name.

Running the code in Listing 2 returns:

Loading DB routines from version 1
Emacs support available.
Enter h or h h for help.
main::(./ require 5.001;

Entering l searchdir allows us to see the text of searchdir, which is the meat of this program.

22 sub searchdir { # takes directory as argument
23: my($dir) = @_;
24: my(@files, @subdirs);
26: opendir(DIR,$dir) or die "Can't open \"
27:     $dir\" for reading: $!\n";
29: while(defined($_ = readdir(DIR))) {
30: /^\./ and next; # if file begins with '.', skip
32 ### SUBTLE HINT ###
As you can see, I left a subtle hint. The bug is that I deleted an important line at this point.

Setting Breakpoints

If we were to step through every line of code in a subroutine that is supposed to be recursive, it would take all day. As I mentioned before, the code as in Listing 2 seems only to list the files in the current directory, and it ignores the files in any subdirectories. Since the code only prints the files in the current, initial directory, maybe the recursive calls aren't working. Invoke the Listing 2 code under the debugger.

Now, set a breakpoint. A breakpoint is a way to tell the debugger that we want normal execution of the program until it gets to a specific point in the code. To specify where the debugger should stop, we insert a breakpoint. In the Perl debugger, there there are two basic ways to insert a breakpoint. The first is by line number, with the syntax b linenum. If linenum is omitted, the breakpoint is inserted at the next line about to be executed. However, we can also specify breakpoints by subroutine, by typing b sub, where sub is the subroutine name. Both forms of breakpointing take an optional second argument, a Perl conditional. If when the flow of execution reached the breakpoint the conditional evaluates to true, the debugger will stop at the breakpoint; otherwise, it will continue. This gives greater control of execution.

For now we'll set a break at the searchdir subroutine with b searchdir. Once the breakpoint is set, we'll just execute until we hit the subroutine. To do this, enter c (for continue).

Adding Actions

Looking at the code in Listing 2, we can see that the first call to searchdir comes in the main code. This seems to works fine, or else nothing would be printed out. Press c again to continue to the next invocation of searchdir, which occurs in the searchdir routine.

We wish to know what is in the $dir variable, which represents the directory that will be searched for files and subdirectories. Specifically, we want to know the contents of this variable each time we cycle through the code. We can do this by setting an action. By looking at the program listing, we see that by line 25, the variable $dir has been assigned. So, set an action at line 25 in this way:

a 25 print "dir is $dir\n"

Now, whenever line 25 comes around, the print command will be executed. Note that for the a command, the line number is optional and defaults to the next line to be executed.

Pressing c will execute the code until we come across a breakpoint, executing action points that are set along the way. In our example, pressing c continuously will yield the following:

main::(../ require 5.001;
 DB<1> b searchdir
 DB<2> a 25 print "dir is $dir\n"
 DB<3> c
main::searchdir(../ my($dir) = @_;
 DB<3> c
dir is .
main::searchdir(../ my($dir) = @_;
 DB<3> c
dir is dir1.0
main::searchdir(../ my($dir) = @_;
 DB<3> c
dir is dir2.0
main::searchdir(../ my($dir) = @_;
 DB<3> c
dir is dir3.0
2043: "Debugged program terminated. Use `q' to quit or `R' to

Note that older versions of the debugger don't output the last line as listed here, but instead exit the debugger. This newer version is nice because when the program has finished it still lets you have control so that you can restart the program.

It still seems that we aren't getting into any subdirectories. Enter D and A to clear all breakpoints and actions, respectively, and enter R to restart. Or, in older debugger versions, simply restart the program to begin again.

We now know that the searchdir subroutine isn't being called for any subdirectories except the first level ones. Looking back at the text of the program, notice in lines 44 through 46 that the only time the searchdir subroutine is called recursively is when there is something in the @subdirs list. Put an action at line 42 that will print the $dir and @subdirs variables by entering:

a 42 print "in $dir is @subdirs \n"

Now, put a breakpoint at line 12 to prevent the program from outputting to our screen (b 12), then enter c. This will tell us all the subdirectories that our program thinks are in the directory.

main::(../ require 5.001;
 DB<1> a 42 print "in $dir is @subdirs \n"
 DB<2> b 12
 DB<3> c
in . is dir1.0 dir2.0 dir3.0
in dir1.0 is
in dir2.0 is
in dir3.0 is
main::(../ foreach (@files) {
This program sees that there are directories in “.”, but not in any of the subdirectories within “.”. Since we are printing out the value of @subdirs at line 42, we know that @subdirs has no elements in it. (Notice that when listing line 42, there is the letter “a” after the line number and a colon. This tells us that there is an action point here.) So, nothing is being assigned to @subdirs in line 37, but should be if the current (as held in $_) file is a directory. If it is, it should be pushed into the @subdirs list. This is not happening.

One error I've committed (intentionally, of course) is on line 38. There is no catch-all “else” statement. I should probably put an error statement here. Instead of doing this, let's put in another action point. Reinitialize the program so that all points are cleared and enter the following:

a 34 if( ! -f $_ and ! -d $_ ) { print "in $dir: $_ is
weird!\n" }
b 12"

which reveals:

main::(../ require 5.001;
 DB<1> a 34 if( ! -f $_ and ! -d $_ ) { print "in $dir:
$_ is weird!\n" }
 DB<2> b 12
 DB<3> c
in dir1.0: dir1.1 is weird!
in dir1.0: dir2.1 is weird!
in dir1.0: file2 is weird!
in dir1.0: file3 is weird!
in dir2.0: dir2.1 is weird!
in dir2.0: dir1.1 is weird!
in dir2.0: file2 is weird!
in dir2.0: file3 is weird!
main::(../ foreach (@files) {
While the program can read (through the readdir call on line 29) that dir1.1 is a file of some type in dir1.0, the file test (the -f construct) on dir1.1 says that it is not.

It would be nice to halt the execution at a point (line 34) where we have a problem. We can use the conditional breakpoint that I mentioned earlier to do this. Reinitialize or restart the debugger, and enter:

b 34 ( ! -f $_ and ! -d $_ )
p $dir

You'll get output that looks like this:

main::(../ require 5.001;
 DB<1> b 34 ( ! -f $_ and ! -d $_ )
 DB<2> c
main::searchdir(../ if( -f $_) { # if its a file...
 DB<2> p
 DB<2> p $dir
The first line sets the breakpoint, the next c executes the program until the break point stops it. The p prints the contents of the variable $_ and the last command, p $dir prints out $dir. So, dir1.1 is a file in dir1.0, but the file tests (-d and -f) don't admit that it exists, and therefore dir1.1 is not being inserted into @subdirs (if it's a directory) or into @files (if it's a file).

Now that we are back at a prompt, we could inspect all sorts of variables, subroutines or any other Perl construct. To save you from banging your heads against your monitors, and thus saving both your heads and your monitors, I'll tell you what is wrong.

All programs have something known as the current working directory (CWD). By default, the CWD is the directory where the program starts. Any and all file accesses (such as file tests or file and directory openings) are made in reference from the CWD. At no time does our program change its CWD. But the values returned by the readdir call on line 29 are simply file names relative to the directory that readdir is reading (which is in $dir). So, when we do the readdir, $_ gets assigned a string representing a file (or directory) within the directory in $dir (which is why it's called a subdirectory). But when running the -f and -d file tests, they look for $_ in the context of the CWD. But it isn't in the CWD, it's in the directory represented by $dir. The moral of the story is that we should be working with $dir/$_, not just $_. So the string


should be replaced by

$_ = "$dir/$_"; # make all path names absolute
That sums it up. Our problem was we were dealing with relative paths, not absolute (from the CWD) paths.

Putting it back into our example, we need to check dir1.0/dir1.1, not dir1.1. To check to make sure that this is what we want, we can put in another action point. Try typing:

a 34 $_ = "$dir/$_"

In effect this temporarily places the corrective measure into our code. Action points are the first item on the line to be evaluated. You should now see the proper results of the execution of the program:

DB<1> a 34 $_ = "$dir/$_"
DB<2> c
2043: "Debugged program terminated. Use `q' to quit or `R' to