14.2.2 Finding the Bug

Let’s say that we are having a problem using (a faulty version of) uniq.awk in “field-skipping” mode, and it doesn’t seem to be catching lines which should be identical when skipping the first field, such as:

awk is a wonderful program!
gawk is a wonderful program!

This could happen if we were thinking (C-like) of the fields in a record as being numbered in a zero-based fashion, so instead of the lines:

clast = join(alast, fcount+1, n)
cline = join(aline, fcount+1, m)

we wrote:

clast = join(alast, fcount, n)
cline = join(aline, fcount, m)

The first thing we usually want to do when trying to investigate a problem like this is to put a breakpoint in the program so that we can watch it at work and catch what it is doing wrong. A reasonable spot for a breakpoint in uniq.awk is at the beginning of the function are_equal(), which compares the current line with the previous one. To set the breakpoint, use the b (breakpoint) command:

gawk> b are_equal
-| Breakpoint 1 set at file `awklib/eg/prog/uniq.awk', line 63

The debugger tells us the file and line number where the breakpoint is. Now type ‘r’ or ‘run’ and the program runs until it hits the breakpoint for the first time:

gawk> r
-| Starting program:
-| Stopping in Rule ...
-| Breakpoint 1, are_equal(n, m, clast, cline, alast, aline)
         at `awklib/eg/prog/uniq.awk':63
-| 63          if (fcount == 0 && charcount == 0)
gawk>

Now we can look at what’s going on inside our program. First of all, let’s see how we got to where we are. At the prompt, we type ‘bt’ (short for “backtrace”), and the debugger responds with a listing of the current stack frames:

gawk> bt
-| #0  are_equal(n, m, clast, cline, alast, aline)
         at `awklib/eg/prog/uniq.awk':68
-| #1  in main() at `awklib/eg/prog/uniq.awk':88

This tells us that are_equal() was called by the main program at line 88 of uniq.awk. (This is not a big surprise, because this is the only call to are_equal() in the program, but in more complex programs, knowing who called a function and with what parameters can be the key to finding the source of the problem.)

Now that we’re in are_equal(), we can start looking at the values of some variables. Let’s say we type ‘p n’ (p is short for “print”). We would expect to see the value of n, a parameter to are_equal(). Actually, the debugger gives us:

gawk> p n
-| n = untyped variable

In this case, n is an uninitialized local variable, because the function was called without arguments (see section Function Calls).

A more useful variable to display might be the current record:

gawk> p $0
-| $0 = "gawk is a wonderful program!"

This might be a bit puzzling at first, as this is the second line of our test input. Let’s look at NR:

gawk> p NR
-| NR = 2

So we can see that are_equal() was only called for the second record of the file. Of course, this is because our program contains a rule for ‘NR == 1’:

NR == 1 {
    last = $0
    next
}

OK, let’s just check that that rule worked correctly:

gawk> p last
-| last = "awk is a wonderful program!"

Everything we have done so far has verified that the program has worked as planned, up to and including the call to are_equal(), so the problem must be inside this function. To investigate further, we must begin “stepping through” the lines of are_equal(). We start by typing ‘n’ (for “next”):

gawk> n
-| 66          if (fcount > 0) {

This tells us that gawk is now ready to execute line 66, which decides whether to give the lines the special “field-skipping” treatment indicated by the -1 command-line option. (Notice that we skipped from where we were before, at line 63, to here, because the condition in line 63, ‘if (fcount == 0 && charcount == 0)’, was false.)

Continuing to step, we now get to the splitting of the current and last records:

gawk> n
-| 67              n = split(last, alast)
gawk> n
-| 68              m = split($0, aline)

At this point, we should be curious to see what our records were split into, so we try to look:

gawk> p n m alast aline
-| n = 5
-| m = untyped variable
-| alast = array, 5 elements
-| aline = untyped variable

(The p command can take more than one argument, similar to awk’s print statement.)

This is kind of disappointing, though. All we found out is that there are five elements in alast; m and aline don’t have values because we are at line 68 but haven’t executed it yet. This information is useful enough (we now know that none of the words were accidentally left out), but what if we want to see inside the array?

The first choice would be to use subscripts:

gawk> p alast[0]
-| "0" not in array `alast'

Oops!

gawk> p alast[1]
-| alast["1"] = "awk"

This would be kind of slow for a 100-member array, though, so gawk provides a shortcut (reminiscent of another language not to be mentioned):

gawk> p @alast
-| alast["1"] = "awk"
-| alast["2"] = "is"
-| alast["3"] = "a"
-| alast["4"] = "wonderful"
-| alast["5"] = "program!"

It looks like we got this far OK. Let’s take another step or two:

gawk> n
-| 69              clast = join(alast, fcount, n)
gawk> n
-| 70              cline = join(aline, fcount, m)

Well, here we are at our error (sorry to spoil the suspense). What we had in mind was to join the fields starting from the second one to make the virtual record to compare, and if the first field were numbered zero, this would work. Let’s look at what we’ve got:

gawk> p cline clast
-| cline = "gawk is a wonderful program!"
-| clast = "awk is a wonderful program!"

Hey, those look pretty familiar! They’re just our original, unaltered input records. A little thinking (the human brain is still the best debugging tool), and we realize that we were off by one!

We get out of the debugger:

gawk> q
-| The program is running. Exit anyway (y/n)? y

Then we get into an editor:

clast = join(alast, fcount+1, n)
cline = join(aline, fcount+1, m)

and problem solved!