gawk: Finding The Bug
14.2.2 Finding the Bug
----------------------
Let's say that we are having a problem using (a faulty version of)
'uniq.awk' in the "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 (
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!
Info Catalog
gawk: Debugger Invocation
gawk: Sample Debugging Session