awk bug

[Daniel R. Levy]

In article <3748 at megaron.arizona.edu>, rupley at arizona.edu (John Rupley) writes:
> In article <672 at pttesac.UUCP>, vanam at pttesac.UUCP (Marnix van Ammers) writes:
> > While trying to install a new program I ran across a bug in our Sys
> > V, release 2.1.1 (AT&T 3B20) awk.  In our awk the following pattern
> > always matches (even if there are 5 or less fields on the current
> > line):
> > if $6 != ""
> > This does not happen on the awk on my 3B1 version 3.51 .
> > Is this a known bug or what?
> Could it be a corrupt copy of awk on your release 2 system?
> The following code excutes properly with my SysV.r2 awk and
> with the new awk (your 3.51 version?):
> echo $* | awk '$6 != ""	{print "$6_!=_zerolength", NR, NF, $6}'
> echo $* | awk '{if ($6 != "")print "$6_!=_zerolength", NR, NF, $6}'

Alas, I must plead guilty (even though I'm not responsible for awk, I'm still
a Death-Starian) for awk's behavior in this manner on the 3B20 (we're running
2.0v3 here).  It's coming from a dereference of a null pointer (the string
"f{\0" is present beginning at location zero in a 3B20 process).  If Rupley
is using a VAX, on the other hand, everything will seem to be hunkey dorey
(location 0 in a VAX [System V UNIX] process contains a zero byte, which is
tantamount to a null string).

I would posit that, just as when programming in C, testing a field without
first knowing that it is valid (the field count is high enough) is poor
programming practice.  I will eat these words if someone can show me awk
documentation that says that an undefined positional parameter is guaranteed
to be null/0 just as an undefined member of an array or previously unused
variable is guaranteed to be.  (I've written many a line of awk code using
much the same care I would use with C, and never tripped over this problem.)
Barring such a guarantee, and certainly in the present situation, it is better
practice, given that one knows that there may be less than six positional
parameters in an input record, to use

	NF >= 6 { action using $6 }

than it is to use

	$6 != "" { action using $6 }

just as you would not blithely want to do (in C):

main(argc,argv)
char **argv;
{
	foo(argv[6]);	/* what if argc < 6 ? */
}
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