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November 2, 1999

The Windowing patent

Software patents strike again, and this time Linux could be directly affected.

Have a look at this News.com article and weep. It seems that McDonnell Douglas has been awarded a patent on a technique called "windowing" which is used for dealing with year 2000 problems. The essence of this highly advanced and obscure algorithm, which converts two-digit years to four digits, is:

  • If the two-digit year is less than a certain value (30, say), assume that it is relative to the year 2000. Thus "01" becomes "2001."

  • Otherwise, assume that it is in the 1900's - "77" becomes "1977."

This patent was filed on October 3, 96 (oops, add 1900: 1996); clearly nobody had ever thought of this approach before.

Thus, we have what simply looks like yet another stupid software patent story. But there is reason for concern here. Consider the following snippit of code from the Intel-specific portion of the Linux kernel source:

	if ((year += 1900) < 1970)
		year += 100;
This comes from arch/i386/kernel/time.c, and deals with getting date values from the real-time clock. This complicated bit of code shows "windowing" in action - the Linux kernel is in violation of this patent.

According to the News.com article, McDonnell Douglas is actively trying to shake down companies using the windowing technique. There is no word yet that they have approached any Linux companies. Now that Linux-oriented companies are frequently found in the news, however, it seems likely that one may draw a lawyer's attention sooner rather than later.

This patent will almost certainly be overturned by demonstration of prior art; see, for example, time.c from kernel 1.2.13, courtesy of the Linux Cross Reference site. The Linux kernel was using windowing in early 1995, if not before. One of the beautiful things about free software is that it makes finding (and documenting) prior art a relatively easy thing to do. (Even so, overturning the patent is not necessarily easy, and the cause is not helped by drawing the attention of the US Patent Office to the problem at this time - please do not contact them about this patent).

But even if this patent falls, it points out yet again the threat posed by software patents. Not very many Linux companies can afford to fight companies like McDonnell Douglas every time one of these silly things comes up. And more of them are coming up all the time. The software patent problem needs to be dealt with, and soon.

(Thanks to Greg Maxwell, who pointed out the offending code on the linux-kernel mailing list. Thanks also to Joey Wetherington who points out that the patent covers the Y2K bug, which maps two-digit years into the 1900-1999 range...).

Update: a few people have pointed out that McDonnell Douglas does not actually own the patent in question - it assigned the patent to the employee Bruce Dickens; it is Mr. Dickens who is pursuing the patent claims against other companies. We regret the error.


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