Hi Kenji,
What a great question! I have encountered problems like this with lots of scanners when repeatedly opening and closing the DS, and have long suspected that there must be a very longstanding memory-management bug either in the DSM or in Windows. When I first raised this a few years ago, the general perception seemed to be that no-one sane ever opened and closed their DS more than a handful of times in a session, so my observation was purely of academic interest – and the TWAIN guys assured me that the DSM was ancient, venerable and watertight code anyway …
Furthermore, I write my apps in FORTRAN, so there is always the suspicion that some cross-compiler issue is to blame. You are the first person I have come across to have found and reported the problem, coding directly in the same language as specified in the TWAIN standard.
It’s particularly interesting that you get the issue with a Fujitsu scanner. I only wish the budgets I get given to play with came anywhere near being able to afford one. The scanners I work with are cheap and cheerful, so my experience can always be discounted on grounds of shoddily-written scanner drivers if not on grounds of mixed-language programming.
Good luck with solving the mystery. You will need it.
Andy