>I would like to write a unique id to the 17th - 20th positions in the header of free DBF's in order to track them as they are processed by our financial analysts. We will lose the key if the structure is ever modified, but otherwise it seems to work ok after some experimenting. There could be a mix of Fox 2X files in there. Is this feasible? What other concerns should I be aware of?
Fox2x dbf doesn't use that positions at all (12-31 reserved). But I don't think it as feasible. You would at least need exclusive access to be able to write there. There are plenty of spaces that you could write this info. Even a dummy file or another table. There are changes in header from FP2x to VFP and no one knows if those positions would be free forever. OTOH if it has a memo file than memo header has much more unused space. But in that case too you should guarantee that operations like SQL copy and delete old, modi stru etc wouldn't be done at all.
I would choose one that I could freely access with lowlevel.
Cetin
