I would suggest that you don't want to use a circular file.  How
about
a flat file with each user assigned a record number?  If your
users
each have a connection of their own this could be the device
number.
If not the update program and the retrieval program could both
access
a file with user names; the position of the user name would
indicate
the record number.  Then both programs would use this file to
determine
the record number to update or retrieve.  Of course an Image data
base
with the user name as the key could accomplish the dame thing.

Hope this helps.

Nick D.