SQLite Forum

Exclusive write-only lock? (Allow only one process read-write, and multiple read-only)
SQLite does not care which process/thread/connection is used for changing the contents of a database file.

You will need to enforce the "designated writer process only" constraint yourself, by setting SQLITE_OPEN_READWRITE only in the designated writer process/thread for the designated writer connection and using SQLITE_OPEN_READONLY everywhere else.

You will also have to handle remedial actions for the writer dieing (convert  reader or start a new writer) yourself, taking care that you do not end up with two writers accidentally.

And you still have to handle transactions properly in readers and the writer so that each has a chance to perform the required operations and access current data.