Specifies a _read (shared) lock. A _read lock denies all processes write access to the specified region of the file, including the process that first locks the region. All processes can _read the locked region. Multiple simultaneous _read locks are allowed, as long as there are no exclusive locks.

Specifies a read/write (exclusive) lock. A read/write lock denies all other processes both read and write access to the locked file region. If a segment has an exclusive lock, it may not have any shared locks or other exclusive locks.