Acquiring an Exclusive Lock While Owning a Shared Lock

When multiple sessions have acquired a shared lock, VISA allows one of the sessions to acquire an exclusive lock as well as the shared lock it is holding. That is, a session holding a shared lock can also acquire an exclusive lock using the viLock() operation. The session holding both the exclusive and shared lock has the same access privileges it had when it was holding only the shared lock. However, the exclusive lock precludes other sessions holding the shared lock from accessing the locked resource. When the session holding the exclusive lock unlocks the resource using the viUnlock() operation, all the sessions (including the one that acquired the exclusive lock) again have all the access privileges associated with the shared lock. This circumstance is useful when you need to synchronize multiple sessions holding a shared lock. A session holding an exclusive and shared lock can also be useful when one of the sessions needs to execute in a critical section.