ObtainSemaphoreShared

gain shared access to a semaphore (V36)

A lock on a signal semaphore may either be exclusive, or shared. Exclusive locks are granted by the ObtainSemaphore() and AttemptSemaphore() functions. Shared locks are granted by ObtainSemaphoreShared(). Calls may be nested.

Any number of Tasks may simultaneously hold a shared lock on a semaphore. Only one Task may hold an exclusive lock. A typical application is a list that is often read, but only occasionally written to.

Any exlusive locker will be held off until all shared lockers release the semaphore. Likewise, if an exlusive lock is held, all potential shared lockers will block until the exclusive lock is released. All shared lockers are restarted at the same time.

This call is guaranteed to preserve all registers, starting with Kickstart 2.04 (V37) exec.

While this function was added for Kickstart 2.0 (V36), the feature magically works with all older semaphore structures.

A Task owning a shared lock must not attempt to get an exclusive lock on the same semaphore.

Starting in Kickstart 3.0 (V39), if the caller already has an exclusive lock on the semaphore it will return with another nesting of the lock. Pre-V39 this would cause a deadlock. For pre-V39 use, you can use the following workaround:

signalSemaphore

an initialized signal semaphore structure

ObtainSemaphore(), InitSemaphore(), ReleaseSemaphore(), AttemptSemaphore(), ObtainSemaphoreList(), Procure(), Vacate()