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ObtainSemaphoreShared.md

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NAME

ObtainSemaphoreShared -- gain shared access to a semaphore (V36)

SYNOPSIS

    ObtainSemaphoreShared(signalSemaphore)
                          a0

    void ObtainSemaphoreShared(struct SignalSemaphore *);

Links: SignalSemaphore

FUNCTION

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.

EXAMPLE ObtainSemaphoreShared(ss); /* read data */ ReleaseSemaohore(ss);

ObtainSemaphore(ss); /* modify data */ ReleaseSemaohore(ss);

NOTES While this function was added for 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.

INPUT signalSemaphore -- an initialized signal semaphore structure

NOTE This call is guaranteed to preserve all registers, starting with V37 exec.

RESULT

SEE ALSO

InitSemaphore, ReleaseSemaphore