For read-only purposes, multiple tasks may have a shared lock on a signal
semaphore.  If a semaphore is already exclusively locked, all attempts to
obtain the semaphore shared will be blocked until the exclusive lock is
released.  At that point, all shared locks will be obtained and the
calling tasks will wake up.

To obtain a shared semaphore, use:

    struct SignalSemaphore *semaphore;
    ObtainSemaphoreShared(semaphore);

To obtain a public shared semaphore, the following code should be used:

    UBYTE *name;
    struct SignalSemaphore *semaphore;

    Forbid();
    if (semaphore = FindSemaphore(name))
        ObtainSemaphoreShared(semaphore);
    Permit();

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