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Class `core.sync.semaphore.Semaphore`

This class represents a general counting semaphore as concieved by Edsger Dijkstra. As per Mesa type monitors however, "signal" has been replaced with "notify" to indicate that control is not transferred to the waiter when a notification is sent.


						
				class Semaphore
				;

Constructors

Name	Description
`this` (count)	Initializes a semaphore object with the specified initial count.

Fields

Name	Type	Description
`m_hndl`	`core.sys.posix.semaphore.sem_t`	Handle to the system-specific semaphore.

Methods

Name	Description
`notify` ()	Atomically increment the current count by one. This will notify one waiter, if there are any in the queue.
`tryWait` ()	If the current count is equal to zero, return. Otherwise, atomically decrement the count by one and return true.
`wait` ()	Wait until the current count is above zero, then atomically decrement the count by one and return.
`wait` (period)	Suspends the calling thread until the current count moves above zero or until the supplied time period has elapsed. If the count moves above zero in this interval, then atomically decrement the count by one and return true. Otherwise, return false.
`factory` (classname)	Create instance of class specified by the fully qualified name classname. The class must either have no constructors or have a default constructor.
`opCmp` (o)	Compare with another Object obj.
`opEquals` (o)	Test whether `this` is equal to `o`. The default implementation only compares by identity (using the `is` operator). Generally, overrides and overloads for `opEquals` should attempt to compare objects by their contents. A class will most likely want to add an overload that takes your specific type as the argument and does the content comparison. Then you can override this and forward it to your specific typed overload with a cast. Remember to check for `null` on the typed overload.
`toHash` ()	Compute hash function for Object.
`toString` ()	Convert Object to a human readable string.

Aliases

Name	Description
`Handle`	Aliases the operating-system-specific semaphore type.

Example

import core.thread, core.atomic;

void testWait()
{
    auto semaphore = new Semaphore;
    shared bool stopConsumption = false;
    immutable numToProduce = 20;
    immutable numConsumers = 10;
    shared size_t numConsumed;
    shared size_t numComplete;

    void consumer()
    {
        while (true)
        {
            semaphore.wait();

            if (atomicLoad(stopConsumption))
                break;
            atomicOp!"+="(numConsumed, 1);
        }
        atomicOp!"+="(numComplete, 1);
    }

    void producer()
    {
        assert(!semaphore.tryWait());

        foreach (_; 0 .. numToProduce)
            semaphore.notify();

        // wait until all items are consumed
        while (atomicLoad(numConsumed) != numToProduce)
            Thread.yield();

        // mark consumption as finished
        atomicStore(stopConsumption, true);

        // wake all consumers
        foreach (_; 0 .. numConsumers)
            semaphore.notify();

        // wait until all consumers completed
        while (atomicLoad(numComplete) != numConsumers)
            Thread.yield();

        assert(!semaphore.tryWait());
        semaphore.notify();
        assert(semaphore.tryWait());
        assert(!semaphore.tryWait());
    }

    auto group = new ThreadGroup;

    for ( int i = 0; i < numConsumers; ++i )
        group.create(&consumer);
    group.create(&producer);
    group.joinAll();
}


void testWaitTimeout()
{
    auto sem = new Semaphore;
    shared bool semReady;
    bool alertedOne, alertedTwo;

    void waiter()
    {
        while (!atomicLoad(semReady))
            Thread.yield();
        alertedOne = sem.wait(dur!"msecs"(1));
        alertedTwo = sem.wait(dur!"msecs"(1));
        assert(alertedOne && !alertedTwo);
    }

    auto thread = new Thread(&waiter);
    thread.start();

    sem.notify();
    atomicStore(semReady, true);
    thread.join();
    assert(alertedOne && !alertedTwo);
}

testWait();
testWaitTimeout();

Authors

Sean Kelly

License

Boost License 1.0