core.sync.semaphore

class 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.

Examples:

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();

this(uint count = 0);

Initializes a semaphore object with the specified initial count.

Parameters:

uint count

The initial count for the semaphore.

Throws:

SyncError on error.

void wait();

Wait until the current count is above zero, then atomically decrement the count by one and return.

Throws:

SyncError on error.

bool wait(Duration 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.

Parameters:

Duration period

The time to wait.

In period must be non-negative.

Throws:

SyncError on error.

Returns:

true if notified before the timeout and false if not.

void notify();

Atomically increment the current count by one. This will notify one waiter, if there are any in the queue.

Throws:

SyncError on error.

bool tryWait();

If the current count is equal to zero, return. Otherwise, atomically decrement the count by one and return true.

Throws:

SyncError on error.

Returns:

true if the count was above zero and false if not.

protected alias Handle = core.sys.posix.semaphore.sem_t;

Aliases the operating-system-specific semaphore type.

protected Handle m_hndl;

Handle to the system-specific semaphore.