Am i using semaphore wrong?

Question

I need to do some task in parallel using semaphore. I try this:

Semaphore sema = new Semaphore(2,2); Thread[] Threads = new Thread[5]; for (int k = 0; k < 5; k++) { sema.WaitOne(); Console.WriteLine((k + 1) + " started"); Threads[k] = new Thread(ThreadMethod1); Threads[k].Start(k + 1); sema.Release(); } static void ThreadMethod1(object id) { Thread.Sleep(50); Console.WriteLine(id + " completed"); }

Output looks like:

1 started 2 started 3 started 4 started 5 started 1 completed 2 completed 4 completed 3 completed 5 completed

Isn't semaphore supposed to let only 2 threads to run? I don't get it or doing something wrong?

Yes, but you are entering exiting the semaphore in the main thread in each cycle, so the semaphore will always be in non-blocking status. — xanatos
– xanatos, Commented Feb 24, 2015 at 9:55

xanatos · Accepted Answer · 2015-02-24 09:58:28Z

You are entering/exiting the semaphore in the "main" thread. It's useless, because in each "cycle" you'll both enter and exit it. In this modified example, you enter the semaphore in the main thread and upon finishing the worker thread you exit it.

Note that I had to pass the semaphore to the worker thread (I used a Tuple, but other methods are ok)

static void Main(string[] args) { Semaphore sema = new Semaphore(2, 2); Thread[] Threads = new Thread[5]; for (int k = 0; k < 5; k++) { sema.WaitOne(); Console.WriteLine((k + 1) + " started"); Threads[k] = new Thread(ThreadMethod1); Threads[k].Start(Tuple.Create(k + 1, sema)); } } static void ThreadMethod1(object tuple) { Tuple<int, Semaphore> tuple2 = (Tuple<int, Semaphore>)tuple; Thread.Sleep(50); Console.WriteLine(tuple2.Item1 + " completed"); tuple2.Item2.Release(); }

You could move the sema.WaitOne "inside" the ThreadMethod1, but it would be different: all the threads would be created but would "wait" and only 2 at a time would do the "real work". As written instead up to two threads are created (and do the work)

Łukasz Rejman · Accepted Answer · 2015-02-24 10:03:05Z

All you have to do is to move operations on the semaphore from main thread. Small correction to your code will solve it.

public static Semaphore sema = new Semaphore(2, 2); static void Main(string[] args) { Thread[] Threads = new Thread[5]; for (int k = 0; k < 5; k++) { Console.WriteLine((k + 1) + " started"); Threads[k] = new Thread(ThreadMethod1); Threads[k].Start(k + 1); } } static void ThreadMethod1(object id) { sema.WaitOne(); Thread.Sleep(1000); Console.WriteLine(id + " completed"); sema.Release(); }

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