a semaphore question

[dullboy]

Semaphore is usually used in guiding the access of, for example, a pool of connections. Now suppose we have 10 threads and 3 connections( they might be database connections or socket connections). In this situation, semaphore is a good candidate for protecting the access of connections. My question is how semaphore could prevent two threads to access the SAME connection? Say connection A, B, C. Thread 1 locks semaphore to access A, how could semaphore prevent thread 2 accessing A?

[joscollin]

Hi,

You must start both threads and do wait thread2 while thread1 is accessing A. When it is finished accessing A release the waiting semaphore for thread2.

[Andreas Masur]

[Moved thread]

[Andreas Masur]

Synchronization (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/base/synchronization.asp)

[dimm_coder]

Originally posted by dullboy
Say connection A, B, C. Thread 1 locks semaphore to access A, how could semaphore prevent thread 2 accessing A?

U should protect each connection with a mutex (this is an object as a semaphore but with a counter = 1). So, before working with some object (describing a connection in your case), any thread should lock a mutex for that object to gain an exclusive access.

For getting more details follow the link pointed out by Andreas or read another docs, describing synchronization aspects.

[dullboy]

Originally posted by dimm_coder
U should protect each connection with a mutex (this is an object as a semaphore but with a counter = 1). So, before working with some object (describing a connection in your case), any thread should lock a mutex for that object to gain an exclusive access.

For getting more details follow the link pointed out by Andreas or read another docs, describing synchronization aspects.
Probably I didn't state my question well. Actually, I don't have problem with synchronization in general, but I do have problem with semaphore. Back to my case, semaphore may allow only at most 3 threads to access the pool of connections, but I just couldn't see how semaphore prevents two threads of them accessing the same connection. So do you mean if I use semaphore, I have to use mutex or critical section in the mean time?

[Kheun]

Quite right. If you are limiting only a thread to access a shared resource at any one time, you can use either a mutex or critical section.