Feb 16, 2011

Mutex vs Semaphores

Key Points

1. Only the thread which acquires the mutex can release the mutex successfully.
2. A binary semaphore (semaphore with a count of 1) is similar to mutex but allows other threads to release it.
A thread which owns a mutex can acquire the same mutex recursively without blocking it's execution. This prevents the thread from deadlocking itself while waiting for the mutex it already owns. To release it's ownership under such circumstances, the thread must release the mutex the same number of times it acquired it.

A semaphore is a synchronization object which maintains a count between zero and a maximum value. The count is decremented each time a thread acquires a semaphore and incremented when a thread releases the semaphore. When the semaphore count reaches 0, the thread attempting to acquire the semaphore will be blocked unless some other thread increments it. Thus, a semaphore is useful in limiting the number of threads sharing a resource.

References:
1. Semaphore Objects
2. Mutex Objects
3. Mutex Vs Semaphores - I
4. Mutex Vs Semaphores - II
5. Mutex Vs Semaphores - III
6. Multithreading in C

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