Difference between mutex and semaphore

[ravind007]

Hi,
I want to the differences between mutex and semaphore.
As i know differences are,
1.Mutex needs to be initialized but semaphore need not be.
2.semaphores will be alive even if the process dies where as mutex dies with the scope where it was declared.
3. mutex can have recurcive lock but semaphores cant.

are these valid ? if not what are the actual differences

[xerophyte80]

Mutex:

Is a key to a toilet. One person can have the key - occupy the toilet - at the time. When finished, the person gives (frees) the key to the next person in the queue.

Officially: "Mutexes are typically used to serialise access to a section of re-entrant code that cannot be executed concurrently by more than one thread. A mutex object only allows one thread into a controlled section, forcing other threads which attempt to gain access to that section to wait until the first thread has exited from that section."
Ref: Symbian Developer Library

(A mutex is really a semaphore with value 1.)
emaphore:

Is the number of free identical toilet keys. Example, say we have four toilets with identical locks and keys. The semaphore count - the count of keys - is set to 4 at beginning (all four toilets are free), then the count value is decremented as people are coming in. If all toilets are full, ie. there are no free keys left, the semaphore count is 0. Now, when eq. one person leaves the toilet, semaphore is increased to 1 (one free key), and given to the next person in the queue.

Officially: "A semaphore restricts the number of simultaneous users of a shared resource up to a maximum number. Threads can request access to the resource (decrementing the semaphore), and can signal that they have finished using the resource (incrementing the semaphore)."
Ref: Symbian Developer Library

[Freston]

I think Verity Stob explains this quite well.

[Rubberman]

I think xerophyte80 has made a very concise explanation of the differences between a mutex and semaphore. Excellent!

[Cabhan]

xerophyte, I must award you my kudos. That's possibly the best explanation I've ever seen of the mutex/semaphore difference.

[elija]

I didn't even know I was looking for an answer to this, but what an answer I found. xerophyte, are you by any chance a teacher?