what is the diff b/w MUTEX and semaphore?
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Answer Posted / krishna
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.)
Semaphore:
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
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