Semaphore vs. mutex

Semaphore vs. mutex

A mutex is essentially the same thing as a binary semaphore
the term "mutex" is used to describe a construct which prevents two processes from executing the same piece of code or accessing the same data, at the same time
The term "binary semaphore" is used to describe a construct which limits access to a single resource.

http://en.wikipedia.org/wiki/Semaphore_%28programming%29







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.

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

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.


"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)."


http://koti.mbnet.fi/niclasw/MutexSemaphore.html










a mutex is locking mechanism used to synchronize access to a resource. Only one task (can be a thread or process based on OS abstraction) can acquire the mutex. It means there will be ownership associated with mutex, and only the owner can release the lock (mutex).

Semaphore is signaling mechanism (“I am done, you can carry on” kind of signal). For example, if you are listening songs (assume it as one task) on your mobile and at the same time your friend called you, an interrupt will be triggered upon which an interrupt service routine (ISR) will signal the call processing task to wakeup.


http://www.geeksforgeeks.org/archives/9102