Mutex is short for mutual exclusion, and is closely related to the semaphore, to the point that it can be implemented using the same assembly routines. A mutex is nothing but a binary semaphore, being initialized with value of 1 to allow the first lock operation.

Due to the property of the semaphore, which would fail any attempt to decrement its counter after its value has reached 0, our quick implementation of the mutex interface renames the semaphore primitives sem_wait and sem_post to mutex_lock and mutex_unlock, respectively.

Two tasks can try to decrement an unlocked mutex at the same time, but only one succeeds; the other will fail. In the blocking version of the mutex for the example scheduler, the wrappers for the mutex API built on top of the semaphore functions are the following:

typedef semaphore mutex;
#define mutex_init(m) sem_init(m, 1)
#define mutex_trylock(m) sem_trywait(m)
#define mutex_lock(x) sem_wait(x)
#define mutex_unlock(x) sem_post(x)

For both semaphores and mutexes, the example operating system written so far offers a complete API for synchronization mechanisms integrated with the scheduler.