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Operating Systems: Virtualization, Concurrency & Persistence

Binary Semaphores (Locks)

Explore how binary semaphores act as locks to control access to critical sections in concurrent programming. Understand the role of semaphore values and thread states in synchronizing multiple threads, helping you manage concurrency effectively.

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We are now ready to use a semaphore. Our first use will be one with which we are already familiar: using a semaphore as a lock. Look at the code snippet below.

C
sem_t m;
sem_init(&m, 0, X); // initialize to X; what should X be?
sem_wait(&m);
// critical section here
sem_post(&m);

Therein, you can see that we simply surround the critical section of interest with a sem_wait()/sem_post() pair. Critical to making this work, though, is the initial value of the semaphore m (initialized to X in the figure). What should X be?

… (Try thinking about it before going on) …

Looking back at the definition of the sem_wait() and sem_post() routines above, we can see that the ...