Operating System (CS2701)

semaphore

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Dr. Rourab Paul

Computer Science Department, SNU University, Chennai

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Operating System

Semaphore

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A semaphore is a synchronization mechanism used in operating systems and concurrent programming to control access to a shared resource by multiple threads or processes using simple integer value.

• A counter (integer) stored by the OS.
• A semaphore is an integer whose value is never allowed to fall below zero.
• This semaphore value can be access only by two atomic operations:
• wait() / P() / sem_wait() → decrements the counter; if counter < 0, the calling thread blocks until it becomes ≥0 again. P for ‘proberen’, means ‘test’
• signal() / V() / sem_post() → increments the counter; if any threads are waiting, one is unblocked. V for verhogen, means ‘increment’

Edsger W. Dijkstra

Semaphore

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Mutex = “I’m using this resource — wait until I’m done.”

Signal/Semaphore = “Hey, I’ve finished something — what you are saying

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line is

busy

Semaphore

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Mutex = “I’m using this resource — wait until I’m done.”

Signal/Semaphore = “Hey, I’ve finished something — you can sta

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bye

Definitions of wait and signal(post)

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wait(S) {

while (S <= 0)

; // busy

wait S--;

}

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signal(S) {

S++;

}

All the modifications to the integer value of the semaphore in the wait() and signal() operation must be executed indivisibly. That is when one process modifies the semaphore value, no other process can simultaneously modify that same semaphore value

Counting vs Binary Semaphores

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Counting Semaphore

• Value ranges over an unrestricted domain
• Used to control access to a resource with multiple instances
• Initialized to the number of available resources�

Binary Semaphore

• Value can only be 0 or 1
• Behaves like a mutex lock
• Can be used for mutual exclusion if no mutex locks exist

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named & unmanned semaphore

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Unnamed semaphore = a physical ticket box inside your office — only people already in your office can see and use it.�

Named semaphore = a public locker with a name/number in the building lobby — any process with the name can use it.

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unnamed semaphore

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In the process’s memory (shared memory or global variable).

• sem_init() is used
• Scope:
• By default for threads within one process.
• 2nd argument ‘0’ = semaphore is shared between threads in the same process.
• 2nd argument ‘1’ (nonzero) = semaphore is shared between processes, and must be placed in shared memory
• No name in the system: It’s just a variable you allocated.
• ✅ Fast, simple for intra-process synchronization.
• ❌ No persistence; you can’t open it by name later.

sem_t sem;

sem_init(&sem, 0, 1); // 0 = shared between threads, 1 = initial value

sem_wait(&sem); // acquire

sem_post(&sem); // release

sem_destroy(&sem);

named semaphore

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In the kernel, identified by a string name (like a file).

• sem_open("/mysem", O_CREAT, 0644, 1) is used
• Any unrelated processes can open the same name and synchronize.
• It appears under /dev/shm/sem.mysem on Linux.
• Needs explicit unlink: sem_unlink("/mysem");
• ✅ Works across unrelated processes easily.
• ✅ Survives even if one process dies (until sem_unlink() is called).
• ❌ Slightly more overhead, needs cleanup.

sem_t *sem = sem_open("/mysem", O_CREAT, 0644, 1);

sem_wait(sem); // acquire

sem_post(sem); // release

sem_close(sem);

sem_unlink("/mysem"); // remove name

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Sample Programs

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unnamed_semaphore.c

named_semaphore.c

process syn with semaphore

process syn with semaphore

Thank You

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