Why are two semaphores and one mutex required in solving bounded buffer instance of producer-consumer?

Question

Why must one use a mutex in addition to a semaphore when using a bounded buffer in producer consumer problem?

Answer 1

empty:semaphore(n)
full: semaphore(0)
mutex: semaphore(1)

"mutex" is used to lock on Buffer.

"full" is used to block consumer if buffer is empty.

"empty" is used to block producer if buffer is full.

That's why you need 3 semaphores.

You can easily google the code so I don't paste it here.