This document discusses semaphores and their use for synchronization between threads or processes. It defines semaphores as consisting of a positive integer value and atomic P() and V() operations that decrement and increment the value. Semaphores can be used for mutual exclusion by initializing a binary semaphore to 1 and having threads call P() before and V() after critical sections. Semaphores can also be used to synchronize producers and consumers in a bounded buffer problem, ensuring the consumer waits if the buffer is empty and the producer waits if the buffer is full. The document outlines implementations of semaphores using spinlocks or queues and the system calls used to manage semaphores in UNIX.

1. Semaphores and
Bounded Buffer

2. Semaphores
• Semaphore is a type of generalized lock
• Defined by Dijkstra in the last 60s
• Main synchronization primitives used in UNIX
• Consist of a positive integer value
• Two operations
• P(): an atomic operation that waits for semaphore to
become positive, then decrement it by 1
• V(): an atomic operation that increments semaphore
by 1 and wakes up a waiting thread at P(), if any.

3. Semaphores vs. Integers
• No negative values
• Only operations are P() and V()
• Cannot read or write semaphore values
• Except at the initialization times
• Operations are atomic
• Two P() calls cannot decrement the value below
zero
• A sleeping thread at P() cannot miss a wakeup
from V()

4. Binary Semaphores
• A binary semaphore is initialized to 1
• P() waits until the value is 1
• Then set it to 0
• V() sets the value to 1
• Wakes up a thread waiting at P(), if any

5. Two Uses of Semaphores
1. Mutual exclusion
• Lock was designed to do this
lock->acquire();
// critical section
lock->release();

6. Two Uses of Semaphores
1. Mutual exclusion
1. The lock function can be realized with a binary
semaphore: semaphore subsumes lock.
• Semaphore has an initial value of 1
• P() is called before a critical section
• V() is called after the critical section
semaphore litter_box = 1;
P(litter_box);
// critical section
V(litter_box);

7. Two Uses of Semaphores
1. Mutual exclusion
• Semaphore has an initial value of 1
• P() is called before a critical section
• V() is called after the critical section
semaphore litter_box = 1;
P(litter_box);
// critical section
V(litter_box);
litter_box = 1

8. Two Uses of Semaphores
1. Mutual exclusion
• Semaphore has an initial value of 1
• P() is called before a critical section
• V() is called after the critical section
semaphore litter_box = 1;
P(litter_box); // purrr…
// critical section
V(litter_box);
litter_box = 1  0

9. Producer-Consumer with a
Bounded Buffer
• A classic problem
• A producer put things into a shared buffer
• A consumer takes them out

10. Problem Constraints
• The solution involves both synchronization
and mutual exclusion
• Constraints
• The consumer must wait if buffers are empty
(synchronization constraint)
• The producer must wait if buffers are full
(synchronization constraint)
• Only one thread can manipulate the buffer at a
time (mutual exclusion)

11. Implementing Semaphore
• How to implement semaphore?
• Almost exactly like lock.
• Using spinlock or queue
• What hardware support is needed?
• Interrupt disable
• Test-and-set

12. Implementing Semaphore
class semaphore {
int value;
}
semaphore::semaphore(int i) {
value = i;
}
semaphore::p() {
// disable interrupts
while (value == 0) {
// enable interrupts
// disable interrupts
}
value --;
// enable interrupts
}
semaphore::v() {
// disable interrupts
value ++;
// enable interrupts
}

13. Implementing Semaphore
with test and set
class semaphore {
int value;
}
semaphore::semaphore(int
i) {
value = i;
}
semaphore::p() {
while
(test_and_set(guard));
while (value == 0) {
// queue the
thread
// guard = 0 and
sleep
}
value --;
guard = 0;
semaphore::v() {
while
(test_and_set(guard));
if (anyone waiting) {
// wake up one thread
// put in on ready
queue
} else {
value ++;
}
guard = 0;
}

14. Semaphore in UNIX
• Managing concurrent access to shared memory.
• Semaphore system calls
• Creation: semget( … )
• Incr/Decr/set : semop(…)
• Deletion: semctl(semid, 0, IPC_RMID, 0);
• See examples: seminit.c, sema.c
semb.c