Semaphores in Concurrent Programming

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Questions and Answers

What are the two routines used to manipulate a semaphore's integer value in the POSIX standard?

sem_wait() and sem_post()

A semaphore's initial value determines how it interacts with the semaphore routines.

True

Before calling other routines to interact with the semaphore, the semaphore must be ______ to a value.

initialized

What two functions can interact with a semaphore after it has been initialized?

sem_wait() and sem_post() Signup and view all the answers

What is the initial value of the semaphore declared in the code snippet shown in figure 31.1?

1 Signup and view all the answers

What is the purpose of the "sem_wait()" and "sem_post()" routines?

They are used to manipulate the integer value of a semaphore. Signup and view all the answers

What is the initial value of the semaphore "m" in Figure 31.3, and why? Hint: It's used as a lock.

1 Signup and view all the answers

A binary semaphore can be used for both locks and condition variables.

False Signup and view all the answers

How does the code in Figure 31.11 fail to correctly implement mutual exclusion?

It introduces a deadlock. Signup and view all the answers

What is the key change in Figure 31.12 that eliminates the deadlock?

The scope of the mutex lock is reduced to only the critical section. Signup and view all the answers

What is the main purpose of the reader-writer lock?

To allow multiple readers to access a shared resource concurrently while ensuring that only one writer can access it at a time. Signup and view all the answers

How do semaphores help solve "thread throttling"?

They limit the number of threads concurrently executing a critical section. Signup and view all the answers

What is the main characteristic of the "Dining Philosophers" problem?

Deadlock Signup and view all the answers

Why is the first attempt at solving "Dining Philosopher's" problem, using a semaphore for each fork, inadequate ?

It can lead to deadlock. Signup and view all the answers

What simple solution for "Dining Philosophers" problem eliminates deadlock?

Breaking the dependency in the way a philosopher acquires forks. Signup and view all the answers

Study Notes

Semaphores

Semaphores are synchronization primitives used to solve concurrency problems
Semaphores use integer values manipulated by sem_wait() and sem_post() routines
Initial semaphore value determines behavior
Binary semaphore: acts like a lock (value initialized to 1)
sem_wait(): decrements the semaphore value; suspends if negative
sem_post(): increments the semaphore value; wakes a waiting thread if any

Binary Semaphores (Locks)

Used to protect critical sections
Initialize to 1
sem_wait() acquires the lock
sem_post() releases the lock

Semaphores for Ordering

Used to order events in concurrent programs
Initialize to 0; a thread waits until the semaphore increments
One thread signals when something happens; another waits for the signal

Producer/Consumer Problem

Producers add items to a buffer; consumers remove them
Semaphores empty and full track buffer status
Use mutexes to avoid race conditions in the buffer

Reader-Writer Locks

Allow multiple readers to access data concurrently
Only one writer can access the data at a time
Readers increment a counter; writers wait for the counter to be zero
Avoid starvation of writers by waiting for all readers to finish before a writer can proceed

Dining Philosophers Problem

Philosophers sit around a table with forks
Each philosopher needs two forks to eat
Problem: deadlock can occur if philosophers grab forks in a certain order
Solutions involve breaking the dependency in acquiring forks

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Description

This quiz covers the concepts of semaphores, specifically their use in synchronization and concurrency problems. It explores binary semaphores, their functions in critical sections, and their application in the producer/consumer problem. Test your knowledge on semaphore operations and their significance in thread management.