Conditional variables

Question 1

⭐️ Give the definition:
No two threads access a critical section at the same time & run at the same time. Enabled by locks.

Answer 1

Mutual exclusion

Question 2

⭐️ What can we use to ensure mutual exclusion?

Answer 2

Locks

Question 3

⭐️ What method ensures that two threads don’t run at the same time?

Answer 3

Locks

Question 4

⭐️ Give the definition:
A thread wishes to check whether a condition is true before execution uses this. It ensures that a thread runs after the one before it completes.

Answer 4

Conditional variable & semaphores

Question 5

⭐️ What sort of primitives are conditional variables?

Answer 5

Synchronization primitives

Question 6

⭐️ What do conditional variables have a queue of?

Answer 6

Waiting threads

Question 7

⭐️ Why are conditional variables used with mutex?

Answer 7

To prevent race conditions

Question 8

⭐️ Which function of a conditional variable is this?
A thread puts itself on the waiting queue

Answer 8

wait(cv …)

Question 9

⭐️ Which function of a conditional variable is this?
A thread sends signal to CV when it’s done

Answer 9

signal(cv …)

Question 10

Which function of a conditional variable is this?
Wakes up all waiting threads

Answer 10

broadcast()

Question 11

Which function of a conditional variable is this?
Wakes up one waiting thread

Answer 11

cond_signal()

Question 12

Which function of a conditional variable is this?
Wakes up all waiting threads

Answer 12

cond_broadcast()

Question 13

What four things are a program with a conditional variable missing if the following problem can arise?
Parent can continue to sleep itself forever

Answer 13

A lock
State variable
Mutex (in child)
Re-check of CV

Question 14

⭐️ What type of problem is the bounded-buffer problem?

Answer 14

Synchronization problem

Question 15

⭐️ A buffer has a bounded size (T/F)

Answer 15

True

Question 16

⭐️ What example of the product/consumer problem is this?
1. Producer puts requests in a queue
2. Consumers picks requests form the queue to process

Answer 16

Web servers

Question 17

⭐️ What problem can both web servers & Unix pipes experience?

Answer 17

Product/consumer problem

Question 18

⭐️ What problem is this a description of?
A buffer has a bounded size meaning that when the buffer is full, producers must wait, and when the buffer is empty, consumers must wait.

Answer 18

Producer/consumer problem or bounded-buffer problem

Question 19

In the producer/consumer problem, when the buffer is full, who must wait?

Answer 19

Producers

Question 20

In the producer/consumer problem, when the buffer is empty, who must wait?

Answer 20

Consumer

Question 21

Because the bounded buffer is a shared variable, there must be … both between producers, & producers and consumers

Answer 21

synchronisation

Question 22

What does producing/consuming to/from the buffer require?

Answer 22

Locking

Question 23

What issue will the p/c (producer/consumer) problem still have if only locks are implemented?

Answer 23

Continue looping due to deadlock

Question 24

To ensure that a producer waits on empty and consumer waits on fill, how many conditional variables are needed to solve the p/c problem?

Answer 24

2 CVs - distinguishes two types of threads

Question 25

⭐️ What 3 things must we ensure to properly use conditional variables?

Answer 25

1. Have a state 2. Use of mutex (ensures no race conditions) 3. Recheck of state (using while)

Question 26

Give the definition: An object with an integer value that we can manipulate with two routines

Answer 26

Semaphore

Question 27

What function of a semaphore is this? Decrements the value of the semaphore integer by one, then waits if the integer is negative

Answer 27

sem_wait() (in POSIX standard)

Question 28

What function of a semaphore is this? Increments the value of the semaphore integer by one, if there are 1/more threads waiting, wakes one

Answer 28

sem_post() (in POSIX standard)

Question 29

What sort of semaphores are locks?

Answer 29

Binary semaphores

Question 30

⭐️ A condition variable must always be used without a mutex to avoid deadlocks (T/F)

Answer 30

False

Question 31

⭐️ When a thread waits on a condition variable, it releases the associated mutex while waiting (T/F)

Answer 31

True