Lecture 07 - Need to finish the bits on semaphores

Question 1

What are the three main ways in which a multi-threaded program can go wrong?

Answer 1

• Deadlock
• Race Conditions
• Starvation

Question 2

What is the name given to source code that leads to a race condition?

Answer 2

A critical section

Question 3

What is a race condition?

Answer 3

When two or more threads are simultaneously reading/working on the same data which can result in data becoming inconsistent, or overwritten.

Question 4

Name two tools used to safeguard critical sections.

Answer 4

Mutexes and semaphores

Question 5

What is a mutex?

Answer 5

A variable which can be locked.

Question 6

How many threads can lock a mutex at once?

Answer 6

One

Question 7

What happens when a thread tries to lock a mutex which is already locked by another thread?

Answer 7

It blocks until the mutex becomes unlocked.

Question 8

What type is used to store mutexes?

Answer 8

pthread_mutex_t

Question 9

What function initialises a mutex?

Answer 9

pthread_mutex_init

Question 10

What is the function prototype for pthread_mutex_init?

Answer 10

int pthread_mutex_init(pthread_mutex_t* mutex, const pthread_mutexattr_t* attr);

Question 11

How do you initialise a GLOBAL mutex?

Answer 11

pthread_mutex_t m = PTHREAD_MUTEX_INITIALISER;

Question 12

What function destroys a mutex?

Answer 12

pthread_mutex_destroy

Question 13

What is the function prototype for pthread_mutex_destroy?

Answer 13

int pthread_mutex_destroy(pthread_mutext_t* mutex);

Question 14

What function is used to lock a mutex?

Answer 14

pthread_mutex_lock

Question 15

What is the function prototype for pthread_mutex_lock?

Answer 15

int pthread_mutex_lock(pthread_mutex_t* mutex);

Question 16

What function is used to unlock a mutex?

Answer 16

pthread_mutex_unlock

Question 17

What is the function prototype for pthread_mutex_unlock?

Answer 17

int pthread_mutex_unlock(pthread_mutex_t* mutex);

Question 18

What function locks a mutex only if it’s available?

Answer 18

trylock

Question 19

What does trylock return if the mutex was successfully locked?

Answer 19

0

Question 20

What does trylock return if the mutex was invalid?

Answer 20

EINVAL

Question 21

What does trylock return if the mutex was busy?

Answer 21

EBUSY

Question 22

What causes deadlock?

Answer 22

When two or more threads are waiting on each other’s resources.