27. Interlude: Thread API

Question 1

Describe the POSIX interface for thread creation

Answer 1

pthread_create(pthread_t thread, const pthread_attr_t attr, void *(start_routine)(void), void *arg);

*thread - pointer to strcture of type pthread_t
*attr - attributes the thread might have (stack size, priority, etc.)
start_routine - a pointer to function which takes void pointer and return void pointer
*arg - function arguments

Question 2

Why does thread creation call uses void argument and void return value for function?

Answer 2

It allows us to pass and return any type of data

Question 3

How do you wait for thread to complete? Describe call interface

Answer 3

int pthread_join(pthread_t thread, void **value_ptr);

thread - specify the thread to wait for
**value_ptr - pointer to return value we expect to get back

**value_ptr is an address of a pointer (i.e pointer to pointer) where we will put the returned pointer from thread

Question 4

What is interface for lock and unlock?

Answer 4

int pthread_mutex_lock(pthread_mutex_t *mutex);

int pthread_mutex_unlock(pthread_mutex_t *mutex);

Question 5

What you need to do before using a lock?

Answer 5

It should be properly initialized. It can be done with following:
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
or
pthread_mutex_init(&lock, NULL) - where the second argument is an optional set of attributes, NULL stands for defaults

Question 6

What should be done after using the lock completely?

Answer 6

pthread_mutex_destroy()

Question 7

What are other interfaces that can be used to acquite the lock?

Answer 7

int pthread_mutex_trylock(pthread_mutex_t *mutex); - return failure if lock is already held

int pthread_mutex_timedlock(pthread_mutex_t *mutex, struct timespec *abs_timeout) - returns after timeout or after acquiring the lock

Question 8

What are conditional variables used for?

Answer 8

For cases when some kind of signaling should take place between threads, if one thread is waiting for another to do something before it can continue