11 Synchronization mechanism in pthreads

Pthreads provides three synchronization mechanisms :

• Joins
• Mutual exclusions (“Mutex)
• Conditional variables

Synchronizing threads using join

• pthread_join() is a blocking function

int pthread_join(
	pthread_t thread_id, // ID of thread to "join"
	void **value_pntr // address of function’s return value
);

• Again, we’ll set value_pntr to NULL

int pthread_join(
	pthread_t thread_id, // ID of thread to "join"
	NULL
);

Mutual exclusion in Pthread : mutex

• Syntax for declaration and initialization of a mutex is

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;

• Creates a mutex ‘mutex1’ and initializes it with the default characteristics.
• Generally, mutexes are declared as global.
• Use of mutex for serializing access to a shared resource.

...
pthread_mutex_lock(&mutex1);
counter++;
pthread_mutex_unlock(&mutex1);
...

• Thread access counter serially one after another.

Conditional variables : syntax

• A conditional variable is a variable of type pthread_cond_t.

pthread_cond_t condition_cond = PTHREAD_COND_INITIALIZER;

• A thread goes to waiting state based on ‘condition to happen’ by:

pthread_cond_wait(&condition_cond, &condition_mutex);

Function takes two arguments: the condition variable and a mutex variable associated with the condition variable.

pthread_cond_signal(&condition_cond);