Syncronizing Threads

Problem

int foo = 0;
void foo_initializer()
{
    if (foo == 0)
	foo = 1;	/* This must only be done once. */
}

 	    Thread 1				    Thread 2
 	if (foo == 0)
 						if (foo == 0)
 						    foo = 1;
 	    foo = 1;

Mutexes

• Stands for Mutual Exclusion
• Serializes access to some critical region of code or data
• Anytime a global resource is accessed by more than one thread the resource should have a Mutex associated with it.

Creating and Destroying Mutexes

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

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

int pthread_mutex_destroy(pthread_mutex_t * mutex);

Using Mutexes

int pthread_mutex_lock(pthread_mutex_t * mutex)

int pthread_mutex_trylock(pthread_mutex_t * mutex)

int pthread_mutex_unlock(pthread_mutex_t * mutex)

Fixed Problem

int foo = 0;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void foo_initializer()
{
    pthread_mutex_lock(&mutex);
    if (foo == 0)
	foo = 1;	/* This must only be done once. */
    pthread_mutex_unlock(&mutex);
}