Syncronizing Threads

Problem

int foo = 0;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void foo_ready()
{
    pthread_mutex_lock(&mutex);
    while (foo != 1) {
        pthread_mutex_unlock(&mutex);
        sleep(1);
        pthread_mutex_lock(&mutex);
   }
   pthread_mutex_unlock(&mutex);
}

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

The thread needs to wait until the state of a shared variable changes, but either the thread spends too much time checking or isn't responsive enough.

Condition Variable

An address for which a thread may wait until the state of a shared variable has changed.
Waiting atomicly puts the current thread to sleep and unlock the mutex.
Multiple threads may wait on the same condition variable.

Creating and Destroying Condition Variables

int pthread_cond_init(pthread_cond_t * cond, pthread_condattr_t *attr);

pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

int pthread_cond_destroy(pthread_cond_t * cond);

Waiting on a Condition Variable

int pthread_cond_wait(pthread_cond_t * cond, pthread_mutex_t * mutex)

int pthread_cond_timedwait(pthread_cond_t * cond, pthread_mutex_t * mutex,
     const struct timespec *abstime)

Waking threads on a Condition Variable

int pthread_cond_signal(pthread_mutex_t * cond)

int pthread_cond_broadcast(pthread_mutex_t * cond)

Fixed Example

int foo = 0;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void foo_ready()
{
    pthread_mutex_lock(&mutex);
    while (foo != 1) {
        pthread_cond_wait(&cond, &mutex);
    }
    pthread_mutex_unlock(&mutex);
}

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

Prepared by Chris Provenzano (proven@mit.edu)