pthreads: a thread that triggers other threads

Question

Hello, I'm new to multi-thread programming. I'm trying to create a code that creates a thread THREAD1, that, after it has done something, it triggers two other threads, say THREAD2 and THREAD3, and then exits.

I wrote two possible solutions.

1) Use of condition variables (DOESN'T WORK: in some case I get a deadlock):

pthread_mutex_t ready_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  ready_cond  = PTHREAD_COND_INITIALIZER;
bool ready = false;

void* trigger(void*);
void* func1(void*);
void* func2(void*);

int main()
{
    pthread_t thread1;
    pthread_t thread2;
    pthread_t thread3;  
    pthread_create(&thread1, 0, &trigger, 0);
    pthread_create(&thread2, 0, &func1, 0);
    pthread_create(&thread3, 0, &func2, 0);
    pthread_join(thread1, 0);
    pthread_join(thread2, 0);
    pthread_join(thread3, 0);
}

void *trigger(void*)
{
    pthread_mutex_lock(&ready_mutex);
    ready = true;
    pthread_cond_broadcast(&ready_cond);
    pthread_mutex_unlock(&ready_mutex);
    return 0;
}

void *func1(void*)
{
    while (!ready) // Needed to avoid spuriuos wake-up
    {
        pthread_mutex_lock(&ready_mutex);
        pthread_cond_wait(&ready_cond, &ready_mutex);
        pthread_mutex_unlock(&ready_mutex);
    }
    std::cout << "In 'func1'>> Do something" << std::endl;
    return 0;
}

void *func2(void*)
{
    while (!ready) // Needed to avoid spuriuos wake-up
    {
        pthread_mutex_lock(&ready_mutex);
        pthread_cond_wait(&ready_cond, &ready_mutex);
        pthread_mutex_unlock(&ready_mutex);
    }
    std::cout << "In 'func2'>> Do something" << std::endl;
    return 0;
}

2) THREAD1 directly creates the two other threads.

pthread_mutex_t ready_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  ready_cond  = PTHREAD_COND_INITIALIZER;

pthread_t thread1;
pthread_t thread2;
pthread_t thread3;

void* trigger(void*);
void* func1(void*);
void* func2(void*);

int main()
{
    pthread_create(&thread1, 0, &trigger, 0);

    pthread_join(thread1, 0);
    pthread_join(thread2, 0);
    pthread_join(thread3, 0);
}

void *trigger(void*)
{
    std::cout << "In 'trigger'>> Do something" << std::endl;

    pthread_create(&thread2, 0, &func1, 0);
    pthread_create(&thread3, 0, &func2, 0);

    return 0;
}

void *func1(void*)
{
    std::cout << "In 'func1'>> Do something" << std::endl;

    return 0;
}

void *func2(void*)
{
    std::cout << "In 'func2'>> Do something" << std::endl;

    return 0;
}

I would like to know your opinion. Thank you very much

Answer 1

Use of condition variables (DOESN'T WORK: in some case I get a deadlock):

The code doesn't lock the mutex when checking the state of shared variable ready . When it does lock the mutex ready may have well changed by that time, this is why you see deadlocks.

The correct version to wait for state change with a condition variable is (error checking omitted):

pthread_mutex_lock(&ready_mutex);
while(!ready) // Needed to avoid spuriuos wake-up
    pthread_cond_wait(&ready_cond, &ready_mutex);
// ready == true here
pthread_mutex_unlock(&ready_mutex);

The above assumes that ready is ever changed only when the same mutex is held.