Thread waiting for signal but never gets it, because of mutex lock

Question

So i am making a program with 1 main thread(my main), which creates a second one. The second one should wait for a signal and then run. Main waits of course to join.

I have a problem in understanding how my main can lock mutex, and then my thread will lock it too and wait for signal. I mean that is the "guideline" everyone gives on how to do it but I don't see how it works. Since main locks mutex, then thread will always come to a locked muted and wait for mutex to be unlocked, no? That way, signal will be sent before thread has acquired mutex and has blocked in wait(so it will never be waken). But then again it is required to lock mutex in order to wait for the variable... Pls help. This is the code, it creates a loop because threadfunction blocks in wait..

static pthread_mutex_t mtx;
pthread_cond_t cond;

void *threadfunction(void* arg){
    pthread_mutex_lock(&mtx);
    pthread_cond_wait(&cond,&mtx);
    pthread_mutex_unlock(&mtx);
}

int main(void){
    pthread_mutex_init(&mtx,NULL);
    pthread_cond_init(&cond,NULL);

    pthread_t tthread;
    pthread_mutex_lock(&mtx);

    pthread_create(&tthread,NULL,threadfunction,NULL);

    pthread_cond_signal(&cond);

    pthread_mutex_unlock(&mtx);

    pthread_join(tthread,NULL);
}

Burak Serdar Burak Serdar · Accepted Answer · 2020-06-15T16:15:17

Probably one detail you missed in pthread_cond_wait is that it releases the mutex when it starts waiting.

The scenario for condition variables is that when a thread needs to wait for a condition to happen, it locks a mutex and calls wait. This will release the mutex so other threads can go into their critical sections, change conditions, and signal so the waiting threads can wake up.

In your example, you create the thread when the mutex is already locked, so the thread has no chance of passing the first lock. That means when you signaled the thread, it wasn't waiting.

Thread waiting for signal but never gets it, because of mutex lock

1 Answers