understanding of pthread_cond_wait() and pthread_cond_signal()

Question

Generally speaking, pthread_cond_wait() and pthread_cond_signal() are called as below:

//thread 1:
pthread_mutex_lock(&mutex);
pthread_cond_wait(&cond, &mutex);
do_something()
pthread_mutex_unlock(&mutex);

//thread 2:
pthread_mutex_lock(&mutex);
pthread_cond_signal(&cond);  
pthread_mutex_unlock(&mutex);

The steps are

1. pthread_cond_wait(&cond, &mutex); is called, it unlocks the mutex

2. Thread 2 locks the mutex and calls pthread_cond_signal(), which unlocks the mutex

3. In thread 1, pthread_cond_wait() is called and locks the mutex again

Now in thread 2, after pthread_cond_signal() is called, pthread_mutex_unlock(&mutex) is going to run, it seems to me that it wants to unlock a the mutex which is now locked by thread 1. Is there anything wrong in my understanding?

Besides, it also seems to me that pthread_cond_wait() can be called by only 1 thread for the same cond-mutex pair. But there is a saying "The pthread_cond_signal() function shall unblock at least one of the threads that are blocked on the specified condition variable cond (if any threads are blocked on cond)." So, it means pthread_cond_wait() can be called by many threads for the same cond-mutex pair?

Answer 1

pthread_cond_signal does not unlock the mutex (it can't as it has no reference to the mutex, so how could it know what to unlock?) In fact, the signal need not have any connection to the mutex; the signalling thread does not need to hold the mutex, though for most algorithms based on condition variables it will.

pthread_cond_wait unlocks the mutex just before it sleeps (as you note), but then it reaquires the mutex (which may require waiting) when it is signalled, before it wakes up. So if the signalling thread holds the mutex (the usual case), the waiting thread will not proceed until the signalling thread also unlocks the mutex.

The common use of condition vars is something like:

thread 1:
    pthread_mutex_lock(&mutex);
    while (!condition)
        pthread_cond_wait(&cond, &mutex);
    /* do something that requires holding the mutex and condition is true */
    pthread_mutex_unlock(&mutex);

thread2:
    pthread_mutex_lock(&mutex);
    /* do something that might make condition true */
    pthread_cond_signal(&cond);
    pthread_mutex_unlock(&mutex);

The two threads have some shared data structure that the mutex is protecting access to. The first thread wants to wait until some condition is true then immediately do some operation (with no race condition opportunity for some other thread to come in between the condition check and action and make the condition false.) The second thread is doing something that might make the condition true, so it needs to wake up anyone that might be waiting for it.

Answer 2

Here is a typical example: thread 1 is waiting for a condition, which may be fulfilled by thread 2.

We use one mutex and one condition.

pthread_mutex_t mutex;
pthread_cond_t condition;

thread 1 :

pthread_mutex_lock(&mutex); //mutex lock
while(!condition){
    pthread_cond_wait(&condition, &mutex); //wait for the condition
}

/* do what you want */

pthread_mutex_unlock(&mutex);

thread 2:

pthread_mutex_lock(&mutex);

/* do something that may fulfill the condition */

pthread_mutex_unlock(&mutex);
pthread_cond_signal(&condition); //wake up thread 1

Edit

As you can see in the pthread_cond_wait manual:

It atomically releases mutex and causes the calling thread to block on the condition variable cond; atomically here means "atomically with respect to access by another thread to the mutex and then the condition variable".

Answer 3

I have taken example from here https://www.geeksforgeeks.org/condition-wait-signal-multi-threading/

and modified to this,

#include <pthread.h> 
#include <stdio.h> 
#include <unistd.h> 

// Declaration of thread condition variable 
pthread_cond_t cond = PTHREAD_COND_INITIALIZER; 

// declaring mutex 
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER; 

// Thread function 
void releaseFun() 
{ 
    // Let's signal condition variable cond
    printf("Signaling condition variable cond\n"); 
    pthread_cond_signal(&cond); 
}

// Thread function 
void* blockedThread() 
{
    // acquire a lock 
    pthread_mutex_lock(&lock); 
    printf("Waiting on condition variable cond\n");
    pthread_cond_wait(&cond, &lock); 
    // release lock 
    pthread_mutex_unlock(&lock); 

    printf("Returning thread\n"); 

    return NULL; 
}    

// Driver code 
int main() 
{ 
    pthread_t tid;

    // Create thread 1 
    pthread_create(&tid, NULL, blockedThread, NULL); 

    // sleep for 1 sec so that thread 1 
    // would get a chance to run first 
    sleep(1); 

    releaseFun();
    // wait for the completion of thread 2 
    pthread_join(tid, NULL); 

    return 0; 
}

Output: gcc test_thread.c -lpthread

Waiting on condition variable cond

Signaling condition variable cond

Returning thread

Locking and unlocking of the thread will happen only when blockThread's pthread_cond_wait() function is signaled to be unblocked.