Trying to understand pthread_cond_signal() - c

Question

Hi i'm new to C and trying to understand mutex, condition and threads. I have the basics of how thread works. Correct me if i'm wrong, from what i understand here is that one thread is waiting on the other 2 thread to send a signal to wake him up. The code can only be run by one thread at a time because of the mutex, thread 2 acquires the lock and runs the code, the sleep(1) helps to alternate thread so thread 2 doesn't run it again, and so thread 3 acquires the lock and runs the code and so on. Only the thread that locks and release the mutex lock. Once it reaches the COUNT_LIMIT 12, it sends a signal to the waiting thread which is thread 1. I understand up till here if i didn't misunderstood anything
So my question is that this few pieces of code here that is in void *watch_count:
pthread_mutex_lock(&count_mutex);
pthread_cond_wait(&count_threshold_cv, &count_mutex);
pthread_mutex_unlock(&count_mutex);
Is it acquiring the same lock as the one in void *inc_count? If so it locks the mutex and the pthread_cond_wait is waiting for the signal in void *watch_count, then continues to execute the code? What is the pthread_mutex_lock and pthread_mutex_unlock doing in void *watch_count, does this play a part in stopping the threads 2 and 3 from executing while waiting for thread 1 to finish it's job?
I hope i am making sense or exaplaining what i understand correctly, thanks in advance for any advice given!
#define NUM_THREADS 3
#define TCOUNT 10
#define COUNT_LIMIT 12
int count = 0;
pthread_mutex_t count_mutex;
pthread_cond_t count_threshold_cv;
void *inc_count(void *t)
{
int i;
long my_id = (long)t;
for (i=0; i < TCOUNT; i++) {
pthread_mutex_lock(&count_mutex);
count++;
/*
Check the value of count and signal waiting thread when condition is
reached. Note that this occurs while mutex is locked.
*/
if (count == COUNT_LIMIT) {
printf("inc_count(): thread %ld, count = %d Threshold reached. ",
my_id, count);
pthread_cond_signal(&count_threshold_cv);
printf("Just sent signal.\n");
}
printf("inc_count(): thread %ld, count = %d, unlocking mutex, i: %d\n",
my_id, count, i);
pthread_mutex_unlock(&count_mutex);
/* Do some work so threads can alternate on mutex lock */
sleep(1);
}
pthread_exit(NULL);
}
void *watch_count(void *t)
{
long my_id = (long)t;
printf("Starting watch_count(): thread %ld\n", my_id);
pthread_mutex_lock(&count_mutex);
while (count < COUNT_LIMIT) {
printf("watch_count(): thread %ld Count= %d. Going into wait...\n", my_id,count);
pthread_cond_wait(&count_threshold_cv, &count_mutex);
printf("watch_count(): thread %ld Condition signal received. Count= %d\n", my_id,count);
printf("watch_count(): thread %ld Updating the value of count...\n", my_id,count);
count += 125;
printf("watch_count(): thread %ld count now = %d.\n", my_id, count);
}
printf("watch_count(): thread %ld Unlocking mutex.\n", my_id);
pthread_mutex_unlock(&count_mutex);
pthread_exit(NULL);
}
int main(int argc, char *argv[])
{
int i, rc;
long t1=1, t2=2, t3=3;
pthread_t threads[3];
pthread_attr_t attr;
/* Initialize mutex and condition variable objects */
pthread_mutex_init(&count_mutex, NULL);
pthread_cond_init (&count_threshold_cv, NULL);
/* For portability, explicitly create threads in a joinable state */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
pthread_create(&threads[0], &attr, watch_count, (void *)t1);
pthread_create(&threads[1], &attr, inc_count, (void *)t2);
pthread_create(&threads[2], &attr, inc_count, (void *)t3);
/* Wait for all threads to complete */
for (i = 0; i < NUM_THREADS; i++) {
pthread_join(threads[i], NULL);
}
printf ("Main(): Waited and joined with %d threads. Final value of count = %d. Done.\n",
NUM_THREADS, count);
/* Clean up and exit */
pthread_attr_destroy(&attr);
pthread_mutex_destroy(&count_mutex);
pthread_cond_destroy(&count_threshold_cv);
pthread_exit (NULL);
}

Answer 1

Yes, all your threads are acquiring the same lock (there is only one mutex in this program - count_mutex).
When watch_count() calls pthread_cond_wait(), it unlocks the mutex and is suspended in one atomic action. This allows other threads to acquire the lock. When the thread that called pthread_cond_wait() wakes up, it will re-acquire the lock before pthread_cond_wait() returns (this means that if another thread has the mutex locked, pthread_cond_wait() can't return until that other thread releases the lock first).
You must have the mutex locked when you call pthread_cond_wait() - this is a requirement of the API. Furthermore, the watch_count() thread locks the count_mutex so that it can access the count variable without introducing a race condition (a concurrent modification of count from another thread). Notice that none of the threads access the count variable - either for read or write - without having the count_mutex locked at the time.