How to reset variable after last thread call? - c

Question

I have been trying to learn pthreads, with mutex and cond, but I can't figure out how to reset a varible ONLY after the last thread that used it doesn't need it anymore.
I have the following code:
int i = 0;
pthread_mutex_t mutex;
pthread_cond_t cond;
int test() {
if (i == 0) {
pthread_mutex_init(&mutex, NULL);
pthread_cond_init(&cond, NULL);
}
pthread_mutex_lock(&mutex);
++i;
if (i % 5 != 0) {
pthread_cond_wait(&cond, &mutex);
}
else {
pthread_cond_broadcast(&cond);
}
int j = i;
i = 0; /* problem line */
pthread_mutex_unlock(&mutex);
//i = 0; /* tried here too */
return j;
}
I have multiple threads calling this test() method.
The first 4 threads will wait until the 5th is called, and then all 5 threads will return 5. But the 6th+ will return '6','7',... etc, I want to reset the i variable i = 0, so the count will restart.
I thought of doing another thread_cond_t to make the threads wait until all j = i are finished. But I don't think that is right (or at least unnecesary).
The actual output is (undetermined order because of threads):
0
0
0
0
5
0
0
0
5
I want them all be 5s.
Edit: I made j global and i is modified after the broadcast and it has worked so far:
int i = 0, j;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
int test() {
pthread_mutex_lock(&mutex);
++i;
if (i % 5 != 0) {
pthread_cond_wait(&cond, &mutex);
} else {
pthread_cond_broadcast(&cond);
j = i;
}
i = 0;
pthread_mutex_unlock(&mutex);
return j;
}
This code successfully resets the variable after reaching 5, and prints that value on all threads, but it seems I suffer from dataraces when I try many threads.

Answer 1

This code successfully resets the variable after reaching 5, and prints that value on all threads, but it seems I suffer from dataraces when I try many threads.
With the edited code, consider this course of events: Four threads are waiting in pthread_cond_wait, the fifth thread just passes pthread_cond_broadcast (causing the first four to suspend on mutex before returning from pthread_cond_wait), and a sixth thread is already waiting in pthread_mutex_lock. Now after setting i = 0 the fifth passes pthread_mutex_unlock and say the sixth resumes with ++i, setting i to 1, and thereafter goes to wait in pthread_cond_wait (causing the mutex to unlock). Then one of the first four threads resumes and passes i = 0, so the counter i is unduly reset.
To rectify this, reset i only in the thread which does the pthread_cond_broadcast, i. e. change
}
i = 0;
to
i = 0;
}
(move the i = 0 into the else { pthread_cond_broadcast block).