Stop thread only if n threads are running including itself - c

Question

I am requested to create some threads in a process(48), but thread number 14 must can stop only if 6 threads are running including itself. However they enter an infinite loop.
Here is the function my threads from the process should execute:
pthread_mutex_lock_t lock;
pthread_mutex_cond_t cond;
reached_6_threads = false;
void *thread_function_P6(void *args)
{
th *t = (th *)args;
printf("started thread %d", t->id);
if (t->id != 14)
{
pthread_mutex_lock(&lock);
while (th_no > 6)
{
pthread_cond_wait(&cond, &lock);
}
if(!reached_6_threads && th_no==6){
pthread_cond_wait(&cond, &lock);
th_no--;
reached_6_threads = true;
}
th_no++;
if (!reached_6_threads && th_no == 6)
{
pthread_cond_broadcast(&cond);
pthread_mutex_unlock(&lock);
}
}
printf("threads running: %d\n", th_no);
printf("stopped thread %d", t->id);
pthread_exit(0);
}
lock and cond are initrialized before creating the threads.

Answer 1

I am not sure I understand you, but note in your code that:
a) Most of the code is under mutex lock, which means they can't really run in parallel
b) Thread 14 runs regardless of the number of running threads
Anyway, the reason it gets stuck is:
a) Your threads are running almost sequentially
b) Threads 1-5 skip while and both ifs, th_no is now 5 (assuming it was initialized to 0?)
c) Thread 6 raises th_no to 6 and enters second if, performing broadcast but there are no threads stuck on that condition lock
d) Threads 7 and above enter first if and wait on a condition lock that will never break
I would suggest the following solution. Since I haven't understood you completely, in this example only 6 threads are allowed to run regardless of their id and you'll have to make only some minor changes.
pthread_mutex_lock(&lock);
while(th_no >= 6){pthread_cond_wait(&cond, &lock);}
th_no++;
pthread_mutex_unlock(&lock); //Raise counter and unlock the mutex
/*
Thread function code here. Pay attention to critical code and use mutex.
*/
pthread_mutex_lock(&lock); //Acquire lock once again, decrease counter and broadcast if number of threads is ok now
th_no--;
if(th_no <= 5){
if(pthread_cond_broadcast(&cond)){
// Error as it should return 0 on success
}
}
pthread_mutex_lock(&unlock);
Let me know if this helps