pthread conditional variable

Question

I'm implementing a thread with a task queue. As soon as as the first task is added to the queue the thread starts running it.

Should I use pthread condition variable to wake up the thread or there is more appropriate mechanism?

If I call pthread_cond_signal() when the other thread is not blocked by pthread_cond_wait() but rather doing something, what happens? Will the signal be lost?

Answer 1

From the pthread_cond_signal Manual:

The pthread_cond_broadcast() and pthread_cond_signal() functions shall have no effect if there are no threads currently blocked on cond.

I suggest you use Semaphores. Basically, each time a task is inserted in the queue, you "up" the semaphore. The worker thread blocks on the semaphore by "down"'ing it. Since it will be "up"'ed one time for each task, the worker thread will go on as long as there are tasks in the queue. When the queue is empty the semaphore is at 0, and the worker thread blocks until a new task arrives. Semaphores also easily handle the case when more than 1 task arrived while the worker was busy. Notice that you still have to lock access to the queue to keep inserts/removes atomic.

Answer 2

Semaphores are good if-and-only-if your queue already is thread safe. Also, some semaphore implementations may be limited by top counter value. Even it is unlikely you would overrun maximal value.

Simplest and correct way to do this is following:

pthread_mutex_t queue_lock;
pthread_cond_t  not_empty;
queue_t queue;

push()
{
  pthread_mutex_lock(&queue_lock);
  queue.insert(new_job);
  pthread_cond_signal(&not_empty)
  pthread_mutex_unlock(&queue_lock);
}
pop()
{
  pthread_mutex_lock(&queue_lock);
  if(queue.empty()) 
     pthread_cond_wait(&queue_lock,&not_empty);
  job=quque.pop();
  pthread_mutex_unlock(&queue_lock);
}