pthread conditional variable

Question

I'm implementing a thread with a queue of tasks. 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

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);
}