c - How to utilize a thread pool with pthreads?

Question

I have a queue of jobs and I want to make a pool of four threads where I can throw my jobs at. What I am stuck at is in how to make the threads and keep them suspended while there is no work.

JOB QUEUE        | job1 | job2 | job3 | job4 | ..

THREAD POOL      | thread1 | thread2 | thread3 | thread4 |

To create the threads I have currently at the initialisation point:

for (t=0; t<num_of_threads; t++){
    pthread_create(&(threads[t]), NULL, doSth2, NULL);
}

Where num_of_threads=4 and doSth2 is a function with nothing inside. So once I have created the 4 threads and they are done with doSth2, how can I give them new work to do, without killing them?

Answer 1

The key to a thread pool is a queue. Here are modified functions for a thread pool I have developed.

Put element in queue

void queue_add(queue q, void *value)
{
    pthread_mutex_lock(&q->mtx);

    /* Add element normally. */

    pthread_mutex_unlock(&q->mtx);

    /* Signal waiting threads. */
    pthread_cond_signal(&q->cond);
}

Get element from queue

void queue_get(queue q, void **val_r)
{
    pthread_mutex_lock(&q->mtx);

    /* Wait for element to become available. */
    while (empty(q))
        rc = pthread_cond_wait(&q->cond, &q->mtx);

    /* We have an element. Pop it normally and return it in val_r. */

    pthread_mutex_unlock(&q->mtx);
}