pthread exit from thread in thread pool

I have a rather simple thread pool, and i have a question regarding thread finalizing.

this is my worker snippet :

static void* threadpool_worker(void* pool_instance)
{
    int rc;
    struct threadpool* pool = (struct threadpool*)pool_instance;
    struct threadpool_task *task;

    for(;;)
    {
        pthread_mutex_lock( &(pool->task_queue_mutex) );

        while( pool->headp->tqh_first == NULL )
        {
            rc = pthread_cond_wait( &(pool->task_queue_cond), &(pool->task_queue_mutex) );
        }

        task = pool->headp->tqh_first;
        TAILQ_REMOVE(pool->headp, pool->headp->tqh_first, entries);

       pthread_mutex_unlock( &(pool->task_queue_mutex) );
       task->routine_cb(task->data);
    }

}

so jobs are executed at this line task->routine_cb(task->data);

and in order to finalize workers threads i'm calling threadpool_enqueue_task

in the following way :

for( i=0 ; i < pool->num_of_workers ; ++i)
{
    threadpool_enqueue_task(pool, pthread_exit, NULL);
}

expecting that pthread_exit will be called in here task->routine_cb(task->data) but it does not work this way, i don't see any explicit error, just memory leak in valgrind

but when i change the worker code like that :

    if(task->routine_cb == pthread_exit)
    {
        pthread_exit(0);
    }
    task->routine_cb(task->data);

everything ends fine. so my question is is there an option to stop the worker just making it to execute pthread_exit in some way,without changing the worker code.

Edit: Thread pool task declared as following :

struct threadpool_task
{
    void (*routine_cb)(void*);
    void *data;
    TAILQ_ENTRY(threadpool_task) entries;          /* List. */
}

As per my understanig there should be no problem to get address of pthread_exit in routine_cb which declared :

extern void pthread_exit (void *__retval) __attribute__ ((__noreturn__));