# 2021-04-06 `Jings1017`

contributed by < `Jings1017` >

###### tags: `linux2021`

## 測驗 2

### dequeue( )

static void dequeue(queue_t *q, int *fd)
{
    node_t *old_head;
    pthread_mutex_lock(q->head_lock);
    /* Wait until signaled that queue is non_empty.
     * Need while loop in case a new thread manages to steal the queue
     * element after the waiting thread is signaled, but before it can
     * re-acquire head_lock.
     */
    
    // Wait until signaled that queue is non_empty.
    while (!q->size) 
        pthread_cond_wait(q->non_empty, q->head_lock); 
    
    old_head = q->head->next;
    if (!old_head->next)
        q->tail = q->head;
    q->head->next = old_head->next;
    
    *fd = old_head->fd;
    q->size--;

    pthread_mutex_unlock(q->head_lock);
    free(old_head);
}

## 問題

1. 執行緒函式 greeter_routine 和 worker_routine 在上述網頁伺服器中，分別負責什麼功能？

greeter_routine : receive the HTTP request
worker_routine : parse the request，回傳相對應的訊息。

2. enqueue 和 dequeue 是單向鏈結串列 (singly-linked list) 所實作的佇列 (queue) 裡頭的基本操作，你如何驗證自己的實作能在並行環境正確無誤？

利用 mutex lock 來對每個 queue 進行存取上的限制，以達到並行下可以正確無誤。