memcached源碼探祕（一）—— hash_table

memcached在刪除hash_table中的單向表的某個節點的時候，非常巧妙的使用了一個二級指針實現了刪除的操作。

void assoc_delete(const char *key, const size_t nkey, const uint32_t hv) {
    item **before = _hashitem_before(key, nkey, hv);

    if (*before) {
        item *nxt;
        hash_items--;
        /* The DTrace probe cannot be triggered as the last instruction
         * due to possible tail-optimization by the compiler
         */
        MEMCACHED_ASSOC_DELETE(key, nkey, hash_items);
        nxt = (*before)->h_next;
        (*before)->h_next = 0;   /* probably pointless, but whatever. */
        *before = nxt;
        return;
    }
    /* Note:  we never actually get here.  the callers don't delete things
       they can't find. */
    assert(*before != 0);
}

要實現單向列表的刪除，按照一般教科書上的方法要聲明兩個指針，pre、cur，用來保存當前的節點和其前置的節點。但是如果應用二級指針，保存指向當前節點指針的地址**before，則*before表示指向當前節點的指針，用這個方法可以非常優雅的解決問題。