memcached源码探秘（一）—— hash_table

memcached在删除hash_table中的单向表的某个节点的时候，非常巧妙的使用了一个二级指针实现了删除的操作。

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);
}

要实现单向列表的删除，按照一般教科书上的方法要声明两个指针，pre、cur，用来保存当前的节点和其前置的节点。但是如果应用二级指针，保存指向当前节点指针的地址**before，则*before表示指向当前节点的指针，用这个方法可以非常优雅的解决问题。