# 2021-04-06 fdfdd12345628

## 測驗 `4`

`gcc -lpthread -march=native -latomic lab.c`

callback 時 refcnt 要加一，結束時要減一，因此需要做兩次的 CAS 才可以

static bool execute_client_callback(bus_client_t *client, void *msg)
{
    /* Load the client with which we are attempting to communicate. */
    bus_client_t local_client;
    __atomic_load(client, &local_client, __ATOMIC_SEQ_CST);

    /* Loop until reference count isupdated or client becomes unregistered */
    while (local_client.registered) {
        /* The expected reference count is the current one + 1 */
        bus_client_t new_client = local_client;
        ++(new_client.refcnt);
        /* If CAS succeeds, the client had the expected reference count, and
         * we updated it successfully. If CAS fails, the client was updated
         * recently. The actual value is copied to local_client.
         */
        if(CAS(&(client->refcnt), &(local_client.refcnt), &(new_client.refcnt))){
            client->callback(client->ctx, msg);

                // Callback done, decrease reference count
            __atomic_load(client, &local_client, __ATOMIC_SEQ_CST);
            while(local_client.registered){
                bus_client_t new_client = local_client;
                --(new_client.refcnt);

                if(CAS(&(client->refcnt), &(local_client.refcnt), &(new_client.refcnt))){
                    return true;
                }

                local_client.refcnt=client->refcnt;
            }// inner while
            
            return false;
        }// first CAS

        local_client.refcnt=client->refcnt;
    }// outer while
    
    /* Client was not registered or got unregistered while we attempted to send
     * a message
     */
    return false;
}

:::warning
注意要更新 reference count
:notes: jserv
:::

:::info
1. 已經將 local_client 的值在 CAS failed 後更新。
2. 已經用半形空白重新排版
> 程式碼排版：4 個半形空白
:::