接上文: libevent(八)bufferevent

在用戶的回調函數中，通過bufferevent_read從輸入緩沖input中讀數據，相應地，通過bufferevent_write向輸出緩沖output中寫數據。

關於寫事件，這裏要多說幾句。

對於一個fd，只要它的寫緩沖區沒有滿，就會觸發寫事件。一般情況下，如果不向這個fd發送大量的數據，它的寫緩沖區是不會滿的。

所以如果一開始就監聽寫事件，那麽寫事件會一直被觸發。

libevent的做法是：當我們確實要寫入數據時，才監聽寫事件。

下面是bufferevent_write的實現：

int
bufferevent_write(
 
struct bufferevent *bufev, const void *data, size_t size)
{
    if (evbuffer_add(bufev->output, data, size) == -1)
        return (-1);

    return 0;
}

/* Adds data to an event buffer */

int
evbuffer_add(struct evbuffer *buf, const void *data_in, size_t datlen)
{
    struct evbuffer_chain *chain, *tmp;
    
 
const unsigned char *data = data_in;
    size_t remain, to_alloc;
    int result = -1;

    EVBUFFER_LOCK(buf);

    if (buf->freeze_end) {
        goto done;
    }
    /* Prevent buf->total_len overflow */
    if (datlen > EV_SIZE_MAX - buf->total_len) {
        goto done;
    }

    chain 
 
= buf->last;

    /* If there are no chains allocated for this buffer, allocate one
     * big enough to hold all the data. */
    if (chain == NULL) {
        chain = evbuffer_chain_new(datlen);
        if (!chain)
            goto done;
        evbuffer_chain_insert(buf, chain);
    }

    if ((chain->flags & EVBUFFER_IMMUTABLE) == 0) {
        /* Always true for mutable buffers */
        EVUTIL_ASSERT(chain->misalign >= 0 &&
            (ev_uint64_t)chain->misalign <= EVBUFFER_CHAIN_MAX);
        remain = chain->buffer_len - (size_t)chain->misalign - chain->off;
        if (remain >= datlen) {
            /* there‘s enough space to hold all the data in the
             * current last chain */
            memcpy(chain->buffer + chain->misalign + chain->off,
                data, datlen);
            chain->off += datlen;
            buf->total_len += datlen;
            buf->n_add_for_cb += datlen;
            goto out;
        } else if (!CHAIN_PINNED(chain) &&
            evbuffer_chain_should_realign(chain, datlen)) {
            /* we can fit the data into the misalignment */
            evbuffer_chain_align(chain);

            memcpy(chain->buffer + chain->off, data, datlen);
            chain->off += datlen;
            buf->total_len += datlen;
            buf->n_add_for_cb += datlen;
            goto out;
        }
    } else {
        /* we cannot write any data to the last chain */
        remain = 0;
    }

    /* we need to add another chain */
    to_alloc = chain->buffer_len;
    if (to_alloc <= EVBUFFER_CHAIN_MAX_AUTO_SIZE/2)
        to_alloc <<= 1;
    if (datlen > to_alloc)
        to_alloc = datlen;
    tmp = evbuffer_chain_new(to_alloc);
    if (tmp == NULL)
        goto done;

    if (remain) {
        memcpy(chain->buffer + chain->misalign + chain->off,
            data, remain);
        chain->off += remain;
        buf->total_len += remain;
        buf->n_add_for_cb += remain;
    }

    data += remain;
    datlen -= remain;

    memcpy(tmp->buffer, data, datlen);
    tmp->off = datlen;
    evbuffer_chain_insert(buf, tmp);
    buf->n_add_for_cb += datlen;

out:
    evbuffer_invoke_callbacks(buf);
    result = 0;
done:
    EVBUFFER_UNLOCK(buf);
    return result;
}

有一點要註意的是：

evbuffer_add將用戶數據寫入輸出緩沖output之後，還要通過evbuffer_invoke_callbacks調用回調函數。

在上文libevent(八)bufferevent中，我們通過bufferevent_socket_new創建bufferevent的時候，設置了輸出緩沖output的回調函數為bufferevent_socket_outbuf_cb，此時就派上用場了。

static void
bufferevent_socket_outbuf_cb(struct evbuffer *buf,
    const struct evbuffer_cb_info *cbinfo,
    void *arg)
{
    struct bufferevent *bufev = arg;
    struct bufferevent_private *bufev_p =
        EVUTIL_UPCAST(bufev, struct bufferevent_private, bev);

    if (cbinfo->n_added &&
        (bufev->enabled & EV_WRITE) &&
        !event_pending(&bufev->ev_write, EV_WRITE, NULL) &&
        !bufev_p->write_suspended) {
        /* Somebody added data to the buffer, and we would like to
         * write, and we were not writing.  So, start writing. */
        if (be_socket_add(&bufev->ev_write, &bufev->timeout_write) == -1) {
            /* Should we log this? */
        }
    }
}

可以看到，在bufferevent_socket_outbuf_cb中，我們將寫事件添加到了event_base中。

總結一下bufferevent_write：

1. 將用戶數據寫入輸出緩沖區output

2. 添加寫事件到事件循環

由於此時fd的寫緩沖區沒有滿，所以寫事件被觸發，開始執行寫事件的回調函數bufferevent_writecb

static void
bufferevent_writecb(evutil_socket_t fd, short event, void *arg)
{
    struct bufferevent *bufev = arg;
    struct bufferevent_private *bufev_p =
        EVUTIL_UPCAST(bufev, struct bufferevent_private, bev);
    int res = 0;
    short what = BEV_EVENT_WRITING;
    int connected = 0;
    ev_ssize_t atmost = -1;

    _bufferevent_incref_and_lock(bufev);

    if (event == EV_TIMEOUT) {
        /* Note that we only check for event==EV_TIMEOUT. If
         * event==EV_TIMEOUT|EV_WRITE, we can safely ignore the
         * timeout, since a read has occurred */
        what |= BEV_EVENT_TIMEOUT;
        goto error;
    }
    if (bufev_p->connecting) {
        int c = evutil_socket_finished_connecting(fd);
        /* we need to fake the error if the connection was refused
         * immediately - usually connection to localhost on BSD */
        if (bufev_p->connection_refused) {
          bufev_p->connection_refused = 0;
          c = -1;
        }

        if (c == 0)
            goto done;

        bufev_p->connecting = 0;
        if (c < 0) {
            event_del(&bufev->ev_write);
            event_del(&bufev->ev_read);
            _bufferevent_run_eventcb(bufev, BEV_EVENT_ERROR);
            goto done;
        } else {
            connected = 1;
#ifdef WIN32
            if (BEV_IS_ASYNC(bufev)) {
                event_del(&bufev->ev_write);
                bufferevent_async_set_connected(bufev);
                _bufferevent_run_eventcb(bufev,
                        BEV_EVENT_CONNECTED);
                goto done;
            }
#endif
            _bufferevent_run_eventcb(bufev,
                    BEV_EVENT_CONNECTED);
            if (!(bufev->enabled & EV_WRITE) ||
                bufev_p->write_suspended) {
                event_del(&bufev->ev_write);
                goto done;
            }
        }
    }

    atmost = _bufferevent_get_write_max(bufev_p);

    if (bufev_p->write_suspended)
        goto done;

    if (evbuffer_get_length(bufev->output)) {
        evbuffer_unfreeze(bufev->output, 1);
        res = evbuffer_write_atmost(bufev->output, fd, atmost);
        evbuffer_freeze(bufev->output, 1);
        if (res == -1) {
            int err = evutil_socket_geterror(fd);
            if (EVUTIL_ERR_RW_RETRIABLE(err))
                goto reschedule;
            what |= BEV_EVENT_ERROR;
        } else if (res == 0) {
            /* eof case
               XXXX Actually, a 0 on write doesn‘t indicate
               an EOF. An ECONNRESET might be more typical.
             */
            what |= BEV_EVENT_EOF;
        }
        if (res <= 0)
            goto error;

        _bufferevent_decrement_write_buckets(bufev_p, res);
    }

    if (evbuffer_get_length(bufev->output) == 0) {
        event_del(&bufev->ev_write);
    }

    /*
     * Invoke the user callback if our buffer is drained or below the
     * low watermark.
     */
    if ((res || !connected) &&
        evbuffer_get_length(bufev->output) <= bufev->wm_write.low) {
        _bufferevent_run_writecb(bufev);
    }

    goto done;

 reschedule:
    if (evbuffer_get_length(bufev->output) == 0) {
        event_del(&bufev->ev_write);
    }
    goto done;

 error:
    bufferevent_disable(bufev, EV_WRITE);
    _bufferevent_run_eventcb(bufev, what);

 done:
    _bufferevent_decref_and_unlock(bufev);
}

三個註意點：

1. 調用evbuffer_write_atmost寫數據

　　evbuffer_write_atmost通過write系統調用將數據寫入fd寫緩沖區。

2. 如果數據寫完，刪除寫事件

3. 最後會調用用戶定義的回調函數

整個寫事件流程如下：

1. 用戶主動調用

libevent(九)bufferevent