在用户的回调函数中，通过bufferevent_read从输入缓冲input中读数据，相应地，通过bufferevent_write向输出缓冲output中写数据。
关于写事件，这里要多说几句。
对于一个fd，只要它的写缓冲区没有满，就会触发写事件。一般情况下，如果不向这个fd发送大量的数据，它的写缓冲区是不会满的。
所以如果一开始就监听写事件，那么写事件会一直被触发。
libevent的做法是：当我们确实要写入数据时，才监听写事件。
下面是bufferevent_write的实现：
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;
}
有一点要注意的是：
evbuffer_add将用户数据写入输出缓冲output之后，还要通过evbuffer_invoke_callbacks调用回调函数。
在上文libevent(八)bufferevent中，我们通过bufferevent_socket_new创建bufferevent的时候，设置了输出缓冲output的回调函数为bufferevent_socket_outbuf_cb，此时就派上用场了。
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，我们将写事件添加到了event_base中。
总结一下bufferevent_write：
1. 将用户数据写入输出缓冲区output
2. 添加写事件到事件循环
由于此时fd的写缓冲区没有满，所以写事件被触发，开始执行写事件的回调函数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);
}
三个注意点：
1. 调用evbuffer_write_atmost写数据
evbuffer_write_atmost通过write系统调用将数据写入fd写缓冲区。
2. 如果数据写完，删除写事件
3. 最后会调用用户定义的回调函数
整个写事件流程如下：
1. 用户主动调用
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时间： 2024-08-08 08:23:33