libevent(九)bufferevent
bufferevent，带buffer的event
struct bufferevent { struct event_base *ev_base; const struct bufferevent_ops *be_ops; // backend struct event ev_read; // 读事件 struct event ev_write; // 写事件 struct evbuffer *input; // 读缓冲 struct evbuffer *output; // 写缓冲 bufferevent_data_cb readcb; // 读事件回调 bufferevent_data_cb writecb; // 写事件回调 bufferevent_event_cb errorcb; // error回调 void *cbarg; // 回调函数参数 struct event_watermark wm_read; struct event_watermark wm_write; struct timeval timeout_read; struct timeval timeout_write; short enabled; };
下面简单分析bufferevent相关函数(示例DEMO)
bufferevent_socket_new
struct bufferevent * bufferevent_socket_new(struct event_base *base, evutil_socket_t fd, int options) { struct bufferevent_private *bufev_p; struct bufferevent *bufev; #ifdef WIN32 if (base && event_base_get_iocp(base)) return bufferevent_async_new(base, fd, options); #endif if ((bufev_p = mm_calloc(1, sizeof(struct bufferevent_private)))== NULL) return NULL; if (bufferevent_init_common(bufev_p, base, &bufferevent_ops_socket, options) < 0) { mm_free(bufev_p); return NULL; } bufev = &bufev_p->bev; evbuffer_set_flags(bufev->output, EVBUFFER_FLAG_DRAINS_TO_FD); event_assign(&bufev->ev_read, bufev->ev_base, fd, EV_READ|EV_PERSIST, bufferevent_readcb, bufev); event_assign(&bufev->ev_write, bufev->ev_base, fd, EV_WRITE|EV_PERSIST, bufferevent_writecb, bufev); evbuffer_add_cb(bufev->output, bufferevent_socket_outbuf_cb, bufev); evbuffer_freeze(bufev->input, 0); evbuffer_freeze(bufev->output, 1); return bufev; }
函数做了4件事：
- 设置backend: bufferevent_ops_socket
- 设置fd的读事件回调: bufferevent_readcb
- 设置fd的写事件回调: bufferevent_writecb
- 设置输出缓冲区的回调: bufferevent_socket_outbuf_cb
backend结构如下:
const struct bufferevent_ops bufferevent_ops_socket = { "socket", evutil_offsetof(struct bufferevent_private, bev), be_socket_enable, be_socket_disable, be_socket_destruct, be_socket_adj_timeouts, be_socket_flush, be_socket_ctrl, };
bufferevent_setcb
void bufferevent_setcb(struct bufferevent *bufev, bufferevent_data_cb readcb, bufferevent_data_cb writecb, bufferevent_event_cb eventcb, void *cbarg) { BEV_LOCK(bufev); bufev->readcb = readcb; bufev->writecb = writecb; bufev->errorcb = eventcb; bufev->cbarg = cbarg; BEV_UNLOCK(bufev); }
该函数主要设置用户回调函数。
bufferevent_enable
int bufferevent_enable(struct bufferevent *bufev, short event) { struct bufferevent_private *bufev_private = EVUTIL_UPCAST(bufev, struct bufferevent_private, bev); short impl_events = event; int r = 0; _bufferevent_incref_and_lock(bufev); if (bufev_private->read_suspended) impl_events &= ~EV_READ; if (bufev_private->write_suspended) impl_events &= ~EV_WRITE; bufev->enabled |= event; if (impl_events && bufev->be_ops->enable(bufev, impl_events) < 0) r = -1; _bufferevent_decref_and_unlock(bufev); return r; } static int be_socket_enable(struct bufferevent *bufev, short event) { if (event & EV_READ) { if (be_socket_add(&bufev->ev_read,&bufev->timeout_read) == -1) return -1; } if (event & EV_WRITE) { if (be_socket_add(&bufev->ev_write,&bufev->timeout_write) == -1) return -1; } return 0; } #define be_socket_add(ev, t) \ _bufferevent_add_event((ev), (t)) int _bufferevent_add_event(struct event *ev, const struct timeval *tv) { if (tv->tv_sec == 0 && tv->tv_usec == 0) return event_add(ev, NULL); else return event_add(ev, tv); }
该函数将fd加入到epoll中。
事件流程
当fd上有读事件发生时，首先调用bufferevent_readcb。
static void bufferevent_readcb(evutil_socket_t fd, short event, void *arg) { struct bufferevent *bufev = arg; struct bufferevent_private *bufev_p = EVUTIL_UPCAST(bufev, struct bufferevent_private, bev); struct evbuffer *input; int res = 0; short what = BEV_EVENT_READING; ev_ssize_t howmuch = -1, readmax=-1; _bufferevent_incref_and_lock(bufev); if (event == EV_TIMEOUT) { /* Note that we only check for event==EV_TIMEOUT. If * event==EV_TIMEOUT|EV_READ, we can safely ignore the * timeout, since a read has occurred */ what |= BEV_EVENT_TIMEOUT; goto error; } input = bufev->input; /* * If we have a high watermark configured then we don't want to * read more data than would make us reach the watermark. */ if (bufev->wm_read.high != 0) { howmuch = bufev->wm_read.high - evbuffer_get_length(input); /* we somehow lowered the watermark, stop reading */ if (howmuch <= 0) { bufferevent_wm_suspend_read(bufev); goto done; } } readmax = _bufferevent_get_read_max(bufev_p); if (howmuch < 0 || howmuch > readmax) /* The use of -1 for "unlimited" * uglifies this code. XXXX */ howmuch = readmax; if (bufev_p->read_suspended) goto done; evbuffer_unfreeze(input, 0); res = evbuffer_read(input, fd, (int)howmuch); /* XXXX evbuffer_read would do better to take and return ev_ssize_t */ evbuffer_freeze(input, 0); if (res == -1) { int err = evutil_socket_geterror(fd); if (EVUTIL_ERR_RW_RETRIABLE(err)) goto reschedule; /* error case */ what |= BEV_EVENT_ERROR; } else if (res == 0) { /* eof case */ what |= BEV_EVENT_EOF; } if (res <= 0) goto error; _bufferevent_decrement_read_buckets(bufev_p, res); /* Invoke the user callback - must always be called last */ if (evbuffer_get_length(input) >= bufev->wm_read.low) _bufferevent_run_readcb(bufev); goto done; reschedule: goto done; error: bufferevent_disable(bufev, EV_READ); _bufferevent_run_eventcb(bufev, what); done: _bufferevent_decref_and_unlock(bufev); }
通过evbuffer_read(内部调用recv)，将fd上的数据读入输入缓冲input中。
通过_bufferevent_run_readcb调用用户回调函数
(如果出错，通过_bufferevent_run_eventcb调用errorcb)
void _bufferevent_run_readcb(struct bufferevent *bufev) { /* Requires that we hold the lock and a reference */ struct bufferevent_private *p = EVUTIL_UPCAST(bufev, struct bufferevent_private, bev); if (bufev->readcb == NULL) return; if (p->options & BEV_OPT_DEFER_CALLBACKS) { p->readcb_pending = 1; if (!p->deferred.queued) SCHEDULE_DEFERRED(p); } else { bufev->readcb(bufev, bufev->cbarg); } }
在用户回调函数中，可以通过 bufferevent_read 取出输入缓冲input中的数据。
size_t bufferevent_read(struct bufferevent *bufev, void *data, size_t size) { return (evbuffer_remove(bufev->input, data, size)); } /* Reads data from an event buffer and drains the bytes read */ int evbuffer_remove(struct evbuffer *buf, void *data_out, size_t datlen) { ev_ssize_t n; EVBUFFER_LOCK(buf); n = evbuffer_copyout(buf, data_out, datlen); if (n > 0) { if (evbuffer_drain(buf, n)<0) n = -1; } EVBUFFER_UNLOCK(buf); return (int)n; }
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