libevent中的缓冲区(二)

1. 缓冲区的chunk的分配
在分配chunk时，其实chunk头包含一些附加信息，如chunk块存放数据的空间大小(buffer_len)，起始地址(buffer)，当前数据尾到起始片的偏移(off)等。chunk的大小算上头部信息，至少是1024个字节。

static struct evbuffer_chain *
evbuffer_chain_new(size_t size)  //这个size不包括chunk头大小
{struct evbuffer_chain *chain;size_t to_alloc;if (size > EVBUFFER_CHAIN_MAX - EVBUFFER_CHAIN_SIZE)   //如果分配空间的大小加上头部大小大于chunk的最大值就直接返回。return (NULL);size += EVBUFFER_CHAIN_SIZE; //加上chunk头大小/* get the next largest memory that can hold the buffer *///计算容纳size大小的最小值if (size < EVBUFFER_CHAIN_MAX / 2) {to_alloc = MIN_BUFFER_SIZE;while (to_alloc < size) {to_alloc <<= 1;}} else {to_alloc = size;}/* we get everything in one chunk */if ((chain = mm_malloc(to_alloc)) == NULL)return (NULL);//chunk头部作息初始化memset(chain, 0, EVBUFFER_CHAIN_SIZE);chain->buffer_len = to_alloc - EVBUFFER_CHAIN_SIZE;/* this way we can manipulate the buffer to different addresses,* which is required for mmap for example.*/chain->buffer = EVBUFFER_CHAIN_EXTRA(u_char, chain);return (chain);
}

2. 列表内容
因为缓冲区是通过evbuffer_chain的next指针连接起来的，这个是链表操作

static void
evbuffer_free_all_chains(struct evbuffer_chain *chain)
{struct evbuffer_chain *next;for (; chain; chain = next) {next = chain->next;evbuffer_chain_free(chain);}
}

3. 缓冲区中添加chunk
将chunk添加到evbuffer的尾部时，会释放掉所有空的chunk，在添加chunk时，不会调用回调

static void
evbuffer_chain_insert(struct evbuffer *buf,struct evbuffer_chain *chain)
{ASSERT_EVBUFFER_LOCKED(buf);if (*buf->last_with_datap == NULL) {  //表示原来的缓冲区不存在为空，不存在chunk时，将first和last指针赋值为chain/* There are no chains data on the buffer at all. */EVUTIL_ASSERT(buf->last_with_datap == &buf->first);EVUTIL_ASSERT(buf->first == NULL);buf->first = buf->last = chain;} else {//在添加时，会释放尾部所有的空chunkstruct evbuffer_chain **ch = buf->last_with_datap;/* Find the first victim chain.  It might be *last_with_datap */while ((*ch) && ((*ch)->off != 0 || CHAIN_PINNED(*ch)))ch = &(*ch)->next;if (*ch == NULL) {/* There is no victim; just append this new chain. */buf->last->next = chain;if (chain->off)  //如果添加的chunk包含有用数据，就修改last_with_datap指针buf->last_with_datap = &buf->last->next;} else {/* Replace all victim chains with this chain. */EVUTIL_ASSERT(evbuffer_chains_all_empty(*ch));evbuffer_free_all_chains(*ch);*ch = chain;}buf->last = chain;}buf->total_len += chain->off;
}

4. chunk块中flag标志的设置及取消
evbuffer_chain中的flag标志主要是锁定读还是写两种情况，在设置取消操作时比较简单

//设置
_evbuffer_chain_pin(struct evbuffer_chain *chain, unsigned flag)
{EVUTIL_ASSERT((chain->flags & flag) == 0);chain->flags |= flag;
}
//取消
void
_evbuffer_chain_unpin(struct evbuffer_chain *chain, unsigned flag)
{EVUTIL_ASSERT((chain->flags & flag) != 0);chain->flags &= ~flag;if (chain->flags & EVBUFFER_DANGLING)evbuffer_chain_free(chain);
}

5. 缓冲区回调的触发
在写操作使用非阻塞模式时，在调用网络接口可写状态监听时，总是满足的，所以在写操作完成后，会将对应的fd事件从事件循环中删除，而当向缓冲区中写数据时，会将对应的事件添加到event_base中，这时，这个回调就显得比较重要了

void
evbuffer_invoke_callbacks(struct evbuffer *buffer)
{if (TAILQ_EMPTY(&buffer->callbacks)) {buffer->n_add_for_cb = buffer->n_del_for_cb = 0;return;}if (buffer->deferred_cbs) {if (buffer->deferred.queued)return;_evbuffer_incref_and_lock(buffer);if (buffer->parent)bufferevent_incref(buffer->parent);EVBUFFER_UNLOCK(buffer);event_deferred_cb_schedule(buffer->cb_queue, &buffer->deferred);  //将缓冲区回调添加加event_base中的回调队列中}evbuffer_run_callbacks(buffer, 0); //调用回调函数
}

6. 缓冲区大小的调节
在I/O上有数据可读时，将I/O上的数据读入到缓冲区，需要调节缓冲区的大小来填装从I/O上读到的数据。主要是函数evbuffer_reserve_space来完成。

int
evbuffer_reserve_space(struct evbuffer *buf, ev_ssize_t size,struct evbuffer_iovec *vec, int n_vecs)
{struct evbuffer_chain *chain, **chainp;int n = -1;EVBUFFER_LOCK(buf);if (buf->freeze_end)  //尾部禁止写，退出goto done;if (n_vecs < 1)goto done;if (n_vecs == 1) {if ((chain = evbuffer_expand_singlechain(buf, size)) == NULL)goto done;vec[0].iov_base = CHAIN_SPACE_PTR(chain);vec[0].iov_len = (size_t) CHAIN_SPACE_LEN(chain);EVUTIL_ASSERT(size<0 || (size_t)vec[0].iov_len >= (size_t)size);n = 1;} else {if (_evbuffer_expand_fast(buf, size, n_vecs)<0)goto done;n = _evbuffer_read_setup_vecs(buf, size, vec, n_vecs,&chainp, 0);}done:EVBUFFER_UNLOCK(buf);return n;}

7. 设置读入的缓冲区
在集中读时，设置iovec的缓冲区

int
_evbuffer_read_setup_vecs(struct evbuffer *buf, ev_ssize_t howmuch,struct evbuffer_iovec *vecs, int n_vecs_avail,struct evbuffer_chain ***chainp, int exact)
{struct evbuffer_chain *chain;struct evbuffer_chain **firstchainp;size_t so_far;int i;ASSERT_EVBUFFER_LOCKED(buf);if (howmuch < 0)return -1;so_far = 0;/* Let firstchain be the first chain with any space on it */firstchainp = buf->last_with_datap;if (CHAIN_SPACE_LEN(*firstchainp) == 0) {firstchainp = &(*firstchainp)->next;}chain = *firstchainp;for (i = 0; i < n_vecs_avail && so_far < (size_t)howmuch; ++i) {size_t avail = (size_t) CHAIN_SPACE_LEN(chain);if (avail > (howmuch - so_far) && exact)avail = howmuch - so_far;vecs[i].iov_base = CHAIN_SPACE_PTR(chain);vecs[i].iov_len = avail;so_far += avail;chain = chain->next;}*chainp = firstchainp;return i;
}

8. 读入缓冲区
在I/O上有数据可读时就将数据读入evbuffer中，函数evbuffer_read来完成，同时设置缓冲区的回调（即应用层的处理）

int
evbuffer_read(struct evbuffer *buf, evutil_socket_t fd, int howmuch)
{struct evbuffer_chain **chainp;int n;int result;#ifdef USE_IOVEC_IMPLint nvecs, i, remaining;
#elsestruct evbuffer_chain *chain;unsigned char *p;
#endifEVBUFFER_LOCK(buf);if (buf->freeze_end) {result = -1;goto done;}n = get_n_bytes_readable_on_socket(fd);if (n <= 0 || n > EVBUFFER_MAX_READ)n = EVBUFFER_MAX_READ;if (howmuch < 0 || howmuch > n)howmuch = n;#ifdef USE_IOVEC_IMPL/* Since we can use iovecs, we're willing to use the last* NUM_READ_IOVEC chains. */if (_evbuffer_expand_fast(buf, howmuch, NUM_READ_IOVEC) == -1) {result = -1;goto done;} else {IOV_TYPE vecs[NUM_READ_IOVEC];
#ifdef _EVBUFFER_IOVEC_IS_NATIVEnvecs = _evbuffer_read_setup_vecs(buf, howmuch, vecs,NUM_READ_IOVEC, &chainp, 1);
#else/* We aren't using the native struct iovec.  Therefore,we are on win32. */struct evbuffer_iovec ev_vecs[NUM_READ_IOVEC];nvecs = _evbuffer_read_setup_vecs(buf, howmuch, ev_vecs, 2,&chainp, 1);for (i=0; i < nvecs; ++i)WSABUF_FROM_EVBUFFER_IOV(&vecs[i], &ev_vecs[i]);
#endif#ifdef WIN32{DWORD bytesRead;DWORD flags=0;if (WSARecv(fd, vecs, nvecs, &bytesRead, &flags, NULL, NULL)) {/* The read failed. It might be a close,* or it might be an error. */if (WSAGetLastError() == WSAECONNABORTED)n = 0;elsen = -1;} elsen = bytesRead;}
#elsen = readv(fd, vecs, nvecs);
#endif}#else /*!USE_IOVEC_IMPL*//* If we don't have FIONREAD, we might waste some space here *//* XXX we _will_ waste some space here if there is any space left* over on buf->last. */if ((chain = evbuffer_expand_singlechain(buf, howmuch)) == NULL) {result = -1;goto done;}/* We can append new data at this point */p = chain->buffer + chain->misalign + chain->off;#ifndef WIN32n = read(fd, p, howmuch);
#elsen = recv(fd, p, howmuch, 0);
#endif
#endif /* USE_IOVEC_IMPL */if (n == -1) {result = -1;goto done;}if (n == 0) {result = 0;goto done;}#ifdef USE_IOVEC_IMPLremaining = n;for (i=0; i < nvecs; ++i) {/* can't overflow, since only mutable chains have* huge misaligns. */size_t space = (size_t) CHAIN_SPACE_LEN(*chainp);/* XXXX This is a kludge that can waste space in perverse* situations. */if (space > EVBUFFER_CHAIN_MAX)space = EVBUFFER_CHAIN_MAX;if ((ev_ssize_t)space < remaining) {(*chainp)->off += space;remaining -= (int)space;} else {(*chainp)->off += remaining;buf->last_with_datap = chainp;break;}chainp = &(*chainp)->next;}
#elsechain->off += n;advance_last_with_data(buf);
#endifbuf->total_len += n;buf->n_add_for_cb += n;/* Tell someone about changes in this buffer */evbuffer_invoke_callbacks(buf);result = n;
done:EVBUFFER_UNLOCK(buf);return result;
}

9. 缓冲区中的写操作
将缓冲区中的数据写入I/O，在写时分为集中写与一般的写。在一般写的情况下，如果要写的字节数分散在多个chunk中，这就要将多个chunk中的数据合并到一个chunk中，而将其它的释放，这种处理是通过函数evbuffer_pullup来完成。将缓冲区的数据写到socket是通过evbuffer_write_atmost来完成。在写完后，会将缓冲区中的一部分chunk释放。

int
evbuffer_write_atmost(struct evbuffer *buffer, evutil_socket_t fd,ev_ssize_t howmuch)
{int n = -1;EVBUFFER_LOCK(buffer);if (buffer->freeze_start) {goto done;}if (howmuch < 0 || (size_t)howmuch > buffer->total_len)howmuch = buffer->total_len;if (howmuch > 0) {
#ifdef USE_SENDFILEstruct evbuffer_chain *chain = buffer->first;if (chain != NULL && (chain->flags & EVBUFFER_SENDFILE))n = evbuffer_write_sendfile(buffer, fd, howmuch);else {
#endif
#ifdef USE_IOVEC_IMPLn = evbuffer_write_iovec(buffer, fd, howmuch);   //集中写
#elif defined(WIN32)                                //一般写/* XXX(nickm) Don't disable this code until we know if* the WSARecv code above works. */void *p = evbuffer_pullup(buffer, howmuch);EVUTIL_ASSERT(p || !howmuch);n = send(fd, p, howmuch, 0);
#elsevoid *p = evbuffer_pullup(buffer, howmuch);EVUTIL_ASSERT(p || !howmuch);n = write(fd, p, howmuch);                  //一般写
#endif
#ifdef USE_SENDFILE}
#endif}if (n > 0)evbuffer_drain(buffer, n);   //释放done:EVBUFFER_UNLOCK(buffer);return (n);
}

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