libevent中的缓冲区(一）



libevent中的缓冲区定义为evbuffer,主要在文件evbuffer-internal.h文件中，定义如下

struct evbuffer {
	/** The first chain in this buffer's linked list of chains. */
	struct evbuffer_chain *first;  //缓冲区中的第一个chain
	/** The last chain in this buffer's linked list of chains. */
	struct evbuffer_chain *last;   //缓冲区中的最后一个chain

	/** Pointer to the next pointer pointing at the 'last_with_data' chain.
	 *
	 * To unpack:
	 *
	 * The last_with_data chain is the last chain that has any data in it.
	 * If all chains in the buffer are empty, it is the first chain.
	 * If the buffer has no chains, it is NULL.
	 *
	 * The last_with_datap pointer points at _whatever 'next' pointer_
	 * points at the last_with_datap chain.  If the last_with_data chain
	 * is the first chain, or it is NULL, then the last_with_datap pointer
	 * is &buf->first.
	 */
	struct evbuffer_chain **last_with_datap; //指向链中包含最后数据的chain

	/** Total amount of bytes stored in all chains.*/
	size_t total_len;   //包含的数据长度

	/** Number of bytes we have added to the buffer since we last tried to
	 * invoke callbacks. */
	size_t n_add_for_cb;  //回调时要处理的字节数
	/** Number of bytes we have removed from the buffer since we last
	 * tried to invoke callbacks. */
	size_t n_del_for_cb;   //回调时要删除的字节数

#ifndef _EVENT_DISABLE_THREAD_SUPPORT
	/** A lock used to mediate access to this buffer. */
	void *lock;
#endif
	/** True iff we should free the lock field when we free this
	 * evbuffer. */
	unsigned own_lock : 1;
	/** True iff we should not allow changes to the front of the buffer
	 * (drains or prepends). */
	unsigned freeze_start : 1; //在缓冲区头允许修改数据的标志
	/** True iff we should not allow changes to the end of the buffer
	 * (appends) */
	unsigned freeze_end : 1;   //在缓冲区尾允许修改数据的标志
	/** True iff this evbuffer's callbacks are not invoked immediately
	 * upon a change in the buffer, but instead are deferred to be invoked
	 * from the event_base's loop.	Useful for preventing enormous stack
	 * overflows when we have mutually recursive callbacks, and for
	 * serializing callbacks in a single thread. */
	unsigned deferred_cbs : 1;  //延迟回调的标志
#ifdef WIN32
	/** True iff this buffer is set up for overlapped IO. */
	unsigned is_overlapped : 1;   //是否设置iocp
#endif
	/** Zero or more EVBUFFER_FLAG_* bits */
	ev_uint32_t flags;   //缓冲区标志

	/** Used to implement deferred callbacks. */
	struct deferred_cb_queue *cb_queue;   //延迟回调队列

	/** A reference count on this evbuffer.	 When the reference count
	 * reaches 0, the buffer is destroyed.	Manipulated with
	 * evbuffer_incref and evbuffer_decref_and_unlock and
	 * evbuffer_free. */
	int refcnt;   //缓冲区引用次数

	/** A deferred_cb handle to make all of this buffer's callbacks
	 * invoked from the event loop. */
	struct deferred_cb deferred;  //延迟回调

	/** A doubly-linked-list of callback functions */
	TAILQ_HEAD(evbuffer_cb_queue, evbuffer_cb_entry) callbacks;	//缓冲区回调队列

	/** The parent bufferevent object this evbuffer belongs to.
	 * NULL if the evbuffer stands alone. */
	struct bufferevent *parent;  //与缓冲区相关的bufferevent
};

缓冲区链的结构evbuffer_chain定义为：

struct evbuffer_chain {
	/** points to next buffer in the chain */
	struct evbuffer_chain *next;  //指向下一个链

	/** total allocation available in the buffer field. */
	size_t buffer_len;		//		链中缓冲区长度

	/** unused space at the beginning of buffer or an offset into a
	 * file for sendfile buffers. */
	ev_misalign_t misalign;  //在缓冲区头中没有使用的个数

	/** Offset into buffer + misalign at which to start writing.
	 * In other words, the total number of bytes actually stored
	 * in buffer. */
	size_t off;   //与缓冲区开始使用处的偏移

	/** Set if special handling is required for this chain */
	unsigned flags; //缓冲区标志
#define EVBUFFER_MMAP		0x0001	/**< memory in buffer is mmaped */
#define EVBUFFER_SENDFILE	0x0002	/**< a chain used for sendfile */
#define EVBUFFER_REFERENCE	0x0004	/**< a chain with a mem reference */
#define EVBUFFER_IMMUTABLE	0x0008	/**< read-only chain */
	/** a chain that mustn't be reallocated or freed, or have its contents
	 * memmoved, until the chain is un-pinned. */
#define EVBUFFER_MEM_PINNED_R	0x0010
#define EVBUFFER_MEM_PINNED_W	0x0020
#define EVBUFFER_MEM_PINNED_ANY (EVBUFFER_MEM_PINNED_R|EVBUFFER_MEM_PINNED_W)
	/** a chain that should be freed, but can't be freed until it is
	 * un-pinned. */
#define EVBUFFER_DANGLING	0x0040

	/** Usually points to the read-write memory belonging to this
	 * buffer allocated as part of the evbuffer_chain allocation.
	 * For mmap, this can be a read-only buffer and
	 * EVBUFFER_IMMUTABLE will be set in flags.  For sendfile, it
	 * may point to NULL.
	 */
	unsigned char *buffer;  //存放数据的缓冲区
};

其逻辑结构图为