一.前言
在前文中，我们分析了libevent库最基本的事件：event的数据结构及相关接口函数，并分析了event背后（backend）的事件库/事件管理核心event_base的数据结构及相关接口函数，由此了解了事件管理的整个循环过程，以及加入、删除、激活、释放事件等基本操作。在此基础上，本文分析缓冲部分，即evbuffer结构。
二.evbuffer结构体
话不多数，直接看源码部分：
struct evbuffer {
/*使用evbuffer_chain存储缓冲：链表形式的多个缓冲区*/
/** The first chain in this buffer's linked list of chains. */
struct evbuffer_chain *first;
/** The last chain in this buffer's linked list of chains. */
struct evbuffer_chain *last;
/** Pointer to the next pointer pointing at the 'last_with_data' chain.
* 该指针指向最后一个有数据的evbuffer_chain结构体
* 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_
* pointing at the last_with_data 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;
/** 存储总数据 Total amount of bytes stored in all chains.*/
size_t total_len;
/** 自上次回调以来增加的buffer
* Number of bytes we have added to the buffer since we last tried to
* invoke callbacks. */
size_t n_add_for_cb;
/** 自上次回调以来清空的buffer
* Number of bytes we have removed from the buffer since we last
* tried to invoke callbacks. */
size_t n_del_for_cb;
/*多线程下保证线程安全的锁*/
#ifndef EVENT__DISABLE_THREAD_SUPPORT
/** A lock used to mediate access to this buffer. */
void *lock;
#endif
/* 标记位，分别作用于：
* 1. 释放缓冲时是否解锁
* 2. 是否允许缓冲区头、尾部的更改
* 3. 是否在改变缓冲区时立刻相应回调函数
*/
/** 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;
/** True iff we should not allow changes to the end of the buffer
* (appends) */
unsigned freeze_end : 1;
/** 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;
/*用于IOCP的IO复用标记位*/
#ifdef _WIN32
/** True iff this buffer is set up for overlapped IO. */
unsigned is_overlapped : 1;
#endif
/** Zero or more EVBUFFER_FLAG_* bits */
ev_uint32_t flags;
/** 对应于标记位回调函数不立刻响应 Used to implement deferred callbacks. */
struct event_base *cb_queue;
/** 标记位，为0的时候清空bufffer
* 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;
/** 回调函数 A struct event_callback handle to make all of this buffer's callbacks
* invoked from the event loop. */
struct event_callback deferred;
/** A doubly-linked-list of callback functions */
LIST_HEAD(evbuffer_cb_queue, evbuffer_cb_entry) callbacks;
/** 涉及到bufferevent 结构体，后文详谈
* The parent bufferevent object this evbuffer belongs to.
* NULL if the evbuffer stands alone. */
struct bufferevent *parent;
};从源码中可以很清楚的看出，libevent的buffer区域并非申请一整块连续空间，而是用一个链表evbuffer_chain的形式存储多个区域，构成了整个evbuffer。这种存储方式的优点在于便于动态增加、减少缓冲区的大小，实现了一个类似于C++的动态数组的容器。
三.evbuffer_chain结构体
evbuffer_chain链表源码如下：
/** 缓冲区链表结构 A single item in an evbuffer. */
struct evbuffer_chain {
/** points to next buffer in the chain */
struct evbuffer_chain *next;
/** 缓冲区长度 total allocation available in the buffer field. */
size_t buffer_len;
/** 缓冲区空闲区域：用于发送buffer
* unused space at the beginning of buffer or an offset into a
* file for sendfile buffers. */
ev_misalign_t misalign;
/** 偏移量 Offset into buffer + misalign at which to start writing.
* In other words, the total number of bytes actually stored
* in buffer. */
size_t off;
/** 标记位：Set if special handling is required for this chain */
unsigned flags;
#define EVBUFFER_FILESEGMENT 0x0001 /**< A chain used for a file segment */
#define EVBUFFER_SENDFILE 0x0002 /**< a chain used with 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
/** a chain that is a referenced copy of another chain */
#define EVBUFFER_MULTICAST 0x0080
/** 对应于evbuffer中同名变量 number of references to this chain */
int refcnt;
/** 真正的缓冲区 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;
};四.小结
本文分析了evbuffer和evbuffer_chain数据结构，和前文event以及event_base一样，evbuffer也有对应的接口函数，在下文中将进行源码的分析和功能的介绍。