libevent源码解析（四）evbuffer结构

ゝ一纸荒年。 2022-05-21 00:05 185阅读 0赞

### 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。