libevent的evbuffer详解(含源码详细注释+测试用例)

前天剖析了libevent的事件链表tail queue，今天来剖析一下它的evbuffer。evbuffer是libevent中的缓冲区模块，支持读写数据，尤其是可以按行读取，特别是TCP这种基于字节流的数据，需要从字节流解析自己的通讯协议，借助缓冲区保存多余的数据，以便和下一次读到的数据合并分析。

evbuffer对外提供的一系列函数简介，可以参看这篇博客：http://blog.sina.com.cn/s/blog_4ab24dd501013d0h.html

我的这篇博客主要用来分析源码，并附上详细注释。

下面是evbuffer结构体：

//libevnet的缓冲模块struct evbuffer {u_char *buffer;    //当前存放有效数据的缓冲区的内存起始地址u_char *orig_buffer;   //整个分配(realloc)用来缓冲的内存起始地址 size_t misalign;  //origin_buffer和buffer之间的字节数 size_t totallen;   //整个分配用来缓冲的内存字节数size_t off;           //当前存放有效数据的缓冲区的长度(字节数)void (*cb)(struct evbuffer *, size_t, size_t, void *);  //缓冲区有变化时调用的回调函数，可以不设置void *cbarg;   //回调函数的参数};

那么怎么理解evbuffer呢？请看下图：

orig_buffer就是指向你申请的总大小，buffer是实际存放数据的大小。读数据从前往后读，写数据相当于追加数据。有以下几种情况：

1.刚开始orign_buffer=buffer,misalign=off=0,放入数据，off增加，即buffer指向的有效数据区增加。

2.读取了一部分数据，一部分数据变为used已经使用了，那么就干掉用过的那部分，buffer向后移，misalign增加，off减小。

3.如果要些数据，那么进行相应的判断，free space是否足够，这是有这几种情况：

     (1)足够，那么直接追加在buffer后面即可。

     (2)free space不够，used区域，也就是已经使用过的但目前没有利用的区域有空间，那么将buffer挪回orig_buffer，相当于扩大free space，再写数据。

     (3)都不够，那么直接扩大内存，以realloc方式，最少一次扩大256字节，后面每次以2倍的方式扩大，和STL的vectot内存策略一样。

下面我将源码附带注释贴出来，这些都在buffer.c中：

#ifdef HAVE_CONFIG_H#include "config.h"#endif#ifdef WIN32#include <winsock2.h>#include <windows.h>#endif#ifdef HAVE_VASPRINTF/* If we have vasprintf, we need to define this before we include stdio.h. */#define _GNU_SOURCE#endif#include <sys/types.h>#ifdef HAVE_SYS_TIME_H#include <sys/time.h>#endif#ifdef HAVE_SYS_IOCTL_H#include <sys/ioctl.h>#endif#include <assert.h>#include <errno.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#ifdef HAVE_STDARG_H#include <stdarg.h>#endif#ifdef HAVE_UNISTD_H#include <unistd.h>#endif#include "event.h"#include "config.h"#include "evutil.h"struct evbuffer *evbuffer_new(void){struct evbuffer *buffer;buffer = calloc(1, sizeof(struct evbuffer));  //动态分配一个evbufferreturn (buffer);}voidevbuffer_free(struct evbuffer *buffer){if (buffer->orig_buffer != NULL)    //先判断orig_buffer是否需要释放，防止内存泄漏free(buffer->orig_buffer);free(buffer);}/*  * This is a destructive add.  The data from one buffer moves into * the other buffer. */#define SWAP(x,y) do { \        //这个是传说中的单向swap???(x)->buffer = (y)->buffer; \(x)->orig_buffer = (y)->orig_buffer; \(x)->misalign = (y)->misalign; \(x)->totallen = (y)->totallen; \(x)->off = (y)->off; \} while (0)//移动数据从一个evbuffer到另一个evbufferintevbuffer_add_buffer(struct evbuffer *outbuf, struct evbuffer *inbuf){int res;/* Short cut for better performance */if (outbuf->off == 0) {      //如果输出outbuf无有效数据，直接交换，无数据outbuf和有数据inbuf交换会清除inbuf中的数据struct evbuffer tmp;size_t oldoff = inbuf->off;/* Swap them directly */SWAP(&tmp, outbuf);    //这里就用了上面那个单向swap，用了3次SWAP(outbuf, inbuf);SWAP(inbuf, &tmp);/*  * Optimization comes with a price; we need to notify the * buffer if necessary of the changes. oldoff is the amount * of data that we transfered from inbuf to outbuf */if (inbuf->off != oldoff && inbuf->cb != NULL)     //如果inbuf->off!=oldoff说明交换成功，若设置回调就调用(*inbuf->cb)(inbuf, oldoff, inbuf->off, inbuf->cbarg);if (oldoff && outbuf->cb != NULL)   //如果老的oldoff有货，且输出outbuf设置就调用(*outbuf->cb)(outbuf, 0, oldoff, outbuf->cbarg);return (0);}res = evbuffer_add(outbuf, inbuf->buffer, inbuf->off);   //将in的evbuffer追加到outbuf中，这里不完美，如果inbuf->off为0，就不用调用if (res == 0) {/* We drain the input buffer on success */evbuffer_drain(inbuf, inbuf->off);}return (res);}intevbuffer_add_vprintf(struct evbuffer *buf, const char *fmt, va_list ap){char *buffer;size_t space;size_t oldoff = buf->off;  int sz;va_list aq;/* make sure that at least some space is available */evbuffer_expand(buf, 64);    //64不是要直接去扩展64字节，而是用64作为基准去衡量有没有free空间for (;;) {                                  //如果连64字节都不够，才进行相应的扩展size_t used = buf->misalign + buf->off;   buffer = (char *)buf->buffer + buf->off;assert(buf->totallen >= used);space = buf->totallen - used;    //空闲空间#ifndef va_copy#defineva_copy(dst, src)memcpy(&(dst), &(src), sizeof(va_list))   //va_list拷贝#endifva_copy(aq, ap);sz = evutil_vsnprintf(buffer, space, fmt, aq);   //交给该函数实现va_end(aq);if (sz < 0)    //失败返回return (-1);if ((size_t)sz < space) {   //返回大小小于spacebuf->off += sz;       //更新偏移if (buf->cb != NULL)    //如果设置了，调用(*buf->cb)(buf, oldoff, buf->off, buf->cbarg);return (sz);}if (evbuffer_expand(buf, sz + 1) == -1)    //确保字符串和\0都被写入了buffer的有效地址，防止\0写入位置越界return (-1);}/* NOTREACHED */}//添加一个格式化的字符串到evbuffer尾部intevbuffer_add_printf(struct evbuffer *buf, const char *fmt, ...)  {int res = -1;va_list ap;va_start(ap, fmt);res = evbuffer_add_vprintf(buf, fmt, ap);   //调用该函数实现va_end(ap);return (res);}/* Reads data from an event buffer and drains the bytes read *///读取evbuffer缓冲区的数据到data中，长度为datlenintevbuffer_remove(struct evbuffer *buf, void *data, size_t datlen){size_t nread = datlen;if (nread >= buf->off)    //如果大，读已有的nread = buf->off;memcpy(data, buf->buffer, nread); evbuffer_drain(buf, nread);    //同样调用消耗函数，清除已读数据return (nread);}/* * Reads a line terminated by either '\r\n', '\n\r' or '\r' or '\n'. * The returned buffer needs to be freed by the called. *///读取以\r或\n结尾的一行数据char *evbuffer_readline(struct evbuffer *buffer){u_char *data = EVBUFFER_DATA(buffer); //(x)->buffersize_t len = EVBUFFER_LENGTH(buffer); //(x)->off,不知道为什么只有此处用了这两个宏，本文件其它可用的地方都没有用char *line;unsigned int i;for (i = 0; i < len; i++) {if (data[i] == '\r' || data[i] == '\n')break;}if (i == len)   //没找到\r或\n直接返回NULLreturn (NULL);if ((line = malloc(i + 1)) == NULL) {fprintf(stderr, "%s: out of memory\n", __func__);return (NULL);}memcpy(line, data, i);    //从buffer拷贝到lineline[i] = '\0';/* * Some protocols terminate a line with '\r\n', so check for * that, too. */if ( i < len - 1 ) {      //如果找到的小于len-1，有些协议可能存在\r\n情况，也要处理一下char fch = data[i], sch = data[i+1];/* Drain one more character if needed */if ( (sch == '\r' || sch == '\n') && sch != fch )i += 1;    //如果是\r\n，再往后走一个}evbuffer_drain(buffer, i + 1);    //注意参数i=1，因为drain函数中用buffer+=len得到新buffer的位置return (line);   //返回数据}/* Adds data to an event buffer *///此函数是evbuffer的调整函数，将evbuffer的buffer段前移到起始位置orig_bufferstatic voidevbuffer_align(struct evbuffer *buf)  {memmove(buf->orig_buffer, buf->buffer, buf->off);    //直接调用memmovebuf->buffer = buf->orig_buffer;buf->misalign = 0;            //更新misalign}/* Expands the available space in the event buffer to at least datlen */intevbuffer_expand(struct evbuffer *buf, size_t datlen){size_t need = buf->misalign + buf->off + datlen;   //先计算总需求/* If we can fit all the data, then we don't have to do anything */if (buf->totallen >= need)    //如果够，不扩展，直接返回return (0);/* * If the misalignment fulfills our data needs, we just force an * alignment to happen.  Afterwards, we have enough space. */if (buf->misalign >= datlen) {  //如果前面0->misalign空间足够datlen，将evbuffer调整，将buffer段内数据前移evbuffer_align(buf);} else {                                     //不够就另外开辟一段空间void *newbuf;size_t length = buf->totallen;     //总大小if (length < 256)       //最少一次性开辟256字节length = 256;while (length < need)     //如果总大小都小于需求，就直接开辟2倍，类似于STL中vector的内存策略length <<= 1;if (buf->orig_buffer != buf->buffer)    //另外开辟前先把buffer前移到起始位置evbuffer_align(buf);if ((newbuf = realloc(buf->buffer, length)) == NULL)    //开辟另外的内存空间return (-1);buf->orig_buffer = buf->buffer = newbuf;   //将两个buffer指针更新到新的内存地址buf->totallen = length;    //更新总大小}return (0);}//将data追加到buffer中   //这个函数有个bug，它没有判断datlen的大小，如果datlen=0，那么直接返回就行了intevbuffer_add(struct evbuffer *buf, const void *data, size_t datlen){size_t need = buf->misalign + buf->off + datlen;  //首先判断缓冲区大小，buf->off是已经存放的有效数据长度size_t oldoff = buf->off;if (buf->totallen < need) {    //如果总大小容纳不下datalen大小，扩充容量if (evbuffer_expand(buf, datlen) == -1)return (-1);}memcpy(buf->buffer + buf->off, data, datlen);    //将data追加到buffer+off后buf->off += datlen;    //off长度增加if (datlen && buf->cb != NULL)     //如果长度大于0且设置了回调函数，则调用回调函数(*buf->cb)(buf, oldoff, buf->off, buf->cbarg);  //没设置就什么也不做return (0);}//该函数有两个作用，其一是完全清除有效缓冲区,len设置为>=off即可，相当于有效缓冲区全部消耗掉//其二是消耗一部分缓冲区，缓冲区前段消耗掉，相当于向后移动，misalign增大,off减小voidevbuffer_drain(struct evbuffer *buf, size_t len){size_t oldoff = buf->off;if (len >= buf->off) {  //如果消耗的len的长度大于等于缓冲区off的长度，清空缓冲区buf->off = 0;buf->buffer = buf->orig_buffer;buf->misalign = 0;goto done;     //这个goto我暂时没看出来有什么用，我认为可以用if-else替换}//如果消耗的len不大于off，有效缓冲区向前移动，前面的一部分被读取，misalign增大，off减小?buf->buffer += len;buf->misalign += len;buf->off -= len; done:/* Tell someone about changes in this buffer */if (buf->off != oldoff && buf->cb != NULL)(*buf->cb)(buf, oldoff, buf->off, buf->cbarg);}/* * Reads data from a file descriptor into a buffer. */#define EVBUFFER_MAX_READ4096       //evbuffer的最大可读字节数//值得注意evbuffer_read并不是相对evbuffer_write，它是从描述符往buffer读数据，evbuffer_remove才是读取evbuffer数据//从fd上往buffer读取数据，如果缓冲区不够，则expandintevbuffer_read(struct evbuffer *buf, int fd, int howmuch){u_char *p;size_t oldoff = buf->off;int n = EVBUFFER_MAX_READ;    //最大字节数#if defined(FIONREAD)#ifdef WIN32long lng = n;if (ioctlsocket(fd, FIONREAD, &lng) == -1 || (n=lng) <= 0) {#elseif (ioctl(fd, FIONREAD, &n) == -1 || n <= 0) {#endifn = EVBUFFER_MAX_READ;} else if (n > EVBUFFER_MAX_READ && n > howmuch) {/* * It's possible that a lot of data is available for * reading.  We do not want to exhaust resources * before the reader has a chance to do something * about it.  If the reader does not tell us how much * data we should read, we artifically limit it. */if ((size_t)n > buf->totallen << 2)n = buf->totallen << 2;if (n < EVBUFFER_MAX_READ)n = EVBUFFER_MAX_READ;}#endifif (howmuch < 0 || howmuch > n)    //如果设置要读的字节数小于0或大于n，就让它读默认的4096个字节howmuch = n;     /* If we don't have FIONREAD, we might waste some space here */if (evbuffer_expand(buf, howmuch) == -1)  //如果不够4096就扩充return (-1);/* We can append new data at this point */p = buf->buffer + buf->off;    //p指向free space#ifndef WIN32n = read(fd, p, howmuch);   //系统调用read#elsen = recv(fd, p, howmuch, 0);#endifif (n == -1)      //失败-1返回return (-1);if (n == 0)              //如果为0返回0，读到0个字节return (0);buf->off += n;      //读到数据后，更新off/* Tell someone about changes in this buffer */if (buf->off != oldoff && buf->cb != NULL)   //如果读到数据且设置了回调就调用 (*buf->cb)(buf, oldoff, buf->off, buf->cbarg);return (n);   //返回读到的字节数}//把buffer的数据写入到文件描述符fd上，如果写入成功，调用evbuffer_drain删除已写数据intevbuffer_write(struct evbuffer *buffer, int fd)  {int n;#ifndef WIN32n = write(fd, buffer->buffer, buffer->off);  //直接写#elsen = send(fd, buffer->buffer, buffer->off, 0);#endifif (n == -1)return (-1);if (n == 0)return (0);evbuffer_drain(buffer, n);    //写入到fd后，删除buffer中的数据，相当于消耗掉了return (n);}//查找字符串whatu_char *evbuffer_find(struct evbuffer *buffer, const u_char *what, size_t len){u_char *search = buffer->buffer, *end = search + buffer->off;u_char *p;while (search < end &&                                            (p = memchr(search, *what, end - search)) != NULL) {   //在search和end的范围内查找*what，注意这实在匹配第一个字符if (p + len > end)     //未找到break;if (memcmp(p, what, len) == 0)   //匹配到第一个字符后比较len长度的内存相等则返回，这两个函数组合也算是一种字符串匹配算法return (p);                              //话说这个查找算法可以用KMPsearch = p + 1;     //不相等后移一个字符，继续查找}return (NULL);}//缓冲区变化时设置的回调回掉函数void evbuffer_setcb(struct evbuffer *buffer,    void (*cb)(struct evbuffer *, size_t, size_t, void *),    void *cbarg){buffer->cb = cb;buffer->cbarg = cbarg;}

下面是我借用上文所说博客的例子，这是一段帮助理解evbuffer的程序。

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <assert.h>#include <event.h>int main(){    struct evbuffer* buff = NULL;    char c, c2[3] = {0};    buff = evbuffer_new();    assert(buff != NULL);    evbuffer_add(buff, "1", 1);    evbuffer_add(buff, "2", 1);    evbuffer_add(buff, "3", 1);    evbuffer_add_printf(buff, "%d%d", 4, 5);    assert(buff->off == 5);    evbuffer_remove(buff, &c, sizeof(char));    assert(c == '1');    evbuffer_remove(buff, &c, sizeof(char));    assert(c == '2');    evbuffer_remove(buff, &c, sizeof(char));    assert(c == '3');    evbuffer_remove(buff, c2, 2);    assert(strcmp(c2, "45") == 0);    assert(buff->off == 0);    evbuffer_add(buff, "test\r\n", 6);    assert(buff->off == 6);    char *line = evbuffer_readline(buff);    assert(strcmp(line, "test") == 0);    assert(buff->off == 0);    free(line);       //notice,dll applied for memory, must free here    evbuffer_free(buff);    printf("well\n");    return 0;}

（转载请注明出处，博客地址：http://blog.csdn.net/freeelinux/article/details/52799951)