基于socket简单通信协议实现

崋果山 2017-07-14

展开全文

1 场景

当用socket进行进程通信，传输数据的时候，会出现以下一些情况：

（1）完整的一条消息被系统拆分成几条发送，例如要发送一条消息：Hello World ，却被系统分成两条消息发送，分别为：Hello 和 World。

（2）几条独立的消息被系统合成一条消息发送，例如要发送两条消息分别为：a memory from my past和it’s been a year，却被系统和成一条消息发送：a memory from my pastit’s been a year。

这个时候，需要为socket通信设计一种通信协议，以保证数据的准确性。

2 协议格式

通信协议设计如下：

Head：帧头，2个字节，此处为0xa5a5

Type：通信类型，1个字节，范围0x00~0xff

Data Length：数据长度，1个字节，即Data的字节总数，

Data：实际传输的数据，长度不定

CS：校验值，1个字节，type、data length、data三个域所有字节的异或值，实际中并没用到校验

End：帧尾，2个字节，此处为0xbeef

3 程序设计

3.1 解析思路

假设socket客户端C和服务端S通信，C向S发送消息M1。

1、 S收到消息M1。S把消息M1拷贝到缓存Q中，Q为循环队列。假如M1的长度大于Q的剩余空间，则只拷贝剩余空间大小的字节到Q。

2、从Q的当前指针开始，查找帧头。如果找到，则当前指针向后移2个字节位置，继续查找；如果没找到，则删除前1个字节，当前指针向后移1个字节位置，继续查找

3、从Q的当前指针开始，查找。如果Q中至少还剩一个字节，则表示找到，当前指针向后移1个字节位置，否则退出解析。

4、从Q的当前指针开始，查找。如果Q中至少还剩一个字节，则表示找到，当前指针向后移1个字节位置，否则退出解析。

5、从Q的当前指针开始，向后移DataLength个字节位置，查找。如果找到，则从Q中取出一条完整的消息P1，并从Q中删除此消息空间，调用外部的回调函数；否则删除帧头的第一个字节a5，当前指针指向帧头第二个字节a5位置，从步骤2开始，重新一轮解析。

3.2 数据结构

查找策略枚举，用于查找时判断查找帧结构的哪个部位：

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typedef enum{
SEARCH_HEAD,
SEARCH_TYPE,
SEARCH_LEN,
//SEARCH_CS,
SEARCH_END,
SEARCH_NONE
}cache_strategy;

消息结构体，用于存储从缓存中解析出的数据：

[cpp] view plain copy

typedef struct{
unsigned char data[SOCKET_MSG_SIZE]; //data
int len;
unsigned char type;
}socket_msg;

回调函数，用于从缓存中解析出消息时调用：

[cpp] view plain copy

typedef void (*tp_socket_msg_handle)(int fd, socket_msg *msg,void *args);

循环队列，用于缓存接收到的数据：

[cpp] view plain copy

typedef struct{
unsigned char buf[SOCKET_MSG_CACHE_SIZE]; //buffer for storing data read from client
int front;
int rear;
int current;
int len;
int tag; //mark that whether the cache is full,1-full,0-not full
cache_strategy strategy;
tp_socket_msg_handle handle;//callback function to invoke when a message is parsed out
void* args; //external parameter
socket_msg recv_msg; //parsed message
}socket_cache;

3.3 关键实现

1、把接收到的数据存储到缓冲中，并准备解析：

[cpp] view plain copy

//copy the unparsed data to cache, and parsed them
int socket_msg_pre_parse(
int fd,
socket_cache *cache,
unsigned char *buf,
int len,
void *args)
{
int n = 0;
unsigned char *p = buf;
//when reading buffer's length is greater than cache's left length,
//we should copy many times.
cache->args = args;
while(1){
n = socket_msg_cpy_in(cache, p, len);
if(n == 0){
return FALSE;//cache is full
}
//parse and handle socket message from cache
socket_msg_parse(fd, cache);
if(n == len){
return TRUE; //copy completed
}
//move the pointer
p = p + n;
len = len - n;
}
return TRUE;
}

2、递归解析消息：

[cpp] view plain copy

//parsed the packaged data, and invoke callback function
void socket_msg_parse(int fd, socket_cache *cache)
{
int current_len;
int p, q;
int i;
int find;
if(cache->front == cache->rear && cache->tag == 0){
//D("socket cache is empty!\n");
return;
}
//calculate the current length of cache
if(cache->current >= cache->front){
current_len = cache->len - (cache->current - cache->front);
}
else{
current_len = cache->rear - cache->current;
}
switch(cache->strategy){
case SEARCH_HEAD://to find a Head format in cache
if(current_len < SOCKET_MSG_HEAD_SIZE){
return;
}
find = FALSE;
for(i = 0; i < current_len - 1; i++){
p = cache->current;
q = (cache->current + 1) % SOCKET_MSG_CACHE_SIZE;
if( (cache->buf[p] == (SOCKET_MSG_HEAD >> 8))&&
(cache->buf[q] == (SOCKET_MSG_HEAD & 0xff))){
find = TRUE;
break; //exit for loop
}
else{
//current pointer move to next
cache->current = q;
//delete one item
cache->front = cache->current;
cache->len --;
cache->tag = 0;
}
}
if(find == TRUE){
//move 2 items towards next
cache->current = (cache->current + 2) % SOCKET_MSG_CACHE_SIZE;
//we found the head format, go on to find Type byte
cache->strategy = SEARCH_TYPE;
}
else{
//if there is no head format ,delete previouse items
LOGE("socket message without head: %x!\n",SOCKET_MSG_HEAD);
//go on to find Head format
cache->strategy = SEARCH_HEAD;
}
break;
case SEARCH_TYPE://to find the type byte in cache
if(current_len < SOCKET_MSG_TYPE_SIZE){
return ;
}
//get the value of type
//cache->type = cache->buf[cache->current];
cache->recv_msg.type = cache->buf[cache->current];
cache->current = (cache->current + 1) % SOCKET_MSG_CACHE_SIZE;
//we found Type byte, go on to find Datalen format
cache->strategy = SEARCH_LEN;
break;
case SEARCH_LEN://to find the datalen byte in cache
if(current_len < SOCKET_MSG_LEN_SIZE){
return ;
}
if(cache->buf[cache->current] > SOCKET_MSG_DATA_SIZE){
LOGE("the data len of message out of size: %d!\n",SOCKET_MSG_DATA_SIZE);
//delete the frist item 'a5'
//move back 2 items
cache->current = cache->current >= 2 ? (cache->current - 2) : (SOCKET_MSG_CACHE_SIZE - 2 + cache->current);
cache->front = cache->current;
//length sub 2
cache->len -= 2;
cache->tag = 0;
//go on to find Head format
cache->strategy = SEARCH_HEAD;
}
else{
//get the value of datalen
//cache->data_len = cache->buf[cache->current];
cache->recv_msg.len = cache->buf[cache->current];
cache->current = (cache->current + 1) % SOCKET_MSG_CACHE_SIZE;
//we found datalen byte, go on to find End format
cache->strategy = SEARCH_END;
}
break;
case SEARCH_END:
if(current_len < (cache->recv_msg.len + SOCKET_MSG_END_SIZE)){
return;
}
//because we have known the data bytes' len, so we move the very
//distance of datalen to see if there is End format.
p = (cache->current + cache->recv_msg.len) % SOCKET_MSG_CACHE_SIZE;
q = (cache->current + cache->recv_msg.len + 1) % SOCKET_MSG_CACHE_SIZE;
if( (cache->buf[p] == (SOCKET_MSG_END >> 8))&&
(cache->buf[q] == (SOCKET_MSG_END & 0xff)) ){
socket_msg_cpy_out(cache, cache->recv_msg.data, cache->current, cache->recv_msg.len);
if(cache->handle != NULL){
//cache->handle(fd, cache->buf + cache->data_index, cache->data_len);
cache->handle(fd, &cache->recv_msg, cache->args);
}
//delete all previous items
cache->current = (q + 1) % SOCKET_MSG_CACHE_SIZE;
cache->front = cache->current;
cache->len -= (cache->recv_msg.len + SOCKET_MSG_FORMAT_SIZE);
cache->tag =0;
}
else{
LOGE("socket message without end: %x!\n",SOCKET_MSG_END);
//delete the frist item 'a5'
//move back 3 items
cache->current = cache->current >= 3 ? (cache->current - 3) : (SOCKET_MSG_CACHE_SIZE - 3 + cache->current);
cache->front = cache->current;
//length sub 3
cache->len -= 3;
cache->tag = 0;
}
//go on to find Head format
cache->strategy = SEARCH_HEAD;
break;
default:
break;
}
//parse new socket message
socket_msg_parse(fd,cache);
}