5. 设计应用层协议（粘包）

Posted 2022-01-03 阿龙亡命天涯

为什么要自己设计协议？

左边是客户端 右边是服务端

服务端输出：

分开三次发送，服务端应该分开收到三次字符，但是结果是一次性收到了三个字符

分析：
发送的时候ABC按序发送、到达。

发送的数据先进入发送缓冲区，再由操作系统发送给远程主机。
接受数据时接受到的数据存入接受缓冲区，程序从缓冲区读取数据。

A B C都在接受缓冲区，所以读取的时候一次性接受。
数据接收端无法知道数据的发送方式（独立发送还是报答报送）


解决方法：自己设计协议，包含长度等信息




柔性数组在《C语言剖析》中记载过…

//message.h
#ifndef _MESSAGE_H_
#define _MESSAGE_H_

typedef struct message

    unsigned short type;
    unsigned short cmd;
    unsigned short index;
    unsigned short total;
    unsigned int length;
    unsigned char payload[];
Message;

Message* Message_New(unsigned short type,unsigned short cmd,
                    unsigned short index,unsigned total,
                    const char* payload,unsigned int length);



#endif

//message.c
#include "message.h"
#include <malloc.h>
#include <string.h>

Message* Message_New(unsigned short type,unsigned short cmd, unsigned short index,unsigned total,const char* payload,unsigned int length)

    Message* ret = malloc(sizeof(Message) + length);
    if(ret)
    
        ret->type=type;
        ret->cmd=cmd;
        ret->index=index;
        ret->total=total;
        ret->length=length;

        if(payload)
        
            memcpy(ret+1,payload,length);
        
    
    return ret;

//client.c
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include "message.h"

int main()

    int sock = 0;
    struct sockaddr_in addr = 0;
    int len = 0;
    char buf[128] = 0;
    char input[32] = 0;
    int r = 0;
    Message* pm = NULL;

    sock = socket(PF_INET, SOCK_STREAM, 0);

    if( sock == -1 )
    
        printf("socket error\\n");
        return -1;
    

    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    addr.sin_port = htons(8888);

    if( connect(sock, (struct sockaddr*)&addr, sizeof(addr)) == -1 )
    
        printf("connect error\\n");
        return -1;
    

    printf("connect success\\n");

    pm = Message_New(0, 0, 1, 3, "A", 1);

    send(sock, pm, sizeof(Message) + 1, 0);

    pm = Message_New(0, 0, 2, 3, "B", 1);

    send(sock, pm, sizeof(Message) + 1, 0);

    pm = Message_New(0, 0, 3, 3, "C", 1);

    send(sock, pm, sizeof(Message) + 1, 0);

    close(sock);

    return 0;

//server.c
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
int main()

    int server = 0;
    struct sockaddr_in saddr = 0;
    int client = 0;
    struct sockaddr_in caddr = 0;
    socklen_t asize = 0;
    int len = 0;
    char buf[32] = 0;
    int r = 0;

    server = socket(PF_INET, SOCK_STREAM, 0);

    if( server == -1 )
    
        printf("server socket error\\n");
        return -1;
    
    saddr.sin_family = AF_INET;
    saddr.sin_addr.s_addr = htonl(INADDR_ANY);
    saddr.sin_port = htons(8888);
    if( bind(server, (struct sockaddr*)&saddr, sizeof(saddr)) == -1 )
    
        printf("server bind error\\n");
        return -1;
    
    if( listen(server, 1) == -1 )
    
        printf("server bind error\\n");
        return -1;
    
    printf("server start success\\n");
    while( 1 )
    
        asize = sizeof(caddr);
        client = accept(server, (struct sockaddr*)&caddr, &asize);
        if( client == -1 )
        
            printf("client accept error\\n");
            return -1;
        
        printf("client: %d\\n", client);
        do
        
            r = recv(client, buf, sizeof(buf), 0);
            if( r > 0 )
            
                int i = 0;
                for(i=0; i<r; i++)
                
                    printf("%02X ", buf[i]);
                
                printf("\\n");
            
         while ( r > 0 );
        close(client);
    
    close(server);
    return 0;

通过眼睛解析打印输出：
前四个字节分别是type cmd，后四个字节为index total，紧接着的四个字节是长度1，之后就是数据41(a)，重复三次