《Linux网络开发必学教程》8_应用协议设计与实现

问题：下面的代码输出什么？为什么？

printf("connect success\n");

send(sock, "A", 1, 0);
send(sock, "B", 1, 0);
send(sock, "C", 1, 0);

close(sock);

do {
    r = recv(client, buf, sizeof(buf), 0);
    if (r > 0) {
        printf("Recv: %s\n", buf);
    }
}while (1);

close(client);

完整代码 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>

int main()
{
    int sock = 0;
    struct sockaddr_in addr = {0};
    int len = 0;
    char buf[128] = {0};
    char input[32] = {0};
    int r = 0;

    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");

    send(sock, "A", 1, 0);

    send(sock, "B", 1, 0);

    send(sock, "C", 1, 0);

    close(sock);

    return 0;
}

完整代码：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 listen 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) {
                printf("Receive: %s\n", buf);  
            } 
        } while (r > 0);

        close(client);
    }

    close(server);

    return 0;
}

输出

server start success
client: 4
Receive: ABC

小知识

• 发送缓冲区

  • 数据先进入发送缓冲区，之后由操作系统送往远端主机

• 接收缓冲区

  • 远端数据被操作系统接受后放入接收缓冲区
  • 之后应用程序从接收缓冲区读取数据

TCP 应用编程中的 “问题”

数据接收端无法知道数据的发送方式

接收端无法知道 "ABC" 是分开发送的！

网络编程中的期望

• 每次发送一条完整的消息，每次接收一条完整的消息
• 即使接收缓冲区中有多条消息，也不会出现消息粘连
• 消息中涵盖了数据类型和数据长度等信息

应用层协议设计

• 什么是协议？

  • 协议是通讯双方为数据交换而建立的规则，标准或预定的集合

• 协议对数据传输的作用

  • 通讯双方根据协议能够正确收发数据
  • 通讯双方根据协议能够解释数据的意义

协议设计示例

• 目标：设计可用于数据传输的协议

• 完整消息包含

  • 数据头：数据类型（即：数据区用途，固定长度）
  • 数据长度：数据区长度（固定长度）
  • 数据区：字节数据（变长区域）

上图可知：
消息至少 12 个字节（消息头 + 数据长度）
通过计算消息的总长度，能够避开数据粘连的问题

typedef struct {
    unsigned short type;
    unsigned short cmd;
    unsigned short index;
    unsigned short total;
    unsigned int length;
    unsigned char payload[];  // 柔性数组
}Message;

编程实验：应用层协议设计与实现

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 listen 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;
}

输出

0000000001000300010000004100000000020003000100000042000000000300
03000100000043

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思考：如何在代码层封装协议细节（仅关系消息本身）？