linux网络编程之socket编程(十四)

经过令国鸡冻的APEC会之后，北京的冬天终于不冷了，有暖气的日子就是倍儿爽呀~~洗完热水澡，舒服的躺在床上欢乐地敲打着键盘，是件多么幸福的事呀，好了，抒发情感后，正题继续。

上节中已经初步学习了UDP的编程，这次主要是进一步加深对UDP的认识，用它来实现一个简易的聊天室程序，下面首先来看一下该程序的总的逻辑架构图：

下面来将其进行分解：

以上就是聊天程序所涉及的一些消息交互的过程，在正式开始代码前，先来看一下该程序的最后效果，对其有一个更加直观的感觉：

接下来再来登录一个用户，这时还是登录aa，用有什么效果呢？

下面给用户发送消息：

接下来客户端退出：

以上就是聊天程序的效果，下面则正式进入代码的阶段，重在分析其流程：

先看一下代码结构：

其中在上面看到了很多状态消息，都利用宏定义在pub.h头文件中：

接下来定义的一些消息结构，也定义在头文件中：

【说明】：关于这里用到的c++知识可以完全理解既可，稍有一点上层编程语言的都很容易理解，例如java，实际上我也还没学过c++的内容，不过将来会扎实地学习它的，这里只是为了实验需要，重在实验的理解。

下面贴出头文件的具体代码：

pub.h:

#ifndef _PUB_H_
#define _PUB_H_

#include <list>
#include <algorithm>
using namespace std;

// C2S
#define C2S_LOGIN            0x01
#define C2S_LOGOUT            0x02
#define C2S_ONLINE_USER        0x03

#define MSG_LEN                512

// S2C
#define S2C_LOGIN_OK        0x01
#define S2C_ALREADY_LOGINED    0x02
#define S2C_SOMEONE_LOGIN    0x03
#define S2C_SOMEONE_LOGOUT    0x04
#define S2C_ONLINE_USER        0x05

// C2C
#define C2C_CHAT            0x06

typedef struct message
{
    int cmd;
    char body[MSG_LEN];
} MESSAGE;

typedef struct user_info
{
    char username[];
    unsigned int ip;
    unsigned short port;
} USER_INFO;

typedef struct chat_msg
{
    char username[];
    char msg[];
}CHAT_MSG;

typedef list<USER_INFO> USER_LIST;

#endif /* _PUB_H_ */

接着开始分析下服务端的代码：

其main函数代码就不做过多解释了，上节UDP编程中已经详细提到过，下面先贴出来：

下面来具体分析该函数：

对应于逻辑图：

下面各个消息处理进行一一分解：

登录do_login：

void do_login(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr)
{
    //从客户端信息中来初使化user结构体
    USER_INFO user;
    strcpy(user.username, msg.body);
    user.ip = cliaddr->sin_addr.s_addr;
    user.port = cliaddr->sin_port;

}

接下来判断用户是否已经登录过：

void do_login(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr)
{
    //从客户端信息中来初使化user结构体
    USER_INFO user;
    strcpy(user.username, msg.body);
    user.ip = cliaddr->sin_addr.s_addr;
    user.port = cliaddr->sin_port;

    /* 查找用户 */
    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
        {
            break;
        }
    }

    if (it == client_list.end())    /* 没找到用户 */
    {
        printf("has a user login : %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        client_list.push_back(user);
    }
    else    /* 找到用户 */
    {
        printf("user %s has already logined\n", msg.body);

    }
}

如果是没有登录过，那就是登录成功了，接下来会进行一系列处理，由于便于理解流程，所以下面说明时会对照着客户端的代码：

void do_login(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr)
{
    //从客户端信息中来初使化user结构体
    USER_INFO user;
    strcpy(user.username, msg.body);
    user.ip = cliaddr->sin_addr.s_addr;
    user.port = cliaddr->sin_port;

    /* 查找用户 */
    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
        {
            break;
        }
    }

    if (it == client_list.end())    /* 没找到用户 */
    {
        printf("has a user login : %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        client_list.push_back(user);//将新的用户插入到集合中

        // 登录成功应答
        MESSAGE reply_msg;
        memset(&reply_msg, 0, sizeof(reply_msg));
        reply_msg.cmd = htonl(S2C_LOGIN_OK);
        sendto(sock, &reply_msg, sizeof(msg), 0, (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

    }
    else    /* 找到用户 */
    {
        printf("user %s has already logined\n", msg.body);

    }
}

这时看一下客户端的代码，登录成功应答时客户端是怎么处理的：

void chat_cli(int sock)
{
    struct sockaddr_in servaddr;
    memset(&servaddr, , sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons();
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");

    struct sockaddr_in peeraddr;
    socklen_t peerlen;

    MESSAGE msg;
    while ()
    {
        //输入用户名
        memset(username,,sizeof(username));
        printf("please inpt your name:");
        fflush(stdout);
        scanf("%s", username);

        //准备向服务端发送登录请求
        memset(&msg, , sizeof(msg));
        msg.cmd = htonl(C2S_LOGIN);
        strcpy(msg.body, username);

        //发送登录请求给服务端
        sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)&servaddr, sizeof(servaddr));

        memset(&msg, , sizeof(msg));
        //接收服务端的消息，其中就是登录请求的应答信息
        recvfrom(sock, &msg, sizeof(msg), 0, NULL, NULL);
        int cmd = ntohl(msg.cmd);
        if (cmd == S2C_ALREADY_LOGINED)//证明用户已经登录过
            printf("user %s already logined server, please use another username\n", username);
        else if (cmd == S2C_LOGIN_OK)
        {//证明用户已经成功登录了
            printf("user %s has logined server\n", username);
            break;
        }

    }
}

接着服务端向客户端发送在线人数及列表：

chatsrv.cpp:

void do_login(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr)
{
    //从客户端信息中来初使化user结构体
    USER_INFO user;
    strcpy(user.username, msg.body);
    user.ip = cliaddr->sin_addr.s_addr;
    user.port = cliaddr->sin_port;

    /* 查找用户 */
    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
        {
            break;
        }
    }

    if (it == client_list.end())    /* 没找到用户 */
    {
        printf("has a user login : %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        client_list.push_back(user);//将新的用户插入到集合中

        // 登录成功应答
        MESSAGE reply_msg;
        memset(&reply_msg, , sizeof(reply_msg));
        reply_msg.cmd = htonl(S2C_LOGIN_OK);
        sendto(sock, &reply_msg, sizeof(msg), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

        int count = htonl((int)client_list.size());
        // 发送在线人数
        sendto(sock, &count, sizeof(int), 0, (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

        printf("sending user list information to: %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        // 发送在线列表
        for (it=client_list.begin(); it != client_list.end(); ++it)
        {
            sendto(sock, &*it/* *it表示USER_INFO */, sizeof(USER_INFO), 0, (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));
        }

    }
    else    /* 找到用户 */
    {
        printf("user %s has already logined\n", msg.body);
    }
}

客户端收到在线列表的处理代码：

chatcli.cpp:

void chat_cli(int sock)
{
    struct sockaddr_in servaddr;
    memset(&servaddr, , sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons();
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");

    struct sockaddr_in peeraddr;
    socklen_t peerlen;

    MESSAGE msg;
    while ()
    {
        //输入用户名
        memset(username,,sizeof(username));
        printf("please inpt your name:");
        fflush(stdout);
        scanf("%s", username);

        //准备向服务端发送登录请求
        memset(&msg, , sizeof(msg));
        msg.cmd = htonl(C2S_LOGIN);
        strcpy(msg.body, username);

        //发送登录请求给服务端
        sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)&servaddr, sizeof(servaddr));

        memset(&msg, , sizeof(msg));
        //接收服务端的消息，其中就是登录请求的应答信息
        recvfrom(sock, &msg, sizeof(msg), , NULL, NULL);
        int cmd = ntohl(msg.cmd);
        if (cmd == S2C_ALREADY_LOGINED)//证明用户已经登录过
            printf("user %s already logined server, please use another username\n", username);
        else if (cmd == S2C_LOGIN_OK)
        {//证明用户已经成功登录了
            printf("user %s has logined server\n", username);
            break;
        }

    }
    int count;
    recvfrom(sock, &count, sizeof(int), 0, NULL, NULL);

    int n = ntohl(count);
    printf("has %d users logined server\n", n);

    for (int i=0; i<n; i++)
    {
        USER_INFO user;
        recvfrom(sock, &user, sizeof(USER_INFO), 0, NULL, NULL);
        client_list.push_back(user);//每接收到一个用户，则插入到聊天成员列表中
        in_addr tmp;
        tmp.s_addr = user.ip;

        printf("%d %s <-> %s:%d\n", i, user.username, inet_ntoa(tmp), ntohs(user.port));
    }

}

下面则向其它用户通知有新用户登录：

void do_login(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr)
{
    //从客户端信息中来初使化user结构体
    USER_INFO user;
    strcpy(user.username, msg.body);
    user.ip = cliaddr->sin_addr.s_addr;
    user.port = cliaddr->sin_port;

    /* 查找用户 */
    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
        {
            break;
        }
    }

    if (it == client_list.end())    /* 没找到用户 */
    {
        printf("has a user login : %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        client_list.push_back(user);//将新的用户插入到集合中

        // 登录成功应答
        MESSAGE reply_msg;
        memset(&reply_msg, , sizeof(reply_msg));
        reply_msg.cmd = htonl(S2C_LOGIN_OK);
        sendto(sock, &reply_msg, sizeof(msg), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

        int count = htonl((int)client_list.size());
        // 发送在线人数
        sendto(sock, &count, sizeof(int), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

        printf("sending user list information to: %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        // 发送在线列表
        for (it=client_list.begin(); it != client_list.end(); ++it)
        {
            sendto(sock, &*it, sizeof(USER_INFO), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));
        }

        // 向其他用户通知有新用户登录
        for (it=client_list.begin(); it != client_list.end(); ++it)
        {
            if (strcmp(it->username,msg.body) == 0)
                continue;

            struct sockaddr_in peeraddr;
            memset(&peeraddr, 0, sizeof(peeraddr));
            peeraddr.sin_family = AF_INET;
            peeraddr.sin_port = it->port;
            peeraddr.sin_addr.s_addr = it->ip;

            msg.cmd = htonl(S2C_SOMEONE_LOGIN);
            memcpy(msg.body, &user, sizeof(user));

            if (sendto(sock, &msg, sizeof(msg), 0, (struct sockaddr *)&peeraddr, sizeof(peeraddr)) < 0)
                ERR_EXIT("sendto");
        }
    }
    else    /* 找到用户 */
    {
        printf("user %s has already logined\n", msg.body);
    }
}

如果发现该用户已经登录了，则给出已登录的提示：

void do_login(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr)
{
    //从客户端信息中来初使化user结构体
    USER_INFO user;
    strcpy(user.username, msg.body);
    user.ip = cliaddr->sin_addr.s_addr;
    user.port = cliaddr->sin_port;

    /* 查找用户 */
    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
        {
            break;
        }
    }

    if (it == client_list.end())    /* 没找到用户 */
    {
        printf("has a user login : %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        client_list.push_back(user);//将新的用户插入到集合中

        // 登录成功应答
        MESSAGE reply_msg;
        memset(&reply_msg, , sizeof(reply_msg));
        reply_msg.cmd = htonl(S2C_LOGIN_OK);
        sendto(sock, &reply_msg, sizeof(msg), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

        int count = htonl((int)client_list.size());
        // 发送在线人数
        sendto(sock, &count, sizeof(int), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

        printf("sending user list information to: %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        // 发送在线列表
        for (it=client_list.begin(); it != client_list.end(); ++it)
        {
            sendto(sock, &*it, sizeof(USER_INFO), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));
        }

        // 向其他用户通知有新用户登录
        for (it=client_list.begin(); it != client_list.end(); ++it)
        {
            if (strcmp(it->username,msg.body) == )
                continue;

            struct sockaddr_in peeraddr;
            memset(&peeraddr, , sizeof(peeraddr));
            peeraddr.sin_family = AF_INET;
            peeraddr.sin_port = it->port;
            peeraddr.sin_addr.s_addr = it->ip;

            msg.cmd = htonl(S2C_SOMEONE_LOGIN);
            memcpy(msg.body, &user, sizeof(user));

            if (sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)&peeraddr, sizeof(peeraddr)) < )
                ERR_EXIT("sendto");

        }
    }
    else    /* 找到用户 */
    {
        printf("user %s has already logined\n", msg.body);

        MESSAGE reply_msg;
        memset(&reply_msg, 0, sizeof(reply_msg));
        reply_msg.cmd = htonl(S2C_ALREADY_LOGINED);
        sendto(sock, &reply_msg, sizeof(reply_msg), 0, (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));
    }
}

接下来，回到客户端这边来，当登录成功之后，会列出该客户端能用到的命令：

chatcli.cpp:

void chat_cli(int sock)
{
    struct sockaddr_in servaddr;
    memset(&servaddr, , sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons();
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");

    struct sockaddr_in peeraddr;
    socklen_t peerlen;

    MESSAGE msg;
    while ()
    {
        //输入用户名
        memset(username,,sizeof(username));
        printf("please inpt your name:");
        fflush(stdout);
        scanf("%s", username);

        //准备向服务端发送登录请求
        memset(&msg, , sizeof(msg));
        msg.cmd = htonl(C2S_LOGIN);
        strcpy(msg.body, username);

        //发送登录请求给服务端
        sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)&servaddr, sizeof(servaddr));

        memset(&msg, , sizeof(msg));
        //接收服务端的消息，其中就是登录请求的应答信息
        recvfrom(sock, &msg, sizeof(msg), , NULL, NULL);
        int cmd = ntohl(msg.cmd);
        if (cmd == S2C_ALREADY_LOGINED)//证明用户已经登录过
            printf("user %s already logined server, please use another username\n", username);
        else if (cmd == S2C_LOGIN_OK)
        {//证明用户已经成功登录了
            printf("user %s has logined server\n", username);
            break;
        }

    }
    int count;
    recvfrom(sock, &count, sizeof(int), , NULL, NULL);

    int n = ntohl(count);
    printf("has %d users logined server\n", n);

    for (int i=; i<n; i++)
    {
        USER_INFO user;
        recvfrom(sock, &user, sizeof(USER_INFO), , NULL, NULL);
        client_list.push_back(user);
        in_addr tmp;
        tmp.s_addr = user.ip;

        printf("%d %s <-> %s:%d\n", i, user.username, inet_ntoa(tmp), ntohs(user.port));
    }

    printf("\nCommands are:\n");
    printf("send username msg\n");
    printf("list\n");
    printf("exit\n");
    printf("\n");

}

接下来用I/O复用模型select函数，来并发处理I/O套接字，因为既有可能产生键盘套接字，也有sock，所以需要用I/O复用模型，如下：

chatcli.cpp:

void chat_cli(int sock)
{
    struct sockaddr_in servaddr;
    memset(&servaddr, , sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons();
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");

    struct sockaddr_in peeraddr;
    socklen_t peerlen;

    MESSAGE msg;
    while ()
    {
        //输入用户名
        memset(username,,sizeof(username));
        printf("please inpt your name:");
        fflush(stdout);
        scanf("%s", username);

        //准备向服务端发送登录请求
        memset(&msg, , sizeof(msg));
        msg.cmd = htonl(C2S_LOGIN);
        strcpy(msg.body, username);

        //发送登录请求给服务端
        sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)&servaddr, sizeof(servaddr));

        memset(&msg, , sizeof(msg));
        //接收服务端的消息，其中就是登录请求的应答信息
        recvfrom(sock, &msg, sizeof(msg), , NULL, NULL);
        int cmd = ntohl(msg.cmd);
        if (cmd == S2C_ALREADY_LOGINED)//证明用户已经登录过
            printf("user %s already logined server, please use another username\n", username);
        else if (cmd == S2C_LOGIN_OK)
        {//证明用户已经成功登录了
            printf("user %s has logined server\n", username);
            break;
        }

    }
    int count;
    recvfrom(sock, &count, sizeof(int), , NULL, NULL);

    int n = ntohl(count);
    printf("has %d users logined server\n", n);

    for (int i=; i<n; i++)
    {
        USER_INFO user;
        recvfrom(sock, &user, sizeof(USER_INFO), , NULL, NULL);
        client_list.push_back(user);
        in_addr tmp;
        tmp.s_addr = user.ip;

        printf("%d %s <-> %s:%d\n", i, user.username, inet_ntoa(tmp), ntohs(user.port));
    }

    printf("\nCommands are:\n");
    printf("send username msg\n");
    printf("list\n");
    printf("exit\n");
    printf("\n");

    fd_set rset;
    FD_ZERO(&rset);
    int nready;
    while (1)
    {
        FD_SET(STDIN_FILENO, &rset);//将标准输入加入到集合中
        FD_SET(sock, &rset);//将sock套接字加入集合中
        nready = select(sock+1, &rset, NULL, NULL, NULL);
        if (nready == -1)
                ERR_EXIT("select");

        if (nready == 0)
                continue;

        if (FD_ISSET(sock, &rset))
        {
            peerlen = sizeof(peeraddr);
            memset(&msg,0,sizeof(msg));
            recvfrom(sock, &msg, sizeof(msg), 0, (struct sockaddr *)&peeraddr, &peerlen);
            int cmd = ntohl(msg.cmd);
            //将服务端发过来的消息进行分发
            switch (cmd)
            {
            case S2C_SOMEONE_LOGIN:
                do_someone_login(msg);
                break;
            case S2C_SOMEONE_LOGOUT:
                do_someone_logout(msg);
                break;
            case S2C_ONLINE_USER:
                do_getlist(sock);
                break;
            case C2C_CHAT:
                do_chat(msg);
                break;
        default:
            break;

            }
        }
        if (FD_ISSET(STDIN_FILENO, &rset))
        {//标准输入产生了事件
            char cmdline[100] = {0};
            if (fgets(cmdline, sizeof(cmdline), stdin) == NULL)
                break;

            if (cmdline[0] == '\n')
                continue;
            cmdline[strlen(cmdline) - 1] = '\0';
            //对用户敲的命令进行解析处理
            parse_cmd(cmdline, sock, &servaddr);
        }
    }
}

下面来看一下parse_cmd函数的实现：

在看具体代码前，先看一下用户输入命令的几种情况：

下面具体来看一下该命令解析函数的实现：

首先从输入的字符中查找空格，并替换成'\0',如下：

void parse_cmd(char* cmdline, int sock, struct sockaddr_in *servaddr)
{
    char cmd[]={};
    char *p;
    p = strchr(cmdline, ' ');//检查空格
    if (p != NULL)
        *p = '\0';//将控格替换成\0

    strcpy(cmd, cmdline);
}

然后下面对其输入的命令进行判断：

void parse_cmd(char* cmdline, int sock, struct sockaddr_in *servaddr)
{
    char cmd[]={};
    char *p;
    p = strchr(cmdline, ' ');
    if (p != NULL)
        *p = '\0';

    strcpy(cmd, cmdline);

    if (strcmp(cmd, "exit") == )
    {//退出

    }
    else if (strcmp(cmd, "send") == )
    {//向用户发送消息

    }
    else if (strcmp(cmd, "list") == )
    {//列出在线用户列表

    }
    else
    {//说明输入命令有误，给出正确命令提示
        printf("bad command\n");
        printf("\nCommands are:\n");
        printf("send username msg\n");
        printf("list\n");
        printf("exit\n");
        printf("\n");
    }
}

当用户敲入了"exit"命令时，会执行下面这段逻辑：

void parse_cmd(char* cmdline, int sock, struct sockaddr_in *servaddr)
{
    char cmd[]={};
    char *p;
    p = strchr(cmdline, ' ');
    if (p != NULL)
        *p = '\0';

    strcpy(cmd, cmdline);

    if (strcmp(cmd, "exit") == )
    {//退出
        MESSAGE msg;
        memset(&msg,,sizeof(msg));
        msg.cmd = htonl(C2S_LOGOUT);//向服务器发送C2S_LOGOUT消息
        strcpy(msg.body, username);

        if (sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)servaddr, sizeof(struct sockaddr_in)) < )
            ERR_EXIT("sendto");

        printf("user %s has logout server\n", username);
        exit(EXIT_SUCCESS);

    }
    else if (strcmp(cmd, "send") == )
    {//向用户发送消息

    }
    else if (strcmp(cmd, "list") == )
    {//列出在线用户列表

    }
    else
    {//说明输入命令有误，给出正确命令提示
        printf("bad command\n");
        printf("\nCommands are:\n");
        printf("send username msg\n");
        printf("list\n");
        printf("exit\n");
        printf("\n");
    }
}

当用户向其它用户发送聊天信息的话：

void parse_cmd(char* cmdline, int sock, struct sockaddr_in *servaddr)
{
    char cmd[]={};
    char *p;
    p = strchr(cmdline, ' ');
    if (p != NULL)
        *p = '\0';

    strcpy(cmd, cmdline);

    if (strcmp(cmd, "exit") == )
    {//退出
        MESSAGE msg;
        memset(&msg,,sizeof(msg));
        msg.cmd = htonl(C2S_LOGOUT);
        strcpy(msg.body, username);

        if (sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)servaddr, sizeof(struct sockaddr_in)) < )
            ERR_EXIT("sendto");

        printf("user %s has logout server\n", username);
        exit(EXIT_SUCCESS);

    }
    else if (strcmp(cmd, "send") == )
    {//向用户发送消息
        char peername[16]={0};//要发送的用户名
        char msg[MSG_LEN]={0};//要发送的消息

        //下面则开始解析命令
        /* send  user  msg  */
        /*       p     p2   */
        while (*p++ == ' ') ;
        char *p2;
        p2 = strchr(p, ' ');
        if (p2 == NULL)
        {
            printf("bad command\n");
            printf("\nCommands are:\n");
            printf("send username msg\n");
            printf("list\n");
            printf("exit\n");
            printf("\n");
            return;
        }
        *p2 = '\0';
        strcpy(peername, p);

        while (*p2++ == ' ') ;
        strcpy(msg, p2);
        //然后将消息发送给对方，这里封装了一个方法
        sendmsgto(sock, peername, msg);
    }
    else if (strcmp(cmd, "list") == )
    {//列出在线用户列表
    }
    else
    {//说明输入命令有误，给出正确命令提示
        printf("bad command\n");
        printf("\nCommands are:\n");
        printf("send username msg\n");
        printf("list\n");
        printf("exit\n");
        printf("\n");
    }
}

下面来看一下sendmsgto方法的具体实现：

bool sendmsgto(int sock, char* name, char* msg)
{
    if (strcmp(name, username) == )
    {//如果向当前用户发送消息，给出错误提示
        printf("can't send message to self\n");
        return false;
    }

    return true;
}

bool sendmsgto(int sock, char* name, char* msg)
{
    if (strcmp(name, username) == )
    {//如果向当前用户发送消息，给出错误提示
        printf("can't send message to self\n");
        return false;
    }

    //下面开始遍历要发送的用户是否已经登录
    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,name) == )
            break;
    }

    if (it == client_list.end())
    {//说明要发送的用户还没有登录，给出错误提示
        printf("user %s has not logined server\n", name);
        return false;
    }
    return true;
}

bool sendmsgto(int sock, char* name, char* msg)
{
    if (strcmp(name, username) == )
    {//如果向当前用户发送消息，给出错误提示
        printf("can't send message to self\n");
        return false;
    }

    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,name) == )
            break;
    }

    if (it == client_list.end())
    {
        printf("user %s has not logined server\n", name);
        return false;
    }

    //流程走到这，证明要发送的用户是已经成功登录过的，所以接下来是组拼消息
    MESSAGE m;
    memset(&m,,sizeof(m));
    m.cmd = htonl(C2C_CHAT);//向服务器发送C2C_CHAT命令

    CHAT_MSG cm;
    strcpy(cm.username, username);
    strcpy(cm.msg, msg);

    memcpy(m.body, &cm, sizeof(cm));
    //strcpy(m.body,msg);

    struct sockaddr_in    peeraddr;
    memset(&peeraddr,,sizeof(peeraddr));
    peeraddr.sin_family      = AF_INET;
    peeraddr.sin_addr.s_addr = it->ip;
    peeraddr.sin_port        = it->port;

    in_addr tmp;
    tmp.s_addr = it->ip;

    printf("sending message [%s] to user [%s] <-> %s:%d\n",  msg, name, inet_ntoa(tmp), ntohs(it->port));

    //发送消息
    sendto(sock, (const char*)&m, sizeof(m), , (struct sockaddr *)&peeraddr, sizeof(peeraddr));
    return true;
}

如果是输入的在线用户列表的命令，则会走如下逻辑：

void parse_cmd(char* cmdline, int sock, struct sockaddr_in *servaddr)
{
    char cmd[]={};
    char *p;
    p = strchr(cmdline, ' ');
    if (p != NULL)
        *p = '\0';

    strcpy(cmd, cmdline);

    if (strcmp(cmd, "exit") == )
    {//退出
        MESSAGE msg;
        memset(&msg,,sizeof(msg));
        msg.cmd = htonl(C2S_LOGOUT);
        strcpy(msg.body, username);

        if (sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)servaddr, sizeof(struct sockaddr_in)) < )
            ERR_EXIT("sendto");

        printf("user %s has logout server\n", username);
        exit(EXIT_SUCCESS);

    }
    else if (strcmp(cmd, "send") == )
    {//向用户发送消息
        char peername[]={};
        char msg[MSG_LEN]={};

        /* send  user  msg  */
        /*       p     p2   */
        while (*p++ == ' ') ;
        char *p2;
        p2 = strchr(p, ' ');
        if (p2 == NULL)
        {
            printf("bad command\n");
            printf("\nCommands are:\n");
            printf("send username msg\n");
            printf("list\n");
            printf("exit\n");
            printf("\n");
            return;
        }
        *p2 = '\0';
        strcpy(peername, p);

        while (*p2++ == ' ') ;
        strcpy(msg, p2);
        sendmsgto(sock, peername, msg);
    }
    else if (strcmp(cmd, "list") == )
    {//列出在线用户列表
        MESSAGE msg;
        memset(&msg, 0, sizeof(msg));
        msg.cmd = htonl(C2S_ONLINE_USER);

        if (sendto(sock, &msg, sizeof(msg), 0, (struct sockaddr *)servaddr, sizeof(struct sockaddr_in)) < 0)
            ERR_EXIT("sendto");
    }
    else
    {//说明输入命令有误，给出正确命令提示
        printf("bad command\n");
        printf("\nCommands are:\n");
        printf("send username msg\n");
        printf("list\n");
        printf("exit\n");
        printf("\n");
    }
}

至此parse_cmd函数的实现就分析到这，客户端还有可能会收到服务端发来的网络的消息，所以下面来看一下这些消息的分发：

下面来具体分析一下实现：

当有用户登录了，do_someone_login函数实现如下：

当有用户登出了，do_someone_logout函数实现如下：

当用户要获取当前在线用户列表时，do_getlist函数实现如下：

void do_getlist(int sock)
{
    int count;
    recvfrom(sock, &count, sizeof(int), , NULL, NULL);//首先得到用户列表的总个数
    printf("has %d users logined server\n", ntohl(count));
    client_list.clear();//将当前的在线列表清空

    int n = ntohl(count);
    for (int i=; i<n; i++)//然后再一个个接收用户，插入到在线列表集合中
    {
        USER_INFO user;
        recvfrom(sock,&user, sizeof(USER_INFO), , NULL, NULL);
        client_list.push_back(user);
        in_addr tmp;
        tmp.s_addr = user.ip;

        printf("%s <-> %s:%d\n", user.username, inet_ntoa(tmp), ntohs(user.port));
    }
}

当要发送消息时，do_chat函数消息实现如下：

void do_chat(const MESSAGE& msg)
{
    CHAT_MSG *cm = (CHAT_MSG*)msg.body;
    printf("recv a msg [%s] from [%s]\n", cm->msg, cm->username);
    //recvfrom(sock, &count, sizeof(int), 0, NULL, NULL);
}

好了，关于聊天程序的大体流程已经分析完毕，具体的一些细节可以细看代码，下面将完整代码贴出来：

chatsrv.cpp:

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <sys/wait.h>

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

#include "pub.h"

#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while()

// 聊天室成员列表
USER_LIST client_list;

void do_login(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr);
void do_logout(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr);
void do_sendlist(int sock, struct sockaddr_in *cliaddr);

void chat_srv(int sock)
{
    struct sockaddr_in cliaddr;
    socklen_t clilen;
    int n;
    MESSAGE msg;
    while ()
    {
        memset(&msg, , sizeof(msg));
        clilen = sizeof(cliaddr);
        n = recvfrom(sock, &msg, sizeof(msg), , (struct sockaddr *)&cliaddr, &clilen);
        if (n < )
        {
            if (errno == EINTR)
                continue;
            ERR_EXIT("recvfrom");
        }

        int cmd = ntohl(msg.cmd);
        switch (cmd)
        {
        case C2S_LOGIN:
            do_login(msg, sock, &cliaddr);
            break;
        case C2S_LOGOUT:
            do_logout(msg, sock, &cliaddr);
            break;
        case C2S_ONLINE_USER:
            do_sendlist(sock, &cliaddr);
            break;
        default:
            break;
        }
    }
}

void do_login(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr)
{
    //从客户端信息中来初使化user结构体
    USER_INFO user;
    strcpy(user.username, msg.body);
    user.ip = cliaddr->sin_addr.s_addr;
    user.port = cliaddr->sin_port;

    /* 查找用户 */
    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
        {
            break;
        }
    }

    if (it == client_list.end())    /* 没找到用户 */
    {
        printf("has a user login : %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        client_list.push_back(user);//将新的用户插入到集合中

        // 登录成功应答
        MESSAGE reply_msg;
        memset(&reply_msg, , sizeof(reply_msg));
        reply_msg.cmd = htonl(S2C_LOGIN_OK);
        sendto(sock, &reply_msg, sizeof(msg), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

        int count = htonl((int)client_list.size());
        // 发送在线人数
        sendto(sock, &count, sizeof(int), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

        printf("sending user list information to: %s <-> %s:%d\n", msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));
        // 发送在线列表
        for (it=client_list.begin(); it != client_list.end(); ++it)
        {
            sendto(sock, &*it, sizeof(USER_INFO), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));
        }

        // 向其他用户通知有新用户登录
        for (it=client_list.begin(); it != client_list.end(); ++it)
        {
            if (strcmp(it->username,msg.body) == )
                continue;

            struct sockaddr_in peeraddr;
            memset(&peeraddr, , sizeof(peeraddr));
            peeraddr.sin_family = AF_INET;
            peeraddr.sin_port = it->port;
            peeraddr.sin_addr.s_addr = it->ip;

            msg.cmd = htonl(S2C_SOMEONE_LOGIN);
            memcpy(msg.body, &user, sizeof(user));

            if (sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)&peeraddr, sizeof(peeraddr)) < )
                ERR_EXIT("sendto");

        }
    }
    else    /* 找到用户 */
    {
        printf("user %s has already logined\n", msg.body);

        MESSAGE reply_msg;
        memset(&reply_msg, , sizeof(reply_msg));
        reply_msg.cmd = htonl(S2C_ALREADY_LOGINED);
        sendto(sock, &reply_msg, sizeof(reply_msg), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));
    }
}

void do_logout(MESSAGE& msg, int sock, struct sockaddr_in *cliaddr)
{
    printf("has a user logout : %s <-> %s:%d\n",  msg.body, inet_ntoa(cliaddr->sin_addr), ntohs(cliaddr->sin_port));

    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
            break;
    }

    if (it != client_list.end())
        client_list.erase(it);

    // 向其他用户通知有用户登出
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
            continue;

        struct sockaddr_in peeraddr;
        memset(&peeraddr, , sizeof(peeraddr));
        peeraddr.sin_family = AF_INET;
        peeraddr.sin_port = it->port;
        peeraddr.sin_addr.s_addr = it->ip;

        msg.cmd = htonl(S2C_SOMEONE_LOGOUT);

        if (sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)&peeraddr, sizeof(peeraddr)) < )
            ERR_EXIT("sendto");

    }
}

void do_sendlist(int sock, struct sockaddr_in *cliaddr)
{
    MESSAGE msg;
    msg.cmd = htonl(S2C_ONLINE_USER);
    sendto(sock, (const char*)&msg, sizeof(msg), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));

    int count = htonl((int)client_list.size());
    /* 发送在线用户数 */
    sendto(sock, (const char*)&count, sizeof(int), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));
    /* 发送在线用户列表 */
    for (USER_LIST::iterator it=client_list.begin(); it != client_list.end(); ++it)
    {
        sendto(sock, &*it, sizeof(USER_INFO), , (struct sockaddr *)cliaddr, sizeof(struct sockaddr_in));
    }
}

int main(void)
{
    int sock;
    if ((sock = socket(PF_INET, SOCK_DGRAM, )) < )
        ERR_EXIT("socket");

    struct sockaddr_in servaddr;
    memset(&servaddr, , sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons();
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(sock, (struct sockaddr*)&servaddr, sizeof(servaddr)) < )
        ERR_EXIT("bind");

    chat_srv(sock);
    return ;
}

chatcli.cpp:

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

#include "pub.h"

#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while()

// 当前用户名
char username[];

// 聊天室成员列表
USER_LIST client_list;

void do_someone_login(MESSAGE& msg);
void do_someone_logout(MESSAGE& msg);
void do_getlist();
void do_chat();

void parse_cmd(char* cmdline, int sock, struct sockaddr_in *servaddr);
bool sendmsgto(int sock, char* username, char* msg);

void parse_cmd(char* cmdline, int sock, struct sockaddr_in *servaddr)
{
    char cmd[]={};
    char *p;
    p = strchr(cmdline, ' ');
    if (p != NULL)
        *p = '\0';

    strcpy(cmd, cmdline);

    if (strcmp(cmd, "exit") == )
    {//退出
        MESSAGE msg;
        memset(&msg,,sizeof(msg));
        msg.cmd = htonl(C2S_LOGOUT);
        strcpy(msg.body, username);

        if (sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)servaddr, sizeof(struct sockaddr_in)) < )
            ERR_EXIT("sendto");

        printf("user %s has logout server\n", username);
        exit(EXIT_SUCCESS);

    }
    else if (strcmp(cmd, "send") == )
    {//向用户发送消息
        char peername[]={};
        char msg[MSG_LEN]={};

        /* send  user  msg  */
        /*       p     p2   */
        while (*p++ == ' ') ;
        char *p2;
        p2 = strchr(p, ' ');
        if (p2 == NULL)
        {
            printf("bad command\n");
            printf("\nCommands are:\n");
            printf("send username msg\n");
            printf("list\n");
            printf("exit\n");
            printf("\n");
            return;
        }
        *p2 = '\0';
        strcpy(peername, p);

        while (*p2++ == ' ') ;
        strcpy(msg, p2);
        sendmsgto(sock, peername, msg);
    }
    else if (strcmp(cmd, "list") == )
    {//列出在线用户列表
        MESSAGE msg;
        memset(&msg, , sizeof(msg));
        msg.cmd = htonl(C2S_ONLINE_USER);

        if (sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)servaddr, sizeof(struct sockaddr_in)) < )
            ERR_EXIT("sendto");
    }
    else
    {//说明输入命令有误，给出正确命令提示
        printf("bad command\n");
        printf("\nCommands are:\n");
        printf("send username msg\n");
        printf("list\n");
        printf("exit\n");
        printf("\n");
    }
}

bool sendmsgto(int sock, char* name, char* msg)
{
    if (strcmp(name, username) == )
    {
        printf("can't send message to self\n");
        return false;
    }

    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,name) == )
            break;
    }

    if (it == client_list.end())
    {
        printf("user %s has not logined server\n", name);
        return false;
    }

    MESSAGE m;
    memset(&m,,sizeof(m));
    m.cmd = htonl(C2C_CHAT);

    CHAT_MSG cm;
    strcpy(cm.username, username);
    strcpy(cm.msg, msg);

    memcpy(m.body, &cm, sizeof(cm));
    //strcpy(m.body,msg);

    struct sockaddr_in    peeraddr;
    memset(&peeraddr,,sizeof(peeraddr));
    peeraddr.sin_family      = AF_INET;
    peeraddr.sin_addr.s_addr = it->ip;
    peeraddr.sin_port        = it->port;

    in_addr tmp;
    tmp.s_addr = it->ip;

    printf("sending message [%s] to user [%s] <-> %s:%d\n",  msg, name, inet_ntoa(tmp), ntohs(it->port));

    sendto(sock, (const char*)&m, sizeof(m), , (struct sockaddr *)&peeraddr, sizeof(peeraddr));
    return true;
}

void do_getlist(int sock)
{
    int count;
    recvfrom(sock, &count, sizeof(int), , NULL, NULL);//首先得到用户列表的总个数
    printf("has %d users logined server\n", ntohl(count));
    client_list.clear();//将当前的在线列表清空

    int n = ntohl(count);
    for (int i=; i<n; i++)//然后再一个个接收用户，插入到在线列表集合中
    {
        USER_INFO user;
        recvfrom(sock,&user, sizeof(USER_INFO), , NULL, NULL);
        client_list.push_back(user);
        in_addr tmp;
        tmp.s_addr = user.ip;

        printf("%s <-> %s:%d\n", user.username, inet_ntoa(tmp), ntohs(user.port));
    }
}

void do_someone_login(MESSAGE& msg)
{
    USER_INFO *user = (USER_INFO*)msg.body;
    in_addr tmp;
    tmp.s_addr = user->ip;
    printf("%s <-> %s:%d has logined server\n", user->username, inet_ntoa(tmp), ntohs(user->port));
    client_list.push_back(*user);
}

void do_someone_logout(MESSAGE& msg)
{
    USER_LIST::iterator it;
    for (it=client_list.begin(); it != client_list.end(); ++it)
    {
        if (strcmp(it->username,msg.body) == )
            break;
    }

    if (it != client_list.end())
        client_list.erase(it);

    printf("user %s has logout server\n", msg.body);
}

void do_chat(const MESSAGE& msg)
{
    CHAT_MSG *cm = (CHAT_MSG*)msg.body;
    printf("recv a msg [%s] from [%s]\n", cm->msg, cm->username);
    //recvfrom(sock, &count, sizeof(int), 0, NULL, NULL);
}

void chat_cli(int sock)
{
    struct sockaddr_in servaddr;
    memset(&servaddr, , sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons();
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");

    struct sockaddr_in peeraddr;
    socklen_t peerlen;

    MESSAGE msg;
    while ()
    {
        //输入用户名
        memset(username,,sizeof(username));
        printf("please inpt your name:");
        fflush(stdout);
        scanf("%s", username);

        //准备向服务端发送登录请求
        memset(&msg, , sizeof(msg));
        msg.cmd = htonl(C2S_LOGIN);
        strcpy(msg.body, username);

        //发送登录请求给服务端
        sendto(sock, &msg, sizeof(msg), , (struct sockaddr *)&servaddr, sizeof(servaddr));

        memset(&msg, , sizeof(msg));
        //接收服务端的消息，其中就是登录请求的应答信息
        recvfrom(sock, &msg, sizeof(msg), , NULL, NULL);
        int cmd = ntohl(msg.cmd);
        if (cmd == S2C_ALREADY_LOGINED)//证明用户已经登录过
            printf("user %s already logined server, please use another username\n", username);
        else if (cmd == S2C_LOGIN_OK)
        {//证明用户已经成功登录了
            printf("user %s has logined server\n", username);
            break;
        }

    }
    int count;
    recvfrom(sock, &count, sizeof(int), , NULL, NULL);

    int n = ntohl(count);
    printf("has %d users logined server\n", n);

    for (int i=; i<n; i++)
    {
        USER_INFO user;
        recvfrom(sock, &user, sizeof(USER_INFO), , NULL, NULL);
        client_list.push_back(user);
        in_addr tmp;
        tmp.s_addr = user.ip;

        printf("%d %s <-> %s:%d\n", i, user.username, inet_ntoa(tmp), ntohs(user.port));
    }

    printf("\nCommands are:\n");
    printf("send username msg\n");
    printf("list\n");
    printf("exit\n");
    printf("\n");

    fd_set rset;
    FD_ZERO(&rset);
    int nready;
    while ()
    {
        FD_SET(STDIN_FILENO, &rset);//将标准输入加入到集合中
        FD_SET(sock, &rset);//将sock套接字加入集合中
        nready = select(sock+, &rset, NULL, NULL, NULL);
        if (nready == -)
                ERR_EXIT("select");

        if (nready == )
                continue;

        if (FD_ISSET(sock, &rset))
        {
            peerlen = sizeof(peeraddr);
            memset(&msg,,sizeof(msg));
            recvfrom(sock, &msg, sizeof(msg), , (struct sockaddr *)&peeraddr, &peerlen);
            int cmd = ntohl(msg.cmd);
            //将服务端发过来的消息进行分发
            switch (cmd)
            {
            case S2C_SOMEONE_LOGIN:
                do_someone_login(msg);
                break;
            case S2C_SOMEONE_LOGOUT:
                do_someone_logout(msg);
                break;
            case S2C_ONLINE_USER:
                do_getlist(sock);
                break;
            case C2C_CHAT:
                do_chat(msg);
                break;
        default:
            break;

            }
        }
        if (FD_ISSET(STDIN_FILENO, &rset))
        {//标准输入产生了事件
            char cmdline[] = {};
            if (fgets(cmdline, sizeof(cmdline), stdin) == NULL)
                break;

            if (cmdline[] == '\n')
                continue;
            cmdline[strlen(cmdline) - ] = '\0';
            //对用户敲的命令进行解析处理
            parse_cmd(cmdline, sock, &servaddr);
        }
    }
}

int main(void)
{
    int sock;
    if ((sock = socket(PF_INET, SOCK_DGRAM, )) < )
        ERR_EXIT("socket");

    chat_cli(sock);

    return ;
}

其中用到的头文件在文章开头已经有了，这次的学习到这，主要是重在体会代码的流程。