服务器：

#include <myhead.h>

#define SER_PORT 6666
#define SER_IP "127.0.0.1"

int find_client(int *client_arr, int len, int client)
{
    for(int i=0; i<len; i++)
    {
        if(client_arr[i] == client)
        {
            return i;
        }
    }

    return -1;
}

void remove_client(int *client_arr, struct sockaddr_in *cin, int *len, int client)
{
    int tar = find_client(client_arr, *len, client);
    if(tar == -1)
        return;
    int i = -1;
    for(i=tar; i<*len-1; i++)
    {
        client_arr[i] = client_arr[i+1];
        cin[i] = cin[i+1];
    }
    client_arr[i] = 0;
    memset(cin+i, 0, sizeof(cin[i]));
    (*len)--;
}

int main(int argc, char const *argv[])
{
    int sfd = socket(AF_INET, SOCK_STREAM, 0);
    if(sfd == -1)
    {
        perror("socket error");
        return -1;
    }
    printf("socket success\n");

    struct sockaddr_in sin;
    sin.sin_family = AF_INET;
    sin.sin_port = htons(SER_PORT);
    sin.sin_addr.s_addr = inet_addr(SER_IP);

    if(bind(sfd, (struct sockaddr *)&sin, sizeof(sin)) == -1)
    {
        perror("bind error");
        return -1;
    }
    printf("bind success\n");

    if(listen(sfd, 128) == -1)
    {
        perror("listen error");
        return -1;
    }
    printf("listen success\n");

    // 定义一个监视列表
    fd_set readfds;
    FD_ZERO(&readfds);
    FD_SET(sfd, &readfds);
    FD_SET(STDIN_FILENO, &readfds);
    int client_arr[128] = {0};  // 客户端套接字文件描述符数组
    int client_count = 0;   // 链接的客户端的个数

    struct sockaddr_in cin[128];
    int size = sizeof(cin[0]);
    while(1)
    {
        fd_set temp = readfds;
        select(FD_SETSIZE, &temp, NULL, NULL, 0);

        // 服务器套接字文件描述符激活
        if(FD_ISSET(sfd, &temp))
        {
            int client = accept(sfd, (struct sockaddr *)&cin[client_count], &size);
            printf("[%s:%d] ：已连接\n", inet_ntoa(cin[client_count].sin_addr), ntohs(cin[client_count].sin_port));

            FD_SET(client, &readfds);
            client_arr[client_count] = client;
            client_count++;
        }

        // 标准输入流文件描述符激活
        if(FD_ISSET(STDIN_FILENO, &temp))
        {
            char buf[128] = "";
            int res = read(STDIN_FILENO, buf, sizeof(buf));
            buf[strlen(buf)-1] = 0;
            for(int i=0; i<client_count; i++)
            {
                sendto(client_arr[i], buf, res, 0, (struct sockaddr *)&cin[i], size);
            }
            if(strcmp(buf, "quit") == 0)
                break;
        }

        // 客户端套接字文件描述符激活
        for(int i=0; i<client_count; i++)
        {
            int client = client_arr[i];
            if(FD_ISSET(client, &temp))
            {
                char buf[128] = "";
                struct sockaddr_in cin_temp;
                int res = read(client, buf, sizeof(buf));
                if(res == 0 || strcmp(buf, "quit") == 0)
                {
                    printf("[%s:%d] ：已断开\n", inet_ntoa(cin[i].sin_addr), ntohs(cin[i].sin_port));
                    FD_CLR(client, &readfds);
                    remove_client(client_arr, cin, &client_count, client);
                    close(client);
                    break;
                }
                printf("[%s:%d] ：%s\n", inet_ntoa(cin[i].sin_addr), ntohs(cin[i].sin_port), buf);
            }
        }
    }

    close(sfd);

    return 0;
}

客户端：

#include <myhead.h>

#define SER_PORT 6666
#define SER_IP "127.0.0.1"

int main(int argc, char const *argv[])
{
    int cfd = socket(AF_INET, SOCK_STREAM, 0);
    if(cfd == -1)
    {
        perror("socket error");
        return -1;
    }
    printf("socket success\n");

    struct sockaddr_in sin;
    sin.sin_family = AF_INET;
    sin.sin_port = htons(SER_PORT);
    sin.sin_addr.s_addr = inet_addr(SER_IP);

    if(connect(cfd, (struct sockaddr *)&sin, sizeof(sin)) == -1)
    {
        perror("connect error");
        return -1;
    }
    printf("connect success\n");

    // 定义一个监视列表
    fd_set readfds;
    FD_ZERO(&readfds);
    FD_SET(cfd, &readfds);
    FD_SET(STDIN_FILENO, &readfds);

    while(1)
    {
        fd_set temp = readfds;
        select(FD_SETSIZE, &temp, NULL, NULL, 0);

        // 标准输入流文件描述符激活
        if(FD_ISSET(STDIN_FILENO, &temp))
        {
            char buf[128] = "";
            int res = read(STDIN_FILENO, buf, sizeof(buf));
            buf[strlen(buf)-1] = 0;
            sendto(cfd, buf, res, 0, (struct sockaddr *)&sin, sizeof(sin));
            if(strcmp(buf, "quit") == 0)
                break;
        }

        // 客户端套接字文件描述符激活
        if(FD_ISSET(cfd, &temp))
        {
            char buf[128] = "";
            int res = read(cfd, buf, sizeof(buf));
            printf("服务器：%s\n", buf);
            if(strcmp(buf, "quit") == 0)
                break;
        }
    }

    close(cfd);

    return 0;
}

 思维导图：