分为两个独立的程序
编译前先确定自己服务器的地址，比如想要在自己的ubuntu下运行，先ip addr获取自己的ip地址，修改chat_server.c，chat_client.c里面的ip地址为自己的ip地址

服务端chat_server.c
编译: gcc chat_server.c -o server
运行: ./server

#include <stdio.h>
#include <sys/types.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <netinet/tcp.h>
#include <errno.h>
#include <signal.h>

#define    MAX_USER        500
#define    MAX_MSG_LEN        1024
#define MAX_IO_RETRY_TIMES    5

int server_fd;

int user_cnt = 0;
int user_fds[MAX_USER];

char msg_buffer[MAX_MSG_LEN];

void down()
{
    int i;
    for(i = 0; i < user_cnt; i++)
    {
        shutdown(user_fds[i], SHUT_RDWR);
        close(user_fds[i]);
        user_fds[i] = 0;
    }
    shutdown(server_fd, SHUT_RDWR);
    close(server_fd);
    server_fd = 0;
}

int go_on = 1;
void sig_handler(int signal)
{
    go_on = 0;
}

int my_safe_read(int fd)
{
    int ret;
    int i;
    
    for(i = 0; i < MAX_IO_RETRY_TIMES; i++)
    {
        errno = 0;
        int ret = read(fd, msg_buffer, MAX_MSG_LEN);
        if(ret < 0)
        {
            if(EAGAIN == errno || EWOULDBLOCK == errno)
            {
                printf("Safe read [%d] cnt\n", i + 1);
                continue;
            }
            else
            {
                return ret;
            }
        }
        else if(0 == ret)
        {
            return 0;
        }
        else
        {
            return ret;
        }
    }
    return ret;
}

int my_safe_write(int fd)
{
    write(fd, msg_buffer, MAX_MSG_LEN);
}

int main(int argc, char *argv[])
{
    signal(SIGINT, sig_handler);
    server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if(-1 == server_fd)
    {
        printf("socket err\n");
        return -1;
    }
    printf("socket ok\n");

    struct sockaddr_in server_addr;
    bzero(&server_addr, sizeof(struct sockaddr_in));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    unsigned short portnum = 0x8888;
    if(argc >= 2)
    {
        int port = atoi(argv[1]);
        if(port > 20000)
        {
            portnum = port;
        }
    }
    printf("port = %d\n", portnum);
    
    server_addr.sin_port = portnum;

    if(-1 == bind(server_fd, (struct sockaddr *)(&server_addr), sizeof(struct sockaddr)))
    {
        printf("bind err\n");
        return -1;
    }
    printf("bind ok\n");

    if(-1 == listen(server_fd, MAX_USER))
    {
        printf("listen err\n");
        return -1;
    }
    printf("listen ok\n");

    int i;
    fd_set fds;
    struct timeval tv;
    while(go_on)
    {
        FD_ZERO(&fds);
        FD_SET(server_fd, &fds);
        for(i = 0; i < user_cnt; i++)
        {
            printf("-%d\n", user_fds[i]);
            if(user_fds[i])
            {
                FD_SET(user_fds[i], &fds);
            }
        }

        int fd_cnt_to_selected = 0;
        for(i = 0; i < user_cnt; i++)
        {
            if(user_fds[i] > fd_cnt_to_selected)
            {
                fd_cnt_to_selected = user_fds[i];
            }
        }
        fd_cnt_to_selected = (0 == user_cnt)? (server_fd + 1) : (fd_cnt_to_selected + 1);

        tv.tv_sec = 2;
        tv.tv_usec = 0;
        int ret = select(fd_cnt_to_selected, &fds, 0, 0, &tv);
        if(ret < 0)
        {
            printf("select err\n");
            break;
        }
        else if(0 == ret)
        {
            continue;
        }

        struct sockaddr_in client_addr;
        for(i = 0; i < user_cnt; i++)
        {
            if(FD_ISSET(user_fds[i], &fds))
            {
                bzero(msg_buffer, MAX_MSG_LEN);
                int ret = my_safe_read(user_fds[i]);
                if(ret <= 0)
                {
                    int connection_state = 1; 
                    int len_of_int = sizeof(int);
                    getsockopt(user_fds[i], IPPROTO_TCP, SO_KEEPALIVE, (void*)&connection_state, &len_of_int); 
                    printf("connection_state = %s\n", (0 == connection_state)? "break" : "alive");
                    if(0 == connection_state)
                    {
                        close(user_fds[i]);
                        FD_CLR(user_fds[i], &fds);
                        if(user_cnt - 1 == i)
                        {
                            user_cnt = user_cnt - 1;
                            user_fds[i] = 0;
                        }
                        else
                        {
                            int j;
                            for(j = i; j < user_cnt; j++)
                            {
                                user_fds[j] = user_fds[j+1];
                            }
                            user_fds[j] = 0;
                            user_cnt = user_cnt -1;
                        }
                    }
                }
                else
                {
                    printf("receive[%s]\n", msg_buffer);
                    int j;
                    for(j = 0; j < user_cnt; j++)
                    {
                        if(j == i)
                        {
                            continue;
                        }
                        my_safe_write(user_fds[j]);
                    }
                }
            }
        }

        if(FD_ISSET(server_fd, &fds))
        {
            int sin_size = sizeof(struct sockaddr);
        
            user_fds[user_cnt] = accept(server_fd, (struct sockaddr *)(&client_addr), &sin_size);
            if(-1 == user_fds[user_cnt])
            {
                printf("accept err\n");
                return -1;
            }
            printf("accept ok!\n");
            printf("user_fds[user_cnt] = %d\n", user_fds[user_cnt]);
            user_cnt++;
        }
    }
    
    down();
    return 0;
}

客户端chat_client.c
编译: gcc chat_client -lpthread -o chat_client
运行: 服务端先运行起来后，再启动客户端，可以在多态机器上启多个客户端，互相聊天
./chat_client

#include <stdlib.h>
#include <sys/types.h>
#include <stdio.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <pwd.h>
#include <signal.h>

int cfd = 0;
struct passwd *pwd;

void sig_handler(int signal)
{
    char bye_msg[1024] = { 0 };
    sprintf(bye_msg, "WARNING------>User[%s] have go, he said all is 'SB'! Oh, what the fuck!", pwd->pw_name);
    write(cfd, bye_msg, 1024);
    
    shutdown(cfd, SHUT_RDWR);
    close(cfd);
    cfd = 0;
    exit(-1);
}

void* f(void* t)
{
    struct timeval tv;
    fd_set fds;
    while(1)
    {
        tv.tv_sec = 0;
        tv.tv_usec = 100;
        FD_ZERO(&fds);
        FD_SET(cfd, &fds);
        
        int r = select(cfd + 1, &fds, 0, 0, &tv);
        if(r<0)
        {
            printf("select err\n");
        }
        else if(r==0)
        {
        }
        
        char buf[1024*10] = { 0 };
        int n = read(cfd, buf, 1024*10);
        if(buf[0] != 0)
            printf("%s\n", buf);
    }    
}

int main(int argc, char * argv[])
{
    signal(SIGINT, sig_handler);
    pwd = getpwuid(getuid());

    int recbytes;
    int sin_size;
    char buffer[1024] ={0}; 
    struct sockaddr_in s_add, c_add;
    unsigned short portnum =0x8888;
    if(argc >= 2)
    {
        int port = atoi(argv[1]);
        if(port > 20000)
        portnum = port;
    }
    printf("port = %d\n", portnum);
    
    cfd = socket(AF_INET, SOCK_STREAM, 0);
    if(-1 == cfd)
    {
        printf("socket err\n");
        return -1;
    }
    //printf("socket ok\n");

    bzero(&s_add, sizeof(struct sockaddr_in));
    s_add.sin_family = AF_INET;
    s_add.sin_addr.s_addr = inet_addr("192.168.60.104");
    s_add.sin_port = htons(portnum);
    
    if(-1 == connect(cfd, (struct sockaddr *)(&s_add), sizeof(struct sockaddr)))
    {
        printf("connect err\n");
        return -1;
    }
    char personal_info[128] = { 0 };
    sprintf(personal_info, "DouBi[%s] is comming!!!HaHaaa^-^", pwd->pw_name);
    write(cfd, personal_info, 128);
    //printf("connect ok\n");

    pthread_t th;
    int rr = pthread_create((pthread_t *)&th, NULL, f, NULL);
    
    while(1)
    {    
        //printf("enter what you want to say:_\n");
        char b[10240] = { 0 };
        sprintf(b, "[%s]: ", pwd->pw_name);
        gets(b + strlen(b));
        //printf("b = %s\n", b);
        if(-1 == (recbytes = write(cfd, b, 1024*10)))
        {
            printf("send data err\n");
            return -1;
        }
        //printf("send ok\n");
    }
    

    //close(cfd);
//    sleep(100);
    
    return 0;
}