服务端流程步骤

socket函数创建监听套接字lfd

bind函数将监听套接字绑定ip和端口

listen函数设置服务器为被动监听状态，同时创建一条未完成连接队列（没走完tcp三次握手流程的连接），和一条已完成连接队列（已完成tcp三次握手的连接）

accept函数循环的从已完成连接队列中提取连接，并返回一个新的套接字cfd跟客户端进行通信

fork函数创建一个子进程，让子进程与客户端进行通信

子进程：read函数循环的从r缓冲区读取客户端发送的数据，write函数将要发送的数据写入w缓冲区

close函数关闭套接字

客户端流程步骤

socket函数创建套接字

connect函数连接服务器

write函数将要发送的数据写入w缓冲区，read函数从r缓冲区读取服务器发送给客户端的数据

close函数关闭套接字

客户端跟服务器通信流程图

服务端代码

#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>

#define BUF_SIZE 256

void do_sigchld(int signo, siginfo_t *siginfo, void *p)
{
    int status;
    pid_t pid;
    while((pid = waitpid(0, &status, WNOHANG)) > 0) //0:回收跟调用进程同组的子进程的资源，WNOHANG:不阻塞
    {
        if (WIFEXITED(status))
            printf("pid = %d, parent pid = %d, child pid = %d, exit status %d\n", pid, getpid(), siginfo->si_pid, WEXITSTATUS(status));
        else if (WEXITSTATUS(status))
            printf("pid = %d, parent pid = %d, child pid = %d, exit by signal %d\n ", pid, getpid(), siginfo->si_pid, WIFSIGNALED(status));
    }
}

void sys_err(const char *str, int err)
{
    perror(str);
    exit(err);
}

int main(int argc, char *argv[])
{
    if (argc < 2)
    {
        printf("%s port\n", argv[0]);
        exit(1);
    }
    //创建流式套接字
    int lfd = socket(AF_INET, SOCK_STREAM, 0);
    if (lfd < 0)
        sys_err("socket", 1);

    //绑定ip端口    
    struct sockaddr_in server_addr;
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons((unsigned short)atoi(argv[1]));
    server_addr.sin_addr.s_addr = 0; //0表示将本机所有ip都绑定上
    int ret = bind(lfd, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (ret < 0)
        sys_err("bind", 1);
    
    //监听
    ret = listen(lfd, 128);
    if (ret < 0)
        sys_err("listen", 1);
    
    //阻塞SIGCHLD信号
    sigset_t set;
    sigemptyset(&set);
    sigaddset(&set, SIGCHLD);
    sigprocmask(SIG_BLOCK, &set, NULL);
    int block = 1;
    //循环提取
    int cfd;
    pid_t pid;
    struct sockaddr_in client_addr; 
    socklen_t len = sizeof(client_addr);
    char buf[BUF_SIZE];
    ssize_t size;
    char ip[INET_ADDRSTRLEN] = "";
    while(1)
    {
        cfd = accept(lfd, (struct sockaddr *)&client_addr, &len);
        if (cfd < 0)
        {
            if (errno == EINTR)
                continue;

            sys_err("accept", 1);
        }

        printf("client ip = %s, port = %d connect success\n", inet_ntop(AF_INET, &client_addr.sin_addr.s_addr, ip, sizeof(ip)), ntohs(client_addr.sin_port));
        pid = fork();
        if (pid == 0)
        {
            //in child
            close(lfd); //关闭不用了的监听套接字
            //解除阻塞SIGCHLD信号
            sigprocmask(SIG_UNBLOCK, &set, NULL);
            block = 0;

            while(1)
            {
                memset(buf, 0, sizeof(buf));
                size = read(cfd, buf, sizeof(buf));
                if (size == 0) //客户端断开连接
                {
                    printf("client close\n");
                    break;
                }

                printf("%s\n", buf);
                write(cfd, buf, size);
            }

            break;
        }
        else if (pid > 0)
        {
            //in parent
            close(cfd); //关闭不用了的跟客户端通讯的套接字
            if (1 == block)
            {
                //先捕捉
                struct sigaction sa;
                sa.sa_sigaction = do_sigchld;
                sigemptyset(&sa.sa_mask);
                sa.sa_flags = SA_SIGINFO;
                sigaction(SIGCHLD, &sa, NULL);
                //后解除阻塞
                sigprocmask(SIG_UNBLOCK, &set, NULL);
                block = 0;
            }
        }
        else
            sys_err("fork", 1);
    }
    
    //关闭套接字
    if (pid == 0)
        close(cfd);
    else if (pid > 0)
        close(lfd);

    return 0;
}

客户端代码

#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
    if (argc < 3)
    {
        printf("%s sever_ip server_port\n", argv[0]);
        exit(1);
    }

    //创建流式套接字
    int fd = socket(AF_INET, SOCK_STREAM, 0);
    if (fd < 0)
    {
        perror("socket");
        exit(1);
    }

    //连接服务器
    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons((unsigned short)atoi(argv[2]));
    addr.sin_addr.s_addr = inet_addr(argv[1]);
    int ret = connect(fd, (struct sockaddr *)&addr, sizeof(addr));
    printf("connect ret = %d\n", ret);
    if (ret < 0)
    {
        perror("connect");
        exit(1);
    }

    //读写数据
    char buf_w[128] = "";
    char buf_r[128] = "";
    ssize_t size_r = 0;
    while(1)
    {
        fgets(buf_w, sizeof(buf_w), stdin);
        buf_w[strlen(buf_w) - 1] = 0;
        write(fd, buf_w, strlen(buf_w));
        size_r = read(fd, buf_r, sizeof(buf_r));
        if (size_r == 0) //服务器断开
            break;
        else
            printf("%s\n", buf_r);

        memset(buf_w, 0, sizeof(buf_w));
        memset(buf_r, 0, sizeof(buf_r));
    }

    //关闭套接字
    close(fd);

    return 0;
}