c语言实现tcp网络通信，客户端显示连接成功后服务端返回要求客户端登录，客户端显示“Please enter user and pw”客户端输入账号密码（正确的应为 abc 和 def），服务端返回登录成功，否则显示登陆失败（密码错误）

服务端代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

#define PORT 8888
#define MAXLINE 1024

int main() {
    int listenfd, connfd;
    struct sockaddr_in servaddr, cliaddr;
    char buff[MAXLINE];
    char account[MAXLINE] = "abc";
    char password[MAXLINE] = "def";
    socklen_t len;
    pid_t pid;

    // 创建套接字
    listenfd = socket(AF_INET, SOCK_STREAM, 0);

    // 初始化地址结构
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(PORT);

    // 绑定地址和端口
    bind(listenfd, (struct sockaddr *) &servaddr, sizeof(servaddr));

    // 监听连接
    listen(listenfd, 5);

    printf("Server is listening at port %d...\n", PORT);

    while (1) {
        len = sizeof(cliaddr);

        // 接受客户端连接
        connfd = accept(listenfd, (struct sockaddr *) &cliaddr, &len);

        // 创建子进程处理客户请求
        pid = fork();

        if (pid == -1) {
            printf("Create fork error\n");
            exit(1);
        }
        else if (pid == 0) {  // child process
            printf("Client connected from %s:%d\n", inet_ntoa(cliaddr.sin_addr), ntohs(cliaddr.sin_port));

            // 连接成功后要求客户端登录
            strcpy(buff, "Please enter user and pw");
            send(connfd, buff, strlen(buff), 0);

            // 处理客户端传来的帐号密码
            bzero(buff, sizeof(buff));
            recv(connfd, buff, MAXLINE, 0);
            char* ptr = strtok(buff, " ");
            char* user = ptr;
            ptr = strtok(NULL, "\n");
            char* pw = ptr;

            // 校验帐号密码
            if (strcmp(user, account) == 0 && strcmp(pw, password) == 0) {
                strcpy(buff, "Login success");
            }
            else {
                strcpy(buff, "Login failed (wrong password)");
            }

            // 返回登录结果
            send(connfd, buff, strlen(buff), 0);

            // 关闭连接
            close(connfd);

            printf("Client disconnected\n");

            exit(0);
        }

        // 父进程关闭连接
        close(connfd);
    }

    return 0;
}

客户端代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

#define SERV_PORT 8888
#define MAXLINE 1024

int main(int argc, char *argv[]) {
    int sockfd;
    struct sockaddr_in servaddr;
    char buff[MAXLINE];

    // 创建套接字
    sockfd = socket(AF_INET, SOCK_STREAM, 0);

    // 初始化地址结构
    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(SERV_PORT);

    // 将点分十进制IP地址转换为网络字节序IP地址
    inet_pton(AF_INET, "127.0.0.1", &servaddr.sin_addr);

    // 连接服务器
    if (connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr)) < 0) {
        printf("Connect error\n");
        exit(1);
    }

    // 接受服务器的要求登录信息
    bzero(buff, sizeof(buff));
    recv(sockfd, buff, MAXLINE, 0);
    printf("%s\n", buff);

    // 输入帐号密码
    printf("Please enter user and pw: ");
    fgets(buff, MAXLINE, stdin);

    // 发送帐号密码给服务器
    send(sockfd, buff, strlen(buff), 0);

    // 接受服务器的登录结果
    bzero(buff, sizeof(buff));
    recv(sockfd, buff, MAXLINE, 0);

    // 显示登录结果
    printf("%s\n", buff);

    // 关闭连接
    close(sockfd);

    return 0;
}

参考资料：

- [TCP/IP网络编程套接字API](https://blog.csdn.net/Wayne_Wang/article/details/52)
- [TCP网络通信之C/S模型（C实现，在linux环境下）](https://blog.csdn.net/qq_37342943/article/details/81505805)