while (1) {
        clientaddr_len = sizeof(clientaddr);
        connfd = accept(listenfd, (SA*) &clientaddr, &clientaddr_len);
        服务逻辑代码
        close(connfd);       //服务结束关闭连接..
}

[tangyujie@VM-4-9-centos Serve]$ cat server.c
#include <sys/types.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <strings.h>
#define SERVE_PORT 12345
#define ERR_EXIT(m) \
  do { perror(m); exit(EXIT_FAILURE); } while (0)
typedef struct sockaddr SA;
int listenfd;               //设置全局监听套接字, 方便关闭
void handle(int signo) {
  fprintf(stdout, "ByeBye!\n");
  close(listenfd);
  exit(EXIT_SUCCESS);
}
int main() {
  signal(SIGINT, handle);
  int connfd;
  struct sockaddr_in serveAdd, clientAdd;
  socklen_t clientAdd_len;
  char ipbuff[256];
  //创建套接字
  if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
    ERR_EXIT("socket"); //协议家族 服务类型(套接字类型), 协议弃用(0)
  }
  //确定服务端地址簇
  bzero(&serveAdd, sizeof(serveAdd));         //清0
  serveAdd.sin_family = AF_INET;
  serveAdd.sin_port = htons(SERVE_PORT);
  serveAdd.sin_addr.s_addr = htonl(INADDR_ANY);   //注意转网络字节序
  //bind 端口 地址信息
  if (bind(listenfd, (SA*)&serveAdd, sizeof(serveAdd)) == -1) {
    ERR_EXIT("bind");
  }
  //开始监听..
  if (listen(listenfd, 3) == -1) {
    ERR_EXIT("listen");
  }
  printf("Accepting connections..\n");
  //循环不断的接收客户的连接请求进行服务
  while (1) {
    clientAdd_len = sizeof(clientAdd);
    if ((connfd = accept(listenfd, (SA*)&clientAdd, &clientAdd_len)) == -1) {
      ERR_EXIT("accept");
    }
    printf("recieve connection from ip is %s and port is %d\n",
      inet_ntop(AF_INET, &clientAdd.sin_addr, ipbuff, sizeof(ipbuff)), 
      ntohs(clientAdd.sin_port));
    //服务
    while (1) {
      char buff[1024] = {0};
      int i;
      int n = read(connfd, buff, sizeof(buff));   //读数据
      if (n == -1) {
        ERR_EXIT("read");
      } 
      if (n == 0) {                 //说明客户端主动断开连接
        break;
      }
      //处理数据, 简单的小写字符转大写
      for (i = 0; i < n; ++i) {
        buff[i] = toupper(buff[i]);
      }
      //写回
      write(connfd, buff, n);
    }
    fprintf(stdout, "ip %s and port is %d interrupt connfd\n",
        ipbuff, ntohs(clientAdd.sin_port));
    close(connfd);                    //服务结束断开连接
  }
    return 0;
}

[tangyujie@VM-4-9-centos Serve]$ cat client.c
#include <sys/types.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <signal.h>
#include <string.h>
#define SERVE_PORT 12345                //端口号
#define ERR_EXIT(m)\
  do { perror(m); exit(EXIT_FAILURE); } while (0)     //错误处理
typedef struct sockaddr SA;               
int sockfd;                       //设置全局, 方便关闭
void handle(int signo) {
  fprintf(stdout, "ByeBye!\n");
  close(sockfd);
  exit(EXIT_SUCCESS);
}
int main(int argc, char* argv[]) {
  if (argc != 2) {
    fprintf(stderr, "%s <ip>", argv[0]);
    close(EXIT_FAILURE);
  }
  signal(SIGINT, handle);
  struct sockaddr_in serveAdd;
  //创建套接字
  if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
    ERR_EXIT("socket"); //协议家族 服务类型(套接字类型), 协议弃用(0)
  }
    //确定服务端地址簇
  bzero(&serveAdd, sizeof(serveAdd));         //清0
  serveAdd.sin_family = AF_INET;
  serveAdd.sin_port = htons(SERVE_PORT);
  //将传入的ip字符串转换为 sin_addr
  if (inet_pton(AF_INET, argv[1], &serveAdd.sin_addr) == -1) {
    ERR_EXIT("inet_ntop");
  } 
  //不需要绑定端口号 系统随机分配一个临时端口号, 直接连接
  if (connect(sockfd, (SA*)&serveAdd, sizeof(serveAdd)) == -1) {
    ERR_EXIT("connect");
  }
  while (1) {                   //死循环, 使用ctrl c信号关闭连接
    char buff[1024];
    printf("请说>>");
    scanf("%s", buff);
    write(sockfd, buff, strlen(buff));
    int n = read(sockfd, buff, sizeof(buff));
    if (n == -1) {
      ERR_EXIT("read");
    }
    buff[n] = 0;
    fprintf(stdout, ">>%s\n", buff);
  }
}