实战 - 服务端不调用 accept() 的情况下能否建立 TCP 连接？(附代码示例)

答：能

为什么？
accpet() 并不参与 TCP 三次握手过程，服务端调用 accept() 只是为了从全连接队列里取出一个已经建立连接的 socket，然后通过该 socket 进行读写操作

一、操作流程

1. 启动服务端
2. sudo tcpdump -i any -s 0 -w no_accept.dat 'tcp and host 192.168.5.28 and port 55535'
3. 启动客户端
4. 停掉 tcpdump 进程
5. 用 Wireshark 解析 no_accept.dat
   

二、代码示例

/* tcp_server.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>

typedef struct sockaddr sockaddr;
typedef struct sockaddr_in sockaddr_in;

#define IP          "192.168.5.28"
#define PORT        55535
#define BUFFER_SIZE 1024

int main()
{
  int         sockfd, client_socket;
  sockaddr_in server_addr, client_addr;
  socklen_t   client_addr_len = sizeof(client_addr);
  char        buffer[BUFFER_SIZE];
  int         recv_len, send_len;
  char        client_ip[INET_ADDRSTRLEN];
  int         client_port;

  /* 创建 TCP 套接字 */
  if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
  {
    perror("socket failed");
    exit(EXIT_FAILURE);
  }

  /* 设置服务器地址结构 */
  memset(&server_addr, 0, sizeof(server_addr));
  server_addr.sin_family = AF_INET;
  server_addr.sin_port = htons(PORT);
  /* 将 IPv4 地址从文本格式转换为网络字节序的二进制格式 */
  if (inet_pton(AF_INET, IP, &server_addr.sin_addr) <= 0)
  {
    perror("inet_pton failed");
    close(sockfd);
    exit(EXIT_FAILURE);
  }

  /* 绑定套接字到指定地址和端口 */
  if (bind(sockfd, (sockaddr *)&server_addr, sizeof(server_addr)) < 0)
  {
    perror("bind failed");
    close(sockfd);
    exit(EXIT_FAILURE);
  }

  /* 监听客户端连接请求 */
  if (listen(sockfd, 5) < 0)
  {
    perror("bind failed");
    close(sockfd);
    exit(EXIT_FAILURE);
  }

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

accept_again:
  /* 模拟不调用 accept() */
  sleep(60);
  client_socket = accept(sockfd, (sockaddr *)&client_addr, &client_addr_len);
  if (client_socket < 0)
  {
    perror("accept failed");
    close(sockfd);
    exit(EXIT_FAILURE);
  }

  if (NULL == inet_ntop(AF_INET, &client_addr.sin_addr, client_ip, INET_ADDRSTRLEN))
    printf("New client connection, sfd[%d]\n", client_socket);
  else
  {
    client_port = ntohs(client_addr.sin_port);
    printf("New client connection from %s:%d, sfd[%d]\n"
        , client_ip, client_port, client_socket);
  }

  while (1)
  {
    recv_len = read(client_socket, buffer, BUFFER_SIZE);
    if (recv_len < 0)
    {
      perror("read failed");
      close(client_socket);
      close(sockfd);
      exit(EXIT_FAILURE);
    }
    else if (0 == recv_len)
    {
      printf("Client disconnected, sfd[%d]\n", client_socket);
      close(client_socket);
      printf("Close fd[%d]\n", client_socket);
      break;
    }
  }

  goto accept_again;

  /* 关闭套接字 */
  close(sockfd);
  return 0;
}

/* tcp_client.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <arpa/inet.h>
#include <netinet/tcp.h>

typedef struct sockaddr sockaddr;
typedef struct sockaddr_in sockaddr_in;

#define SERVER_IP    "192.168.5.28"
#define SERVICE_PORT 55535
#define BUFFER_SIZE  1024

int main()
{
  int         sockfd;
  sockaddr_in server_addr;
  socklen_t   server_addr_len = sizeof(server_addr);
  char        buffer[BUFFER_SIZE];
  int         len, send_len, recv_len;

  /* 创建 TCP 套接字 */
  if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
  {
    perror("socket failed");
    exit(EXIT_FAILURE);
  }

  /* 设置服务器地址结构 */
  memset(&server_addr, 0, sizeof(server_addr));
  server_addr.sin_family = AF_INET;
  server_addr.sin_port = htons(SERVICE_PORT);
  /* 将 IPv4 地址从文本格式转换为网络字节序的二进制格式 */
  if (inet_pton(AF_INET, SERVER_IP, &server_addr.sin_addr) <= 0)
  {
    perror("inet_pton failed");
    close(sockfd);
    exit(EXIT_FAILURE);
  }

  if (connect(sockfd, (sockaddr *)&server_addr, server_addr_len) < 0)
  {
    perror("connect failed");
    close(sockfd);
    exit(EXIT_FAILURE);
  }

  printf("Connectied to server[%s:%d]\n", SERVER_IP, SERVICE_PORT);

  /* 要比 server 端 sleep 小 */
  sleep(10);

  /* 关闭套接字 */
  close(sockfd);
  return 0;
}