关于STM32F4+LWIP,socket接口接收失败

使用了STM32F407+LWIP+FreeRTOS，需要与服务器进行tcp通信，采用了socket接口，连接都成功了，但是使用函数recv接收时，一直返回0，即断开连接。怎么会这样子啊啊，我要疯了啊啊。谁能帮帮我啊啊？代码如下：

while(gnetif.ip_addr.addr == 0)
      {//获取到本机ip
            osDelay(1000);
      }

      server_addr.sin_family = AF_INET;

      server_addr.sin_addr.s_addr = inet_addr("119.103.33.44");
      server_addr.sin_port = htons(1234);


      while(1)
      {
      sock_fd = socket(AF_INET, SOCK_STREAM, 0);
            if(sock_fd == -1)
            {
                     osDelay(1000); //1秒
            }
            else
            {
               res = connect(sock_fd, (struct sockaddr *)&server_addr, sizeof(struct sockaddr));
                     if(0 == res)
                     {
                              static s8 recv_flag;
                              while(1)
                              {
                                    //接收
                                    recv_flag = recv(sock_fd,recv_data_buf,RECV_INSTR_DATA_NUM,0);
                                    if(recv_flag == 0)
                                    {//另一端已关闭
                                             break;
                                    }
                                    else if(recv_flag == -1)
                                    {//接收失败,下次再接收

                                    }
                                    else
                                    {
                                       //接收正确
                                    }

                              osDelay(50);
                              }
                              close(sock_fd);
                     }
                     else
                     {
                              close(sock_fd);
                              osDelay(500);
                     }
            }
            osDelay(500);
      }