实际是要判断 numbytes 是否等于-1，而不是要判断结果赋给numbytes
if((numbytes = recv(sockfd, buf, sizeof buf,0)) == -1) if(numbytes = recv(sockfd, buf, sizeof buf,0) == -1) // 类似于 numbytes = (recv(sockfd, buf, sizeof buf,0) == -1)
