Linux UDP 套接字 : select() is not notified?

Question

我有这样的 Linux UDP 套接字代码:

//create the UDP socket and bind to a local port 5001
//sockfd = socket(AF_INET, UDP, 0);
//bind(sockfd, .., ...);
recvfrom(sockfd, buffer, 1024, 0, (struct sockaddr *)&remote_addr, &addrlen);
printf("%s\n", buffer);  

到这里，recvfrom 工作正常，上面的printf 可以打印我从ncat 客户端发送的内容。

但是如果我把 recvfrom 放在 select() 之后，看下面的代码，当我从此套接字的 ncat 客户端。

max_fd = 0
FD_ZERO(&read_set);
FD_SET(sockfd, &read_set);

while (1) {
    i = select(max_fd + 1, &read_set, NULL, NULL, NULL);  //==> !!! select() is not returned from ncat sent data to this socket
    if (i < 0 && errno != EINTR) { 
         .............
    }
    for (current_fd = 0; current_fd <= max_fd; current_fd++){
        if ( !FD_ISSET(current_fd, &read_set) )
                continue;
         nbytes = recvfrom(sockfd, buffer, 1024, 0, (struct sockaddr *)&remote_addr, &addrlen);
         printf("%d bytes received \n", nbytes );
    }
}

Answer 1

while (1) {
 FD_ZERO(&read_set);
 FD_SET(sockfd, &read_set);
 i = select(max_fd + 1, &read_set, NULL, NULL, NULL);  
if (i < 0 && errno != EINTR) { 
     .............
}
for (current_fd = 0; current_fd <= max_fd; current_fd++){
    if ( !FD_ISSET(current_fd, &read_set) )
            continue;
     nbytes = recvfrom(sockfd, buffer, 1024, 0, (struct sockaddr    *)&remote_addr, &addrlen);
     printf("%d bytes received \n", nbytes );
}
}