when tcp in close_wait, select always return 1

Question

I wrote a server/client program. And use select check socket. But when client close socket(tcp status in server will get in close_wait), select always return 1 and errno is 0.

Why select return 1? Tcp socket have nothing to read now!

server:

int sock = socket(AF_INET, SOCK_STREAM, 0);

struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY);
addr.sin_port = htons(6999);
socklen_t socklen = sizeof(struct sockaddr_in);
bind(sock, (struct sockaddr *)&addr, socklen);
listen(sock, 0);

int clisock;
clisock = accept(sock, NULL, NULL);

fd_set backset, rcvset;
struct timeval timeout;
timeout.tv_sec = 3;

int maxfd = clisock+1;
FD_SET(clisock, &rcvset);
backset = rcvset;

int ret;
while(1) {
    rcvset = backset;
    timeout.tv_sec = 3;
    ret = select(maxfd, &rcvset, NULL, NULL, &timeout);
    if(ret <= 0)
        continue;

    sleep(1);
    printf("ret:%d, %s\n", 
        ret, strerror(errno));
}

client:

int sock = socket(AF_INET, SOCK_STREAM, 0);

struct sockaddr_in addr;
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr("127.0.0.1");
addr.sin_port = htons(6999);

socklen_t socklen = sizeof(struct sockaddr_in);
connect(sock, (struct sockaddr *)&addr, socklen);

sleep(3);
close(sock);
sleep(100);

output:

./server 
ret:1, Success
ret:1, Success
ret:1, Success

When the remote end-point closes its connection, the local end-point will become readable, but with read returning 0. That is the indication that the remote end-point has been closed, read returning 0. — Some programmer dude
Regarding your use of errno, don't check it unless there actually is an error, and always check it directly after the erroneous function call (i.e. if select fail you have to check errno directly after select). Think about what would happen if sleep failed? Then errno would contain the error from the sleep function. — Some programmer dude
You have told select() that you want a notification when you can read from the socket. You should read from the socket, then you can check if reading from the socket give you an error on that socket. (select() can watch many sockets, select() is not going to indicate an error on one particular socket when you watch the socket for read events). If you chose to not read from the socket when select() tells you that you can, select() will tell you that you still can read from it on the next iteration too. — nos
And lastly, remember that select modifies the sets, so if you have multiple descriptors in the sets you pass to select, you have to clear the set and add all descriptors again. — Some programmer dude
The value of errno if there is no error is undefined, don't check it unless there is an error. — Some programmer dude

user207421 user207421 · Accepted Answer · 2014-09-03T09:50:49

The socket is readable because the peer has closed it and when you read from it you will get an end of stream. Rather than nothing.

CLOSE_WAIT means that TCP is waiting for you to close the socket. So close it.

when tcp in close_wait, select always return 1

2 Answers