Check if socket is connected or not [duplicate]

问题

I have an application which needs to send some data to a server at some time. The easy way would be to close the connection and then open it again when I want to send something. But I want to keep the connection open so when I want to send data, I first check the connection using this function:

bool is_connected(int sock)
{
    unsigned char buf;
    int err = recv(sock,&buf,1,MSG_PEEK);
    return err == -1 ? false : true;
}

The bad part is that this doesn't work. It hangs when there is no data to receive. What can I do? How can I check if the connection is still open?

回答1:

Don't check first and then send. It's wasted effort and won't work anyway -- the status can change between when you check and when you send. Just do what you want to do and handle the error if it fails.

To check status, use:

int error_code;
int error_code_size = sizeof(error_code);
getsockopt(socket_fd, SOL_SOCKET, SO_ERROR, &error_code, &error_code_size);

回答2:

You need to enable non-blocking behavior, by setting O_NONBLOCK using fcntl. One easy but non-standard way to do a non-blocking read would be to use:

recv(sock, &buf, 1, MSG_PEEK | MSG_DONTWAIT);

Afterwards, you must check errno if it fails. It can fail with EAGAIN or it can fail with EBADF or ENOTCONN etc.

Obviously, the simplest and cleanest way to deal with this would be to avoid "forgetting" if the socket is connected or not. You'll notice if the socket becomes disconnected once a recv returns 0 or a send returns EPIPE.

回答3:

Default use of TCP doesn't allow very timely detection of dead sockets (outside of normal closure) so I'll suggest that an "is_connected" function like this is largely useless for all practical purposes. Consider implementing an application-layer keep-alive and track if it's alive based on timely responses (or lack thereof).

edit: after posting i see BoBTFish's link, which is effectively the same thing.

来源：https://stackoverflow.com/questions/12402549/check-if-socket-is-connected-or-not

标签

c++

sockets

connection