Check if socket is connected or not [duplicate]

Question

This question already has an answer here:

• How to find the socket connection state in C? 9 answers

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?

Answer 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);

Answer 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 .

Answer 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.