Check if socket is connected or not [duplicate]

Answer 1

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.

Answer 2

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 3

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.

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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.