c - Bind failed: Address already in use

Question

I am attempting to bind a socket to a port below:

if( bind(socket_desc,(struct sockaddr *) &server, sizeof(server)) < 0)
{
    perror("bind failed. Error");
    return 1;
}
puts("bind done");

But it gives:

$ ./serve   
Socket created    
bind failed. Error: Address already in use

Why does this error occur?

Answer 1

Everyone is correct. However, if you're also busy testing your code your own application might still "own" the socket if it starts and stops relatively quickly. Try SO_REUSEADDR as a socket option:

What exactly does SO_REUSEADDR do?

This socket option tells the kernel that even if this port is busy (in the TIME_WAIT state), go ahead and reuse it anyway. If it is busy, but with another state, you will still get an address already in use error. It is useful if your server has been shut down, and then restarted right away while sockets are still active on its port. You should be aware that if any unexpected data comes in, it may confuse your server, but while this is possible, it is not likely.

It has been pointed out that "A socket is a 5 tuple (proto, local addr, local port, remote addr, remote port). SO_REUSEADDR just says that you can reuse local addresses. The 5 tuple still must be unique!" by Michael Hunter (mphunter@qnx.com). This is true, and this is why it is very unlikely that unexpected data will ever be seen by your server. The danger is that such a 5 tuple is still floating around on the net, and while it is bouncing around, a new connection from the same client, on the same system, happens to get the same remote port. This is explained by Richard Stevens in ``2.7 Please explain the TIME_WAIT state.''.