Fight Against the 'Address alrady in use' Error

You have probably seen this error quite often. The detailed reason why this error occurs is explained in detail here.

In short, if a TCP socket is not closed properly when the program exits, the OS will put that socket in a TIME_WAIT state for a period of time (2MSL, usually a couple of minutes). During that time, if you want to bind to the same port, you'll get the "Address already in use" error, even though technically no body is actually using that port.

In practice, especially when you're debugging, it's very annoying to wait (even a few minutes) before you can re-run your program if it crashes previously. And you often very sure you're the only one that will use that certain port number.

The solution is, you can use the SO_REUSEADDR option to avoid that binding error.

/* reuse server port, since the OS will prevent us to bind to this port
 * immediately after we close the sock */
int optval = 1;
if (setsockopt(server_sock, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval)) != 0) {
    perror("setsockopt");
    return -1;
}

Now you can happily bind to that port again, again, and again...

Resources

Here is a few stackoverflow threads that discussing what happend to the old open socket, and the difference between SO_REUSEADDR and SO_REUSEPORT.