Error: Address already in use while binding socket with address but the port number is shown free by `netstat`

I've run into that same issue as well. It's because you're closing your connection to the socket, but not the socket itself. The socket can enter a TIME_WAIT state (to ensure all data has been transmitted, TCP guarantees delivery if possible) and take up to 4 minutes to release.

or, for a REALLY detailed/technical explanation, check this link

It's certainly annoying, but it's not a bug. See the comment from @Vereb on this answer below on the use of SO_REUSEADDR.

Try netstat like this: netstat -ntp, without the -l. It will show tcp connection in TIME_WAIT state.

I know its been a while since the question was asked but I was able to find a solution:

int sockfd;
int option = 1;
sockfd = socket(AF_INET, SOCK_STREAM, 0);
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &option, sizeof(option));

This set the socket able to be reused immediately.

I apologize if this is "wrong". I'm not very experienced with sockets