bind() with SO_REUSEADDR fails

Question

My task is to implement a two player game played between two computers connected via TCP. One of the requirement is that only the winner is given the choice play again or not. In case the server wins and decides not to play further, the client should restart as a server and accept new connections.

My approach: If game LOST (in client mode), close sockfd and recreate another one. Then use setsockopt to allow rebinding using SO_REUSEADDR, then calling bind.

int yes = 1;
if ( setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1 )
{
    perror("setsockopt");
}

if ( bind(sockfd, (struct sockaddr*)&svr, sizeof(svr) ) == -1 )
{
    perror("server: bind");
}

But still, I'm getting the same "Address already in use" error. I've tried sleeping for 150 seconds before recreating the socket and this method works.

NOTE: I'm testing this on the same PC. It may work on two linked PCs but its a necessity to make it work on the same PC. Please help.

Answer 1

SO_REUSEADDR only allows you to simultaneously bind to a more a specific address, i.e. first server listens on INADDR_ANY (all interfaces) and subsequent servers listen on different specific interface addresses.

Second scenario is when listening TCP socket accepts a connection, which is kept in use, but the listening socket itself is closed and then re-opened - say parent server process exits and starts again.

In both cases you need to always set SO_REUSEADDR option on the listening socket before you call bind(2).

Answer 2

Since you are running this on the same system, it sounds like you have a race condition. The client is trying to bind() the socket just before the server has closed it (assuming both server and client are setting SO_REUSEADDR on their sockets).

You need to implement some kind of handshake that allows the server to inform the client after it has closed the listening socket - perhaps the server should close the listening socket before it closes the active socket from the last game?