Wait for a socket to change status. A wait timeout timeval or int microseconds may be specified; if a timeout is not specified or the timeval is null, the maximum timeout is used. The timeval timeout has an unspecified value when select returns. Returns the number of sockets with status changes, 0 on timeout, or -1 on interruption. If the return value is greater than 0, the SocketSets are updated to only contain the sockets having status changes. For a connecting socket, a write status change means the connection is established and it's able to send. For a listening socket, a read status change means there is an incoming connection request and it's able to accept.