s
    A socket descriptor returned by socket(), subsequently bound to
    an address with bind(), which is listening for connections after
    a listen().
 addr
    A result argument filled in with the address of the connecting
    entity, as known to the TCP/IP kernel. The exact format of the
    structure to which the address parameter points is determined by
    the address family. Specify either the IPv4 address family (AF_
    INET) or the IPv6 address family (AF_INET6).
 addrlen
    A value/result argument. It should initially contain the size of
    the structure pointed to by addr. On return it will contain the
    actual length, in bytes, of the sockaddr structure that has been
    filled in by the TCP/IP kernel.