s
    A socket descriptor created by the socket() function.
 level
    The protocol level for which the socket options are desired. It
    can have one of the following values:
    SOL_SOCKET         Get the options at the socket level.
    p                  Any protocol number. Get the options for
                       protocol level specified by p. The IPPROTO
                       values are defined in the IN.H header file
                       (for IPv4), or the IN6.H header file (for
                       IPv6).
 optname
    Interpreted by the protocol specified in the level. Options at
    each protocol level are documented with the protocol.
    For descriptions of the supported socket level options, see the
    description of setsockopt() in this chapter.
 optval
    Points to a buffer in which the value of the specified option
    should be placed by getsockopt().
 optlen
    Points to an integer containing the size of the buffer pointed
    to by optval. On return, the integer is modified to indicate the
    actual size of the option value returned.