VMS Help TCPIP Services, Programming Interfaces, Sockets API, setsockopt(), Arguments  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 s
    A socket descriptor created by the socket() function.
 level
    The protocol level for which the socket options are to be
    modified. It can have one of the following values:
    SOL_SOCKET         Set the options at the socket level.
    p                  Any protocol number. Set the options for
                       protocol level p. See the IN.H header file
                       for the various IPPROTO values.
 optname
    Interpreted by the protocol specified in level. Options at each
    protocol level are documented with the protocol.
    Refer to:
    o  <REFERENCE>(op_setsock_tab) for a list of socket options
    o  <REFERENCE>(tcp_set_tab_opt) for a list of TCP options
    o  <REFERENCE>(ip_set_tab_opt) for a list of IP options
 optval
    Points to a buffer containing the arguments of the specified
    option.
    All socket-level options other than SO_LINGER should be nonzero
    if the option is to be enabled, or zero if it is to be disabled.
    SO_LINGER uses a linger structure argument defined in the
    SOCKET.H header file. This structure specifies the desired state
    of the option and the linger interval. The option value for the
    SO_LINGER command is the address of a linger structure.
    If the socket promises the reliable delivery of data and l_onoff
    is nonzero, the system blocks the process on the close() attempt
    until it is able to transmit the data or until it decides it is
    unable to deliver the information. A timeout period, called the
    linger interval, is specified in l_linger.
    If l_onoff is set to zero and a close() is issued, the system
    processes the close in a manner that allows the process to
    continue as soon as possible.
 optlen
    An integer specifying the size of the buffer pointed to by
    optval.