Linger option "on/off" has no meaning here.

The on/off boolean in linger SockOpt options is not useful here:

• By using the LWIP software layer, as the Lwip_Socket_Close() function cannot be blocked, the closing cannot be "lingered".
• By using the Socket API, the close confirmation is always transmitted after all socket resources are freed (after the TIME-WAIT state is terminated). The closing is always "lingered".
  In both case, the linger timeout is still useful to set a maximum time before freed the socket resources.

A decision has to be taken: Either this value is relevant just in order to have the same interface as POSIX socket or that value has to be removed.
If this option is keeped, the behaviors have to be documented in the Socket API and LWIP documentation.