The handling of LWIP_SOCKET_OFFSET looks inconsistent. Suppose LWIP_SOCKET_OFFSET is 100 and LWIP_SOCKET_OPEN_COUNT is defined. The first alloc_socket call sets newsock->count = 100 and returns 100. However, if get_socket(100) is then called, it first subtracts LWIP_SOCKET_OFFSET from s, resulting in 0, and then compares sock->count to 0; won't match.
I guess this mismatch does not harm the Hurd if you define LWIP_SOCKET_OFFSET as 0 there, but I'd like to see it fixed anyway.
