Ancillary Data

data structure to handle various optional data.
cmsghdr + cmsg_data
set to the msg_control member of msghdr{} for sendmsg() and recvmsg() system calls.

   |<--------------------------- msg_controllen -------------------------->|
   |                                  OR                                   |
   |<--------------------------- msg_controllen ----------------------->|
   |                                                                       |
   |<----- ancillary data object ----->|<----- ancillary data object ----->|
   |<------ min CMSG_SPACE() --------->|<------ min CMSG_SPACE() --------->|
   |                                   |                                   |
   |<---------- cmsg_len ---------->|  |<--------- cmsg_len ----------->|  |
   |<--------- CMSG_LEN() --------->|  |<-------- CMSG_LEN() ---------->|  |
   |                                |  |                                |  |
   +-----+-----+-----+--+-----------+--+-----+-----+-----+--+-----------+--+
   |cmsg_|cmsg_|cmsg_|XX|           |XX|cmsg_|cmsg_|cmsg_|XX|           |XX|
   |len  |level|type |XX|cmsg_data[]|XX|len  |level|type |XX|cmsg_data[]|XX|
   +-----+-----+-----+--+-----------+--+-----+-----+-----+--+-----------+--+
   ^
   |
   msg_control
   points here