in example code generated by rpcgen (appname_server.c), there was union used as “define”.

  // "define"
  union {
    internal_type internal_name; // special alias
  } arg; // new alias
  
  // arg is accessed by pointer to avoid member access

have you ever seen it or used it in your code ?? i dont see many advantages unless you are a code generator like rpcgen, since it could have been:

  internal_type arg;

from the start.